Biology and the Engineer
Subra Suresh is borrowing tools from physics to understand nanoscale changes in diseased cells.
John Mills fiddles with the knobs on a microscope, but instead of looking into the eyepiece, he stares at a sphere displayed on a laptop’s screen. The laptop is connected to a video feed coming from the microscope, and Mills watches as fluids on a slide flow past the sphere, a tiny silica bead. After a few seconds, something that looks like a dented doughnut appears on the screen. It’s a red blood cell, and Mills quickly adjusts the microscope’s knobs until the bead “catches” the cell. He turns the knobs again, and a second silica bead appears and attaches to the cell. Then Mills slowly maneuvers the silica beads, which are coated with proteins that stick to the blood cell, so that the cell stretches out into the shape of a cigar.
Mills, a PhD student in materials science and engineering at MIT, is demonstrating what are probably the world’s most powerful optical tweezers, which he built as part of his thesis work with his advisor, Subra Suresh. Optical tweezers, which were developed in the mid-1980s, use the force of light to manipulate tiny objects. In this case, Mills uses a pair of lasers to control the silica beads. Using the beads as “handles,” Mills can apply a force of up to 500 pico-newtons to the red blood cell – several times that possible with previous optical tweezers – to test the elasticity of the cell’s wall.
[Click here for images of Suresh and his team at work.]
Using such ultrasensitive tools to measure the physical properties of a cell, such as its stiffness, “will let us look at things in ways we haven’t done before,” says Suresh, a professor in MIT’s Department of Materials Science and Engineering. For example, Suresh and his team are examining the way a force applied by the tweezers affects a healthy red blood cell, and then comparing the way a cell infected with a malaria parasite responds to a similar force. Suresh hopes that knowing precisely how malaria changes the physical properties of a cell could lead to better ways to treat the disease, or even to prevent it.
Studying such properties of biological components is relatively new ground for Suresh. As a materials scientist, he has spent much of his career studying the structural properties of materials used, say, as coatings or in thin films. While in France on sabbatical in April 2004, he spoke at a prestigious technical school in Paris, where he had a chance encounter at the cafeteria with a biology professor. The biologist did research at the Institut Pasteur and invited Suresh to speak there about his work.
The reaction of the biologists was enthusiastic. They weren’t using the precise tools – such as optical tweezers – that are relatively common in Suresh’s field. And they saw that Suresh’s expertise, experiments, and computer modeling might help them understand some of the physical changes that diseased cells undergo.
Although the malaria parasite, which attacks red blood cells, is one of the deadliest killers on the planet, there is a great deal scientists don’t know about how it works. Scientists do know that malaria makes red blood cells stiffer, which impedes their ability to move through the bloodstream, and it makes them stickier, which causes them to clump together and stick to blood vessel walls. But Suresh’s work has yielded far more precise knowledge of just how stiff red blood cells get. Researchers had believed infected cells to be about three times as stiff as healthy cells, but Suresh showed that they are in fact up to 10 times as stiff.
Malaria parasites grow to maturity within 48 hours. Suresh wants to know how the stiffness of affected red blood cells changes as the parasite matures.Experiments on such a time scale would have been almost impossible in the past: Suresh and his colleagues previously used optical tweezers that might need an hour to catch a single blood cell, and it took hours more to process the data they collected. With the new tweezers, Mills can grab a cell in seconds, and improved modeling software lets the team analyze the data in real time. The improvements in the technique make practical a series of experiments designed to study the action of specific proteins responsible for altering cells infected with malaria.
The first protein the Suresh group is studying is the RESA protein, which a malaria parasite introduces into an infected cell. The protein affects the cell membrane, and Suresh and his collaborators at the Institut Pasteur and the National University of Singapore want to see how the cell’s elasticity varies at different stages of the parasite’s development. The researchers hope to learn whether the protein is an attractive target for treating or preventing malaria.
In an effort to determine what role the RESA protein plays in malaria, Mills uses infected cells in which the protein is deactivated and then measures the stiffness of the cells at various points in the parasite’s 48-hour growth cycle. As a control, he also measures cells in which the protein is active; the comparison should show whether the protein’s inactivity at different stages of the parasite’s growth has any effect on cell structure.
The 30-minute setup of the tweezers includes a series of calibrations to make sure that the force exerted by the laser is small and precise enough for experiments on the nanoscale. “My biggest complaint is that parasites don’t sleep,” says Mills, who has to get up at all hours to test the stiffness of cells in different stages of infection. That test involves turning on his 10-watt laser, focusing the laser on the beads, and capturing a red blood cell. He then spends about half an hour applying various degrees of force to the cell, with the data and video being fed into his computer.
Suresh points back to the computer screen, where Mills has captured another cell. But red blood cells are not solitary things. The parasite creates “knobs” on the surface of a red blood cell that make it stick to healthy cells, sometimes causing clumping in the bloodstream. Such clumping can cause tremendous internal damage and even death.
“We think we can measure the force of adhesion between two cells – a measure of the stickiness, which also plays a huge role in the development of the disease,” Suresh says. “As far as we know, nobody has quantified that stickiness.” Suresh hopes that determining the force of adhesion will help lead to a malaria treatment that improves blood flow.
Although Suresh is excited about the biological work he’s doing, he’s also circumspect. Nanoscale measurement of the physical properties of biological cells is really still in its early phases, he says. “We’re just starting to put this together. It’ll be five years before we start to see where we can go. We still have to understand the science. Then we can figure out the potential for treatments.”