In vitro assembly of novel cholera toxin-like complexes

Piggybacking on the Cholera Toxin: Identification of a CTB-Binding Protein as an Approach for Targeted Delivery of Proteins to Motor Neurons

A significant unmet need exists for the delivery of biologic drugs such as polypeptides or nucleic acids to the central nervous system for the treatment and understanding of neurodegenerative diseases. Naturally occurring bacterial toxins have been considered as tools to meet this need. However, due to the complexity of tethering macromolecular drugs to toxins and the inherent dangers of working with large quantities of recombinant toxins, no such route has been successfully exploited. Developing a method where a bacterial toxin's nontoxic targeting subunit can be assembled with a drug immediately prior to in vivo administration has the potential to circumvent some of these issues. Using a phage-display screen, we identified two antibody mimetics, anticholera toxin Affimer (ACTA)-A2 and ACTA-C6 that noncovalently associate with the nonbinding face of the cholera toxin B-subunit. In a first step toward the development of a nonviral motor neuron drug-delivery vehicle, we show that Affimers can be selectively delivered to motor neurons in vivo.

Purification and characterization of Yersinia enterocolitica and Yersinia pestis LcrV-cholera toxin A(2)/B chimeras

Yersinia pestis is a virulent human pathogen and potential biological weapon. Despite a long history of research on this organism, there is no licensed vaccine to protect against pneumonic forms of Y. pestis disease. In the present study, plasmids were constructed to express cholera toxin A(2)/B chimeric molecules containing the LcrV protective antigen from Yersinia enterocolitica and Y. pestis. These chimeras were expressed and purified to high yields from the supernatant of transformed Escherichia coli. Western and GM(1) ELISA assays were used to characterize the composition, receptor-binding and relative stability of the LcrV-CTA(2)/B chimera in comparison to cholera toxin. In addition, we investigated the ability of the Y. pestis LcrV-CTA(2)/B chimera to bind to and internalize into cultured epithelial cells and macrophages by confocal microscopy. These studies indicate that the uptake and trafficking of the LcrV antigen from the chimera is comparable to the trafficking of native toxin. Together these findings report that stable, receptor-binding, non-toxic LcrV-cholera toxin A(2)/B chimeras can be expressed at high levels in E. coli and purified from the supernatant. In addition, the internalization of antigen in vitro reported here supports the development of these molecules as novel mucosal vaccine candidates.

Heteropentameric cholera toxin B subunit chimeric molecules genetically fused to a vaccine antigen induce systemic and mucosal immune responses: a potential new strategy to target recombinant vaccine antigens to mucosal immune systems

Noninvasive mucosal vaccines are attractive alternatives to parenteral vaccines. Although the conjugation of vaccine antigens with the B subunit of cholera toxin (CTB) is one of the most promising strategies for vaccine delivery to mucosal immune systems, the molecule cannot tolerate large-protein fusion, as it severely impairs pentamerization and loses affinity for GM1-ganglioside. Here we report a new strategy, in which steric hindrance between CTB-antigen fusion subunits is significantly reduced through the integration of unfused CTB "molecular buffers" into the pentamer unit, making them more efficiently self-assemble into biologically active pentamers. In addition, the chimeric protein took a compact configuration, becoming small enough to be secreted, and one-step affinity-purified proteins, when administered through a mucosal route, induced specific immune responses in mice. Since our results are not dependent on the use of a particular expression system or vaccine antigen, this strategy could be broadly applicable to bacterial enterotoxin-based vaccine design.

Characterization of fluorescent chimeras of cholera toxin and Escherichia coli heat-labile enterotoxins produced by use of the twin arginine translocation system

Cholera toxin (CT) is an AB(5) toxin responsible for the profuse secretory diarrhea resulting from Vibrio cholerae infection. CT consists of a pentameric, receptor-binding B subunit (CTB) and a monomeric A subunit (CTA) that has latent enzymatic activity. In addition to its enterotoxicity, CT has potent mucosal adjuvant activity and can also function as a carrier molecule with many potential applications in cell biology. In earlier studies, the toxic CTA(1) domain was replaced by several other antigenic protein domains to produce holotoxin-like chimeras for use as potential mucosal vaccines. In the present study we utilized the twin arginine translocation (tat) system to produce fluorescent CT chimeras, as well as fluorescent chimeras of Escherichia coli heat-labile toxins LTI and LTIIb. Fusion proteins containing either green fluorescent protein (GFP) or monomeric red fluorescent protein (mRFP) and the A(2) domain of CT, LTI, or LTIIb were transported to the periplasm of E. coli by the tat system, and the corresponding B polypeptides of CT, LTI, and LTIIb were transported to the periplasm by the sec system. The fluorescent fusion proteins were shown to assemble spontaneously and efficiently with the corresponding B polypeptides in the periplasm to form chimeric holotoxin-like molecules, and these chimeras bound to and entered cultured cells in a manner similar to native CT, LTI, or LTIIb. The GFP and mRFP derivatives of CT, LT, and LTIIb developed here are useful tools for studies on the cell biology of trafficking of the CT/LT family of bacterial enterotoxins. In addition, these constructs provide proof in principle for the development of novel chimeric CT-like or LT-like vaccine candidates containing CTA(2) fusion proteins that cannot be delivered to the periplasm of E. coli by use of the sec secretion pathway.

Cholera holotoxin assembly requires a hydrophobic domain at the A-B5 interface: mutational analysis and development of an in vitro assembly system

Cholera toxin (CT) and related Escherichia coli enterotoxins LTI and LTIIb have a conserved hydrophobic region at the AB(5) interface postulated to be important for toxin assembly. Hydrophobic residue F223 in the A subunit of CT (CTA) as well as residues 174, L77, and T78 in the B subunit of CT (CTB) were replaced individually with aspartic acid, and the resulting CTA and CTB variants were analyzed for their ability to assemble into holotoxin in vivo. CTA-F223D holotoxin exhibited decreased stability and toxicity and increased susceptibility to proteolysis by trypsin. CTB-L77D was unable to form functional pentamers. CTB-I74D and CTB-T78D formed pentamers that bound to GM(1) and D-galactose but failed to assemble with CTA to form holotoxin. In contrast, CTB-T78D and CTA-F223H interacted with each other to form a significant amount of holotoxin in vivo. Our findings support the importance of hydrophobic interactions between CTA and CTB in holotoxin assembly. We also developed an efficient method for assembly of CT in vitro, and we showed that CT assembled in vitro was comparable to wild-type CT in toxicity and antigenicity. CTB-I74D and CTB-T78D did not form pentamers or holotoxin in vitro, and CTA-F223D did not form holotoxin in vitro. The efficient system for in vitro assembly of CT described here should be useful for future studies on the development of drugs to inhibit CT assembly as well as the development of chimeric CT-like molecules as potential vaccine candidates.