Chemists make cellular forces visible at the molecular scale

Date:
September 22, 2020
Source:
Emory Health Sciences
Summary:
Scientists have developed a new technique using tools made of
luminescent DNA, lit up like fireflies, to visualize the mechanical
forces of cells at the molecular level.



FULL STORY ========================================================================== Scientists have developed a new technique using tools made of luminescent
DNA, lit up like fireflies, to visualize the mechanical forces of cells at
the molecular level. Nature Methods published the work, led by chemists
at Emory University, who demonstrated their technique on human blood
platelets in laboratory experiments.


========================================================================== "Normally, an optical microscope cannot produce images that resolve
objects smaller than the length of a light wave, which is about 500 nanometers," says Khalid Salaita, Emory professor of chemistry and senior author of the study.

"We found a way to leverage recent advances in optical imaging along
with our molecular DNA sensors to capture forces at 25 nanometers. That resolution is akin to being on the moon and seeing the ripples caused
by raindrops hitting the surface of a lake on the Earth." Almost every biological process involves a mechanical component, from cell division
to blood clotting to mounting an immune response. "Understanding how
cells apply forces and sense forces may help in the development of new therapies for many different disorders," says Salaita, whose lab is a
leader in devising ways to image and map bio-mechanical forces.

The first authors of the paper, Joshua Brockman and Hanquan Su, did
the work as Emory graduate students in the Salaita lab. Both recently
received their PhDs.

The researchers turned strands of synthetic DNA into molecular tension
probes that contain hidden pockets. The probes are attached to receptors
on a cell's surface. Free-floating pieces of DNA tagged with fluorescence
serve as imagers.

As the unanchored pieces of DNA whizz about they create streaks of light
in microscopy videos.

When the cell applies force at a particular receptor site, the attached
probes stretch out causing their hidden pockets to open and release
tendrils of DNA that are stored inside. The free-floating pieces of DNA
are engineered to dock onto these DNA tendrils. When the florescent DNA
pieces dock, they are briefly demobilized, showing up as still points
of light in the microscopy videos.



========================================================================== Hours of microscopy video are taken of the process, then speeded
up to show how the points of light change over time, providing the molecular-level view of the mechanical forces of the cell.

The researchers use a firefly analogy to describe the process.

"Imagine you're in a field on a moonless night and there is a tree that
you can't see because it's pitch black out," says Brockman, who graduated
from the Wallace H. Coulter Department of Biomedical Engineering, a joint program of Georgia Tech and Emory, and is now a post-doctoral fellow at Harvard. "For some reason, fireflies really like that tree. As they land
on all the branches and along the trunk of the tree, you could slowly
build up an image of the outline of the tree. And if you were really
patient, you could even detect the branches of the tree waving in the wind
by recording how the fireflies change their landing spots over time."
"It's extremely challenging to image the forces of a living cell at
a high resolution," says Su, who graduated from Emory's Department of
Chemistry and is now a post-doctoral fellow in the Salaita lab. "A big advantage of our technique is that it doesn't interfere with the normal behavior or health of a cell." Another advantage, he adds, is that DNA
bases of A, G, T and C, which naturally bind to one another in particular
ways, can be engineered within the probe-and- imaging system to control specificity and map multiple forces at one time within a cell.



========================================================================== "Ultimately, we may be able to link various mechanical activities of
a cell to specific proteins or to other parts of cellular machinery,"
Brockman says.

"That may allow us to determine how to alter the cell to change and
control its forces." By using the technique to image and map the
mechanical forces of platelets, the cells that control blood clotting
at the site of a wound, the researchers discovered that platelets
have a concentrated core of mechanical tension and a thin rim that
continuously contracts. "We couldn't see this pattern before but now we
have a crisp image of it," Salaita says. "How do these mechanical forces control thrombosis and coagulation? We'd like to study them more to see
if they could serve as a way to predict a clotting disorder." Just as increasingly high-powered telescopes allow us to discover planets, stars
and the forces of the universe, higher-powered microscopy allows us to
make discoveries about our own biology.

"I hope this new technique leads to better ways to visualize not just
the activity of single cells in a laboratory dish, but to learn about cell-to-cell interactions in actual physiological conditions," Su
says. "It's like opening a new door onto a largely unexplored realm --
the forces inside of us." Co-authors of the study include researchers
from Children's Healthcare of Atlanta, Ludwig Maximilian University
in Munich, the Max Planck Institute and the University of Alabama at Birmingham. The work was funded by grants from the National Institutes
of Health, the National Science Foundation, the Naito Foundation and
the Uehara Memorial Foundation.


========================================================================== Story Source: Materials provided by Emory_Health_Sciences. Original
written by Carol Clark.

Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Joshua M. Brockman, Hanquan Su, Aaron T. Blanchard, Yuxin Duan,
Travis
Meyer, M. Edward Quach, Roxanne Glazier, Alisina Bazrafshan,
Rachel L.

Bender, Anna V. Kellner, Hiroaki Ogasawara, Rong Ma, Florian
Schueder, Brian G. Petrich, Ralf Jungmann, Renhao Li, Alexa
L. Mattheyses, Yonggang Ke, Khalid Salaita. Live-cell super-resolved
PAINT imaging of piconewton cellular traction forces. Nature
Methods, 2020; DOI: 10.1038/s41592-020- 0929-2 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/09/200922112234.htm

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