Cholera toxin: A paradigm of a multifunctional protein.

Cholera toxin (CT) was discovered exactly half a century ago by S.N. De. We have come a long way since this epoch-making discovery. Retrospectively, science had to wait a long time since Koch’s prediction of the existence of a toxin, and its actual discovery by De. CT is not just another enterotoxin that causes the signs and symptoms of the dreaded disease, cholera. It is unique in many respects, starting from its structure to its functions. CT is a multifunctional protein that is capable of influencing the immune system in many ways. It not only has remarkable adjuvant properties, but also acts as an anti-inflammatory agent, by modulating specific signal transduction pathways. Its immunomodulatory properties can be harnessed for treatment of various autoimmune disorders, and have shown great promise in the area of immunotherapeutics. CT can truly be considered as a paradigm of a multifunctional protein.

Cholera toxin – A foe & a friend.

After De’s pivotal demonstration in 1959 of a diarrhoeogenic exo-enterotoxin in cell-free culture filtrates from Vibrio cholerae (of classical biotype), much insight has been gained about cholera toxin (CT), which is arguably now the best known of all microbial toxins. The subunit structure and function of CT, its receptor (the GM1 ganglioside), and its effects on the cyclic AMP system and on intestinal secretion were defined in the 1970s, and the essential aspects of the genetic organization in the 1980s. Recent findings have generated additional perspectives. The 3D-crystal structure of CT has been established, the CT-encoding operon has been shown to be carried by a non-lytic bacteriophage, and in depth knowledge has been gained on how the bacterium controls CT gene expression in response to cell density and various environmental signals. The mode of entry into target cells and the intracellular transport of CT are becoming clearer. CT has become the prototype enterotoxin and a widely used tool for elucidating important aspects of cell biology and physiology, e.g., cell membrane receptors, the cyclic AMP system, G proteins, as well as normal and pathological ion transport mechanisms. In immunology, CT has emerged as a potent, widely used experimental adjuvant, and the strong oral-mucosal immunogenicity of the non-toxic B-subunit (CTB) has led to the use of CTB as a protective antigen together with killed vibrios in a widely licensed oral cholera vaccine. CTB has also been shown to promote immunological tolerance against certain types of mucosally co-administered antigens, preferably tissue antigens linked to the CTB molecule; this has stimulated research and development to use CTB in this context for treatment of autoimmune and allergic diseases. In summary, in the 50 years after De’s discovery of CT, this molecule has emerged from being the cholera patient’s “foe” to also becoming a highly useful scientist’s “friend”.

Generation of transgenic corn-derived Actinobacillus pleuropneumoniae ApxIIA fused with the cholera toxin B subunit as a vaccine candidate

Corn, one of the most important forage crops worldwide, has proven to be a useful expression vehicle due to the availability of established transformation procedures for this well-studied plant. The exotoxin Apx, a major virulence factor, is recognized as a common antigen of Actinobacillus (A.) pleuropneumoniae, the causative agent of porcine pleuropneumonia. In this study, a cholera toxin B (CTB)-ApxIIA#5 fusion protein and full-size ApxIIA expressed in corn seed, as a subunit vaccine candidate, were observed to induce Apx-specific immune responses in mice. These results suggest that transgenic corn-derived ApxIIA and CTB-ApxIIA#5 proteins are potential vaccine candidates against A. pleuropneumoniae infection.

Mechanism of action of cholera toxin & other toxins.

Vibrio cholerae produce a variety of extracellular products that have deleterious effects on eukaryotic cells. The massive diarrhoea produced by V. cholerae is caused by cholera toxin (CT). CT is composed of 1A and 5B units. CT causes a significant amount of fluid secretion and haemorrhage in the ligated rabbit ileal loops. Its action involves the role of various biochemical pathways. CT acts by activation of adenylate cyclase-cAMP system located at the basolateral membrane of intestinal epithelial cells. The increase in cyclic AMP levels is mainly responsible for the altered transport of Na+ and Cl-. Besides activating cAMP, CT is also known to act through release of prostaglandins and involvement of intramural nerves. Besides CT, other bacterial toxins like Escherichia coli LT, Salmonella toxin, Shigella toxin and Campylobacter toxin also possess A-B structure. The structure and function of E. coli LT resembles closely that of CT. Most of the bacterial toxins exert their effect through involvement of ADP-ribosylating proteins whereas other toxins involve guanylate cyclase system, calcium and protein kinases for their ultimate action.