Vol 6. Issue 29 / Oct 2, 2006
Twice Told Tales
By Eric Sauter
The viruses that Juan C. de la Torre has been studying in one form or another since he joined The Scripps Research Institute in 1989 have been saddled with the prosaic acronyms LCMV and BDV, which makes them sound more like throw offs from the Department of Motor Vehicles than potentially lethal bits of stray RNA.
The archetypal arenavirus lymphocytic choriomeningitis virus, or LCMV, is a rodent-borne virus, first isolated during a 1933 epidemic of St. Louis encephalitis; since then, LCMV has become a primary workhorse in the fields of viral immunology and pathogenesis. Arenaviruses also include important human pathogens such as Lassa fever virus and other causative agents of hemorrhagic fever disease. Moreover, evidence indicates that LCMV is likely a neglected human pathogen, especially in cases of congenital infections.
The neurotropic Borna disease virus (BDV) is named for Borna in Saxony, Germany, where an epidemic of "head disease" in horses was first described in the late 1880s. Naturally occurring infections with BDV were thought to be limited to horses and sheep within certain endemic areas of central Europe, but more recent data indicate that BDV is more widespread, both geographically and in terms of types of hosts, than originally thought. Now there's evidence that BDV can infect humans, as well as a proposal—still highly controversial and not yet proven—that BDV might be associated with certain neuropsychiatric disorders.
Taken together, like a viral yin and yang, these two viruses provide investigators with an open doorway to the complex threats of viruses in general and eventually, so Juan C. de la Torre hopes, new ways to fight them.
"A remarkable thing about these two, and many other viruses, is that they have a very simple genome, coupled with very low proteomic complexity," says de la Torre, an associate professor in the Department of Molecular and Integrative Neurosciences. "LCMV, for example, has only four gene products—expressed proteins. Notably, despite its limited genome complexity, LCMV manages to negotiate with its natural host a situation where the virus can live for a long time, so-called persistent infection. It's a remarkable standoff."
There are two key requirements for persistent infection. First, the virus must be able to escape from the host immune surveillance system. Second, the viral gene expression program has to be regulated so that it does not compromise the survival of its host. Disruption of this balance frequently occurs when an arenavirus jumps from its natural host to humans, as exemplified by the severe disease associated with Lassa fever virus in humans, which is acquired by exposure to rodent secretions.
Linking Viruses and Central Nervous System Disorders
And even in persistent infections, the standoff might not be as balanced as it first appears. While at first glance mice chronically infected with LCMV look fine, they exhibit a number of significant deficiencies in both memory and cognition. These type of findings, which have been made with a variety of viruses, have led to the hypothesis that viruses can contribute to a variety of human central nervous system (CNS) disorders whose etiology remain elusive.
"In some cases, like with BDV, epidemiological data have provided suggestive evidence of a viral association with such disorders, but what this means we don't know," de la Torre says, choosing his words carefully. "What that means has been debated for years. Viruses are unlikely to directly cause any of these disorders, but they could contribute to them. I think it is a combination of factors. The host's genetic make up, various environment factors, and the virus all work together somehow. Imagine someone with a borderline tendency toward a particular CNS disorder who gets hit with a viral infection that disrupts the production of a given neurotransmitter. That could be the last straw, setting the disease in motion."
To uncover the definitive answer to this question would be "Eureka" science of the first order and, when de la Torre talks, you get the feeling he'd very much like to be part of it. But, as he points out, almost wistfully, the resources and work needed for that kind of study are beyond the capacity of any single laboratory, and answers will likely come only from the combined efforts of many different investigators working in different areas of research.
Over the course of the last 10 years, de la Torre has managed to outmaneuver a number of these complications. This fall, in a paper published in the journal Nature Reviews Microbiology (Vol. 4), de la Torre discusses in extraordinary depth techniques that he and his colleagues developed in an earlier study published by the Journal of General Virology using reverse genetics (an approach that begins with a piece of DNA and proceeds to find out what it does). Using these reverse genetic systems, it is now feasible to rescue LCMV or BDV entirely from cloned cDNAs, which provide investigators with the ability to manipulate these viral genomes at will.
The wide-ranging review makes clear the importance of BDV as a model for understanding how the virus produces behavioral and neurodevelopmental aberrations, and points to reverse genetics as "providing a unique and powerful approach" to reaching that understanding.
In two other studies, both published in the Journal of Virology (Vol. 80, No. 18), de la Torre describes how a gene product of LCMV, the virus nucleoprotein, interferes with the host's ability to produce type I interferon, one of the body's primary defenses against viral infection. The other study, which adds a bit more intellectual fuel to the fire, describes how a congenitally acquired chronic LCMV infection permanently alters CNS gene expression in mice, leaving them with learning and memory problems.
De la Torre and his colleagues have also developed a reverse genetics system for LCMV that provides another investigative tool to study the molecular mechanisms that these extremely pathogenic viruses use against their human hosts, including RNA replication, control of gene expression, assembly, and viral interactions with cell factors. As de la Torre explained in the 2004 Scripps Research Scientific Report, "LCMV is a Rosetta stone for the investigation of virus-host interactions. We can now generate predetermined specific mutations within the LCMV genome and analyze their phenotypic expression in vivo... [to help elucidate] the molecular mechanisms that underlie interactions between arenavirus and host cells and associated disease."
While the scientists in his lab are engaged in basic research, de la Torre says, they're not just in it for the intellectual tingle: "We hope that eventually the things that we do will contribute to improving our knowledge of viruses and potential ways to control them."
A Dynamic Environment
He was already well on that path when he stepped off the plane from Madrid in 1987.
De la Torre was born in the Canary Islands (one travel guide lauded them with ungrammatical praise as a "paradisiac group of islands…") and lived there until he was 16, when he moved to Madrid for his college education and later Ph.D. at the "Universidad Autonoma de Madrid" under the direction of Professor Esteban Domingo, a world leader in the field of RNA virus evolution. In 1987, at age 25, he moved to San Diego to conduct postdoctoral research at the University of California, San Diego (UCSD) in the laboratory of Professor John Holland.
It was like déjà vu all over again when he stepped off the plane.
"I came here without knowing very much about the city," he says. "I mean, I knew it was south. So I was shocked when I looked out of the plane window and saw that it was just like the island I grew up on. It was like going back home."
De la Torre met his wife in San Diego—they arrived at nearly the same time—where she was pursuing her own Ph.D. at UCSD. They were married in 1991. Once a member of the Salk Institute, she is now involved in stem cell research at a San Diego biotechnology company.
De la Torre came to UCSD on a Fulbright scholarship and, after finishing several research projects in Holland's lab related to the field of RNA virus evolution, he realized he still had a year to go on his grant. De la Torre used up the remainder of his grant working with Scripps Research Professor Michael Oldstone—and was then asked to stay on at the institute.
"Scripps Research is a fantastic place," de la Torre says. "Scientifically, I doubt if you can have it better in terms of collegiality and interactions. Really the whole La Jolla area, with all these exceptional scientific powerhouses located within walking distance of one another, provides a scientifically very rich and dynamic environment."
So Many Questions
Moving from FMDV to VSV, and subsequently to LCMV and BDV, kept him on the same research track, trying to uncover how these viruses manage to navigate through their host world and survive, sometimes leaving the host intact, sometimes not.
"It's obvious what arenaviruses like LCMV do in humans," he says. "But in terms of their mechanisms there are still so many questions—which is why LCMV is such a fantastic model to study the cell biology of arenaviruses. With the newly developed reverse genetics system for LCMV, we can now dissect each of the steps of the viruses' life cycle and figure out ways to disrupt them."
Thus, the hope is that by studying the basic aspects of arenavirus molecular and cell biology the investigators will be able to come up with effective anti-viral therapies to combat pathogenic arenaviruses, needed now more than ever. While Lassa virus infections are rare in the developed world, in West Africa they are not. The Centers for Disease Control puts the number of infections at between 100,000 and 300,000 per year with approximately 5,000 deaths. In parts of Sierra Leone and Liberia, between 10 and 16 percent of patients admitted to hospitals have Lassa fever. If the infection rate rises to the epidemic level, which it does occasionally, according to the agency, the fatality rate can reach as high as 50 per cent.
"What's important is that now, in the 21st century, these viruses are no longer restricted to isolated areas," de la Torre says. "If you have the right sequence of events, it's not far fetched that a lethal virus could move from Kenya to New York in a matter of hours or days. It would be easy to spread because of our high mobility. Some of the things we've found in studying LCMV and BDV could help in the development of a vaccine or a pill you could take before you travel that would protect you from infection."
So, as de la Torre observes, it's not a bad thing to study these viruses.
Such wariness comes naturally to people who study viruses for a living, even though it's not always easy to keep the big picture in mind.
"It's sometimes difficult to see that what you're doing can somehow translate into future benefits, especially when you get caught up in the daily routine or the next paper you're writing," de la Torre says. "These two viruses are RNA viruses, and one of the amazing things about RNA viruses is their ability to adapt and evolve. They circulate around the world, trying out random mutations all the time. A bad combination could occur at any moment—maybe never, maybe 100 years from now, maybe tomorrow."
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