Viral Keys for Molecular Locks
All scientific discoveries, large and small, start with one common goal—to advance knowledge, be it of our bodies and health, our environment, or even our universe. But the most exciting discoveries, built on years of research and collaboration, fundamentally change how we understand our world.
In a special summer series, we'll talk with five Northwestern scientists whose work is already changing their fields, and could potentially change our lives.
Over the course of more than thirty years in science, Patricia Spear, professor emeritus of microbiology-immunology at Northwestern’s Feinberg School of Medicine, has discovered much of what we know about the herpes simplex virus, and shed important light on the complicated process through which a virus infects a cell. We asked Spear to tell us more about her ground-breaking research.
Tell me about the focus of your work.
The focus of my work has been on herpesviruses—in particular, on herpes simplex viruses (HSV), which are the viruses that cause cold sores and fever blisters and can also cause encephalitis. I’ve been trying to understand at the finest level—the molecular level—how this virus is able to invade a cell, because if it can’t invade a cell, it can’t initiate infection, and it would be harmless.
Basically, we wanted to know for which “locks” on the cell’s surface the virus has keys, enabling it to invade the cell.
What did you find?
We discovered a number of different cell surface molecules—proteins—that serve as locks which the virus can open through interaction of its own surface proteins with the cell’s surface proteins.
Additionally, a new way of looking a viral entry emerged through work with herpesviruses and also with HIV. Before these viruses were studied in great detail, viral entry was viewed as a rather simplistic process—the virus binds to something on the cell’s surface and presto-chango, it’s in the cell. Studies with HIV and HSV—and I think our work contributed to it—made it clear that it’s a very complicated process, and that there’s a cascade of interactions that must occur in order for the virus to actually open up the cell, and be able to enter the cell.
Can you tell me about these interactions?
We discovered that HSV can bind to cells by binding to certain sugar molecules on the cell’s surface. This binding, although not absolutely essential, significantly enhanced the efficiency of infection. So, if you didn’t have these sugars on the cell, the cell was pretty resistant to viral infection.
Binding to those sugars puts the virus in the proximity of specific protein receptors that the virus has to interact with in order to unlock its way into the cell. We discovered three different classes of cell surface proteins that could actually serve as the lock, and that they act independently of one another. A practical consequence of what we found is that one could design drugs to block these interactions and thereby block entry into the cell.
Why three different proteins?
Toward the end of my career, we were able to answer that question by using an animal model—mice—for infection. Mice can be infected by HSV, and they get disease that is similar in many respects to what humans get. The cell surface proteins that allow for entry into human cells are also the same three classes that enable entry into mouse cells. That makes the mouse a good model for studying the role of these receptors in viral disease.
We had learned in the meantime that two out of these three classes were the most important. One is called nectin and the other is called HVEM – let’s call one “N” and one “H.” We were able to get from colleagues mice whose H or N gene had been deleted, or made non-functional, so they could not express either H or N. We mated these mice together, so that we also had mice that failed to produce both H and N.
Therefore we had four different, what we call, “genotypes” of mice – wild type, which express both H and N, double-mutant, which express neither H nor N, and single-mutants, which express one but not the other.
We discovered that the vaginal epithelium (vaginal skin cells) could be infected provided the mouse expressed H or N, but not if it expressed neither. However, a high frequency of neurological disease, for the most part, required N. In other words, the virus required N for efficient spread to parts of the nervous system that can cause neurologic disease.
We also wanted to understand which of these receptors was really important in the brain, because the virus can cause encephalitis. And what we discovered is that, as long as the mouse expressed N—it didn’t matter whether it expressed H or not—it developed encephalitis. If it didn’t express N, it didn’t get ill. There was a little bit of infection if the mouse expressed H, but it didn’t get any disease. If the mouse expressed neither H nor N, there was no infection, no disease, nothing.
This project is being continued by one of my colleagues, Richard Longnecker, a graduate student in his lab, Andrew Karaba, and my former technician, Sarah Kopp. They’re finding some very interesting results by the ocular route of infection – onto the cornea of the eye, which is an important mode of HSV infection. In fact, a disease called herpes keratitis, where the cornea becomes basically non-functional, is the most common form of infectious blindness that occurs in developed countries. So it’s an important cause of disease. And they’re discovering that both H and N are equally important in the eye.
Now we can begin to get a view as to why there are multiple alternative receptors for the virus to enter cells, because each receptor is important for different cell types at different body surfaces or in the brain. This is more complicated than we would ever have dreamed, but it turned out to be an interesting story.
How did you become interested in studying viruses?
I became interested in the herpesviruses when I was a graduate student, in the late sixties, at the University of Chicago. Looking through the microscope at cells that were infected with the virus, and noticing the new shapes they took, fascinated me. So it was a morphological change at the time that was interesting to me.
How or when did you fall in love with science?
For me it was a gradual evolution, and it didn’t happen until college. Some people decide when they’re six years old that they want to do science, and that wasn’t me. I went through high school totally unscathed by any thinking of science [laughing].
I went to a collegiate nursing program at Florida State University, and learned right away that nursing wasn’t for me. But I liked the science courses I was taking, so I decided to switch my major to microbiology with a minor in chemistry. When I graduated, I realized the only thing I could do with my degree was to go somewhere and be a lab technician, and I wanted to do more than that. I went back to Florida State for a master’s program, but decided after one year that I wanted to broaden my horizons. So I decided to come to the University of Chicago [to get my PhD], did a rotation in a lab that was studying herpesviruses, and became entranced.
What advice do you have for young people?
To make sure that you establish a career where you can do what you like to do. Do something that you’re really passionate about. Science, for example, would be a terrible career if you don’t like doing it, because it’s hard work, and you have so many disappointments. But when it works, it can be very, very gratifying, if you’re really passionate about the process. I just think you have to be really invested in what you’re going to devote your working life to.
Is there an aspect of your work of which you’re most proud?
I’m very proud that we discovered all these receptors—I think my lab is almost single-handedly responsible for that. But most of that early work was done in cell culture, and you don’t necessarily know how that translates to disease. So I think I’m most proud of the fact that we were able to translate our in vitro work to the mouse, and show that in the mouse, at least two of these receptors are clearly critical in order for disease to occur. And I think it’s not a long stretch to say that that’s going be true for humans as well.
Also, I’d like to note that all of this work was done by students and fellows in my lab. One thing that’s been really gratifying is to work with some excellent students and fellows who are now doing excellent work in their own rights, and in their own laboratories. Some have gone off on different tacks—one former fellow has become a well-regarded science writer. But some have continued on in research also, and are doing very, very well. In a career like mine, you have two products—new information and new scientists, and both are equally important.
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