Molecular Tug-of-war Gives Cells Their Shape

In a new study, University of Maryland researchers have demystified the process by which cells receive their shape—and it all starts with a protein called actin.

Actin is a key component of the cytoskeleton that provides structure to cells, much like how our skeletons support our bodies. However, unlike our skeleton, the actin cytoskeleton is a highly malleable structure that can rapidly assemble and disassemble in response to biochemical and biophysical cues. 

It is well known that actin can form both 3D spherical shell-like structures that protect cells from external pressure and 2D rings that modify intracellular functions. But whenever researchers tried to recreate these structures outside the cell, they almost always ended up with clusters of actin. No one knew why—until now.

The researchers used computer simulations to show that actin and its partner protein, myosin, engage in a tug-of-war, with myosin trying to trap actin in local clusters and actin attempting to flee. If actin wins, actin filaments escape myosin’s pulling force and spontaneously form rings and spherical shells. If myosin wins, the actin network collapses and forms dense clusters. 

Actin (shown in magenta and in box “a”) and myosin (shown in green and in box “b”) are depicted in the actin rings of live T cells (box “c”). Box “d” provides a snapshot of MEDYAN simulations, which resemble the actin ring found in T cells. Credit: Haoran Ni.  Actin (shown in magenta and in box “a”) and myosin (shown in green and in box “b”) are depicted in the actin rings of live T cells (box “c”). Box “d” provides a snapshot of MEDYAN simulations, which resemble the actin ring found in T cells. Credit: Haoran Ni.  

“Actin rings and spherical shells are ubiquitous in almost all cell types across species. We think that understanding the mechanism behind the formation of these structures unlocks the door to how cells sense and respond to their environment,” said Garegin Papoian, a co-author of the study and a UMD Monroe Martin Professor in the Department of Chemistry and Biochemistry and the Institute for Physical Science and Technology (IPST).

Their findings, published Oct. 21, 2022 in the journal eLife, could have important implications for human health. Because actin rings are central to our bodies’ ability to fight off foreign cells—with defects potentially resulting in impaired immunity or autoimmune disorders—the findings of this study could aid the development of future drugs.

Actin monomers can be thought of as railroad cars, which link up to form a train-like actin filament. These actin trains move through the cell because of a process called treadmilling. Also at play are the myosin motors, which pull oppositely oriented trains toward each other. Papoian, Qin Ni (Ph.D. ’21, chemical engineering) and biophysics Ph.D. student Haoran Ni believed that a competition between myosin’s pulling force and the rate of treadmilling was responsible for the formation of actin rings.

Fine-tuning these parameters in living cells is not possible, so the researchers turned to a simulation software called MEDYAN, developed by the Papoian Lab. MEDYAN uses physics and chemistry rules to simulate the dynamics of cytoskeletal proteins. They simulated an actin and myosin network (collectively referred to as actomyosin) in a thin disc and spherical shell.

They found that if the actin trains move slowly, the myosin pulling force causes traffic jams, which are the actomyosin clusters that have been observed in networks reconstituted outside cells. On the other hand, if the actin trains move fast, they can escape myosin’s pull. Once they reach the boundary of the disc, myosin’s pulling force makes the actin trains turn, preventing a head-on collision with the disc edge. Repeated occurrence of these events results in all the trains moving in a circle along the perimeter of the disc, which forms the actin ring.

Further analysis offers a thermodynamic theory to explain why cells form rings and shells. According to the laws of physics, systems favor the lowest energy configuration. Myosin proteins generate a lot of mechanical energy by bending actin filaments, which can only be released if actin can run away and relax. In living cells, actin’s ability to move fast enough to escape myosin and run to the edge allows for this built-up energy to be released, allowing for the formation of rings or shells, which, thermodynamically speaking, is the lowest energy configuration.

“The reason rings were not previously seen outside the cell is because actin just wasn’t moving fast enough,” Papoian said. “Myosin was winning 10 times out of 10.”

Together with Professor Arpita Upadhyaya and physics graduate student Kaustubh Wagh, biological sciences graduate student Aashli Pathni and biophysics graduate student Vishavdeep Vashisht, the team set out to test this model in living cells by turning their attention to T cells, where rings naturally form.

T cells are the cells in our body that hunt down foreign cells. When they recognize a cell as foreign and become activated, the T cell cytoskeleton rapidly reorganizes itself to form an actin ring at the cell-cell interface. Starting with cells that had formed rings, the researchers investigated the effect of perturbing actin and myosin using high-resolution live-cell imaging.

Reducing the actin train speed resulted in dissolution of the ring into small clusters, while increasing myosin’s pulling force led to rapid contraction of the ring, in remarkable agreement with associated simulations.

As a follow-up to this study, the team plans to add more complexity to the model and include other cytoskeletal components and organelles.

“We have been able to capture one fundamental aspect of cytoskeletal organization,” Papoian said. “Piece by piece, we plan to build a computational model of a complete cell using fundamental principles from physics and chemistry.”


Original story:

This article is adapted from text provided by Qin Ni and Kaustubh Wagh.

The research paper, “A tug of war between filament treadmilling and myosin induced contractility generates actin rings,” was published in eLife on Oct. 21, 2022.  

This work was supported by the National Science Foundation (Award Nos. CHE-1800418, PHY-1806903 and PHY-1607645) and the National Institutes of Health (Award No. R01 GM131054). This story does not necessarily reflect the views of these organizations.

Media Relations Contact: Emily C. Nunez, 301-405-9463, This email address is being protected from spambots. You need JavaScript enabled to view it.

UMD Team Leads a New Test of Universality of Leptons at the LHCb Experiment

The LHCb collaboration has presented a new test of the universality of the electroweak properties of leptons.

Nearly seven years of analysis of LHCb data by University of Maryland physicists Phoebe Hamilton and Hassan Jawahery and CERN collaborator Greg Ciezarek led to the results unveiled October 18, 2022 at a seminar at CERN and presented the next day at a flavor physics workshop at CERN.The results reflect analysis of seven years of data by Phoebe Hamilton and Hassan Jawahery and their CERN collaborator, Greg Ciezarek.The results reflect analysis of seven years of data by Phoebe Hamilton and Hassan Jawahery and their CERN collaborator, Greg Ciezarek.

In 2015, the LHCb team reported on a test of lepton universality with the measurement of a key observable. But the new results represent the first simultaneous measurements of two correlated observables at the LHC collider, significantly improving the sensitivity to new physics effects. This is particularly important for the extensions of the Standard Model that contain additional Higgs bosons. The results are consistent with previous measurements, which hinted at deviation from lepton universality.  The combined values are at 3.2 standard deviation from the Standard Model. 

These studies are a major theme of the physics program of the Maryland group in LHCb with the current and the future data with the upgraded detector. Over the past decade they have carried out a broad program of studies of lepton flavor universality in decays of particles containing the b quark, which have been published in PRL and highlighted in the CERN Courier and Symmetry magazine.  Professor Manuel Franco Sevilla, whose PhD thesis work at BaBar provided the first hint of deviation of these observables from universality, has recently co-authored a comprehensive review of the past studies and the prospects for future measurements in Review of Modern Physics


Dragt Awarded Robert R. Wilson Prize

The American Physical Society (APS) has honored Professor Emeritus Alex Dragt with the 2023 Robert R. Wilson Prize for Achievement in the Physics of Particle Accelerators. He was cited "for pioneering contributions to the development and application of Lie methods in accelerator physics and nonlinear dynamics," and will receive a $10,000 award.

Dragt studied chemistry and mathematics at Calvin University before earning his Ph.D. in theoretical physics at the University of California, Berkeley, under Robert Karplus.  After an appointment at the Institute for Advanced Study, Dragt joined the Department of Physics as an assistant professJohn Toll and Alex DragtJohn Toll and Alex Dragtor in 1965, and served as department chair from 1975-78.  He led the Dynamical Systems and Accelerator Theory Group, whose work included the computation of charged particle beam transport and the computation of electromagnetic fields and beam-cavity interactions.  He received the University of Maryland Regents' Award for Excellence in Teaching in 1967 and was named a University of Maryland Distinguished Scholar-Teacher in 1984.

Dragt (rear, white shirt) with colleagues in the Center for Superconductivity Research (now QMC).Dragt (rear, white shirt) with colleagues in the Center for Superconductivity Research (now QMC).In 2002, Dragt served as Chair of the Executive Committee of the APS Division of Physics of Beams.  From 1985-1993 he was as an editor of Physica D: Nonlinear Phenomena. He was recognized with the 2013 IEEE Particle Accelerator Science and Technology (PAST) award for substantial contributions to the analysis of nonlinear phenomena in accelerator beam optics.

He has had several visiting appointments, including at the Institut des Hautes Etudes Scientifiques; the Institute for Theoretical Physics of the University of California, Santa Barbara; Los Alamos National Laboratory; the SSC Design Center at the Lawrence Berkeley Laborator; and the Stanford Linear Accelerator Center.

Dragt is a Fellow of the APS and the American Association for the Advancement of Science and a member of the IEEE, the American Geophysical Union, and the American Mathematical Society.

APS Wilson prize announcement:

Maryland Quantum-Thermodynamics Hub Launches With $2M Grant

The Maryland Quantum-Thermodynamics Hub, supported by a grant from the Templeton Foundation, will bring together researchers from several universities to galvanize a field that is central to understanding the workings of our universe and to developing robust quantum technologies, ranging from a new class of computers to secure communications networks. It will be based in the Institute for Physical Science and Technology at UMD.

Leading the project at UMD are Christopher Jarzynski, a Distinguished University Professor with appointments in chemistry and biochemistry and in physics, and Nicole Yunger Halpern, a fellow in the Joint Center for Quantum Information and Computer Science (QuICS) and a scientist at the National Institute of Standards and Technology.

“We’re delighted at the support that UMD institutions, including IPST, have demonstrated for establishing this hub and for quantum thermodynamics in general,” said Yunger Halpern. “We look forward to building a North American lodestone for quantum thermodynamics, inspired by our international peers.”

[Mapping the Quantum Frontier: UMD Expands Its Footprint as the ‘Capital of Quantum’]

In the quantum world, you don’t need to just think about the movement of energy and particles—as in traditional thermodynamics—but also about the movement of information in uniquely quantum ways. The thermodynamic rules of quantum systems require significant additional research before physicists can understand them as well as they do the established laws of thermodynamics that govern heat flow in things like refrigerators, steam engines and rocket thrusters.

The Maryland hub team is not scrapping traditional thermodynamics completely, however. Instead, it is combining physicists’ established understanding with the modern tools of quantum physics and information processing to develop deeper insights.

The scientists involved in the Maryland Quantum-Thermodynamics Hub say they’re not only interested in a richer understanding of quantum physics and its use in technology, but also how it connects to the flow of time and the laws of classical physics we constantly see playing out in everyday life.

In addition to cutting-edge research, the hub team also plans to organize symposia, seminars, an international conference, a visitors program and a science-fiction short-story contest.

In addition to Jarzynski and Yunger Halpern, senior personnel involved include University of Maryland, Baltimore County Associate Professor of Physics Sebastian Deffner; UMD Assistant Research Scientist Luis Pedro García-Pintos, who is also a member of the Joint Quantum Institute and QuICS; University College Dublin Assistant Professor Steve Campbell; University of Southern California Quantum Information Scientist Amir Kalev, who was formerly a Hartree postdoctoral fellow at QuICS; and Arizona State University Assistant Professor Kanu Sinha, Ph.D. ’15.

Story by Bailey Bedford

Read a blog post on the establishment of the hub by Nicole Yunger Halpern, or Maryland Today’s previous story on her recent book on the field of quantum steampunk.