Researchers at Washington University School of Medicine in St. Louis and the University of Pittsburgh Center for Vaccine Research and School of Public Health have discovered how the mosquito-borne virus responsible for Rift Valley fever gets inside cells, by taking advantage of a protein called Lrp1, which is normally involved in taking up low-density lipoproteins (LDL) from the blood. The discovery, the team suggested, could lead to therapies that prevent Rift Valley fever or reduce its impact by interfering with the ability of the virus to get into cells.
“This finding is the key to understanding how Rift Valley fever virus spreads not only throughout the human body but also how it is able to infect mosquitoes and different species of mammals, said Amy Hartman, PhD, an associate professor of infectious diseases & microbiology at the University of Pittsburgh. “Knowing how the virus spreads will help us develop targeted therapies, which currently do not exist for Rift Valley fever. This discovery opens up new opportunities to study virus-host interactions at the cellular and organismal level and enriches our understanding of the basic biology of mosquito-transmitted emerging viruses.” Hartman is co-senior author of the team’s published study, in Cell, which is titled “6Lrp1 is a host entry factor for Rift Valley fever virus.”
The mosquito-borne Rift Valley fever virus (RVFV) causes economically devastating outbreaks of hemorrhagic fever in livestock such as sheep, goats, and cattle. These outbreaks can also lead to infection in people, especially among those working with dead or dying animals, who are infected by mosquito bites. Outbreaks can sometimes cause hundreds of human cases and dozens of deaths. “RVFV is zoonotically transmitted from animals to people, and human infections can result in severe health consequences, including hepatitis, hemorrhagic fever, encephalitis, and retinal vasculitis,” the authors noted.
There is no specific treatment for RVFV, which to date has been limited to Africa and the Arabian Peninsula. But mosquitoes capable of transmitting the virus can be found all over the world, necessitating a need to understand and control the virus. The geographic range of RVFV includes most of Africa, Madagascar, and the Saudi Arabian peninsula,” the investigators noted. “Importantly, competent mosquito species are found in North America and Europe, and climate change is rapidly altering the natural habitat of RVFV-competent mosquito and reservoir species.” The World Health Organization has listed Rift Valley fever as a prioritized disease likely to cause epidemics in the near future.
“For people in areas where Rift Valley fever is endemic, an outbreak threatens not only their livelihood but their health,” said co-senior author Gaya K. Amarasinghe, PhD, a professor of pathology & immunology and of biochemistry & molecular biophysics at Washington University. “People have a 1% to 2% chance of death if they get infected with this virus, which doesn’t sound like much, but it’s about the same as COVID-19. The disease is much more severe in domesticated animals, especially young animals, which get very ill and die in large numbers. This virus has been flying under the radar, but given that it’s transmitted by mosquitoes that are found everywhere, it could spread into other parts of the world and become a serious issue.”
To find out how the virus invades cells, first author Safder Ganaie, PhD, a postdoctoral researcher who works with Amarasinghe, grew the virus in mouse cells, and carried out an unbiased, genome-wide CRISPR-Cas9 screen to identify which host factors that were either proviral or antiviral. By systematically disrupting normal mouse genes, Ganaie and colleagues found that the virus failed to infect mouse cells that lacked certain genes, notably the gene for LDL receptor-related protein 1 (Lrp1). Further experiments showed that the virus requires the LRP1 protein to infect mouse, hamster, cow, monkey, and human cells, indicating that the virus uses the same protein across distantly related species.
The finding constitutes an opportunity to explore potential therapeutic approaches. If the virus needs LRP1 to infect cells, then temporarily taking LRP1 out of commission may limit its ability to spread in the body, thereby reducing disease. The researchers tested this strategy using a protein, RAP, which attaches to LRP1 and so blocks anything else that tries to attach. “RAP is an important molecular chaperone of Lrp1 that universally inhibits ligand interaction to ensure passage of Lrp1 from the ER to the cell surface,” they explained.
The researchers infected a group of mice with the virus and simultaneously treated them with murine RAP (mRAP). A second group of mice also was infected but was left untreated for comparison. Most of the treated mice survived, while all of the untreated mice died. Further, the treated mice had lower levels of virus throughout their bodies on the third day after infection compared with the untreated mice. Mice treated using a mutant form of RAP that displays only reduced binding to LRP1 also succumbed to RFFV infection. “Important controls including a mutant mRAP that shows reduced Lrp1 binding were not able to rescue mice from lethal infection,” the authors noted. “These results provide further support for a role for Lrp1 as a major cellular factor required for RVFV infection in a rodent model.”
And while RAP itself is not a good prospect for drug development, because the protein plays a role in many important biological processes, the study findings nevertheless suggest that targeting LRP1 may lead to therapeutics for Rift Valley fever. “The discovery of Lrp1 as a major cellular factor for RVFV provides a framework to better understand the molecular basis for RVFV attachment and internalization,” the investigators further concluded. “Although the exact mechanism by which Lrp1 mediates RVFV entry requires further study, our findings provide a foundation for answering many open questions related to RVFV, including mechanisms associated with zoonotic transmission, tropism, spread, and pathogenesis. Knowledge gained from these studies positions us to explore Lrp1, a conserved cell-surface protein, as a target for prophylactic and therapeutic development of RVFV infections.”
The discovery that Rift Valley fever virus uses LRP1 to get inside cells is also interesting because the protein is better known for its role in cholesterol metabolism. It also is thought to play a role in Alzheimer’s disease and possibly in infections by the intestinal bacterium Clostridium difficile. It’s not clear why these disparate biological processes are linked, but Amarasinghe, Hartman, and their collaborators already have several projects underway to explore these connections.