Tetanus and Botulism Neurotoxins

Botulinum Neurotoxin Induces Neurotoxic Microglia Mediated by Exogenous Inflammatory Responses

Botulinum neurotoxin serotype A (BoNT/A) is widely used in therapeutics and cosmetics. The effects of multi-dosed BoNT/A treatment are well documented on the peripheral nervous system (PNS), but much less is known on the central nervous system (CNS). Here, the mechanism of multi-dosed BoNT/A leading to CNS neurodegeneration is explored by using the 3D human neuron-glia model. BoNT/A treatment reduces acetylcholine, triggers astrocytic transforming growth factor beta, and upregulates C1q, C3, and C5 expression, inducing microglial proinflammation. The disintegration of the neuronal microtubules is escorted by microglial nitric oxide, interleukin 1β, tumor necrosis factor α, and interleukin 8. The microglial proinflammation eventually causes synaptic impairment, phosphorylated tau (pTau) aggregation, and the loss of the BoNT/A-treated neurons. Taking a more holistic approach, the model will allow to assess therapeutics for the CNS neurodegeneration under the prolonged use of BoNT/A.

Aggregates Sealed by Ions

The chapter draws a line connecting some recent results where the role of ions is found essential in sealing more or less pre-organized assemblies of macromolecules. We draw some dots along the line that starts from the effect of the ionic atmosphere and ends with the chemical bonds formed by multivalent ions acting as bridges between macromolecules. Many of these dots involve structurally disordered peptides and disordered regions of proteins. A broad perspective of the role of multivalent ions in assisting the assembly process, shifting population in polymorphic states, and sealing protein aggregates, is suggested.

Solubility of the catalytic domains of Botulinum neurotoxin serotype E subtypes

The Clostridium botulinum neurotoxins (BoNTs) are the most potent protein toxins known to humans. There are seven serotypes of the BoNTs (A-G), among which serotypes A, B, E and F are known to cause natural human intoxication. To date, eleven subtypes of LC/E, termed E1∼E11, have been identified. The LCs of BoNT/E were insoluble, prohibiting studies towards understanding the mechanisms of toxin action and substrate recognition. In this work, the molecular basis of insolubility of the recombinant LCs of two representative subtypes of BoNT/E, E1(Beluga) and E3 (Alaska), was determined. Hydrophobicity profile and structural modeling predicted a C-terminal candidate region responsible for the insolubility of LC/Es. Deletion of C-terminal 19 residues of LC/E(1-400) resulted in enhanced solubility, from 2 to ∼50% for LC/EAlaska and from 16 to ∼95% for LC/EBeluga. In addition, resides 230-236 were found to contribute to a different solubility level of LC/EAlaska when compared to LC/EBeluga. Substituting residues (230)TCI(232) in LC/EAlaska to the corresponding residues of (230)KYT(232) in LC/EBeluga enhanced the solubility of LC/EAlaska to a level approaching that of LC/EBeluga. Among these LC/Es and their derivatives, LC/EBeluga 1-400 was the most soluble and stable protein. Each LC/E derivative possessed similar catalytic activity, suggesting that the C-terminal region of LC/Es contributed to protein solubility, but not catalytic activity. In conclusion, this study generated a soluble and stable recombinant LC/E and provided insight into the structural components that govern the solubility and stability of the LCs of other BoNT serotypes and Tetanus toxin.

Development of Human-Like scFv-Fc Neutralizing Botulinum Neurotoxin E

Background

Botulinum neurotoxins (BoNTs) are considered to be the most toxic substances known on earth and are responsible for human botulism, a life-threatening disease characterized by flaccid muscle paralysis that occurs naturally by food-poisoning or colonization of the gastrointestinal tract by BoNT-producing clostridia. BoNTs have been classified as category A agent by the Centers of Disease Control and Prevention (CDC) and are listed among the six agents with the highest risk to be used as bioweapons. Neutralizing antibodies are required for the development of effective anti-botulism therapies to deal with the potential risk of exposure.

Results

In this study, a macaque (Macaca fascicularis) was immunized with recombinant light chain of BoNT/E3 and an immune phage display library was constructed. After a multi-step panning, several antibody fragments (scFv, single chain fragment variable) with nanomolar affinities were isolated, that inhibited the endopeptidase activity of pure BoNT/E3 in vitro by targeting its light chain. Furthermore, three scFv were confirmed to neutralize BoNT/E3 induced paralysis in an ex vivo mouse phrenic nerve-hemidiaphragm assay. The most effective neutralization (20LD₅₀/mL, BoNT/E3) was observed with scFv ELC18, with a minimum neutralizing concentration at 0.3 nM. Furthermore, ELC18 was highly effective in vivo when administered as an scFv-Fc construct. Complete protection of 1LD₅₀ BoNT/E3 was observed with 1.6 ng/dose in the mouse flaccid paralysis assay.

Conclusion

These scFv-Fcs antibodies are the first recombinant antibodies neutralizing BoNT/E by targeting its light chain. The human-like nature of the isolated antibodies is predicting a good tolerance for further clinical development.

Non-proteolytic functions of microbial proteases increase pathological complexity

Proteases are enzymes that catalyse hydrolysis of peptide bonds thereby controlling the shape, size, function, composition, turnover and degradation of other proteins. In microbes, proteases are often identified as important virulence factors and as such have been targets for novel drug design. It is emerging that some proteases possess additional non‐proteolytic functions that play important roles in host epithelia adhesion, tissue invasion and in modulating immune responses. These additional “moonlighting” functions have the potential to obfuscate data interpretation and have implications for therapeutic design. Moonlighting enzymes comprise a subcategory of multifunctional proteins that possess at least two distinct biological functions on a single polypeptide chain. Presently, identifying moonlighting proteins relies heavily on serendipitous empirical data with clues arising from proteins lacking signal peptides that are localised to the cell surface. Here, we describe examples of microbial proteases with additional non‐proteolytic functions, including streptococcal pyrogenic exotoxin B, PepO and C5a peptidases, mycoplasmal aminopeptidases, mycobacterial chaperones and viral papain‐like proteases. We explore how these non‐proteolytic functions contribute to host cell adhesion, modulate the coagulation pathway, assist in non‐covalent folding of proteins, participate in cell signalling, and increase substrate repertoire. We conclude by describing how proteomics has aided in moonlighting protein discovery, focusing attention on potential moonlighters in microbial exoproteomes.

Architecture and function of metallopeptidase catalytic domains

The cleavage of peptide bonds by metallopeptidases (MPs) is essential for life. These ubiquitous enzymes participate in all major physiological processes, and so their deregulation leads to diseases ranging from cancer and metastasis, inflammation, and microbial infection to neurological insults and cardiovascular disorders. MPs cleave their substrates without a covalent intermediate in a single-step reaction involving a solvent molecule, a general base/acid, and a mono- or dinuclear catalytic metal site. Most monometallic MPs comprise a short metal-binding motif (HEXXH), which includes two metal-binding histidines and a general base/acid glutamate, and they are grouped into the zincin tribe of MPs. The latter divides mainly into the gluzincin and metzincin clans. Metzincins consist of globular ∼ 130-270-residue catalytic domains, which are usually preceded by N-terminal pro-segments, typically required for folding and latency maintenance. The catalytic domains are often followed by C-terminal domains for substrate recognition and other protein-protein interactions, anchoring to membranes, oligomerization, and compartmentalization. Metzincin catalytic domains consist of a structurally conserved N-terminal subdomain spanning a five-stranded β-sheet, a backing helix, and an active-site helix. The latter contains most of the metal-binding motif, which is here characteristically extended to HEXXHXXGXX(H,D). Downstream C-terminal subdomains are generally shorter, differ more among metzincins, and mainly share a conserved loop--the Met-turn--and a C-terminal helix. The accumulated structural data from more than 300 deposited structures of the 12 currently characterized metzincin families reviewed here provide detailed knowledge of the molecular features of their catalytic domains, help in our understanding of their working mechanisms, and form the basis for the design of novel drugs.

Simultaneous and sensitive detection of six serotypes of botulinum neurotoxin using enzyme-linked immunosorbent assay-based protein antibody microarrays

Botulinum neurotoxins (BoNTs), produced by Clostridium botulinum, are a group of seven (A-G) immunologically distinct proteins and cause the paralytic disease botulism. These toxins are the most poisonous substances known to humans and are potential bioweapon agents. Therefore, it is necessary to develop highly sensitive assays for the detection of BoNTs in both clinical and environmental samples. In the current study, we have developed an enzyme-linked immunosorbent assay (ELISA)-based protein antibody microarray for the sensitive and simultaneous detection of BoNT serotypes A, B, C, D, E, and F. With engineered high-affinity antibodies, the BoNT assays have sensitivities in buffer ranging from 1.3fM (0.2pg/ml) to 14.7fM (2.2pg/ml). Using clinical and food matrices (serum and milk), the microarray is capable of detecting BoNT serotypes A to F to similar levels as in standard buffer. Cross-reactivity between assays for individual serotype was also analyzed. These simultaneous, rapid, and sensitive assays have the potential to measure botulinum toxins in a high-throughput manner in complex clinical, food, and environmental samples.

Detection of botulinum neurotoxin serotype A, B, and F proteolytic activity in complex matrices with picomolar to femtomolar sensitivity

Rapid, high-throughput assays that detect and quantify botulinum neurotoxin (BoNT) activity in diverse matrices are required for environmental, clinical, pharmaceutical, and food testing. The current standard, the mouse bioassay, is sensitive but is low in throughput and precision. In this study, we present three biochemical assays for the detection and quantification of BoNT serotype A, B, and F proteolytic activities in complex matrices that offer picomolar to femtomolar sensitivity with small assay volumes and total assay times of less than 24 h. These assays consist of magnetic beads conjugated with BoNT serotype-specific antibodies that are used to purify BoNT from complex matrices before the quantification of bound BoNT proteolytic activity using the previously described BoTest reporter substrates. The matrices tested include human serum, whole milk, carrot juice, and baby food, as well as buffers containing common pharmaceutical excipients. The limits of detection were below 1 pM for BoNT/A and BoNT/F and below 10 pM for BoNT/B in most tested matrices using 200-μl samples and as low as 10 fM for BoNT/A with an increased sample volume. Together, these data describe rapid, robust, and high-throughput assays for BoNT detection that are compatible with a wide range of matrices.

The receptor binding domain of botulinum neurotoxin serotype C binds phosphoinositides

Botulinum neurotoxins (BoNTs) are the most toxic proteins known for humans and animals with an extremely low LD(50) of ∼1 ng/kg. BoNTs generally require a protein and a ganglioside on the cell membrane surface for binding, which is known as a "dual receptor" mechanism for host intoxication. Recent studies have suggested that in addition to gangliosides, other membrane lipids such as phosphoinositides may be involved in the interactions with the receptor binding domain (HCR) of BoNTs for better membrane penetration. Using two independent lipid-binding assays, we tested the interactions of BoNT/C-HCR with lipids in vitro domain. BoNT/C-HCR was found to bind negatively charged phospholipids, preferentially phosphoinositides in both assays. Interactions with phosphoinositides may facilitate tighter binding between neuronal membranes and BoNT/C.

Basic tetrapeptides as potent intracellular inhibitors of type A botulinum neurotoxin protease activity

Botulinum neurotoxins (BoNT) are the most potent of all toxins that cause flaccid muscle paralysis leading to death. They are also potential biothreat agents. A systematic investigation of various short peptide inhibitors of the BoNT protease domain with a 17-residue peptide substrate led to arginine-arginine-glycine-cysteine having a basic tetrapeptide structure as the most potent inhibitor. When assayed in the presence of dithiothreitol (DTT), the inhibitory effect was drastically reduced. Replacing the terminal cysteine with one hydrophobic residue eliminated the DTT effect but with two hydrophobic residues made the pentapeptide a poor inhibitor. Replacing the first arginine with cysteine or adding an additional cysteine at the N terminus did not improve inhibition. When assessed using mouse brain lysates, the tetrapeptides also inhibited BoNT/A cleavage of the endogenous SNAP-25. The peptides penetrated the neuronal cell lines, N2A and BE(2)-M17, without adversely affecting metabolic functions as measured by ATP production and P-38 phosphorylation. Biological activity of the peptides persisted within cultured chick motor neurons and rat and mouse cerebellar neurons for more than 40 h and inhibited BoNT/A protease action inside the neurons in a dose- and time-dependent fashion. Our results define a tetrapeptide as the smallest peptide inhibitor in the backdrop of a large substrate protein of 200+ amino acids having multiple interaction regions with its cognate enzyme. The inhibitors should also be valuable candidates for drug development.

Structural analysis of the receptor binding domain of botulinum neurotoxin serotype D

Botulinum neurotoxins (BoNTs) are the most toxic proteins known. The mechanism for entry into neuronal cells for serotypes A, B, E, F, and G involves a well understood dual receptor (protein and ganglioside) process, however, the mechanism of entry for serotypes C and D remains unclear. To provide structural insights into how BoNT/D enters neuronal cells, the crystal structure of the receptor binding domain (S863-E1276) for this serotype (BoNT/D-HCR) was determined at 1.65Å resolution. While BoNT/D-HCR adopts an overall fold similar to that observed in other known BoNT HCRs, several major structural differences are present. These structural differences are located at, or near, putative receptor binding sites and may be responsible for BoNT/D host preferences. Two loops, S1195-I1204 and K1236-N1244, located on both sides of the putative protein receptor binding pocket, are displaced >10Å relative to the corresponding residues in the crystal structures of BoNT/B and G. Obvious clashes were observed in the putative protein receptor binding site when the BoNT/B protein receptor synaptotagmin II was modeled into the BoNT/D-HCR structure. Although a ganglioside binding site has never been unambiguously identified in BoNT/D-HCR, a shallow cavity in an analogous location to the other BoNT serotypes HCR domains is observed in BoNT/D-HCR that has features compatible with membrane binding. A portion of a loop near the putative receptor binding site, K1236-N1244, is hydrophobic and solvent-exposed and may directly bind membrane lipids. Liposome-binding experiments with BoNT/D-HCR demonstrate that this membrane lipid may be phosphatidylethanolamine.

Substrate recognition of VAMP-2 by botulinum neurotoxin B and tetanus neurotoxin

Botulinum neurotoxin (BoNT; serotypes A-G) and tetanus neurotoxin elicit flaccid and spastic paralysis, respectively. These neurotoxins are zinc proteases that cleave SNARE proteins to inhibit synaptic vesicle fusion to the plasma membrane. Although BoNT/B and tetanus neurotoxin (TeNT) cleave VAMP-2 at the same scissile bond, their mechanism(s) of VAMP-2 recognition is not clear. Mapping experiments showed that residues 60-87 of VAMP-2 were sufficient for efficient cleavage by BoNT/B and that residues 40-87 of VAMP-2 were sufficient for efficient TeNT cleavage. Alanine-scanning mutagenesis and kinetic analysis identified three regions within VAMP-2 that were recognized by BoNT/B and TeNT: residues adjacent to the site of scissile bond cleavage (cleavage region) and residues located within N-terminal and C-terminal regions relative to the cleavage region. Analysis of residues within the cleavage region showed that mutations at the P7, P4, P2, and P1' residues of VAMP-2 had the greatest inhibition of LC/B cleavage (> or =32-fold), whereas mutations at P7, P4, P1', and P2' residues of VAMP-2 had the greatest inhibition of LC/TeNT cleavage (> or =64-fold). Residues within the cleavage region influenced catalysis, whereas residues N-terminal and C-terminal to the cleavage region influenced binding affinity. Thus, BoNT/B and TeNT possess similar organization but have unique residues to recognize and cleave VAMP-2. These studies provide new insights into how the clostridial neurotoxins recognize their substrates.

Mechanism of substrate recognition by botulinum neurotoxin serotype A

Botulinum neurotoxins (BoNTs) are zinc proteases that cleave SNARE proteins to elicit flaccid paralysis by inhibiting neurotransmitter-carrying vesicle fusion to the plasma membrane of peripheral neurons. Unlike other zinc proteases, BoNTs recognize extended regions of SNAP25 for cleavage; however, the molecular basis for this extended substrate recognition is unclear. Here, we define a multistep mechanism for recognition and cleavage of SNAP25 by BoNT/A. SNAP25 initially binds along the belt region of BoNT/A, which aligns the P5 residue to the S5 pocket at the periphery of the active site. Although the exact order of each step of recognition of SNAP25 by BoNT/A at the active site is not clear, the initial binding could subsequently orient the P4'-residue of SNAP25 to form a salt bridge with the S4'-residue, which opens the active site allowing the P1'-residue access to the S1'-pocket. Subsequent hydrophobic interactions between the P3 residue of SNAP25 and the S3 pocket optimize alignment of the scissile bond for cleavage. This explains how the BoNTs recognize and cleave specific coiled SNARE substrates and provides insight into the development of inhibitors to prevent botulism.

Unique Substrate Recognition by Botulinum Neurotoxins Serotypes A and E

Botulinum neurotoxins (BoNTs) are zinc proteases that cleave SNARE proteins to elicit flaccid paralysis by inhibiting the fusion of neurotransmitter-carrying vesicles to the plasma membrane of peripheral neurons. There are seven serotypes of BoNT, termed A-G. BoNT serotype A and serotype E cleave SNAP25 at residues 197-198 and 180-181, respectively. Unlike other zinc proteases, the BoNTs recognize extended regions of SNAP25 for cleavage. The basis for this extended substrate recognition and specificity is unclear. Saturation mutagenesis and deletion mapping identified residues 156-202 of SNAP25 as the optimal cleavage domain for BoNT/A, whereas the optimal cleavage domain for BoNT/E was shorter, comprising residues 167-186 of SNAP25. Two sub-sites were resolved within each optimal cleavage domain, which included a recognition or active site (AS) domain that contained the site of cleavage and a binding (B) domain, which contributed to substrate affinity. Within the AS domains, the P1', P3, and P5 sites of SNAP25 contributed to scissile bond cleavage by LC/A, whereas the P1' and P2 sites of SNAP25 contributed to scissile bond cleavage by LC/E. These studies provide insight into the development of strategies for small molecule inhibitors of the BoNTs.

Brain-derived neurotrophic factor-induced potentiation of glutamate and GABA release: different dependency on signaling pathways and neuronal activity

The mechanisms underlying BDNF-modulated neurotransmitter release remain elusive. Here, we found that 24-h exposure of postnatal cortical neurons to BDNF potentiated depolarization-evoked glutamate and GABA release in a protein synthesis-dependent manner. BDNF-potentiated glutamate release occurred through the PLC-gamma and MAPK pathways. The expression of synapsin I, synaptotagmin, and synaptophysin, but not of syntaxin or SNAP25, increased through the PLC-gamma and MAPK pathways. In contrast, BDNF-up-regulated GABA release and GAD65/67 expression depended on MAPK. Furthermore, neuronal activity was necessary for the up-regulation of glutamate release and synapsin I, synaptotagmin, and synaptophysin expression, but not of GABA or GAD65/67. PLC-gamma inhibitor attenuated BDNF-stimulated long-lasting MAPK activation. As BDNF rapidly potentiates glutamatergic transmission through PLC-gamma (J. Biol. Chem. 277, (2002) 6520-6529), PLC-gamma-mediated neuronal activity might sustain MAPK activation, resulting in BDNF-potentiated glutamate release. In conclusion, BDNF potentiates the excitatory and inhibitory system separately, which may be important for the regulation of synaptic plasticity.

Characterization of the antibody response to the receptor binding domain of botulinum neurotoxin serotypes A and E

Clostridium botulinum neurotoxins (BoNTs) are the most toxic proteins for humans. The current clostridial-derived vaccines against BoNT intoxication have limitations including production and accessibility. Conditions were established to express the soluble receptor binding domain (heavy-chain receptor [HCR]) of BoNT serotypes A and E in Escherichia coli. Sera isolated from mice and rabbits immunized with recombinant HCR/A1 (rHCR/A1) from the classical type A-Hall strain (ATCC 3502) (BoNT/A1) and rHCR/E from BoNT serotype E Beluga (BoNT/E(B)) neutralized the homologous serotype of BoNT but displayed differences in cross-recognition and cross-protection. Enzyme-linked immunosorbent assay and Western blotting showed that alpha-rHCR/A1 recognized epitopes within the C terminus of the HCR/A and HCR/E, while alpha-rHCR/E recognized epitopes within the N terminus or interface between the N and C termini of the HCR proteins. alpha-rHCR/E(B) sera possessed detectable neutralizing capacity for BoNT/A1, while alpha-rHCR/A1 did not neutralize BoNT/E. rHCR/A was an effective immunogen against BoNT/A1 and the Kyoto F infant strain (BoNT/A2), but not BoNT serotype E Alaska (BoNT/E(A)), while rHCR/E(B) neutralized BoNT/E(A), and under hyperimmunization conditions protected against BoNT/A1 and BoNT/A2. The protection elicited by rHCR/A1 to BoNT/A1 and BoNT/A2 and by rHCR/E(B) to BoNT/E(A) indicate that immunization with receptor binding domains elicit protection within sub-serotypes of BoNT. The protection elicited by hyperimmunization with rHCR/E against BoNT/A suggests the presence of common neutralizing epitopes between the serotypes E and A. These results show that a receptor binding domain subunit vaccine protects against serotype variants of BoNTs.

Comparison of proteolytic activities produced by entomopathogenic Photorhabdus bacteria: strain- and phase-dependent heterogeneity in composition and activity of four enzymes

Twenty strains (including eight phase variant pairs) of nematode-symbiotic and insect-pathogenic Photorhabdus bacteria were examined for the production of proteolytic enzymes by using a combination of several methods, including gelatin liquefaction, zymography coupled to native and sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and activity measurement with two chromogen substrate types. Four protease activities (approximately 74, approximately 55, approximately 54, and approximately 37 kDa) could be separated. The N-terminal sequences of three of the proteases were determined, and a comparison with sequences in databases allowed identification of these proteases as HEXXH metallopeptidases. Thus, the 74-kDa protease (described formerly as Php-B [J. Marokhazi, G. Koczan, F. Hudecz, L. Graf, A. Fodor, and I. Venekei, Biochem. J. 379:633-640, 2004) is an ortholog of OpdA, a member the thimet oligopeptidase family, and the 55-kDa protease is an ortholog of PrtA, a HEXXH+H peptidase in clan MB (metzincins), while the 37-kDa protease (Php-C) belongs to the HEXXH+E peptidases in clan MA. The 54-kDa protease (Php-D) is a nonmetalloenzyme. PrtA and Php-C were zymographically detected, and they occurred in several smaller forms as well. OpdA could not be detected by zymography. PrtA, Php-C, and Php-D were secreted proteases; OpdA, in contrast, was an intracellular enzyme. OpdA activity was found in every strain tested, while Php-D was detected only in the Brecon/1 strain. There was significant strain variation in the secretion of PrtA and Php-C activities, but reduced activity or a lack of activity was not specific to secondary-phase variants. The presence of PrtA, OpdA, and Php-C activities could be detected in the hemolymph of Galleria melonella larvae 20 to 40 h postinfection. These proteases appear not to be directly involved in the pathogenicity of Photorhabdus, since strains or phase variants lacking any of these proteases do not show reduced virulence when they are injected into G. melonella larvae.

Basic immunological aspects of botulinum toxin therapy

Previous studies in this laboratory had mapped the immune recognition profile of the regions recognized antibodies (Abs) and by T cells on the protective H(C) domain (C-terminal fragment corresponding to residues 855-1296 of the heavy chain) of botulinum neurotoxin serotype A (BoNT/A). The localization of these regions has several potential applications and has provided a basis for the understanding of immunoresistance to treatment. We briefly outline these localized regions and discuss the impact of these findings on the immunotherapeutic applications of BoNT/A. Immunoresistance to toxin therapy can appear in some patients after a few injections with the toxin. Our epitope mapping studies have shown that several factors can influence the immune response to the toxin. These factors include dose, duration of treatment, frequency of immunization, and quality of the toxin. The immune response to the whole toxin is under genetic control, and the response to each epitope is under separate genetic control. Therefore, the appearance of blocking Abs (i.e., immunoresistance) in patients might be controlled by the major histocompatability of the host. Once a patient becomes immunoresistant to one toxin then switching to another toxin will most often be of limited and short-lived benefit, because the patient becomes rapidly immunoresistant to the second toxin. Finally, because of the considerable structural homology between tetanus neurotoxin (TeNT) and BoNTs, it is possible, although not certain, that a prior active immune response to TeNT might play some role in the early appearance on anti-BoNT Abs in some patients.

Effect of Inherited Abnormalities of Calcium Regulation on Human Neuromuscular Transmission

Synaptotagmins are abundant synaptic proteins that represent the best candidate for the calcium sensor at the nerve terminal. The pore-forming, voltage-sensing transmembrane alpha-1 subunit of the P/Q voltage-gated calcium channel (or Ca(v)2.1) encoded by the CACNA1A gene is another major component of the process of action potential-evoked exocytosis at the adult mammalian neuromuscular junction. Defects of these proteins, in nonhuman species, result in severe disruption of rapid synaptic transmission. This paper investigates the molecular bases of inherited presynaptic deficits of neuromuscular transmission in humans. Patients with congenital presynaptic failure, including two patients with episodic ataxia type 2 (EA-2) due to CACNA1A mutations, were studied with muscle biopsy, microelectrode studies, electron microscopy, DNA amplification, and sequencing. All patients, including EA-2 patients, showed selective failure of the action potential-dependent release without reduction of the spontaneous release of neurotransmitter. In addition, patients with EA-2 showed partial blockade of neuromuscular transmission with the N-type blocker omega-conotoxin not seen in controls. The EM showed a varied degree of increased complexity of postsynaptic folds. Mutational analysis in candidate genes, including human synaptotagmin II, syntaxin 1A, synaptobrevin I, SNAP 25, CACNA1A, CACNB2, and Rab3A, was unrevealing. Although no mutations in candidate genes were found in patients with inborn presynaptic failure, functional and structural similarities between this group and patients with EA-2 due to CACNA1A mutations suggest a common pathogenic mechanism.

A capillary electrophoresis technique for evaluating botulinum neurotoxin B light chain activity

Botulinum neurotoxin B (BoNT/B) produces muscle paralysis by cleaving synaptobrevin/vesicle-associated membrane protein (VAMP), an 18-kDa membrane-associated protein located on the surface of small synaptic vesicles. A capillary electrophoresis (CE) assay was developed to evaluate inhibitors of the proteolytic activity of BoNT/B with the objective of identifying suitable candidates for treatment of botulism. The assay was based on monitoring the cleavage of a peptide that corresponds to residues 44-94 of human VAMP-2 (V51) following reaction with the catalytic light chain (LC) of BoNT/B. Cleavage of V51 generated peptide fragments of 18 and 33 amino acids by scission of the bond between Q76 and F77. The fragments and parent peptide were clearly resolved by CE, allowing accurate quantification of the BoNT/B LC-mediated reaction rates. The results indicate that CE is suitable for assessing the enzymatic activity of BoNT/B LC.

Genetic and biochemical characterization of PrtA, an RTX-like metalloprotease from Photorhabdus

Proteases play a key role in the interaction between pathogens and their hosts. The bacterial entomopathogen Photorhabdus lives in symbiosis with nematodes that invade insects. Following entry into the insect, the bacteria are released from the nematode gut into the open blood system of the insect. Here they secrete factors which kill the host and also convert the host tissues into food for the replicating bacteria and nematodes. One of the secreted proteins is PrtA, which is shown here to be a repeats-in-toxin (RTX) alkaline zinc metalloprotease. PrtA has high affinity for artificial substrates such as casein and gelatin and can be inhibited by zinc metalloprotease inhibitors. The metalloprotease also shows a calcium- and temperature-dependent autolysis. The prtA gene carries the characteristic RTX repeated motifs and predicts high similarity to proteases from Erwinia chrysanthemi, Pseudomonas aeruginosa and Serratia marcescens. The prtA gene resides in a locus encoding both the protease ABC transporter (prtBCD) and an intervening ORF encoding a protease inhibitor (inh). PrtA activity is detectable 24 h after artificial bacterial infection of an insect, suggesting that the protease may play a key role in degrading insect tissues rather than in overcoming the insect immune system. Purified PrtA also shows cytotoxicity to mammalian cell cultures, supporting its proposed role in bioconversion of the insect cadaver into food for bacterial and nematode development.

Role of vesicle-associated membrane protein-2, through Q-soluble N-ethylmaleimide-sensitive factor attachment protein receptor/R-soluble N-ethylmaleimide-sensitive factor attachment protein receptor interaction, in the exocytosis of specific and tertiary granules of human neutrophils

We have examined the role of the R-soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) synaptobrevin-2/vesicle-associated membrane protein (VAMP)-2 in neutrophil exocytosis. VAMP-2, localized in the membranes of specific and gelatinase-containing tertiary granules in resting human neutrophils, resulted translocated to the cell surface following neutrophil activation under experimental conditions that induced exocytosis of specific and tertiary granules. VAMP-2 was also found on the external membrane region of granules docking to the plasma membrane in activated neutrophils. Specific Abs against VAMP-2 inhibited Ca(2+) and GTP-gamma-S-induced exocytosis of CD66b-enriched specific and tertiary granules, but did not affect exocytosis of CD63-enriched azurophilic granules, in electropermeabilized neutrophils. Tetanus toxin disrupted VAMP-2 and inhibited exocytosis of tertiary and specific granules. Activation of neutrophils led to the interaction of VAMP-2 with the plasma membrane Q-SNARE syntaxin 4, and anti-syntaxin 4 Abs inhibited exocytosis of specific and tertiary granules in electropermeabilized neutrophils. Immunoelectron microscopy showed syntaxin 4 on the plasma membrane contacting with docked granules in activated neutrophils. These data indicate that VAMP-2 mediates exocytosis of specific and tertiary granules, and that Q-SNARE/R-SNARE complexes containing VAMP-2 and syntaxin 4 are involved in neutrophil exocytosis.

A zinc metalloprotease inhibitor, Inh, from the insect pathogen Photorhabdus luminescens

The entomopathogen Photorhabdus luminescens secretes many proteins during the late stages of insect larvae infection and during in vitro laboratory culture. The authors have previously characterized and purified a 55 kDa zinc metalloprotease, PrtA, from culture supernatants of P. luminescens. PrtA is secreted via a classical type I secretory pathway and is encoded within the operon prtA-inh-prtBCD. The 405 bp inh gene encodes a 14.8 kDa pre-protein that is translocated to the periplasm by the classical signal-peptide-dependent sec pathway, yielding the mature 11.9 kDa inhibitor Inh. Inh is a specific inhibitor of the protease PrtA. This study describes the purification of Inh and the initial characterization of its in vitro protease inhibition properties.

Zinc-mediated dimerization and its effect on activity and conformation of staphylococcal enterotoxin type C

Staphylococcal enterotoxins are superantigen exotoxins that mediate food poisoning and toxic shock syndrome in humans. Despite their structural and functional similarities, superantigens display subtle differences in biological properties and modes of receptor binding as a result of zinc atoms bound differently in their crystal structures. For example, the crystal structures of the staphylococcal enterotoxins in the type C serogroup (SECs) contain a zinc atom coordinated by one aspartate and two histidine residues from one molecule and another aspartate residue from the next molecule, thus forming a dimer. This type of zinc ligation and zinc-mediated dimerization occurs in several SECs, but not in most other staphylococcal enterotoxin serogroups. This prompted us to investigate the potential importance of zinc in SEC-mediated pathogenesis. Site-directed mutagenesis was used to replace SEC zinc binding ligands with alanine. SEC mutants unable to bind zinc did not have major conformational alterations although they failed to form dimers. Zinc binding was not essential for T cell stimulation, emesis, or lethality although in general the mutants were less pyrogenic. Thus the zinc atom in SECs might represent a non-functional heavy atom in an exotoxin group that has diverged from related bacterial toxins containing crucial zinc atoms.

Hyperosmolarity reduces GLUT4 endocytosis and increases its exocytosis from a VAMP2-independent pool in l6 muscle cells

The intracellular traffic of the glucose transporter 4 (GLUT4) in muscle cells remains largely unexplored. Here we make use of L6 myoblasts stably expressing GLUT4 with an exofacially directed Myc-tag (GLUT4myc) to determine the exocytic and endocytic rates of the transporter. Insulin caused a rapid (t(12) = 4 min) gain, whereas hyperosmolarity (0.45 m sucrose) caused a slow (t(12) = 20 min) gain in surface GLUT4myc molecules. With prior insulin stimulation followed by addition of hypertonic sucrose, the increase in surface GLUT4myc was partly additive. Unlike the effect of insulin, the GLUT4myc gain caused by hyperosmolarity was insensitive to wortmannin or to tetanus toxin cleavage of VAMP2 and VAMP3. Disappearance of GLUT4myc from the cell surface was rapid (t(12) = 1.5 min). Insulin had no effect on the initial rate of GLUT4myc internalization. In contrast, hyperosmolarity almost completely abolished GLUT4myc internalization. Surface GLUT4myc accumulation in response to hyperosmolarity was only partially blocked by inhibition of tyrosine kinases with erbstatin analog (erbstatin A) and genistein. However, neither inhibitor interfered with the ability of hyperosmolarity to block GLUT4myc internalization. We propose that hyperosmolarity increases surface GLUT4myc by preventing GLUT4 endocytosis and stimulating its exocytosis via a pathway independent of phosphatidylinositol 3-kinase activity and of VAMP2 or VAMP3. A tetanus toxin-insensitive v-SNARE such as TI-VAMP detected in these cells, might mediate membrane fusion of the hyperosmolarity-sensitive pool.

High-affinity, protective antibodies to the binding domain of botulinum neurotoxin type A

Monoclonal antibodies (MAbs) were prepared against the putative binding domain of botulinum neurotoxin A (BoNT/A), a nontoxic 50-kDa fragment. Initially, all fusion products were screened against the holotoxin BoNT/A and against the binding fragment, BoNT/A H(C). Eleven neutralizing hybridomas were cloned, and their specific binding to BoNT/A H(C) was demonstrated by surface plasmon resonance, with dissociation constants ranging from 0.9 to <0.06 nM. Epitope mapping by real-time surface plasmon resonance showed that the antibodies bound to at least two distinct regions of the BoNT/A H(C) fragment. These MAbs will be useful tools for studying BoNT/A interactions with its receptor, and they have potential diagnostic and therapeutic applications.

VAMP2, but not VAMP3/cellubrevin, mediates insulin-dependent incorporation of GLUT4 into the plasma membrane of L6 myoblasts

Like neuronal synaptic vesicles, intracellular GLUT4-containing vesicles must dock and fuse with the plasma membrane, thereby facilitating insulin-regulated glucose uptake into muscle and fat cells. GLUT4 colocalizes in part with the vesicle SNAREs VAMP2 and VAMP3. In this study, we used a single-cell fluorescence-based assay to compare the functional involvement of VAMP2 and VAMP3 in GLUT4 translocation. Transient transfection of proteolytically active tetanus toxin light chain cleaved both VAMP2 and VAMP3 proteins in L6 myoblasts stably expressing exofacially myc-tagged GLUT4 protein and inhibited insulin-stimulated GLUT4 translocation. Tetanus toxin also caused accumulation of the remaining C-terminal VAMP2 and VAMP3 portions in Golgi elements. This behavior was exclusive to these proteins, because the localization of intracellular myc-tagged GLUT4 protein was not affected by the toxin. Upon cotransfection of tetanus toxin with individual vesicle SNARE constructs, only toxin-resistant VAMP2 rescued the inhibition of insulin-dependent GLUT4 translocation by tetanus toxin. Moreover, insulin caused a cortical actin filament reorganization in which GLUT4 and VAMP2, but not VAMP3, were clustered. We propose that VAMP2 is a resident protein of the insulin-sensitive GLUT4 compartment and that the integrity of this protein is required for GLUT4 vesicle incorporation into the cell surface in response to insulin.

Nerve growth factor-induced phosphorylation of SNAP-25 in PC12 cells: a possible involvement in the regulation of SNAP-25 localization

Synaptosomal-associated protein of 25 kDa (SNAP-25), a t-SNARE protein essential for neurotransmitter release, is phosphorylated at Ser187 following activation of cellular protein kinase C by treatment with phorbol 12-myristate 13-acetate. However, it remains unclear whether neuronal activity or an endogenous ligand induces the phosphorylation of SNAP-25. Here we studied the phosphorylation of SNAP-25 in PC12 cells using a specific antibody for SNAP-25 phosphorylated at Ser187. A small fraction of SNAP-25 was phosphorylated when cells were grown in the absence of nerve growth factor (NGF). A brief treatment with NGF that was enough to activate the mitogen-activated protein kinase signal transduction pathway did not increase the phosphorylation of SNAP-25; however, phosphorylation was up-regulated after a prolonged incubation with NGF. Up-regulation was transitory, and maximum phosphorylation (a fourfold increase over basal phosphorylation) was achieved between 36 and 48 h after the addition of NGF. Immunofluorescent microscopy showed that SNAP-25 was localized primarily in the plasma membrane, although a significant population was also present in the cytoplasm. Quantitative microfluorometry revealed that prolonged treatment with NGF resulted in a preferential localization of SNAP-25 in the plasma membrane. A mutational study using a fusion protein with green fluorescent protein as a tag indicated that the point mutation of Ser187 to Ala abolished the NGF-dependent relocalization. A population of SNAP-25 in the plasma membrane was not increased by a point mutation at Ser187 to Glu; however, it was increased by prolonged treatment with NGF, indicating that the SNAP-25 phosphorylation is essential, but not sufficient, for the NGF-induced relocation to the plasma membrane. Our results suggest a close temporal relationship between the up-regulation of SNAP-25 phosphorylation and its relocation, and NGF-induced differentiation of PC12 cells.

Bacterial proteinases as targets for the development of second-generation antibiotics

The emergence of bacterial pathogen resistance to common antibiotics strongly supports the necessity to develop alternative mechanisms for combating drug-resistant forms of these infective organisms. Currently, few pharmaceutical companies have attempted to investigate the possibility of interrupting metabolic pathways other than those that are known to be involved in cell wall biosynthesis. In this review, we describe multiple, novel roles for bacterial proteinases during infection using, as a specific example, the enzymes from the organism Porphyromonas gingivalis, a periodontopathogen, which is known to be involved in the development and progression of periodontal disease. In this manner, we are able to justify the concept of developing synthetic inhibitors against members of this class of enzymes as potential second-generation antibiotics. Such compounds could not only prove valuable in retarding the growth and proliferation of bacterial pathogens but also lead to the use of this class of inhibitors against invasion by other infective organisms.

The effects of tetanus toxin on the orbicularis oculi muscle

PURPOSE

Tetanus toxin can cause localized neuromuscular weakness, but it also can produce systemic tetany. The action of tetanus toxin on the orbicularis muscle has not been studied in animals immunized to prevent systemic tetany. Our objective was to determine whether tetanus toxin could be used to treat orbicularis oculi muscle spasms.

METHODS

We analyzed the clinical, electrophysiologic, and histopathologic effects of tetanus toxin injected into the orbicularis oculi muscle of rabbits with passive immunity to tetanus toxin. In six rabbits, the orbicularis oculi function in both eyes was assessed clinically, and the baseline orbicularis oculi muscle action potential was measured physiologically with electromyography (EMG). The rabbits then were immunized against tetanus toxin with tetanus immunoglobulin for immediate and definitive immunity. Tetanus toxin was injected into the left orbicularis oculi muscles, leaving the right eyes as controls. Ten days later, the rabbits were again assessed by clinical examination and with EMGs on both the injected side and the noninjected side. The animals were killed at 14 days, and the orbicularis muscle was removed from both sides. The injected and control tissues were examined microscopically for signs of neuromuscular denervation.

RESULTS

All six rabbits showed weakness in eye closure on the side injected with tetanus toxin. In addition, four rabbits developed complete ear ptosis on the tetanus toxin injected side because of spread of the toxin to adjacent ear muscles. EMGs showed both a denervation of the orbicularis oculi muscle and a poor blink potential on the side injected with tetanus toxin. Histopathologic studies of the orbicularis oculi muscle injected with tetanus toxin showed angulation of both slow and fast types of muscle fibers compatible with neuromuscular denervation.

CONCLUSIONS

Tetanus toxin can cause localized orbicularis oculi weakness, as documented clinically, physiologically, and microscopically, without producing systemic tetany in immunized rabbits. Tetanus toxin may have a potential application in the treatment of blepharospasm and hemifacial spasm.

Differential phosphorylation of syntaxin and synaptosome-associated protein of 25 kDa (SNAP-25) isoforms

The synaptic plasma membrane proteins syntaxin and synaptosome-associated protein of 25 kDa (SNAP-25) are central participants in synaptic vesicle trafficking and neurotransmitter release. Together with the synaptic vesicle protein synaptobrevin/vesicle-associated membrane protein (VAMP), they serve as receptors for the general membrane trafficking factors N-ethylmaleimide-sensitive factor (NSF) and soluble NSF attachment protein (alpha-SNAP). Consequently, syntaxin, SNAP-25, and VAMP (and their isoforms in other membrane trafficking pathways) have been termed SNAP receptors (SNAREs). Because protein phosphorylation is a common and important mechanism for regulating a variety of cellular processes, including synaptic transmission, we have investigated the ability of syntaxin and SNAP-25 isoforms to serve as substrates for a variety of serine/threonine protein kinases. Syntaxins 1 A and 4 were phosphorylated by casein kinase II, whereas syntaxin 3 and SNAP-25 were phosphorylated by Ca2+- and calmodulin-dependent protein kinase II and cyclic AMP-dependent protein kinase, respectively. The biochemical consequences of SNARE protein phosphorylation included a reduced interaction between SNAP-25 and phosphorylated syntaxin 4 and an enhanced interaction between phosphorylated syntaxin 1A and the synaptic vesicle protein synaptotagmin I, a potential Ca2+ sensor in triggering synaptic vesicle exocytosis. No other effects on the formation of SNARE complexes (comprised of syntaxin, SNAP-25, and VAMP) or interactions involving n-Sec1 or alpha-SNAP were observed. These findings suggest that although phosphorylation does not directly regulate the assembly of the synaptic SNARE complex, it may serve to modulate SNARE complex function through other proteins, including synaptotagmin I.

Bacteroides fragilis enterotoxin cleaves the zonula adherens protein, E-cadherin

Strains of Bacteroides fragilis associated with diarrheal disease (enterotoxigenic B. fragilis) produce a 20-kDa zinc-dependent metalloprotease toxin (B. fragilis enterotoxin; BFT) that reversibly stimulates chloride secretion and alters tight junctional function in polarized intestinal epithelial cells. BFT alters cellular morphology and physiology most potently and rapidly when placed on the basolateral membrane of epithelial cells, suggesting that the cellular substrate for BFT may be present on this membrane. Herein, we demonstrate that BFT specifically cleaves within 1 min the extracellular domain of the zonula adherens protein, E-cadherin. Cleavage of E-cadherin by BFT is ATP-independent and essential to the morphologic and physiologic activity of BFT. However, the morphologic changes occurring in response to BFT are dependent on target-cell ATP. E-cadherin is shown here to be a cellular substrate for a bacterial toxin and represents the identification of a mechanism of action, cell-surface proteolytic activity, for a bacterial toxin.