April 18, 2006 – The cell is understood to be highly organized, with specialized areas for different functions and molecular motors shuttling components around. Researchers from the University of Illinois’ Chicago and Urbana-Champaign campuses now offer the first imaging evidence from live cells of ongoing organization and transport within the cell nucleus.
Genes that are active are located mainly in the central region of the nucleus, while inactive genes are at the periphery. But scientists have had no way to track chromosome movement inside the nucleus or to determine whether the location of the chromosomes was the result of random diffusion or if they are moved around by molecular motors.
In a study published in the April 17 issue of Current Biology, UIC and UIUC researchers show that chromosomes in the cell nucleus are capable of directed, long-range movement that depends on actin and myosin, the major molecular motor complex in the cytoplasm.
Developing a system for observing nuclear motion was difficult because chromosome movement is extremely light-sensitive. One exposure to light, and the chromosome would not move, even though the cell appeared undamaged. After extensive experimentation, researchers at UIUC developed a method that allowed them to take pictures without killing the movement.
Using this system, UIUC graduate student Chien-Hui Chuang studied a chromosome that is normally found in an inactive state near the nuclear periphery and moves to the interior of the nucleus when it receives an activating signal.
"The movement following activation was radically different from the rapid, but short-range, diffuse movement previously observed in these nuclei," said Dr. Andrew Belmont, professor of cell and developmental biology at UIUC, principle investigator and co-author of the study. It was clear that this was directed movement that required a motor, Belmont said, because the chromosome was moving in a nearly straight line perpendicular to the nuclear envelope. The chromosome traveled further in several minutes than ever observed, even over several hours, in the absence of activation.
"It looked nothing like the random, but localized, bouncing around that had been previously observed," he said.
Belmont’s group collaborated with Primal de Lanerolle, professor of physiology and biophysics at UIC, who had discovered a type of myosin in the nucleus. Most myosin molecules are found in the cytoplasm, where they interact with actin filaments to do physical work. Because these molecules can contract muscles or move things around, they are called molecular motors. de Lanerolle and his colleagues were able to offer the Belmont laboratory a number of ways to test whether the chromosome movement was actin/myosin-dependent.
When the researchers introduced a mutant form of myosin protein to the nucleus, the movement slowed. Introducing a mutant actin that does not form filaments stopped the movement, while the introduction of an actin mutant that enhances filament formation accelerated the movement. In addition, when a drug that inhibits actin/myosin interactions was added to the cells, the chromosome movement was stopped completely. These experiments conclusively established that actin and myosin are involved in this chromosome movement.
"While we have known for a long time that actin is present in the nucleus and we had shown that myosin is also present in the nucleus, nobody really knew if they worked together," said de Lanerolle.
"There has been tantalizing evidence of organization in the nucleus-active genes found in the central region associated with nucleus complexes of transcription machinery necessary for gene expression, while inactive genes are found at the periphery," Belmont said. "For the first time, we have been able to observe an active mechanism for directed long-range chromosome movements that depend directly or indirectly on actin and myosin."
Other authors include graduate students Anne Carpenter at UIUC, currently at a post-doctoral fellow at M.I.T., and Beata Fuchsova and Terezina Johnson at UIC.