Interplay between chemotaxis and contact inhibition of locomotion determines exploratory cell migration

Gating and tunable confinement of active colloids within patterned environments

Active colloidal particles typically exhibit a pronounced affinity for accumulating and being captured at boundaries. Here, we engineer long-range repulsive interactions between colloids that self-propel under an electric field and patterned obstacles. As a result of these interactions, particles turn away from obstacles and avoid accumulation. We show that by tuning the applied field frequency, we precisely and rapidly control the effective size of the obstacles and therefore modulate the particle approach distance. This feature allows us to achieve gating and tunable confinement of our active particles whereby they can access regions between obstacles depending on the applied field. Our work provides a versatile means to directly control confinement and organization, paving the way towards applications such as sorting particles based on motility or localizing active particles on demand.

Bioanalytic Hybrid System Merging 3-Dimensional Cell Culture and Chromatographic Precision for Unprecedented Preclinical Insights in Molecular Imaging

We introduce a unique bioanalytic hybrid system for the preclinical assessment of radiotracer candidates, combining a 3-dimensional cell culture on an artificial extracellular matrix functioning as a stationary phase and a chromatographiclike system array. Methods: Silk fibroin sponges were applied to simulate an extracellular matrix and to function as a stationary phase. Different cell lines were grown on the silk scaffold and used to investigate radiopharmaceuticals in a small-animal PET/CT system. Results: Our system integrates with the strength of chromatographic systems, allowing high throughput, full automation, and online processing with the complexity of an advanced 3-dimensional cell culture for in vitro modeling of real tissuelike geometry, morphology, and dynamics. Conclusion: This system holds great potential to study newly developed radiotracers for applications in binding studies and assessment of unspecific binding. It might help to decrease the translational gap from in vitro cell cultures to in vivo studies while it aligns with the 3R principle (reduce, replace, refine) of animal testing.

Collective mechanics of small migrating cell groups

Migration of adhesive cell groups is a fundamental part of wound healing, development and carcinogenesis. Intense research has been conducted on mechanisms of collective migration of adhesive groups of cells. Here we focus on mechanical and mechanistic lessons from small migrating cell groups. We review forces and locomotory dynamics of two- and three-cell clusters, rotation of cell doublets, self-organization of one-dimensional cell trains, nascent efforts to understand three-dimensional collective migration and border cell clusters in Drosophila embryo.

Excitable Rho dynamics control cell shape and motility by sequentially activating ERM proteins and actomyosin contractility

Migration of endothelial and many other cells requires spatiotemporal regulation of protrusive and contractile cytoskeletal rearrangements that drive local cell shape changes. Unexpectedly, the small GTPase Rho, a crucial regulator of cell movement, has been reported to be active in both local cell protrusions and retractions, raising the question of how Rho activity can coordinate cell migration. Here, we show that Rho activity is absent in local protrusions and active during retractions. During retractions, Rho rapidly activated ezrin-radixin-moesin proteins (ERMs) to increase actin-membrane attachment, and, with a delay, nonmuscle myosin 2 (NM2). Rho activity was excitable, with NM2 acting as a slow negative feedback regulator. Strikingly, inhibition of SLK/LOK kinases, through which Rho activates ERMs, caused elongated cell morphologies, impaired Rho-induced cell contractions, and reverted Rho-induced blebbing. Together, our study demonstrates that Rho activity drives retractions by sequentially enhancing ERM-mediated actin-membrane attachment for force transmission and NM2-dependent contractility.

Limits on the accuracy of contact inhibition of locomotion

Cells that collide with each other repolarize away from contact, in a process called contact inhibition of locomotion (CIL), which is necessary for correct development of the embryo. CIL can occur even when cells make a micron-scale contact with a neighbor-much smaller than their size. How precisely can a cell sense cell-cell contact and repolarize in the correct direction? What factors control whether a cell recognizes it has contacted a neighbor? We propose a theoretical model for the limits of CIL where cells recognize the presence of another cell by binding the protein ephrin with the Eph receptor. This recognition is made difficult by the presence of interfering ligands that bind nonspecifically. Both theoretical predictions and simulation results show that it becomes more difficult to sense cell-cell contact when it is difficult to distinguish ephrin from the interfering ligands, or when there are more interfering ligands, or when the contact width decreases. However, the error of estimating contact position remains almost constant when the contact width changes. This happens because the cell gains spatial information largely from the boundaries of cell-cell contact. We study using statistical decision theory the likelihood of a false-positive CIL event in the absence of cell-cell contact, and the likelihood of a false negative where CIL does not occur when another cell is present. Our results suggest that the cell is more likely to make incorrect decisions when the contact width is very small or so large that it nears the cell's perimeter. However, in general, we find that cells have the ability to make reasonably reliable CIL decisions even for very narrow (micron-scale) contacts, even if the concentration of interfering ligands is ten times that of the correct ligands.

Learning dynamical models of single and collective cell migration: a review

Single and collective cell migration are fundamental processes critical for physiological phenomena ranging from embryonic development and immune response to wound healing and cancer metastasis. To understand cell migration from a physical perspective, a broad variety of models for the underlying physical mechanisms that govern cell motility have been developed. A key challenge in the development of such models is how to connect them to experimental observations, which often exhibit complex stochastic behaviours. In this review, we discuss recent advances in data-driven theoretical approaches that directly connect with experimental data to infer dynamical models of stochastic cell migration. Leveraging advances in nanofabrication, image analysis, and tracking technology, experimental studies now provide unprecedented large datasets on cellular dynamics. In parallel, theoretical efforts have been directed towards integrating such datasets into physical models from the single cell to the tissue scale with the aim of conceptualising the emergent behaviour of cells. We first review how this inference problem has been addressed in both freely migrating and confined cells. Next, we discuss why these dynamics typically take the form of underdamped stochastic equations of motion, and how such equations can be inferred from data. We then review applications of data-driven inference and machine learning approaches to heterogeneity in cell behaviour, subcellular degrees of freedom, and to the collective dynamics of multicellular systems. Across these applications, we emphasise how data-driven methods can be integrated with physical active matter models of migrating cells, and help reveal how underlying molecular mechanisms control cell behaviour. Together, these data-driven approaches are a promising avenue for building physical models of cell migration directly from experimental data, and for providing conceptual links between different length-scales of description.

Micro-engineering and nano-engineering approaches to investigate tumour ecosystems

The interactions among tumour cells, the tumour microenvironment (TME) and non-tumour tissues are of interest to many cancer researchers. Micro-engineering approaches and nanotechnologies are under extensive exploration for modelling these interactions and measuring them in situ and in vivo to investigate therapeutic vulnerabilities in cancer and extend a systemic view of tumour ecosystems. Here we highlight the greatest opportunities for improving the understanding of tumour ecosystems using microfluidic devices, bioprinting or organ-on-a-chip approaches. We also discuss the potential of nanosensors that can transmit information from within the TME or elsewhere in the body to address scientific and clinical questions about changes in chemical gradients, enzymatic activities, metabolic and immune profiles of the TME and circulating analytes. This Review aims to connect the cancer biology and engineering communities, presenting biomedical technologies that may expand the methodologies of the former, while inspiring the latter to develop approaches for interrogating cancer ecosystems.

Collective chemotaxis in a Voronoi model for confluent clusters

Collective chemotaxis, where single cells cannot climb a biochemical signaling gradient but clusters of cells can, has been observed in different biological contexts, including confluent tissues where there are no gaps or overlaps between cells. Although particle-based models have been developed that predict important features of collective chemotaxis, the mechanisms in those models depend on particle overlaps, and so it remains unclear if they can explain behavior in confluent systems. Here, we develop an open-source code that couples a two-dimensional Voronoi simulation for confluent cell mechanics to a dynamic chemical signal that can diffuse, advect, and/or degrade and use the code to study potential mechanisms for collective chemotaxis in cellular monolayers. We first study the impact of advection on collective chemotaxis and delineate a regime where advective terms are important. Next, we investigate two possible chemotactic mechanisms, contact inhibition of locomotion and heterotypic interfacial tension, and demonstrate that both can drive collective chemotaxis in certain parameter regimes. We further demonstrate that the scaling behavior of cluster motion is well captured by simple analytic theories.

Cell Repolarization: A Bifurcation Study of Spatio-Temporal Perturbations of Polar Cells

The intrinsic polarity of migrating cells is regulated by spatial distributions of protein activity. Those proteins (Rho-family GTPases, such as Rac and Rho) redistribute in response to stimuli, determining the cell front and back. Reaction-diffusion equations with mass conservation and positive feedback have been used to explain initial polarization of a cell. However, the sensitivity of a polar cell to a reversal stimulus has not yet been fully understood. We carry out a PDE bifurcation analysis of two polarity models to investigate routes to repolarization: (1) a single-GTPase ("wave-pinning") model and (2) a mutually antagonistic Rac-Rho model. We find distinct routes to reversal in (1) vs. (2). We show numerical simulations of full PDE solutions for the RD equations, demonstrating agreement with predictions of the bifurcation results. Finally, we show that simulations of the polarity models in deforming 1D model cells are consistent with biological experiments.

A Thromboxane A2 Receptor-Driven COX-2-Dependent Feedback Loop That Affects Endothelial Homeostasis and Angiogenesis

BACKGROUND

TP (thromboxane A₂ receptor) plays an eminent role in the pathophysiology of endothelial dysfunction and cardiovascular disease. Moreover, its expression is reported to increase in the intimal layer of blood vessels of cardiovascular high-risk individuals. Yet it is unknown, whether TP upregulation per se has the potential to affect the homeostasis of the vascular endothelium.

METHODS

We combined global transcriptome analysis, lipid mediator profiling, functional cell analyses, and in vivo angiogenesis assays to study the effects of endothelial TP overexpression or knockdown/knockout on the angiogenic capacity of endothelial cells in vitro and in vivo.

RESULTS

Here we report that endothelial TP expression induces COX-2 (cyclooxygenase-2) in a G_i/o- and G_q/11-dependent manner, thereby promoting its own activation via the auto/paracrine release of TP agonists, such as PGH₂ (prostaglandin H₂) or prostaglandin F₂ but not TxA₂ (thromboxane A₂). TP overexpression induces endothelial cell tension and aberrant cell morphology, affects focal adhesion dynamics, and inhibits the angiogenic capacity of human endothelial cells in vitro and in vivo, whereas TP knockdown or endothelial-specific TP knockout exerts opposing effects. Consequently, this TP-dependent feedback loop is disrupted by pharmacological TP or COX-2 inhibition and by genetic reconstitution of PGH₂-metabolizing prostacyclin synthase even in the absence of functional prostacyclin receptor expression.

CONCLUSIONS

Our work uncovers a TP-driven COX-2-dependent feedback loop and important effector mechanisms that directly link TP upregulation to angiostatic TP signaling in endothelial cells. By these previously unrecognized mechanisms, pathological endothelial upregulation of the TP could directly foster endothelial dysfunction, microvascular rarefaction, and systemic hypertension even in the absence of exogenous sources of TP agonists.

Dynamic self-organization of migrating cells under constraints by spatial confinement and epithelial integrity

Understanding how migrating cells can establish both dynamic structures and coherent dynamics may provide mechanistic insights to study how living systems acquire complex structures and functions. Recent studies revealed that intercellular contact communication plays a crucial role for establishing cellular dynamic self-organization (DSO) and provided a theoretical model of DSO for migrating solitary cells in a free space. However, to apply those understanding to situations in living organisms, we need to know the role of cell-cell communication for tissue dynamics under spatial confinements and epithelial integrity. Here, we expand the previous numerical studies on DSO to migrating cells subjected spatial confinement and/or epithelial integrity. An epithelial monolayer is simulated by combining the model of cellular DSO and the cellular vertex model in two dimensions for apical integrity. Under confinement to a small space, theoretical models of both solitary and epithelial cells exhibit characteristic coherent dynamics, including apparent swirling. We also find that such coherent dynamics can allow the cells to overcome the strong constraint due to spatial confinement and epithelial integrity. Furthermore, we demonstrate how epithelial cell clusters behave without spatial confinement and find various cluster dynamics, including spinning, migration and elongation.

Complex effects of kinase localization revealed by compartment-specific regulation of protein kinase A activity

Kinase activity in signaling networks frequently depends on regulatory subunits that can both inhibit activity by interacting with the catalytic subunits and target the kinase to distinct molecular partners and subcellular compartments. Here, using a new synthetic molecular interaction system, we show that translocation of a regulatory subunit of the protein kinase A (PKA-R) to the plasma membrane has a paradoxical effect on the membrane kinase activity. It can both enhance it at lower translocation levels, even in the absence of signaling inputs, and inhibit it at higher translocation levels, suggesting its role as a linker that can both couple and decouple signaling processes in a concentration-dependent manner. We further demonstrate that superposition of gradients of PKA-R abundance across single cells can control the directionality of cell migration, reversing it at high enough input levels. Thus, complex in vivo patterns of PKA-R localization can drive complex phenotypes, including cell migration.

Regulation of cell locomotion by nanosecond-laser-induced hydroxyapatite patterning

Hydroxyapatite, an essential mineral in human bones composed mainly of calcium and phosphorus, is widely used to coat bone graft and implant surfaces for enhanced biocompatibility and bone formation. For a strong implant-bone bond, the bone-forming cells must not only adhere to the implant surface but also move to the surface requiring bone formation. However, strong adhesion tends to inhibit cell migration on the surface of hydroxyapatite. Herein, a cell migration highway pattern that can promote cell migration was prepared using a nanosecond laser on hydroxyapatite coating. The developed surface promoted bone-forming cell movement compared with the unpatterned hydroxyapatite surface, and the cell adhesion and movement speed could be controlled by adjusting the pattern width. Live-cell microscopy, cell tracking, and serum protein analysis revealed the fundamental principle of this phenomenon. These findings are applicable to hydroxyapatite-coated biomaterials and can be implemented easily by laser patterning without complicated processes. The cell migration highway can promote and control cell movement while maintaining the existing advantages of hydroxyapatite coatings. Furthermore, it can be applied to the surface treatment of not only implant materials directly bonded to bone but also various implanted biomaterials implanted that require cell movement control.

Advances in the study of cancer metastasis and calcium signaling as potential therapeutic targets

Metastasis is still the primary cause of cancer-related mortality. However, the underlying mechanisms of cancer metastasis are not yet fully understood. Currently, the epithelial-mesenchymal transition, metabolic remodeling, cancer cell intercommunication and the tumor microenvironment including diverse stromal cells, are reported to affect the metastatic process of cancer cells. Calcium ions (Ca²⁺) are ubiquitous second messengers that manipulate cancer metastasis by affecting signaling pathways. Diverse transporter/pump/channel-mediated Ca²⁺ currents form Ca²⁺ oscillations that can be decoded by Ca²⁺-binding proteins, which are promising prognostic biomarkers and therapeutic targets of cancer metastasis. This paper presents a review of the advances in research on the mechanisms underlying cancer metastasis and the roles of Ca²⁺-related signals in these events.

SUN-MKL1 Crosstalk Regulates Nuclear Deformation and Fast Motility of Breast Carcinoma Cells in Fibrillar ECM Microenvironment

Aligned collagen fibers provide topography for the rapid migration of single tumor cells (streaming migration) to invade the surrounding stroma, move within tumor nests towards blood vessels to intravasate and form distant metastases. Mechanisms of tumor cell motility have been studied extensively in the 2D context, but the mechanistic understanding of rapid single tumor cell motility in the in vivo context is still lacking. Here, we show that streaming tumor cells in vivo use collagen fibers with diameters below 3 µm. Employing 1D migration assays with matching in vivo fiber dimensions, we found a dependence of tumor cell motility on 1D substrate width, with cells moving the fastest and the most persistently on the narrowest 1D fibers (700 nm-2.5 µm). Interestingly, we also observed nuclear deformation in the absence of restricting extracellular matrix pores during high speed carcinoma cell migration in 1D, similar to the nuclear deformation observed in tumor cells in vivo. Further, we found that actomyosin machinery is aligned along the 1D axis and actomyosin contractility synchronously regulates cell motility and nuclear deformation. To further investigate the link between cell speed and nuclear deformation, we focused on the Linker of Nucleoskeleton and Cytoskeleton (LINC) complex proteins and SRF-MKL1 signaling, key regulators of mechanotransduction, actomyosin contractility and actin-based cell motility. Analysis of The Cancer Genome Atlas dataset showed a dramatic decrease in the LINC complex proteins SUN1 and SUN2 in primary tumor compared to the normal tissue. Disruption of LINC complex by SUN1 + 2 KD led to multi-lobular elongated nuclei, increased tumor cell motility and concomitant increase in F-actin, without affecting Lamin proteins. Mechanistically, we found that MKL1, an effector of changes in cellular G-actin to F-actin ratio, is required for increased 1D motility seen in SUN1 + 2 KD cells. Thus, we demonstrate a previously unrecognized crosstalk between SUN proteins and MKL1 transcription factor in modulating nuclear shape and carcinoma cell motility in an in vivo relevant 1D microenvironment.

Rules of contact inhibition of locomotion for cells on suspended nanofibers

Contact inhibition of locomotion (CIL), in which cells repolarize and move away from contact, is now established as a fundamental driving force in development, repair, and disease biology. Much of what we know of CIL stems from studies on two-dimensional (2D) substrates that do not provide an essential biophysical cue-the curvature of extracellular matrix fibers. We discover rules controlling outcomes of cell-cell collisions on suspended nanofibers and show them to be profoundly different from the stereotyped CIL behavior on 2D substrates. Two approaching cells attached to a single fiber do not repolarize upon contact but rather usually migrate past one another. Fiber geometry modulates this behavior; when cells attach to two fibers, reducing their freedom to reorient, only one cell repolarizes on contact, leading to the cell pair migrating as a single unit. CIL outcomes also change when one cell has recently divided and moves with high speed-cells more frequently walk past each other. Our computational model of CIL in fiber geometries reproduces the core qualitative results of the experiments robustly to model parameters. Our model shows that the increased speed of postdivision cells may be sufficient to explain their increased walk-past rate. We also identify cell-cell adhesion as a key mediator of collision outcomes. Our results suggest that characterizing cell-cell interactions on flat substrates, channels, or micropatterns is not sufficient to predict interactions in a matrix-the geometry of the fiber can generate entirely new behaviors.

Learning the dynamics of cell-cell interactions in confined cell migration

The migratory dynamics of cells in physiological processes, ranging from wound healing to cancer metastasis, rely on contact-mediated cell-cell interactions. These interactions play a key role in shaping the stochastic trajectories of migrating cells. While data-driven physical formalisms for the stochastic migration dynamics of single cells have been developed, such a framework for the behavioral dynamics of interacting cells still remains elusive. Here, we monitor stochastic cell trajectories in a minimal experimental cell collider: a dumbbell-shaped micropattern on which pairs of cells perform repeated cellular collisions. We observe different characteristic behaviors, including cells reversing, following, and sliding past each other upon collision. Capitalizing on this large experimental dataset of coupled cell trajectories, we infer an interacting stochastic equation of motion that accurately predicts the observed interaction behaviors. Our approach reveals that interacting noncancerous MCF10A cells can be described by repulsion and friction interactions. In contrast, cancerous MDA-MB-231 cells exhibit attraction and antifriction interactions, promoting the predominant relative sliding behavior observed for these cells. Based on these experimentally inferred interactions, we show how this framework may generalize to provide a unifying theoretical description of the diverse cellular interaction behaviors of distinct cell types.

Filopodia-based contact stimulation of cell migration drives tissue morphogenesis

Cells migrate collectively to form tissues and organs during morphogenesis. Contact inhibition of locomotion (CIL) drives collective migration by inhibiting lamellipodial protrusions at cell-cell contacts and promoting polarization at the leading edge. Here, we report a CIL-related collective cell behavior of myotubes that lack lamellipodial protrusions, but instead use filopodia to move as a cohesive cluster in a formin-dependent manner. We perform genetic, pharmacological and mechanical perturbation analyses to reveal the essential roles of Rac2, Cdc42 and Rho1 in myotube migration. These factors differentially control protrusion dynamics and cell-matrix adhesion formation. We also show that active Rho1 GTPase localizes at retracting free edge filopodia and that Rok-dependent actomyosin contractility does not mediate a contraction of protrusions at cell-cell contacts, but likely plays an important role in the constriction of supracellular actin cables. Based on these findings, we propose that contact-dependent asymmetry of cell-matrix adhesion drives directional movement, whereas contractile actin cables contribute to the integrity of the migrating cell cluster.

Dynamic Self-Organization of Idealized Migrating Cells by Contact Communication

This Letter investigates what forms of cellular dynamic self-organization are caused through intercellular contact communication based on a theoretical model in which migrating cells perform contact following and contact inhibition and attraction of locomotion. Tuning those strengths causes varieties of dynamic patterns. This further includes a novel form of collective migration, snakelike dynamic assembly. Scrutinizing this pattern reveals that cells in this state can accurately respond to an external directional cue but have no spontaneous global polar order.

Cell Migration Guided by Cell-Cell Contacts in Innate Immunity

The directed migration of leukocytes to sites of damage or infection is necessary for a productive immune response. There is substantial evidence supporting a key role for chemoattractants in directed migration, however, less is known about how cell-cell contacts affect the migratory behavior of leukocytes in innate immunity. Here, we explore how cell-cell contacts can affect the directed migration of innate immune cells, including their role in attracting, repelling, or stopping cell motility. Further investigation of cell contact dynamics as guidance cues may yield new insights into the regulation of innate immunity.

Bridging from single to collective cell migration: A review of models and links to experiments

Mathematical and computational models can assist in gaining an understanding of cell behavior at many levels of organization. Here, we review models in the literature that focus on eukaryotic cell motility at 3 size scales: intracellular signaling that regulates cell shape and movement, single cell motility, and collective cell behavior from a few cells to tissues. We survey recent literature to summarize distinct computational methods (phase-field, polygonal, Cellular Potts, and spherical cells). We discuss models that bridge between levels of organization, and describe levels of detail, both biochemical and geometric, included in the models. We also highlight links between models and experiments. We find that models that span the 3 levels are still in the minority.

Periodic propagating waves coordinate RhoGTPase network dynamics at the leading and trailing edges during cell migration

Migrating cells need to coordinate distinct leading and trailing edge dynamics but the underlying mechanisms are unclear. Here, we combine experiments and mathematical modeling to elaborate the minimal autonomous biochemical machinery necessary and sufficient for this dynamic coordination and cell movement. RhoA activates Rac1 via DIA and inhibits Rac1 via ROCK, while Rac1 inhibits RhoA through PAK. Our data suggest that in motile, polarized cells, RhoA–ROCK interactions prevail at the rear, whereas RhoA-DIA interactions dominate at the front where Rac1/Rho oscillations drive protrusions and retractions. At the rear, high RhoA and low Rac1 activities are maintained until a wave of oscillatory GTPase activities from the cell front reaches the rear, inducing transient GTPase oscillations and RhoA activity spikes. After the rear retracts, the initial GTPase pattern resumes. Our findings show how periodic, propagating GTPase waves coordinate distinct GTPase patterns at the leading and trailing edge dynamics in moving cells.

Matrix degradation regulates osteoblast protrusion dynamics and individual migration

Protrusions are one of the structures that cells use to sense their surrounding environment in a probing and exploratory manner as well as to communicate with other cells. In particular, osteoblasts embedded within a 3D matrix tend to originate a large number of protrusions compared to other type of cells. In this work, we study the role that mechanochemical properties of the extracellular matrix (ECM) play on the dynamics of these protrusions, namely, the regulation of the size and number of emanating structures. In addition, we also determine how the dynamics of the protrusions may lead the 3D movement of the osteoblasts. Significant differences were found in protrusion size and cell velocity, when degradation activity due to metalloproteases was blocked by means of an artificial broad-spectrum matrix metalloproteinase inhibitor, whereas stiffening of the matrix by introducing transglutaminase crosslinking, only induced slight changes in both protrusion size and cell velocity, suggesting that the ability of cells to create a path through the matrix is more critical than the matrix mechanical properties themselves. To confirm this, we developed a cell migration computational model in 3D including both the mechanical and chemical properties of the ECM as well as the protrusion mechanics, obtaining good agreement with experimental results.

Ultrasound enhanced synthetic platelet therapy for augmented wound repair

Native platelets perform a number of functions within the wound healing process, including interacting with fibrin fibers at the wound site to bring about retraction after clot formation. Clot retraction improves clot stability and enhances the function of the fibrin network as a provisional matrix to support cellular infiltration of the wound site, thus facilitating tissue repair and remodeling after hemostasis. In cases of traumatic injury or disease, platelets can become depleted and this process disrupted. To that end, our lab has developed synthetic platelet-like particles (PLPs) that recapitulate the clot retraction abilities of native platelets through a Brownian-wrench driven mechanism that drives fibrin network densification and clot retraction over time, however, this Brownian-motion driven process occurs on a longer time scale than native active actin/myosin-driven platelet-mediated clot retraction. We hypothesized that a combinatorial therapy comprised of ultrasound stimulation of PLP motion within fibrin clots would facilitate a faster induction of clot retraction on a more platelet-mimetic time scale and at a lower dosage than required for PLPs acting alone. We found that application of ultrasound in combination with a subtherapeutic dosage of PLPs resulted in increased clot density and stiffness, improved fibroblast migration in vitro and increased epidermal thickness and angiogenesis in vivo, indicating that this combination therapy has potential to facilitate multiphase pro-healing outcomes. Additionally, while these particular studies focus on the role of ultrasound in enhancing specific interactions between fibrin-binding synthetic PLPs embedded within fibrin networks, these studies have wide applicability in understanding the role of ultrasound stimulation in enhancing multi-scale colloidal interactions within fibrillar matrices.

Unique Cancer Migratory Behaviors in Confined Spaces of Microgroove Topography with Acute Wall Angles

In recent years, many types of micro-engineered platform have been fabricated to investigate the influences of surrounding microenvironments on cell migration. Previous researches demonstrated that microgroove-based topographies can influence cell motilities of normal and cancerous cells differently. In this study, the microgroove wall angle was altered from obtuse to acute angles and the resulting differences in the responses of normal and cancer cells were investigated to explore the geometrical characteristics that can efficiently distinguish normal and cancer cells. Interestingly, different trends in cell motilities of normal and cancer cells were observed as the wall angles were varied between 60–120°, and in particular, invasive cancer cells exhibited a unique, oscillatory migratory behavior. Results from the immunostaining of cell mechanotransduction components suggested that this difference stemmed from directional extensions and adhesion behaviors of each cell type. In addition, the specific behaviors of invasive cancer cells were found to be dependent on the myosin II activity, and modulating the activity could revert cancerous behaviors to normal ones. These novel findings on the interactions of acute angle walls and cancer cell migration provide a new perspective on cancer metastasis and additional strategies via microstructure geometries for the manipulations of cell behaviors in microscale biodevices.

Prognostic value of ephrin B receptors in breast cancer: An online survival analysis using the microarray data of 3,554 patients

The roles of Ephrin B (EphB) receptors in cancer are relatively unknown as these receptors are associated with complex signaling pathways. A limited number of studies have investigated the association between EphB receptors and prognosis. Using the Kaplan-Meier plotter database, the present study investigated the associations between the mRNA expression levels of five EphB receptors and the outcomes of 3,554 patients with breast cancer who had been followed-up for 20 years. Hazard ratios (HR) and 95% confidence intervals (CI) were calculated to assess the relative risk of survival. The results demonstrated that high mRNA expression levels of EphB2 (HR, 0.74; 95% CI, 0.66-0.84; P=2.1×10-6), EphB4 (HR, 0.82; 95% CI, 0.72-0.93; P=0.0023) and EphB6 (HR, 0.69; 95% CI, 0.61-0.78; P=3×10-9) were significantly associated with improved survival, while a high mRNA expression level of EphB3 (HR, 1.14; 95% CI, 1.01-1.28; P=0.029) was associated with worse survival for patients with breast cancer. High expression levels of all EphB receptors, including EphB1 (HR, 1.4; 95% CI, 1.02-1.94; P=0.039), EphB2 (HR, 1.34; 95% CI, 1.07-1.67; P=0.011), EphB3 (HR, 1.39; 95% CI, 1.11-1.73, P=0.0038), EphB4 (HR, 1.33; 95% CI, 1.06-1.67; P=0.013) and EphB6 (HR, 1.32; 95% CI, 1.05-1.65; P=0.016), were associated with an increased risk of mortality in patients with lymph-node-positive breast cancer. High mRNA expression levels of EphB1 were not associated with survival for all patients with breast cancer (HR, 0.85; 95% CI, 0.72-1.01; P=0.058). The results of the present suggested that EphB receptors may be useful as prognostic biomarkers of breast cancer.

Immature dendritic cells navigate microscopic mazes to find tumor cells

Dendritic cells (DCs) are potent antigen-presenting cells with high sentinel ability to scan their neighborhood and to initiate an adaptive immune response. Whereas chemotactic migration of mature DCs (mDCs) towards lymph nodes is relatively well documented, the migratory behavior of immature DCs (imDCs) in tumor microenvironments is still poorly understood. Here, microfluidic systems of various geometries, including mazes, are used to investigate how the physical and chemical microenvironment influences the migration pattern of imDCs. Under proper degree of confinement, the imDCs are preferentially recruited towards cancer vs. normal cells, accompanied by increased cell speed and persistence. Furthermore, a systematic screen of cytokines, reveals that Gas6 is a major chemokine responsible for the chemotactic preference. These results and the accompanying theoretical model suggest that imDC migration in complex tissue environments is tuned by a proper balance between the strength of the chemical gradients and the degree of spatial confinement.

Randomly Distributed K14+ Breast Tumor Cells Polarize to the Leading Edge and Guide Collective Migration in Response to Chemical and Mechanical Environmental Cues

Collective cell migration is an adaptive, coordinated interactive process involving cell-cell and cell-extracellular matrix (ECM) microenvironmental interactions. A critical aspect of collective migration is the sensing and establishment of directional movement. It has been proposed that a subgroup of cells known as leader cells localize at the front edge of a collectively migrating cluster and are responsible for directing migration. However, it is unknown how and when leader cells arrive at the front edge and what environmental cues dictate leader cell development and behavior. Here, we addressed these questions by combining a microfluidic device design that mimics multiple tumor microenvironmental cues concurrently with biologically relevant primary, heterogeneous tumor cell organoids. Prior to migration, breast tumor leader cells (K14⁺) were present throughout a tumor organoid and migrated (polarized) to the leading edge in response to biochemical and biomechanical cues. Impairment of either CXCR4 (biochemical responsive) or the collagen receptor DDR2 (biomechanical responsive) abrogated polarization of leader cells and directed collective migration. This work demonstrates that K14⁺ leader cells utilize both chemical and mechanical cues from the microenvironment to polarize to the leading edge of collectively migrating tumors. SIGNIFICANCE: These findings demonstrate that pre-existing, randomly distributed leader cells within primary tumor organoids use CXCR4 and DDR2 to polarize to the leading edge and direct migration.

Density-dependent ERK MAPK expression regulates MMP-9 and influences growth

Previous work has shown that expression of the extracellular signal-regulated kinase (ERK) is decreased by high density in normal fibroblast cells, and this was correlated with increased expression of mitogen-activated protein kinase phosphatases. Because of these differences in ERK regulation upon contact inhibition, it is likely that other cellular responses may be influenced by the attainment of a contact-inhibited state. Expression of matrix metalloproteinase-9 and cadherin cleavage were both found to be decreased upon reaching high culture density. Inhibition of ERK activity with the MEK inhibitor PD98059 resulted in increased expression of cadherins, while constitutive activation of ERK through the use of expression of an ERK construct with a D319N sevenmaker mutation resulted in decreased expression of cadherins and enhanced colony formation of HT-1080 fibrosarcoma cells. Taken together, these results corroborate a role for the regulation of ERK upon the attainment of a contact-inhibited state with increased expression of cadherins.

Comprehensive circRNA expression profile during ischemic postconditioning attenuating hepatic ischemia/reperfusion injury

Ischemic postconditioning (IPO) attenuates hepatic ischemia/reperfusion (I/R) injury. The aim of this study was to explore the role of circular RNAs (circRNAs) in the protective mechanism of IPO. In this study, microarray hybridization analysis was performed to determine the circRNA expression profile. Briefly, a total of 1599 dysregulated circRNAs were detected. The competitive endogenous RNA (ceRNA) network, including 6 circRNAs, 47 miRNAs and 90 mRNAs, indicated that the potential “housekeeping” function of circRNAs is dysregulated in hepatic I/R injury. Based on the validation results of selected circRNAs, miRNAs and mRNAs following qRT-PCR amplification, the mmu_circRNA_005186-miR-124-3p-Epha2 pathway was constructed. Dual-luciferase reporter analysis showed that miR-124-3p interacted directly with mmu_circRNA_005186 and Epha2 through the predicted binding sites, which suggested that mmu_circRNA_005186, serving as a miRNA sponge for miR-124-3p, regulated the expression of Epha2. Functionally, we explored the mechanism of mmu_circRNA_005186 in LPS-treated RAW264.7 cells which simulated the inflammation in hepatic I/R injury. We found that mmu_circRNA_005186 silencing attenuated the LPS-induced inflammation and was associated with miR-124-3p upregulation and Epha2 downregulation. Our study is the first to show that circRNAs are closely related to hepatic I/R injury and IPO and suggests that targeting mmu_circRNA_005186-miR-124-3p-Epha2 pathway might attenuate hepatic I/R injury.

Reversible Control of Protein Localization in Living Cells Using a Photocaged-Photocleavable Chemical Dimerizer

Many dynamic biological processes are regulated by protein-protein interactions and protein localization. Experimental techniques to probe such processes with temporal and spatial precision include photoactivatable proteins and chemically induced dimerization (CID) of proteins. CID has been used to study several cellular events, especially cell signaling networks, which are often reversible. However, chemical dimerizers that can be both rapidly activated and deactivated with high spatiotemporal resolution are currently limited. Herein, we present a novel chemical inducer of protein dimerization that can be rapidly turned on and off using single pulses of light at two orthogonal wavelengths. We demonstrate the utility of this molecule by controlling peroxisome transport and mitotic checkpoint signaling in living cells. Our system highlights and enhances the spatiotemporal control offered by CID. This tool addresses biological questions on subcellular levels by controlling protein-protein interactions.

Caught in the "Akt": Cross-talk between EphA2 and EGFR through the Akt-PIKfyve axis maintains cellular sensitivity to EGF

Activation of EGFR (epidermal growth factor receptor) and Eph receptor often exerts opposing effects on cell functions. In this issue of Science Signaling, Stallaert et al reveal how cells maintain sustained response to EGF stimulation by replenishing EGFR at the plasma membrane and how conflicting signals from the EphA-ephrin system and EGFR are integrated to coordinate cellular responses, including cell migration and proliferation.

PDGF-A suppresses contact inhibition during directional collective cell migration

The leading-edge mesendoderm (LEM) of the Xenopus gastrula moves as an aggregate by collective migration. However, LEM cells on fibronectin in vitro show contact inhibition of locomotion by quickly retracting lamellipodia upon mutual contact. We found that a fibronectin-integrin-syndecan module acts between p21-activated kinase 1 upstream and ephrin B1 downstream to promote the contact-induced collapse of lamellipodia. To function in this module, fibronectin has to be present as puncta on the surface of LEM cells. To overcome contact inhibition in LEM cell aggregates, PDGF-A deposited in the endogenous substratum of LEM migration blocks the fibronectin-integrin-syndecan module at the integrin level. This stabilizes lamellipodia preferentially in the direction of normal LEM movement and supports cell orientation and the directional migration of the coherent LEM cell mass.

Redistribution of Adhesive Forces through Src/FAK Drives Contact Inhibition of Locomotion in Neural Crest

Contact inhibition of locomotion is defined as the behavior of cells to cease migrating in their former direction after colliding with another cell. It has been implicated in multiple developmental processes and its absence has been linked to cancer invasion. Cellular forces are thought to govern this process; however, the exact role of traction through cell-matrix adhesions and tension through cell-cell adhesions during contact inhibition of locomotion remains unknown. Here we use neural crest cells to address this and show that cell-matrix adhesions are rapidly disassembled at the contact between two cells upon collision. This disassembly is dependent upon the formation of N-cadherin-based cell-cell adhesions and driven by Src and FAK activity. We demonstrate that the loss of cell-matrix adhesions near the contact leads to a buildup of tension across the cell-cell contact, a step that is essential to drive cell-cell separation after collision.

•

Focal adhesions disassemble at cell-cell contacts in contact inhibition of locomotion

•

FA disassembly at the cell contact during CIL requires N-cadherin/Src/FAK signaling

•

Cell separation during CIL involves a buildup of tension across the cell contact

Engineering Approaches to Study Cellular Decision Making

In their native environment, cells are immersed in a complex milieu of biochemical and biophysical cues. These cues may include growth factors, the extracellular matrix, cell-cell contacts, stiffness, and topography, and they are responsible for regulating cellular behaviors such as adhesion, proliferation, migration, apoptosis, and differentiation. The decision-making process used to convert these extracellular inputs into actions is highly complex and sensitive to changes both in the type of individual cue (e.g., growth factor dose/level, timing) and in how these individual cues are combined (e.g., homotypic/heterotypic combinations). In this review, we highlight recent advances in the development of engineering-based approaches to study the cellular decision-making process. Specifically, we discuss the use of biomaterial platforms that enable controlled and tailored delivery of individual and combined cues, as well as the application of computational modeling to analyses of the complex cellular decision-making networks.

Cell migration in microengineered tumor environments

Recent advances in microengineered cell migration platforms are discussed critically with a focus on how cell migration is influenced by engineered tumor microenvironments, the medical relevance being to understand how tumor microenvironments may promote or suppress the progression of cancer. We first introduce key findings in cancer cell migration under the influence of the physical environment, which is systematically controlled by microengineering technology, followed by multi-cues of physico-chemical factors, which represent the complexity of the tumor environment. Recognizing that cancer cells constantly communicate not only with each other but also with tumor-associated cells such as vascular, fibroblast, and immune cells, and also with non-cellular components, it follows that cell motility in tumor microenvironments, especially metastasis via the invasion of cancer cells into the extracellular matrix and other tissues, is closely related to the malignancy of cancer-related mortality. Medical relevance of forefront research realized in microfabricated devices, such as single cell sorting based on the analysis of cell migration behavior, may assist personalized theragnostics based on the cell migration phenotype. Furthermore, we urge development of theory and numerical understanding of single or collective cell migration in microengineered platforms to gain new insights in cancer metastasis and in therapeutic strategies.

The minor histocompatibility antigen 1 (HMHA1)/ArhGAP45 is a RacGAP and a novel regulator of endothelial integrity

Endothelial cells line the vasculature and act as gatekeepers that control the passage of plasma, macromolecules and cells from the circulation to the interstitial space. Dysfunction of the endothelial barrier can lead to uncontrolled leak or edema. Vascular leakage is a hallmark of a range of diseases and despite its large impact no specialized therapies are available to prevent or reduce it. RhoGTPases are known key regulators of cellular behavior that are directly involved in the regulation of the endothelial barrier. We recently performed a comprehensive analysis of the effect of all RhoGTPases and their regulators on basal endothelial integrity. In addition to novel positive regulators of endothelial barrier function, we also identified novel negative regulators, of which the ArhGAP45 (also known as HMHA1) was the most significant. We now demonstrate that ArhGAP45 acts as a Rac-GAP (GTPase-Activating Protein) in endothelial cells, which explains its negative effect on endothelial barrier function. Silencing ArhGAP45 not only promotes basal endothelial barrier function, but also increases cellular surface area and induces sprout formation in a 3D-fibrin matrix. Our data further shows that loss of ArhGAP45 promotes migration and shear stress adaptation. In conclusion, we identify ArhGAP45 (HMHA1) as a novel regulator, which contributes to the fine-tuning of the regulation of basal endothelial integrity.

The balance between Gαi-Cdc42/Rac and Gα12/13-RhoA pathways determines endothelial barrier regulation by sphingosine-1-phosphate

The bioactive sphingosine-1-phosphatephosphate (S1P) is present in plasma, bound to carrier proteins, and involved in many physiological processes, including angiogenesis, inflammatory responses, and vascular stabilization. S1P can bind to several G-protein-coupled receptors (GPCRs) activating a number of different signaling networks. At present, the dynamics and relative importance of signaling events activated immediately downstream of GPCR activation are unclear. To examine these, we used a set of fluorescence resonance energy transfer-based biosensors for different RhoGTPases (Rac1, RhoA/B/C, and Cdc42) as well as for heterotrimeric G-proteins in a series of live-cell imaging experiments in primary human endothelial cells. These experiments were accompanied by biochemical GTPase activity assays and transendothelial resistance measurements. We show that S1P promotes cell spreading and endothelial barrier function through S1PR1-Gαi-Rac1 and S1PR1-Gαi-Cdc42 pathways. In parallel, a S1PR2-Gα12/13-RhoA pathway is activated that can induce cell contraction and loss of barrier function, but only if Gαi-mediated signaling is suppressed. Our results suggest that Gαq activity is not involved in S1P-mediated regulation of barrier integrity. Moreover, we show that early activation of RhoA by S1P inactivates Rac1 but not Cdc42, and vice versa. Together, our data show that the rapid S1P-induced increase in endothelial integrity is mediated by a S1PR1-Gαi-Cdc42 pathway.

A CDC42-centered signaling unit is a dominant positive regulator of endothelial integrity

Endothelial barrier function is carefully controlled to protect tissues from edema and damage inflicted by extravasated leukocytes. RhoGTPases, in conjunction with myriad regulatory proteins, exert both positive and negative effects on the endothelial barrier integrity. Precise knowledge about the relevant mechanisms is currently fragmented and we therefore performed a comprehensive analysis of endothelial barrier regulation by RhoGTPases and their regulators. Combining RNAi with electrical impedance measurements we quantified the relevance of 270 Rho-associated genes for endothelial barrier function. Statistical analysis identified 10 targets of which six promoted- and four reduced endothelial barrier function upon downregulation. We analyzed in more detail two of these which were not previously identified as regulators of endothelial integrity. We found that the Rac1-GEF (Guanine nucleotide Exchange Factor) TIAM2 is a positive regulator and the Cdc42(Rac1)-GAP (GTPase-Activating Protein) SYDE1 is a negative regulator of the endothelial barrier function. Finally, we found that the GAP SYDE1 is part of a Cdc42-centered signaling unit, also comprising the Cdc42-GEF FARP1 and the Cdc42 effector PAK7 which controls the integrity of the endothelial barrier. In conclusion, using a siRNA-based screen, we identified new regulators of barrier function and found that Cdc42 is a dominant positive regulator of endothelial integrity.

Connecting individual to collective cell migration

Collective cell migration plays a pivotal role in the formation of organs, tissue regeneration, wound healing and many disease processes, including cancer. Despite the considerable existing knowledge on the molecular control of cell movements, it is unclear how the different observed modes of collective migration, especially for small groups of cells, emerge from the known behaviors of individual cells. Here we derive a physical description of collective cellular movements from first principles, while accounting for known phenomenological cell behaviors, such as contact inhibition of locomotion and force-induced cell repolarization. We show that this theoretical description successfully describes the motion of groups of cells of arbitrary numbers, connecting single cell behaviors and parameters (e.g., adhesion and traction forces) to the collective migration of small groups of cells and the expansion of large cell colonies. Specifically, using a common framework, we explain how cells characterized by contact inhibition of locomotion can display coherent collective behavior when in groups, even in the absence of biochemical signaling. We find an optimal group size leading to maximal group persistence and show that cell proliferation prevents the buildup of intercellular forces within cell colonies, enabling their expansion.

Characterizing Cell Migration Within Three-dimensional In Vitro Wound Environments

Currently, most in vitro models of wound healing, such as well-established scratch assays, involve studying cell migration and wound closure on two-dimensional surfaces. However, the physiological environment in which in vivo wound healing takes place is three-dimensional rather than two-dimensional. It is becoming increasingly clear that cell behavior differs greatly in two-dimensional vs. three-dimensional environments; therefore, there is a need for more physiologically relevant in vitro models for studying cell migration behaviors in wound closure. The method described herein allows for the study of cell migration in a three-dimensional model that better reflects physiological conditions than previously established two-dimensional scratch assays. The purpose of this model is to evaluate cell outgrowth via the examination of cell migration away from a spheroid body embedded within a fibrin matrix in the presence of pro- or anti-migratory factors. Using this method, cell outgrowth from the spheroid body in a three-dimensional matrix can be observed and is easily quantifiable over time via brightfield microscopy and analysis of spheroid body area. The effect of pro-migratory and/or inhibitory factors on cell migration can also be evaluated in this system. This method provides researchers with a simple method of analyzing cell migration in three-dimensional wound associated matrices in vitro, thus increasing the relevance of in vitro cell studies prior to the use of in vivo animal models.

Topography-Guided Control of Local Migratory Behaviors and Protein Expression of Cancer Cells

In vivo cancer cell migration and invasion are directed by biophysical guidance mechanisms such as pre-existing microtracks and basement membrane extracellular matrices. Here, this paper reports the correlation of the local migratory behavior of cancer cells and the biochemical signal expression using the topography that can guide or inhibit cell behaviors. To this end, the local apparent migration and the protein expression level are investigated with respect to the topographical feature size (flat, nanoline, and microline) and orientation (microline, microconcentric, and microradial) with the collectively migrating (A431) and individually migrating (MDA-MB-231 and U-87-MG) cancer cells. The results show that the migration and the protein expression of focal adhesion kinase, rho-associated protein kinase, and extracellular signal-regulated kinase are localized in the periphery of cell colony. Furthermore, the inhibition of migratory behavior at the periphery recues the protein expression, while the guidance of migration enhances the aforementioned protein expression. The results may imply the employ of biophysical inhibitory factors can help to control invasiveness of cancer cells during the progression state.

Collective cell migration: a physics perspective

Cells have traditionally been viewed either as independently moving entities or as somewhat static parts of tissues. However, it is now clear that in many cases, multiple cells coordinate their motions and move as collective entities. Well-studied examples comprise development events, as well as physiological and pathological situations. Different ex vivo model systems have also been investigated. Several recent advances have taken place at the interface between biology and physics, and have benefitted from progress in imaging and microscopy, from the use of microfabrication techniques, as well as from the introduction of quantitative tools and models. We review these interesting developments in quantitative cell biology that also provide rich examples of collective out-of-equilibrium motion.

Mechanisms of Cell Polarization

Cell polarization is a key step in the migration, development, and organization of eukaryotic cells, both at the single cell and multicellular level. Research on the mechanisms that give rise to polarization of a given cell, and organization of polarity within a tissue has led to new understanding across cellular and developmental biology. In this review, we describe some of the history of theoretical and experimental aspects of the field, as well as some interesting questions and challenges for the future.

Physical models of collective cell motility: from cell to tissue

In this article, we review physics-based models of collective cell motility. We discuss a range of techniques at different scales, ranging from models that represent cells as simple self-propelled particles to phase field models that can represent a cell's shape and dynamics in great detail. We also extensively review the ways in which cells within a tissue choose their direction, the statistics of cell motion, and some simple examples of how cell-cell signaling can interact with collective cell motility. This review also covers in more detail selected recent works on collective cell motion of small numbers of cells on micropatterns, in wound healing, and the chemotaxis of clusters of cells.

Cancer cell motility: lessons from migration in confined spaces

Time-lapse, deep-tissue imaging made possible by advances in intravital microscopy has demonstrated the importance of tumour cell migration through confining tracks in vivo. These tracks may either be endogenous features of tissues or be created by tumour or tumour-associated cells. Importantly, migration mechanisms through confining microenvironments are not predicted by 2D migration assays. Engineered in vitro models have been used to delineate the mechanisms of cell motility through confining spaces encountered in vivo. Understanding cancer cell locomotion through physiologically relevant confining tracks could be useful in developing therapeutic strategies to combat metastasis.

Stromal Androgen Receptor in Prostate Cancer Development and Progression

Prostate cancer development and progression is the result of complex interactions between epithelia cells and fibroblasts/myofibroblasts, in a series of dynamic process amenable to regulation by hormones. Whilst androgen action through the androgen receptor (AR) is a well-established component of prostate cancer biology, it has been becoming increasingly apparent that changes in AR signalling in the surrounding stroma can dramatically influence tumour cell behavior. This is reflected in the consistent finding of a strong association between stromal AR expression and patient outcomes. In this review, we explore the relationship between AR signalling in fibroblasts/myofibroblasts and prostate cancer cells in the primary site, and detail the known functions, actions, and mechanisms of fibroblast AR signaling. We conclude with an evidence-based summary of how androgen action in stroma dramatically influences disease progression.

Modeling Contact Inhibition of Locomotion of Colliding Cells Migrating on Micropatterned Substrates

In cancer metastasis, embryonic development, and wound healing, cells can coordinate their motion, leading to collective motility. To characterize these cell-cell interactions, which include contact inhibition of locomotion (CIL), micropatterned substrates are often used to restrict cell migration to linear, quasi-one-dimensional paths. In these assays, collisions between polarized cells occur frequently with only a few possible outcomes, such as cells reversing direction, sticking to one another, or walking past one another. Using a computational phase field model of collective cell motility that includes the mechanics of cell shape and a minimal chemical model for CIL, we are able to reproduce all cases seen in two-cell collisions. A subtle balance between the internal cell polarization, CIL and cell-cell adhesion governs the collision outcome. We identify the parameters that control transitions between the different cases, including cell-cell adhesion, propulsion strength, and the rates of CIL. These parameters suggest hypotheses for why different cell types have different collision behavior and the effect of interventions that modulate collision outcomes. To reproduce the heterogeneity in cell-cell collision outcomes observed experimentally in neural crest cells, we must either carefully tune our parameters or assume that there is significant cell-to-cell variation in key parameters like cell-cell adhesion.

Many cells cooperate with their neighbors to move as a group. However, the mechanisms of these cell-cell interactions are not well understood. One experimental tool to analyze interactions is to allow cells to collide with one another, and see what happens. In order to better understand what features these experiments measure, we develop a computational model of cell-cell collisions, and identify the biochemical and mechanical parameters that lead to different outcomes of collisions. We can recreate all known types of collisions seen in experiments, including cells reversing on contact, sticking, or walking past each other. Our model suggests that what happens in a collision may depend strongly on the mechanical forces between the two cells.

An Innovative Assay for the Analysis of In Vitro Endothelial Remodeling: Experimental and Computational Evidence

The molecular and cellular mechanisms underlying vascular remodeling are currently investigated by experimental strategies which aim to mimic the complex environmental conditions found in vivo. Some of them focus on the tubulogenic activity of dispersed endothelial cell populations, while others evaluate vascular sprouting. Here we propose a new method to assess matrigel invasion starting from confluent or subconfluent monolayers of human microvascular ECs (HMVEC) seeded on different substrates. The experimental setting is also validated by an improved hybrid multiscale mathematical approach, which integrates a mesoscopic grid-based cellular Potts model, that describes HMVEC phenomenology, with a continuous one, accounting for the kinetics of diffusing growth factors. Both experimental and theoretical approaches show that the endothelial potential to invade, migrate, and organize in tubule structures is a function of selected environmental parameters. The present methodology is intended to be simple to use, standardized for rapid screening and suitable for mechanistic studies. J. Cell. Physiol. 232: 243-248, 2017. © 2016 Wiley Periodicals, Inc.