Fireflies, glowworms, jellyfish and some types of plankton have a special glow that scientists, especially over the past decade, have discovered how to harness.
Bioluminescence imaging (BLI) is part of the burgeoning field of molecular imaging that aims to “see” not just anatomy, but specific molecular or cellular processes.
Light production depends on the presence of a protein enzyme called a luciferase. The luciferase performs a biochemical reaction on its substrate—luciferin for the firefly protein—usually requiring energy, oxygen and other co-factors, with the end products including the release of a single photon of light.
Investigators at Vanderbilt embraced bioluminescence imaging early on to follow cells and gene expression in living animals. Watching cells as they migrate through a living animal, take up residence, multiply, and in the case of tumor cells, metastasize to new sites, has been the most popular application of bioluminescence to date.
“What bioluminescence gives you is a level of sensitivity of detection that is not attainable by any other current method,” says E. Duco Jansen, Ph.D., professor of Biomedical Engineering and Neurological Surgery, who is affiliated with the Biophotonics Center at Vanderbilt.
The way it works is conceptually quite simple, Jansen explains. Cells of any sort can be infected with viruses or engineered to incorporate a luciferase gene. After being injected into small animals, usually mice, the cells begin to produce the luciferase protein. Investigators then inject the substrate molecule—such as luciferin—into the animals, and the luciferase acts on it, releasing photons of light.
“So, we have effectively a light bulb inside the cell,” Jansen says.
That light bulb is really quite weak—the mice do not actually glow like fireflies. But some of the photons of light do make their way out of the animal, and sophisticated charge couple device (CCD) cameras, cooled with liquid nitrogen to minimize noise, can capture them.
Imaging systems produced by Xenogen, a company that markets unique instrumentation and biological reagents that use luminescent signals for studying biology in animals, are available to Vanderbilt scientists through the Institute of Imaging Science.
While bioluminescence imaging offers excellent sensitivity for tracking cells and seeing gene expression in living animals, it suffers from poor spatial resolution. Because light is absorbed and scattered by tissues as it makes its way out of the animal, images become “fuzzy.” Jansen likens it to having a pencil in a glass of water and adding a few drops of milk—you can still make out an image, but it’s no longer clear that it’s a pencil.
“All of the imaging modalities have strengths, and they all have weaknesses,” Jansen says. “Bioluminescence is great at sensitivity, but it’s lousy at resolution. So, in many cases we combine it with something like CT or MR, or even fluorescence.”
BLI is not currently used in humans for clinical diagnostics due to limitations like poor tissue penetration and scattering of light, making it unsuitable for deep tissue imaging within the body. It is primarily used in preclinical research on animal models to study processes like cancer growth and metastasis.
Whether or not bioluminescence imaging makes it to the clinic, researchers see it as one of the cornerstone technologies for developing new ways to treat disease and to visualize biology as it occurs in the living body.
Editor’s note: This article is adapted from Seeing the shimmer of biology in action by Leigh MacMillan, Ph.D., September 18, 2024