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Genetically Altered Mice Have Glowing Hearts

By Pippa Wysong, Access Excellence

Ithaca, New York (04/26/06)- Researchers have found a way to make mouse hearts glow green each time they beat. The feat was accomplished by creating genetically altered mice by modifying and inserting a gene from a jellyfish known for its ability to fluoresce. Not only that, researchers can turn the glowing feature on and off, at will.

But this is more than just a party trick. The reason for having such a mouse is so that specific features of beating hearts can be studied. In this case, the fluorescence is associated with changes in calcium levels at the cellular level, and so allows researchers to observe these changes in living, beating hearts, according to Michael Kotlikoff, PhD, professor and chair of the department of biomedical sciences at the College of Veterinary Medicine at Cornell University.

Calcium levels increase when heart cells contract, and decrease when the cells relax. "The fluorescence is proportional to the level of calcium in the cell," he told Access Excellence. "Every time the heart beats, all the cells in the heart sequentially release calcium and calcium goes up about ten-fold within the cell." Changes in fluorescence are recorded through use of a specially designed high-speed digital video camera.

Fluorescing hearts can be observed in adult mice as well as ones that are still in embryonic form. "The heart is unique in that it's the first organ that functions in the body and it has to function before it's completely developed," he said. In the embryonic mouse, cells that form the heart start to beat even before the heart is fully developed. While this is already a well established fact, there are still a lot of unanswered questions about how those cells combine with each other and morph into a fully developed heart.

The ability to track changes in levels of a chemical such as calcium gives researchers a tool they can use to study the biology of the heart, and gather information about what happens when cells communicate with each other.

There is a lot of complex biology that can't be studied with a single cell on a microscope slide. Scientists want to learn what happens when various cells interact with each other; things such as nerves interacting with muscles, or vessels interacting with their surrounding environment. "It's the signals between those cells that we're interested in eavesdropping on. And it's through genetic engineering that this can be done in a stable way," he said. "It really opens a new window in biology and tells us about complex events that we have no other way of knowing about."

The project is exciting because until now there haven't been effective ways to observe chemical changes in live, beating hearts, Dr. Kotlikoff said. "Now, we have molecular-scale signals that we can record in vivo, in live working hearts. There's no other way to do this, other than genetically. (Otherwise we have to) take the heart out, take apart all the cells and load them with dye just to measure calcium in individual cells...What I really want to see is how those cells are talking to each other – what the overall event is," he said.

The gene used in the study was derived from the green fluorescent protein (GFP) found in the Aequorea jellyfish. The researchers altered the gene so that the protein it produced would bind and respond to calcium. The gene was also designed so it could be turned off. This is done by giving the genetically altered mice a compound (in this case a common antibiotic) that blocks the expression of the GFP gene. When the antibiotic wears off, the heart cells start to fluoresce again.

The researchers are also creating other kinds of fluorescing genes to detect other chemicals besides calcium in order to study tissue repair, cell function and more. The study was published recently in Proceedings of the National Academy of Sciences (PNAS 2006 103:4753-4758).

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