Actin-ring segment switching drives nonadhesive gap closure

Mechanoresponse of Curved Epithelial Monolayers Lining Bowl-Shaped 3D Microwells

The optimal functioning of many organs relies on the curved architecture of their epithelial tissues. However, the mechanoresponse of epithelia to changes in curvature remains misunderstood. Here, bowl-shaped microwells in hydrogels are designed via photopolymerization to faithfully replicate the shape and dimensions of lobular structures. Leveraging these hydrogel-based microwells, curved epithelial monolayers are engineered, and how in-plane and Gaussian curvatures at the microwell entrance influence epithelial behavior is investigated. Cells and nuclei around the microwell edge display a more pronounced centripetal orientation as the in-plane curvature decreases, and enhanced cell straightness and speed. Moreover, cells reorganize their actin cytoskeleton by forming a supracellular actin cable at the microwell edge, with its size becoming more pronounced as the in-plane curvature decreases. The Gaussian curvature at the microwell entrance enhances the maturation of the supracellular actin cable architecture and leads to a vertical orientation of nuclei toward the bottom of the microwell. Increasing Gaussian curvature results in flattened and elongated nuclear morphologies characterized by highly compacted chromatin states. This approach provides better understanding of the mechanoresponse of curved epithelial monolayers curvatures lining lobular structures. In addition, bowl-shaped microwells offer a powerful platform to study curvature-dependent mechanotransduction pathways in anatomically relevant 3D structures.

Large Polyacrylamide Hydrogels for Large-Batch Cell Culture and Mechanobiological Studies

The rigidity of a cell's substrate or extracellular matrix plays a vital role in regulating cell and tissue functions. Polyacrylamide (PAAm) hydrogels are one of the most widely used cell culture substrates that provide a physiologically relevant range of stiffness. However, it is still arduous and time-consuming to prepare PAAm substrates in large batches for high-yield or multiscale cell cultures. In this communication, a simple method to prepare PAAm hydrogels with less time cost and easily accessible materials is presented. The hydrogel is mechanically uniform and supports cell culture in a large batch. It is further shown that the stiffness of the hydrogel covers a large range of Young's modulus and is sensed by cells, regulating various cell features including changes in cell morphology, proliferation, and contractility. This method improves the reproducibility of mechanobiology studies and can be easily applied for mechanobiology research requiring large numbers of cells or experimental groups.

Physical forces guide curvature sensing and cell migration mode bifurcating

The ability of cells to sense and adapt to curvy topographical features has been implicated in organ morphogenesis, tissue repair, and tumor metastasis. However, how individual cells or multicellular assemblies sense and differentiate curvatures remains elusive. Here, we reveal a curvature sensing mechanism in which surface tension can selectively activate either actin or integrin flows, leading to bifurcating cell migration modes: focal adhesion formation that enables cell crawling at convex front edges and actin cable assembly that pulls cells forward at concave front edges. The molecular flows and curved front morphogenesis are sustained by coordinated cellular tension generation and transmission. We track the molecular flows and mechanical force transduction pathways by a phase-field model, which predicts that multicellular curvature sensing is more efficient than individual cells, suggesting collective intelligence of cells. The unique ability of cells in curvature sensing and migration mode bifurcating may offer insights into emergent collective patterns and functions of living active systems at different length scales.

The analytical solution to the migration of an epithelial monolayer with a circular spreading front and its implications in the gap closure process

The coordinated behaviors of epithelial cells are widely observed in tissue development, such as re-epithelialization, tumor growth, and morphogenesis. In these processes, cells either migrate collectively or organize themselves into specific structures to serve certain purposes. In this work, we study a spreading epithelial monolayer whose migrating front encloses a circular gap in the monolayer center. Such tissue is usually used to mimic the wound healing process in vitro. We model the epithelial sheet as a layer of active viscous polar fluid. With an axisymmetric assumption, the model can be analytically solved under two special conditions, suggesting two possible spreading modes for the epithelial monolayer. Based on these two sets of analytical solutions, we assess the velocity of the spreading front affected by the gap size, the active intercellular contractility, and the purse-string contraction acting on the spreading edge. Several critical values exist in the model parameters for the initiation of the gap closure process, and the purse-string contraction plays a vital role in governing the gap closure kinetics. Finally, the instability of the morphology of the spreading front was studied. Numerical calculations show how the perturbated velocities and the growth rates vary with respect to different model parameters.

Geometry-mediated bridging drives nonadhesive stripe wound healing

Wound healing through reepithelialization of gaps is of profound importance to the medical community. One critical mechanism identified by researchers for closing non-cell-adhesive gaps is the accumulation of actin cables around concave edges and the resulting purse-string constriction. However, the studies to date have not separated the gap-edge curvature effect from the gap size effect. Here, we fabricate micropatterned hydrogel substrates with long, straight, and wavy non-cell-adhesive stripes of different gap widths to investigate the stripe edge curvature and stripe width effects on the reepithelialization of Madin-Darby canine kidney (MDCK) cells. Our results show that MDCK cell reepithelization is closely regulated by the gap geometry and may occur through different pathways. In addition to purse-string contraction, we identify gap bridging either via cell protrusion or by lamellipodium extension as critical cellular and molecular mechanisms for wavy gap closure. Cell migration in the direction perpendicular to wound front, sufficiently small gap size to allow bridging, and sufficiently high negative curvature at cell bridges for actin cable constriction are necessary/sufficient conditions for gap closure. Our experiments demonstrate that straight stripes rarely induce cell migration perpendicular to wound front, but wavy stripes do; cell protrusion and lamellipodia extension can help establish bridges over gaps of about five times the cell size, but not significantly beyond. Such discoveries deepen our understanding of mechanobiology of cell responses to curvature and help guide development of biophysical strategies for tissue repair, plastic surgery, and better wound management.

Size-dependent response of cells in epithelial tissue modulated by contractile stress fibers

Although cells with distinct apical areas have been widely observed in epithelial tissues, how the size of cells affects their behavior during tissue deformation and morphogenesis as well as key physical factors modulating such influence remains elusive. Here, we showed that the elongation of cells within the monolayer under anisotropic biaxial stretching increases with their size because the strain released by local cell rearrangement (i.e., T1 transition) is more significant for small cells that possess higher contractility. On the other hand, by incorporating the nucleation, peeling, merging, and breakage dynamics of subcellular stress fibers into classical vertex formulation, we found that stress fibers with orientations predominantly aligned with the main stretching direction will be formed at tricellular junctions, in good agreement with recent experiments. The contractile forces generated by stress fibers help cells to resist imposed stretching, reduce the occurrence of T1 transitions, and, consequently, modulate their size-dependent elongation. Our findings demonstrate that epithelial cells could utilize their size and internal structure to regulate their physical and related biological behaviors. The theoretical framework proposed here can also be extended to investigate the roles of cell geometry and intracellular contraction in processes such as collective cell migration and embryo development.

Leading-edge elongation by follower cell interruption in advancing epithelial cell sheets

Collective cell migration is seen in many developmental and pathological processes, such as morphogenesis, wound closure, and cancer metastasis. When a fish scale is detached and adhered to a substrate, epithelial keratocyte sheets crawl out from it, building a semicircular pattern. All the keratocytes at the leading edge of the sheet have a single lamellipodium, and are interconnected with each other via actomyosin cables. The leading edge of the sheet becomes gradually longer as it crawls out from the scale, regardless of the cell-to-cell connections. In this study, we found leading-edge elongation to be realized by the interruption of follower cells into the leading edge. The follower cell and the two adjacent leader cells are first connected by newly emerging actomyosin cables. Then, the contractile forces along the cables bring the follower cell forward to make it a leader cell. Finally, the original cables between the two leader cells are stretched to tear by the interruption and the lamellipodium extension from the new leader cell. This unique actomyosin-cable reconnection between a follower cell and adjacent leaders offers insights into the mechanisms of collective cell migration.

Active chemo-mechanical feedbacks dictate the collective migration of cells on patterned surfaces

Recent evidence has demonstrated that, when cultured on micro-patterned surfaces, living cells can move in a coordinated manner and form distinct migration patterns, including flowing chain, suspended propagating bridge, rotating vortex, etc. However, the fundamental question of exactly how and why cells migrate in these fashions remains elusive. Here, we present a theoretical investigation to show that the tight interplay between internal cellular activities, such as chemo-mechanical feedbacks and polarization, and external geometrical constraints are behind these intriguing experimental observations. In particular, on narrow strip patterns, strongly force-dependent cellular contractility and intercellular adhesion were found to be critical for reinforcing the leading edge of the migrating cell monolayer and eventually result in the formation of suspended cell bridges flying over nonadhesive regions. On the other hand, a weak force-contractility feedback led to the movement of cells like a flowing chain along the adhesive strip. Finally, we also showed that the random polarity forces generated in migrating cells are responsible for driving them into rotating vortices on strips with width above a threshold value (~10 times the size of the cell).