Is the light chain subcellular localization an important factor in botulinum toxin duration of action?

Neurotoxin-Derived Optical Probes for Elucidating Molecular and Developmental Biology of Neurons and Synaptic Connections. Toxin-Derived Optical Probes for Neuroimaging

Botulinum neurotoxins (BoNTs) and tetanus toxin (TeTX) are the deadliest biological substances that cause botulism and tetanus, respectively. Their astonishing potency and capacity to enter neurons and interfere with neurotransmitter release at presynaptic terminals have attracted much interest in experimental neurobiology and clinical research. Fused with reporter proteins or labelled with fluorophores, BoNTs and TeTX and their non-toxic fragments also offer remarkable opportunities to visualize cellular processes and functions in neurons and synaptic connections. This study presents the state-of-the-art optical probes derived from BoNTs and TeTX and discusses their applications in molecular and synaptic biology and neurodevelopmental research. It reviews the principles of the design and production of probes, revisits their applications with advantages and limitations and considers prospects for future improvements. The versatile characteristics of discussed probes and reporters make them an integral part of the expanding toolkit for molecular neuroimaging, promoting the discovery process in neurobiology and translational neurosciences.

Update on Non-Interchangeability of Botulinum Neurotoxin Products

The growing use of botulinum neurotoxins (BoNTs) for medical and aesthetic purposes has led to the development and marketing of an increasing number of BoNT products. Given that BoNTs are biological medications, their characteristics are heavily influenced by their manufacturing methods, leading to unique products with distinct clinical characteristics. The manufacturing and formulation processes for each BoNT are proprietary, including the potency determination of reference standards and other features of the assays used to measure unit potency. As a result of these differences, units of BoNT products are not interchangeable or convertible using dose ratios. The intrinsic, product-level differences among BoNTs are compounded by differences in the injected tissues, which are innervated by different nerve fiber types (e.g., motor, sensory, and/or autonomic nerves) and require unique dosing and injection sites that are particularly evident when treating complex therapeutic and aesthetic conditions. It is also difficult to compare across studies due to inherent differences in patient populations and trial methods, necessitating attention to study details underlying each outcome reported. Ultimately, each BoNT possesses a unique clinical profile for which unit doses and injection paradigms must be determined individually for each indication. This practice will help minimize unexpected adverse events and maximize efficacy, duration, and patient satisfaction. With this approach, BoNT is poised to continue as a unique tool for achieving individual goals for an increasing number of medical and aesthetic indications.

Botox (onabotulinumtoxinA) mechanism of action

Studies in the 1920s found that botulinum neurotoxin type A (BoNT/A) inhibited the activity of motor and parasympathetic nerve endings, confirmed several decades later to be due to decreased acetylcholine release. The 1970s were marked by studies of cellular mechanisms aided by use of neutralizing antibodies as pharmacologic tools: BoNT/A disappeared from accessibility to neutralizing antibodies within minutes, although it took several hours for onset of muscle weakness. The multi-step mechanism was experimentally confirmed and is now recognized to consist broadly of binding to nerve terminals, internalization, and lysis or cleavage of a protein (SNAP-25: synaptosomal associated protein-25 kDa) that is part of the SNARE (Soluble NSF Attachment protein REceptor) complex needed for synaptic vesicle docking and fusion. Clinical use of the BoNT/A product onabotulinumtoxinA was based on its ability to reduce muscle contractions via inhibition of acetylcholine from motor terminals. Sensory mechanisms of onabotulinumtoxinA have now been identified, supporting its successful treatment of chronic migraine and urgency in overactive bladder. Exploration into migraine mechanisms led to anatomical studies documenting pain fibers that send axons through sutures of the skull to outside the head-a potential route by which extracranial injections could affect intracranial processes. Several clinical studies have also identified benefits of onabotulinumtoxinA in major depression, which have been attributed to central responses induced by feedback from facial muscle and skin movement. Overall, the history of BoNT/A is distinguished by basic science studies that stimulated clinical use and, conversely, clinical observations that spurred basic research into novel mechanisms of action.

How Botulinum Neurotoxin Light Chain A1 Maintains Stable Association with the Intracellular Neuronal Plasma Membrane

Botulinum neurotoxin serotype A (BoNT/A) is the most potent protein toxin for humans and is utilized as a therapy for numerous neurologic diseases. BoNT/A comprises a catalytic Light Chain (LC/A) and a Heavy Chain (HC/A) and includes eight subtypes (BoNT/A1-/A8). Previously we showed BoNT/A potency positively correlated with stable localization on the intracellular plasma membrane and identified a low homology domain (amino acids 268-357) responsible for LC/A1 stable co-localization with SNAP-25 on the plasma membrane, while LC/A3 was present in the cytosol of Neuro2A cells. In the present study, steady-state- and live-imaging of a cytosolic LC/A3 derivative (LC/A3V) engineered to contain individual structural elements of the A1 LDH showed that a 59 amino acid region (275-334) termed the MLD was sufficient to direct LC/A3V from the cytosol to the plasma membrane co-localized with SNAP-25. Informatics and experimental validation of the MLD-predicted R1 region (an α-helix, residues 275-300) and R2 region (a loop, α-helix, loop, residues 302-334) both contribute independent steps to the stable co-localization of LC/A1 with SNAP-25 on the plasma membrane of Neuro-2A cells. Understanding how these structural elements contribute to the overall association of LC/A1 on the plasma membrane may identify the molecular basis for the LC contribution of BoNT/A1 to high potency.

Clostridial Neurotoxins: Structure, Function and Implications to Other Bacterial Toxins

Gram-positive bacteria are ancient organisms. Many bacteria, including Gram-positive bacteria, produce toxins to manipulate the host, leading to various diseases. While the targets of Gram-positive bacterial toxins are diverse, many of those toxins use a similar mechanism to invade host cells and exert their functions. Clostridial neurotoxins produced by Clostridial tetani and Clostridial botulinum provide a classical example to illustrate the structure-function relationship of bacterial toxins. Here, we critically review the recent progress of the structure-function relationship of clostridial neurotoxins, including the diversity of the clostridial neurotoxins, the mode of actions, and the flexible structures required for the activation of toxins. The mechanism clostridial neurotoxins use for triggering their activity is shared with many other Gram-positive bacterial toxins, especially molten globule-type structures. This review also summarizes the implications of the molten globule-type flexible structures to other Gram-positive bacterial toxins. Understanding these highly dynamic flexible structures in solution and their role in the function of bacterial toxins not only fills in the missing link of the high-resolution structures from X-ray crystallography but also provides vital information for better designing antidotes against those toxins.

Crystal structure of the catalytic domain of botulinum neurotoxin subtype A3

Botulinum neurotoxins (BoNTs) are among the most widely used therapeutic proteins; however, only two subtypes within the seven serotypes, BoNT/A1 and BoNT/B1, are currently used for medical and cosmetic applications. Distinct catalytic properties, substrate specificities, and duration of enzymatic activities potentially make other subtypes very attractive candidates to outperform conventional BoNTs in particular therapeutic applications. For example, BoNT/A3 has a significantly shorter duration of action than other BoNT/A subtypes. Notably, BoNT/A3 is the subtype with the least conserved catalytic domain among BoNT/A subtypes. This suggests that the sequence differences, many of which concern the α-exosite, contribute to the observed functional differences in toxin persistence by affecting the binding of the substrate SNAP-25 and/or the stability of the catalytic domain fold. To identify the molecular determinants accounting for the differences in the persistence observed for BoNT/A subtypes, we determined the crystal structure of the catalytic domain of BoNT/A3 (LC/A3). The structure of LC/A3 was found to be very similar to that of LC/A1, suggesting that the overall mode of SNAP-25 binding is common between these two proteins. However, circular dichroism (CD) thermal unfolding experiments demonstrated that LC/A3 is significantly less stable than LC/A1, implying that this might contribute to the reduced toxin persistence of BoNT/A3. These findings could be of interest in developing next-generation therapeutic toxins.

The Light Chain Defines the Duration of Action of Botulinum Toxin Serotype A Subtypes

Botulinum neurotoxin (BoNT) is the causative agent of botulism and a widely used pharmaceutical to treat a variety of neurological diseases. BoNTs are 150-kDa protein toxins organized into heavy chain (HC) and light chain (LC) domains linked by a disulfide bond. The HC selectively binds to neurons and aids cell entry of the enzymatically active LC. There are seven immunological BoNT serotypes (A to G); each serotype includes genetic variants, termed subtypes. Only two subtypes, BoNT/A1 and BoNT/B1, are currently used as therapeutics. BoNT serotype A (BoNT/A) subtypes A2 to A8 show distinct potency, duration of action, and pathology relative to BoNT/A1. Specifically, BoNT/A3 possesses shorter duration of action and elicits distinct symptoms in mice at high toxin doses. In this report, we analyzed the roles of LC and HC of BoNT/A3 for duration of action, neuronal cell entry, and mouse pathology by using clostridium-derived recombinant hybrid BoNTs consisting of reciprocal LC and HC (BoNTA1/A3 and BoNTA3/A1). Hybrid toxins were processed in their expression host to a dichain BoNT consisting of LC and HC linked via a disulfide bond. The LC and HC defined BoNT potency in mice and BoNT toxicity for cultured neuronal cells, while the LC defined the duration of BoNT action in cell and mouse models. Protein alignment identified a previously unrecognized region within the LC subtype A3 (LC/A3) relative to the other LC serotype A (LC/A) subtypes (low primary acid homology [LPH]) that correlated to intracellular LC localization. This study shows the utility of recombinant hybrid BoNTs with new therapeutic potential, while remaining sensitive to antitoxins and therapies to native BoNT.IMPORTANCE Botulinum neurotoxins are the most potent protein toxins for humans and potential bioterrorism threats, but they are also widely used as pharmaceuticals. Within the large family of BoNTs, only two subtypes are currently used as pharmaceuticals, with a large number of BoNT subtypes remaining as untapped potential sources for unique pharmaceuticals. Here, two recombinant hybrid toxins were engineered, consisting of domains from two BoNT subtypes that possess distinct duration of action and activity in human neurons and mice. We define the functional domains responsible for BoNT action and demonstrate creation of functional hybrid BoNTs with new therapeutic potential, while remaining sensitive to antitoxins and therapies to native BoNT.

Deubiquitinating enzyme VCIP135 dictates the duration of botulinum neurotoxin type A intoxication

Botulism is characterized by flaccid paralysis, which can be caused by intoxication with any of the seven known serotypes of botulinum neurotoxin (BoNT), all of which disrupt synaptic transmission by endoproteolytic cleavage of SNARE proteins. BoNT serotype A (BoNT/A) has the most prolonged or persistent effects, which can last several months, and exerts its effects by specifically cleaving and inactivating SNAP25. A major factor contributing to the persistence of intoxication is the long half-life of the catalytic light chain, which remains enzymatically active months after entry into cells. Here we report that BoNT/A catalytic light chain binds to, and is a substrate for, the ubiquitin ligase HECTD2. However, the light chain evades proteasomal degradation by the dominant effect of a deubiquitinating enzyme, VCIP135/VCPIP1. This deubiquitinating enzyme binds BoNT/A light chain directly, with the two associating in cells through the C-terminal 77 amino acids of the light chain protease. The development of specific DUB inhibitors, together with inhibitors of BoNT/A proteolytic activity, may be useful for reducing the morbidity and public health costs associated with BoNT/A intoxication and could have potential biodefense implications.

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.

Apoptotic action of botulinum toxin on masseter muscle in rats: early and late changes in the expression of molecular markers

The purpose of this study was to compare the early or late expression levels of p65, Bcl-2, and type II myosin and the frequency of TUNEL-positive nuclei in the rat masseter muscle after injection of different concentrations of botulinum toxin-A (BTX-A). We injected either 5 U or 10 U of BTX-A into both masseter muscles of the rat. As a control group, the same volume of saline was injected. After 2 or 12 weeks, the animals were sacrificed. Subsequently, a biopsy and immunohistochemical staining of the samples were performed using a p65, Bcl-2, or type II myosin antibody. Additionally, a TUNEL assay and transmission electron microscopic analysis were performed. The expression of p65, Bcl-2, and type II myosin increased significantly with increasing concentrations of BTX-A at 2 weeks after BTX-A injection (P < 0.05). The number of TUNEL-positive nuclei was also significantly increased in the BTX-A-treated groups in comparison to the saline-treated control at 2 weeks after BTX-A injection (P < 0.05). Elevated expression of Bcl-2 was also observed in 10-unit BTX-A-treated group at 12 weeks after injection (P < 0.05). At 12 weeks after injection, the number of enlarged mitochondria was increased, and many mitochondria displayed aberrations in cristae morphology after BTX-A injection. In conclusion, BTX-A injection into the masseter muscle increased the expression level of p65, Bcl-2, and type II myosin at an early stage. The morphological changes of mitochondria were more evident at 12 weeks after injection.

Persistence of botulinum neurotoxin a subtypes 1-5 in primary rat spinal cord cells

Botulinum neurotoxins (BoNTs) are the most poisonous substances known and cause the severe disease botulism. BoNTs have also been remarkably effective as therapeutics in treating many neuronal and neuromuscular disorders. One of the hallmarks of BoNTs, particularly serotype A, is its long persistence of 2-6 months in patients at concentrations as low as fM or pM. The mechanisms for this persistence are currently unclear. In this study we determined the persistence of the BoNT/A subtypes 1 through 5 in primary rat spinal neurons. Remarkably, the duration of intracellular enzymatic activity of BoNT/A1, /A2, /A4 and /A5 was shown to be at least 10 months. Conversely, the effects of BoNT/A3 were observed for up to ∼5 months. An intermittent dosing with BoNT/E showed intracellular activity of the shorter acting BoNT/E for 2–3 weeks, followed by reoccurrence and persistence of BoNT/A-induced SNAP-25 cleavage products.

Anti-nociceptive effect of a conjugate of substance P and light chain of botulinum neurotoxin type A

Neuropathic pain is a debilitating condition resulting from damage to sensory transmission pathways in the peripheral and central nervous system. A potential new way of treating chronic neuropathic pain is to target specific pain-processing neurons based on their expression of particular receptor molecules. We hypothesized that a toxin-neuropeptide conjugate would alter pain by first being taken up by specific receptors for the neuropeptide expressed on the neuronal cells. Then, once inside the cell the toxin would inhibit the neurons' activity without killing the neurons, thereby providing pain relief without lesioning the nervous system. In an effort to inactivate the nociceptive neurons in the trigeminal nucleus caudalis in mice, we targeted the NK1 receptor (NK1R) using substance P (SP). The catalytically active light chain of botulinum neurotoxin type A (LC/A) was conjugated with SP. Our results indicate that the conjugate BoNT/A-LC:SP is internalized in cultured NK1R-expressing neurons and also cleaves the target of botulinum toxin, a component-docking motif necessary for release of neurotransmitters called SNAP-25. The conjugate was next tested in a murine model of Taxol-induced neuropathic pain. An intracisternal injection of BoNT/A-LC:SP decreased thermal hyperalgesia as measured by the operant orofacial nociception assay. These findings indicate that conjugates of the light chain of botulinum toxin are extremely promising agents for use in suppressing neuronal activity for extended time periods, and that BoNT/A-LC:SP may be a useful agent for treating chronic pain.

Persistence of Botulinum neurotoxin inactivation of nerve function

The extraordinary persistence of intoxication occurring after exposure to some Botulinum neurotoxin (BoNT) serotypes is both a therapeutic marvel and a biodefense nightmare. Understanding the mechanisms underlying BoNT persistence will offer new strategies for improving the efficacy and extending the applications of BoNT therapeutic agents as well as for treating the symptoms of botulism. Research indicates that the persistence of BoNT intoxication can be influenced both by the ability of the toxin protease or its cleaved soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein substrate to resist turnover. Protease turnover seems to be mediated in part by the ubiquitin-proteasome system (UPS) and efforts to manipulate the UPS may prove to be an effective strategy for improving therapeutic utility of BoNT products and in the development of botulism antidotes.

Development of a Cell-Based Functional Assay for the Detection of Clostridium botulinum Neurotoxin Types A and E

The standard procedure for definitive detection of BoNT-producing Clostridia is a culture method combined with neurotoxin detection using a standard mouse bioassay (MBA). The mouse bioassay is highly sensitive and specific, but it is expensive and time-consuming, and there are ethical concerns due to use of laboratory animals. Cell-based assays provide an alternative to the MBA in screening for BoNT-producing Clostridia. Here, we describe a cell-based assay utilizing a fluorescence reporter construct expressed in a neuronal cell model to study toxin activity in situ. Our data indicates that the assay can detect as little as 100 pM BoNT/A activity within living cells, and the assay is currently being evaluated for the analysis of BoNT in food matrices. Among available in vitro assays, we believe that cell-based assays are widely applicable in high-throughput screenings and have the potential to at least reduce and refine animal assays if not replace it.

Botulinum neurotoxins A and E undergo retrograde axonal transport in primary motor neurons

The striking differences between the clinical symptoms of tetanus and botulism have been ascribed to the different fate of the parental neurotoxins once internalised in motor neurons. Tetanus toxin (TeNT) is known to undergo transcytosis into inhibitory interneurons and block the release of inhibitory neurotransmitters in the spinal cord, causing a spastic paralysis. In contrast, botulinum neurotoxins (BoNTs) block acetylcholine release at the neuromuscular junction, therefore inducing a flaccid paralysis. Whilst overt experimental evidence supports the sorting of TeNT to the axonal retrograde transport pathway, recent findings challenge the established view that BoNT trafficking is restricted to the neuromuscular junction by highlighting central effects caused by these neurotoxins. These results suggest a more complex scenario whereby BoNTs also engage long-range trafficking mechanisms. However, the intracellular pathways underlying this process remain unclear. We sought to fill this gap by using primary motor neurons either in mass culture or differentiated in microfluidic devices to directly monitor the endocytosis and axonal transport of full length BoNT/A and BoNT/E and their recombinant binding fragments. We show that BoNT/A and BoNT/E are internalised by spinal cord motor neurons and undergo fast axonal retrograde transport. BoNT/A and BoNT/E are internalised in non-acidic axonal carriers that partially overlap with those containing TeNT, following a process that is largely independent of stimulated synaptic vesicle endo-exocytosis. Following intramuscular injection in vivo, BoNT/A and TeNT displayed central effects with a similar time course. Central actions paralleled the peripheral spastic paralysis for TeNT, but lagged behind the onset of flaccid paralysis for BoNT/A. These results suggest that the fast axonal retrograde transport compartment is composed of multifunctional trafficking organelles orchestrating the simultaneous transfer of diverse cargoes from nerve terminals to the soma, and represents a general gateway for the delivery of virulence factors and pathogens to the central nervous system.

Botulinum neurotoxins are the most toxic molecules known to mankind, and as a result, are currently listed among the top bio-threats. However, their ability to bind specifically to neurons and their inhibitory effects on regulated secretion prompted their clinical use in pathologies characterised by increased muscular tone, such as dystonia and various forms of spasticity, or abnormal secretion, such as drooling and excessive sweating, to cite a few. As a consequence, botulinum neurotoxin A, which is the serotype most commonly used in human therapy, has become the treatment of choice for an ever-expanding number of pathological and non-pathological (e.g. cosmetic) conditions. All current indications show that the systemic effects and toxicity of botulinum neurotoxin A are minimised by the specific route of administration (local injection) and the low diffusion of this molecule in tissues. However, recent reports suggest that in contrast to this common belief, botulinum neurotoxin A is able to reach distal sites in the body and may have previously unanticipated effects in the central nervous system. In this study, we demonstrate that botulinum neurotoxin A and E enter alternative endocytic pathway(s) in addition to synaptic vesicle recycling, and undergo long-range transport in a non degradative compartment in spinal cord motor neurons. Our results show that axonal retrograde transport is a common pathway for the dissemination in the central nervous system of pathogens and virulence factors important for human and animal health.

Emerging opportunities for serotypes of botulinum neurotoxins

Background: Two decades ago, botulinum neurotoxin (BoNT) type A was introduced to the commercial market. Subsequently, the toxin was approved by the FDA to address several neurological syndromes, involving muscle, nerve, and gland hyperactivity. These syndromes have typically been associated with abnormalities in cholinergic transmission. Despite the multiplicity of botulinal serotypes (designated as types A through G), therapeutic preparations are currently only available for BoNT types A and B. However, other BoNT serotypes are under study for possible clinical use and new clinical indications; Objective: To review the current research on botulinum neurotoxin serotypes A-G, and to analyze potential applications within basic science and clinical settings; Conclusions: The increasing understanding of botulinal neurotoxin pathophysiology, including the neurotoxin’s effects on specific neuronal populations, will help us in tailoring treatments for specific diagnoses, symptoms and patients. Scientists and clinicians should be aware of the full range of available data involving neurotoxin subtypes A-G.

Clinical uses of botulinum neurotoxins: current indications, limitations and future developments

Botulinum neurotoxins (BoNTs) cause flaccid paralysis by interfering with vesicle fusion and neurotransmitter release in the neuronal cells. BoNTs are the most widely used therapeutic proteins. BoNT/A was approved by the U.S. FDA to treat strabismus, blepharospam, and hemificial spasm as early as 1989 and then for treatment of cervical dystonia, glabellar facial lines, axillary hyperhidrosis, chronic migraine and for cosmetic use. Due to its high efficacy, longevity of action and satisfactory safety profile, it has been used empirically in a variety of ophthalmological, gastrointestinal, urological, orthopedic, dermatological, secretory, and painful disorders. Currently available BoNT therapies are limited to neuronal indications with the requirement of periodic injections resulting in immune-resistance for some indications. Recent understanding of the structure-function relationship of BoNTs prompted the engineering of novel BoNTs to extend therapeutic interventions in non-neuronal systems and to overcome the immune-resistance issue. Much research still needs to be done to improve and extend the medical uses of BoNTs.

Reexamining hydroxamate inhibitors of botulinum neurotoxin serotype A: extending towards the β-exosite

Botulinum neurotoxins (BoNTs) are the most toxic proteins known to man, exposure to which results in flaccid paralysis. Given their extreme potency, these proteins have become studied as possible weapons of bioterrorism; however, effective treatments that function after intoxication have not progressed to the clinic. Here, we have reexamined one of the most effective inhibitors, 2,4-dichlorocinnamyl hydroxamate, in the context of the known plasticity of the BoNT/A light chain metalloprotease. Our studies have shown that modifications of this compound are tolerated and result in improved inhibitors, with the best compound having an IC(50) of 0.23 μM. Given the inconsistency of structure-activity relationship trends observed across similar compounds, this data argues for caution in extrapolating across structural series.

Association of botulinum neurotoxin serotype A light chain with plasma membrane-bound SNAP-25

The Clostridium botulinum neurotoxins (BoNTs) cleave SNARE proteins, which inhibit binding and thus fusion of neurotransmitter vesicles to the plasma membrane of peripheral neurons. BoNTs comprise an N-terminal light chain (LC) and C-terminal heavy chain, which are linked by a disulfide bond. There are seven serotypes (A-G) of BoNTs based upon immunological neutralization. Although the binding and entry of BoNT/A into neurons has been subjected to considerable investigation, the intracellular events that allow BoNT/A to efficiently cleave SNAP-25 within neurons is less well understood. Earlier studies showed that intracellular LC/A bound to the plasma membrane of neurons. In this study, intracellular LC/A is shown to directly bind SNAP-25 on the plasma membrane. Solid phase binding showed that the N-terminal residues of LC/A bound residues 80-110 of SNAP-25, which was also observed in cultured neurons. Association of the N-terminal 8 amino acids of LC/A and residues 80-110 of SNAP-25 also enhanced substrate cleavage. These findings explain how LC/A associates with SNAP-25 on the plasma membrane and provide a basis for LC/A cleavage of SNAP-25 within the SNARE complex.

Towards new uses of botulinum toxin as a novel therapeutic tool

The uses of botulinum toxin in the fields of neurology, ophthalmology, urology, rehabilitation medicine and aesthetic applications have been revolutionary for the treatment of patients. This non-invasive therapeutic has continually been developed since first discovered in the 1970s as a new approach to what were previously surgical treatments. As these applications develop, so also the molecules are developing into tools with new therapeutic properties in specific clinical areas. This review examines how the botulinum toxin molecule is being adapted to new therapeutic uses and also how new areas of use for the existing molecules are being identified. Prospects for future developments are also considered.

Targeting botulinum neurotoxin persistence by the ubiquitin-proteasome system

Botulinum neurotoxins (BoNTs) are the most potent natural toxins known. The effects of BoNT serotype A (BoNT/A) can last several months, whereas the effects of BoNT serotype E (BoNT/E), which shares the same synaptic target, synaptosomal-associated protein 25 (SNAP25), last only several weeks. The long-lasting effects or persistence of BoNT/A, although desirable for therapeutic applications, presents a challenge for medical treatment of BoNT intoxication. Although the mechanisms for BoNT toxicity are well known, little is known about the mechanisms that govern the persistence of the toxins. We show that the recombinant catalytic light chain (LC) of BoNT/E is ubiquitylated and rapidly degraded in cells. In contrast, BoNT/A LC is considerably more stable. Differential susceptibility of the catalytic LCs to ubiquitin-dependent proteolysis therefore might explain the differential persistence of BoNT serotypes. In this regard we show that TRAF2, a RING finger protein implicated in ubiquitylation, selectively associates with BoNT/E LC and promotes its proteasomal degradation. Given these data, we asked whether BoNT/A LC could be targeted for rapid proteasomal degradation by redirecting it to characterized ubiquitin ligase domains. We describe chimeric SNAP25-based ubiquitin ligases that target BoNT/A LC for degradation, reducing its duration in a cellular model for toxin persistence.

Camelid single domain antibodies (VHHs) as neuronal cell intrabody binding agents and inhibitors of Clostridium botulinum neurotoxin (BoNT) proteases

Botulinum neurotoxins (BoNTs) function by delivering a protease to neuronal cells that cleave SNARE proteins and inactivate neurotransmitter exocytosis. Small (14 kDa) binding domains specific for the protease of BoNT serotypes A or B were selected from libraries of heavy chain only antibody domains (VHHs or nanobodies) cloned from immunized alpacas. Several VHHs bind the BoNT proteases with high affinity (K(D) near 1 nM) and include potent inhibitors of BoNT/A protease activity (K(i) near 1 nM). The VHHs retain their binding specificity and inhibitory functions when expressed within mammalian neuronal cells as intrabodies. A VHH inhibitor of BoNT/A protease was able to protect neuronal cell SNAP25 protein from cleavage following intoxication with BoNT/A holotoxin. These results demonstrate that VHH domains have potential as components of therapeutic agents for reversal of botulism intoxication.

Lipid and cationic polymer based transduction of botulinum holotoxin, or toxin protease alone, extends the target cell range and improves the efficiency of intoxication

Botulinum neurotoxin (BoNT) heavy chain (Hc) facilitates receptor-mediated endocytosis into neuronal cells and transport of the light chain (Lc) protease to the cytosol where neurotransmission is inhibited as a result of SNARE protein cleavage. Here we show that the role of BoNT Hc in cell intoxication can be replaced by commercial lipid-based and polycationic polymer DNA transfection reagents. BoNT "transduction" by these reagents permits efficient intoxication of neuronal cells as well as some non-neuronal cell lines normally refractory to BoNT. Surprisingly, the reagents facilitate delivery of recombinant BoNT Lc protease to the cytosol of both neuronal and non-neuronal cells in the absence of BoNT Hc, and with sensitivities approaching that of BoNT holotoxin. Transduction of BoNT, as with natural intoxication, is inhibited by bafilomycin A1, methylamine and ammonium chloride indicating that both pathways require endosome acidification. DNA transfection reagents facilitate intoxication by holotoxins, or isolated Lc proteases, of all three BoNT serotypes tested (A, B, E). These results suggest that lipid and cationic polymer transfection reagents facilitate cytosolic delivery of BoNT holotoxins and isolated Lc proteases by an endosomal uptake pathway.

Presynaptic neurotoxins with enzymatic activities

Toxins that alter neurotransmitter release from nerve terminals are of considerable scientific and clinical importance. Many advances were recently made in the understanding of their molecular mechanisms of action and use in human therapy. Here, we focus on presynaptic neurotoxins, which are very potent inhibitors of the neurotransmitter release because they are endowed with specific enzymatic activities: (1) clostridial neurotoxins with a metallo-proteolytic activity and (2) snake presynaptic neurotoxins with a phospholipase A2 activity.

The structure and mode of action of different botulinum toxins

The seven serotypes (A-G) of botulinum neurotoxin (BoNT) are proteins produced by Clostridium botulinum and have multifunctional abilities: (i) they target cholinergic nerve endings via binding to ecto-acceptors (ii) they undergo endocytosis/translocation and (iii) their light chains act intraneuronally to block acetylcholine release. The fundamental process of quantal transmitter release occurs by Ca2+-regulated exocytosis involving sensitive factor attachment protein-25 (SNAP-25), syntaxin and synaptobrevin. Proteolytic cleavage by BoNT-A of nine amino acids from the C-terminal of SNAP-25 disables its function, causing prolonged muscle weakness. This unique combination of activities underlies the effectiveness of BoNT-A haemagglutinin complex in treating human conditions resulting from hyperactivity at peripheral cholinergic nerve endings. In vivo imaging and immunomicroscopy of murine muscles injected with type A toxin revealed that the extended duration of action results from the longevity of its protease, persistence of the cleaved SNAP-25 and a protracted time course for the remodelling of treated nerve-muscle synapses. In addition, an application in pain management has been indicated by the ability of BoNT to inhibit neuropeptide release from nociceptors, thereby blocking central and peripheral pain sensitization processes. The widespread cellular distribution of SNAP-25 and the diversity of the toxin's neuronal acceptors are being exploited for other therapeutic applications.

Botulinum neurotoxin structure, engineering, and novel cellular trafficking and targeting

Botulinum neurotoxins are multifaceted molecules, which are truly unique not only in their mode of action, but also their utility as a drug carrier either across the gut wall or to the nerve terminals. The molecule is divided in clear functional domains that can operate independently. This feature can be used to employ them as cargo carrier by linking other drugs or vaccines with the binding and translocation domains of BoNT. While the domain structures are largely independent of each other, the dynamic structure of these domains, especially that of the enzymatic domain (L chain), is quite different from the reported crystal structures for several BoNT serotypes and their enzymatic domain. This review discusses the comparative structures of BoNT in crystal and solution for their relevance to the molecular mechanism of BoNT action, especially in view of our recent discovery that the enzymatically active structure of the BoNT exists as a molten-globule and that of the endopeptidase domain as a novel PRIME conformation. Finally, a non-exhaustive discussion has been included to explain the long-lasting biological effects of certain serotypes of BoNT, based on the current knowledge of the structure-function of different serotypes of botulinum neurotoxins.

Hemagglutinin-33 of type A botulinum neurotoxin complex binds with synaptotagmin II

Botulinum neurotoxin type A (BoNT/A), the most toxic substance known to mankind, is produced by Clostridium botulinum type A as a complex with a group of neurotoxin-associated proteins (NAPs) through polycistronic expression of a clustered group of genes. NAPs are known to protect BoNT against adverse environmental conditions and proteolytic digestion. Hemagglutinin-33 (Hn-33) is a 33 kDa subcomponent of NAPs that is resistant to protease digestion, a feature likely to be involved in the protection of the botulinum neurotoxin from proteolysis. However, it is not known whether Hn-33 plays any role other than the protection of BoNT. Using immunoaffinity column chromatography and pull-down assays, we have now discovered that Hn-33 binds to synaptotagmin II, the putative receptor of botulinum neurotoxin. This finding provides important information relevant to the design of novel anti-botulism therapeutic agents targeted to block the entry of botulinum neurotoxin into nerve cells.

The Role of Exoproteases in Governing Intraneuronal Metabolism of Botulinum Toxin

Botulinum toxin type A has a long duration of action, and thus it can block transmitter release for several weeks to several months. However, little is known about the precise mechanism that accounts for termination of toxin action. Therefore, experiments were done to gauge the effects of aminopeptidases and carboxypeptidases on the structure and function of the toxin. Exoproteases were added to the holotoxin, the native light chain, and a recombinant light chain. Treated toxin and light chain were examined for their effects on neuromuscular transmission and on isolated substrate. The data showed that aminopeptidase attack did not alter the N-terminus of the toxin/light chain, nor did it produce losses in biological activity. Carboxypeptidase attack did alter the C-terminus of the light chain, but not sufficiently to alter biological activity. The data suggest that the tertiary structure of the light chain confers upon the molecule substantial resistance to exoproteases.