Of Fireflies and Viruses

On a winter morning, Susan Cook takes the freight elevator from her fifth floor lab in the School of Public Health to a storage room on the second floor. Inside that room, she places three anesthetized mice under a camera lens fitted inside a black cabinet the size of a milk crate, closes the door securely, and hits a few strokes on a nearby computer keyboard. 

"Now we wait and see," says Cook, a fourth-year doctoral student who studies the effects of single-point genetic mutations on the virulence of the Sindbis virus. One minute later, a photographic image of the three mice appears on the computer screen. But it's no ordinary photograph because superimposed onto the mice's bodies are patches of color: blue and green throughout the mice's midsections, and red over their tiny right feet. "The feet are lighting up," says Cook, as she saves the image to the computer's hard drive, "because the virus hasn't had much time to move."

Three days ago, Cook injected the right foot of each mouse with a strain of Sindbis she'd engineered to express the gene that produces luciferase. Then, just before placing the mice under the camera lens today, she injected each with the substrate luciferin, which reacts with the luciferase in the virus and produces light - just as it does in fireflies. In the mice's bodies, the luciferase in the virus cells becomes a kind of homing device, lighting up wherever the virus is replicating. 

Though Cook can't see the light from the virus cells like she can see a firefly's glow, the camera inside the black cabinet can. In fact, the camera — which is a highly sophisticated imaging system created by a biotech company called Xenogen — literally counts the number of photons emitted from the virus cells in the mice's bodies, and displays those numbers as colors. When Cook sees the mice's feet are covered in red, the color designating a high number of emitted photons, she knows the virus is active there. "I can quantify this data in a minute," says Cook. "Before, it would take me two days."

Until a year ago, when Cook began using luciferase in conjunction with the camera, a method known as in vivo biophotonic imaging, her research required her to inject large numbers of mice, kill them daily, and grind up their organs to assay the location and virulence of the Sindbis cells. "Before this technology, it was impossible to link early data and an outcome in a single animal. Now we're getting more and better data, and having to sacrifice fewer animals," she says. "Also, before you had to know where in the body to look for the virus. Now we can see it even if it shows up someplace unexpected." 

Indeed, minutes after Cook repositions the mice in the black cabinet and programs the camera for a longer exposure, something unexpected does appear on her computer screen: a red patch over the neck area of one mouse. "He's already replicating in his spinal cord and brain," she says. "It's earlier than I expected."

Over the next four days, the Sindbis virus will run its course in the mice. And each day, Cook will descend to the storage room, inject the mice with luciferin and anesthesia, and place them in the black cabinet to measure the photons emitted from their bodies. After the mice die, Cook will use the daily snapshots to construct a narrative of the virus that killed them. Each time she injects a new set of mice with a slightly different strain of Sindbis, she'll have the ability to compare its effects with the effects of the other strains. "This is a new way of looking at a virus in a whole animal over time," she says. "It's really exciting."

Diane Griffin, professor and chair of the Department of Molecular Microbiology and Immunology, shares Cook's enthusiasm. "This method had been used before to study bacteria and tumor growth, but no one had applied it to the study of viruses," says Griffin, MD, PhD. "Susan has proved it works. And I think it's going to be broadly applicable for people interested in how viruses cause disease, because almost every virus can be engineered to express luciferase."