In the book The Hot Zone, author Richard Preston called viruses like Ebola “molecular sharks”—mindless attackers made of almost nothing. Ebola virus, which causes often-fatal hemorrhagic fevers, carries just seven genes, none of which can do much without first stealing the smarts inside a host cell.
Virologists in the lab of John Connor have recently discovered one previously unknown way that Ebola hijacks a cell’s machinery and uses it to replicate and spread. When the virus invades, it takes over a pathway that normally helps the host cell turn its own genes into proteins. The virus uses this pathway—in particular, an activated form of a protein called eIF5A—to turn its own viral genes into proteins. When those viral proteins accumulate, the virus is able to unleash the rest of its deadly viral proteins.
By figuring out how the virus hijacks a host cell, Connor and his team have found a new lead for the treatment of Ebola infections, though more research is required to pinpoint a treatment strategy. “My lab tries to shine light on underlying mechanisms,” says Connor, associate professor of microbiology at Boston University’s National Emerging Infectious Diseases Laboratories (NEIDL). “Once you understand the different parts of a cell the virus is using, you can come up with ways to stop it.”
The findings were published in the American Society for Microbiology’s mBio journal in July 2016. Funders of the research include the National Institutes of Health and the US Department of Health and Human Services.
The team began the project by working with a disabled form of Ebola called a minigenome system. This system, originally developed by BU School of Medicine associate professor and NEIDL microbiologist Elke Mühlberger, includes the four Ebola genes that get a foothold in the cell. A gene that codes for a luminescent marker that glows upon a chemical reaction has replaced the rest of the viral genes.
“It’s a little different from the virus, but it allows us to do the work at a Biosafety Level 2 laboratory,” says Michelle Olsen (GRS’15), a postdoctoral fellow in Connor’s lab and lead author on the study. Work on the actual Ebola virus can only be done in a lab designated by the Centers for Disease Control and Prevention as meeting the much stricter Biosafety Level 4 standards.
Olsen began by looking at how the Ebola minigenome depends on tiny molecules in host cells called polyamines, which help replicate other viruses. Using an inhibitor, she blocked the creation of the polyamine that activates eIF5A. This, in turn, blocked the Ebola minigenome system. The disabled virus wasn’t able to replicate its glowing stand-in for an infection.
Further testing with different inhibitors showed that specifically blocking the polyamine’s activation of eIF5A blocks viral replication by interfering with the creation of VP30, one of the four viral proteins required for the virus to replicate itself. “This data says that eIF5A functions somewhat like a gate,” says Connor. “If you inactivate eIF5A, that gate is closed and there isn’t enough VP30 created to drive the virus.”
It isn’t known yet exactly what causes VP30 to stop accumulating. So far, Olsen has shown that the viral gene for VP30 is still being transcribed into RNA instructions used to build the protein. But then the production line falters. “Somewhere along the line the protein is either not being produced or it’s disappearing because it’s not stable,” says Olsen.
Once the details are fleshed out, the finding could point to a clear therapeutic strategy. But before digging into those details, Connor, Olsen, and co-lead author Claire Marie Filone, a former postdoctoral fellow in Connor’s lab, wanted to make sure that their method for stopping infection using a minigenome system also worked on live Ebola virus.
They tapped Chad Mire and colleagues at the University of Texas Medical Branch, who run a Biosafety Level 4 lab and have access to Ebola. Mire and Filone tested one of the small molecule inhibitors used at BU, ciclopirox, against live Ebola virus and against the closely related Marburg virus, another “molecular shark” in the filovirus family that also causes deadly hemorrhagic fevers. The drug decreased viral infections in cultured cells by approximately 2.5 orders of magnitude in both viruses, a notable effect, according to Mire.
“This was a really interesting result because it worked against Ebola and against another very important virus called Marburg,” says Mire, who works with many labs to test antivirals against live virus. “What we have here is a positive first step toward a broad-spectrum filovirus antiviral.”
Connor and Olsen are already working toward the next step of understanding what is happening to VP30 when the eIF5A gate closes. “This pathway is important,” says Connor. “The question is why. Understanding that can be critical to knowing how to effectively target it to treat Ebola.”