Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session S31: Emergent Collective Dynamics in Biology: from Microbes to Organs IFocus
|
Hide Abstracts |
Sponsoring Units: DSOFT DBIO GSNP Chair: Miguel Ruiz Garcia, University of Pennsylvania Room: 503 |
Thursday, March 5, 2020 11:15AM - 11:51AM |
S31.00001: Dynamics and self-organization of active surfaces Invited Speaker: Frank Julicher Biological cells are active systems and exhibit complex dynamic behaviours such as cell division, cell polarity establishment and cell locomotion. Such dynamic processes emerge from the collective interplay of many molecular components far from thermodynamic equilibrium. Active molecular processes such as the force generation of molecular motors along filaments of the cytoskeleton transduce chemical free energy to generate movements and mechanical work. The cell cortex, a thin film of active material assembled below the cell membrane, plays a key role in cellular symmetry breaking processes such as cell polarity establishment and cell division. I will present a minimal model of the mechano-chemical self-organization of the cell cortex that is based on a hydrodynamic theory of curved active surfaces and that can capture the emergence of shapes. Active stresses on this surface are regulated by a diffusing molecular species. We show that coupling of the active surface to a passive bulk fluid enables spontaneous polarization and the formation of a contractile ring on the surface via mechano-chemical instabilities. We discuss the role of external fields in guiding such pattern formation. Our work reveals that key features of cellular symmetry breaking and cell division can emerge in a minimal model via general dynamic instabilities. Self-organised active surfaces provide minimal models for the generation of shape and the emergence of dynamic patterns in basic cellular processes. |
Thursday, March 5, 2020 11:51AM - 12:03PM |
S31.00002: Microtubule Motility on Fixed and Diffusive Motor Proteins Fereshteh Memarian, Joseph Lopes, Linda S. Hirst Motor proteins transporting cargo by walking on microtubules (MTs). MTs are part of the cytoskeleton and are important in cell division, giving shape to the cell membrane. Here we investigate motor-based transportation and behavior of MTs. In a MT gliding assay, motor proteins adhere to the glass while MTs glide on them. MT behavior in gliding assay experiments with high MT concentration is compared with behavior on diffusing motor proteins and at similar concentration. An interesting twist on the standard gliding assay is to attach the motors to a lipid bilayer and let them diffuse on the membrane. Using fluorescence recovery after photobleaching (FRAP), we measured motor diffusion constants. We demonstrate the MT “snuggling” effect, i.e. they spontaneously align locally while gliding on the membrane-bound motor proteins. When MTs encounter each other for gliding assays, they simply cross over in the absence of a crowding agent. Furthermore, we observe that MTs exhibit collective motion, much like bird flocking, with diffusing kinesins, which is not replicable with fixed kinesins. |
Thursday, March 5, 2020 12:03PM - 12:15PM |
S31.00003: Modeling of the collective motion of microtubules with mobile kinesin motors. Madhuvanthi Athani, Fereshteh Memarian, Linda S. Hirst, Daniel Beller The microtubule gliding assay, an in-vitro technique commonly used to study motility of motor proteins, also provides an opportunity to study the collective motion of microtubules as an active fluid. Computational modeling of this system usually studies the microtubule positions assuming a spatially uniform distribution of the motor proteins, which models the experiments where the positions of motors are fixed. We explore a modified setup where kinesin motors are mobile. We use Brownian Dynamics simulations to model the collective motions of microtubules with explicit consideration of the concentration of the motors. We investigate spontaneous dynamical spatial variation in microtubule density that is introduced by allowing diffusion and transport of the motors. We also address how the mobile kinesin motors change the phase diagram as a function of kinesin and microtubule densities. |
Thursday, March 5, 2020 12:15PM - 12:27PM |
S31.00004: Multi-functional crystalline frameworks self-assembled from amphiphilic DNA nanostructures. Ryan Brady, Nicholas Brooks, Pietro Cicuta, Lorenzo Di Michele Several emerging technologies require the production of porous frameworks with a precisely controlled nanoscale morphology and stimuli-responsiveness. Due to the binding selectivity of nucleic acids, their facile synthesis and functionalization, DNA nanotechnology has emerged as a prime route for the production of programmable nanoscale materials. Nonetheless, the reliable preparation of crystalline, highly porous and fucntional 3D DNA frameworks remains elusive. |
Thursday, March 5, 2020 12:27PM - 12:39PM |
S31.00005: Controlled membrane remodeling by DNA origami nanorods: Experiments targeting the design principles for membrane-based materials Sarah Zuraw, Anthony Duprat Dinsmore, Mahsa Siavashpouri, Zvonimir Dogic, Thomas Gerling, Hendrik Dietz Membrane remodeling facilitated by the self-assembly of proteins on the membrane is essential for cellular function. Inspired by this system, we use DNA origami nanorods to illuminate the role of particle shape and adhesion on membrane reconfiguration. We combine giant unilamellar vesicles with oppositely charged nanorods and observe them with optical and electron microscopy. The binding affinity of the nanorods to the membrane is tunable via lipid composition, which reveals three primary behaviors. For weak particle binding vesicles adhere to one another and form a stable gel. At intermediate binding strengths the gel forms but is subsequently destroyed by avid binding of the nanorods. At higher binding strengths the vesicles rupture without forming a gel. Cryo transmission electron microscopy reveals in-plane ordering of rods on the membrane. These responses are robust and repeatable providing a physical understanding of the dependence on shape, binding affinity and concentration in membrane remodeling. The design principles derived from these experiments will lead to bio-inspired membrane materials that are stimuli-responsive and reconfigurable. |
Thursday, March 5, 2020 12:39PM - 12:51PM |
S31.00006: A coarse-grained model for lipid bilayer formation, fusion, and its hydrodynamics Yuan-nan Young, Szu-Pei Fu, Rolf Ryham In this paper a theoretical model for long-range, hydrophobic attraction between amphiphilic particles is developed to quantify the macroscopic assembly and mechanics of a lipid bilayer membrane in solvents. The non-local interactions between amphiphilic particles are obtained from the first domain variation of a hydrophobicity functional, giving rise to forces and torques (between particles) that dictate the motion of both particles and the surrounding solvent. The functional minimizer (that accounts for hydrophobicity at molecular-aqueous interfaces) is a solution to a boundary value problem of the screened Laplace equation. We reformulate the boundary value problem as a second-kind integral equation. Solving a mobility problem in Stokes flow is incorporated to obtain corresponding rigid body motion. The simulated fluid-particle systems exhibit a variety of multiscale behaviors over both time and length: Over short time scales, the numerical results show self-assembly for model lipid particles. For large system simulations, the particles form realistic configurations like micelles and bilayers. Over long time scales, the bilayer shapes emerging from the simulation appear to minimize a form of bending energy. |
Thursday, March 5, 2020 12:51PM - 1:03PM |
S31.00007: Phase separation and domain registry in giant multilamellar vesicles prepared with biologically-relevant lipid compositions. Dylan Steer, Cecilia Leal Lipids are amphiphiles that commonly form bilayer sheets in the form of vesicles when exposed to excess water. Most commonly lipids have been studied in the context of the plasma membrane and other unilamellar vesicles containing a single bilayer. However, several key biological systems such as the myelin sheath, tubular myelin, or pulmonary lipid-based membranes are known to rely on correlated stacks of lipid bilayers into onion-like multi-lamellar vesicles. Lateral ordering of lipids is well-understood mostly in single-bilayer systems. Most often, the macroscopic distribution of phases is only well-characterized for giant unilamellar vesicles. Here we report on the macroscopic arrangement of phase-separated domains in multilamellar forms with biologically-relevant compositions. We will show how the registration of individual domains results in large changes in macroscopic vesicle structure and properties. Confocal fluorescence microscopy allows direct visualization of phase separation, supported by SAXS and spectrometry studies of lipid ordering. |
Thursday, March 5, 2020 1:03PM - 1:15PM |
S31.00008: Clustering dynamics of collectively migrating malignant lymphocytes Farnaz Golnaraghi, David A. Quint, Nir Schachna Gov, Ajay Gopinathan Multi-cellular aggregates such as cell clusters and tissues exhibit collective migration with complex emergent behaviors that are very different from the behavior of the constituent single cells. We focus on the migration of clusters of malignant lymphocytes, responsible for the metastases of lymphomas. Previous work has shown that, in chemokine gradients, these clusters show a number of novel collective phases including rotations that enhance their chemotactic efficiency. Here we study the attachment and detachment dynamics that ultimately guides the formation of the clusters, which is similar to the gas-liquid transition of passive (thermal equilibrium) systems. We aim to quantify cell cluster shape dynamics and chemotactic efficiency as a function of cellular parameters such as, cell-cell adhesion and alignment, contact inhibition and chemotactic response. In particular, we show that cell-cell adhesion and alignment are important regulators of cell cluster size and shape, which in turn affects their chemotactic efficiency. Our systematic approach will allow us to identify regions of parameter space that may shed light on candidate strategies for suppressing metastatic potential. |
Thursday, March 5, 2020 1:15PM - 1:27PM |
S31.00009: Imaging the emergence of bacterial turbulence using light-powered E. coli Yi Peng, Zhengyang Liu, Xiang Cheng The collective motion of bacteria leads to intermittent jets and swarming vortices, a fluid pattern often referred as bacterial turbulence. We investigate the emergence of the collective motion of Escherichia coli suspensions and explore the kinetic pathway towards bacterial turbulence. We map the phase diagram of bacterial flows as functions of bacterial concentration, bacterial swimming speed and the number fraction of active bacteria. A simple model based on two-body hydrodynamic interaction quantitatively predicts the phase boundary of the 3D phase diagram. Furthermore, we trigger bacterial turbulence by using genetically engineered light-powered E. coli, whose swimming speeds vary with light intensity. The kinetics show one step near the phase boundary and two steps with an intermediate state far above the phase boundary. The transition rate increases as the system moves deep inside the turbulent phase. Our research reveals the microscopic origin of bacterial turbulence and provides new insights into nonequilibrium phase transitions in active matter. |
Thursday, March 5, 2020 1:27PM - 1:39PM |
S31.00010: Activity-induced phase transitions in confined bacterial suspensions Dipanjan Ghosh, Zhengyang Liu, Xiang Cheng Active matter exhibits fascinating emergent ordered structures absent in passive equilibrium systems and, therefore, has been extensively studied as a model to explore nonequilibrium phase transitions. Here, we study the emergence of collective order in confined suspensions of genetically engineered Escherichia coli, whose propulsion speed can be controlled via intensity of light. The suspensions, when placed in a Hele-Shaw cell having a gap thickness of 10 microns, show various ordered phases unseen in their three dimensional counterparts. Using a bright-field microscope, we image the positions and orientations of individual bacterium through different phase transitions. We identify a transition from a disordered phase to a nematic phase characterized by a long-range orientational order and the formation of lanes. The dependence of transition dynamics on the concentration and swimming speed of bacteria is explored. Moreover, we examine the rise of bacterial polar ordering within the nematic phase. Our study provides an experimental benchmark for understanding the role of complex interplays between hydrodynamic and steric interactions responsible for the emergence of ordered phases in confined active systems. |
Thursday, March 5, 2020 1:39PM - 1:51PM |
S31.00011: Comparing sperm collective swimming with flocking transition Allan Oduor, Yishak Bililign, Daniel Sussman, Soon Hon Cheong, Susan S Suarez, M. Lisa Manning, Chih-Kuan Tung In viscoelastic fluid, bovine sperm are able to interact and align with their neighbors to swim in clusters. The formation of the polar liquid phase, or large flocks of sperm, however, did not solely depend on the number density and the alignment mediated by the fluid, but was strongly influenced by the initial conditions. If a pulse of flow was used to create an aligned initial condition, hundreds of sperm were able to form a flock swimming in the same direction. This suggests that the transition is first-order, with strong hysteresis. Analyzing these flocks, we found the decay of the orientation correlation function to be linear on a log-log plot. From our finite flock sizes, there was no indication that the correlation function decayed to a non-zero value, as suggested theoretically for the polar liquid phase. Further, the effective exponents of the correlation function were found to vary for the same flock at different time points, which made us wonder about some of the premises of continuum theoretical models. From tracking individually swimming sperm, we found that the rotational noise is an exponential decay, while the speed follows a Gamma distribution. Neither is commonly used in theoretical models. |
Thursday, March 5, 2020 1:51PM - 2:03PM |
S31.00012: Correlations in suspensions of microswimmers Alexander Morozov Recent years witnessed a significant interest in physical properties of self-propelled particles that can extract energy from the environment and convert it into directed motion. One of the most striking consequences of this ability is the appearance of collective motion in self-propelled particles suspended in a fluid observed in recent experiments and simulations: at low densities particles move around in an uncorrelated fashion, while at higher densities they organise into jets and vortices comprising many individual swimmers. |
Thursday, March 5, 2020 2:03PM - 2:15PM |
S31.00013: Collective migration of bacteria in disordered media Tapomoy Bhattacharjee, Daniel Amchin, Felix S Kratz, Jenna A Ott, Sujit Datta While bacterial motility is well-studied on flat surfaces or in unconfined liquid media, most bacteria are found in disordered porous media, such as biological gels and tissues, soils, sediments, and subsurface formations. Understanding how porous confinement alters bacterial motility is therefore critical to modeling the progression of infections, applying beneficial bacteria for drug delivery, and bioremediation. We recently discovered that isolated cells of E. coli move through disordered media via intermittent hopping and trapping, reminiscent of thermally-activated transport. Here, we use direct visualization and 3D bioprinting to investigate how this behavior manifests in multicellular communities in porous media. We find that cellular chemotaxis drives collective migration—and that this process depends sensitively on pore-scale confinement, colony density, and differential metabolism of nutrients. Our results thus expand the current understanding of collective migration, which focuses on populations in homogeneous environments, to the case of bacteria in disordered porous media. |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700