Biomimetic emulsions reveal the effect of mechanical forces on cell–cell adhesion

Abstract

Cell-cell contacts in tissues are continuously subject to mechanical forces due to homeostatic pressure and active cytoskeleton dynamics. In the process of cellular adhesion, the molecular pathways are well characterized but the role of mechanics is less well understood. To isolate the role of pressure we present a dense packing of functionalized emulsion droplets in which surface interactions are tuned to mimic those of real cells. By visualizing the microstructure in 3D we find that a threshold compression force is necessary to overcome electrostatic repulsion and surface elasticity and establish protein-mediated adhesion. Varying the droplet interaction potential maps out a phase diagram for adhesion as a function of force and salt concentration. Remarkably, fitting the data with our theoretical model predicts binder concentrations in the adhesion areas that are similar to those found in real cells. Moreover, we quantify the dependence of the area of adhesion on the applied force and thus reveal adhesion strengthening with increasing external pressure even in the absence of active cellular processes. This biomimetic approach reveals a physical origin of pressure-sensitive adhesion and its strength across cell-cell junctions.cell mechanics | protein emulsion | lipid emulsion C ell-cell adhesion is important in biology because it underlies the structure of tissues and their dynamic reorganization during processes as important as morphogenesis (1, 2), cell locomotion (3, 4) and signaling (5, 6). In addition to the high level of complexity in the identified biochemical pathways, it has recently become clear that mechanical effects also play an important role. For example, pushing cells together or increasing their contractile forces by changing the substrate stiffness reinforces the strength of contacts (7-9). Furthermore, because homeostatic pressure arising from the balance of cell division and cell death is important in achieving the mechanical integrity of tissues (10) it should also affect cell-cell adhesion. Despite these important observations, the physical origin of force-sensitive adhesion remains an open question. In fact, theoretical models are derived from the behavior of simplified model membranes that lack mechanical resilience (11). Although these models successfully describe the kinetics and energetics of adhesion in the absence of rigidity (12, 13), they cannot address the effects of force. In an individual cell, the cytoskeleton scaffold coupled to the membrane gives rise to a cortical tension of ≈0.035 mN∕m (14). Moreover, the interplay between cortical tension and adhesive interactions with surrounding neighbors gives rise to a different surface tension in cellular aggregates (15, 16) and measures ≈1-20 mN∕m (17, 18).To mimic the dense packing of cells in tissue with a homeostatic pressure of ≈10 kPa (19, 20), we use a 3D assembly of biomimetic emulsion droplets under an external compression. In the emulsion system the compression of individual droplets with an interfacial tension of ≈10 mN∕m ...

Rheology of foams and highly concentrated emulsions

“…The interaction energy (typically (10-40) k B T) is strong enough to stick particles together so that packing structure and rheology are significantly modified [10,11]. Alternatively, adhesion between neighboring droplets can be promoted by ligands which connect receptor molecules adsorbed on each water-oil interface [6] (Fig. 1e).…”

“…1e). Minimization of the total emulsion free energy including the long range interaction potential energy, the interfacial energy and adhesion energy allows the thickness and the radius of the contact films to be predicted [6].…”

“…Therefore, emulsions with sub-micron sized droplets can be stable over months in contrast to foams where bubbles Area with marked fluorescence reveals adhesive patches between neighboring droplets. Adapted from [6]. Courtesy L.L.…”

Specific and reversible DNA-directed self-assembly of oil-in-water emulsion droplets

“…By exploiting the well-characterized ligand-receptor pair biotin-streptavidin, EDs were immobilized on glass surfaces (9) and actively transported along microtubules (10). In the context of self-assembly, both polymer-mediated (11-13) and biotin-streptavidin-mediated aggregation of EDs was shown (14). Despite the advantages a combination of EDs as building blocks and DNA oligonucleotides as linking agent offers, a protocol for DNA-directed self-assembly of EDs is currently missing.…”

One-pot system for synthesis, assembly, and display of functional single-span membrane proteins on oil–water interfaces

“…We apply our one-pot approach to produce natural apoptosis-inducing receptor ligands and show that these single-span membrane receptor ligands efficiently killed cultured cancer cells. Oil drops have been used to interact with cells (12,13) and study protein interactions (14)(15)(16)(17); here, we produce oil drops coated with membrane proteins. By in vitrosynthesizing proteins from DNA templates, we can also rapidly engineer synthetic ssMPs to modulate their assembly and function (18) on oil drops and therefore, show a unique way to manipulate aggregation-prone hydrophobic proteins.…”