Molecular beacons shine light on how cells 'crawl'

Adherent cells, the kind that form the architecture of all multi-cellular organisms, are mechanically engineered with precise forces that allow them to move around and stick to things. Proteins called integrin receptors act like little hands and feet to pull these cells across a surface or to anchor them in place. When groups of these cells are put into a petri dish with a variety of substrates they can sense the differences in the surfaces and they will "crawl" toward the stiffest one they can find.

Now chemists have devised a method using DNA-based tension probes to zoom in at the molecular level and measure and map these phenomena: How cells mechanically sense their environments, migrate and adhere to things.

Nature Communications published the research, led by the lab of Khalid Salaita, assistant professor of biomolecular chemistry at Emory University. Co-authors include mechanical and biological engineers from Georgia Tech.

Using their new method, the researchers showed how the forces applied by fibroblast cells is actually distributed at the individual molecule level. "We found that each of the integrin receptors on the perimeter of cells is basically 'feeling' the mechanics of its environment," Salaita says. "If the surface they feel is softer, they will unbind from it and if it's more rigid, they will bind. They like to plant their stakes in firm ground."

Each cell has thousands of these integrin receptors that span the cellular membrane. Cell biologists have long been focused on the chemical aspects of how integrin receptors sense the environment and interact with it, while the understanding of the mechanical aspects lagged. Cellular mechanics is a relatively new but growing field, which also involves biophysicists, engineers, chemists and other specialists.

"Lots of good and bad things that happen in the body are mediated by these integrin receptors, everything from wound healing to metastatic cancer, so it's important to get a more complete picture of how these mechanisms work," Salaita says.

The Salaita lab previously developed a fluorescent-sensor technique to visualize and measure mechanical forces on the surface of a cell using flexible polymers that act like tiny springs. These springs are chemically modified at both ends. One end gets a fluorescence-based turn-on sensor that will bind to an integrin receptor on the cell surface. The other end is chemically anchored to a microscope slide and a molecule that quenches fluorescence. As force is applied to the polymer spring, it extends. The distance from the quencher increases and the fluorescent signal turns on and grows brighter. Measuring the amount of fluorescent light emitted determines the amount of force being exerted.

Yun Zhang, a co-author of the Nature Communications paper and a graduate student in the Salaita lab, had the idea of using DNA molecular beacons instead of flexible polymers. "She was new to the lab and brought a fresh perspective," Salaita says.

The molecular beacons are short pieces of lab-synthesized DNA, each consisting of about 20 base pairs, used in clinical diagnostics and research. The beacons are called DNA hairpins because of their shape.

The thermodynamics of DNA, its double-strand helix structure and the energy needed for it to fold are well understood, making the DNA hairpins more refined instruments for measuring force. Another key advantage is the fact that their ends are consistently the same distance apart, Salaita says, unlike the random coils of flexible polymers.

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Molecular beacons shine light on how cells 'crawl'