Viruses are freeloaders.
Unable to replicate by themselves, they must infect a cell - a bacterium or a cell that is part of a multicellular organism - and use that cell's protein-making machinery to make copies of themselves.
Targeting that replication machinery for antiviral drug development - ideally, the parts of it that are essential to the virus but nonessential, at least in the short run, to the cell - might cause fewer problems with resistance than drugs targeting the virus itself. That's because "resistance would arise from a complex adaptation to use a different cellular protein for viral replication," as researchers pointed out in an article published in the September 2005 issue of Journal of Virology. That article, as well as another paper published in the October 2005 issue of Nature Immunology and now available online, reported research on the opposite ends of the viral hijacking process.
In the Nature Immunology paper, scientists from the National Institutes of Health in Bethesda, Md., and the University of California at Los Angeles reported on the mechanism by which a certain type of defensin, retrocyclin-2, prevents viruses from entering cells.
Defensins are immune system proteins that bind to and weaken the membranes of viruses, bacteria and fungi. Retrocyclin-2 is a defensin that is known to inhibit infection with a range of viruses, independent of the specific receptors a virus uses for entry. Like other defensins, retrocyclin is active against some types of bacteria and fungi, as well as viruses. "I can't think of a drug that's antiviral and antibacterial, as well," said Robert Lehrer, professor of medicine at the UCLA School of Medicine and a co-author of the paper. But "that doesn't mean the immune system can't come up with multifunctional molecules."
In the experiments reported in Nature Immunology, the scientists set out to decipher how retrocyclin-2 works. They knew that it somehow blocks viruses from entering cells. Many viruses enter cells in a three-stage process. First the virus binds to a receptor on the surface of the cell to be infected. That's followed by fusion of the membranes of the virus with that of the cell, which allows the contents of the virus to be dumped into the cell by endocytosis.
The scientists showed that while retrocyclin-2 inhibited infection of cells by influenza virus, it influenced neither receptor binding nor endocytosis. They then turned their attention to the fusion stage in more detail and found that retrocylin-2 inhibits fusion of several different viruses by crosslinking glycoproteins on the cell-surface membrane.
That cross-linking immobilizes the proteins within the cell membrane; since the membrane needs to be cleared of proteins to enable the tight contact between virus and cell that is necessary for fusion, such immobilization essentially formed a mechanical barrier that prevented the virus from fusing.
Lehrer refused to give details on how retrocyclin-2 achieved that crosslinking, citing the necessity for peer-reviewed publication first. But he said that when he and his colleagues looked at how other defensins work, two of them showed a similar mechanism of action to retrocyclin-2.
"We are now talking about a class of action," he told BioWorld Today.
In the study published in Journal of Virology, scientists investigated the other end of the viral hijacking process - the way in which viral articles exit a host cell after replication.
Viruses have different ways of getting out of cells. Some use a brute force method that might be described as breaking and exiting: They simply lyse the cell they have infected. For others, getting out of the cell is as complicated for the daughter viruses as getting in was for mom: They use the cell's exocytosis machinery to leave. In the experiments reported in Journal of Virology, the scientists focused on viruses that use exocytosis; within the exocytosis pathway, they concentrated on a protein known as Rab9, which is involved in vesicle transport within the cell.
The scientists first identified Rab9 as one of several genes that, through disruption, allowed the survival of Marburg virus-infected cells.
Because the Rab9 protein also interacts with a protein that in turn is important for HIV to exit cells, they then investigated whether other types of viruses could be inhibited by interfering with Rab9. Using small interfering RNAs (siRNAs), they found that HIV, as well as Ebola and measles viruses, were sensitive to Rab9 inhibition. Reovirus - a virus that does not have an envelope and thus does not use the cellular endosome to escape from the infected cell - was insensitive to Rab9 siRNAs, though. Additional experiments showed that HIV replication also was dependent on other proteins in the exocytosis pathway, including the related Rab11A.
The research reported in the Journal of Virology study was conducted by scientists at the Centers for Disease Control and Prevention, Georgia Tech and Emory University, all of Atlanta; the University of Georgia at Athens; the National Institutes of Health; Vanderbilt University, Avatar BioSci Inc. and the Veteran's Administration Tennessee Valley Healthcare System, all of Nashville, Tenn.; and the Hudson-Alpha Institute for Biotechnology in Huntsville, Ala.