A substrate sensor chip to assay the enzymatic activity of Botulinum neurotoxin A

Detection of botulinum neurotoxin A (BoNT/A) enzymatic activity by pregnancy test strips based on hCG-modified magnetic nanoparticles

The detection of botulinum neurotoxin A (BoNT/A) endopeptidase activity by pregnancy test paper based on human chorionic gonadotropin (hCG)-functionalized peptide-modified magnetic nanoparticles (MNs) is described for the first time. HCG-functionalized SNAP-25 peptide substrate with hydrolysis recognition sites was optimally designed. HCG can be recognized by pregnancy test strips. BoNT/A light chain (BoNT-LcA) is the central part of the endopeptidase function in holotoxin, which can specifically hydrolyze SNAP-25 peptide to release the hCG-peptide probe, and the hCG-peptide probe released can be quantitatively detected by pregnancy test strips, achieving indirect determination of BoNT/A. By quantifying the T-line color intensity of test strips, the visual detection limit for BoNT-LcA is 12.5 pg/mL, and the linear range of detection for BoNT-LcA and BoNT/A holotoxin was 100 pg/mL to 1 ng/mL and 25 to 250 ng/mL. The ability of the method to quantify BoNT/A was validated in human serum samples. This method shows the potential for sensitive detecting BoNT/A and has prospects for the diagnosis and prognosis of clinical botulism.

Optimization of Application-Driven Development of In Vitro Neuromuscular Junction Models

Neuromuscular junctions (NMJs) are specialized synapses responsible for signal transduction between motor neurons (MNs) and skeletal muscle tissue. Malfunction at this site can result from developmental disorders, toxic environmental exposures, and neurodegenerative diseases leading to severe neurological dysfunction. Exploring these conditions in human or animal subjects is restricted by ethical concerns and confounding environmental factors. Therefore, in vitro NMJ models provide exciting opportunities for advancements in tissue engineering. In the last two decades, multiple NMJ prototypes and platforms have been reported, and each model system design is strongly tied to a specific application: exploring developmental physiology, disease modeling, or high-throughput screening. Directing the differentiation of stem cells into mature MNs and/or skeletal muscle for NMJ modeling has provided critical cues to recapitulate early-stage development. Patient-derived inducible pluripotent stem cells provide a personalized approach to investigating NMJ disease, especially when disease etiology cannot be resolved down to a specific gene mutation. Having reproducible NMJ culture replicates is useful for high-throughput screening to evaluate drug toxicity and determine the impact of environmental threat exposures. Cutting-edge bioengineering techniques have propelled this field forward with innovative microfabrication and design approaches allowing both two-dimensional and three-dimensional NMJ culture models. Many of these NMJ systems require further validation for broader application by regulatory agencies, pharmaceutical companies, and the general research community. In this summary, we present a comprehensive review on the current state-of-art research in NMJ models and discuss their ability to provide valuable insight into cell and tissue interactions. Impact statement In vitro neuromuscular junction (NMJ) models reveal the specialized mechanisms of communication between neurons and muscle tissue. This site can be disrupted by developmental disorders, toxic environmental exposures, or neurodegenerative diseases, which often lead to fatal outcomes and is therefore of critical importance to the medical community. Many bioengineering approaches for in vitro NMJ modeling have been designed to mimic development and disease; other approaches include in vitro NMJ models for high-throughput toxicology screening, providing a platform to limit or replace animal testing. This review describes various NMJ applications and the bioengineering advancements allowing for human NMJ characteristics to be more accurately recapitulated. While the extensive range of NMJ device structures has hindered standardization attempts, there is still a need to harmonize these devices for broader application and to continue advancing the field of NMJ modeling.

Recent Developments in Botulinum Neurotoxins Detection

Botulinum neurotoxins (BoNTs) are produced as protein complexes by bacteria of the genus Clostridium that are Gram-positive, anaerobic and spore forming (Clostridium botulinum, C. butyricum, C. baratii and C. argentinense spp.). BoNTs show a high immunological and genetic diversity. Therefore, fast, precise, and more reliable detection methods are still required to monitor outbreaks and ensure surveillance of botulism. The botulinum toxin field also comprises therapeutic uses, basic research studies and biodefense issues. This review presents currently available detection methods, and new methods offering the potential of enhanced precision and reproducibility. While the immunological methods offer a range of benefits, such as rapid analysis time, reproducibility and high sensitivity, their implementation is subject to the availability of suitable tools and reagents, such as specific antibodies. Currently, the mass spectrometry approach is the most sensitive in vitro method for a rapid detection of active or inactive forms of BoNTs. However, these methods require inter-laboratory validation before they can be more widely implemented in reference laboratories. In addition, these surrogate in vitro models also require full validation before they can be used as replacement bioassays of potency. Cell-based assays using neuronal cells in culture recapitulate all functional steps of toxin activity, but are still at various stages of development; they are not yet sufficiently robust, due to high batch-to-batch cell variability. Cell-based assays have a strong potential to replace the mouse bioassay (MBA) in terms of BoNT potency determination in pharmaceutical formulations; they can also help to identify suitable inhibitors while reducing the number of animals used. However, the development of safe countermeasures still requires the use of in vivo studies to complement in vitro immunological or cell-based approaches.

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.

Sensitive detection of type G botulinum neurotoxin through Endopep-MS peptide substrate optimization

Clostridium botulinum produces botulinum neurotoxins (BoNTs) that are one of the most poisonous substances. In order to respond to public health emergencies, there is a need to develop sensitive and specific methods for detecting botulinum toxin in various clinical matrices. Our laboratory has developed a mass spectrometry-based Endopep-MS assay that is able to rapidly detect and differentiate BoNT serotypes A–G by immunoaffinity capture of toxins and detection of unique cleavage products of peptide substrates. To improve the sensitivity of the Endopep-MS assay for the detection of BoNT serotype G, we report here the optimization of synthetic peptide substrates through systematic substitution, deletion, and incorporation of unnatural amino acids. Our data show that the resulting optimized peptides produced a significant improvement (two orders of magnitude) in assay sensitivity and allowed the detection of 0.01 mouseLD₅₀ toxin present in buffer solution.

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.

Content/Potency Assessment of Botulinum Neurotoxin Type-A by Validated Liquid Chromatography Methods and Bioassays

Botulinum neurotoxin type-A (BoNTA) is one of the seven different serotypes (A to G) produced by Clostridium botulinum. A stability-indicating size-exclusion chromatography (SEC) method was developed and validated, and the specificity was confirmed by forced degradation study, interference of the excipients, and peaks purity. The method was applied to assess the content and high-molecular-weight (HMW) forms of BoNTA in biopharmaceutical products, and the results were compared with those of the LD₅₀ mouse bioassay, the T−47D cell culture assay, and the reversed-phase chromatography (RPC) method, giving mean values of 0.71% higher, 0.36% lower, and 0.87% higher, respectively. Aggregated forms showed significant effects on cytotoxicity, as well as a decrease in the bioactivity (p < 0.05). The employment of the proposed method in conjunction with the optimized analytical technologies for the analysis of the intact and altered forms of the biotechnology-derived medicines, in the correlation studies, enabled the demonstration of the capability of each one of the methods and allowed for great improvements, thereby assuring their safe and effective use.

Real-Time Monitoring of a Botulinum Neurotoxin Using All-Carbon Nanotube-Based Field-Effect Transistor Devices

The possibility of exposure to botulinum neurotoxin (BoNT), a powerful and potential bioterrorism agent, is considered to be ever increasing. The current gold-standard assay, live-mouse lethality, exhibits high sensitivity but has limitations including long assay times, whereas other assays evince rapidity but lack factors such as real-time monitoring or portability. In this study, we aimed to devise a novel detection system that could detect BoNT at below-nanomolar concentrations in the form of a stretchable biosensor. We used a field-effect transistor with a p-type channel and electrodes, along with a channel comprising aligned carbon nanotube layers to detect the type E light chain of BoNT (BoNT/E-Lc). The detection of BoNT/E-Lc entailed observing the cleavage of a unique peptide and the specific bonding between BoNT/E-Lc and antibody BoNT/E-Lc (Anti-BoNT/E-Lc). The unique peptide was cleaved by 60 pM BoNT/E-Lc; notably, 52 fM BoNT/E-Lc was detected within 1 min in the device with the antibody in the bent state. These results demonstrated that an all-carbon nanotube-based device (all-CNT-based device) could be produced without a complicated fabrication process and could be used as a biosensor with high sensitivity, suggesting its potential development as a wearable BoNT biosensor.

A computational and experimental study to develop E-selectin targeted peptides for molecular imaging applications

Aim: E-selectin is overexpressed on angiogenic and inflamed endothelium. Molecules binding to E-selectin with high affinity and specificity enable its use as a molecular imaging biomarker. Material & methods: The interactions of four different peptides (i.e., Ac-P1 [Acetyl-IELLQAR-CONH2], H2N-P2 [H2N-DITWDQLWDLMK-CONH2], H2N-P3A5 [H2N-YRNWAGRW-CONH2], and Ac-P4 [Acetyl-YRNWDGRW-CONH2]) with E-selectin were analyzed by computational methodologies, surface plasmon resonance and in vitro using activated human umbilical vein endothelial cells. Poly(butyl cyanoacrylate) microbubbles were functionalized with the best candidates and evaluated as molecular ultrasound probes in cultured cells and explanted carotid arteries. Results: H2N-P3A5 and Ac-P4 peptides bound stronger to E-selectin than Ac-P1 and H2N-P2, but with lower specificity. H2N-P2 bound with higher specificity and affinity than Ac-P1. Conclusion: H2N-P2 is a good candidate for designing E-selectin-targeted molecular imaging agents.

Detecting Biothreat Agents: From Current Diagnostics to Developing Sensor Technologies

Although a fundamental understanding of the pathogenicity of most biothreat agents has been elucidated and available treatments have increased substantially over the past decades, they still represent a significant public health threat in this age of (bio)terrorism, indiscriminate warfare, pollution, climate change, unchecked population growth, and globalization. The key step to almost all prevention, protection, prophylaxis, post-exposure treatment, and mitigation of any bioagent is early detection. Here, we review available methods for detecting bioagents including pathogenic bacteria and viruses along with their toxins. An introduction placing this subject in the historical context of previous naturally occurring outbreaks and efforts to weaponize selected agents is first provided along with definitions and relevant considerations. An overview of the detection technologies that find use in this endeavor along with how they provide data or transduce signal within a sensing configuration follows. Current "gold" standards for biothreat detection/diagnostics along with a listing of relevant FDA approved in vitro diagnostic devices is then discussed to provide an overview of the current state of the art. Given the 2014 outbreak of Ebola virus in Western Africa and the recent 2016 spread of Zika virus in the Americas, discussion of what constitutes a public health emergency and how new in vitro diagnostic devices are authorized for emergency use in the U.S. are also included. The majority of the Review is then subdivided around the sensing of bacterial, viral, and toxin biothreats with each including an overview of the major agents in that class, a detailed cross-section of different sensing methods in development based on assay format or analytical technique, and some discussion of related microfluidic lab-on-a-chip/point-of-care devices. Finally, an outlook is given on how this field will develop from the perspective of the biosensing technology itself and the new emerging threats they may face.

Fluorescent Neuraminidase Assay Based on Supramolecular Dye Capture After Enzymatic Cleavage

A conceptually new type of enzymatic cleavage assay is reported that utilizes in situ supramolecular capture of the fluorescent product. A squaraine-derived substrate with large blocking groups at each end of its structure cannot be threaded by a tetralactam macrocycle until the blocking groups are removed by enzyme cleavage. A prototype design responds to viral neuraminidase, an indicator of influenza infection, and also measures susceptibility of the sample to neuraminidase inhibitor drugs. The substrate structure incorporates three key features: (a) a bis(4-amino-3-hydroxyphenyl)squaraine core with bright deep-red fluorescence and excellent photostability, (b) an N-methyl group at each end of the squaraine core that ensures fast macrocycle threading kinetics, and (c) sialic acid blocking groups that prevent macrocycle threading until they are removed by viral neuraminidase. The enzyme assay can be conducted in aqueous solution where dramatic colorimetric and fluorescence changes are easily observed by the naked eye. Alternatively, affinity capture beads coated with macrocycle can be used to immobilize the liberated squaraine and enable a range of heterogeneous analysis options. With further optimization, this new type of neuraminidase assay may be useful in a point of care clinic to rapidly diagnose influenza infection and also determine which of the approved antiviral inhibitor drugs is likely to be the most effective treatment for an individual patient. The assay design is generalizable and can be readily modified to monitor virtually any type of enzyme-catalyzed cleavage reaction.

An optical biosensor assay for rapid dual detection of Botulinum neurotoxins A and E

The enzymatic activity of the pathogenic botulinum neurotoxins type A and E (BoNT/A and E) leads to potentially lethal paralytic symptoms in humans and their prompt detection is of crucial importance. A chip assay based on Surface Plasmon Resonance monitoring of the cleavage products is a simple method that we have previously established to detect BoNT/A activity. We have now developed a similar format assay to measure BoNT/E activity. A monoclonal antibody specifically recognizing SNAP25 cleaved by BoNT/E was generated and used to measure the appearance of the neo-epitope following injection of BoNT/E over SNAP-25 immobilized on a chip. This assay detects BoNT/E activity at 1 LD₅₀/ml within minutes and linear dose-responses curves were obtained using a multiplexed biosensor. A threshold of 0.01 LD₅₀/ml was achieved after 5 h of cleavage. This assay is 10-fold more sensitive than the in vivo assay for direct detection of BoNT/E in serum samples. The SNAP25 chip assay is able to discriminate in an automated manner the presence of BoNT/E, BoNT/A or a combination of both toxins.

Autonomously Sensing Hydrogels for the Rapid and Selective Detection of Pathogenic Bacteria

The development of a versatile approach for the rapid and sensitive detection of relevant pathogenic bacteria and autonomous signaling of the detection events in reporter hydrogel film coatings is reported. Exploiting chitosan hydrogel films equipped with chromogenic or fluorogenic reporter moieties, the presence of the Gram-negative bacterium Pseudomonas aeruginosa and the Gram-positive bacterium Staphylococcus aureus is sensed within 1 h by detecting the characteristic enzymes α-glucosidase and elastase with limits of detection (LOD) <45 × 10(-9) M and <20 × 10(-9) M, respectively, for this observation time. The values for the LOD are two to three orders of magnitude smaller than the concentrations of the enzymes detected in the corresponding bacterial supernatants. The results show that the covalently conjugated reporter moieties are exclusively and efficiently reacted by the associated enzyme, allowing in principle for discrimination among different types of bacteria. Since high enzyme concentrations are a result of proliferating bacteria, e.g., in wounds or food, and since the selectivity of the reporting function is easily adapted to bacteria of choice, these reporter hydrogels comprise an interesting platform for the rapid detection of bacteria.

Rapid Detection of Escherichia coli via Enzymatically Triggered Reactions in Self-Reporting Chitosan Hydrogels

In this work, a self-reporting hydrogel for the rapid in situ detection of bacterial enzymes is reported. To implement the reporting function for the bacterium Escherichia coli into a film-based sensing format, chitosan hydrogel films on solid backing supports were equipped with a reporting function for the enzyme β-glucuronidase (β-GUS), which is secreted by >98% of all known E. coli strains. Covalent coupling of the fluorogenic substrate 4-methylumbelliferyl-β-D-glucuronide or the complementary chromogenic substrate 4-nitrophenyl-β-D-glucuronide via amide bond formation afforded an attachment that is stable for >24 h under physiological conditions. By contrast, in the presence of β-GUS, the reporter dyes were very rapidly cleaved and produced a signal for the presence of the enzyme, which was detectable by bare eye under appropriate illumination. Detailed investigations of the enzymatic reaction for both types of substrates in neat enzyme solution as well as in bacterial supernatant revealed the apparent reaction kinetics and allowed us to determine the concentration of β-GUS in the supernatant. Under optimized conditions, the 4-methylumbelliferyl-β-D-glucuronide-functionalized hydrogel reported the presence of β-GUS within 15 min with a limit of detection of <1 nM. Finally, the function of the generally applicable hydrogel-film-based sensing approach, which is compatible with polymer-film-based applications, including wound dressings and packaging materials, and is also amenable to address noncultivatable pathogenic bacteria by using appropriate fluorogenic or chromogenic substrates, was demonstrated by direct application with bacterial medium.

A chip-based assay for botulinum neurotoxin A activity in pharmaceutical preparations

The production of botulinum neurotoxin A (BoNT/A) for therapeutic and cosmetic applications requires precise determination of batch potency, and the enzymatic activity of BoNT/A light chain is a crucial index that can be measured in vitro. We previously established a SNAP-25 chip-based assay using surface plasmon resonance (SPR) that is more sensitive than the standard mouse bioassay for the quantification of BoNT/A activity. We have now adapted this procedure for pharmaceutical preparations. The optimized SPR assay allowed multiple measurements on a single chip, including the kinetics of substrate cleavage. The activity of five different batches of a pharmaceutical BoNT/A preparation was determined in a blind study by SPR and found to be in agreement with data from the in vivo mouse lethality assay. Biosensor detection of specific proteolytic products has the potential to accurately monitor the activity of pharmaceutical BoNT/A preparations, and a single chip can be used to assay more than 100 samples.

A graphene oxide based fluorescence resonance energy transfer (FRET) biosensor for ultrasensitive detection of botulinum neurotoxin A (BoNT/A) enzymatic activity

Botulinum neurotoxins (BoNTs) are among the most potent toxic bacterial proteins for humans, which make them potential agents for bioterrorism. Therefore, an ultrasensitive detection of BoNTs and their active states is in great need as field-deployable systems for anti-terrorism applications. We report the construction of a novel graphene oxide (GO)-peptide based fluorescence resonance energy transfer (FRET) biosensor for ultrasensitive detection of the BoNT serotype A light chain (BoNT-LcA) protease activity. A green fluorescence protein (GFP) modified SNAP-25 peptide substrate (SNAP-25-GFP) was optimally designed and synthesized with the centralized recognition/cleavage sites. This FRET platform was constructed by covalent immobilization of peptide substrate on GO with BSA passivation which have advantages of low non-specific adsorption and high stability in protein abundant solution. BoNT-LcA can specifically cleave SNAP-25-GFP substrate covalently immobilized on GO to release the fragment with GFP. Based on fluorescence signal recovery measurement, the target BoNT-LcA was detected sensitively and selectively with the linear detection range from 1fg/mL to 1pg/mL. The limit of detection (LOD) for BoNT-LcA is around 1fg/mL.

Antigen detection via the rate of ion current rectification change of the antibody-modified glass nanopore membrane

Ion current rectification (ICR), defined as an increase in ion conduction at a given polarity and a decrease in ion conduction for the same voltage at the opposite polarity, i.e., a deviation from a linear ohmic response, occurs in conical shaped pores due to the voltage dependent solution conductivity within the aperture. The degree to which the ionic current rectifies is a function of the size and surface charge of the nanopore, with smaller and more highly charged pores exhibiting greater degrees of rectification. The ICR phenomenon has previously been exploited for biosensing applications, where the level of ICR for a nanopore functionalized with an analyte-specific binding molecule (e.g., an antibody, biotin, etc.) changes upon binding its target analyte (e.g., an antigen, streptavidin, etc.) due to a resulting change in the size and/or charge of the aperture. While this type of detection measurement is typically qualitative, for the first time, we demonstrate that the rate at which the nanopore ICR response changes is dependent on the concentration of the target analyte introduced. Utilizing a glass nanopore membrane (GNM) internally coated with a monoclonal antibody specific to the cleaved form of synaptosomal-associated protein 25 (cSNAP-25), creating the antibody-modified glass nanopore membrane (AMGNM), we demonstrate a correlation between the rate of ICR change and the concentration of introduced cSNAP-25, over a range of 500 nM-100 μM. The methodology presented here significantly expands the applications of nanopore ICR biosensing measurements and demonstrates that these measurements can be quantitative in nature.

Enzyme-sensing chitosan hydrogels

We report on a chitosan hydrogel-based platform for the detection of enzymes, which is compatible with the implementation in infection-sensing wound dressings. Thin films of the established wound dressing biopolymer chitosan were functionalized with a fluorogenic substrate, which is released upon enzymatic degradation, resulting in a pronounced increase in fluorescence emission intensity. In this first model study, the fluorogenic substrate alanyl-alanyl-phenylalanine-7-amido-4-methylcoumarin (AAP-AMC) was covalently conjugated via amide bond formation to chitosan and was shown to facilitate the detection of the serine protease α-chymotrypsin. Systematic investigations established the dependence of hydrogel thickness and substrate loading on the hydrogel preparation conditions, as well as the dependence of the rate of the reaction on the initial enzyme concentration and the loading of AAP-AMC in the hydrogel. The initial release rate of the fluorophore 7-AMC was found to be linear with enzyme concentration and substrate loading and was independent of hydrogel thickness. Under optimized conditions the hydrogel reports the presence of α-chymotrypsin in <5 min with a limit of detection of ≤10 nM. This generic approach, which can be adapted to detect different kinds of enzymes by using appropriate fluorogenic or chromogenic substrates, is highly interesting for targeting the detection of specific pathogenic bacteria, e.g., in wound dressings.

Nanoporous membrane based impedance sensors to detect the enzymatic activity of botulinum neurotoxin A

Botulinum neurotoxins are among the most potent toxic bacterial proteins for humans and there is a great need to develop simple, rapid and sensitive methods for toxin detection and protease activity quantification in field deployment. In this paper, a nanoporous membrane based impedance sensor was developed to monitor the activity of the BoNT serotype A light chain protease (LcA). Synaptosomal-associated protein 25 (SNAP-25) was first immobilized inside nanopore walls via silane linkers. BoNT LcA was then injected over the nanoporous membrane substrate sensor and specifically cleaved SNAP-25. The cleavage activity could be monitored by measuring impedance signals across nanoporous membranes which represented the nanopore blockage degree. This initial device could achieve a 500 pM LcA detection limit within 25 minutes.