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Medical physics

Optical trap reveals molecular tension

17 Oct 2017 Aaron Blanchard 
Left to right: William Weis and Nick Bax (co-advised by Alex Dunn) of the Weis lab; Alex Dunn, Derek Huang and Craig Buckley of the Dunn lab.
Left to right: William Weis and Nick Bax (co-advised by Alex Dunn) of the Weis lab; Alex Dunn, Derek Huang and Craig Buckley of the Dunn lab.

Cells must sense, respond to, and exert mechanical signals to navigate their physical surroundings. Recent advances in single-molecule manipulation techniques are now allowing researchers to study how mechanical signals interact with individual biomolecules. And an interdisciplinary team at Stanford University in the US has now exploited these techniques to reveal how one key biomolecule strengthens under mechanical tension in an orientation-dependent manner. Their findings, reported in Science, have important implications for our understanding of cell movement (Sciencedoi: 10.1126/science.aan2556).

According to William Weis, a professor in structural biology at Stanford, the importance of these mechanical phenomena cannot be understated. “Mechanical signals are key in countless critical biological processes, including embryonic development, immunity and cancer metastasis,” he explains.

Two components of the cell have received significant attention in this field: the cytoskeleton and adhesion complexes. “The cytoskeleton gives the cell its structure and generates the mechanical force needed for movement, while adhesion complexes are found on the cell’s exterior surface and anchor cells to surrounding tissues,” continues Weis. These complexes also contain mechanosensors – biomolecular machines that measure the mechanical properties of nearby objects.

You can compare a cell crawling across a surface to an ice-climber using axes to climb up a wall of ice. The climber’s back and arm muscles are like the cytoskeleton, generating the force needed to pull her up. As the climber ascends, she uses her axe both to check the strength of the ice and to pull herself up, analogous to the way a cell uses an adhesion complex for mechanosensing and adhesion.

However, a critical factor in this process is the climber’s hand, which links her force-generating muscles to the axe. Similarly, the cell has special biomolecular machinery that links the force-generating cytoskeleton to the adhesion complex.

Vinculin is a protein that does just that. While there are several different types of adhesion complexes containing vastly different biomolecular machinery, vinculin is one of the few known biomolecules that plays a role in all of them. Recent studies have shown that vinculin’s role as a one-size-fits all connector is critical for several cellular tasks such as unidirectional movement.

The team sought to study the effect of mechanical force on individual vinculin molecules. “Using a single-molecule technique called the optical trap, we attached single cytoskeleton fibres to single vinculin molecules and then pulled the two apart at a constant force,” explains Alex Dunn, a professor in chemical engineering. “Importantly, the high precision of the optical trap enabled us to tug with forces between 1 and 30 piconewtons, which is comparable to forces exerted by single biomolecules in live cells.”

The interdisciplinary team then measured the length of time that the vinculin molecule stayed attached to the fibre. Surprisingly, the team found that the vinculin could hold on for longer as they tugged with stronger forces. That result is counterintuitive; if our ice climber concentrates all her weight on one of her hands, then her forearm will tire out and she’ll be able to hang on for less time.

The team revealed that vinculin forms what’s known as a “catch bond” with the cytoskeleton fibre. It turns out that many biomolecules form catch bonds, and we now know that catch bonds are a critical factor in explaining how cells respond to mechanical signals.

As the team analysed the data, they started noticing that the catch-bond behaviour depended heavily on the orientation of the cytoskeleton fibre. This finding suggested that vinculin interacts differently with the cytoskeleton, depending on whether the fibre is pushing out to the cell’s edge or pulling in to the cell’s centre.

To illustrate how this finding could help explain cell behaviour, the team ran simulations of cytoskeleton fibres stochastically drifting over stationary vinculin molecules. The team found that the simulated fibres met much higher resistance due to vinculin binding when drifting backwards, which would result in a faster net motion of the polymerizing cytoskeleton out towards the cell’s outer edge. These findings could help explain how cells are able to crawl unidirectionally across a surface.

“This paper really points out that it’s important to think about how the cytoskeleton is organized. You have an adhesion molecule, vinculin, that not only senses force, but also senses its local cytoskeletal architecture,” says co-author and biophysics graduate student Derek Huang. The team is currently digging deeper into their findings to understand the biomolecular machinery that enables directionally-dependent catch-bond behaviour. They are also searching for this behaviour in other important adhesion molecules.

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