In the world of preclinical research, bioluminescence imaging is creating a stir. Like fluorescence imaging, it enables the visualization of genetic expression and physiological processes at the molecular level in living tissues. But because there is no competing background signal, it can be used to detect much lower levels of light.
Another key difference between the two optical imaging methods is that bioluminescence does not rely on an external light source. Instead, borrowing from nature (usually a firefly), bioluminescence imaging detects light that is emitted from within experimental animals through the action of luciferase on its substrate, luciferin. Finally, because luciferase imaging requires genetic transfection, it is unlikely to ever find a place in human studies, although it can be useful for predicting human response to therapy in the early stages of drug development.
During imaging, an anesthetized mouse is placed in a dark chamber, and a photographic image is taken. Sensitive CCD cameras are used to capture the bioluminescent data, which are superimposed on the photographic image for easier interpretation.
Luciferase can be incorporated into a wide array of cells, organisms, and genes. Bioluminescence imaging has been used primarily to track tumor and immune system cells, bacterial and fungal infections, and gene expression.
Prostate tumor cells, for example, can be transfected with the luciferase gene. Once they are injected into mice, bioluminescence imaging can be used to monitor tumor growth and metastasis and to gauge the response to chemotherapy.
Xenogen in Alameda, CA, has even incorporated luciferase into a line of light-producing transgenic animals, including one that can detect tumor-associated angiogenesis. In this case, the gene for a vascular endothelial growth factor (VEGF) surface receptor promoter is incorporated into a mouse chromosome along with a gene for luciferase. When VEGF is secreted during angiogenesis, induction of the VEGF promoter prompts production of luciferase as well.