ÃÈÃÃÉçÇø

A new study reveals the biological secret to the Zika virus's infectious success: Zika uses host cells' own "self-care" system of clearing away useless molecules to suppress the host proteins that the virus has employed to get into those cells in the first place.

While these cell surface proteins are valuable for viral entry, they also have roles in producing an antiviral response. Before that can happen, the virus manipulates a process cells use to keep themselves healthy to lower the proteins' activity, clearing the way for unfettered viral infection.

Though other viruses, such as HIV, are known to silence host receptors that let them into cells, Zika is unusual for having at least three of its own proteins that can get the job done, said Shan-Lu Liu, senior author of the study and a virology professor in the Department of Veterinary Biosciences at The Ohio State University.

"That's the most interesting part: It's amazing that not only one, but several Zika proteins can do this," said Liu, also a professor in the Department of Microbial Infection and Immunity. "We looked at two Zika virus strains and examined three physiologically relevant cell types. With both strains, we could see the downregulation in all three cell types. It looks like this is an important mechanism."

The study was in Proceedings of the National Academy of Sciences.

The Zika virus, transmitted to humans primarily by Aedes aegypti mosquitoes, has caused infectious outbreaks in Africa, the Americas, Asia and the Pacific since 2007, according to the World Health Organization. Though cases have declined globally since 2017, virus transmission continues at low levels in the Americas and other endemic regions.

A large epidemic in Brazil in 2015 led to confirmation of a link between Zika infection during pregnancy and babies born with congenital problems including microcephaly, or smaller than normal head size. While most infected people develop no or only mild symptoms, the virus is also associated with Guillain-Barré syndrome, neuropathy and myelitis (spinal cord inflammation) in adults and older children.

Previous research has shown that specific cell surface proteins known as PS receptors are important entry points for many viruses, including Zika. This study focused on two of these proteins, known as AXL and TIM-1, that had previously been linked to Zika infection. In this work, Liu and colleagues set out to explain how Zika sustains infection after gaining entry through AXL and TIM-1.

The team completed cell culture experiments using African and Asian strains of Zika virus in three types of cells related to respiratory, reproductive and neurological systems targeted by the pathogen: human cells that line the lungs, embryo-supporting cells called trophoblasts and glioblastoma brain cancer cells.

Experiments showed that both AXL and TIM-1 were downregulated on the three types of cells after infection by Zika. The researchers expected to find this suppression occurred through two common protein degradation processes, but found instead that the Zika virus makes use of a cellular self-preservation routine: autophagy.

"Autophagy is a fundamental physiological mechanism to conserve cellular processes by degrading host components. It's also called self-eating—the host needs to remove their own damaged organelles or misfolded proteins because they're not good for the host," said Liu, also associate director of Ohio State's Center for Retrovirus Research and a program co-director of the Viruses and Emerging Pathogens Program in Ohio State's Infectious Diseases Institute.

In this case, the virus's infectious process manipulated the host cells into suppressing their own protective proteins—a viral adaptive tactic that allows Zika to control its own destiny.

Without this suppression, AXL and TIM-1 would begin producing inflammatory molecules as part of an antiviral response. Their normal level of facilitating viral entry could also enable more Zika particles to access already-infected cells, setting up a competitive scenario called a superinfection—something viruses want to avoid because the overcrowding threatens to kill cells, which kills infecting pathogens.

Further experiments identified three Zika proteins that prompt host cell autophagy, all of which are located on the virus's membrane.

"Normally those proteins mediate viral entry or are involved in viral replication, but they're also responsible for this downregulation—kind of a new function, which is not so surprising, because viruses encode something that's important for them, either for their own replication or to modulate the host," Liu said.

Though further research is needed to know for sure, there is a chance this mechanism is relevant to the Ebola virus, which uses the TIM-1 protein to access host cells, or to other pathogens in the same flavivirus family as Zika, including West Nile, yellow fever and dengue viruses.

"The bottom line is that this speaks to the co-evolution of viral-host interactions. The more important a host factor is to a virus, the more a virus is going to do to take control of it," Liu said. "Understanding these mechanisms is an important part of being prepared for emerging or reemerging viruses that cause infectious diseases."

This work was primarily conducted by Jingyou Yu, a former graduate student in the Liu lab and now a principal investigator at the Guangzhou National Laboratory in China. Additional contributions were made by Yi-Min Zheng, a senior scientist, and Pei Li, a postdoctoral fellow in the Liu lab. Additional co-authors are Megan Sheridan, Toshihiko Ezashi and R. Michael Roberts of the University of Missouri.