Tiny ‘hairpin’ probes are made of DNA

The new cell traction force probes consist of single strands of DNA shaped as hairpins of different lengths and DNA sequences—each tuned to unfold when subject to a specific amount of force, and attached to a light-emitting fluorophore (a fluorescent chemical compound) and a "quencher" that effectively dims the fluorophore's emitted light. (Credit: iStockphoto)

A new force probe, shaped like a hairpin and made from DNA, offers higher-resolution measurements of cell traction forces.

The way individual cells tug on their immediate environment plays a key role in fundamental biological processes like cell division, differentiation, and migration, as well as more complex ones like embryonic development and inflammation.

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When receptors on the cell surface called integrins adhere to the cell’s surrounding network of protein scaffolding known as the extracellular matrix, the forces they exert signal to the rest of the cell that important cellular processes are underway.

Measuring those forces over time could reveal more about the inner workings of normal and abnormal cellular development, and enable clinicians to more effectively diagnose and treat cancer, scarring, and inflammation.

The researchers describe and demonstrate this advance in Nature Methods.

“Traditional methods that rely on watching cells pull on soft, synthetic materials could only resolve these forces at the micron or cellular scale, but the new method can do so at the nanoscale level where individual integrins adhere to the extracellular matrix, enabling researchers to obtain a much clearer picture of these forces,” says Professor Christopher Chen of Boston University.

Fluorophores and ‘quenchers’

The research team engineered a new class of cell traction force probes consisting of single strands of DNA shaped as hairpins of different lengths and DNA sequences—each tuned to unfold when subject to a specific amount of force, and attached to a light-emitting fluorophore (a fluorescent chemical compound) and a “quencher” that effectively dims the fluorophore’s emitted light.

When a cell’s integrin pulls on the extracellular matrix (ECM), the force that it exerts causes one of the DNA hairpin probes to unfold, thereby separating the quencher from the fluorophore and freeing it to emit light.

Based on the intensity of this light, a microscope measures the forces exerted by clusters of integrins at each site where a cell adheres to the ECM, and shows how these forces are distributed throughout the cell.

As microscopy advances, the new method will enable measurement of cell traction forces at the single integrin level as well.

Chen conceived of the new approach with David Liu, a Harvard University professor of chemistry and chemical biology, over a long flight, and now aims to implement it to help uncover new knowledge about the mechanisms underlying cellular development.

“By engineering individual molecules as light-emitting force sensors, we hope to be able to use these as coatings to better understand how cells generate forces in a variety of settings where they have not been characterized,” he says.

Coauthors of the study contributed from Harvard, Stanford, Penn, the Wyss Institute for Biologically Inspired Engineering, and the Howard Hughes Medical Institute contributed to the study, which the National Institutes of Health and HHMI funded.

Source: Boston University