William J. Todd, Mcmanus, Jacqueline M., Cooper, Richard K., Smith, Brandy S., Spring, Laynette | 10/21/2004 10:30:08 PM
William Todd, Laynette Spring, Jackie McManus, Brandye Smith and Richard K. Cooper
Viruses cause lost productivity in all species of agricultural plants and animals. Viruses work by entering a cell and subverting the essential functions of that host cell to replicate their own kind. Implicit in this strategy for survival are consequences for their hosts, ranging from pain and suffering to possible death, depending on the nature of the virus and the particular host.
Because viruses are so small, they must accomplish their entire life cycle using a limited number of genes and structural elements. Most viruses consist of only a nucleic acid core, either DNA or RNA, surrounded by at least one protein coat, and sometimes, depending upon the type of virus, a second overcoat consisting of a membrane with inserts of viral encoded proteins. These coats protect the viral nucleic acids as the virus travels from cell to cell within the host and during transport between hosts. Some of the specific proteins on the viral surface are also designed as molecular keys that enable the virus to enter the specific host cell of its choosing.
Occasionally, virus infestations wreak havoc on a product and cause the imposition of regulatory limitations, preventing transport of the product from the farm. With animal agriculture, additional problems can and do arise. Because animals can serve as reservoirs for the genetic mixing of genes from different viruses, the potential exists to generate more virulent strains of viruses capable of infecting people and causing disease. Clearly, there is a need to limit viral infections of our agriculturally important species.
Historically and to date, most attempts to control indigenous viruses have depended on stimulation of the host immune system to recognize and inactivate the structural elements and protein keys on the surface of each virus through vaccination strategies. Vaccination is often effective. But success is typically limited to a particular strain of virus. Furthermore, viruses have evolved along with their hosts and have developed ways to escape the immune response by altering the protein keys to confuse host immunity. Vaccinations, though useful, are often short-term solutions of limited effectiveness.
In addition to vaccines, some progress has been made to design drugs specific enough to interfere with unique viral functions without causing too many side effects for the host; however, because of the price of these drugs, agricultural applications are not realistic.
Despite limitations, there has never been a better time to be optimistic about
our ability to overcome these pesky viral infections. Because of recent advances in molecular genetics and biotechnology, strategies to prevent viral infections are within our future. Instead of basing prevention on the proteins of viruses, opportunities exist to modify genetic interactions between viruses and host cells, to the detriment of the virus.
Gene wars between viruses and host cells have always existed at every level—from viruses that infect bacteria to humans. The innate genetic ability to inhibit most viruses is what prevents each species from being inundated with all but the select few viruses that have figured how to work around the genetic mechanisms of inhibition used by that particular host. With advances in biotechnology, it is now possible to assist genetics to overcome the remaining recalcitrant viruses.
Biotechnology to the Rescue
Each type of virus has unique signals used to control the host cell of its choosing. The trick is to modify the genetics of the host in ways that will prevent the targeted class of virus from fulfilling its quest to produce more of its own kind. Once a virus enters a cell, it exists as nucleic acid, separate from the protective protein coat, and is now entirely dependent on the cell. We believe that each class of virus encodes genetic signals that are unique to the virus and not present in uninfected cells. These unique genetic signals can be exploited by altering the host cell so that expression of the viral signals can no longer lead to the production of progeny virus. Each virus, therefore, has the genetic equivalent of an Achilles’ heel, either encoded in gene regulatory mechanisms or in unique methods of the viral nucleic acid replication.
Consider the common influenza type viruses that cause so many problems for animals and humans. All influenza viruses use single strands of RNA as their genomes, and the single strands are in a form called negative RNA, a complementary copy of the messenger RNA that host cells normally translate into gene products. At the early stages of infection, the viral negative RNA must first be converted to messenger RNA so its encoded commands can be understood by the host cell. This conversion can be accomplished only because the virus provides the unique enzyme, an RNA-dependent polymerase, that allows the host cell to convert the viral negative RNA into a translatable form. The virus carries this essential polymerase with it as it enters the host cell. Without this enzyme, any form of negative RNA remains as nonsense to the host.
Unique functions of pathogens, such as this, provide opportunities for disruption without harming the host. The unique RNA polymerase can be exploited. For example, the host cells of plants and animals can be engineered to produce small amounts of negative RNA encoding a product that would shut the cell down upon infection with the virus. Without the virus, the negative RNA could never be transcribed, so uninfected cells could never be harmed. The consequence of being infected with a negative RNA virus would be to shut down the few initially infected cells; the virus would be prevented from producing progeny and causing disease.
To reach this goal, we have engineered plasmids to express both negative RNA encoding genes to shut down cells, along with marker genes, and we have expressed these plasmids within cells cultured in the laboratory. When the molecular dynamics are sufficiently understood at the cultured cell level, such constructs could be engineered into farm animals. The need for such protection is significant to agriculture and of potential importance to human health. For example, it is the genetic mixing of human and bird forms of influenza in pigs that generates new forms of human influenza. Limiting influenza virus infections of domesticated pigs and birds would decrease opportunities for the influenza viruses to recombine into more virulent forms that could infect humans. We believe that genetic strategies to modify host cells of agriculturally important plants and animals for resistance to specific pathogens will contribute to the future profitability of agriculture and will improve human health.
At the LSU AgCenter we envision a time when viral infections of plants and animals will terminate at the beginning of the infectious cycle before production of the viral progeny essential to cause disease. The modified plants and animals will be genetically incapable of being infected by specific viral pathogens. As agricultural scientists we look forward to being part of the research programs that will be responsible for limiting viral infections of our plant and animal commodity products.
William Todd and Richard K. Cooper are professors in the Department of Veterinary Science. Laynette Spring, Jackie McManus and Brandye Smith are research associates.
(This article was published in the fall 2003 issue of Louisiana Agriculture.)