Researchers use laser tweezers and sub-micron focus to pinpoint the rhythmic rigidity of cell skeletons

Endothelial cells, which line the body's blood vessels and regulate the exchange of material between the blood stream and surrounding tissue, are one of the most closely studied types of cell in the body.

The cells play an important role in cardiovascular disease. And a greater knowledge of their interior functions may help scientists develop new cancer treatments that curb or suppress the growth of tumors by cutting off their blood supply.

Daniel Ou-Yang's research group at Lehigh is the first to use a laser tool known as optical tweezers to study the interior of an endothelial cell in a non-invasive way without introducing foreign particles into the cell or around it.

Achieving a resolution of 0.5 microns, Ou-Yang and his group can pinpoint and "trap" an organelle - a specialized part of a cell that resembles and functions like an organ - without damaging it.

They have discovered that the rigidity of the cytoskeleton, or cell skeleton, in the vicinity of the cell's organelles, appears to change by a factor of four in a rhythmical pattern with a periodicity of 20 to 30 seconds.

"This rhythm tells us something is alive," says Ou-Yang, a professor of physics, co-director of Lehigh's bioengineering program and a member of Lehigh's Center for Optical Technologies. "But it raises other questions. What triggers this rhythm? And what is its significance?"

Ou-Yang is collaborating with Linda Lowe-Krentz, professor of biological sciences. He also works with Profs. Ivan Biaggio and Volkmar Dierolf of the physics department and the COT, who specialize in the advanced imaging techniques necessary to measure the intracellular molecular signals.

Dierolf incorporates Raman spectroscopy scattering to see molecules without labeling (dyeing) them. Biaggio measure the mechanical properties of cells using nonlinear optical effects, which generate ultrasound waves to measure mechanical properties.

The work of Ou-Yang, Biaggio and Dierolf is supported by the COT. Ou-Yang and Lowe-Krentz are seeking a grant from the National Science Foundation.

Ou-Yang's group also includes several students. Meron Mengistu is a graduate student in molecular biology. Elizabeth Rickter, a graduate student in physics, was the first person to observe the rhythmic behaviors that appear to originate from endothelial cytoskeletons. And Laura Morkowchuk, a sophomore  bioengineering major, is studying the effect of the cytoskeletal rhythm on the transport of proteins from the blood stream to a cell's interior substrate tissues.

The overall goal of Ou-Yang's group is ambitious: To understand the mechanisms and functions of a cell in a quantitative way, and to map cell functions as scientists have already mapped such major body functions as respiration and digestion.

Ou-Yang has used optical tweezers in his research for more than 10 years, and is one of the pioneers in the technique. The tweezers, also called laser tweezers or optical traps, focus a laser beam through an optical microscope to trap micron-sized dielectric objects, which can then be manipulated by externally steering the laser beams.

Optical tweezers can pinpoint organelles at a resolution of 0.5 microns. The resulting vibration of the cell part is 0.5 nanometers, a measurement that Ou-Yang's group makes with an innovative application of optical diffraction.

The researchers are interested in cytoskeletal rigidity for several reasons. The cytoskeleton plays an important role in cell division. If scientists can learn how to suppress the rearrangement of the cytoskeleton that is necessary for mitosis to occur, they might be able to obstruct the growth of cancerous tumors, which depends on the often runaway rate of mitosis in cancerous cells.

Cytoskeletal rigidity has also been observed as a response to the chemical treatments used on cancer patients, Ou-Yang says. And tumor growth can be choked by depriving cancer cells of their blood supply, which is regulated by endothelial cells.

Two other Lehigh students have contributed to Ou-Yang's work with laser tweezers. Larry Hough, who received his Ph.D. in physics in August, is now a research scientist at the University of Pennsylvania. Megan Valentine, who earned a B.S. in physics from Lehigh in 1996, recently completed a Ph.D. in physics at Harvard, and is going to Stanford to become a research scientist in biophysics.