Moscow researchers have developed a bright-red fluorescent protein that can be seen from deep within the bodies of small animals. The new imaging agent will allow biologists to monitor the progression of diseases like cancer noninvasively and over time in live animals.

Fluorescent proteins have been a basic tool for biologists since the early 1990s, allowing them to track the molecular activity of cells in lab dishes and of tissues just beneath the skin of mice and other animals. But even with the best of these red, green, and yellow light-emitting proteins, researchers haven’t been able to see deep inside the bodies of live, whole animals. The new red protein was developed by Dmitriy Chudakov of the Shemiakin-Ovchinnikov Institute of Bioorganic Chemistry, in Moscow, which is part of the Russian Academy of Sciences. The work is described online in Nature Methods.

Biologists can use the glowing proteins in two ways. First, they can engineer organisms that produce the red dyes in conjunction with another protein. This allows them to track the movement of cells during the development of the embryo and the movement of tumor cells during cancer progression and treatment. Second, biologists can join the red proteins to antibodies that bind to specific target molecules in the body, such as biomarkers on the surface of cancer cells. These targeted imaging proteins can be injected intravenously to spotlight specific cells.

Chudakov happened on the protein by chance. One of his collaborators spotted “an extremely bright-red sea anemone” in a Moscow pet shop. By inducing both random and directed mutations in the anemone protein’s molecular sequence, the Moscow researchers made new proteins that are stable inside the body and even brighter than the original one. They made two versions of the brightest and stablest of these proteins, and named them Katushka and mKate.

Red light can pass through tissues far better than any other color can. For this reason, says Zhen Cheng, a research scientist at Stanford University’s Molecular Imaging Program, “a lot of people have been trying to develop proteins with high stability that emit light at this wavelength.” But other red fluorescent proteins haven’t been as stable and bright as the Russian protein, says Cheng, who suspects that the new markers will have broad applications in real-time tracking of the molecular activities of tissues deep inside animals. For example, says Cheng, Chudakov’s probe should allow researchers to monitor the origins, spread, and treatment of cancer deep within the body and “extend [our] understanding of disease progression.”

Chudakov has tested the new proteins in human cells and in frogs. In the animal studies, he found that the new proteins shone brightly from deep muscle tissue while the others were barely visible. The Moscow lab will soon begin testing the proteins in mice.

Cheng says that the new protein should allow researchers to perform noninvasive monitoring of cancer progression in living animals in real time. Using other fluorescent proteins, researchers have been limited to performing these kinds of studies in tumors implanted just beneath the skin or under other unrealistic conditions–by cutting into mice to image their tumors, for example, or by imaging tissue biopsies. The most successful of these proteins, called enhanced green fluorescent protein, is not nearly as stable over time, and its light can’t penetrate through tissues the way that light from the new red proteins can.

Cheng says that the Russian proteins might eventually be used clinically. Light from the red proteins can’t penetrate far enough through tissue for whole-body imaging in humans, but he says that they might be useful for imaging superficial tumors like melanoma and breast cancer.