Session K27: Mechanobiology of Cell Matrix Interactions

Coordinating Adhesion with Repulsion: How Cells Leverage Hyaluronan Glycocalyx

Cell rearrangements during proliferation, migration, synaptogenesis, and other dynamic physiological processes are often accompanied by alteration in hyaluronan synthesis and the associated hyaluronan-rich glycocalyx. Previously, we have shown that this bulky glycocalyx significantly weakens cell adhesion and tunes cell migration speed in a predictable, adhesion-dependent fashion. Building on these results, I will present new and consistent data quantifying the impact of hyaluronan expression in neural crest cell migration in vivo and ex vivo. I will then show how we use a biomimetic dynamic hyaluronan glycocalyx assay (made with templated hyaluronan synthase) to interrogate the role of glycocalyx in mechanobiology. We quantified the kinetics of cell adhesion versus the glycocalyx thickness and found that the HA glycocalyx modulates cell adhesion in a thickness-dependent manner, slowing but not altering the final number of adherent cells. Fluorescent labeling confirms localized removal of HA to promote focal adhesion formation by hyaluronidase. We also investigated the impact of the force generated by dynamic glycocalyx growth and found that cells become dramatically deformed, the number of focal adhesions is reduced, and thirty percent of adhered cells detach from the substrate. These results support the hypothesis that hyaluronan synthase can extrude polymers and drive glycocalyx assembly to generate forces large enough to influence cell mechanobiology, including focal adhesion turnover, size, and cell morphology.   

Using 3D Discrete Fiber Models to Investigate Cell-Mediated Extracellular Matrix Remodeling

Motile cells within a fibrous matrix not only move but also rearrange the matrix around them, with potential consequences for the local and global tissue structure. Because cells also sense and respond to changes in the matrix, this rearrangement provides a mechanism for cell-cell communication. We present a coarse-grained computational model of collagen matrix rearrangement by one or more cells, focusing on the changes in the matrix and comparing the results to experimental observation. The model represents collagen fibers as strings of beads that intersect at some beads to form a large network; an important feature of the beads is that they can bond to beads from nearby fiber segments, leading to irreversible rearrangement of the fibers. The action of cellular pseudopodia is represented by a collection of "tractors," which extend from the cell, bind to the surrounding matrix, and then retract, pulling the matrix with them. The combined effect of the tractors and the interfibrillar bonding is a large-scale rearrangement and, in the case of multiple cells, local realignment of the surrounding fiber network.   

Cell collision outcomes in suspended fiber environments as controlled by physical and geometric factors

Cell migrations and interactions are essential for many physiological processes and pathologies, and are usually studied through cellular motion and collisions on 2D flat substrates, where colliding cells reverse directions and move away from contact (termed contact inhibition of locomotion or CIL). Unlike 2D surfaces, suspended nanofibers closely resembling extracellular matrix provide a more biologically native environment for cell-cell collisions, whose outcomes are more various than classical CIL, such as cells sticking to or walking past each other while crawling along fibers. Inspired by experiments, we use a phase-field model to numerically simulate two-cell collisions in fiber geometries, especially the effects of cellular mechanics and geometric factors on their outcomes. We focus on cell behaviors on two parallel fibers, showing that greater abilities of cells to deform and polarize, in addition to larger fiber spacing, can lead to more walk-past rather than sticking together upon collision. This is consistent with a simple linear stability analysis on the cell-cell interface at collision. Our results illustrate the roles of both cell-cell and cell-matrix interactions in controlling cell motility.   

Collective cell migration driven by mechanical coupling

Cells in multicellular organisms migrate during physiological processes such as tissue formation, regeneration, and immune defense. In their natural environment in vivo, cells closely interact with their surrounding tissue and the extracellular matrix (ECM) - a dynamic structure essential for mechanical support. Given the extensive contact between the cells and the surrounding tissue, we ask how the mechanical coupling between the cell and the substrate facilitates collective cell migration. We will discuss experimental evidence, a computational model verified with experimental observations and hypothesized principles linking collective cell migration to cell-substrate mechanical coupling. We show that cell stiffness is dynamically reduced in response to the temporal stiffening of the substrate, with consequent collective cell migration well characterized by the stiffness ratio between the cell and the substrate. On the basis of computational predictions tested by experiments, we provide evidence that an optimal cell-to-substrate stiffness ratio is important in allowing for collective cell migration rather than a fixed value of substrate stiffness.   

Dynamics of Early Cell Spreading with Hyaluronan Glycocalyx

Newly adhering cells in the extracellular matrix are enriched with hyaluronan glycocalyx, especially cells that have just completed cell division, a process during which hyaluronan synthase expression is increased. Yet, quantitative studies of early cell spreading have largely ignored the presence of glycocalyx. A puzzle remains regarding how receptor-ligand bonds are formed between integrins and the extracellular matrix in the presence of giant and densely-assembled hyaluronan molecules associated with the cell plasma membrane. In this study, we will compare the dynamics of isotropic spreading MCF-7 cells with and without glycocalyx during early cell adhesion.

    

Cell motility self-regulated by secreted footprints

Eukaryotic cell migration is essential to biological processes like embryonic development, immune response, wound healing, or cancer metastasis. During migration, there is a complex interplay between cells and their environment, as cells respond to environmental signals and actively alter their surroundings. Recent experiments observed that MDCK epithelial cells, when placed on 1D fibronectin micropatterned stripes, leave a footprint on the substrate that modifies their own motility, resulting in oscillatory motion. This talk will explore how footprint secretion affects cell motility patterns by combining mathematical modeling and experiments. We assume that cells secrete a footprint that activates signaling pathways that regulate cell polarity. The model reproduces the observed oscillatory motion and predicts new 2D motility patterns, which are experimentally verified. We show that minor changes in footprint interactions can cause cells to switch from confinement to complex exploratory dynamics. This study highlights the potential of cells to self-regulate their motility using footprints and provides insight into the mechanisms guiding cell migration.   

Myxococcus xanthus colony expansion depends on substrate stiffness and surface coating

Many cellular functions depend on the physical properties of the cell's environment. Many bacteria have different surface appendages to enable adhesion and motion on a variety of surfaces. Myxococcus xanthus is a social soil bacterium with two distinctly regulated modes of surface motility, termed the social motility mode driven by type iv pili and the adventurous motility mode based on focal adhesion complexes. How bacteria sense different surfaces and subsequently coordinate their collective motion remains largely unclear. Using polyacrylamide hydrogels of tunable stiffness, we found that wild-type M. xanthus spreads faster on stiffer substrates. Here, we show using motility mutants that disrupting adventurous motility suppresses this substrate-stiffness response, suggesting focal-adhesion-based adventurous motility is substrate-stiffness dependent. We also show that modifying surface adhesion by adding adhesive ligands, chitosan, and extracellular DNA, increases the amount of M.xanthus flairs, a characteristic feature of adventurous motility. Taken together, we hypothesize a central role of M. xanthus adventurous motility as a driving mechanism for surface and surface stiffness sensing.   

Effect of cell softness and polydispersity on the viscosity of non-confluent tissue

The collective motion of cells in a tissue depends on the mechanical properties of the individual

cells. The stiffness of the cells and the size dispersity are crucial to determining the fate of the

tissue dynamics as they are directly related to the motility of each cell. Using agent(particle)-

based simulations of three-dimensional tissue, we showed that depending on the cell elasticity and

the size dispersity, the tissue exhibits fluid-like, glass-like, and glass-like dynamics followed by a

viscosity saturation. Furthermore, for a low value of polydispersity, the tissue crystallizes. In order

to visualize this complex behavior, we draw a phase diagram in the polydispersity-cell elasticity

plane.