Characterization of botulinum neurotoxin A subtypes 1 through 5 by investigation of activities in mice, in neuronal cell cultures, and in vitro.

Botulinum neurotoxins (BoNTs) are synthesized by Clostridium botulinum and exist as seven immunologically distinct serotypes designated A through G. For most serotypes, several subtypes have now been described based on nominal differences in the amino acid sequences. BoNT/A1 is the most well-characterized subtype of the BoNT/A serotype, and many of its properties, including its potency, its prevalence as a food poison, and its utility as a pharmaceutical, have been thoroughly studied. In contrast, much remains unknown of the other BoNT/A subtypes. In this study, BoNT/A subtype 1 (BoNT/A1) to BoNT/A5 were characterized utilizing a mouse bioassay, an in vitro cleavage assay, and several neuronal cell-based assays. The data indicate that BoNT/A1 to -5 have distinct in vitro and in vivo toxicological properties and that, unlike those for BoNT/A1, the neuronal and mouse results for BoNT/A2 to -5 do not correlate with their enzymatic activity. These results indicate that BoNT/A1 to -5 have distinct characteristics, which are of importance for a greater understanding of botulism and for pharmaceutical applications.

Moderate temperature fluctuations rapidly reduce the viability of Ralstonia solanacearum race 3, biovar 2, in infected geranium, tomato, and potato plants.

Most Ralstonia solanacearum strains are tropical plant pathogens, but race 3, biovar 2 (R3bv2), strains can cause bacterial wilt in temperate zones or tropical highlands where other strains cannot. R3bv2 is a quarantine pathogen in North America and Europe because of its potential to damage the potato industry in cooler climates. However, R3bv2 will not become established if it cannot survive temperate winters. Previous experiments showed that in water at 4°C, R3bv2 does not survive as long as native U.S. strains, but R3bv2 remains viable longer than U.S. strains in potato tubers at 4°C. To further investigate the effects of temperature on this high-concern pathogen, we assessed the ability of R3bv2 and a native U.S. strain to survive typical temperate winter temperature cycles of 2 days at 5°C followed by 2 days at -10°C. We measured pathogen survival in infected tomato and geranium plants, in infected potato tubers, and in sterile water. The population sizes of both strains declined rapidly under these conditions in all three plant hosts and in sterile water, and no culturable R. solanacearum cells were detected after five to seven temperature cycles in plant tissue. The fluctuations played a critical role in loss of bacterial viability, since at a constant temperature of -20°C, both strains could survive in infected geranium tissue for at least 6 months. These results suggest that even when sheltered in infected plant tissue, R3bv2 is unlikely to survive the temperature fluctuations typical of a northern temperate winter.

Ralstonia solanacearum Dps contributes to oxidative stress tolerance and to colonization of and virulence on tomato plants.

Ralstonia solanacearum, an economically important soilborne plant pathogen, infects host roots to cause bacterial wilt disease. However, little is known about this pathogen's behavior in the rhizosphere and early in pathogenesis. In response to root exudates from tomato, R. solanacearum strain UW551 upregulated a gene resembling Dps, a nonspecific DNA binding protein from starved cells that is critical for stress survival in other bacteria. An R. solanacearum dps mutant had increased hydrogen peroxide sensitivity and mutation rate under starvation. Furthermore, dps expression was positively regulated by the oxidative stress response regulator OxyR. These functional results are consistent with a Dps annotation. The dps mutant caused slightly delayed bacterial wilt disease in tomato after a naturalistic soil soak inoculation. However, the dps mutant had a more pronounced reduction in virulence when bacteria were inoculated directly into host stems, suggesting that Dps helps R. solanacearum adapt to conditions inside plants. Passage through a tomato plant conferred transient increased hydrogen peroxide tolerance on both wild-type and dps mutant strains, demonstrating that R. solanacearum acquires Dps-independent oxidative stress tolerance during adaptation to the host environment. The dps mutant strain was also reduced in adhesion to tomato roots and tomato stem colonization. These results indicate that Dps is important when cells are starved or in stationary phase and that Dps contributes quantitatively to host plant colonization and bacterial wilt virulence. They further suggest that R. solanacearum must overcome oxidative stress during the bacterial wilt disease cycle.

Botulinum Neurotoxins Can Enter Cultured Neurons Independent of Synaptic Vesicle Recycling.

Botulinum neurotoxins (BoNTs) are the causative agent of the severe and long-lasting disease botulism. At least seven different serotypes of BoNTs (denoted A-G) have been described. All BoNTs enter human or animal neuronal cells via receptor mediated endocytosis and cleave cytosolic SNARE proteins, resulting in a block of synaptic vesicle exocytosis, leading to the flaccid paralysis characteristic of botulism. Previous data have indicated that once a neuronal cell has been intoxicated by a BoNT, further entry of the same or other BoNTs is prevented due to disruption of synaptic vesicle recycling. However, it has also been shown that cultured neurons exposed to BoNT/A are still capable of taking up BoNT/E. In this report we show that in general BoNTs can enter cultured human or mouse neuronal cells that have previously been intoxicated with another BoNT serotype. Quantitative analysis of cell entry by assessing SNARE cleavage revealed none or only a minor difference in the efficiency of uptake of BoNTs into previously intoxicated neurons. Examination of the endocytic entry pathway by specific endocytosis inhibitors indicated that BoNTs are taken up by clathrin coated pits in both non pre-exposed and pre-exposed neurons. LDH release assays indicated that hiPSC derived neurons exposed consecutively to two different BoNT serotypes remained viable and healthy except in the case of BoNT/E or combinations of BoNT/E with BoNT/B, /D, or /F. Overall, our data indicate that previous intoxication of neuronal cells with BoNT does not inhibit further uptake of BoNTs.

Activity of botulinum neurotoxin type D (strain 1873) in human neurons.

Botulinum Neurotoxin type D (BoNT/D) causes periodic outbreaks of botulism in cattle and horses, but is rarely associated with human botulism. Previous studies have shown that humans responded poorly to peripheral injection of up to 10U of BoNT/D. Isolated human pyramidalis muscle preparations were resistant to BoNT/D, whereas isolated human intercostal muscle preparations responded to BoNT/D similarly as to other BoNT serotypes. In vitro data indicate that BoNT/D does not cleave human VAMP1 efficiently, and differential expression of the VAMP 1 and 2 isoforms may be responsible for the above observations. Here we examined sensitivity of cultured human neurons derived from human induced pluripotent stem cells to BoNT/D. Our data indicate that BoNT/D can enter and cleave VAMP 2 in human neurons, but at significantly lower efficiency than other BoNT serotypes. In addition, BoNT/D had a short duration of action in the cultured neurons, similar to that of BoNT/E. In vivo analyses indicated a slower time to death in mice, as well as a later onset and shorter duration of action than BoNT/A1. Finally, examination of BoNT/D activity in various rodent and human cell models resulted in dramatic differences in sensitivity, indicating a unique cell entry mechanism of BoNT/D.

Human Induced Pluripotent Stem Cell Derived Neuronal Cells Cultured on Chemically-Defined Hydrogels for Sensitive In Vitro Detection of Botulinum Neurotoxin.

Botulinum neurotoxin (BoNT) detection provides a useful model for validating cell-based neurotoxicity screening approaches, as sensitivity is dependent on functionally competent neurons and clear quantitative endpoints are available for correlating results to approved animal testing protocols. Here, human induced pluripotent stem cell (iPSC)-derived neuronal cells were cultured on chemically-defined poly(ethylene glycol) (PEG) hydrogels formed by "thiol-ene" photopolymerization and tested as a cell-based neurotoxicity assay by determining sensitivity to active BoNT/A1. BoNT/A1 sensitivity was comparable to the approved in vivo mouse bioassay for human iPSC-derived neurons and neural stem cells (iPSC-NSCs) cultured on PEG hydrogels or treated tissue culture polystyrene (TCP) surfaces. However, maximum sensitivity for BoNT detection was achieved two weeks earlier for iPSC-NSCs that were differentiated and matured on PEG hydrogels compared to TCP. Therefore, chemically-defined synthetic hydrogels offer benefits over standard platforms when optimizing culture conditions for cell-based screening and achieve sensitivities comparable to an approved animal testing protocol.

Analysis of gene expression in induced pluripotent stem cell-derived human neurons exposed to botulinum neurotoxin A subtype 1 and a type A atoxic derivative.

Botulinum neurotoxin type A1 (BoNT/A1) is a potent protein toxin responsible for the potentially fatal human illness botulism. Notwithstanding, the long-lasting flaccid muscle paralysis caused by BoNT/A has led to its utility as a powerful and versatile bio-pharmaceutical. The flaccid paralysis is due to specific cleavage of neuronal SNAREs by BoNTs. However, actions of BoNTs on intoxicated neurons besides the cleavage of SNAREs have not been studied in detail. In this study we investigated by microarray analysis the effects of BoNT/A and a catalytically inactive derivative (BoNT/A ad) on the transcriptome of human induced pluripotent stem cell (hiPSC)-derived neurons at 2 days and 2 weeks after exposure. While there were only minor changes in expression levels at 2 days post exposure, at 2 weeks post exposure 492 genes were differentially expressed more than 2-fold in BoNT/A1-exposed cells when compared to non-exposed populations, and 682 genes were differentially expressed in BoNT/A ad-exposed cells. The vast majority of genes were similarly regulated in BoNT/A1 and BoNT/A ad-exposed neurons, and the few genes differentially regulated between BoNT/A1 and BoNT/A ad-exposed neurons were differentially expressed less than 3.5 fold. These data indicate a similar response of neurons to BoNT/A1 and BoNT/A ad exposure. The most highly regulated genes in cells exposed to either BoNT/A1 or BoNT/A ad are involved in neurite outgrowth and calcium channel sensitization.