Scientists reveal how deadly toxin hijacks cells

Scientists have pinpointed exactly how botulinum neurotoxin A – a potential agent of biological warfare and one of the most lethal toxins known to humans – is able to sneak into cells.

The finding is crucial for the development of new treatments against botulism, a paralytic illness caused by the toxin more commonly known as botox. As small amounts of botox are also known to alleviate many medical problems, the recent work could help to quell any risks associated with the toxin’s clinical use.

Writing in the current online edition of Science, a team of researchers at the UW–Madison and the University of Texas report that botox latches onto a protein known as SV2 to gain entry into neurons.

“Our work shows that botox is really smart and clever,” says senior author Edwin Chapman, a UW–Madison professor of physiology and an investigator of the Howard Hughes Medical Institute. “It uses SV2 to sneak into nerves like a Trojan horse.”

“Botulinum neurotoxins are among the six most dangerous bioterrorism threats,” adds lead author Min Dong, a UW–Madison postdoctoral researcher in the department of physiology. “Knowing the protein receptor for [botulinum toxins] can pave the way for developing anti-toxin reagents which may block the entry of toxins into cells.”

The botulinum toxins, of which there are seven types, are made by a bacterium commonly found in soil, known as Clostridium botulinum. Of the seven-identified by the letters A through G-botox A lasts a particularly long time in neurons. While that feature makes it especially useful in the clinic, it also means that botox A may pose a particularly dangerous threat as a biological weapon.

The toxin enters neurons by binding to nerve endings and preventing the release of crucial chemical messengers, known as neurotransmitters, that communicate with muscles. When enough nerve endings are invaded, botox can lead to paralysis and death.

By capitalizing on the ability of botox to act on a localized group of muscles, doctors have strategically used the toxin to treat an array of medical troubles, including migraine headaches, chronic inflammation and even stuttering. “I don’t think there’s a neuromuscular junction that hasn’t been inhibited by injecting with botox A,” says Chapman.

Chapman and his team located the exact molecular gateway through which botox penetrates cells by gathering clues from earlier research that pointed to the potential importance of tiny neural storage bins known as “synaptic vesicles.” Situated at nerve endings, synaptic vesicles continually work to store and release neurotransmitters.

Dozens of proteins, including SV2, work to ensure that vesicles function properly. With standard screening experiments known as “entry assays,” the scientists were able to zero in on SV2. To confirm that result, they acquired mice that were genetically engineered to carry reduced amounts of SV2. Without that protein around, the researchers found that botox was unable to wreak havoc.

Co-author Felix Yeh, a UW–Madison graduate student who works with Chapman, says that researchers have so far found three of the seven protein receptors that allow the different types of botulinum toxins into cells. “One goal at the Chapman laboratory is to identify the remaining receptors,” Yeh says.

Other participating co-authors included Eric Johnson, a UW–Madison professor of food microbiology and toxicology; William Tepp, a UW–Madison senior research specialist in the department of food microbiology and toxicology; Camin Dean, a UW–Madison postdoctoral fellow in physiology; and Roger Janz, a researcher with joint appointments at W.M. Keck Center for Learning and Memory and the department of neurobiology and anatomy at the University of Texas-Houston Medical School.