Receptor and substrate interactions of clostridial neurotoxins

EGFP/RFP-based FRET sensors for botulinum neurotoxin A biological activity detection and methodological validation

BACKGROUND

Botulinum neurotoxin type A (BoNT/A) is the most potent and prevalent neurotoxin known to cause botulism, and is also widely used in medical and cosmetic applications. The detection of BoNT/A is of great significance for botulism diagnosis and drug potency determination. Currently, the mouse bioassay (MBA) has long been the gold standard method but has disadvantages of ethical concerns, long testing duration, and high costs. CFP/YFP-based FRET sensors for detecting BoNT/A biological activity require complicated experimental control due to crosstalk, as well as inconvenient operation, intricate data analysis and UV-induced phototoxicity.

RESULTS

In present work, a FRET-based EGFP-SNAP25(141-206)-DsRED molecular sensor was developed to analyze the endopeptidase activity of BoNT/A, and methodological validation was performed. The results showed that the sensor has a high sensitivity of 4 U, with a linear range from 7.8 U to 125 U, a recovery rate of 96.8 % to 122.7 %, and a precision within 20 %. Furthermore, a cellular sensor for FRET-based BoNT/A biological activity detection was successfully constructed with a Neuro-2a cell line stably expressing the EGFP-SNAP25-tDimer2 variant, enabling the monitoring of the entire process of BoNT/A action. The cell-based assay exhibited a sensitivity of up to 100 pM of BoNT/A with a linear range from 3.125 nM to 50 nM and a recovery rate of 86.3 % to 117.2 %.

SIGNIFICANCE

The FRET-based molecular and cellular sensors provided a convenient and rapid method for measuring BoNT/A biological activity, making them suitable for assessing BoNT/A raw material biological activities in clinical and environmental samples, as well as batches during pharmaceutical production processes.

New Engineered-Chimeric Botulinum Neurotoxin Mutant Acts as an Effective Bivalent Vaccine Against Botulinum Neurotoxin Serotype A and E

Botulinum neurotoxins (BoNTs), including serotypes A and E, are potent biotoxins known to cause human poisoning. In addition to the critical protective antigen found in the full BoNT molecule, the receptor binding domain (Hc domain), BoNTs also harbour another essential protective antigen-the light chain-translocation domain (L-HN domain). Leveraging these pivotal protective antigens, we genetically engineered a series of inactivated chimeric molecules incorporating L-HN and Hc domains of BoNT/A and E. The structure of these chimeric molecules, mirror BoNT/A and E, but are devoid of enzyme activity. Experimental findings demonstrated that a lead candidate mEL-HN-mAHc harnessing the inactivated protease LCHN/E with the mutated gangliosides binding site Hc/A (mE-mA) elicited robust immune protection against BoNT/A and E simultaneously in a mouse model, requiring low immune dosages and minimal immunisations. Moreover, mE-mA exhibited high protective efficacy against BoNT/A and E in guinea pigs and New Zealand white rabbits, resulting in elevated neutralising antibody titres. Furthermore, mE-mA proved to be a more stable and safer vaccine compared to formaldehyde-inactivated toxoid. Our data underscore the genetically engineered mE-mA as a highly effective bivalent vaccine against BoNT/A and E, paving the way for the development of polyvalent vaccines against biotoxins.

Toxicology and pharmacology of botulinum and tetanus neurotoxins: an update

Tetanus and botulinum neurotoxins cause the neuroparalytic syndromes of tetanus and botulism, respectively, by delivering inside different types of neurons, metalloproteases specifically cleaving the SNARE proteins that are essential for the release of neurotransmitters. Research on their mechanism of action is intensively carried out in order to devise improved therapies based on antibodies and chemical drugs. Recently, major results have been obtained with human monoclonal antibodies and with single chain antibodies that have allowed one to neutralize the metalloprotease activity of botulinum neurotoxin type A1 inside neurons. In addition, a method has been devised to induce a rapid molecular evolution of the metalloprotease domain of botulinum neurotoxin followed by selection driven to re-target the metalloprotease activity versus novel targets with respect to the SNARE proteins. At the same time, an intense and wide spectrum clinical research on novel therapeutics based on botulinum neurotoxins is carried out, which are also reviewed here.

Interaction between a Novel Oligopeptide Fragment of the Human Neurotrophin Receptor TrkB Ectodomain D5 and the C-Terminal Fragment of Tetanus Neurotoxin

This article presents experimental evidence and computed molecular models of a potential interaction between receptor domain D5 of TrkB with the carboxyl-terminal domain of tetanus neurotoxin (Hc-TeNT). Computational simulations of a novel small cyclic oligopeptide are designed, synthesized, and tested for possible tetanus neurotoxin-D5 interaction. A hot spot of this protein-protein interaction is identified in analogy to the hitherto known crystal structures of the complex between neurotrophin and D5. Hc-TeNT activates the neurotrophin receptors, as well as its downstream signaling pathways, inducing neuroprotection in different stress cellular models. Based on these premises, we propose the Trk receptor family as potential proteic affinity receptors for TeNT. In vitro, Hc-TeNT binds to a synthetic TrkB-derived peptide and acts similar to an agonist ligand for TrkB, resulting in phosphorylation of the receptor. These properties are weakened by the mutagenesis of three residues of the predicted interaction region in Hc-TeNT. It also competes with Brain-derived neurotrophic factor, a native binder to human TrkB, for the binding to neural membranes, and for uptake in TrkB-positive vesicles. In addition, both molecules are located together in vivo at neuromuscular junctions and in motor neurons.

Botulinum Toxin: From Poison to Possible Treatment for Spasticity in Spinal Cord Injury

Botulism has been known for about three centuries, and since its discovery, botulinum toxin has been considered one of the most powerful toxins. However, throughout the 20th century, several medical applications have been discovered, among which the treatment of spasticity stands out. Botulinum toxin is the only pharmacological treatment recommended for spasticity of strokes and cerebral palsy. Although its use as an adjuvant treatment against spasticity in spinal cord injuries is not even approved, botulinum toxin is being used against such injuries. This article describes the advances that have been made throughout history leading to the therapeutic use of botulinum toxin and, in particular, its application to the treatment of spasticity in spinal cord injury.

Botulinum Neurotoxins: Mechanism of Action

Botulinum neurotoxins (BoNTs) are a growing family of bacterial protein toxins that cause botulism, a rare but often fatal animal and human disease. They are the most potent toxins known owing to their molecular architecture, which underlies their mechanism of action. BoNTs target peripheral nerve terminals by a unique mode of binding and enter into their cytosol where they cleave SNARE proteins, thus inhibiting the neurotransmitter release. The specificity and rapidity of binding, which limits the anatomical area of its neuroparalytic action, and its reversible action make BoNT a valuable pharmaceutical to treat neurological and non-neurological diseases determined by hyperactivity of cholinergic nerve terminals. This review reports the progress on our understanding of how BoNTs cause nerve paralysis highlighting the different steps of their molecular mechanism of action as key aspects to explain their extreme toxicity but also their unique pharmacological properties.

Optimization of SNAP-25 and VAMP-2 Cleavage by Botulinum Neurotoxin Serotypes A-F Employing Taguchi Design-of-Experiments

The detection of catalytically active botulinum neurotoxins (BoNTs) can be achieved by monitoring the enzymatic cleavage of soluble NSF (N-ethylmaleimide-sensitive-factor) attachment protein receptor (SNARE) proteins by the toxins’ light chains (LC) in cleavage-based assays. Thus, for sensitive BoNT detection, optimal cleavage conditions for the clinically relevant A–F serotypes are required. Until now, a systematic evaluation of cleavage conditions for the different BoNT serotypes is still lacking. To address this issue, we optimized cleavage conditions for BoNT/A–F using the Taguchi design-of-experiments (DoE) method. To this aim, we analyzed the influence of buffer composition (pH, Zn²⁺, DTT (dithiothreitol), NaCl) as well as frequently used additives (BSA (bovine serum albumin), Tween 20, trimethylamine N-oxide (TMAO)) on BoNT substrate cleavage. We identified major critical factors (DTT, Zn²⁺, TMAO) and were able to increase the catalytic efficiency of BoNT/B, C, E, and F when compared to previously described buffers. Moreover, we designed a single consensus buffer for the optimal cleavage of all tested serotypes. Our optimized buffers are instrumental to increase the sensitivity of cleavage-based assays for BoNT detection. Furthermore, the application of the Taguchi DoE approach shows how the method helps to rationally improve enzymatic assays.

Botulinum neurotoxins A, B, C, E, and F preferentially enter cultured human motor neurons compared to other cultured human neuronal populations

Human-induced pluripotent stem cell (hiPSC)-derived neurons can be exquisitely sensitive to botulinum neurotoxins (BoNTs), exceeding sensitivity of the traditionally used mouse bioassay. In this report, four defined hiPSC-derived neuronal populations including primarily GABAergic, glutamatergic, dopaminergic, and motor neurons were examined for BoNT/A, B, C, D, E, and F sensitivity. The data indicate that sensitivity varies markedly for the BoNTs tested. Motor neurons are significantly more sensitive than other neuron types for all BoNTs except BoNT/D. Examination of SNARE protein levels and BoNT-specific cell surface protein receptors reveals few differences between the cell types except greater expression levels of the receptor protein SV2C and synapsin-IIa in motor neurons. This indicates that differential toxicity of BoNTs for motor neurons compared to other neuronal cell types involves multiple mechanisms.

Functional detection of botulinum neurotoxin serotypes A to F by monoclonal neoepitope-specific antibodies and suspension array technology

Botulinum neurotoxins (BoNTs) are the most potent toxins known and cause the life threatening disease botulism. Sensitive and broad detection is extremely challenging due to the toxins' high potency and molecular heterogeneity with several serotypes and more than 40 subtypes. The toxicity of BoNT is mediated by enzymatic cleavage of different synaptic proteins involved in neurotransmitter release at serotype-specific cleavage sites. Hence, active BoNTs can be monitored and distinguished in vitro by detecting their substrate cleavage products. In this work, we developed a comprehensive panel of monoclonal neoepitope antibodies (Neo-mAbs) highly specific for the newly generated N- and/or C-termini of the substrate cleavage products of BoNT serotypes A to F. The Neo-mAbs were implemented in a set of three enzymatic assays for the simultaneous detection of two BoNT serotypes each by monitoring substrate cleavage on colour-coded magnetic Luminex-beads. For the first time, all relevant serotypes could be detected in parallel by a routine in vitro activity assay in spiked serum and food samples yielding excellent detection limits in the range of the mouse bioassay or better (0.3-80 pg/mL). Therefore, this work represents a major step towards the replacement of the mouse bioassay for botulism diagnostics.

Botulinum neurotoxin serotype D - A potential treatment alternative for BoNT/A and B non-responding patients

OBJECTIVES

Botulinum neurotoxin serotypes A and B (BoNT/A & B) are highly effective medicines to treat hyperactive cholinergic neurons. Due to neutralizing antibody formation, some patients may become non-responders. In these cases, the serotypes BoNT/C-G might become treatment alternatives. BoNT/D is genetically least related to BoNT/A & B and thereby circumventing neutralisation in A/B non-responders. We produced BoNT/D and compared its pharmacology with BoNT/A ex vivo in mice tissue and in vivo in human volunteers.

METHODS

BoNT/D was expressed recombinantly in E. coli, isolated by chromatography and its ex vivo potency was determined at mouse phrenic nerve hemidiaphragm preparations. Different doses of BoNT/D or incobotulinumtoxinA were injected into the extensor digitorum brevis (EDB) muscles (n = 30) of human volunteers. Their compound muscle action potentials were measured 11 times by electroneurography within 220 days.

RESULTS

Despite a 3.7-fold lower ex vivo potency in mice, a 110-fold higher dosage of BoNT/D achieved the same clinical effect as incobotulinumtoxinA while showing a 50% shortened duration of action.

CONCLUSIONS

BoNT/D blocks dose-dependently acetylcholine release in human motoneurons upon intramuscular administration, but its potency and duration of action is inferior to approved BoNT/A based drugs.

SIGNIFICANCE

BoNT/D constitutes a potential treatment alternative for BoNT/A & B non-responders.

Why Are Botulinum Neurotoxin-Producing Bacteria So Diverse and Botulinum Neurotoxins So Toxic?

Botulinum neurotoxins (BoNTs) are the most lethal toxins among all bacterial, animal, plant and chemical poisonous compounds. Although a great effort has been made to understand their mode of action, some questions are still open. Why, and for what benefit, have environmental bacteria that accidentally interact with their host engineered so diverse and so specific toxins targeting one of the most specialized physiological processes, the neuroexocytosis of higher organisms? The extreme potency of BoNT does not result from only one hyperactive step, but in contrast to other potent lethal toxins, from multi-step activity. The cumulative effects of the different steps, each having a limited effect, make BoNTs the most potent lethal toxins. This is a unique mode of evolution of a toxic compound, the high potency of which results from multiple steps driven by unknown selection pressure, targeting one of the most critical physiological process of higher organisms.

Variations in the Botulinum Neurotoxin Binding Domain and the Potential for Novel Therapeutics

Botulinum neurotoxins (BoNTs) are categorised into immunologically distinct serotypes BoNT/A to /G). Each serotype can also be further divided into subtypes based on differences in amino acid sequence. BoNTs are ~150 kDa proteins comprised of three major functional domains: an N-terminal zinc metalloprotease light chain (LC), a translocation domain (H_N), and a binding domain (H_C). The H_C is responsible for targeting the BoNT to the neuronal cell membrane, and each serotype has evolved to bind via different mechanisms to different target receptors. Most structural characterisations to date have focussed on the first identified subtype within each serotype (e.g., BoNT/A1). Subtype differences within BoNT serotypes can affect intoxication, displaying different botulism symptoms in vivo, and less emphasis has been placed on investigating these variants. This review outlines the receptors for each BoNT serotype and describes the basis for the highly specific targeting of neuronal cell membranes. Understanding receptor binding is of vital importance, not only for the generation of novel therapeutics but also for understanding how best to protect from intoxication.

Light Chain Diversity among the Botulinum Neurotoxins

Botulinum neurotoxins (BoNT) are produced by several species of clostridium. There are seven immunologically unique BoNT serotypes (A⁻G). The Centers for Disease Control classifies BoNTs as 'Category A' select agents and are the most lethal protein toxins for humans. Recently, BoNT-like proteins have also been identified in several non-clostridia. BoNTs are di-chain proteins comprised of an N-terminal zinc metalloprotease Light Chain (LC) and a C-terminal Heavy Chain (HC) which includes the translocation and receptor binding domains. The two chains are held together by a disulfide bond. The LC cleaves Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs). The cleavage of SNAREs inhibits the fusion of synaptic vesicles to the cell membrane and the subsequent release of acetylcholine, which results in flaccid paralysis. The LC controls the catalytic properties and the duration of BoNT action. This review discusses the mechanism for LC catalysis, LC translocation, and the basis for the duration of LC action. Understanding these properties of the LC may expand the applications of BoNT as human therapies.

Structural basis for the unique ganglioside and cell membrane recognition mechanism of botulinum neurotoxin DC

Botulinum neurotoxins (BoNTs), the most potent toxins known, are potential bioterrorism agents. It is well established that all seven serotypes of BoNTs (BoNT/A-G) require complex gangliosides as co-receptors. Here, we report that BoNT/DC, a presumed mosaic toxin between BoNT/D and BoNT/C1, binds and enters efficiently into neurons lacking complex gangliosides and shows no reduction in toxicity in mice deficient in complex gangliosides. The co-crystal structure of BoNT/DC with sialyl-Thomsen-Friedenreich antigen (Sialyl-T) suggests that BoNT/DC recognizes only the sialic acid, but not other moieties in gangliosides. Using liposome flotation assays, we demonstrate that an extended loop in BoNT/DC directly interacts with lipid membranes, and the co-occurring sialic acid binding and loop-membrane interactions mediate the recognition of gangliosides in membranes by BoNT/DC. These findings reveal a unique mechanism for cell membrane recognition and demonstrate that BoNT/DC can use a broad range of sialic acid-containing moieties as co-receptors.

Botulinum Neurotoxins: Biology, Pharmacology, and Toxicology

The study of botulinum neurotoxins (BoNT) is rapidly progressing in many aspects. Novel BoNTs are being discovered owing to next generation sequencing, but their biologic and pharmacological properties remain largely unknown. The molecular structure of the large protein complexes that the toxin forms with accessory proteins, which are included in some BoNT type A1 and B1 pharmacological preparations, have been determined. By far the largest effort has been dedicated to the testing and validation of BoNTs as therapeutic agents in an ever increasing number of applications, including pain therapy. BoNT type A1 has been also exploited in a variety of cosmetic treatments, alone or in combination with other agents, and this specific market has reached the size of the one dedicated to the treatment of medical syndromes. The pharmacological properties and mode of action of BoNTs have shed light on general principles of neuronal transport and protein-protein interactions and are stimulating basic science studies. Moreover, the wide array of BoNTs discovered and to be discovered and the production of recombinant BoNTs endowed with specific properties suggest novel uses in therapeutics with increasing disease/symptom specifity. These recent developments are reviewed here to provide an updated picture of the biologic mechanism of action of BoNTs, of their increasing use in pharmacology and in cosmetics, and of their toxicology.

Embryonic Stem Cell-Derived Neurons Grown on Multi-Electrode Arrays as a Novel In vitro Bioassay for the Detection of Clostridium botulinum Neurotoxins

Clostridium botulinum neurotoxins (BoNTs) are the most poisonous naturally occurring protein toxins known to mankind and are the causative agents of the severe and potentially life-threatening disease botulism. They are also known for their application as cosmetics and as unique bio-pharmaceuticals to treat an increasing number of neurological and non-neurological disorders. Currently, the potency of biologically active BoNT for therapeutic use is mainly monitored by the murine LD₅₀-assay, an ethically disputable test causing suffering and death of a considerable number of mice. The aim of this study was to establish an in vitro assay as an alternative to the widely used in vivo mouse bioassay. We report a novel BoNT detection assay using mouse embryonic stem cell-derived neurons (mESN) cultured on multi-electrode arrays (MEAs). After 21 days in culture, the mESN formed a neuronal network showing spontaneous bursting activity based on functional synapses and express the necessary target proteins for BoNTs. Treating cultures for 6 h with 16.6 pM of BoNT serotype A and incubation with 1.66 pM BoNT/A or 33 Units/ml of Botox^® for 24 h lead to a significant reduction of both spontaneous network bursts and average spike rate. This data suggests that mESN cultured on MEAs pose a novel, biologically relevant model that can be used to detect and quantify functional BoNT effects, thus accelerating BoNT research while decreasing animal use.

BoNT/AB hybrid maintains similar duration of paresis as BoNT/A wild-type in murine running wheel assay

The highly potent Botulinum neurotoxins (BoNT) are successful drugs to treat neuromuscular disorders. Efforts are being made to further reduce the injected BoNT dose and to lengthen the interval between treatments. Detailed knowledge of the BoNT structure-activity relationship (SAR) allows combining the best features of the different BoNT serotypes. Of all seven BoNT serotypes A-G, BoNT/A displays the highest potency despite low neuronal binding affinity, while BoNT/B exhibits much higher affinity. Recently, a new BoNT/AB hybrid (AABB) was constructed comprising the catalytic and translocation domain of BoNT/A and the 50kDa cell binding domain of BoNT/B. Here, we compared BoNT/A wild-type (AAAA) and AABB with regard to ex vivo potency and in vivo potency, efficacy and duration of action using the mouse phrenic nerve hemidiaphragm assay and the murine running wheel assay, respectively. The ex vivo potency of AABB was found to be 8.4-fold higher than that of AAAA. For the latter, two and 5 pg each of AAAA and AABB, respectively, were bilaterally injected into the calf muscles and mouse running wheel performance was automatically monitored during the following weeks to determine potency, efficacy and duration. Mice displayed a dose-dependent impairment of running performance. AABB showed potency, efficacy and duration equal to AAAA demonstrating successful exchange of the cell binding domain. AABB might combine the higher potency and longer duration of BoNT/A with the target specificity for the autonomic nervous system of BoNT/B. AABB might therefore constitute an improved treatment option for acetylcholine-mediated autonomic disorders such as hypersalivation or hyperhidrosis.

Glycosphingolipid-Protein Interaction in Signal Transduction

Glycosphingolipids (GSLs) are a class of ceramide-based glycolipids essential for embryo development in mammals. The synthesis of specific GSLs depends on the expression of distinctive sets of GSL synthesizing enzymes that is tightly regulated during development. Several reports have described how cell surface receptors can be kept in a resting state or activate alternative signalling events as a consequence of their interaction with GSLs. Specific GSLs, indeed, interface with specific protein domains that are found in signalling molecules and which act as GSL sensors to modify signalling responses. The regulation exerted by GSLs on signal transduction is orthogonal to the ligand–receptor axis, as it usually does not directly interfere with the ligand binding to receptors. Due to their properties of adjustable production and orthogonal action on receptors, GSLs add a new dimension to the control of the signalling in development. GSLs can, indeed, dynamically influence progenitor cell response to morphogenetic stimuli, resulting in alternative differentiation fates. Here, we review the available literature on GSL–protein interactions and their effects on cell signalling and development.

Circumventing Brain Barriers: Nanovehicles for Retroaxonal Therapeutic Delivery

In addition to safeguarding the central nervous system (CNS) from the vast majority of pathogens and toxins, transvascular barriers impose immense challenges to the delivery of beneficial cargo. A few toxins and neurotropic viruses capable of penetrating the brain have proved to be potentially valuable for neuron targeting and enhanced transfer of restorative medicine and therapeutic genes. Here we review molecular concepts and implications of the highly neurotropic tetanus toxin (TeTx) and botulinum neurotoxins (BoNTs) and their ability to infiltrate and migrate throughout neurons. We discuss recent applications of their detoxified variants as versatile nanovehicles for retroaxonal delivery of therapeutics to motor neurons and synapses. Continued advances in research on these remarkable agents in preclinical trials might facilitate their future use for medical benefit.

Tetanus Neurotoxin Neutralizing Antibodies Screened from a Human Immune scFv Antibody Phage Display Library

Tetanus neurotoxin (TeNT) produced by Clostridiumtetani is one of the most poisonous protein substances. Neutralizing antibodies against TeNT can effectively prevent and cure toxicosis. Using purified Hc fragments of TeNT (TeNT-Hc) as an antigen, three specific neutralizing antibody clones recognizing different epitopes were selected from a human immune scFv antibody phage display library. The three antibodies (2-7G, 2-2D, and S-4-7H) can effectively inhibit the binding between TeNT-Hc and differentiated PC-12 cells in vitro. Moreover, 2-7G inhibited TeNT-Hc binding to the receptor via carbohydrate-binding sites of the W pocket while 2-2D and S-4-7H inhibited binding of the R pocket. Although no single mAb completely protected mice from the toxin, they could both prolong survival when challenged with 20 LD₅₀s (50% of the lethal dose) of TeNT. When used together, the mAbs completely neutralized 1000 LD₅₀s/mg Ab, indicating their high neutralizing potency in vivo. Antibodies recognizing different carbohydrate-binding pockets could have higher synergistic toxin neutralization activities than those that recognize the same pockets. These results could lead to further production of neutralizing antibody drugs against TeNT and indicate that using TeNT-Hc as an antigen for screening human antibodies for TeNT intoxication therapy from human immune antibody library was convenient and effective.

Two Feet on the Membrane: Uptake of Clostridial Neurotoxins

The extraordinary potency of botulinum neurotoxins (BoNT) and tetanus neurotoxin (TeNT) is mediated by their high neurospecificity, targeting peripheral cholinergic motoneurons leading to flaccid and spastic paralysis, respectively, and successive respiratory failure. Complex polysialo gangliosides accumulate BoNT and TeNT on the plasma membrane. The ganglioside binding in BoNT/A, B, E, F, G, and TeNT occurs via a conserved ganglioside-binding pocket within the most carboxyl-terminal 25 kDa domain H_CC, whereas BoNT/C, DC, and D display here two different ganglioside binding sites. This enrichment step facilitates subsequent binding of BoNT/A, B, DC, D, E, F, and G to the intraluminal domains of the synaptic vesicle glycoprotein 2 (SV2) isoforms A-C and synaptotagmin-I/-II, respectively. Whereas an induced α-helical 20-mer Syt peptide binds via side chain interactions to the tip of the H_CC domain of BoNT/B, DC and G, the preexisting, quadrilateral β-sheet helix of SV2C-LD4 binds the clinically most relevant serotype BoNT/A mainly through backbone-backbone interactions at the interface of H_CC and H_CN. In addition, the conserved, complex N559-glycan branch of SV2C establishes extensive interactions with BoNT/A resulting in delayed dissociation providing BoNT/A more time for endocytosis into synaptic vesicles. An analogous interaction occurs between SV2A/B and BoNT/E. Altogether, the nature of BoNT-SV2 recognition clearly differs from BoNT-Syt. Subsequently, the synaptic vesicle is recycled and the bound neurotoxin is endocytosed. Acidification of the vesicle lumen triggers membrane insertion of the translocation domain, pore formation, and finally translocation of the enzymatically active light chain into the neuronal cytosol to halt release of neurotransmitters.

Pharmaceutical, biological, and clinical properties of botulinum neurotoxin type A products

Botulinum neurotoxin injections are a valuable treatment modality for many therapeutic indications and have revolutionized the field of aesthetic medicine so that they are the leading cosmetic procedure performed worldwide. Studies show that onabotulinumtoxinA, abobotulinumtoxinA, and incobotulinumtoxinA are comparable in terms of clinical efficacy. Differences between the products relate to the botulinum neurotoxin complexes, specific biological potency, and their immunogenicity. Protein complex size and molecular weight have no effect on biological activity, stability, distribution, or side effect profile. Complexing proteins and inactive toxin (toxoid) content increase the risk of neutralizing antibody formation, which can cause secondary treatment failure, particularly in chronic disorders that require frequent injections and long-term treatment. These attributes could lead to differences in therapeutic outcomes, and, given the widespread aesthetic use of these three neurotoxin products, physicians should be aware of how they differ to ensure their safe and effective use.

Generation and Characterization of Six Recombinant Botulinum Neurotoxins as Reference Material to Serve in an International Proficiency Test

The detection and identification of botulinum neurotoxins (BoNT) is complex due to the existence of seven serotypes, derived mosaic toxins and more than 40 subtypes. Expert laboratories currently use different technical approaches to detect, identify and quantify BoNT, but due to the lack of (certified) reference materials, analytical results can hardly be compared. In this study, the six BoNT/A1–F1 prototypes were successfully produced by recombinant techniques, facilitating handling, as well as improving purity, yield, reproducibility and biosafety. All six BoNTs were quantitatively nicked into active di-chain toxins linked by a disulfide bridge. The materials were thoroughly characterized with respect to purity, identity, protein concentration, catalytic and biological activities. For BoNT/A₁, B₁ and E₁, serotypes pathogenic to humans, the catalytic activity and the precise protein concentration were determined by Endopep-mass spectrometry and validated amino acid analysis, respectively. In addition, BoNT/A₁, B₁, E₁ and F₁ were successfully detected by immunological assays, unambiguously identified by mass spectrometric-based methods, and their specific activities were assigned by the mouse LD₅₀ bioassay. The potencies of all six BoNT/A1–F1 were quantified by the ex vivo mouse phrenic nerve hemidiaphragm assay, allowing a direct comparison. In conclusion, highly pure recombinant BoNT reference materials were produced, thoroughly characterized and employed as spiking material in a worldwide BoNT proficiency test organized by the EQuATox consortium.

Intrastriatal injection of botulinum neurotoxin-A is not cytotoxic in rat brain - A histological and stereological analysis

Parkinson's disease (PD) is caused by progressive degeneration of dopaminergic neurons in the substantia nigra pars compacta, resulting in a deficiency of dopamine in the striatum and an increased release of acetylcholine by tonically active interneurons. Botulinum neurotoxin-A (BoNT-A) is well known for blocking transmitter release by cholinergic presynaptic terminals. Treating striatal hypercholinism by local application of BoNT-A could be a possible new local therapy option of PD. In previous studies of our group, we analyzed the effect of BoNT-A injection into the CPu of 6-OHDA lesioned hemiparkinsonian rats. Our studies showed that BoNT-A application in hemiparkinson rat model is capable of abolishing apomorphine induced rotations for approximately 3 months. Regularly occurring axonal swellings in the BoNT-A infiltrated striata were also discovered, which we named BoNT-A induced varicosities (BiVs). Résumé: Here we investigated the long-term effect of the injection of 1ng BoNT-A into the right CPu of naive Wistar rats on the number of ChAT-ir interneurons as well as on the numeric density and the volumetric size of the BiVs in the CPu. Significant differences in the number of ChAT-ir neurons between the right BoNT-A treated CPu and the left untreated CPu were not detected up to 12 month post BoNT-A injection. The numeric density of BiVs in the treated CPu reached a maximum 3 months after BoNT-A treatment and decreased afterwards, whereas the volume of single BiVs increased steadily throughout the whole time course of the experiment.

Botulinum neurotoxin: Progress in negating its neurotoxicity; and in extending its therapeutic utility via molecular engineering. MiniReview

While the poisonous effects of botulinum neurotoxin (BoNT) have been recognized since antiquity, the overall actions and mechanisms of effects of BoNT have been elucidated primarily over the past several decades. The general utility of BoNT is described in the paper, but the focus is mainly on the approaches towards negating the toxic effects of BoNT, and on the projection of an engineered BoNT molecule serving as a Trojan Horse to deliver a therapeutic load for treatment of a host of medical disorders. The BoNT molecule is configured with a binding domain, a zinc-dependent protease with specificity primarily for vesicular proteins, and a translocation domain for delivery of the metalloprotease into the cytoplasm. The anti-toxin approaches for BoNT include the use of vaccines, antibodies, block of BoNT binding or translocation, inhibition of metalloprotease activity, impeded translocation of the protease/catalytic domain, and inhibition of the downstream Src signaling pathway. Projections of BoNT as a therapeutic include its targeting to non-cholinergic nerves, also targeting to non-neuronal cells for treatment of hypersecretory disorders (e.g., cystic fibrosis), and treatment of hormonal disorders (e.g., acromegaly). Still in the exploratory phase, there is the expectation of major advances in BoNT neuroprotective strategies and burgeoning utility of engineered BoNTs as therapeutics.

The long journey of botulinum neurotoxins into the synapse

Botulinum neurotoxins (BoNT) cause the disease botulism, a flaccid paralysis of the muscle. They are also very effective, widely used medicines applied locally in sub-nanogram quantities. BoNTs are released together with several non-toxic, associated proteins as progenitor toxin complexes (PCT) by Clostridium botulinum to become highly potent oral poisons ingested via contaminated food. They block the neurotransmission in susceptible animals and humans already in nanogram quantities due to their specific ability to enter motoneurons and to cleave only selected neuronal proteins involved in neuroexocytosis. BoNTs have developed a sophisticated strategy to passage the gastrointestinal tract and to be absorbed in the intestine of the host to finally attack neurons. A non-toxic non-hemagglutinin (NTNHA) forms a binary complex with BoNT to protect it from gastrointestinal degradation. This binary M-PTC is one component of the bi-modular 14-subunit ∼760 kDa large progenitor toxin complex. The other component is the structurally and functionally independent dodecameric hemagglutinin (HA) complex which facilitates the absorption on the intestinal epithelium by glycan binding. Subsequent to its transcytosis the HA complex disrupts the tight junction of the intestinal barrier from the basolateral side by binding to E-cadherin. Now, the L-PTC can also enter the circulation by paracellular routes in much larger quantities. From here, the dissociated BoNTs reach the neuromuscular junction and accumulate via interaction with polysialo gangliosides, complex glycolipids, on motoneurons at the neuromuscular junction. Subsequently, additional specific binding to luminal segments of synaptic vesicles proteins like SV2 and synaptotagmin leads to their uptake. Finally, the neurotoxins shut down the synaptic vesicle cycle, which they had exploited before to enter their target cells, via specific cleavage of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins, which constitute the core components of the cellular membrane fusion machinery.

On the translocation of botulinum and tetanus neurotoxins across the membrane of acidic intracellular compartments

Tetanus and botulinum neurotoxins are produced by anaerobic bacteria of the genus Clostridium and are the most poisonous toxins known, with 50% mouse lethal dose comprised within the range of 0.1-few nanograms per Kg, depending on the individual toxin. Botulinum neurotoxins are similarly toxic to humans and can therefore be considered for potential use in bioterrorism. At the same time, their neurospecificity and reversibility of action make them excellent therapeutics for a growing and heterogeneous number of human diseases that are characterized by a hyperactivity of peripheral nerve terminals. The complete crystallographic structure is available for some botulinum toxins, and reveals that they consist of four domains functionally related to the four steps of their mechanism of neuron intoxication: 1) binding to specific receptors of the presynaptic membrane; 2) internalization via endocytic vesicles; 3) translocation across the membrane of endocytic vesicles into the neuronal cytosol; 4) catalytic activity of the enzymatic moiety directed towards the SNARE proteins. Despite the many advances in understanding the structure-mechanism relationship of tetanus and botulinum neurotoxins, the molecular events involved in the translocation step have been only partially elucidated. Here we will review recent advances that have provided relevant insights on the process and discuss possible models that can be experimentally tested. This article is part of a Special Issue entitled: Pore-Forming Toxins edited by Mauro Dalla Serra and Franco Gambale.

OnabotulinumtoxinA for chronic migraine: a critical appraisal

Chronic migraine (CM) is a severe disabling condition with a few available evidence-based management options. OnabotulinumtoxinA (onaBoNTA) is approved for use in a number of disorders. Its benefits and potential use in migraine were observed incidentally while treating patients cosmetically for wrinkles. The mechanism of action of onaBoNTA in CM is not fully understood, but there is evidence that this involves axonal transport via sensory fibers. The Phase III REsearch Evaluating Migraine Prophylaxis Therapy trials have established the efficacy as well as the long-term safety and tolerability of onaBoNTA in CM. This review will discuss the evidence behind its use in this setting.

A Highly Specific Monoclonal Antibody for Botulinum Neurotoxin Type A-Cleaved SNAP25

Botulinum neurotoxin type-A (BoNT/A), as onabotulinumtoxinA, is approved globally for 11 major therapeutic and cosmetic indications. While the mechanism of action for BoNT/A at the presynaptic nerve terminal has been established, questions remain regarding intracellular trafficking patterns and overall fate of the toxin. Resolving these questions partly depends on the ability to detect BoNT/A’s location, distribution, and movement within a cell. Due to BoNT/A’s high potency and extremely low concentrations within neurons, an alternative approach has been employed. This involves utilizing specific antibodies against the BoNT/A-cleaved SNAP25 substrate (SNAP25₁₉₇) to track the enzymatic activity of toxin within cells. Using our highly specific mouse monoclonal antibody (mAb) against SNAP25₁₉₇, we generated human and murine recombinant versions (rMAb) using specific backbone immunoglobulins. In this study, we validated the specificity of our anti-SNAP25₁₉₇ rMAbs in several different assays and performed side-by-side comparisons to commercially-available and in-house antibodies against SNAP25. Our rMAbs were highly specific for SNAP25₁₉₇ in all assays and on several different BoNT/A-treated tissues, showing no cross-reactivity with full-length SNAP25. This was not the case with other reportedly SNAP25₁₉₇-selective antibodies, which were selective in some, but not all assays. The rMAbs described herein represent effective new tools for detecting BoNT/A activity within cells.

Novel therapeutic uses and formulations of botulinum neurotoxins: a patent review (2012 - 2014)

INTRODUCTION

Botulinum neurotoxins (BoNTs) are among the most toxic of known biological molecules and function as acetylcholine release inhibitors and neuromuscular blocking agents. Paradoxically, these properties also make them valuable therapeutic agents for the treatment of movement disorders, urological conditions and hypersecretory disorders. Greater understanding of their molecular mechanism of action and advances in protein engineering has led to significant efforts to improve and expand their function with a view towards broadening their therapeutic potential.

AREAS COVERED

Searches of Espacenet and Google Patent have revealed a number of patents related to BoNTs. This review will focus on novel therapeutic uses and formulations disclosed during 2012 - 2014. The seven patents discussed will include nanoformulations of FDA-approved BoNTs, additional BoNT subtypes and novel BoNT variants and chimeras created through protein engineering. Supporting patents and related publications are also briefly discussed.

EXPERT OPINION

The clinical and commercial success of BoNTs has prompted investigation into novel BoNTs or BoNT-mediated chimeras with promising in vitro results. Distinct strategies including the use of nanoformulations and targeted delivery have been implemented to identify new indication and improved functionality. Greater understanding of their systemic exposure, efficacy and safety profiles will be required for further development.

Internalization and retrograde axonal trafficking of tetanus toxin in motor neurons and trans-synaptic propagation at central synapses exceed those of its C-terminal-binding fragments

The prominent tropism of tetanus toxin (TeTx) towards peripheral nerves with retrograde transport and transfer to central neurons render it an invaluable probe for exploring fundamental neuronal processes such as endocytosis, retrograde trafficking and trans-synaptic transport to central neurons. While the specificity of TeTx to nerve cells has been attributed to its binding domains (HC and HCC), molecular determinants of the long-range trafficking that ensure its central delivery and induction of spastic paralysis remain elusive. Here, we report that a protease-inactive TeTx mutant (TeTIM) fused to core streptavidin (CS) proved superior to CS-HC and CS-HCC fragments in antagonizing the internalization of the active toxin in cultured spinal cord neurons. Also, in comparison to CS-HC and CS-HCC, CS-TeTIM undergoes faster clearance from motor nerve terminals after peripheral injection, and is detected in a greater number of neurons in the spinal cord and brain stem ipsi-lateral to the administration site. Consistent with trans-synaptic transfer from motor neurons to inter-neurons, CS-TeTIM infiltrated non-cholinergic cells in the spinal cord; in contrast, the retrograde spread of CS-HC was largely restricted to neurons stained for choline acetyltransferase. Peripheral injection of CS-TeTIM conjugated to a lentivirus encoding mutated SNAP-25, resistant to cleavage by botulinum neurotoxin A, E and C1, rendered spontaneous excitatory postsynaptic currents in motor neurons resilient to challenge by type A toxin in vitro, whereas the same virus conjugated to CS-HC proved ineffective. These findings indicate that full-length inactive TeTx greatly exceeds HC and HCC in targeting and invading motor nerve terminals at the periphery and exploits more efficiently the retrograde transport and trans-synaptic transfer mechanisms of motor neurons to arrive at central neurons. Such qualities render TeTIM a more suitable research probe and neuron-targeting vehicle for retro-axonal delivery of viral vectors to the CNS.

Resolution of sub-nanosecond motions in botulinum neurotoxin endopeptidase: An evidence of internal flexibility

Botulinum neurotoxins (BoNTs) are the most poisonous substances known to mankind, which act on the peripheral nervous system leading to flaccid paralysis. Although co-crystal structure of BoNT/A light chain (LC) reveals some unique features of the biological function of this molecule, structural characteristics in solution reveal its dynamic features, not available through the published crystal structures. In this study, we have examined internal flexibility of this molecule by measuring rotational correlation time as a function of viscosity, using frequency domain fluorescence anisotropy decay technique. Fluorescence anisotropy decay of BoNT/A LC resolved sub-nanosecond local motion (faster component), interpreted as internal flexibility of the molecule was affected significantly with viscosity. Both local and global movements were affected by viscosity, which indicates the accessibility of protein core and flexibility of overall structure. In conclusion, this work demonstrates the presence of flexibility in the internal peptide segments, which appears to play a significant role in BoNT/A LC biological function.

On botulinum neurotoxin variability

The rapidly growing number of botulinum neurotoxin sequences poses the problem of the possible evolutionary significance of the variability of these superpotent neurotoxins for toxin-producing Clostridium species. To progress in the understanding of this remarkable phenomenon, we suggest that researchers should (i) abandon an anthropocentric view of these neurotoxins as human botulism-causing agents or as human therapeutics, (ii) begin to investigate in depth the role of botulinum neurotoxins in animal botulism in the wilderness, and (iii) devote large efforts to next-generation sequencing of soil samples to identify novel botulinum neurotoxins. In order to compare the fitness of the different toxins, we suggest that assays of all the steps from toxin production to animal death should be performed.

Botulinum neurotoxins: new questions arising from structural biology

Botulinum neurotoxins (BoNTs) are the most toxic substances known and cause botulism in vertebrates. They have also emerged as effective and powerful reagents for cosmetic and medical applications. One important prerequisite for understanding BoNT function in disease, and the further development of the toxins for cosmetic and medical applications, is a detailed knowledge of BoNT interactions with non-toxic neurotoxin-associated proteins and cell surface receptors. Based on the substantial recent progress in obtaining high-resolution crystal structures of key BoNT complexes, we summarize the major advances in understanding BoNT interactions and discuss the resulting potential implications, in particular those relating to BoNT serotype A.

Structural insight into exosite binding and discovery of novel exosite inhibitors of botulinum neurotoxin serotype A through in silico screening

Botulinum neurotoxin serotype A (BoNT/A) is the most lethal toxin among the Tier 1 Select Agents. Development of potent and selective small molecule inhibitors against BoNT/A zinc metalloprotease remains a challenging problem due to its exceptionally large substrate binding surface and conformational plasticity. The exosites of the catalytic domain of BoNT/A are intriguing alternative sites for small molecule intervention, but their suitability for inhibitor design remains largely unexplored. In this study, we employed two recently identified exosite inhibitors, D-chicoric acid and lomofungin, to probe the structural features of the exosites and molecular mechanisms of synergistic inhibition. The results showed that D-chicoric acid favors binding at the α-exosite, whereas lomofungin preferentially binds at the β-exosite by mimicking the substrate β-sheet binding interaction. Molecular dynamics simulations and binding interaction analysis of the exosite inhibitors with BoNT/A revealed key elements and hotspots that likely contribute to the inhibitor binding and synergistic inhibition. Finally, we performed database virtual screening for novel inhibitors of BoNT/A targeting the exosites. Hits C1 and C2 showed non-competitive inhibition and likely target the α- and β-exosites, respectively. The identified exosite inhibitors may provide novel candidates for structure-based development of therapeutics against BoNT/A intoxication.

The blockade of the neurotransmitter release apparatus by botulinum neurotoxins

The high toxicity of the seven serotypes of botulinum neurotoxins (BoNT/A to G), together with their specificity and reversibility, includes them in the list A of potential bioterrorism weapons and, at the same time, among the therapeutics of choice for a variety of human syndromes. They invade nerve terminals and cleave specifically the three proteins which form the heterotrimeric SNAP REceptors (SNARE) complex that mediates neurotransmitter release. The BoNT-induced cleavage of the SNARE proteins explains by itself the paralysing activity of the BoNTs because the truncated proteins cannot form the SNARE complex. However, in the case of BoNT/A, the most widely used toxin in therapy, additional factors come into play as it only removes a few residues from the synaptosomal associate protein of 25 kDa C-terminus and this results in a long duration of action. To explain these facts and other experimental data, we present here a model for the assembly of the neuroexocytosis apparatus in which Synaptotagmin and Complexin first assist the zippering of the SNARE complex, and then stabilize and clamp an octameric radial assembly of the SNARE complexes.

Effects of enzymatically inactive recombinant botulinum neurotoxin type A at the mouse neuromuscular junctions

Botulinum neurotoxin A (BoNT/A) is used clinically to treat several neurological and metabolic diseases. However, the mechanisms that underlie the clinical use of the toxin remain still to be elusive. BoNT/A inhibits acetylcholine (ACh) release at the motor nerve terminals (MNT) and causes neuroparalysis. The toxic effects of BoNT/A at the MNT occur in sub-pico molar range, and it is invaluable to determine the half-life and the persistence of catalytic activity of the toxin to develop therapeutics against BoNT/A intoxication. However, the use of extremely low concentrations of BoNT/A in cellular, or animal models due to high toxicity makes it difficult to determine new cellular mechanisms and binding or interacting partners of BoNT/A. In order to address this, a catalytically deactivated, non-toxic version of BoNT/A, designated as DrBoNT/A, was characterized. DrBoNT/A lacks endoprotease activity (SNAP-25 cleavage) at concentrations as high as 46,875-fold, compared to wild-type BoNT/A. Unlike BoNT/A injection (3.2 pg), injection of the recombinant product (150 ng or 3.2 pg) into mouse hind limbs failed to cause neuroparalysis as exhibited by the lack of inhibition of toe spread reflex (ability of the mouse to spread its hindlimb toes), and inhibit ACh release at the MNT. The in vitro experiments also demonstrate that DrBoNT/A uptake (at concentrations equivalent to BoNT/A), internalization and localization at the MNT remained unaltered. In addition, modeling studies support that DrBoNT/A lacked the zinc binding ability, and the ability to directly participate in the hydrolysis of SNAP-25 substrate. Collectively, we demonstrate that DrBoNT/A is non-toxic to the MNT and can be used as a surrogate tool to understand the mechanism by which BoNT/A modulates signal transduction mechanisms.

Interaction of membrane/lipid rafts with the cytoskeleton: impact on signaling and function: membrane/lipid rafts, mediators of cytoskeletal arrangement and cell signaling

The plasma membrane in eukaryotic cells contains microdomains that are enriched in certain glycosphingolipids, gangliosides, and sterols (such as cholesterol) to form membrane/lipid rafts (MLR). These regions exist as caveolae, morphologically observable flask-like invaginations, or as a less easily detectable planar form. MLR are scaffolds for many molecular entities, including signaling receptors and ion channels that communicate extracellular stimuli to the intracellular milieu. Much evidence indicates that this organization and/or the clustering of MLR into more active signaling platforms depends upon interactions with and dynamic rearrangement of the cytoskeleton. Several cytoskeletal components and binding partners, as well as enzymes that regulate the cytoskeleton, localize to MLR and help regulate lateral diffusion of membrane proteins and lipids in response to extracellular events (e.g., receptor activation, shear stress, electrical conductance, and nutrient demand). MLR regulate cellular polarity, adherence to the extracellular matrix, signaling events (including ones that affect growth and migration), and are sites of cellular entry of certain pathogens, toxins and nanoparticles. The dynamic interaction between MLR and the underlying cytoskeleton thus regulates many facets of the function of eukaryotic cells and their adaptation to changing environments. Here, we review general features of MLR and caveolae and their role in several aspects of cellular function, including polarity of endothelial and epithelial cells, cell migration, mechanotransduction, lymphocyte activation, neuronal growth and signaling, and a variety of disease settings. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

Role of pore-forming toxins in bacterial infectious diseases

Pore-forming toxins (PFTs) are the most common bacterial cytotoxic proteins and are required for virulence in a large number of important pathogens, including Streptococcus pneumoniae, group A and B streptococci, Staphylococcus aureus, Escherichia coli, and Mycobacterium tuberculosis. PFTs generally disrupt host cell membranes, but they can have additional effects independent of pore formation. Substantial effort has been devoted to understanding the molecular mechanisms underlying the functions of certain model PFTs. Likewise, specific host pathways mediating survival and immune responses in the face of toxin-mediated cellular damage have been delineated. However, less is known about the overall functions of PFTs during infection in vivo. This review focuses on common themes in the area of PFT biology, with an emphasis on studies addressing the roles of PFTs in in vivo and ex vivo models of colonization or infection. Common functions of PFTs include disruption of epithelial barrier function and evasion of host immune responses, which contribute to bacterial growth and spreading. The widespread nature of PFTs make this group of toxins an attractive target for the development of new virulence-targeted therapies that may have broad activity against human pathogens.

Botulinum toxin for the treatment of myofascial pain syndromes involving the neck and back: a review from a clinical perspective

Introduction. Botulinum toxin inhibits acetylcholine (ACh) release and probably blocks some nociceptive neurotransmitters. It has been suggested that the development of myofascial trigger points (MTrP) is related to an excess release of ACh to increase the number of sensitized nociceptors. Although the use of botulinum toxin to treat myofascial pain syndrome (MPS) has been investigated in many clinical trials, the results are contradictory. The objective of this paper is to identify sources of variability that could explain these differences in the results. Material and Methods. We performed a content analysis of the clinical trials and systematic reviews of MPS. Results and Discussion. Sources of differences in studies were found in the diagnostic and selection criteria, the muscles injected, the injection technique, the number of trigger points injected, the dosage of botulinum toxin used, treatments for control group, outcome measures, and duration of followup. The contradictory results regarding the efficacy of botulinum toxin A in MPS associated with neck and back pain do not allow this treatment to be recommended or rejected. There is evidence that botulinum toxin could be useful in specific myofascial regions such as piriformis syndrome. It could also be useful in patients with refractory MPS that has not responded to other myofascial injection therapies.

Facing glycosphingolipid-Shiga toxin interaction: dire straits for endothelial cells of the human vasculature

The two major Shiga toxin (Stx) types, Stx1 and Stx2, produced by enterohemorrhagic Escherichia coli (EHEC) in particular injure renal and cerebral microvascular endothelial cells after transfer from the human intestine into the circulation. Stxs are AB(5) toxins composed of an enzymatically active A subunit and the pentameric B subunit, which preferentially binds to the glycosphingolipid globotriaosylceramide (Gb3Cer/CD77). This review summarizes the current knowledge on Stx-caused cellular injury and the structural diversity of Stx receptors as well as the initial molecular interaction of Stxs with the human endothelium of different vascular beds. The varying lipoforms of Stx receptors and their spatial organization in lipid rafts suggest a central role in different modes of receptor-mediated endocytosis and intracellular destiny of the toxins. The design and development of tailored Stx neutralizers targeting the oligosaccharide-toxin recognition event has become a very real prospect to ameliorate or prevent life-threatening renal and neurological complications.