Imaging in mice with fluorescent proteins: From macro to subcellular

Whole-body imaging with fluorescent proteins has been shown to be a powerful technology with many applications in small animals. Brighter, red-shifted proteins can make whole-body imaging even more sensitive due to reduced absorption by tissues and less scatter. For example, a new protein called Katushka has been isolated that is the brightest known protein with emission at wavelengths longer than 620 nm. This new protein offers potential for non-invasive whole-body macro imaging such as of tumor growth. For subcellular imaging, to observe cytoplasmic and nuclear dynamics in the living mouse, cancer cells were labeled in the nucleus with green fluorescent protein and with red fluorescent protein in the cytoplasm. The nuclear and cytoplasmic behavior of cancer cells in real time in blood vessels was imaged as they trafficked by various means or adhered to the vessel surface in the abdominal skin flap. During extravasation, real-time dual-color imaging showed that cytoplasmic processes of the cancer cells exited the vessels first, with nuclei following along the cytoplasmic projections. Both cytoplasm and nuclei underwent deformation during extravasation. Cancer cells trafficking in lymphatic vessels was also imaged. To noninvasively image cancer cell/stromal cell interaction in the tumor microenvironment as well as drug response at the cellular level in live animals in real time, we developed a new imageable three-color animal model. The model consists of GFP-expressing mice transplanted with the dual-color cancer cells. With the dual-color cancer cells and a highly sensitive small animal imaging system, subcellular dynamics can now be observed in live mice in real time. Fluorescent proteins thus enable both macro and micro imaging technology and thereby provide the basis for the new field of in vivo cell biology.