Primary cell culture for evaluation of botulinum neurotoxin antagonists

Critical analysis in the advancement of cell-based assays for botulinum neurotoxin

The study on botulinum neurotoxins (BoNTs) has rapidly evolved for their structure and functions as opposed to them being poisons or cures. Since their discoveries, the scientific community has come a long way in understanding BoNTs' structure and biological activity. Given its current application as a tool for understanding neurocellular activity and as a drug against over 800 neurological disorders, relevant and sensitive assays have become critical for biochemical, physiological, and pharmacological studies. The natural entry of the toxin being ingestion, it has also become important to examine its mechanism while crossing the epithelial cell barrier. Several techniques and methodologies have been developed, for its entry, pharmacokinetics, and biological activity for identification, and drug efficacy both in vivo and in vitro conditions. However, each of them presents its own challenges. The cell-based assay is a platform that exceeds the sensitivity of mouse bioassay while encompassing all the steps of intoxication including cell binding, transcytosis, endocytosis, translocation and proteolytic activity. In this article we review in detail both the neuronal and nonneuronal based cellular interaction of BoNT involving its transportation, and interaction with the targeted cells, and intracellular activities.

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.

A comparison of biological activity of commercially available purified native botulinum neurotoxin serotypes A1 to F1 in vitro, ex vivo, and in vivo

Botulinum neurotoxin (BoNT) is a major therapeutic agent. Of seven native BoNT serotypes (A to G), only A and B are currently used in the clinic. Here we compared the potency of commercially available purified native serotypes A1 to F1 across in vitro, ex vivo, and in vivo assays. BoNT potency in vitro was assessed in rat primary cells (target protein cleavage and neurotransmitter release assays) in supraspinal, spinal, and sensory systems. BoNT potency ex vivo was measured in the mouse phrenic nerve hemidiaphragm (PNHD) assay, measuring muscle contractility. In vivo, BoNT-induced muscle relaxation in mice and rats was assessed in the Digit Abduction Score (DAS) test, while effects on body weight (BW) gain were used to assess tolerability. In all assays, all BoNT serotypes were potent toxins, except serotype D1 in vivo which failed to produce significant muscle flaccidity in mice and rats. In rats, all serotypes were well-tolerated, whereas in mice, reductions in BW were detected at high doses. Serotype A1 was the most potent serotype across in vitro, ex vivo, and in vivo assays. The rank order of potency of the serotypes revealed differences among assays. For example, species-specificity was seen for serotype B1, and to a lesser extent for serotype C1. Serotypes F1 and C1, not currently in the clinic, showed preference for sensory over motor models and therefore could be considered for development in conditions involving the somatosensory system.

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

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

Botulinum Neurotoxins: Qualitative and Quantitative Analysis Using the Mouse Phrenic Nerve Hemidiaphragm Assay (MPN)

The historical method for the detection of botulinum neurotoxin (BoNT) is represented by the mouse bioassay (MBA) measuring the animal survival rate. Since the endpoint of the MBA is the death of the mice due to paralysis of the respiratory muscle, an ex vivo animal replacement method, called mouse phrenic nerve (MPN) assay, employs the isolated N. phrenicus-hemidiaphragm tissue. Here, BoNT causes a dose-dependent characteristic decrease of the contraction amplitude of the indirectly stimulated muscle. Within the EQuATox BoNT proficiency 13 test samples were analysed using the MPN assay by serial dilution to a bath concentration resulting in a paralysis time within the range of calibration curves generated with BoNT/A, B and E standards, respectively. For serotype identification the diluted samples were pre-incubated with polyclonal anti-BoNT/A, B or E antitoxin or a combination of each. All 13 samples were qualitatively correctly identified thereby delivering superior results compared to single in vitro methods like LFA, ELISA and LC-MS/MS. Having characterized the BoNT serotype, the final bath concentrations were calculated using the calibration curves and then multiplied by the respective dilution factor to obtain the sample concentration. Depending on the source of the BoNT standards used, the quantitation of ten BoNT/A containing samples delivered a mean z-score of 7 and of three BoNT/B or BoNT/E containing samples z-scores <2, respectively.

Recent developments in cell-based assays and stem cell technologies for botulinum neurotoxin research and drug discovery

Botulinum neurotoxins (BoNTs) are exceptionally potent inhibitors of neurotransmission, causing muscle paralysis and respiratory failure associated with the disease botulism. Currently, no drugs are available to counter intracellular BoNT poisoning. To develop effective medical treatments, cell-based assays provide a valuable system to identify novel inhibitors in a time- and cost-efficient manner. Consequently, cell-based systems including immortalized cells, primary neurons and stem cell-derived neurons have been established. Stem cell-derived neurons are highly sensitive to BoNT intoxication and represent an ideal model to study the biological effects of BoNTs. Robust immunoassays are used to quantify BoNT activity and play a central role during inhibitor screening. In this review, we examine recent progress in physiologically relevant cell-based assays and high-throughput screening approaches for the identification of both direct and indirect BoNT inhibitors.

Morphological and functional differentiation in BE(2)-M17 human neuroblastoma cells by treatment with Trans-retinoic acid

Background

Immortalized neuronal cell lines can be induced to differentiate into more mature neurons by adding specific compounds or growth factors to the culture medium. This property makes neuronal cell lines attractive as in vitro cell models to study neuronal functions and neurotoxicity. The clonal human neuroblastoma BE(2)-M17 cell line is known to differentiate into a more prominent neuronal cell type by treatment with trans-retinoic acid. However, there is a lack of information on the morphological and functional aspects of these differentiated cells.

Results

We studied the effects of trans-retinoic acid treatment on (a) some differentiation marker proteins, (b) types of voltage-gated calcium (Ca²⁺) channels and (c) Ca²⁺-dependent neurotransmitter ([³H] glycine) release in cultured BE(2)-M17 cells. Cells treated with 10 μM trans-retinoic acid (RA) for 72 hrs exhibited marked changes in morphology to include neurite extensions; presence of P/Q, N and T-type voltage-gated Ca²⁺ channels; and expression of neuron specific enolase (NSE), synaptosomal-associated protein 25 (SNAP-25), nicotinic acetylcholine receptor α7 (nAChR-α7) and other neuronal markers. Moreover, retinoic acid treated cells had a significant increase in evoked Ca²⁺-dependent neurotransmitter release capacity. In toxicity studies of the toxic gas, phosgene (CG), that differentiation of M17 cells with RA was required to see the changes in intracellular free Ca²⁺ concentrations following exposure to CG.

Conclusion

Taken together, retinoic acid treated cells had improved morphological features as well as neuronal characteristics and functions; thus, these retinoic acid differentiated BE(2)-M17 cells may serve as a better neuronal model to study neurobiology and/or neurotoxicity.

Progress in cell based assays for botulinum neurotoxin detection

Botulinum neurotoxins (BoNTs) are the most potent human toxins known and the causative agent of botulism, and are widely used as valuable pharmaceuticals. The BoNTs are modular proteins consisting of a heavy chain and a light chain linked by a disulfide bond. Intoxication of neuronal cells by BoNTs is a multi-step process including specific cell binding, endocytosis, conformational change in the endosome, translocation of the enzymatic light chain into the cells cytosol, and SNARE target cleavage. The quantitative and reliable potency determination of fully functional BoNTs produced as active pharmaceutical ingredient (API) requires an assay that considers all steps in the intoxication pathway. The in vivo mouse bioassay has for years been the 'gold standard' assay used for this purpose, but it requires the use of large numbers of mice and thus causes associated costs and ethical concerns. Cell-based assays are currently the only in vitro alternative that detect fully functional BoNTs in a single assay and have been utilized for years for research purposes. Within the last 5 years, several cell-based BoNT detection assays have been developed that are able to quantitatively determine BoNT potency with similar or greater sensitivity than the mouse bioassay. These assays now offer an alternative method for BoNT potency determination. Such quantitative and reliable BoNT potency determination is a crucial step in basic research, in the development of pharmaceutical BoNTs, and in the quantitative detection of neutralizing antibodies.

Time course and temperature dependence of the membrane translocation of tetanus and botulinum neurotoxins C and D in neurons

Tetanus and botulinum neurotoxins act inside nerve terminals and, therefore, they have to translocate across a membrane to reach their targets. This translocation is driven by a pH gradient, acidic on the cis side and neutral on the cytosol. Recently, a protocol to induce translocation from the plasma membrane was established. Here, we have used this approach to study the temperature dependence and time course of the entry of the L chain of tetanus neurotoxin and of botulinum neurotoxins type C and D across the plasma membrane of cerebellar granular neurons. The time course of translocation of the L chain varies for the three neurotoxins, but it remains in the range of minutes at 37 °C, whilst it takes much longer at 20 °C. BoNT/C does not enter neurons at 20 °C. Translocation also depends on the dimension of the pH gradient. These data are discussed with respect to the contribution of the membrane translocation step to the total time to paralysis and to the low toxicity of these neurotoxins in cold-blood vertebrates.

Botulinum neurotoxin serotype A specific cell-based potency assay to replace the mouse bioassay

Botulinum neurotoxin serotype A (BoNT/A), a potent therapeutic used to treat various disorders, inhibits vesicular neurotransmitter exocytosis by cleaving SNAP25. Development of cell-based potency assays (CBPAs) to assess the biological function of BoNT/A have been challenging because of its potency. CBPAs can evaluate the key steps of BoNT action: receptor binding, internalization-translocation, and catalytic activity; and therefore could replace the current mouse bioassay. Primary neurons possess appropriate sensitivity to develop potential replacement assays but those potency assays are difficult to perform and validate. This report describes a CBPA utilizing differentiated human neuroblastoma SiMa cells and a sandwich ELISA that measures BoNT/A-dependent intracellular increase of cleaved SNAP25. Assay sensitivity is similar to the mouse bioassay and measures neurotoxin biological activity in bulk drug substance and BOTOX® product (onabotulinumtoxinA). Validation of a version of this CBPA in a Quality Control laboratory has led to FDA, Health Canada, and European Union approval for potency testing of BOTOX®, BOTOX® Cosmetic, and Vistabel®. Moreover, we also developed and optimized a BoNT/A CBPA screening assay that can be used for the discovery of novel BoNT/A inhibitors to treat human disease.

Embryonic stem cell-derived motoneurons provide a highly sensitive cell culture model for botulinum neurotoxin studies, with implications for high-throughput drug discovery

Botulinum neurotoxins (BoNTs) inhibit cholinergic synaptic transmission by specifically cleaving proteins that are crucial for neurotransmitter exocytosis. Due to the lethality of these toxins, there are elevated concerns regarding their possible use as bioterrorism agents. Moreover, their widespread use for cosmetic purposes, and as medical treatments, has increased the potential risk of accidental overdosing and environmental exposure. Hence, there is an urgent need to develop novel modalities to counter BoNT intoxication. Mammalian motoneurons are the main target of BoNTs; however, due to the difficulty and poor efficiency of the procedures required to isolate the cells, they are not suitable for high-throughput drug screening assays. Here, we explored the suitability of embryonic stem (ES) cell-derived motoneurons as a renewable, reproducible, and physiologically relevant system for BoNT studies. We found that the sensitivity of ES-derived motoneurons to BoNT/A intoxication is comparable to that of primary mouse spinal motoneurons. Additionally, we demonstrated that several BoNT/A inhibitors protected SNAP-25, the BoNT/A substrate, in the ES-derived motoneuron system. Furthermore, this system is compatible with immunofluorescence-based high-throughput studies. These data suggest that ES-derived motoneurons provide a highly sensitive system that is amenable to large-scale screenings to rapidly identify and evaluate the biological efficacies of novel therapeutics.

Embryonic stem cell-derived neurons are a novel, highly sensitive tissue culture platform for botulinum research

There are no pharmacological treatments to rescue botulinum neurotoxin (BoNT)-mediated paralysis of neuromuscular signaling. In part, this failure can be attributed to the lack of a cell culture model system that is neuron-based, allowing detailed elucidation of the mechanisms underlying BoNT pathogenesis, yet still compatible with modern cellular and molecular approaches. We have developed a method to derive highly enriched, glutamatergic neurons from suspension-cultured murine embryonic stem (ES) cells. Hypothesizing that ES cell-derived neurons (ESNs) might comprise a novel platform to investigate the neurotoxicology of BoNTs, we evaluated the susceptibility of ESNs to BoNT/A and BoNT/E using molecular and functional assays. ESNs express neuron-specific proteins, develop synapses and release glutamate in a calcium-dependent manner under depolarizing conditions. They express the BoNT substrate SNARE proteins SNAP25, VAMP2 and syntaxin, and treatment with BoNT/A and BoNT/E holotoxin results in proteolysis of SNAP25 within 24 h with EC50s of 0.81 and 68.6 pM, respectively. Intoxication with BoNT/A results in the functional inhibition of potassium-induced, calcium-dependent glutamate release. ESNs remain viable and susceptible to intoxication for up to 90 days after plating, enabling longitudinal screens exploring toxin-specific mechanisms underlying persistence of synaptic blockade. The evidence suggests that derived neurons are a novel, biologically relevant model system that combines the verisimilitude of primary neurons with the genetic tractability and scalable expansion of a continuous cell line, and thus should significantly accelerate BoNT research and drug discovery while dramatically decreasing animal use.

An efficient drug delivery vehicle for botulism countermeasure

Background

Botulinum neurotoxin (BoNT) is the most potent poison known to mankind. Currently no antidote is available to rescue poisoned synapses. An effective medical countermeasure strategy would require developing a drug that could rescue poisoned neuromuscular synapses and include its efficient delivery specifically to poisoned presynaptic nerve terminals. Here we report a drug delivery strategy that could directly deliver toxin inhibitors into the intoxicated nerve terminal cytosol.

Results

A targeted delivery vehicle was developed for intracellular transport of emerging botulinum neurotoxin antagonists. The drug delivery vehicle consisted of the non-toxic recombinant heavy chain of botulinum neurotoxin-A coupled to a 10-kDa amino dextran via the heterobifunctional linker 3-(2-pyridylthio)-propionyl hydrazide. The heavy chain served to target botulinum neurotoxin-sensitive cells and promote internalization of the complex, while the dextran served as a platform to deliver model therapeutic molecules to the targeted neurons. Our results indicated that the drug delivery vehicle entry into neurons was via BoNT-A receptor mediated endocytosis. Once internalized into neurons, the drug carrier component separated from the drug delivery vehicle in a fashion similar to the separation of the BoNT-A light chain from the holotoxin. This drug delivery vehicle could be used to deliver BoNT-A antidotes into BoNT-A intoxicated cultured mouse spinal cord cells.

Conclusion

An effective BoNT-based drug delivery vehicle can be used to directly deliver toxin inhibitors into intoxicated nerve terminal cytosol. This approach can potentially be utilized for targeted drug delivery to treat other neuronal and neuromuscular disorders. This report also provides new knowledge of endocytosis and exocytosis as well as of BoNT trafficking.

Botulism diagnostics: from clinical symptoms to in vitro assays

Botulinum neurotoxin (BoNT), which cause the deadly neuroparalytic disease, botulism, is the most toxic substance known to man. BoNT can be used as potential bioterrorism agents, and therefore, pose great threat to national security and public health. Rapid and sensitive detection of BoNTs using molecular and biochemical techniques is an essential component in the diagnosis of botulism, and is yet to be achieved. The most sensitive and widely accepted assay method for BoNTs is mouse bioassay, which takes 4 days to complete. This clearly can not meet the need for clinical diagnosis of botulism, botulinum detection in field conditions, and screening of large scale samples. Consequently, the clinical diagnosis of botulism relies on the clinical symptom development, thus limiting the effectiveness of antitoxin treatment. In response to this critical need, many in vitro methods for BoNT detection are under development. This review is focused on recently developed in vitro detection methods for BoNTs, and emerging new technologies with potential for sensitive and rapid in vitro diagnostics for botulism.

Comparison of extracellular and intracellular potency of botulinum neurotoxins

Levels of botulinum neurotoxin (BoNT) proteolytic activity were compared using a cell-free assay and living neurons to measure extracellular and intracellular enzymatic activity. Within the cell-free reaction model, BoNT serotypes A and E (BoNT/A and BoNT/E, respectively) were reversibly inhibited by chelating Zn2+ with N,N,N',N'-tetrakis (2-pyridylmethyl) ethylenediamine (TPEN). BoNT/E required relatively long incubation with TPEN to achieve total inhibition, whereas BoNT/A was inhibited immediately upon mixing. When naïve Zn2+-containing BoNTs were applied to cultured neurons, the cellular action of each BoNT was rapidly inhibited by subsequent addition of TPEN, which is membrane permeable. Excess Zn2+ added to the culture medium several hours after poisoning fully restored intracellular toxin activity. Unlike TPEN, EDTA irreversibly inhibited both BoNT/A and -E within the cell-free in vitro reaction. Excess Zn2+ did not reactivate the EDTA-treated toxins. However, application of EDTA-treated BoNT/A or -E to cultured neurons demonstrated normal toxin action in terms of both blocking neurotransmission and SNAP-25 proteolysis. Different concentrations of EDTA produced toxin preparations with incrementally reduced in vitro proteolytic activities, which, when applied to living neurons showed undiminished cellular potency. This suggests that EDTA renders the BoNT proteolytic domain conformationally inactive when tested with the cell-free reaction, but this change is corrected during entry into neurons. The effect of EDTA is unrelated to Zn2+ because TPEN could be applied to living cells before or after poisoning to produce rapid and reversible inhibition of both BoNTs. Therefore, bound Zn2+ is not required for toxin entry into neurons, and removal of Zn2+ from cytosolic BoNTs does not irreversibly alter toxin structure or function. We conclude that EDTA directly alters both BoNTs in a manner that is independent of Zn2+.

Growth factor dependent cholinergic function and survival in primary mouse spinal cord cultures

In primary embryonic spinal cord cultures, synaptic transmission can be conveniently studied by monitoring radiolabeled neurotransmitter release or by recording of electrophysiological responses. However, while the mature spinal cord contains an appreciable number of cholinergic motoneurons, cultures of embryonic spinal cord have a paucity of these neurons and release little or no acetylcholine upon stimulation. To determine whether the proportion of cholinergic neurons in primary mouse spinal cord cultures can be augmented, the effects of several classes of growth factors were examined on depolarization- and Ca(2+)-evoked release of choline/acetylcholine (Ch/ACh). In the absence of growth factors, little or no evoked release of radiolabeled Ch/ACh could be demonstrated. Media supplemented with brain-derived neurotrophic factor (BDNF), ciliary neurotrophic factor (CNTF) or basic fibroblast growth factor (bFGF) were examined for their ability to preserve the population of neurons in culture. CNTF was found to increase the number of surviving neurons and to enhance the release of radiolabeled Ch/ACh; the other factors were without effect. The action of CNTF was transient, and the neuronal population decreased to levels observed in cultures lacking growth factor after 20 days in vitro. The correlation between enhanced neuron survival and increased Ch/ACh release suggests that CNTF protected cholinergic neurons, albeit transiently, from cell death.

Recovery from botulinum neurotoxin poisoning in vivo

Botulinum neurotoxins cause the disease botulism, which is characterized by prolonged muscle paralysis. In contrast, injections of low doses of purified botulinum neurotoxins do not cause systemic illness but produce localized muscle paralysis that is beneficial for treating several human medical disorders involving uncontrollable muscle contraction. Optimizing the therapeutic efficacy while diminishing adverse reactions requires precise knowledge of toxin potency as well as a clear understanding of how each toxin causes disease. A novel in vivo mouse assay has been used to correlate toxin dosage with the duration of muscle paralysis. Voluntary running activity performed by mice was proportional to the amount of toxin injected into the hind limbs and the subsequent rate of recovery over the ensuing days or weeks was a function of botulinum neurotoxin serotype A or B concentration. Botulinum neurotoxin A produced longer paralysis than botulinum neurotoxin B consistent with human observations. A third serotype, botulinum neurotoxin E, had the shortest duration of action, but unlike the other two toxins, dosage did not influence recovery time. Botulinum neurotoxin A recovery appeared biphasic with the initial phase about two-fold faster than the final phase. Over four weeks, muscle activity had gradually improved following the highest botulinum neurotoxin A dose, reaching about half of the normal running activity. Lower botulinum neurotoxin A doses led to incrementally faster and complete recovery. Persistence of maximum paralysis was exponentially related to botulinum neurotoxin A dosage, with a doubling of the paralysis time occurring with every 25% increase of the toxin concentration. In contrast, the rate of recovery from botulinum neurotoxin B was monophasic relative to toxin dosage and the duration of maximum paralysis was linear relative to dosage. Combinations of botulinum neurotoxin A and B and botulinum neurotoxin A and E were tested and shown to exacerbate paralysis compared with individually administered serotypes.