Session P66: Force Generation, Biomechanics and Tissue Dynamics

Active wetting of epithelial tissues

Development, regeneration, and cancer involve drastic transitions in tissue morphology. In analogy with the behavior of inert fluids, some of these transitions have been interpreted as wetting transitions. The validity and scope of this analogy are unclear, however, because the active cellular forces that drive tissue wetting have been neither measured nor theoretically accounted for. We show that the transition between two-dimensional epithelial monolayers and three-dimensional spheroidal cell aggregates can be understood as an active wetting transition whose physics differs fundamentally from that of passive wetting phenomena. By combining an active polar fluid model with measurements of physical forces as a function of tissue size, contractility, cell-cell and cell-substrate adhesion, and substrate stiffness, we show that the wetting transition results from the competition between active traction forces and contractile intercellular stresses. This competition defines a new intrinsic length scale of active polar fluids that gives rise to a critical size for the wetting transition: Tissues larger than the critical size wet the substrate whereas smaller tissues dewet — a striking feature that has no counterpart in classical wetting. Finally, we show that active fluctuations of the tissue shape are dynamically amplified during the dewetting process. Overall, we conclude that tissue spreading constitutes a prominent example of active wetting — a novel physical scenario that may explain morphological transitions during tissue morphogenesis and tumor progression.

C. Pérez-González*, R.Alert*, C. Blanch-Mercader, M. Gómez-González, T. Kolodziej, E. Bazellieres, J. Casademunt*, and X. Trepat*. Active wetting of epithelial tissues. Nat. Phys. (2018)   

A Computational Model of Calcium Signals Around Laser-Induced Epithelial Wounds

Epithelial wounds heal in multiple stages that involve wound detection, cell migration, and cell proliferation. One of the earliest signals of wound detection is an increase in cytosolic calcium concentration. Laser-ablation wounds in Drosophila epithelia trigger complex calcium signaling dynamics on multiple spatiotemporal scales. Given multiple hypothesized mechanisms that may drive calcium signals, we have developed a computational model to test the plausibility of these hypotheses and further understand the underlying biology. The model consists of intracellular exchange of calcium between cytosol and endoplasmic reticulum (ER), as well as exchange of calcium and other ions with the extracellular space and neighboring cells. The model thus couples calcium concentrations and membrane potentials among gap-junction-connected epithelial cells. These ion exchanges are initiated in the model by microtears in the plasma membranes of cells near the wound and by the activation of G-protein coupled receptors via a wound-induced diffusible ligand. We will discuss the model in detail, evaluate the plausibility of its hypotheses , and describe its experimentally testable predictions.   

Controlling collective cell migration using geometric boundary perturbations

We work at the intersection of the active matter and tissue engineering communities, where our goal is to learn more about emergent behaviors of cellular collectives in response to perturbations of the microenvironment and to exploit these to develop design rules for controlling tissues. Here, we try to bias MDCK tissue expansion using asymmetric geometric boundary perturbations, a known strategy for biasing transport in several active matter systems. We aim to find the minimal boundary perturbation that can influence behavior in the bulk of a cell collective. Using a high throughput, high-resolution tissue-patterning assay, we can study the effects of these patterns on tissue expansion, wound healing (tissue collisions), and collective cell migration in general without typical microfabrication processes.   

Coupling between symmetry and motion in 3D printed microtissue arrays

Symmetry plays a major role in the emergence of collective phases of inanimate materials. In magnetism, for example, geometric frustration leads to spin-glass phases. If similar principles could be leveraged to control collective phases of biological materials, like living cell assemblies, a new set of design strategies could be developed in tissue engineering applications. Building symmetry into biological systems is often challenging and is sometimes achieved through surface micropatterning; achieving such patterns in 3D is even more challenging. To fabricate 3D multi-cellular systems of designed symmetry and spatial patterning, we 3D print geometric patterns of collagen and 3T3 fibroblast cells. These cell/ECM patterns are printed directly into a 3D growth media made from jammed microgels, providing a well-defined yet reconfigurable environment on all sides of the structures. By comparing the collective motion arising in systems with hexagonal and square symmetries, we probe the potential role of geometric frustration in multicellular structure maturation. Preliminary data and analysis will be presented.
  

Instabilities on the leading front of collectively migrating tissue

Instabilities are often observed on the leading front of a collectively migrating cellular sheet both in vivo and in vitro. Instead of moving forward uniformly, the leading front often destabilizes into multicellular fingering-like structures. To decipher the mechanical origin of the instabilities, we develop a continuous model based on the Toner-Tu equations for active fluids, in which the collectively moving cells are modeled as a system of self-propelled particles. We perform different perturbations on the front edge and analyze the linear stability. We show the instabilities are due to some kinds of perturbations, for example, leader cells. Finally, we show our simulation of figuring-like protrusions with the guidance of leader cells.   

Different modes of fluidization in Human Bronchial Epithelial Cells -- the Unjamming Transition vs. the Epithelial-Mesenchymal Transition

Epithelial tissues are non-migratory and behave as a jammed system under homeostatic conditions. Using a jammed layer of human lung epithelial cells we compare a new mode of tissue fluidization, induced by mechanical compression, namely the unjamming transition (UJT) with the epithelial-mesenchymal transition (EMT). Our analyses of experimental data reveal the following: Strong cellular elongation and large fast moving Nematic swirls during the UJT while retaining epithelial nature. All these features are lost during the EMT when the cells become mesenchymal. To further our understanding we developed a dynamic vertex model (DVM) which differs from previous vertex models in that the cell edges can now become curved and can thus reflect the competition of the forces acting on the edge locally. These forces include cortical tension, intracellular-pressure differences, and polarized motility forces. We explore different paths of solid-to-fluid transitions based on different parameters in the model, such as individual cell motility and preferred cell shapes, and compare our predictions with the experimental observations on UJT and EMT. Based on our comparisons, we propose that the UJT could be an alternative route to fluidization of jammed epithelial tissues, independent of the EMT.   

How the mechanics of extracellular matrices interacts with cells

In the last twenty years, we have seen all-round progress in modeling the mechanics of the biopolymer network. Its stability is understood based on Maxwell counting arguments. The lattice-based models numerically predict the phase diagrams and other bulk properties that are consistent with experimental observations. Other analytical theories, such as mean-field approximation and scaling laws, also enlighten our understanding.

Part of the reason behind it being so intensively investigated is that, a good example of biopolymer networks, extra-cellular matrices(ECM), plays a central role in many cellular processes. On the single-cell level, experiments have shown that the ECM interact with biochemical networks in a cell, e.g. inducing the epithelial-mesenchymal transition. On multi-cellular levels, the ECM modulates collective biophysical processes like the durotaxis. Deciphering the force pattern behind many collective cellular behaviors is still under its way. We hereby showcase our recent efforts in modeling the couplings between the mechanics of ECM and biology of cells embedded inside the former.   

The origin of solidity and fluidity in cellular materials and biological tissues.

Models for confluent biological tissues often describe the network formed by cells as a triple-junction network, similar to foams. However, higher order vertices or multicellular rosettes exist prevalently in developmental and in vitro processes and have been recognized as crucial in many important aspects of development, disease and physiology. In this work, we study the influence of rosettes on the mechanics of a confluent tissue. We find that the existence of rosettes in a tissue can greatly influence its rigidity. Using a generalized vertex model and effective medium theory we find a fluid-to-solid transition driven by rosette density and intracellular tensions. This phase transition exhibits several hallmarks of a second-order phase transition such as a growing correlation length and a universal critical scaling in the vicinity a critical point. Further, we elucidate the nature of rigidity transitions in dense tissues using a generalized Maxwell constraint counting to answer a long-standing puzzle of the origin of solidity in cellular materials.   

The dynamics of laminin-mediated red blood cell adhesion in sickle cell disease

Understanding red blood cell (RBC) adhesion to blood vessel walls is crucial to treat sickle cell disease. We have developed a microfluidic RBC adhesion assay [Alapan et al., Translational Research, 173, 74-91, (2016)] and performed experiments on clinical whole blood samples to probe RBC adhesion biomechanics. Microchannels functionalized with laminin (an endothelial protein) were integrated with a programmable syringe pump to mimic physiological flow conditions in microvessels. A novel set of methods for automated identification and tracking of adherent RBCs from video data was developed. We analyzed the dynamical data using a theoretical approach adapted from a prior study on heterogeneity in protein adhesion in Atomic Force Microscopy (AFM) experiments [Hinczewski et al.,Proc.Natl.Acad.Sci.,113,E3852 (2016)]. Our analysis yielded a minimal physical model of the RBC detachment process, and comprehensively characterized the adhesion dynamics across patient samples. Model parameters also revealed statistically significant correlations with patient clinical data, opening the possibility of diagnostic applications suited to automated, high-throughput and low cost diagnostic platforms with clinical utility.   

Shape Fluctuations and Curvature-Driven Mechanics of Heterogeneous Lipid Vesicles

We investigate heterogeneous vesicles consisting of mixtures of different lipids. We develop single-bead implicit-solvent coarse-grained models based on anisotropic pair potentials to capture vesicle shapes arising from species with different preferred curvatures. We develop theory and methods for mapping our molecular results to continuum mechanics descriptions, useful for analysis of fluctuation spectra to estimate bending elasticities and other mechanical parameters. For mixed lipid species, we find that passive fluctuation spectra are significantly influenced by the formation of small curved segregated domains. We actively probe the vesicle mechanics with compression and narrow-passage transport experiments. When compressed, we find that high-curvature domains arrange circumferentially, leading to a smaller resistance force. When transporting through a narrow passage, we find that mixed vesicles are characterized by much higher variances in passage times. We also find that geometric effects can delay phase-separation in the bulk and promote phase-separation during channel transport, resulting in occasional budding events. Our results may have implications for biological systems, fluidics utilizing vesicles, and other experimental assays.   

Theory of cytoskeletal rearrangement and force generation

Cytoskeletal networks that include microtubules, crosslinkers, and kinesin motors are the basis of the mitotic spindle and cytoplasmic transport. The large separation of time scales between motor and crosslinker activity (sub-second to second) and network function (minutes to hours) is a challenge for theoretical approaches. We developed a minimal model, building on the separation of time scales between relatively fast crosslinker and motor rearrangement and relatively slow filament movement. The model reproduces experimentally measured force, torque, and self-organization in cytoskeletal networks of different length scales. With this model, we study mitotic spindle assembly from a monopolar initial condition and compare the mechanisms of motor- versus crosslinker-mediated spindle assembly. The results show how torque-balance and force-balance properly align microtubule bundles. In another study, we propose that oscillating optically trapped microtubule pairs crosslinked with motors and/or crosslinkers can be used to determine the kinetics and mechanics of crosslinking proteins.
  

Force Generation by Curvature-Generating Molecules

Curvature-generating molecules (CGMs) are central to a variety of biological processes. In particular, proteins such as clathrin help provide bending forces and moments to drive endocytosis. We develop a discrete mechanical model of the shape of a small CGM-membrane complex that incorporates the effects of cell wall elasticity and high turgor pressure. We study the dependence of the force generated by the CGMs on several parameters, including the bending modulus, the complex size, and the turgor pressure. We find evidence of transitions as a function of external turgor pressure and intrinsic curvature. This work also compares the discrete model to previous continuous models of CGM forces, finding that the distribution of the forces depends on the strength of the turgor pressure relative to the bending energy. The forces are localized at the edges for high turgor pressure, and more widely distributed for low turgor pressure. In addition, the energy exhibits a minimum at small numbers of molecules. Further, for realistic values of the bending rigidity and curvature, CGMs alone are insufficient to initiate endocytosis against turgor pressure, in agreement with previous findings.