Session S27: Biological Active Matter III

Modulating the living cell dynamics by surface functionalization

During various physiological processes, such as morphogenesis, wound healing, and cancer invasion, collective cell migration occurs in a coordinated manner involving mechanical force generation. The forces balance with the cell-cell junctions and cell-substrate adhesion. By modulating cell-substrate adhesion, the collective migration of the cells can be tuned. Here, we vary the cell-substrate adhesion by surface functionalizing the substrate. We impose topological defects using a ridged topographical substrate coated with a negatively charged molecule. In this condition, fibroblast cells align azimuthally following the ridges. However, in the case of the positively charged ridged substrate, the cells change shape and become more isotropic. Moreover, the cell dynamics reveals different degree of collective migration leading the cells towards the topological defects with much higher average velocity on the positively charged surface than on the negatively charged one, due to the weaker cell-substrate adhesion. This work aims at elucidating the relation between adhesion and collective migration, while also providing a useful platform to tune cell migration.   

Balancing reaction-diffusion network for cell polarization pattern with stability and asymmetry

Cell polarization is a critical process that separates molecules into two distinct regions in prokaryotic and eukaryotic cells, guiding biological events like cell division and cell differentiation. Although some underlying antagonistic reaction-diffusion networks capable of setting up cell polarization have been identified experimentally and theoretically, it is still elusive how the pattern stability and asymmetry can be modulated. Here, we first numerically demonstrate that the polarized pattern generated by an antagonistic 2-node network would collapse into a homogeneous distribution when single-sided self-regulation, single-sided additional regulation, or unequal system parameters are added. Interestingly, the combination of two of those unbalanced modifications can stabilize the polarized pattern. To test if this fundamental rule governs the network programming in the real system, we conduct an elaborate literature search to reconstruct the cell polarization network in the nematode Caenorhabditis elegans zygote, where a 4-node network with full mutual inhibitions between anterior and posterior is modified by a mutual activation in the anterior and an additional mutual inhibition between the anterior and the posterior, constituting a 5-node network. Numerical simulation further reveals the balance between these two modifications, which jointly maintain pattern stability and enhance pattern asymmetry. Our computational framework successfully simulates the simple 2-node network and C. elegans 5-node network in wild-type and perturbed embryos, providing new insight into the design principles of both natural and artificial cell polarization systems. Last but not least, we build user-friendly software, PolarSim, to facilitate the exploration of networks with alternative node numbers, parameter values, and regulation pathways.   

Long-Range Repulsion Between Chromosomes in Mammalian Oocyte Spindles

During eukaryotic cell division, a microtubule-based structure called the spindle organizes and segregates chromosomes. According to the best-studied models of spindle force generation, including microtubule depolymerization and molecular-motor-driven sliding, forces are generated by nanometer-scale molecular processes and then transmitted over distances of microns by rigid microtubules lying parallel to the spindle long axis. While many important questions about force generation, regulation, and transmission remain unsolved, these mechanisms can in principle explain chromosome motion along the spindle axis, including congression to the metaphase plate (i.e. the spindle mid-plane), and the separation of sister chromatids in anaphase. However, they cannot account for forces in the perpendicular direction, and it remains unclear which physical principles determine the chromosome configuration within the metaphase plate. Here, we use quantitative live-cell microscopy to show that metaphase chromosomes are spatially anti-correlated in mouse oocyte spindles, consistent with the existence of hitherto unknown long-range forces acting perpendicular to microtubules. We demonstrate that a simple continuum model can account for this observation, as well as several other measurements of the structure and dynamics of the microtubule network itself. In this model, the spindle is treated as an active nematic liquid crystal droplet, the orientation of microtubules throughout the spindle is determined by nematic elasticity, condensed chromosomes act like micron-scale inclusions in a continuous nematic phase, and long- range repulsive forces arise due to deformation of the nematic field around embedded chromosomes. Our work highlights the surprising relevance of materials physics in understanding the structure, dynamics, and mechanics of cellular structures, and presents a novel and potentially generic mode of chromosome self-organization in large spindles.   

The flow patterns of ink released by cuttlefish

Cuttlefish mix ink and water and release six ink patterns using the same funnel. Previous studies examine the chemical defense mechanisms of cuttlefish ink or intercommunication within the same species, which could benefit aquaculture. However, the physics of cuttlefish ink ejection remains poorly understood. Here we investigate how cuttlefish mix two liquids (water and ink) and jet various ink patterns with the same funnel. In this study, we observe live cuttlefish inking and mimic the process with a table-top. Cuttlefish's ink visualizes the flow field and demonstrates distinct characteristics. The rope-like ink could be laminar or transition flow, the ink blob is a vortex ring, and the puff ink is turbulent flow. The ejection velocity and the relative position between the ink and the involved water determine the patterns. The velocity of ink ejection determines how fast the pattern disperses. At low ejection velocities, the patterns remain for at least six seconds. At high velocities of ejection, the patterns disperse in one second. Our findings would advance liquid mixing methods applied in the pharmaceutical industry and inkjet technology.   

Malignant brain tumors behave in vivo as liquid crystals: quasi long-range nematic order and topological defects.

Glioblastomas (GBM) are the most common adult brain tumors with worst prognosis, given their capacity to invade normal brain structures, and become resistant to all treatments. Our data demonstrated that GBM tumors exhibit self-organized, nematically aligned, multicellular structures, termed “oncostreams,” that influence tumor invasion and malignancy. GBM tumor cells grown in vitro exhibit topological defects and nematic correlation, defining GBMs as displaying liquid crystal organization. Using 3D brain reconstruction, we study how in-plane nematic alignment propagates in space throughout the tumor. Interestingly, nematic alignment length scale correlates with tumor aggression based on 4 unique mouse models and a human gliosarcoma patient. Ongoing work is utilizing whole brain clearing and light-sheet scanning microscopy to fully reconstruct 3D oncostream ordered organization and topological defects. We believe our work will demonstrate that GBMs in vivo behave as liquid crystals thus demonstrating a novel ordering of malignant brain tumors. This work will define the role of liquid crystal organization as novel physical structures in GBM, eventually leading to the development of therapeutic strategies targeting the structure underlying brain tumor organization, namely, the liquid crystalline order.   

Modeling Cell Division-Induced Motion in Cell Aggregates

Cell division is an important stage of the cell cycle. Cell division induces differentiation into multiple cell and tissue types during development and sustains tissues beyond developmental stages. Although the biochemistry and microscale biophysics of cell division are well-understood, questions remain concerning the effects of cell division on larger scale mechanical properties of confluent tissues. For example, do sites of cell division act as defects that affect the mechanical properties of tissues or to what extent do cell divisions induce cell rearrangements? We develop a novel deformable particle model for cell division, which includes cell-cell adhesion and energy costs for changes in cell area and perimeter in two dimensions (2D). We perform discrete element method (DEM) simulations of cells undergoing division in isolation and confluent 2D cell packings. We calculate cell shape and non-affine displacement fields of all cells as a function of time during division. We then compare our results to the cell shape parameter and non-affine displacements in experiments of MDCK cells dividing in confluent monolayers. Preliminary results show that our simulations accurately capture the shape changes during cell division and mechanical stresses mitotic cells induce in aggregates.   

Curious Run-and-Tumble Behavior in Multicellular Organisms on the Water's Surface

We uncover a "run-and-tumble" motion in Rhagovelia, a water-walking predatory insect, a behavior previously well-documented only in microorganisms like E. coli. This motion consists of persistent directional movement (the run) followed by random reorientation (the tumble). We record the insect's position and orientation on water to understand the advantages of this unique locomotion. Our study also evaluates how environmental factors such as water flow and the presence of food or conspecifics influence this behavior. Our findings extend the concept of "run-and-tumble" to macroscopic organisms, offering insights into its evolutionary advantages. This research has implications for the development of search robots mimicking this efficient movement strategy and suggests a unifying principle behind "run-and-tumble" motion across different length scales.   

Spatio-temporal dynamics of nutrient exchanges in microbial active matter

Microorganisms inhabit highly fluctuating environments and survive in a low-nutrient resource bath. It is now well recognized that symbiotic relationships between microbes play a vital role in their survival. The existence of such interaction raises general questions about the spatio-temporal dynamics of nutrient exchanges. Here we experimentally and theoretically examine a model system of this problem – bacteria, an obligate microbe capable of chemotactic response towards oxygen, in a co-culture with green algae, which produce oxygen when illuminated. Even in their simplest arrangement in a localized illuminated domain, we find a complex dynamics involving nutrient exchanges, enhanced algal diffusivity due to the bacteria, and a stochastic version of  “flux expulsion”.   

Understanding the Collective Chemotactic Behavior of Malignant Lymphocyte Clusters

Single lymphocyte cells under physiological conditions exhibit positive chemotactic migration in response to weak chemical gradients (of the chemokine CCL19) and negative migration (opposing the gradient) in the presence of strong gradients. In contrast, clusters of malignant lymphocytes display positive chemotaxis even when exposed to high chemical gradients. This distinct collective behavior is not well understood and is linked to the increased metastatic potential of lymphocyte clusters over that of individual cells. To understand these cluster dynamics, we develop an agent-based model that incorporates contact inhibition, adhesive interactions, and gradient sensing capabilities. Our analysis reveals that multiple elements contribute to the behavior of these cell clusters. Besides inherent cellular properties such as contact inhibition and gradient sensing, the internal dynamics of the cells are crucial. Specifically, depending on the dynamics of switching from chemoattraction to chemorepulsion and the rate of cell exchanges between the cluster's periphery and interior, clusters can show both chemo-attractive and chemo-repulsive behavior. Insights from our modeling help in understanding the collective behavior of these cells in experiments and can potentially shed light on physiological observations.   

Universal distribution of cell shapes in active systems

We use agent-based modeling of cells as deformable particles, to explore the distribution of cell shapes in active-driven systems.  The deformable cells move using an active brownian force model in a quasi-two-dimensional geometry. We characterize the cell shape using S=P²/4πA, the non-dimensional ratio of perimeter P to area A. S=1 for a circle and is larger for any other shape. We compare systems containing cells with fixed (elastic) shape parameter to those with dynamic (plastic) shape parameter. The shape parameter must be greater than 1.15 for the cells to completely cover space. In these dense states, the cells must deform to move.  In the fixed S₀ systems, the area of the cell is fixed and the perimeter is an elastic spring with  rest-length P₀=√4πAS₀. In the dynamic system, the rest-length of the perimeter can relax based on the local stress. For fixed S₀ systems, a critical shape parameter S^* =1.34 emerges. For S₀*, the cells tend to stretch their perimeter, increasing S. For S₀>S^*, the cells tend to compress their perimeter. In the dynamic systems, a broad universal distribution of shape parameters emerges regardless of the initial distribution of shapes. This broad distribution is peaked near S=1.15, but has very long high S tails extending above S=3. The broad distribution matches that found in high mobility MDCK cell monolayers.             

Chaos of Cardiac Tissue under Periodic Stimuli and Fibrillation: Experiments and Control

Several cellular cardiac voltage models exhibit chaotic dynamics suggesting that the irregular heart rhythms of cardiac disease may have a nonlinear mechanism of control. However, little experimental evidence has documented the chaotic behavior of cardiac tissue excitations. This study aims to both quantify and qualify the chaotic nature of cardiac tissue from the system's arrhythmic electrical response to fast periodic forcing. Leading Lyapunov Exponents were estimated from action potential duration (APD) time series from single cells of bullfrogs yielding negative exponents for frequencies near period doubling cascades and positive exponents for arrhythmic responses to periodic forcing. Additionally, several stable period-three orbits and unstable periodic orbits were identified. Further, a biphasic perturbation around a forcing frequency resulting in arrhythmic behavior appears to be able to stabilize an unstable periodic orbit of the response. On a multicellular scale, unstable periodic behavior is also observed in the ventricle fibrillation of pigs and humans in isolated regions across the heart. These findings indicate cardiac tissue is governed in part by chaotic factors and nonlinear control can be employed to terminate arrhythmias.

  

Surface Chemistry Influence on Orientations, Contact Area, and Deformation of Adhering E. coli Cells

Motivated by the significant role of cell adhesion in biofilm formation, we investigated the impact of surface chemistry starting at the single-cell level. We employed flagella-free Escherichia coli (E. coli) to examine how interactions between the negatively charged cell body and engineered cationic, hydrophobic, and anionic surfaces influenced cell orientation. Cationic surfaces resulted in diverse adhered cell orientations due to rapid electrostatic attractions of cells tumbling in shear flow as they approached the surface.  In contrast, hydrophobic and anionic surfaces produced greater cell alignment with the plane of the surface and with the flow direction. Increases in wall shear rate had no impact on the orientation of cells on the cationic surface which were firmly bound; however, increased flow over anionic and hydrophobic surfaces produced reversible alignment changes. These findings suggest mechanisms by which surface chemistry may play a role in the evolving structure and function of microbial communities. Additionally, in studies of bacterial cell adhesion to surfaces, we introduced a novel technique to measure the cell-substrate contact region, gap separation, and curvature near the contact zone. This method provided insights into the undersides of adhered bacterial cells, revealing that the small contact areas of end-adhered cells may pose limitations for antimicrobial surfaces designed for direct bactericidal contact.   

Flying fish taxiing on the water's surface

Flying fish is a fascinating creature which is known for their locomotion between water and air. Flying fish walk on the water's surface with their tails slapping the water surface at high frequency. This behavior is the so-called taxiing. By taxiing, flying fish propel themselves above water and re-initiate gliding. Previous researchers have studied western grebes and basilisk lizards walking on the water surface by slapping the water surface [1,2], but the working principle for flying fish taxiing remains unknown. In this study, we investigate the tail-slapping frequency of flying fish experimentally and theoretically. Flying fish beat their tails on the water's surface at a frequency of 13.5 Hz, which is higher than most other water-walking animals. With the force analysis, we predict that the minimum tail beat force to sustain a 200-g flying fish's locomotion above the water surface is around 2 newtons. This study will contribute to the mechanics of taxiing and advance the design of hydro-aero robots.



  

Chemical gradients coupled to self-generated flow exhibit large-scale pattern formation in active fluids.

Active materials are composed of a dense ensemble of motile particles that consume chemical energy on the microscopic scale to drive collective material reorganization and autonomous flows. We investigate the mechano-chemical coupling between spatial distributions of energy sources and self-organized flows. We build a model system composed of microtubule-based active fluids. Into such systems, we embed colloidal beads which act as sources of fuel that are advected by the active flow. The resulting connection between material flows and the spatial gradients in fuel generate spatiotemporal patterns. We parameterize the behavior of our system by changing motor, fuel, and colloidal bead concentration. Varying these, we identify two qualitatively distinct limits of organization. In one extreme, active flows are localized, leaving the network largely undisturbed. In the other, material flows overlap, homogeneously fluidizing the system. Between these, there is a steady-state regime characterized by emergence of large-scale density patterns.   

Principles of cellular behavior: integrating cellular structure, dynamics, and decision making

Although it may be easy to think of cells as the simple building blocks of more complex organisms such as animals, single cells are capable of remarkably sophisticated behaviors such as navigating dynamic environments, hunting prey, and evading predation. These behaviors emerge from the interactions among myriad molecular components in conjunction with physical constraints and mechanisms that dictate interactions between the cell and its environment. The ciliate Euplotes, a cell that walks across surfaces using motile appendages (cirri) composed of bundles of cilia, is an ideal system for navigating this mechanistic complexity due to its extensive behavioral repertoire that is amenable to rigorous analysis. Analyses drawing on ideas from non-equilibrium physics and computer science revealed finite state machine-like processing embodied in walking Euplotes eurystomus cells. Cellular walking entails regulated transitions between a discrete set of gait states with stereotypy in sequential patterns of state transitions. Simulations and experiments suggest that the sequential logic of the gait is functionally important. Taken together, these findings implicate a finite-state machine-like process. Cirri are connected by microtubule bundles (fibers), and the dynamics of cirri involved in different state transitions are associated with the structure of the fiber system. Perturbative experiments revealed that the fibers mediate gait coordination at fast timescales, suggesting a mechanical basis of gait control, and implicate intracellular signaling involving Ca²⁺ and cGMP in dictating overall cirral activity levels. Comparisons among Euplotes species show a complex scaling relationship between cell structure and movement patterns. Cell movement patterns are stereotyped, species specific, and dictated by environmental conditions as well as cell state over multiple timescales. These results highlight the role of physical and developmental constraints in the evolution of cellular behavior.  Ultimately, we aim to elucidate general principles of the regulation and evolution of cellular behavior by integrating understanding across scales of biological organization, linking cellular structure and physiology to patterns of behavior to environmental contexts.