Phenomenal Russian launch leads fairy-tale international start for Shrek The Third

A toxin is defined as a poison that is of animal, plant or microbial origin causing disease when present even in low concentrations. Animal toxins are generally for predation (snake and spider venoms) and defense (bee and wasp stings). Examples of plant toxins are ricin and RCA (Ricinus communis agglutinin) from castor bean extracts (one seed can be fatal to a child). And then there are bacterial toxins that help these microorganisms colonize their hosts. This article will introduce the bacterial protein toxins and highlight their importance in biology and the medical sciences.

I have been studying the vacuolating toxin (VacA) since my graduate work in the Institute of Tropical Medicine in Nagasaki University. The causative organism for most gastric ulcers, Helicobacter pylori, produces it.  VacA is a large molecule that induces large vacuole formation in the eukaryotic cells, eventually leading to cell death. Thus, VacA is very cytotoxic to mammalian cells, but they do have their uses in biomedical science. In fact, with our discovery of a potential receptor candidate for the toxin in the laboratory, VacA is now a very valuable tool in studying cell vacuole formation, cell signaling from membrane receptors and pathways leading to programmed cell death (apoptosis). It is noteworthy that only the purified native form of the toxin has the ability to induce vacuole formation. Unlike other more common bacterial toxins, the recombinant or cloned VacA is inactive.

The most frequently used example of a bacterial toxin of medical importance is the potent neurotoxin produced by Clostridium botulinum: botulinum toxin or more commonly known by its drug brand name: “botox.” Ingestion of the toxin from contaminated food (food-borne botulism), infection of open wounds by the bacteria (wound botulism) and ingestion of bacterial spores by infants (infantile botulism) are considered medical emergencies because it can rapidly lead to paralysis and death. Nowadays, because of its ability to cause muscle paralysis, very tiny amounts are being used by dermatologists and plastic surgeons to make skin wrinkles disappear. Expanding this “botox effect,” it is now being used for a diverse number of neuromuscular disorders like crossed eyes (strabismus), involuntary muscular spasm of the neck (cervical dystonia), writer’s cramp, facial spasm, tremors, lower back pain, epilepsy, tennis elbow and even excessive underarm sweating. One area that is important to Filipino patients is its use to alleviate the debilitating involuntary muscular spasms of lubag (X-linked dystonia-Parkinsonism or XDP), a hereditary lethal neuromuscular disorder endemic on Panay Island, specifically in Capiz province and its environs.

The tetanus toxin is another example of a neurotoxin. Infection of wounds by Clostridium tetani is the principal cause of tetanus (lockjaw).  Its action is the direct opposite of botulinum toxin: instead of flaccid paralysis, it causes spasms of the muscle due to excessive firing of signals from the neurons (spastic paralysis). Both the tetanus and botulinum toxins have contributed to our understanding of how our nervous system functions. The tetanus toxin itself is used as an ingredient in vaccines to prevent tetanus.

Toxins are also defined and grouped according to their protein structure. The following are members of a family of toxins called A/B toxins which are composed of two distinct protein molecules (A and B) with the former serving as the vehicle for cell entry (by binding to its receptor) and the latter bringing about the biochemical event leading to the disease state: cholera toxin, Shiga toxin (Stx), E. coli heat labile (LT) and heat stable (ST) toxins, diptheria toxin, pertussis toxin, Pseudomonas exotoxin A, and anthrax toxin.

The cholera toxin is responsible for the excessive watery diarrhea (“rice water stools”) in cholera, which is brought about by consumption of food or water contaminated by Vibrio cholerae. Other toxins that attack the gastrointestinal system, giving rise to some type of diarrhea, are the Shiga toxin (Stx) that is secreted by Shigella dysenteriae and the enterohemorrhagic E. coli (EHEC), and enterotoxigenic E. coli  (ETEC) LT and ST toxins. Patients infected with Shigella dysenteriae (shigellosis) exhibit fever, abdominal cramps, and bloody diarrhea due to the effects of the Shiga toxin. Although symptoms could resolve after five days in adults, infants and toddlers can be severely affected so that they need hospitalization. EHEC with the specific serotype 0157:H7 is the main culprit in numerous fatalities involving food poisoning cases in the US and Japan because it can lead to severe renal failure (hemolytic-uremic syndrome). ETEC infection is the most common cause of diarrhea cases for tourists visiting Third World countries (travelers’ diarrhea or TD).

The diptheria (Corynebacterium diphtheriae) and pertussis (Bordetella pertussis) toxins produce extensive respiratory mucosal cell death in diptheria and whooping cough. Another toxin that is produced by a respiratory pathogen is the Pseudomonas exotoxin A. Pseudomonas aeruginosa infection can lead to severe pneumonia. Biochemical studies on these toxins by virtue of their common structure has lead to our understanding of how cellular signals are transmitted from the plasma membrane to the cytoplasm (the role of secondary messengers like adenyl cyclase in the activation of cAMP). And because of their ability to induce a strong immune response, members of the A/B family of toxins (whole or fragments) are incorporated in vaccine preparations to prevent the diseases that they cause, e.g., DPT vaccine (composed of detoxified diphtheria toxin, pertussis antigens and toxoid and tetanus toxin). Because of their ability to stimulate a strong immune response (adjuvant property), the cholera toxin and E. coli LT are being incorporated in experimental vaccines for flu, HIV and even H. pylori.

The Pseudomonas toxin and Shiga toxins (Stx) have been used for cancer immunotherapy. They are used as “missiles” (or “magic bullets”) that target cancer cells for destruction. By targeting specific receptors found in cancer cells, they bring about cytotoxicity. These toxins are usually attached to radioactive materials and antibodies that further enhance their lethality. Clinical trials are now ongoing to test how effective these are for leukemia and other related cancers. 

The most well known bacterial toxin in this post-9/11 world is the anthrax toxin because of its use for bio-terrorism. Anthrax can cause skin and pulmonary anthrax with the latter being almost fatal. A few weeks after hijackers crashed two commercial airplanes into the World Trade Center buildings in New York, letters with white powder were mailed to several TV and print media headquarters and the US Senate. The white powder turned out to be a weaponized form of the anthrax spores. Once inhaled, the bacteria multiply rapidly producing their lethal toxin. Five people died in connection with these letters, including two postal workers, with another 17 people injured seriously. Interestingly, in contrast to the other members of the A/B toxin family, the anthrax toxin has three components: Factor I (edema factor), Factor II (protective antigen), Factor III (lethal factor). Each produces an adverse reaction to the host cell but is not deadly. All three factors combine to produce toxicity by attacking the lungs and white blood cells (lymphocytes). The anthrax toxin is currently being used as a tool to study the relationship between edema and cAMP, and its effect in the immune system, i.e., macrophage phagocytosis, contributing to our knowledge of how our immune system is regulated when a particular pathogen invades our bodies.

These microbes and their associated toxins have evolved to adapt in disrupting numerous biochemical pathways in eukaryotic cells. Despite the fact that these toxins are harmful to human beings, they have been used and exploited as probes in biomedical research to further our understanding of not only basic cell biology and microbial pathogenesis, but also to find and develop promising treatments for numerous medical maladies.

* * *

Dr. Philip Ian Padilla is a graduate of UP College of Medicine, Class 1992. He finished his Ph.D. in Medical Science (Bacteriology) at the Institute of Tropical Medicine, Nagasaki University. After leaving Japan, he did a postdoctoral research fellowship in cell biology at the National Heart, Lung and Blood Institute (NHLBI), National Institutes of Health (NIH) in Bethesda, Maryland. He is currently on study leave as an associate professor at the University of the Philippines in the Visayas (UPV), Miag-ao, Iloilo in order to finish his experiments at NHLBI. E-mail him at [email protected] or [email protected].