VAMP/synaptobrevin cleavage by tetanus and botulinum neurotoxins is strongly enhanced by acidic liposomes

TRIM67 regulates exocytic mode and neuronal morphogenesis via SNAP47

Neuronal morphogenesis involves dramatic plasma membrane expansion, fueled by soluble N-ethylmaleimide-sensitive factor attachment protein eceptors (SNARE)-mediated exocytosis. Distinct fusion modes described at synapses include full-vesicle fusion (FVF) and kiss-and-run fusion (KNR). During FVF, lumenal cargo is secreted and vesicle membrane incorporates into the plasma membrane. During KNR, a transient fusion pore secretes cargo but closes without membrane addition. In contrast, fusion modes are not described in developing neurons. Here, we resolve individual exocytic events in developing murine cortical neurons and use classification tools to identify four distinguishable fusion modes: two FVF-like modes that insert membrane material and two KNR-like modes that do not. Discrete fluorescence profiles suggest distinct behavior of the fusion pore. Simulations and experiments agree that FVF-like exocytosis provides sufficient membrane material for morphogenesis. We find the E3 ubiquitin ligase TRIM67 promotes FVF-like exocytosis in part by limiting incorporation of the Qb/Qc SNARE SNAP47 into SNARE complexes and, thus, SNAP47 involvement in exocytosis.

Urbina et al. identify four exocytic modes in developing neurons: KNRd, KNRi, FVFd, FVFi. Simulations and experiments suggest that FVFi and FVFd provide material for plasma membrane expansion. Deletion of Trim67 decreases FVFi and FVFd while reducing surface area. SNAP47 incorporation into SNARE complexes alters fusion pore behavior, increasing KNRd.

Electrophysiological Characterization of the Antarease Metalloprotease from Tityus serrulatus Venom

Scorpions are among the oldest venomous living organisms and the family Buthidae is the largest and most medically relevant one. Scorpion venoms include many toxic peptides, but recently, a metalloprotease from Tityus serrulatus called antarease was reported to be capable of cleaving VAMP2, a protein involved in the neuroparalytic syndromes of tetanus and botulism. We have produced antarease and an inactive metalloprotease mutant in a recombinant form and analyzed their enzymatic activity on recombinant VAMP2 in vitro and on mammalian and insect neuromuscular junction. The purified recombinant antarease paralyzed the neuromuscular junctions of mice and of Drosophila melanogaster whilst the mutant was inactive. We were unable to demonstrate any cleavage of VAMP2 under conditions which leads to VAMP proteolysis by botulinum neurotoxin type B. Antarease caused a reduced release probability, mainly due to defects upstream of the synaptic vesicles fusion process. Paired pulse experiments indicate that antarease might proteolytically inactivate a voltage-gated calcium channel.

Past, Present and Future of Chemodenervation with Botulinum Toxin in the Treatment of Overactive Bladder

PURPOSE

We systematically reviewed preclinical and clinical studies on bladder chemodenervation with onabotulinumtoxin A to highlight current limitations and future drug delivery approaches.

MATERIALS AND METHODS

We identified peer reviewed basic and clinical research studies of onabotulinumtoxin A in the treatment of neurogenic bladder and refractory idiopathic overactive bladder published between March 2000 and March 2016. Paired investigators independently screened 125 English language articles to identify controlled studies on onabotulinumtoxin A administration in the MEDLINE® database and abstracts presented at annual American Urological Association meetings. The review yielded an evidence base of more than 50 articles relevant to the approach of injection-free onabotulinumtoxin A chemodenervation.

RESULTS

The efficacy and safety of intradetrusor injection of onabotulinumtoxin A for the treatment of overactive bladder are sensitive to injection volume and depth, and this issue has motivated researchers to study injection-free modes of drug delivery into the bladder. Urothelial denudation with protamine sulfate or dimethyl sulfoxide, liposome encapsulated onabotulinumtoxin A and other physical approaches are being studied to increase toxin permeability and avoid intradetrusor injections. Liposome encapsulated onabotulinumtoxin A enhances toxin activity while reducing its toxin degradation. The safety and efficacy of liposome encapsulated onabotulinumtoxin A were tested in a multicenter, placebo controlled study. Although this treatment successfully reduced urinary frequency and urgency, it did not significantly reduce urgency urinary incontinence episodes.

CONCLUSIONS

Intradetrusor injection of onabotulinumtoxin A is a safe and effective treatment as reported in several large multicenter, randomized controlled trials. Injection of the toxin into the bladder wall impairs afferent and efferent nerves, but injection-free drug delivery approaches only impair the bladder afferent nerves. Further studies are needed to develop better drug delivery platforms that overcome the drawbacks of intradetrusor injection, increase patient acceptance and reduce treatment costs.

In vitro potency determination of botulinum neurotoxin B based on its receptor-binding and proteolytic characteristics

Botulinum neurotoxins (BoNTs) are the most potent toxins known. However, the paralytic effect caused by BoNT serotypes A and B is taken advantage of to treat different forms of dystonia and in cosmetic procedures. Due to the increasing areas of application, the demand for BoNTs A and B is rising steadily. Because of the high toxicity, it is mandatory to precisely determine the potency of every produced BoNT batch, which is usually accomplished by performing toxicity testing (LD50 test) in mice. Here we describe an alternative in vitro assay for the potency determination of the BoNT serotype B. In this assay, the toxin is first bound to its specific receptor molecules. After the proteolytic subunit of the toxin has been released and activated by chemical reduction, it is exposed to synaptobrevin, its substrate protein. Finally the proteolytic cleavage is quantified by an antibody-mediated detection of the neoepitope, reaching a detection limit below 0.1mouseLD50/ml. Thus, the assay, named BoNT/B binding and cleavage assay (BoNT/B BINACLE), takes into account the binding as well as the protease function of the toxin, thereby measuring its biological activity.

Investigational drugs for bladder pain syndrome (BPS) / interstitial cystitis (IC)

INTRODUCTION

Bladder pain syndrome (BPS)/interstitial cystitis (IC) is associated with sensory lower urinary tract symptoms. Unfortunately, many of the existing oral treatments are ineffective in most patients of BPS/IC, which is the motivation for developing new drugs and therapeutic approaches. This review covers the latest drugs that have been investigated in BPS/IC patients. Intravesical treatments offer the opportunity to directly target the painful bladder with less systemic side effects.

AREAS COVERED

In this review, the authors analyze the existing literature supporting the treatment of BPS/IC with conventional drugs including heparin, hyaluronic acid, chondroitin sulfate, and dimethylsulfoxide (DMSO). Furthermore, investigational drugs such as tanezumab and adalimumab, capable of sequestering nerve growth factor (NGF), and Tumor necrosis factor-α (TNF- α) are discussed. Investigational treatments such as liposomes, botulinum toxin (BTX), liposomal BTX, PD-0299685 (a Ca(2+) channel ɑ2δ ligand), continuous intravesical lidocaine, and AQX-1125 (a novel SHIP1 activating compound) are also covered.

EXPERT OPINION

New investigational drugs offer promising improvements in clinical outcomes for BPS/IC patients; however, BPS/IC is a chronic pain disorder that is very vulnerable to a strong placebo effect. In addition, BPS/IC is a heterogeneous disorder that can be classified into several phenotypes. Since different phenotypes of BPS/IC respond differently to systemic and intravesical treatments, the authors believe that new drugs developed for BPS/IC are more likely to meet their predetermined clinical endpoints if the inclusion/exclusion criterion is tailored to specific phenotype of BPS/IC patients.

Advances in intravesical therapy for urinary tract disorders

INTRODUCTION

Intravesical therapy is a valuable option in the clinical management of urinary tract disorders such as interstitial cystitis/ painful bladder syndrome (IC/PBS) and refractory overactive bladder. This review will cover the latest advances in this field using polymer and liposomes as delivery platform for drugs, protein and nucleic acids.

AREAS COVERED

This review summarizes the significance of intravesical therapy for lower urinary tract disorders. The recent advancement of liposomes as a drug delivery platform for botulinum toxin, tacrolimus and small interfering RNA is discussed. The importance of polymers forming indwelling devices and hydrogels are also discussed, where all preparations improved efficacy parameters in rodent models. Clinical experience of treating IC/PBS with indwelling devices and liposomes are summarized and preclinical evidence about the downregulation of target gene expression in rodent bladder with liposomes complexed with siRNA is also reviewed.

EXPERT OPINION

There have been several advances in the field of intravesical therapy for improving clinical outcomes. One of the most promising research avenues is the repurposing of drugs, given previously by other routes of administration, such as tacrolimus. Intravesical therapy also opens up novel therapeutic targets with improved efficacy and safety for underactive bladder.

The blockade of the neurotransmitter release apparatus by botulinum neurotoxins

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

Intravesical liposome and antisense treatment for detrusor overactivity and interstitial cystitis/painful bladder syndrome

Purpose. The following review focuses on the recent advancements in intravesical drug delivery, which brings added benefit to the therapy of detrusor overactivity and interstitial cystitis/painful bladder syndrome (IC/PBS). Results. Intravesical route is a preferred route of administration for restricting the action of extremely potent drugs like DMSO for patients of interstitial cystitis/painful bladder syndrome (IC/PBS) and botulinum toxin for detrusor overactivity. Patients who are either refractory to oral treatment or need to mitigate the adverse effects encountered with conventional routes of administration also chose this route. Its usefulness in some cases can be limited by vehicle (carrier) toxicity or short duration of action. Efforts have been underway to overcome these limitations by developing liposome platform for intravesical delivery of biotechnological products including antisense oligonucleotides. Conclusions. Adoption of forward-thinking approaches can achieve advancements in drug delivery systems targeted to future improvement in pharmacotherapy of bladder diseases. Latest developments in the field of nanotechnology can bring this mode of therapy from second line of treatment for refractory cases to the forefront of disease management.

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

Botulinum neurotoxin A (BoNT/A) induces muscle paralysis by enzymatically cleaving the presynaptic SNARE protein SNAP-25, which results in lasting inhibition of acetylcholine release at the neuromuscular junction. A rapid and sensitive in vitro assay for BoNT/A is required to replace the mouse lethality assay (LD50) in current use. We have developed a fully automated sensor to assay the endoprotease activity of BoNT/A. We produced monoclonal antibodies (mAbs) that recognize SNAP-25 neo-epitopes specifically generated by BoNT/A action. Recombinant SNAP-25 was coupled to the sensor surface of a surface plasmon resonance (SPR) system and samples containing BoNT/A were injected over the substrate sensor. Online substrate cleavage was monitored by measuring binding of mAb10F12 to a SNAP-25 neo-epitope. The SNAP-25-chip assay was toxin serotype-specific and detected 55 fM BoNT/A (1 LD50/ml) in 5 min and 0.4 fM (0.01 LD50/ml) in 5h. Time-course and dose-response curves were linear, yielding a limit of quantification of 0.03 LD50/ml. This label-free method is 100 times more sensitive than the mouse assay, potentially providing rapid read-out of small amounts of toxin for environmental surveillance and the quality control of pharmaceutical preparations.

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.

Bacterial protein toxins and lipids: role in toxin targeting and activity

All bacterial toxins, which globally are hydrophilic proteins, interact first with their target cells by recognizing a surface receptor, which is either a lipid or a lipid derivative, or another compound but in a lipid environment. Intracellular active toxins follow various trafficking pathways, the sorting of which is greatly dependent on the nature of the receptor, notably lipidic receptor or receptor embedded into a distinct environment such as lipid microdomains. Numerous other toxins act locally on cell membrane. Indeed, phospholipase activity is a common mechanism shared by several membrane-damaging toxins. In addition, many toxins active intracellularly or on cell membrane modulate host cell phospholipid pathways. Unusually, a few bacterial toxins require a lipid post-translational modification to be active. Thereby, lipids are obligate partners of bacterial toxins.

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.

Sphingosine facilitates SNARE complex assembly and activates synaptic vesicle exocytosis

Synaptic vesicles loaded with neurotransmitters fuse with the plasma membrane to release their content into the extracellular space, thereby allowing neuronal communication. The membrane fusion process is mediated by a conserved set of SNARE proteins: vesicular synaptobrevin and plasma membrane syntaxin and SNAP-25. Recent data suggest that the fusion process may be subject to regulation by local lipid metabolism. Here, we have performed a screen of lipid compounds to identify positive regulators of vesicular synaptobrevin. We show that sphingosine, a releasable backbone of sphingolipids, activates synaptobrevin in synaptic vesicles to form the SNARE complex implicated in membrane fusion. Consistent with the role of synaptobrevin in vesicle fusion, sphingosine upregulated exocytosis in isolated nerve terminals, neuromuscular junctions, neuroendocrine cells and hippocampal neurons, but not in neurons obtained from synaptobrevin-2 knockout mice. Further mechanistic insights suggest that sphingosine acts on the synaptobrevin/phospholipid interface, defining a novel function for this important lipid regulator.

A yeast assay probes the interaction between botulinum neurotoxin serotype B and its SNARE substrate

The seven functionally distinct serotypes (A-G) of botulinum neurotoxin (BoNT) are dichains consisting of light chain (LC) with zinc-dependent endoprotease activity connected by one disulfide bond to heavy chain with neuronal-cell translocation and receptor-binding domains. LC-mediated proteolysis of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins and consequent inhibition of synaptic vesicle fusion to the presynaptic membrane of human motor neurons are responsible for flaccid paralysis associated with botulism. LC endoproteolysis is complex, requiring highly extended SNARE sequences at the surface of intracellular membranes and prompting our development of a genetically amenable assay to monitor the interaction between BoNT/LC and its SNARE substrate. Using BoNT serotype B as a model, the assay employs a chimeric SNARE protein where a portion of neuronal synaptobrevin (Sb) is fused to Snc2p, a Sb ortholog required for protein secretion from yeast cells. Regulated expression of serotype B-LC in yeast leads to cleavage of the chimera and a conditional growth defect. To assess utility of this assay for monitoring SNARE protein cleavage, we growth-selected chimeric SNARE mutations that inhibited proteolysis. When these mutations were introduced into Sb and examined for cleavage, substrate residues located near and distal to the cleavage site were important, including residues positioned near the Sb transmembrane domain, an unexplored aspect of BoNT cell intoxication. Additional mutations were positioned in a nine-residue SNARE motif, supporting a previously assigned role for this motif in LC recognition and providing proof of principle for the application of yeast-based technology to study intracellular BoNT/LC endoproteases.

Synaptic vesicle chips to assay botulinum neurotoxins

BoNTs (botulinum neurotoxins), considered to be the most toxic of all biological substances, inhibit neurotransmission through proteolytic cleavage of SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins [VAMP (vesicle-associated membrane protein, or synaptobrevin), SNAP-25 (25 kDa synaptosome-associated protein) or syntaxin]. Expansion in the use of BoNTs as therapeutic and cosmetic agents, and the potential threat they constitute as biological weapons, underlines the need for rapid and sensitive in vitro assays. Here, we present new automatized bioassays to detect VAMP cleavage by BoNT/B and F. Western blotting and SPR (surface plasmon resonance) methods revealed that BoNT/B and F totally cleave their substrate on immunoisolated SVs (synaptic vesicles). Real-time monitoring of the immunocapture of native SVs from crude lysates on SPR sensor chips enabled the detection of picogram amounts of different SV proteins. Pre-incubation of a membrane fraction containing SVs with BoNT specifically inhibited capture by anti-VAMP antibodies, and amounts as low as 0.1 pg of BoNT/B were detected. This automated SPR assay is approx. 200 times more sensitive, and 25 times more rapid, than the in vivo BoNT/B test currently used. Moreover, the method can be performed using a few thousand cultured neurons and constitutes a new screening assay for inhibitors. Our data indicate that native VAMP is an optimal substrate for in vitro BoNT assays that can be monitored by SPR.

A Phosphatidylserine-binding Site in the Cytosolic Fragment of Clostridium sordellii Lethal Toxin Facilitates Glucosylation of Membrane-bound Rac and Is Required for Cytotoxicity

Large clostridial toxins glucosylate some small G proteins on a threonine residue, thereby preventing their interactions with effector molecules and regulators. We show that the glucosyltransferase domain of lethal toxin from Clostridium sordellii (LT(cyt); amino acids 1-546), which is released into the cytosol during cell infection, binds preferentially to liposomes containing phosphatidylserine as compared with other anionic lipids. The binding of LT(cyt) to phosphatidylserine increases by two orders of magnitude the rate of glucosylation of liposome-bound geranyl-geranylated Rac-GDP. Limited proteolysis and deletion studies show that the binding site for phosphatidylserine lies within the first 18 N-terminal residues of LT(cyt). Deletion of these residues abolishes the effect of phosphatidylserine on the activity of LT(cyt) on liposome-bound geranyl-geranylated Rac-GDP and prevents the morphological effects induced by LT(cyt) microinjection into various cells, but it does not affect the intrinsic activity of LT(cyt) on non-geranyl-geranylated Rac-GDP in solution. We conclude that the avidity of LT(cyt) for phosphatidylserine facilitates its targeting to the cytosolic leaflet of cell membranes and, notably, the plasma membrane, where this anionic lipid is abundant and where several targets of lethal toxin reside.

Using fluorescent sensors to detect botulinum neurotoxin activity in vitro and in living cells

Botulinum neurotoxins (BoNTs) act as zinc-dependent endopeptidases that cleave proteins required for neurotransmitter release. To detect toxin activity, fragments of the toxin substrate proteins, synaptobrevin (Syb) or synaptosome-associated protein of 25 kDa (SNAP-25), were used to link cyan fluorescent protein (CFP) to yellow fluorescent protein (YFP). Cleavage of these fusion proteins by BoNTs abolished fluorescence resonance energy transfer between the CFP and YFP, providing a sensitive means to detect toxin activity in real-time in vitro. Furthermore, using full-length SNAP-25 and Syb as the linkers, we report two fluorescent biosensors that can detect toxin activity within living cells. Cleavage of the SNAP-25 fusion protein abolished fluorescence resonance energy transfer between CFP and YFP, and cleavage of Syb resulted in spatial redistribution of YFP fluorescence in cells. This approach provides a means to carry out cell-based screening of toxin inhibitors and to study toxin activity in situ. By using these biosensors, we found that the subcellular localizations of SNAP-25 and Syb are critical for efficient cleavage by BoNT/A and B, respectively.

A common mechanism for the regulation of vesicular SNAREs on phospholipid membranes

The SNARE (soluble N -ethylmaleimide-sensitive fusion protein attachment protein receptor) family of proteins is essential for membrane fusion in intracellular traffic in eukaryotic organisms. v-SNAREs (vesicular SNAREs) must engage target SNAREs in the opposing membrane to form the fusogenic SNARE complex. Temporal and spatial control of membrane fusion is important for many aspects of cell physiology and may involve the regulation of the SNAREs resident on intracellular membranes. Here we show that the v-SNARE synaptobrevin 2, also known as VAMP (vesicle-associated membrane protein) 2, is restricted from forming the SNARE complex in chromaffin granules from adrenal medullae to the same degree as in brain-purified synaptic vesicles. Our analysis indicates that the previously reported synaptophysin-synaptobrevin interaction is not likely to be involved in regulation of the v-SNARE. Indeed, the restriction can be reproduced for two distinct v-SNARE homologues, synaptobrevin 2 and cellubrevin/VAMP3, by reconstituting them in pure liposomal membranes. Overall, our data uncover a common mechanism for the control of SNARE engagement where intact phospholipid membranes rather than proteins down-regulate vesicular SNAREs in different cellular organelles.