Penn Researchers Discover Key To How SARS Virus Infects Cells

This study alsodemonstrates a new mechanism for how viral proteins are activatedwithin host cells, states senior author Paul Bates, PhD, an AssociateProfessor in the Department of Microbiology. Bates and first authorGraham Simmons, PhD, Research Associate, also in the Department ofMicrobiology, published their findings in the early August issue of theProceedings of the National Academy of Sciences.

To gain entry, avirus binds to receptors on the surface of the host cell, and is takenup into a vesicle, or sphere, inside the cell. (Click on thumbnail toview full-size images). Unlike most known viruses, the SARS coronavirus(like the Ebola virus) needs one more step to infect the cell. Theproteins within the membrane of both SARS and Ebola need to be cut byspecial cellular enzymes (cathepsins) in order to replicate within thehost cell. Cathepsins act in the low pH (acidic) environment inside thevesicle, facilitating fusion of the viral membrane and the vesiclemembrane, so that viral proteins and nucleic acids can enter the cellwhere viral replication occurs.

“This paper changes the thinkingof the field,” says Bates. “Up to this point, everyone thought all ofthe activation steps were at the cell surface or due to the low pHenvironment in the vesicle. Our paper shows that it’s not just low pH,but the cathepsin proteases in the vesicles that clip the viralprotein. This gives us a new target to address in the development oftherapeutics against the SARS virus.”

The researchers found thatseveral chemical inhibitors of cathepsin activity blocked infection ofhuman cell lines by the SARS virus, which were grown in a high-levelsafety laboratory. In general, these findings, say the researchers,have led to a better understanding that the cutting of viral protein bycathepsins is necessary for infectivity and is likely not uniquebecause both the SARS and Ebola viruses are now known to use a similarmechanism to invade their host cells. (In June 2005, a group fromHarvard School of Medicine discovered that the Ebola viral membraneprotein is similarly activated by cathepsin L and B.)

If theseproteases are important for other viruses, they represent a new way tostop viral infection. SARS and Ebola are the first examples of the needfor these proteins to be cleaved during infection of the host cell.

Thiswork is a joint collaboration between the Bates lab and the researchgroup led by Scott L. Diamond, PhD, Director of the Penn Center forMolecular Discovery, one of nine facilities that the NationalInstitutes of Health (NIH) is establishing as part of the MolecularLibrary Screening Center Network. Diamond is also Professor of Chemicaland Biomolecular Engineering within the Institute for Medicine andEngineering at Penn. While independently screening for inhibitors,Diamond’s lab found a cathepsin L inhibitor called MDL28170, whichBates and Simmons tested for efficacy in inhibiting SARS coronavirusinfection. The cellular cathepsin enzymes have many other roles withinthe body, including mediating the inflammatory immune response in thelungs and antigen processing in T cells.

The Bates researchgroup, in collaboration with the Diamond group, has identified a fewcompounds, including MDL28170, which they plan to test in animals forSARS inhibition. “We’re now searching for other viruses that also usethis cleaving mechanism for activating their proteins,” says Bates. “Ifthere are a number of other viruses that do that, and we have somepreliminary evidence to suggest this, then we can develop smallmolecule inhibitors as possible therapeutics.” One advantage of thisapproach is that oral medications made from small-molecule inhibitorsare more readily made and distributed in the developing world-asopposed to a vaccine, suggests Bates. Protease inhibitors activeagainst cathepsins have been tested in mice with no ill side effects,which bodes well for their eventual testing in humans.

Co-authorsare Dhaval N. Gosalia, Andrew J. Rennekamp, and Jacqueline D. Reeves,all from Penn. This study was funded by NIH and the NIH Mid-AtlanticRegional Center of Excellence for Biodefense and Emerging InfectiousDiseases.