Research on key host pathways has implications for Ebola and beyond

Mortality rates from Ebola outbreaks can be as high as 90%, according to the Centers for Disease Control and Prevention, and 55 people died in the most recent outbreak in Uganda in 2022. The virus continues to evolve, but currently approved vaccines and therapeutics remain limited. And Ronald N. Harty, professor of pathobiology and microbiology at the University of Pennsylvania School of Veterinary Medicine,and Jingjing Liang, a research associate in the Harty Lab, still have a lot of questions.

How would currently approved vaccines fare if the virus were to spread widely? Why, in central Africa where Ebola outbreaks occur, is the virus able to persist in fruit bats as a natural reservoir, instead of killing them like it kills humans? How is the virus able to sometimes stay dormant in different types of tissues for weeks, months, or even years before it starts replicating again, only then causing symptoms and potentially death?

Harty has been working on Ebola for more than two decades. In trying to better understand virus-host interactions to identify better treatments, he has focused on VP40, the viral matrix protein that plays a critical role in how the virus escapes from a cell. His lab has looked at key pieces of this protein, including the L domain, a short stretch of amino acids that hijacks host proteins the Ebola virus uses to exit the cell.

By looking at host proteins that bind specifically to the Ebola L domain, Harty, Liang, and researchers from the Biosafety Level 4 lab at Texas Biomedical Research Institute-Olena Shtanko, Marija A. Djurkovic, and Carson G. Leavitt-found a connection between the Hippo pathway, a pivotal signaling pathway that controls cell proliferation and movement, and transcription and egress of Ebola virus. Their findings are published in Nature Communications.

"It's a novel finding and the first indication that the Hippo pathway and Ebola are intersecting," Harty says. "We think it can have a major impact for the field, and our findings open up many new areas of investigation for Ebola virus pathogenesis and biology."

Specifically, the study shows that the proteinsLATS1, LATS2, and YAP, key components of the Hippo pathway, affect multiple stages of the Ebola virus life cycle. YAP is a downstream protein in the pathway that regulates transcription of other genes in the cell and LATS1 and LATS2 are upstream enzymes that regulate YAP localization and activity, Harty and Liang explain.

The researchers found that when LATS1/2 phosphorylates YAP, or attaches a phosphate group, YAP stays in the cytoplasm of a cell and sequesters VP40, preventing egress of the Ebola virus from the cell. But when YAP is unphosphorylated, YAP travels to the nucleus and promotes virus egress. In these two scenarios, the Hippo pathway is respectively "on" and "off."

Liang, the first and lead author on the paper, says the Hippo pathway could also be a good direction of research for virologists studying viruses other than Ebola. Liang notes that other recent studies have found a Hippo pathway connection with Zika and SARS-CoV-2, the novel coronavirus that led to the COVID-19 pandemic. "We think it could be sort of a pioneering paper in that regard," Harty says.

Liang says this paper builds on a study the same group published last year in Proceedings of the National Academy of Sciences, showing that the Ebola virus also exploits mTORC1, another key pathway. Now that researchers know these two cellular pathways are important for viral spread, Liang says she wants to tackle further unanswered questions: How does the virus exploit these two pathways, and why these two in particular?

"If we know how the virus can exploit these two pathways, we might find a target or a strategy to interfere with the virus-host interaction," Liang says. "If we have some compound or chemical or drugs to target this interaction, we might earn more time for our immune system to fight back."

Harty says the two approved vaccines are only effective against the ZEBOV strain of the virus, but other strains can cause severe disease. The few therapeutics that are approved are all monoclonal antibodies that are designed to bind to the surface protein of Ebola and prevent entry into cells. Many researchers are trying to develop treatments that target not only entry, but also additional stages in the virus lifecycle.

Harty's group is working on inhibitors that block budding, the last stage of the replication cycle. This approach, which he says is similar in concept to that of some approved influenza drugs, would dampen the ability of the virus to spread and give a person's immune system more time to recognize and clear the virus. Intervir, a company he founded with Penn Vet professor Bruce Freedman, is working is collaboration with Texas Biomedical Research Instituteand Fox Chase Therapeutics Discovery Inc. to develop these inhibitors.

"What I would envision is a drug cocktail that could not only target the entry step, but also maybe a budding inhibitor like ours that targets the last step of replication, and maybe there's a third one that targets a middle step in the lifecycle, like gene expression," Harty says. "Now you've got a cocktail that gives a one-two-three punch."

Ronald N. Harty is professor of pathobiology and microbiology at the University of Pennsylvania School of Veterinary Medicine.

Jingjing Liang is a research associate at Penn Vet.

The other co-authors are Marija A. Djurkovic, Carson G. Leavitt, and Olena Shtanko of Texas Biomedical Research Institute.

This research was supported by the National Institutes of Health (grants AI138052, AI139392, AI153815, AI154336, and AI1517170).