A new Duke study suggests that typhoid fever invades human cells with the help of cholesterol.
After analyzing the genomes of cells that were particularly susceptible to typhoid infection, the team centered their research around the protein VAC14. They ultimately found that cells producing less VAC14 had more cholesterol in their cell membranes and were also more likely to become infected by invading bacteria.
“This genetic difference affects the level of VAC14 in your cell,” said Dennis Ko, assistant professor of molecular genetics and microbiology and senior author of the paper. “VAC14 is involved in lipid signaling, but we didn’t know how that connected to salmonella [bacteria] invasion.”
He explained that the first part of the experiment was dedicated to pinpointing which genes were associated with an increase in infection of Salmonella typhi, the bacteria that causes typhoid fever.
By exposing cells taken from hundreds of volunteers to the bacteria, the group determined which genes may make cells more conducive to bacterial invasion. Ko compared this work to a “needle-in-the-haystack scenario,” since there are many possible genes that could play a role in the infection process.
After isolating VAC14 as a significant gene for Salmonella infection, Monica Alvarez, a graduate student and lead author of the study, identified that cells producing less VAC14 had more bacteria attached to their membranes.
In other words, the team's findings showed that VAC14 was affecting the first step of bacterial invasion, as salmonella must first attach to the cell membrane and use a “molecular needle” to inject the viral molecules into the cell, Ko explained.
“That was important, because it told us that the initial binding was the step involved,” he said. “That’s when we remembered that salmonella had been previously shown to bind directly to cholesterol as part of this initial attachment process.”
To further test this theory, the researchers turned to Sarah Dunstan, who studies typhoid fever susceptibility in Vietnam. Compiling the DNA information for about 1,000 Vietnamese patients, Ko and the team again found that the VAC14 gene was tied to the risk of Salmonella infection.
After confirming VAC14's importance, the team next looked at how this process played out in a zebrafish model. They centered their tests around the hypothesis that decreasing the levels of cholesterol would lead to less Salmonella invasion.
“Zebrafish are a phenomenal animal model system because they are optically transparent making them ideal for imaging studies,” Alvarez wrote in an email.
The zebrafish were first treated with ezetimibe—a drug that lowers cholesterol levels—and then exposed to the typhoid fever bacteria. Alvarez found that the fish treated with the cholesterol-lowering drug had improved survival rates and also cleared the bacteria from their system with more efficiency.
Both Ko and Alvarez noted that their success in zebrafish may not be entirely predictive for humans but were optimistic about the initial results. They said they hoped to move to a different animal to show that the treatment may be more broadly useful.
“The reason we still have to be cautious is because it’s a zebrafish—not a person, not a mouse, it’s a zebrafish,” Ko said. “And we were delivering the bacteria through an injection, so people usually get salmonella infection because they eat something that’s tainted—so the route of delivery isn’t quite the same.”
He explained that the exact mechanism behind VAC14’s ability to affect the amount of cholesterol in the cell is still unknown. There is also a potential to apply these findings to other diseases and examine whether other pathogens are similarly affected by cholesterol.
“The ultimate goal in terms of a utility standpoint would be to be able to say, ‘yeah, we started these initial observations based on cell biology, and we were able to take it to animal models and then eventually, we wanted to see whether or not they had any usefulness in people,’” Ko said.
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