Nov. 3, 1997
DURHAM -- Most people might figure an eerie, green glowing fly for a Halloween prank, but scientists at Duke University Medical Center have inserted a glowing jellyfish protein tag onto a key cell structural protein in fruit flies to reveal how they transform from embryos to larvae to adults. The scientists believe their research will also help in understanding birth defects in humans.
The researchers have just published studies that use the glowing flies to shed light on how cells cinch shut in the fly's dorsal flank during development, a process comparable to neural tube closure in developing mammalian fetuses.
The Duke scientists used a time-lapse camera and high-resolution light microscope to take the first video of dorsal closure in flies as it occurred. They believe the new information gained will help them understand the causes of spina bifida, a birth defect in which the spinal column doesn't close properly during development, leaving a hole in the spine that must be closed surgically after birth.
The Duke researchers, including Kevin Edwards, Maddy Demsky, Ruth Montague and Nate Weymouth, and led by Daniel Kiehart, associate professor of cell biology, report their findings in a Nov. 1 cover story of the journal Developmental Biology. The research was funded by grants from the National Institutes of Health and the March of Dimes.
The researchers created the glowing flies by inserting a gene constructed in the lab into fly eggs. The new gene is a hybrid between a fly gene that contributes to cell structure during development and the green fluorescent protein (GFP) gene from the jellyfish Aequorea victoria. GFP emits bright green light when exposed to ultraviolet or blue light.
The research team used GFP to tag the protein, which attaches to the cell's actin cytoskeleton, a rich meshwork that helps cells keep their shape and migrate from one place to another during the transformation from fertilized egg to adult fly. All the flies that made the fluorescent protein appeared remarkably normal, researchers said.
"Previous cell staining methods required toxic fixatives, which means each image is only a snapshot of what is happening in the cell," Kiehart said. "We wanted to follow movement in a dynamic way, and this fluorescent protein allowed us to do that. It's like going from photographs to a full-length motion picture."
Fruit flies contain much of the same basic genetic programming that choreographs the intricate journey from a fertilized human egg into a healthy baby. But because mammals gestate their young inside the body, it is very difficult to follow key developmental steps. So studying fruit flies, technically known as Drosophila melanogaster, can tell us about our own development, Kiehart said.
Kiehart and his colleagues are zeroing in on how and why cells move during development. He wants to identify which genes are crucial for normal movements and cell shape changes during development, and why, when gene products don't function at the right time, birth defects can result.
One key protein, he says, is non-muscle myosin, a kind of molecular motor that drives changes in cell shape and powers cell movements as a fertilized fly egg grows and develops legs, eyes, wings and all its other body parts. Scientists also know that myosin is vital to daily cell maintenance in both flies and people. Kiehart has already identified one type of non-muscle myosin that, when missing in flies, results in a defect in the way cells change shape, comparable to spina bifida in people.
By watching the fate of the glowing cells in his experimental flies, Kiehart and his colleagues have already confirmed some of their previous hypotheses about cell movement during dorsal closure. They also have provided a powerful tool for other researchers studying development, because the glowing protein is also concentrated in the developing eye, nervous system, the forming gut, the sensory organs, and particularly, the leading edges of migrating cells in all organ systems.
For example, the Duke researchers can now observe directly the actin-rich microvilli -- or little fingers -- form in the developing eye, particularly in the light receptor cells, retina and optic lobe. "This localization may make it easier to study formation of the eye, and to find genes involved in eye development," Kiehart said.
Kiehart and his lab group are expanding the use of this glowing protein to help find out how skin cells move to cover an open wound. He has begun experiments to put the glowing protein in human and mouse skin cells in laboratory dishes. The human cells actively produce the fluorescent protein, and it doesn't appear to be toxic to them, Kiehart said.
"We believe this new tool for studying cell shape change will provide a rich source of information that will open up one of the final frontiers of developmental biology: morphogenesis or cell growth and maturation," he said. "The ability to observe cell shape and structure should contribute to our understanding of human disease as well."
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