Moving tiny things by optical tweezers.

Moving tiny things by optical tweezers 

A living cell is best handled with cell-sized tools. For three years or so, researchers have been honing their jeweler-like control over the movements of cells, microbes and even organelles within cells by using tightly focused, low-power laser beams as "optical tweezers."

At a meeting of MIT's nanotechnology study group this week, biophysicist Steven M. Block of the Rowland Institute for Science in Cambridge, Mass., reported using optical tweezers to probe the physical properties of "mechanoenzymes," proteins responsible for cellular movements such as the rotary motions of flagella, which propel bacteria.

Laser tweezers don't actually squeeze, but they allow researchers to lift up, move, and position microscopic objects, using the pressure of the laser light itself -- a phenomenon akin to a blast of air levitating a plastic ball.

"Laser tweezers are very much like a [science-fiction] tractor beam, except they work in the microscopic rather than the macroscopic domain," Block told SCIENCE NEWS. "You can manipulate living things without damaging them," adds physicist and optical-tweezer developer Arthur Ashkin of AT&T Bell Laboratories in Holmdel, N.J.

Last spring, Block and his co-workers reported experiments in which they used an optical tweezer to twist flagella -- the minuscule motor/propeller assemblies that bacteria and other microbes use to get around -- in order to measure their flexibility under applied forces.

Block, physiologist Bruce Schnapp of Boston University, biologist Lawrence Goldstein of Harvard University and others now are training optical tweezers on motion-making proteins -- such as myosin, kinesin, and dynein. Myosin works in muscle contraction. Kinesin helps move organelles (a term for a variety of substances within cells) along microtubules -- major components of the microscopic "skeletal" systems inside nerve cells. Dynein enables sperm tails to wiggle.

"The molecular mechanisms by which any biological motor works remain obscure," Block notes. One can study kinesin by coating bacteria-sized glass beads with the protein and observing how the coated particles hook onto and move along a microtubule. In these experiments, the beads appear to glide slowly along in a smooth motion that Block suspects emerges from the collective action of many kinesin molecules. "But if each of the kinesin molecules were actually doing a chiggedy-chiggedy moving along [as some models of kinesin-mediated movement propose], you might expect to see some sort of jerkiness."

Fade in optical tweezers. Beads adorned with only one or two kinesin molecules may never encounter a microtubule with an orientation that leads to an interaction. "So you grab a bead with the optical tweezers, and then you physically place it on the microtubule," Block says. Such control enables the scientists to test models of the mechanisms underlying molecular machines.

"As soon as you touch [kinesin] down to the microtubule, it just starts taking off," Block says. But the movements appear jerky, he reported this week. The sparsely adorned beads also spontaneously detach from the microtubule, a process that may reflect the kinesin molecules' natural cycle of operation. In a collaboration with other scientists, Block and his co-workers are studying the molecular mechanisms by which hair cells -- the sensory cells of the auditory system -- change their positions slightly as they adapt to become sensitive to different wavelengths of sound.

Scientists also use optical tweezers for sorting cells, moving organelles from one place to another within a single living cell, and for moving isolated chromosomes on a microscope slide, Ashkin says. The ability to move organelles from their normal positions opens doors to sophisticated studies of cell function. "Things are where they are [in cells] for particular reasons," Ashkin notes. What happens when you relocate them? Stay tuned, he says.