scientists have identified a protein that allows the hepatitis C virus to enter mouse cells. Previously, researchers had figured out that three proteins—CD81, CLDN1, and SR-BI—have key roles in shuttling the virus into cells. At least one factor, however, was still missing, because when mouse cells were engineered to overexpress all three proteins, the cells still denied the virus entry.
Using a genetic screen, the team identified a human protein called occludin that makes mouse cells susceptible to the virus. Thus far scientists have been unable to develop a useful small animal model for the disease largely due to its inability to infect animals other than humans and chimpanzees.
Since the hepatitis C virus exclusively targets human liver cells, suggesting that these cells express genes that allow uptake of the virus, the team first cloned all the genes that were expressed in liver cells and then delivered them to mouse cells during their DNA screen. “Then, going through an iterative screening process, we honed in on the genes that made the mouse cells permissive,” says Alexander Ploss, a research associate.
To further establish occludin's role as a required entry factor, the group showed that human liver cells naturally express high levels of occludin and that by silencing its expression, they could give these once highly susceptible liver cells the ability to completely block infection.
Because mice and humans each have a species-specific version of the four factors, the group used hamster cells to see which combination of factors best infected cells. The researchers found that in the case of two of the proteins, occludin and CD81, only the human versions worked. For SR-BI and CLDN1, the human and mouse versions did an equally good job. This suggests that there may be more than one potential animal model and also that there are several specific combinations of entry factors that could generate them.
The work is published in the January 29 advance online issue of Nature.