CARNEGIE MELLON (US) — An element commonly found in nature might be a way to neutralize the potentially lethal effects of a compound known as Shiga toxin..
Published in the journal Science, a new study shows how manganese completely protects against Shiga toxicosis in animal models. Produced by certain bacteria, including Shigella and some strains of E. coli, Shiga toxin can cause symptoms ranging from mild intestinal disease to kidney failure.
The new findings could pave the way for future research aimed at creating an inexpensive treatment for infections caused by bacteria that produce the lethal Shiga toxin. Currently there is no treatment for the infection that afflicts more than 150 million people each year, resulting in more than one million deaths worldwide.
Produced by certain bacteria, including Shigella and some strains of E. coli, Shiga toxin causes infections that are common in developing countries where they cause waterborne epidemics. The infections can be particularly deadly, especially in children, causing dysentery and severe hemorrhagic diarrhea, which cannot be adequately treated in areas without access to clean water.
In developed countries, Shiga toxicosis is most common during foodborne outbreaks — like the widespread E. coli outbreak this past summer in Germany and Western Europe, where more than 3,700 people were infected and 45 died.
After entering the body, Shiga toxin is secreted by the infecting bacteria. The toxin then attaches itself to a surface receptor on a cell’s plasma membrane and enters the cell through endocytosis. Normally, when a harmful substance enters a cell in this way, it’s wrapped in a package called an endosome and sent directly to the cell’s lysosome where it is degraded and discarded.
“That’s exactly the process that Shiga toxin avoids. It would be neutralized if it were to get degraded, so it had to find some way to get out of being sent to the lysosome,” says Adam Linstedt, professor of biological sciences at Carnegie Mellon University.
A phone call and a protein
Linstedt and colleague Somshuvra Mukhopadhyay discovered exactly how Shiga toxin avoided the lysosome as they were doing basic biological research to understand how components of the cell function. “If we weren’t focused on answering fundamental biological questions, we wouldn’t have made this discovery,” Linstedt says.
Fifteen years ago Linstedt discovered GPP130, a protein found in the Golgi apparatus, a kind of post office for the cell that sorts and packages molecules made in the endoplasmic reticulum and delivers them to their final destinations within the cell.
Linstedt found that GPP13 didn’t behave like most Golgi proteins. Rather than remaining in the Golgi, GPP130 constantly cycles to the endosomes and back to the Golgi. As it returns, it avoids the pathway that takes a substance to the lysosome to be degraded.
In the new study, Linstedt and Mukhopadhyay report that Shiga toxin exploits this unique quality of GPP130 to its advantage. As it starts its return to the Golgi, GPP130 moves into a tubular extension off of the endosome’s membrane and avoids being sent to the degradation pathway.
Shiga toxin binds to GPP130, hitching a ride on a route that doesn’t go to the lysosome. Instead, the toxin is carried to the Golgi apparatus and then to the endoplasmic reticulum, where it gets released into the cell’s cytoplasm. It’s here that it does its damage, shutting down protein synthesis and causing the cell to die.
Because Shiga toxin was dependent on GPP130 and manganese caused loss of GPP130, Linstedt and Mukhopadhyay decided to see whether manganese would protect against Shiga toxin infection.
In cell cultures, manganese treatment yielded an almost 4,000-fold increase in the amount of Shiga toxin required to induce cell death. In a mouse model, mice exposed to a high dose of Shiga toxin and treated with non-toxic doses of manganese were 100 percent resistant to the toxin.
By introducing manganese, Linstedt and Mukhopadhyay were able to remove Shiga toxin’s vehicle for avoiding degradation— GPP130. This could be a promising treatment for neutralizing the effects of Shiga toxin in humans, the researchers say.
“Manganese is inexpensive. While Shiga toxin infection affects people in the developed world, it affects far more people in the developing world. An inexpensive, accessible treatment—not a designer drug—is the ideal solution,” Linstedt says.
“We know the toxicity levels of manganese in humans; we know ways to administer it. While further testing is needed to determine if manganese is a suitable treatment for humans, I’m optimistic that trials should move forward quickly.”
Manganese might also be able to be used in conjunction with antibiotics. Currently, if an infected person is given an antibiotic, the antibiotic kills the bacteria (E. coli or Shigella) that produce the toxin. This releases the toxin in larger amounts and causes a higher percentage of patients to die. Linstedt believes manganese can be used to block the toxin and an antibiotic to kill the bacteria, making for an extremely effective therapy.
The next step is to study situations that more closely mimic Shiga toxin infection in humans, and by testing their antibiotic/manganese combination therapy in mouse models., paying attention to manganese toxicity.
High doses of manganese can have severe neurotoxic effects. The amounts used in the current study were low enough that they didn’t cause any side effects in the mouse models.
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