Researchers from the University of Pennsylvania School of Medicine have found that inhibitors of an enzyme called cathepsin L prevent the SARS (severe acute respiratory syndrome) virus from entering target cells. SARS is caused by an emergent coronavirus. There is no effective treatment at this time.
This study also demonstrates a new mechanism for how viral proteins are activated within host cells, states senior author Paul Bates, PhD, an Associate Professor in the Department of Microbiology. Bates and first author Graham Simmons, PhD, Research Associate, also in the Department of Microbiology, published their findings in the early August issue of the Proceedings of the National Academy of Sciences.
To gain entry, a virus binds to receptors on the surface of the host cell, and is taken up into a vesicle, or sphere, inside the cell. Unlike most known viruses, the SARS coronavirus (like the Ebola virus) needs one more step to infect the cell. The proteins within the membrane of both SARS and Ebola need to be cut by special cellular enzymes (cathepsins) in order to replicate within the host cell. Cathepsins act in the low pH (acidic) environment inside the vesicle, facilitating fusion of the viral membrane and the vesicle membrane, so that viral proteins and nucleic acids can enter the cell where viral replication occurs.
"This paper changes the thinking of the field," says Bates. "Up to this point, everyone thought all of the activation steps were at the cell surface or due to the low pH environment in the vesicle. Our paper shows that it's not just low pH, but the cathepsin proteases in the vesicles that clip the viral protein. This gives us a new target to address in the development of therapeutics against the SARS virus."
The researchers found that several chemical inhibitors of cathepsin activity blocked infection of human cell lines by the SARS virus, which were grown in a high-level safety laboratory. In general, these findings, say the researchers, have led to a better understanding that the cutting of viral protein by cathepsins is necessary for infectivity and is likely not unique because both the SARS and Ebola viruses are now known to use a similar mechanism to invade their host cells. (In June 2005, a group from Harvard School of Medicine discovered that the Ebola viral membrane protein is similarly activated by cathepsin L and B.)
If these proteases are important for other viruses, they represent a new way to stop viral infection. SARS and Ebola are the first examples of the need for these proteins to be cleaved during infection of the host cell.
This work is a joint collaboration between the Bates lab and the research group led by Scott L. Diamond, PhD, Director of the Penn Center for Molecular Discovery, one of nine facilities that the National Institutes of Health (NIH) is establishing as part of the Molecular Library Screening Center Network. Diamond is also Professor of Chemical and Biomolecular Engineering within the Institute for Medicine and Engineering at Penn. While independently screening for inhibitors, Diamond's lab found a cathepsin L inhibitor called MDL28170, which Bates and Simmons tested for efficacy in inhibiting SARS coronavirus infection. The cellular cathepsin enzymes have many other roles within the body, including mediating the inflammatory immune response in the lungs and antigen processing in T cells.
The Bates research group, in collaboration with the Diamond group, has identified a few compounds, including MDL28170, which they plan to test in animals for SARS inhibition. "We're now searching for other viruses that also use this cleaving mechanism for activating their proteins," says Bates. "If there are a number of other viruses that do that, and we have some preliminary evidence to suggest this, then we can develop small molecule inhibitors as possible therapeutics." One advantage of this approach is that oral medications made from small-molecule inhibitors are more readily made and distributed in the developing world-as opposed to a vaccine, suggests Bates. Protease inhibitors active against cathepsins have been tested in mice with no ill side effects, which bodes well for their eventual testing in humans.