ABSTRACTBotulinum neurotoxins (BoNTs) and tetanus neurotoxin (TeNT) are the most toxic proteins for humans. While BoNTs cause flaccid paralysis, TeNT causes spastic paralysis. Characterized BoNT serotypes enter neurons upon binding dual receptors, a ganglioside and a neuron-specific protein, either synaptic vesicle protein 2 (SV2) or synaptotagmin, while TeNT enters upon binding gangliosides as dual receptors. Recently, TeNT was reported to enter central nervous system (CNS) neurons upon synaptic vesicle cycling that was mediated by the direct binding to SV2, implying that TeNT and BoNT utilize common mechanisms to enter CNS neurons. This prompted an assessment of TeNT entry into CNS neurons, using the prototypic BoNT serotype A as a reference for SV2-mediated entry into synaptic vesicles, analyzing the heavy-chain receptor binding domain (HCR) of each toxin. Synaptic vesicle cycling stimulated the entry of HCR/A into neurons, while HCR/T entered neurons with similar levels of efficiency in depolarized and nondepolarized neurons. ImageJ analysis identified two populations of cell-associated HCR/T in synaptic vesicle cycling neurons, a major population which segregated from HCR/A and a minor population which colocalized with HCR/A. HCR/T did not inhibit HCR/A entry into neurons in competition experiments and did not bind SV2, the protein receptor for BoNT/A. Intoxication experiments showed that TeNT efficiently cleaved VAMP2 in depolarized neurons and neurons blocked for synaptic vesicle cycling. These experiments demonstrate that TeNT enters neurons by two pathways, one independent of stimulated synaptic vesicle cycling and one by synaptic vesicles independent of SV2, showing that TeNT and BoNT/A enter neurons by unique mechanisms.

Infection with Helicobacter pylori is a major risk factor for development of gastric disease, including gastric cancer. Patients infected with H. pylori strains that express CagA are at even greater risk of gastric carcinoma. Given the importance of CagA, this report describes a new molecular mechanism by which the cagA copy number dynamically expands and contracts in H. pylori. Analysis of strain PMSS1 revealed a heterogeneous population in terms of numbers of cagA copies; strains carried from zero to four copies of cagA that were arranged as direct repeats within the chromosome. Each of the multiple copies of cagA was expressed and encoded functional CagA; strains with more cagA repeats exhibited higher levels of CagA expression and increased levels of delivery and phosphorylation of CagA within host cells. This concomitantly resulted in more virulent phenotypes as measured by cell elongation and interleukin-8 (IL-8) induction. Sequence analysis of the repeat region revealed three cagA homologous areas (CHAs) within the cagA repeats. Of these, CHA-ud flanked each of the cagA copies and is likely important for the dynamic variation of cagA copy numbers. Analysis of a large panel of clinical isolates showed that 7.5% of H. pylori strains isolated in the United States harbored multiple cagA repeats, while none of the tested Korean isolates carried more than one copy of cagA. Finally, H. pylori strains carrying multiple cagA copies were differentially associated with gastric disease. Thus, the dynamic expansion and contraction of cagA copy numbers may serve as a novel mechanism by which H. pylori modulates gastric disease development.

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The nickel-containing enzymes of Helicobacter pylori, urease and hydrogenase, are essential for efficient colonization in the human stomach. The insertion of nickel into urease and hydrogenase is mediated by the accessory protein HypA. HypA contains an N-terminal nickel-binding site and a dynamic structural zinc-binding site. The coordination of nickel and zinc within HypA is known to be critical for urease maturation and activity. Herein, we test the hydrogenase activity of a panel of H. pylori mutant strains containing point mutations within the nickel-and zinc-binding sites. We found that the residues that are important for hydrogenase activity are those that were similarly vital for urease activity. Thus, the zinc and metal coordination sites of HypA play similar roles in urease and hydrogenase maturation. In other pathogenic bacteria, deletion of hydrogenase leads to a loss in acid resistance. Thus, the acid resistance of two strains of H. pylori containing a hydrogenase deletion was also tested. These mutant strains demonstrated wild-type levels of acid resistance, suggesting that in H. pylori, hydrogenase does not play a role in acid resistance.

Botulinum neurotoxins (BoNTs) are the most toxic proteins for humans but also are common therapies for neurological diseases. BoNTs are dichain toxins, comprising an N-terminal catalytic domain (LC) disulfide bond linked to a C-terminal heavy chain (HC) which includes a translocation domain (H N ) and a receptor binding domain (H C ). Recently, the BoNT serotype A (BoNT/A) subtypes A1 and A2 were reported to possess similar potencies but different rates of cellular intoxication and pathology in a mouse model of botulism. The current study measured H C A1 and H C A2 entry into rat primary neurons and cultured Neuro2A cells. We found that there were two sequential steps during the association of BoNT/A with neurons. The initial step was ganglioside dependent, while the subsequent step involved association with synaptic vesicles. H C A1 and H C A2 entered the same population of synaptic vesicles and entered cells at similar rates. The primary difference was that H C A2 had a higher degree of receptor occupancy for cells and neurons than HcA1. Thus, H C A2 and H C A1 share receptors and entry pathway but differ in their affinity for receptor. The initial interaction of H C A1 and H C A2 with neurons may contribute to the unique pathologies of BoNT/A1 and BoNT/A2 in mouse models.KEYWORDS botulinum toxin, gangliosides, synaptic vesicles, synaptic vesicle protein 2, TIRF microscopy, clostridial neurotoxins, Clostridium botulinum, toxins B otulinum neurotoxins (BoNTs) are AB exotoxins secreted by several species of the genus Clostridium. BoNTs are single-chain 150-kDa proteins cleaved by either bacterial or host proteases to a dichain comprising a 50-kDa light chain (LC) and a 100-kDa heavy chain (HC) linked by an interchain disulfide bond. The HC includes a translocation domain (H N ) and receptor binding domain (H C ) (1). The H C includes an N-terminal subdomain (H CN ) of limited known function and a C-terminal subdomain (H CC ) that confers neuron specificity by binding dual neuron-specific receptors. There are seven BoNT serotypes (A to G) (2). BoNT serotype A (BoNT/A) binds a ganglioside and synaptic vesicle glycoprotein 2 (SV2) (3-6), which allows rapid toxin entry via synaptic vesicles. Acidification of the synaptic vesicle lumen triggers H N to form a channel to facilitate LC translocation into the cytosol. Intracellular LC cleaves a SNARE (soluble N-ethylmaleimide-sensitive factor [NSF] attachment protein receptor) protein (7-12). SNARE protein cleavage inhibits exocytosis of cholinergic synaptic vesicles at neuromuscular junctions. BoNT LCs are long-lived proteases, sustaining paralysis in humans for several months depending on the serotype (2), which led to the licensing of BoNT/A and BoNT/B for human therapies (13,14).BoNT serotypes include subtypes neutralized by serotype-specific antisera (15-17). Informatics has classified eight BoNT/A subtypes (A1 to A8), which vary by ϳ10 to 15% in amino acid identity (15,16,(18)(19)(20)(21). While BoNT/A1 and BoNT/A2 cleave SNAP25 with similar kinetics (22-2...

Helicobacter pylori HypA (HpHypA) is a metallochaperone necessary for maturation of [Ni,Fe]-hydrogenase and urease, the enzymes required for colonization and survival of H. pylori in the gastric mucosa. HpHypA contains a structural Zn(II) site and a unique Ni(II) binding site at the N-terminus. X-ray absorption spectra suggested that the Zn(II) coordination depends on pH and on the presence of Ni(II). This study was performed to investigate the structural properties of HpHypA as a function of pH and Ni(II) binding, using NMR spectroscopy combined with DFT and molecular dynamics calculations. The solution structure of apo, Zn-HpHypA, containing Zn(II) but devoid of Ni(II), was determined using 2D, 3D and 4D NMR spectroscopy. The structure suggests that a Ni-binding and a Zn-binding domain, joined through a short linker, could undergo mutual reorientation. This flexibility has no physiological effect on acid viability or urease maturation in H. pylori. Atomistic molecular dynamics simulations suggest that Ni(II) binding is important for the conformational stability of the N-terminal helix. NMR chemical shift perturbation analysis indicates that no structural changes occur in the Zn-binding domain upon addition of Ni(II) in the pH 6.3–7.2 range. The structure of the Ni(II) binding site was probed using 1H NMR spectroscopy experiments tailored to reveal hyperfine-shifted signals around the paramagnetic metal ion. On this basis, two possible models were derived using quantum-mechanical DFT calculations. The results provide a comprehensive picture of the Ni(II) mode to HpHypA, important to rationalize, at the molecular level, the functional interactions of this chaperone with its protein partners.

Tetanus neurotoxin (TeNT) and botulinum neurotoxin (BoNT) are clostridial neurotoxins (CNTs) that are the most toxic proteins for humans (1). TeNT and BoNT share ϳ35% identity and ϳ65% similarity and overall structure-function properties (2). BoNT intoxication results in flaccid paralysis through the inhibition of acetylcholine release by motor neurons, while TeNT intoxication yields a spastic paralysis due to inhibition of glycine release by inhibitory neurons (3). TeNT and BoNT are expressed as ϳ150-kDa single-chain proteins that are cleaved to form dichain proteins linked by a disulfide bond (2). The N-terminal 50-kDa light chain (LC) is a zinc-metalloprotease that cleaves neuronspecific soluble NSF attachment protein (SNAP) receptor (SNARE) proteins (4). TeNT and BoNT serotype B cleave the same residue within vesicle-associated membrane protein 2 (VAMP2), a SNARE protein of synaptic vesicles (SVs). The C-terminal 100-kDa heavy chain (HC) contains two structurally distinct domains with separate functions. The translocation domain (HCT) facilitates LC translocation from the SV lumen into the cell cytosol, and the receptor binding domain (HCR) binds dual host receptors.BoNT/A binds a ganglioside and synaptic vesicle protein 2 (SV2) and enters neurons upon SV recycling from the plasma membrane (5). Upon SV acidification within the periphery of motor neurons, the HCT undergoes a pH-dependent conformational change and inserts into the SV membrane, forming a channel that allows the LC to escape into the cytosol. Within the periphery of the motor neuron, the LC cleaves SNARE proteins, resulting in loss of stimulatory signaling between neurons and muscles, yielding flaccid paralysis. The LC of BoNT/A localizes to the plasma membrane to target synaptosomal-associated protein 25 (SNAP25) for cleavage within neurons (6).TeNT binds two gangliosides as functional receptors (7). TeNT can bind a glycophosphatidylinositol (GPI)-anchored protein (8) or SV2 (9), but the significance of these interactions has not been defined (10) or reproduced (11), respectively. TeNT enters motor neurons upon endocytosis (12) and traffics through motor neurons associated with Rab7-enriched endosomes that are of neutral pH (13,14). Retrograde trafficking proceeds from the axon to the soma, where TeNT transcytoses from the motor neuron into an inhibitory neuron of the central nervous system (CNS). Upon vesicle acidification, the LC is translocated into the cytosol and cleaves VAMP2. The block in signaling between the inhibitory neurons and motor neurons leads to the spastic paralysis characteristic of tetanus. The molecular mechanism responsible for the unique entry of BoNT and TeNT is not clearly understood.The modular structural domains of the CNTs have permitted the study of individual domains to assess protein structure-function in vitro; the LC is a functional SNARE protease (15), the HCT inserts into lipid bilayers and forms a channel (16), and the HCR binds host receptors (17). The tetanus HCR (HCR/T) can retrograde traffic in both cultu...

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