UCSD researchers stretched cells in a workout chamber the size of a credit card to gain a better understanding of how repetitive stretching of endothelial cells that line arteries can make them healthy and resistant to vascular diseases.
Bioengineering researchers at UCSD's Jacobs School of Engineering will report in the Nov. 1 issue of Proceedings of the National Academy of Sciences (PNAS) that arterial endothelial cells subjected to repeated stretching (10 percent of their length, 60 times per minute) produced intracellular arrays of parallel "stress fibers" in a few hours.
The tests were performed on endothelial cells lining the aorta of a cow, but the endothelial cells of the human aorta are expected to react similarly. The stress fibers were made of actin, a fibrous protein that is part of the machinery that gives muscle its ability to contract. Actin also gives virtually all cells their ability to make an internal "cytoskeleton." The stress fibers of endothelial cells in arteries are aligned parallel to the long axis of blood vessels, and this alignment is perpendicular to the direction of rhythmic stretching caused by a beating heart. Such an orientation of stress fibers is a hallmark of healthy blood vessels, but scientists currently understand few of the factors responsible for generating that configuration.
Rubber bands and most other flexible materials react to stretching by forming stress wrinkles parallel to the direction in which they are being pulled. However, the healthy bovine aorta endothelial cells did not behave that way in tests performed in the laboratory of Shu Chien, a coauthor of the PNAS paper and a professor of bioengineering and medicine and director of the Whitaker Institute of Biomedical Engineering at UCSD. When Chien and his collaborators stretched the cells back and forth along one axis in the miniature workout chambers, the cells formed stress fibers perpendicular to the direction of stretch. "This orientation of actin fibers can be thought of as a feedback control in which the external stresses imposed on the cell are felt internally to a much reduced degree," said Chien.
Post-doctoral fellow Roland Kaunas, now an assistant professor of biomedical engineering at Texas A&M University, with the help of UCSD laboratory assistant Phu Nguyen, found that unstretched cells or cells that were stretched only 1 percent of their length contained actin fibers with no directional orientation. However, as they increased the rhythmic stretching from 3 percent of a cell's length to 10 percent, stretch fibers became increasingly oriented perpendicular to the stretching direction.
In the most significant finding in the PNAS article, which was made available online Oct. 24, Chien's group reported that when an intracellular protein called Rho was chemically inhibited, stress fibers grew in the "wrong" direction; they grew parallel rather than perpendicular to the direction of cell stretching. Without Rho, the cells lost their ability to orient stress fibers properly. "Rho is a very important molecule," said Chien. "It works in response to, and in concert with, physical stretching to generate the healthy alignment of stress fibers." Indeed, when Chien's group used a genetic technique to increase the activity of Rho, those cells grew stress fibers in the healthy direction at a lower threshold of stretching.
"Until now, it has not been shown that there is an equivalence and cooperation between mechanical and biochemical stimuli to regulate the proper orientation of these stress fibers," said Kaunas. "Indeed, we found that the stress fibers oriented in such a way to control their level of stress -- not too little and not too much."
Chien and Kaunas collaborated with UCSD research scientist Shunichi Usami, who contributed to the design of the miniature workout chambers. Silicone rubber membranes inside the chambers were coated with a protein that allowed the endothelial cells to adhere to the membranes in a manner similar to how they attach to underlying blood vessel tissue in the body. The researchers isolated endothelial cells from the bovine aorta, grew the cells in culture flasks, and seeded them onto the silicone membranes. After the cells grew into confluent layers, a piston-like "indenter" was programmed to repeatedly push into the underside of the membranes and retract. The 60-cycle-per-minute motion of the indenter simulated the stretching movements of a blood vessel in response to the rising-and-falling blood pressure produced by a beating heart.
The researchers also demonstrated that inhibition of either Rho or a related protein called Rho kinase resulted in loss of the healthy alignment of stress fibers as well as alignment of adhesion sites where those stress fibers would attach to the cell membrane. These new results clearly show that Rho and physical stresses cooperate to produce healthy alignments of stress fibers," said Chien. "We need to understand how cells can sense the mechanical force and achieve this beneficial effect through the activation of Rho, and we also need to identify other proteins that may be involved in this feedback control mechanism."
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