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Seeing is believing: Powerful new microscopy fuels studies in cell dynamics
March 2005--Denise Montell has been studying cell migration for a decade, trying to figure out why some epithelial cells stay put in their tissue of origin while a more adventurous group restlessly seeks opportunities for growth elsewhere. Migratory cells are industrious workers—essential for development, wound healing and immune surveillance—but a small subset travels for less benevolent purposes, fueling metastases that can kill.
“We’d like to understand how epithelial cells become migratory and invasive,” Montell says, “from both a basic science and a clinical point of view.”
Her pursuit will be greatly assisted by advances in live-cell imaging now enabling investigators to track the movement of individual protein molecules as they initiate, carry out and conclude their journeys.
“We’re going to see things,” says Montell, who directs the IBBS Center for Cell Dynamics, “that we just wouldn’t be able to detect any other way.”
Montell has long employed a forward genetics approach—inducing random mutations, selecting ones that cause problems, and isolating the respective genes—to identify the proteins that drive cell migration. Using Drosophila ovaries as a model system, her lab first looked for mutations that prevented or inhibited a group of six to 10 epithelial cells from migrating to the oocyte border as they normally do during the process of egg formation. The group identified the STAT protein, known to work with a tyrosine kinase called JAK to regulate transcription in the mammalian immune system.
They found a much simpler version of the STAT-JAK pathway in the Drosophila ovary and eventually discovered that it is precisely this pathway that determines which six cells out of thousands will migrate.
“And if we express it in more cells,” she says, “then they think they should go too.”
Montell and her students then became curious to see if proteins that seemed to control border cell migration in development might play a similar role in metastases. They decided to look at a motor protein, Myosin-VI, that another group of researchers had found highly expressed in border cells as they initiate migration. Researchers in the Montell lab soon found that limiting expression of the Myosin-VI protein produced a severe migration defect.
But the real shock came when they started looking at the human homologue of Myosin-VI. With Honami Naora, then a postdoc in Richard Roden’s lab here, they stained human tissue with a Myosin-VI antibody that revealed not a trace of the protein in normal ovaries and very little in borderline tumors but a spreading dark brown stain in high-grade ovarian carcinomas.
“Data from a large number of patients confirms that we don’t detect it at all in normal cells, but we always see a lot of it in high-grade tumors,” says Montell.
Cell culture and in vivo experiments with mice confirmed that Myosin-VI enhances cell migration and that curbing its expression greatly reduces the spread of such tumor cells in mice.
The next step for the Montell lab will be to use live-cell imaging of the very first stages of migration in the Drosophila ovary, when cells begin to extend long processes that chart the course to their new location. These studies require a mix of highly advanced microscopy and biosensors that tell investigators when and where a specific protein is active.
For example, Jin Zhang, an assistant professor of pharmacology here, recently developed a sensor for protein kinase A that flashes red when the protein is activated. The technique enables investigators to chart the dynamics of this protein in response to simuli or during a behavior such as cell migration.
Identifying just the right biosensors—molecules capable of reporting on the activities of various proteins—is critical. “The fluorescent sensors have to be specific and nontoxic, and they can’t interfere with the normal workings of the cell,” Montell says.
It is no longer adequate merely to know the distribution and location of proteins. In the past, researchers would kill cells, fix them, bring them in to take their picture. That tell where things are located, but nothing about process. ImMontell and her colleagues in the Center for Cell Dynamics are applying new imaging techniques to observe precise temporal and spatial locations of specific protein activities in living cells during critical processes like cytokinesis, chemotaxis, neuronal pathfinding and organogenesis.
Cell dynamics is still an emerging discipline, and there aren’t more than a dozen investigators throughout the country developing these techniques, Montell says. Their goals are ambitious, but given the power of the new microscopy, they seem increasingly within reach.
“We’ll be able to analyze mutant phenotypes in a whole new way,” Montell says. “Instead of just saying that migration is defective, we will be able to say precisely how it is defective.”