Researchers at Cornell University, the University of Pittsburgh, and Japan’s RIKEN Brain Science Institute have created transgenic mice whose hearts produce a fluorescent protein that’s turned on by calcium ions. Calcium concentrations in heart cells skyrocket and then plummet during electrical signalling and muscle contraction in animals, including humans and mice. Using a fluorescence microscope, the scientists were able to film these waves of activity moving through the four chambers of the mice’s hearts with each beat. Cornell University’s Michael Kotlikoff, who led the research, calls the fluorescent protein a “molecular spy.”
[To see images of fluorescent hearts, click here.]
Biologists have long dreamed of being able to see the activities of cells in living, functioning animals. Now the techniques that will make this possible are coming to fruition. These new genetically engineered mice are providing a window into what happens during a heart beat and also illuminating the electrical signals that coordinate the beating. The mice have provided insights into the development of the mammalian heart, and are also being used to evaluate heart stem cell transplants. And, finally, the researchers are also breeding mice with glowing nerve cells to study signalling in the brain.
“The goal for all of us is to see the changes in ion concentration or other aspects of cellular function” in living animals, says Withrow Gil Wier, a professor of physiology at the University of Maryland School of Medicine, who in 1980 first showed the transience of calcium in heart muscles.
“We learn a lot by taking cells out, chopping them up, and looking at individual molecules. But it doesn’t tell us what we need to know about the complex interactions between cells,” says Kotlikoff, who is chair of the department of biomedical sciences at Cornell’s College of Veterinary Medicine.
RIKEN scientist Junichi Nakai created the fluorescent protein (called “GCaMP2”) by modifying an existing one that was not bright or stable enough to use for in vivo heart experiments. Cornell scientists genetically engineered mice that produce this protein day after day and at a steady concentration, in their hearts and nowhere else in the body.
Kotlikoff says the advantage of the new fluorescent protein over its predecessors is the speed at which it can turn on and off, like a light bulb, its stability at body temperature, and its brightness. The mouse heart beats up to 600 times per minute. “There’s so much background light and reflection and motion that you need something very bright to be able to detect these cellular signals,” he says.