Engineering Botulinum Neurotoxins for Enhanced Therapeutic Applications and Vaccine Development

Effectiveness of a Bivalent Recombinant Vaccine on the Production of Neutralizing Antibodies Against BoNT/C, BoNT/D, BoNT/CD e BoNT/DC in Bovines

Background/Objectives. Bovine botulism, although relatively rare, presents significant economic losses due to high mortality rates and restrictions on livestock product trade. Vaccination remains the most effective strategy for preventing botulism-related mortality. This study evaluated the efficacy of a bivalent recombinant vaccine targeting the C-terminal portion of the heavy chain (Hc) of botulinum neurotoxin serotype C (BoNT/C) (Hc BoNT/C) and botulinum neurotoxin serotype D (BoNT/D) (Hc BoNT/D) in inducing neutralizing antibodies against these toxins and their mosaic variants BoNT/CD and BoNT/DC in cattle. This comparison aims to improve the design of an optimal recombinant vaccine for preventing bovine botulism caused by the most common serotypes. Methods. Twenty, four-month-old Holstein Friesian calves were randomly assigned to two groups of ten animals: vaccinated group and control group. Sera were collected at various time points to assess antibody titers using ELISA and neutralizing antibody titers using a mouse protection assay. Neutralizing antibody titers were compared to those obtained with a commercially available toxoid vaccine. Results. The recombinant vaccine elicited significant increases in anti-HcBoNT/C and anti-HcBoNT/D IgG antibody levels in vaccinated animals compared to controls animals with no adverse effects. Specifically, post-vaccination, the calves showed no local reactions (swelling, warmth) or behavioral changes suggestive of systemic illness. Neutralizing antibody titers against BoNT/C and BoNT/D were significantly higher in the recombinant vaccine group compared to the toxoid vaccine group. However, the recombinant vaccine showed lower neutralizing activity against BoNT/DC compared to the toxoid vaccine. Conclusions. The bivalent recombinant vaccine demonstrated promising immunogenicity in cattle, inducing high neutralizing antibody titers against BoNT/C and BoNT/D. While effective against these toxins, the lower efficacy against BoNT/DC highlights the need for further research to optimize the vaccine formulation, potentially by incorporating a BoNT/DC Hc component, to provide broader protection against bovine botulism.

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.

Novel platform for engineering stable and effective vaccines against botulinum neurotoxins A, B and E

Botulinum neurotoxin (BoNT), produced by Clostridium botulinum, is the most toxic protein known, capable of causing severe paralysis and posing a significant bioterrorism threat due to its extreme lethality even in minute quantities. Despite this, there are currently no FDA-approved vaccines for widespread public use. To address this urgent need, we have developed an innovative vaccine platform by fusing the neuronal binding domain of BoNT/E (Hc/E) with core-streptavidin (CS), resulting in a stable CS-Hc/E vaccine. Mice vaccinated with CS-Hc/E exhibited superior antibody titers compared to those receiving Hc/E alone. To develop a trivalent vaccine against BoNT/A, BoNT/B, and BoNT/E- key contributors to the vast majority of human botulism-we conjugated CS-Hc/E with a biotinylated atoxic chimeric protein incorporating neutralizing epitopes from BoNT/A and BoNT/B. This chimeric protein includes the binding domain of BoNT/A, along with the protease-inactive light chain and translocation domains of BoNT/B. The interaction between CS and biotin formed a stable tetrameric antigen, EBA. Vaccination with EBA in mice elicited robust antibody responses and provided complete protection against lethal doses of BoNT/A, BoNT/B, and BoNT/E. Our findings highlight EBA's potential as a stable and effective broad-spectrum vaccine against BoNT. Moreover, our technology offers a versatile platform for developing multivalent, stable vaccines targeting various biological threats by substituting the BoNT domain(s) with neutralizing epitopes from other life-threatening pathogens, thereby enhancing public health preparedness and biodefense strategies.

Botulinum toxin intoxication requires retrograde transport and membrane translocation at the ER in RenVM neurons

Botulinum neurotoxin A (BoNT/A) is a highly potent proteolytic toxin specific for neurons with numerous clinical and cosmetic uses. After uptake at the synapse, the protein is proposed to translocate from synaptic vesicles to the cytosol through a self-formed channel. Surprisingly, we found that after intoxication proteolysis of a fluorescent reporter occurs in the neuron soma first and then centrifugally in neurites. To investigate the molecular mechanisms at play, we use a genome-wide siRNA screen in genetically engineered neurons and identify over three hundred genes. An organelle-specific split-mNG complementation indicates BoNT/A traffic from the synapse to the soma-localized Golgi in a retromer-dependent fashion. The toxin then moves to the ER and appears to require the Sec61 complex for retro-translocation to the cytosol. Our study identifies genes and trafficking processes hijacked by the toxin, revealing a new pathway mediating BoNT/A cellular toxicity.

Delivery of endolysin across outer membrane of Gram-negative bacteria using translocation domain of botulinum neurotoxin

The emergence of multidrug-resistant pathogens has outpaced the development of new antibiotics, leading to renewed interest in endolysins. Endolysins have been investigated as novel biocontrol agents for Gram-positive bacteria. However, their efficacy against Gram-negative species is limited by the barrier presented by their outer membrane, which prevents endolysin access to the peptidoglycan substrate. Here, we used the translocation domain of botulinum neurotoxin to deliver endolysin across the outer membrane of Gram-negative bacteria. The translocation domain selectively interacts with and penetrates membranes composed of anionic lipids, which have been used in nature to deliver various proteins into animal cells. In addition to the botulinum neurotoxin translocation domain, we have fused bacteriophage-derived receptor binding protein to endolysins. This allows the attached protein to efficiently bind to a broad spectrum of Gram-negative bacteria. By attaching these target-binding and translocation machineries to endolysins, we aimed to develop an engineered endolysin with broad-spectrum targeting and enhanced antibacterial activity against Gram-negative species. To validate our strategy, we designed engineered endolysins using two well-known endolysins, T5 and LysPA26, and tested them against 23 strains from six species of Gram-negative bacteria, confirming that our machinery can act broadly. In particular, we observed a 2.32 log reduction in 30 min with only 0.5 µM against an Acinetobacter baumannii isolate. We also used the SpyTag/SpyCatcher system to easily attach target-binding proteins, thereby improving its target-binding ability. Overall, our newly developed endolysin engineering strategy may be a promising approach to control multidrug-resistant Gram-negative bacterial strains.

Neurobiological mechanisms of botulinum neurotoxin-induced analgesia for neuropathic pain

Botulinum neurotoxins (BoNTs) are a family of neurotoxins produced by Clostridia and other bacteria that induce botulism. BoNTs are internalized into nerve terminals at the site of injection and cleave soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins to inhibit the vesicular release of neurotransmitters. BoNTs have been approved for multiple therapeutic applications, including the treatment of migraines. They have also shown efficacies for treating neuropathic pain, such as diabetic neuropathy, and postherpetic and trigeminal neuralgia. However, the mechanisms underlying BoNT-induced analgesia are not well understood. Peripherally administered BoNT is taken up by the nerve terminals and reduces the release of glutamate, calcitonin gene-related peptide, and substance P, which decreases neurogenic inflammation in the periphery. BoNT is retrogradely transported to sensory ganglia and central terminals in a microtubule-dependent manner. BoNTs decrease the expression of pronociceptive genes (ion channels or cytokines) from sensory ganglia and the release of neurotransmitters and neuropeptides from primary afferent central terminals, which likely leads to decreased central sensitization in the dorsal horn of the spinal cord or trigeminal nucleus. BoNT-induced analgesia is abolished after capsaicin-induced denervation of transient receptor potential vanilloid 1 (TRPV1)-expressing afferents or the knockout of substance P or the neurokinin-1 receptor. Although peripheral administration of BoNT leads to changes in the central nervous system (e.g., decreased phosphorylation of glutamate receptors in second-order neurons, reduced activation of microglia, contralateral localization, and cortical reorganization), whether such changes are secondary to changes in primary afferents or directly mediated by trans-synaptic, transcytotic, or the hematogenous transport of BoNT is controversial. To enhance their therapeutic potential, BoNTs engineered for specific targeting of nociceptive pathways have been developed to treat chronic pain. Further mechanistic studies on BoNT-induced analgesia can enhance the application of native or engineered BoNTs for neuropathic pain treatment with improved safety and efficacy.

Intramuscular Botulinum Neurotoxin Serotypes E and A Elicit Distinct Effects on SNAP25 Protein Fragments, Muscular Histology, Spread and Neuronal Transport: An Integrated Histology-Based Study in the Rat

Botulinum neurotoxins E (BoNT/E) and A (BoNT/A) act by cleaving Synaptosome-Associated Protein 25 (SNAP25) at two different C-terminal sites, but they display very distinct durations of action, BoNT/E being short acting and BoNT/A long acting. We investigated the duration of action, spread and neuronal transport of BoNT/E (6.5 ng/kg) and BoNT/A (125 pg/kg) after single intramuscular administrations of high equivalent efficacious doses, in rats, over a 30- or 75-day periods, respectively. To achieve this, we used (i) digit abduction score assay, (ii) immunohistochemistry for SNAP25 (N-ter part; SNAP25N-ter and C-ter part; SNAP25C-ter) and its cleavage sites (cleaved SNAP25; c-SNAP25E and c-SNAP25A) and (iii) muscular changes in histopathology evaluation. Combined in vivo observation and immunohistochemistry analysis revealed that, compared to BoNT/A, BoNT/E induces minimal muscular changes, possesses a lower duration of action, a reduced ability to spread and a decreased capacity to be transported to the lumbar spinal cord. Interestingly, SNAP25C-ter completely disappeared for both toxins during the peak of efficacy, suggesting that the persistence of toxin effects is driven by the persistence of proteases in tissues. These data unveil some new molecular mechanisms of action of the short-acting BoNT/E and long-acting BoNT/A, and reinforce their overall safety profiles.

Foodborne botulism outbreak involving different nationalities during the Rugby World Cup: critical role of credit card data and rapid international cooperation, France, September 2023

In September 2023, a severe outbreak of type B botulism with fifteen cases was linked to consumption of canned sardines at a restaurant in Bordeaux, France, during the Rugby World Cup. The cases were from seven countries. One death was recorded. Outbreak investigation using credit card data, rapid communication between health authorities of the affected countries and broad media communication allowed identification of cases and exposed persons and prevented further severe outcomes.

Molecular landscape of BoNT/B bound to a membrane-inserted synaptotagmin/ganglioside complex

Botulinum neurotoxin serotype B (BoNT/B) uses two separate protein and polysialoglycolipid-binding pockets to interact with synaptotagmin 1/2 and gangliosides. However, an integrated model of BoNT/B bound to its neuronal receptors in a native membrane topology is still lacking. Using a panel of in silico and experimental approaches, we present here a new model for BoNT/B binding to neuronal membranes, in which the toxin binds to a preassembled synaptotagmin-ganglioside GT1b complex and a free ganglioside allowing a lipid-binding loop of BoNT/B to interact with the glycone part of the synaptotagmin-associated GT1b. Furthermore, our data provide molecular support for the decrease in BoNT/B sensitivity in Felidae that harbor the natural variant synaptotagmin2-N59Q. These results reveal multiple interactions of BoNT/B with gangliosides and support a novel paradigm in which a toxin recognizes a protein/ganglioside complex.

High Cell Density Cultivation Process for the Expression of Botulinum Neurotoxin a Receptor Binding Domain

The receptor-binding domain of botulinum neurotoxin (H_C fragment), is a promising botulism vaccine candidate. In the current study, fermentation strategies were evaluated to upscale H_C fragment expression. A simple translation of the growth conditions from shake flasks to a batch fermentation process resulted in limited culture growth and protein expression (OD of 11 and volumetric protein yields of 123 mg/L). Conducting fed-batch fermentation with rich media and continuous nutrient supplementation significantly improved culture growth (OD of 40.3) and protein expression (1093 mg/L). A further increase in H_C fragment yield was achieved by high cell density cultivation (HCDC). The bacterium was grown in a defined medium and with a combined bolus/continuous feed of nutrients to maintain desired oxygen levels and prevent acetate accumulation. The final OD of the process was 260, and the volumetric yield of the H_C fragment was 2065 mg/L, which reflects improvement by an order of magnitude. Purified H_C fragments, produced by HCDC, exhibited typical biochemical and protective characteristics in mice. Taken together, the advancements achieved in this study promote large-scale production of the H_C fragment in E. coli for use in anti-botulism vaccines.

Gene expression profile and injury sites in mice treated with Shiga toxin 2 and lipopolysaccharide as a Shiga toxin-associated hemolytic uremic syndrome model

Shiga toxin 2 (Stx2) and lipopolysaccharide (LPS) contribute to the development of hemolytic uremic syndrome (HUS). Mouse models of HUS induced by LPS/Stx2 have been used for elucidating HUS pathophysiology and for therapeutic development. However, the underlying molecular mechanisms and detailed injury sites in this model remain unknown. We analyzed mouse kidneys after LPS/Stx2 administration using microarrays. Decreased urinary osmolality and urinary potassium were observed after LPS/Stx2 administration, suggestive of distal nephron disorders. A total of 1212 and 1016 differentially expressed genes were identified in microarrays at 6 and 72 h after LPS/Stx2 administration, respectively, compared with those in controls. Ingenuity pathway analysis revealed activation of TNFR1/2, iNOS, and IL-6 signaling at both time points, and inhibition of pathways associated with lipid metabolism at 72 h only. The strongly downregulated genes in the 72-h group were expressed in the distal nephrons. In particular, genes associated with distal convoluted tubule (DCT) 2 /connecting tubule (CNT) and principal cells of the cortical collection duct (CCD) were downregulated to a greater extent than those associated with DCT1 and intercalated cells. Stx receptor globotriaosylceramide 3 (Gb3) revealed no colocalization with DCT1-specific Pvalb and intercalated cell-specific Slc26a4 but did present colocalization with Slc12a3 (present in both DCT1 and DCT2), and Aqp2 in principal cells. Gb3 localization tended to coincide with the segment in which the downregulated genes were present. Thus, the LPS/Stx2-induced kidney injury model represents damage to DCT2/CNT and principal cells in the CCD, based on molecular, biological, and physiological findings.

The Isolated Mouse Jejunal Afferent Nerve Assay as a Tool to Assess the Effect of Botulinum Neurotoxins in Visceral Nociception

For the past two decades, botulinum neurotoxin A (BoNT/A) has been described as a strong candidate in the treatment of pain. With the production of modified toxins and the potential new applications at the visceral level, there is a real need for tools allowing the assessment of these compounds. In this study, we evaluated the jejunal mesenteric afferent nerve assay to investigate BoNT/A effects on visceral nociception. This ex vivo model allowed the continuous recording of neuronal activity in response to various stimuli. BoNT/A was applied intraluminally during three successive distensions, and the jejunum was distended every 15 min for 3 h. Finally, samples were exposed to external capsaicin. BoNT/A intoxication was validated at the molecular level with the presence of cleaved synaptosomal-associated protein of 25 (SNAP25) in nerve terminals in the mucosa and musculosa layers 3 h after treatment. BoNT/A had a progressive inhibitory effect on multiunit discharge frequency induced by jejunal distension, with a significant decrease from 1 h after application without change in jejunal compliance. The capsaicin-induced discharge was also affected by the toxin. This assay allowed the description of an inhibitory effect of BoNT/A on afferent nerve activity in response to distension and capsaicin, suggesting BoNT/A could alleviate visceral nociception.

NGF Enhances CGRP Release Evoked by Capsaicin from Rat Trigeminal Neurons: Differential Inhibition by SNAP-25-Cleaving Proteases

Nerve growth factor (NGF) is known to intensify pain in various ways, so perturbing pertinent effects without negating its essential influences on neuronal functions could help the search for much-needed analgesics. Towards this goal, cultured neurons from neonatal rat trigeminal ganglia—a locus for craniofacial sensory nerves—were used to examine how NGF affects the Ca²⁺-dependent release of a pain mediator, calcitonin gene-related peptide (CGRP), that is triggered by activating a key signal transducer, transient receptor potential vanilloid 1 (TRPV1) with capsaicin (CAP). Measurements utilised neurons fed with or deprived of NGF for 2 days. Acute re-introduction of NGF induced Ca²⁺-dependent CGRP exocytosis that was inhibited by botulinum neurotoxin type A (BoNT/A) or a chimera of/E and/A (/EA), which truncated SNAP-25 (synaptosomal-associated protein with Mr = 25 k) at distinct sites. NGF additionally caused a Ca²⁺-independent enhancement of the neuropeptide release evoked by low concentrations (<100 nM) of CAP, but only marginally increased the peak response to ≥100 nM. Notably, BoNT/A inhibited CGRP exocytosis evoked by low but not high CAP concentrations, whereas/EA effectively reduced responses up to 1 µM CAP and inhibited to a greater extent its enhancement by NGF. In addition to establishing that sensitisation of sensory neurons to CAP by NGF is dependent on SNARE-mediated membrane fusion, insights were gleaned into the differential ability of two regions in the C-terminus of SNAP-25 (181–197 and 198–206) to support CAP-evoked Ca²⁺-dependent exocytosis at different intensities of stimulation.

Botulinum Neurotoxins in Central Nervous System: An Overview from Animal Models to Human Therapy

Botulinum neurotoxins (BoNTs) are potent inhibitors of synaptic vesicle fusion and transmitter release. The natural target of BoNTs is the peripheral neuromuscular junction (NMJ) where, by blocking the release of acetylcholine (ACh), they functionally denervate muscles and alter muscle tone. This leads them to be an excellent drug for the therapy of muscle hyperactivity disorders, such as dystonia, spasticity, and many other movement disorders. BoNTs are also effective in inhibiting both the release of ACh at sites other than NMJ and the release of neurotransmitters other than ACh. Furthermore, much evidence shows that BoNTs can act not only on the peripheral nervous system (PNS), but also on the central nervous system (CNS). Under this view, central changes may result either from sensory input from the PNS, from retrograde transport of BoNTs, or from direct injection of BoNTs into the CNS. The aim of this review is to give an update on available data, both from animal models or human studies, which suggest or confirm central alterations induced by peripheral or central BoNTs treatment. The data will be discussed with particular attention to the possible therapeutic applications to pathological conditions and degenerative diseases of the CNS.

Neurocosmetics in Skincare—The Fascinating World of Skin–Brain Connection: A Review to Explore Ingredients, Commercial Products for Skin Aging, and Cosmetic Regulation

The “modern” cosmetology industry is focusing on research devoted to discovering novel neurocosmetic functional ingredients that could improve the interactions between the skin and the nervous system. Many cosmetic companies have started to formulate neurocosmetic products that exhibit their activity on the cutaneous nervous system by affecting the skin’s neuromediators through different mechanisms of action. This review aims to clarify the definition of neurocosmetics, and to describe the features of some functional ingredients and products available on the market, with a look at the regulatory aspect. The attention is devoted to neurocosmetic ingredients for combating skin stress, explaining the stress pathways, which are also correlated with skin aging. “Neuro-relaxing” anti-aging ingredients derived from plant extracts and neurocosmetic strategies to combat inflammatory responses related to skin stress are presented. Afterwards, the molecular basis of sensitive skin and the suitable neurocosmetic ingredients to improve this problem are discussed. With the aim of presenting the major application of Botox-like ingredients as the first neurocosmetics on the market, skin aging is also introduced, and its theory is presented. To confirm the efficacy of the cosmetic products on the market, the concept of cosmetic claims is discussed.

Botulinum Toxin in Movement Disorders: An Update

Since its initial approval in 1989 by the US Food and Drug Administration for the treatment of blepharospasm and other facial spasms, botulinum toxin (BoNT) has evolved into a therapeutic modality for a variety of neurological and non-neurological disorders. With respect to neurologic movement disorders, BoNT has been reported to be effective for the treatment of dystonia, bruxism, tremors, tics, myoclonus, restless legs syndrome, tardive dyskinesia, and a variety of symptoms associated with Parkinson's disease. More recently, research with BoNT has expanded beyond its use as a powerful muscle relaxant and a peripherally active drug to its potential central nervous system applications in the treatment of neurodegenerative disorders. Although BoNT is the most potent biologic toxin, when it is administered by knowledgeable and experienced clinicians, it is one of the safest therapeutic agents in clinical use. The primary aim of this article is to provide an update on recent advances in BoNT research with a focus on novel applications in the treatment of movement disorders. This comprehensive review of the literature provides a critical review of evidence-based clinical trials and highlights recent innovative pilot studies.