Advancing Edge Speeds of Epithelial Monolayers Depend on Their Initial Confining Geometry

Matrix Anisotropy Promotes a Transition of Collective to Disseminated Cell Migration via a Collective Vortex Motion

Cells detached and disseminated away from collectively migrating cells are frequently found during tumor invasion at the invasion front, where extracellular matrix (ECM) fibers are parallel to the cell migration direction. However, it remains unclear how anisotropic topography promotes the transition of collective to disseminated cell migration. This study applies a collective cell migration model with and without 800 nm wide aligned nanogrooves parallel, perpendicular, or diagonal to the cell migration direction. After 120 hour migration, MCF7-GFP-H2B-mCherry breast cancer cells display more disseminated cells at the migration front on parallel topography than on other topographies. Notably, a fluid-like collective motion with high vorticity is enhanced at the migration front on parallel topography. Furthermore, high vorticity but not velocity is correlated with disseminated cell numbers on parallel topography. Enhanced collective vortex motion colocalizes with cell monolayer defects where cells extend protrusions into the free space, suggesting that topography-driven cell crawling for defect closure promotes the collective vortex motion. In addition, elongated cell morphology and frequent protrusions induced by topography may further contribute to the collective vortex motion. Overall, a high-vorticity collective motion at the migration front promoted by parallel topography suggests a cause of the transition of collective to disseminated cell migration.

Collective motility and mechanical waves in cell clusters

Epithelial cell clusters often move collectively on a substrate. Mechanical signals play a major role in organizing this behavior. There are a number of experimental observations in these systems which await a comprehensive explanation. These include: the internal strains are tensile even for clusters that expand by proliferation; the tractions on the substrate are often confined to the edges of the cluster; there can exist density waves within the cluster; and for cells in an annulus, there is a transition between expanding clusters with proliferation and the case where cells fill the annulus and rotate around it. We formulate a mechanical model to examine these effects. We use a molecular clutch picture which allows "stalling"-inhibition of cell contraction by external forces. Stalled cells are passive from a physical point of view and the un-stalled cells are active. By attaching cells to the substrate and to each other, and taking into account contact inhibition of locomotion, we get a simple picture for many of these findings as well as predictions that could be tested.

Multicellular scale front-to-rear polarity in collective migration

Collective cell migration does not only reflect the migration of cells at a similar speed and in the same direction, it also implies the emergence of new properties observed at the level of the cell group. This collective behavior relies on interactions between the cells and the establishment of a hierarchy amongst cells with leaders driving the group of followers. Here, we make the parallel between the front-to-rear polarity axis in single cell and the front-to-rear multicellular-scale polarity of a migrating collective which established through exchange of biochemical and mechanical information from the front to the rear and vice versa. Such multicellular-scale polarity gives the migrating group the possibility to better sense and adapt to energy, biochemical and mechanical constraints and facilitates migration over long distances in complex and changing environments.

Extracellular matrix micropatterning technology for whole cell cryogenic electron microscopy studies

Cryogenic electron tomography is the highest resolution tool available for structural analysis of macromolecular organization inside cells. Micropatterning of extracellular matrix (ECM) proteins is an established in vitro cell culture technique used to control cell shape. Recent traction force microscopy studies have shown correlation between cell morphology and the regulation of force transmission. However, it remains unknown how cells sustain increased strain energy states and localized stresses at the supramolecular level. Here, we report a technology to enable direct observation of mesoscale organization in epithelial cells under morphological modulation, using a maskless protein photopatterning method (PRIMO) to confine cells to ECM micropatterns on electron microscopy substrates. These micropatterned cell culture substrates can be used in mechanobiology research to correlate changes in nanometer-scale organization at cell-cell and cell-ECM contacts to strain energy states and traction stress distribution in the cell.

Continuum descriptions of spatial spreading for heterogeneous cell populations: Theory and experiment

Variability in cell populations is frequently observed in both in vitro and in vivo settings. Intrinsic differences within populations of cells, such as differences in cell sizes or differences in rates of cell motility, can be present even within a population of cells from the same cell line. We refer to this variability as cell heterogeneity. Mathematical models of cell migration, for example, in the context of tumour growth and metastatic invasion, often account for both undirected (random) migration and directed migration that is mediated by cell-to-cell contacts and cell-to-cell adhesion. A key feature of standard models is that they often assume that the population is composed of identical cells with constant properties. This leads to relatively simple single-species homogeneous models that neglect the role of heterogeneity. In this work, we use a continuum modelling approach to explore the role of heterogeneity in spatial spreading of cell populations. We employ a three-species heterogeneous model of cell motility that explicitly incorporates different types of experimentally-motivated heterogeneity in cell sizes: (i) monotonically decreasing; (ii) uniform; (iii) non-monotonic; and (iv) monotonically increasing distributions of cell size. Comparing the density profiles generated by the three-species heterogeneous model with density profiles predicted by a more standard single-species homogeneous model reveals that when we are dealing with monotonically decreasing and uniform distributions a simple and computationally efficient single-species homogeneous model can be remarkably accurate in describing the evolution of a heterogeneous cell population. In contrast, we find that the simpler single-species homogeneous model performs relatively poorly when applied to non-monotonic and monotonically increasing distributions of cell sizes. Additional results for heterogeneity in parameters describing both undirected and directed cell migration are also considered, and we find that similar results apply.

Switch-like enhancement of epithelial-mesenchymal transition by YAP through feedback regulation of WT1 and Rho-family GTPases

Collective cell migration occurs in many patho-physiological states, including wound healing and invasive cancer growth. The integrity of the expanding epithelial sheets depends on extracellular cues, including cell-cell and cell-matrix interactions. We show that the nano-scale topography of the extracellular matrix underlying epithelial cell layers can strongly affect the speed and morphology of the fronts of the expanding sheet, triggering partial and complete epithelial-mesenchymal transitions (EMTs). We further demonstrate that this behavior depends on the mechano-sensitivity of the transcription regulator YAP and two new YAP-mediated cross-regulating feedback mechanisms: Wilms Tumor-1-YAP-mediated downregulation of E-cadherin, loosening cell-cell contacts, and YAP-TRIO-Merlin mediated regulation of Rho GTPase family proteins, enhancing cell migration. These YAP-dependent feedback loops result in a switch-like change in the signaling and the expression of EMT-related markers, leading to a robust enhancement in invasive cell spread, which may lead to a worsened clinical outcome in renal and other cancers.

Reorganisation of the extracellular matrix (ECM) controls processes involving epithelial-mesenchymal transition (EMT). Here, the authors show that EMT occurring in epithelial cells on a fabricated nano-engineered cell adhesion surface is triggered by mechanical cues from the ECM.

Controlling cell shape on hydrogels using lift-off protein patterning

Polyacrylamide gels functionalized with extracellular matrix proteins are commonly used as cell culture platforms to evaluate the combined effects of extracellular matrix composition, cell geometry and substrate rigidity on cell physiology. For this purpose, protein transfer onto the surface of polyacrylamide hydrogels must result in geometrically well-resolved micropatterns with homogeneous protein distribution. Yet the outcomes of micropatterning methods have not been pairwise evaluated against these criteria. We report a high-fidelity photoresist lift-off patterning method to pattern ECM proteins on polyacrylamide hydrogels with elastic moduli ranging from 5 to 25 kPa. We directly compare the protein transfer efficiency and pattern geometrical accuracy of this protocol to the widely used microcontact printing method. Lift-off patterning achieves higher protein transfer efficiency, increases pattern accuracy, increases pattern yield, and reduces variability of these factors within arrays of patterns as it bypasses the drying and transfer steps of microcontact printing. We demonstrate that lift-off patterned hydrogels successfully control cell size and shape and enable long-term imaging of actin intracellular structure and lamellipodia dynamics when we culture epithelial cells on these substrates.

Leaders in collective migration: are front cells really endowed with a particular set of skills?

Collective cell migration is the coordinated movement emerging from the interaction of at least two cells. In multicellular organisms, collective cell migration is ubiquitous. During development, embryonic cells often travel in numbers, whereas in adults, epithelial cells close wounds collectively. There is often a division of labour and two categories of cells have been proposed: leaders and followers. These two terms imply that followers are subordinated to leaders whose proposed broad range of actions significantly biases the direction of the group of cells towards a specific target. These two terms are also tied to topology. Leaders are at the front while followers are located behind them. Here, we review recent work on some of the main experimental models for collective cell migration, concluding that leader-follower terminology may not be the most appropriate. It appears that not all collectively migrating groups are driven by cells located at the front. Moreover, the qualities that define leaders (pathfinding, traction forces and matrix remodelling) are not specific to front cells. These observations indicate that the terms leaders and followers are not suited to every case. We think that it would be more accurate to dissociate the function of a cell from its position in the group. The position of cells can be precisely defined with respect to the direction of movement by purely topological terms such as “front” or “rear” cells. In addition, we propose the more ample and strictly functional definition of “steering cells” which are able to determine the directionality of movement for the entire group. In this context, a leader cell represents only a specific case in which a steering cell is positioned at the front of the group.

Stereolithographic printing of ionically-crosslinked alginate hydrogels for degradable biomaterials and microfluidics

3D printed biomaterials with spatial and temporal functionality could enable interfacial manipulation of fluid flows and motile cells. However, such dynamic biomaterials are challenging to implement since they must be responsive to multiple, biocompatible stimuli. Here, we show stereolithographic printing of hydrogels using noncovalent (ionic) crosslinking, which enables reversible patterning with controlled degradation. We demonstrate this approach using sodium alginate, photoacid generators and various combinations of divalent cation salts, which can be used to tune the hydrogel degradation kinetics, pattern fidelity, and mechanical properties. This approach is first utilized to template perfusable microfluidic channels within a second encapsulating hydrogel for T-junction and gradient devices. The presence and degradation of printed alginate microstructures were further verified to have minimal toxicity on epithelial cells. Degradable alginate barriers were used to direct collective cell migration from different initial geometries, revealing differences in front speed and leader cell formation. Overall, this demonstration of light-based 3D printing using non-covalent crosslinking may enable adaptive and stimuli-responsive biomaterials, which could be utilized for bio-inspired sensing, actuation, drug delivery, and tissue engineering.

Deletion of the cytoplasmic domain of N-cadherin reduces, but does not eliminate, traction force-transmission

Collective migration of epithelial cells is an integral part of embryonic development, wound healing, tissue renewal and carcinoma invasion. While previous studies have focused on cell-extracellular matrix adhesion as a site of migration-driving, traction force-transmission, cadherin mediated cell-cell adhesion is also capable of force-transmission. Using a soft elastomer coated with purified N-cadherin as a substrate and a Hepatocyte Growth Factor-treated, transformed MDCK epithelial cell line as a model system, we quantified traction transmitted by N-cadherin-mediated contacts. On a substrate coated with purified extracellular domain of N-cadherin, cell surface N-cadherin proteins arranged into puncta. N-cadherin mutants (either the cytoplasmic deletion or actin-binding domain chimera), however, failed to assemble into puncta, suggesting the assembly of focal adhesion like puncta requires the cytoplasmic domain of N-cadherin. Furthermore, the cytoplasmic domain deleted N-cadherin expressing cells exerted lower traction stress than the full-length or the actin binding domain chimeric N-cadherin. Our data demonstrate that N-cadherin junctions exert significant traction stress that requires the cytoplasmic domain of N-cadherin, but the loss of the cytoplasmic domain does not completely eliminate traction force transmission.