The first World Cell Race

Memory and Friction: From the Nanoscale to the Macroscale

Friction is a phenomenon that manifests across all spatial and temporal scales, from the molecular to the macroscopic scale. It describes the dissipation of energy from the motion of particles or abstract reaction coordinates and arises in the transition from a detailed molecular-level description to a simplified, coarse-grained model. It has long been understood that time-dependent (non-Markovian) friction effects are critical for describing the dynamics of many systems, but that they are notoriously difficult to evaluate for complex physical, chemical, and biological systems. In recent years, the development of advanced numerical friction extraction techniques and methods to simulate the generalized Langevin equation has enabled exploration of the role of time-dependent friction across all scales. We discuss recent applications of these friction extraction techniques and the growing understanding of the role of friction in complex equilibrium and nonequilibrium dynamic many-body systems.

Biophysical and Molecular mechanisms that control active wetting and tissue fluidification in epithelial tissues

Tissue-level phase transitions are emerging as a crucial mechanism in tumour development and metastasis. This study aims to identify molecular determinants and physical conditions that control active wetting and solid-to-fluid transition of epithelial tissues. We focused on IRSp53, a protein linking plasma membranes to the cytoskeleton. Depleting IRSp53, in MCF10 DCIS.com cells, disrupts coordinated collective movement by promoting local fluctuations in cell velocity resulting in increased tissue fluidity. In dense monolayers, IRSp53 ablation allows cells to escape the physical constraint imposed by cell crowding resulting in a delayed transition toward a jammed state. In 3D spheroids, IRSp53 loss fosters active wetting of a rigid substrate, shifting spheroid behaviour to a more fluid-like state. Biophysical modelling of the spreading cells as an active polar fluid indicates that IRSp53 depletion reduces bulk viscosity and contractility in spheroids. This effect is the result of reduced supracellular tension and disrupted organization of cell-cell junctions, which lead to decreased intercellular friction and enhanced local cell rearrangements. Molecularly, IRSp53 physically and functionally interacts with the junctional protein Afadin in the regulation of tissue tensile state and active wetting in tumour spheroids. These findings identify IRSp53 and Afadin as key regulators of tissue viscosity in breast cancer tumoroid undergoing solid-to-fluid transition linked to tumour progression. They further provide the molecular basis to causally relate subcellular and cell scale processes to tissue-levels dynamics.

Contractility-Driven Cell Motility against a Viscoelastic Resistance

We study a model of contraction-based cell motility inside a microchannel to investigate the regulation of cell polarization and motion by the mechanical resistance of the environment. A positive feedback between the asymmetry of the acto-myosin cortex density and cell motion gives rise to spontaneous symmetry breaking and motility beyond a threshold contractility that depends on the resistance of extracellular medium. In highly viscous environments, we predict bistability under moderate contractility, so that symmetry breaking needs to be activated. In viscoelastic environments, we find the possibility for periodic oscillations in cortex density polarization and velocity. At the boundary between viscous and viscoelastic environments, the cell may cross, bounce back, or become trapped, depending on the viscoelastic relaxation time. These results are summarized in phase diagrams obtained by combining linear stability analysis and numerical simulations.

Ovarian cancer cells exhibit diverse migration strategies on stiff collagenous substrata

In homoeostasis, the shape and sessility of untransformed epithelial cells are intricately linked together. Variations of this relationship in migrating cancer cells as they encounter different microenvironments are as yet ill understood. Here, we explore the interdependency of such traits in two morphologically distinct invasive ovarian cancer cell lines (OVCAR-3 and SK-OV-3) under mechanically variant contexts. We first established a metric toolkit that assessed traits associated with cell motion and shape, and rigorously measured their dynamical variation across trajectories of migration using a Shannon entropic distribution. Two stiffness conditions on polymerized collagen I with Young's moduli of 0.5 kPa (soft) and 20 kPa (stiff) were chosen. Both the epithelioid OVCAR-3 and mesenchymal SK-OV-3 cells on soft substrata exhibited slow and undirected migration. On stiff substrata, SK-OV-3 showed faster persistent directed motion. Surprisingly, OVCAR-3 cells on stiffer substrata moved even faster than SK-OV-3 cells but showed a distinct angular motion. The polarity of SK-OV-3 cells on stiff substrata was well correlated with their movement, whereas, for OVCAR-3, we observed an unusual "slip" behavior, wherein the axes of cell shape and movement were poorly correlated. Whereas SK-OV-3 and OVCAR-3 showed greater mean deformation on stiffer substrata, the latter was anticorrelated with variation in angular motion or the mean deviation between shape and motility axis for SK-OV-3 but poorly correlated for OVCAR-3. Moreover, on softer substrata OVCAR-3 and SK-OV-3 were relatively rigid but showed greater shape variation (with OVCAR-3 showing a higher fold change) on stiffer substrata. Our findings suggest that greater deformability on stiffer milieu allow epithelioid cells to overcome constraints on the congruence in axis of shape and motion seen for mesenchymal cells and display distinct motile behaviors across this phenotypic spectrum.

Light-guided actin polymerization drives directed motility in protocells

Motility is a hallmark of life's dynamic processes, enabling cells to actively chase prey, repair wounds, and shape organs. Recreating these intricate behaviors using well-defined molecules remains a major challenge at the intersection of biology, physics, and molecular engineering. Although the polymerization force of the actin cytoskeleton is characterized as a primary driver of cell motility, recapitulating this process in protocellular systems has proven elusive. The difficulty lies in the daunting task of distilling key components from motile cells and integrating them into model membranes in a physiologically relevant manner. To address this, we developed a method to optically control actin polymerization with high spatiotemporal precision within cell-mimetic lipid vesicles known as giant unilamellar vesicles (GUVs). Within these active protocells, the reorganization of actin networks triggered outward membrane extensions as well as the unidirectional movement of GUVs at speeds of up to 0.43 μm/min, comparable to typical adherent mammalian cells. Notably, our findings reveal a synergistic interplay between branched and linear actin forms in promoting membrane protrusions, highlighting the cooperative nature of these cytoskeletal elements. This approach offers a powerful platform for unraveling the intricacies of cell migration, designing synthetic cells with active morphodynamics, and advancing bioengineering applications, such as self-propelled delivery systems and autonomous tissue-like materials.

Methods and computational tools to study eukaryotic cell migration in vitro

Cellular movement is essential for many vital biological functions where it plays a pivotal role both at the single cell level, such as during division or differentiation, and at the macroscopic level within tissues, where coordinated migration is crucial for proper morphogenesis. It also has an impact on various pathological processes, one for all, cancer spreading. Cell migration is a complex phenomenon and diverse experimental methods have been developed aimed at dissecting and analysing its distinct facets independently. In parallel, corresponding analytical procedures and tools have been devised to gain deep insight and interpret experimental results. Here we review established experimental techniques designed to investigate specific aspects of cell migration and present a broad collection of historical as well as cutting-edge computational tools used in quantitative analysis of cell motion.

Mesenchymal cell migration on one-dimensional micropatterns

Quantitative studies of mesenchymal cell motion are important to elucidate cytoskeleton function and mechanisms of cell migration. To this end, confinement of cell motion to one dimension (1D) significantly simplifies the problem of cell shape in experimental and theoretical investigations. Here we review 1D migration assays employing micro-fabricated lanes and reflect on the advantages of such platforms. Data are analyzed using biophysical models of cell migration that reproduce the rich scenario of morphodynamic behavior found in 1D. We describe basic model assumptions and model behavior. It appears that mechanical models explain the occurrence of universal relations conserved across different cell lines such as the adhesion-velocity relation and the universal correlation between speed and persistence (UCSP). We highlight the unique opportunity of reproducible and standardized 1D assays to validate theory based on statistical measures from large data of trajectories and discuss the potential of experimental settings embedding controlled perturbations to probe response in migratory behavior.

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.

Polarization and motility of one-dimensional multi-cellular trains

Collective cell migration, whereby cells adhere to form multi-cellular clusters that move as a single entity, play an important role in numerous biological processes, such as during development and cancer progression. Recent experimental work focused on migration of one-dimensional cellular clusters, confined to move along adhesive lanes, as a simple geometry in which to systematically study this complex system. One-dimensional migration also arises in the body when cells migrate along blood vessels, axonal projections, and narrow cavities between tissues. We explore here the modes of one-dimensional migration of cellular clusters ("trains") by implementing cell-cell interactions in a model of cell migration that contains a mechanism for spontaneous cell polarization. We go beyond simple phenomenological models of the cells as self-propelled particles by having the internal polarization of each cell depend on its interactions with the neighboring cells that directly affect the actin polymerization activity at the cell's leading edges. Both contact inhibition of locomotion and cryptic lamellipodia interactions between neighboring cells are introduced. We find that this model predicts multiple motility modes of the cell trains, which can have several different speeds for the same polarization pattern. Compared to experimental data, we find that Madin-Darby canine kidney cells are poised along the transition region where contact inhibition of locomotion and cryptic lamellipodia roughly balance each other, where collective migration speed is most sensitive to the values of the cell-cell interaction strength.

Microtubule detyrosination drives symmetry breaking to polarize cells for directed cell migration

To initiate directed movement, cells must become polarized, establishing a protrusive leading edge and a contractile trailing edge. This symmetry-breaking process involves reorganization of cytoskeleton and asymmetric distribution of regulatory molecules. However, what triggers and maintains this asymmetry during cell migration remains largely elusive. Here, we established a micropatterning-based 1D motility assay to investigate the molecular basis of symmetry breaking required for directed cell migration. We show that microtubule (MT) detyrosination drives cell polarization by directing kinesin-1-based transport of the adenomatous polyposis coli (APC) protein to cortical sites. This is essential for the formation of cell's leading edge during 1D and 3D cell migration. These data, combined with biophysical modeling, unveil a key role for MT detyrosination in the generation of a positive feedback loop linking MT dynamics and kinesin-1-based transport. Thus, symmetry breaking during cell polarization relies on a feedback loop driven by MT detyrosination that supports directed cell migration.

Interpreting T-cell search "strategies" in the light of evolution under constraints

Two decades of in vivo imaging have revealed how diverse T-cell motion patterns can be. Such recordings have sparked the notion of search "strategies": T cells may have evolved ways to search for antigen efficiently depending on the task at hand. Mathematical models have indeed confirmed that several observed T-cell migration patterns resemble a theoretical optimum; for example, frequent turning, stop-and-go motion, or alternating short and long motile runs have all been interpreted as deliberately tuned behaviours, optimising the cell's chance of finding antigen. But the same behaviours could also arise simply because T cells cannot follow a straight, regular path through the tight spaces they navigate. Even if T cells do follow a theoretically optimal pattern, the question remains: which parts of that pattern have truly been evolved for search, and which merely reflect constraints from the cell's migration machinery and surroundings? We here employ an approach from the field of evolutionary biology to examine how cells might evolve search strategies under realistic constraints. Using a cellular Potts model (CPM), where motion arises from intracellular dynamics interacting with cell shape and a constraining environment, we simulate evolutionary optimization of a simple task: explore as much area as possible. We find that our simulated cells indeed evolve their motility patterns. But the evolved behaviors are not shaped solely by what is functionally optimal; importantly, they also reflect mechanistic constraints. Cells in our model evolve several motility characteristics previously attributed to search optimisation-even though these features are not beneficial for the task given here. Our results stress that search patterns may evolve for other reasons than being "optimal". In part, they may be the inevitable side effects of interactions between cell shape, intracellular dynamics, and the diverse environments T cells face in vivo.

Polarity in immune cells

Immune cells are responsible for pathogen detection and elimination, as well as for signaling to other cells the presence of potential danger. In order to mount an efficient immune response, they need to move and search for a pathogen, interact with other cells, and diversify the population by asymmetric cell division. All these actions are regulated by cell polarity: cell polarity controls cell motility, which is crucial for scanning peripheral tissues to detect pathogens, and recruiting immune cells to sites of infection; immune cells, in particular lymphocytes, communicate with each other by a direct contact called immunological synapse, which entails a global polarization of the cell and plays a role in activating lymphocyte response; finally, immune cells divide asymmetrically from a precursor, generating a diversity of phenotypes and cell types among daughter cells, such as memory and effector cells. This review aims at providing an overview from both biology and physics perspectives of how cell polarity shapes the main immune cell functions.

EGFR-dependent aerotaxis is a common trait of breast tumour cells

BACKGROUND

Aerotaxis, the chemotactism to oxygen, is well documented in prokaryotes. We previously reported for the first time that non-tumorigenic breast epithelial cells also display unequivocal directional migration towards oxygen. This process is independent of the hypoxia-inducible factor (HIF)/prolyl hydroxylase domain (PHD) pathway but controlled by the redox regulation of epidermal growth factor receptor (EGFR), with a reactive oxygen species (ROS) gradient overlapping the oxygen gradient at low oxygen concentration. Since hypoxia is an acknowledged hallmark of cancers, we addressed the putative contribution of aerotaxis to cancer metastasis by studying the directed migration of cancer cells from an hypoxic environment towards nearby oxygen sources, modelling the in vivo migration of cancer cells towards blood capillaries.

METHODS

We subjected to the aerotactic test described in our previous papers cells isolated from fresh breast tumours analysed by the Pathology Department of the Saint-Etienne University Hospital (France) over a year. The main selection criterion, aside from patient consent, was the size of the tumour, which had to be large enough to perform the aerotactic tests without compromising routine diagnostic tests. Finally, we compared the aerotactic properties of these primary cells with those of commonly available breast cancer cell lines.

RESULTS

We show that cells freshly isolated from sixteen human breast tumour biopsies, representative of various histological characteristics and grades, are endowed with strong aerotactic properties similar to normal mammary epithelial cell lines. Strikingly, aerotaxis of these primary cancerous cells is also strongly dependent on both EGFR activation and ROS. In addition, we demonstrate that aerotaxis can trigger directional invasion of tumour cells within the extracellular matrix contrary to normal mammary epithelial cells. This contrasts with results obtained with breast cancer cell lines, in which aerotactic properties were either retained or impaired, and in some cases, even lost during the establishment of these cell lines.

CONCLUSIONS

Altogether, our results support that aerotaxis may play an important role in breast tumour metastasis. In view of these findings, we discuss the prospects for combating metastatic spread.

TRIAL REGISTRATION

IRBN1462021/CHUSTE.

Distinct speed and direction memories of migrating dendritic cells diversify their search strategies

Migrating cells exhibit various motility patterns, resulting from different migration mechanisms, cell properties, or cell-environment interactions. The complexity of cell dynamics is reflected, e.g., in the diversity of the observed forms of velocity autocorrelation function-which has been widely served as a measure of diffusivity and spreading. By analyzing the dynamics of migrating dendritic cells in vitro, we disentangle the contributions of direction θ and speed v to the velocity autocorrelation. We find that the ability of cells to maintain their speed or direction of motion is unequal, reflected in different temporal decays of speed and direction autocorrelation functions, ACv(t)∼t-1.2 and ACθ(t)∼t-0.5, respectively. The larger power-law exponent of ACv(t) indicates that the cells lose their speed memory considerably faster than the direction memory. Using numerical simulations, we investigate the influence of ACθ and ACv as well as the direction-speed cross correlation Cθ-v on the search time of a persistent random walker to find a randomly located target in confinement. Although ACθ and Cθ-v play the major roles, we find that the speed autocorrelation ACv can be also tuned to minimize the search time. Adopting an optimal ACv can reduce the search time even up to 10% compared with uncorrelated spontaneous speeds. Our results suggest that migrating cells can improve their search efficiency, especially in crowded environments, through the directional or speed persistence or the speed-direction correlation.

Effects of vimentin on the migration, search efficiency, and mechanical resilience of dendritic cells

Dendritic cells use amoeboid migration to pass through narrow passages in the extracellular matrix and confined tissue in search for pathogens and to reach the lymph nodes and alert the immune system. Amoeboid migration is a migration mode that, instead of relying on cell adhesion, is based on mechanical resilience and friction. To better understand the role of intermediate filaments in ameboid migration, we studied the effects of vimentin on the migration of dendritic cells. We show that the lymph node homing of vimentin-deficient cells is reduced in our in vivo experiments in mice. Lack of vimentin also reduces the cell stiffness, the number of migrating cells, and the migration speed in vitro in both 1D and 2D confined environments. Moreover, we find that lack of vimentin weakens the correlation between directional persistence and migration speed. Thus, vimentin-expressing dendritic cells move faster in straighter lines. Our numerical simulations of persistent random search in confined geometries verify that the reduced migration speed and the weaker correlation between the speed and direction of motion result in longer search times to find regularly located targets. Together, these observations show that vimentin enhances the ameboid migration of dendritic cells, which is relevant for the efficiency of their random search for pathogens.

How cell migration helps immune sentinels

The immune system relies on the migratory capacity of its cellular components, which must be mobile in order to defend the host from invading micro-organisms or malignant cells. This applies in particular to immune sentinels from the myeloid lineage, i.e. macrophages and dendritic cells. Cell migration is already at work during mammalian early development, when myeloid cell precursors migrate from the yolk sac, an extra embryonic structure, to colonize tissues and form the pool of tissue-resident macrophages. Later, this is accompanied by a migration wave of precursors and monocytes from the bone marrow to secondary lymphoid organs and the peripheral tissues. They differentiate into DCs and monocyte-derived macrophages. During adult life, cell migration endows immune cells with the ability to patrol their environment as well as to circulate between peripheral tissues and lymphoid organs. Hence migration of immune cells is key to building an efficient defense system for an organism. In this review, we will describe how cell migratory capacity regulates the various stages in the life of myeloid cells from development to tissue patrolling, and migration to lymph nodes. We will focus on the role of the actin cytoskeletal machinery and its regulators, and how it contributes to the establishment and function of the immune system.

Cell motility as an energy minimization process

The dynamics of active matter driven by interacting molecular motors has a nonpotential structure at the local scale. However, we show that there exists a quasipotential effectively describing the collective self-organization of the motors propelling a cell at a continuum active gel level. Such a model allows us to understand cell motility as an active phase transition problem between the static and motile steady-state configurations that minimize the quasipotential. In particular, both configurations can coexist in a metastable fashion and a small stochastic disorder in the gel is sufficient to trigger an intermittent cell dynamics where either static or motile phases are more probable, depending on which state is the global minimum of the quasipotential.

Persistent cell migration emerges from a coupling between protrusion dynamics and polarized trafficking

Migrating cells present a variety of paths, from random to highly directional ones. While random movement can be explained by basal intrinsic activity, persistent movement requires stable polarization. Here, we quantitatively address emergence of persistent migration in (hTERT)–immortalizedRPE1 (retinal pigment epithelial) cells over long timescales. By live cell imaging and dynamic micropatterning, we demonstrate that the Nucleus-Golgi axis aligns with direction of migration leading to efficient cell movement. We show that polarized trafficking is directed toward protrusions with a 20-min delay, and that migration becomes random after disrupting internal cell organization. Eventually, we prove that localized optogenetic Cdc42 activation orients the Nucleus-Golgi axis. Our work suggests that polarized trafficking stabilizes the protrusive activity of the cell, while protrusive activity orients this polarity axis, leading to persistent cell migration. Using a minimal physical model, we show that this feedback is sufficient to recapitulate the quantitative properties of cell migration in the timescale of hours.

Cancer cell migration on straight, wavy, loop and grid microfibre patterns

Cell migration plays an important role in physiological and pathological processes where the fibrillar morphology of extracellular matrices (ECM) could regulate the migration dynamics. To mimic the morphological characteristics of fibrillar matrix structures, low-voltage continuous electrospinning was adapted to construct straight, wavy, looped and gridded fibre patterns made of polystyrene (of fibre diameter ca. 3 μm). Cells were free to explore their different shapes in response to the directly-adhered fibre, as well as to the neighbouring patterns. For all the patterns studied, analysing cellular migration dynamics of MDA-MB-231 (a highly migratory breast cancer cell line) demonstrated two interesting findings: first, although cells dynamically adjust their shapes and migration trajectories in response to different fibrillar environments, their average step speed is minimally affected by the fibre global pattern; secondly, a switch in behaviour was observed when the pattern features approach the upper limit of the cell body's minor axis, reflecting that cells' ability to divert from an existing fibre track is limited by the size along the cell body's minor axis. It is therefore concluded that the upper limit of cell body's minor axis might act as a guide for the design of microfibre patterns for different purposes of cell migration.

Crowdsourced analysis of fungal growth and branching on microfluidic platforms

Fungal hyphal growth and branching are essential traits that allow fungi to spread and proliferate in many environments. This sustained growth is essential for a myriad of applications in health, agriculture, and industry. However, comparisons between different fungi are difficult in the absence of standardized metrics. Here, we used a microfluidic device featuring four different maze patterns to compare the growth velocity and branching frequency of fourteen filamentous fungi. These measurements result from the collective work of several labs in the form of a competition named the "Fungus Olympics." The competing fungi included five ascomycete species (ten strains total), two basidiomycete species, and two zygomycete species. We found that growth velocity within a straight channel varied from 1 to 4 μm/min. We also found that the time to complete mazes when fungal hyphae branched or turned at various angles did not correlate with linear growth velocity. We discovered that fungi in our study used one of two distinct strategies to traverse mazes: high-frequency branching in which all possible paths were explored, and low-frequency branching in which only one or two paths were explored. While the high-frequency branching helped fungi escape mazes with sharp turns faster, the low-frequency turning had a significant advantage in mazes with shallower turns. Future work will more systematically examine these trends.

Local actin dynamics couple speed and persistence in a cellular Potts model of cell migration

Cell migration is astoundingly diverse. Molecular signatures, cell-cell interactions, and environmental structures each play their part in shaping cell motion, yielding numerous morphologies and migration modes. Nevertheless, in recent years, a simple unifying law was found to describe cell migration across many different cell types and contexts: faster cells turn less frequently. This universal coupling between speed and persistence (UCSP) was explained by retrograde actin flow from front to back, but it remains unclear how this mechanism generalizes to cells with complex shapes and cells migrating in structured environments, which may not have a well-defined front-to-back orientation. Here, we present an in-depth characterization of an existing cellular Potts model, in which cells polarize dynamically from a combination of local actin dynamics (stimulating protrusions) and global membrane tension along the perimeter (inhibiting protrusions). We first show that the UCSP emerges spontaneously in this model through a cross talk of intracellular mechanisms, cell shape, and environmental constraints, resembling the dynamic nature of cell migration in vivo. Importantly, we find that local protrusion dynamics suffice to reproduce the UCSP-even in cases in which no clear global, front-to-back polarity exists. We then harness the spatial nature of the cellular Potts model to show how cell shape dynamics limit both the speed and persistence a cell can reach and how a rigid environment such as the skin can restrict cell motility even further. Our results broaden the range of potential mechanisms underlying the speed-persistence coupling that has emerged as a fundamental property of migrating cells.

Amoebic Foraging Model of Metastatic Cancer Cells

The Lévy walk is a pattern that is often seen in the movement of living organisms; it has both ballistic and random features and is a behavior that has been recognized in various animals and unicellular organisms, such as amoebae, in recent years. We proposed an amoeba locomotion model that implements Bayesian and inverse Bayesian inference as a Lévy walk algorithm that balances exploration and exploitation, and through a comparison with general random walks, we confirmed its effectiveness. While Bayesian inference is expressed only by P(h) = P(h|d), we introduce inverse Bayesian inference expressed as P(d|h) = P(d) in a symmetry fashion. That symmetry contributes to balancing contracting and expanding the probability space. Additionally, the conditions of various environments were set, and experimental results were obtained that corresponded to changes in gait patterns with respect to changes in the conditions of actual metastatic cancer cells.

Deep neural net tracking of human pluripotent stem cells reveals intrinsic behaviors directing morphogenesis

Lineage tracing is a powerful tool in developmental biology to interrogate the evolution of tissue formation, but the dense, three-dimensional nature of tissue limits the assembly of individual cell trajectories into complete reconstructions of development. Human induced pluripotent stem cells (hiPSCs) can recapitulate aspects of developmental processes, providing an in vitro platform to assess the dynamic collective behaviors directing tissue morphogenesis. Here, we trained an ensemble of neural networks to track individual hiPSCs in time-lapse microscopy, generating longitudinal measures of cell and cellular neighborhood properties on timescales from minutes to days. Our analysis reveals that, while individual cell parameters are not strongly affected by pluripotency maintenance conditions or morphogenic cues, regional changes in cell behavior predict cell fate and colony organization. By generating complete multicellular reconstructions of hiPSC behavior, our tracking pipeline enables fine-grained understanding of morphogenesis by elucidating the role of regional behavior in early tissue formation.

Multi-Factor Clustering Incorporating Cell Motility Predicts T Cell Expansion Potential

Expansion of an initial population of T cells is essential for cellular immunotherapy. In Chronic Lymphocytic Leukemia (CLL), expansion is often complicated by lack of T cell proliferation, as these cells frequently show signs of exhaustion. This report seeks to identify specific biomarkers or measures of cell function that capture the proliferative potential of a starting population of cells. Mixed CD4+/CD8+ T cells from healthy donors and individuals previously treated for CLL were characterized on the basis of proliferative potential and in vitro cellular functions. Single-factor analysis found little correlation between the number of populations doublings reached during expansion and either Rai stage (a clinical measure of CLL spread) or PD-1 expression. However, inclusion of in vitro IL-2 secretion and the propensity of cells to align onto micropatterned features of activating proteins as factors identified three distinct groups of donors. Notably, these group assignments provided an elegant separation of donors with regards to proliferative potential. Furthermore, these groups exhibited different motility characteristics, suggesting a mechanism that underlies changes in proliferative potential. This study describes a new set of functional readouts that augment surface marker panels to better predict expansion outcomes and clinical prognosis.

On the adhesion-velocity relation and length adaptation of motile cells on stepped fibronectin lanes

The biphasic adhesion-velocity relation is a universal observation in mesenchymal cell motility. It has been explained by adhesion-promoted forces pushing the front and resisting motion at the rear. Yet, there is little quantitative understanding of how these forces control cell velocity. We study motion of MDA-MB-231 cells on microlanes with fields of alternating Fibronectin densities to address this topic and derive a mathematical model from the leading-edge force balance and the force-dependent polymerization rate. It reproduces quantitatively our measured adhesion-velocity relation and results with keratocytes, PtK1 cells, and CHO cells. Our results confirm that the force pushing the leading-edge membrane drives lamellipodial retrograde flow. Forces resisting motion originate along the whole cell length. All motion-related forces are controlled by adhesion and velocity, which allows motion, even with higher Fibronectin density at the rear than at the front. We find the pathway from Fibronectin density to adhesion structures to involve strong positive feedbacks. Suppressing myosin activity reduces the positive feedback. At transitions between different Fibronectin densities, steady motion is perturbed and leads to changes of cell length and front and rear velocity. Cells exhibit an intrinsic length set by adhesion strength, which, together with the length dynamics, suggests a spring-like front-rear interaction force. We provide a quantitative mechanistic picture of the adhesion-velocity relation and cell response to adhesion changes integrating force-dependent polymerization, retrograde flow, positive feedback from integrin to adhesion structures, and spring-like front-rear interaction.

Non-Markovian data-driven modeling of single-cell motility

Trajectories of human breast cancer cells moving on one-dimensional circular tracks are modeled by the non-Markovian version of the Langevin equation that includes an arbitrary memory function. When averaged over cells, the velocity distribution exhibits spurious non-Gaussian behavior, while single cells are characterized by Gaussian velocity distributions. Accordingly, the data are described by a linear memory model which includes different random walk models that were previously used to account for various aspects of cell motility such as migratory persistence, non-Markovian effects, colored noise, and anomalous diffusion. The memory function is extracted from the trajectory data without restrictions or assumptions, thus making our approach truly data driven, and is used for unbiased single-cell comparison. The cell memory displays time-delayed single-exponential negative friction, which clearly distinguishes cell motion from the simple persistent random walk model and suggests a regulatory feedback mechanism that controls cell migration. Based on the extracted memory function we formulate a generalized exactly solvable cell migration model which indicates that negative friction generates cell persistence over long timescales. The nonequilibrium character of cell motion is investigated by mapping the non-Markovian Langevin equation with memory onto a Markovian model that involves a hidden degree of freedom and is equivalent to the underdamped active Ornstein-Uhlenbeck process.

Critical role of lipid membranes in polarization and migration of cells: a biophysical view

Cell migration plays vital roles in many biologically relevant processes such as tissue morphogenesis and cancer metastasis, and it has fascinated biophysicists over the past several decades. However, despite an increasing number of studies highlighting the orchestration of proteins involved in different signaling pathways, the functional roles of lipid membranes have been essentially overlooked. Lipid membranes are generally considered to be a functionless two-dimensional matrix of proteins, although many proteins regulating cell migration gain functions only after they are recruited to the membrane surface and self-organize their functional domains. In this review, we summarize how the logistical recruitment and release of proteins to and from lipid membranes coordinates complex spatiotemporal molecular processes. As predicted from the classical framework of the Smoluchowski equation of diffusion, lipid/protein membranes serve as a 2D reaction hub that contributes to the effective and robust regulation of polarization and migration of cells involving several competing pathways.

Bioimage Analysis and Cell Motility

Bioimage analysis (BIA) has historically helped study how and why cells move; biological experiments evolved in intimate feedback with the most classical image processing techniques because they contribute objectivity and reproducibility to an eminently qualitative science. Cell segmentation, tracking, and morphology descriptors are all discussed here. Using ameboid motility as a case study, these methods help us illustrate how proper quantification can augment biological data, for example, by choosing mathematical representations that amplify initially subtle differences, by statistically uncovering general laws or by integrating physical insight. More recently, the non-invasive nature of quantitative imaging is fertilizing two blooming fields: mechanobiology, where many biophysical measurements remain inaccessible, and microenvironments, where the quest for physiological relevance has exploded data size. From relief to remedy, this trend indicates that BIA is to become a main vector of biological discovery as human visual analysis struggles against ever more complex data.

Of Cell Shapes and Motion: The Physical Basis of Animal Cell Migration

Motile cells have developed a variety of migration modes relying on diverse traction-force-generation mechanisms. Before the behavior of intracellular components could be easily imaged, cell movements were mostly classified by different types of cellular shape dynamics. Indeed, even though some types of cells move without any significant change in shape, most cell propulsion mechanisms rely on global or local deformations of the cell surface. In this review, focusing mostly on metazoan cells, we discuss how different types of local and global shape changes underlie distinct migration modes. We then discuss mechanical differences between force-generation mechanisms and finish by speculating on how they may have evolved.

Stick-slip model for actin-driven cell protrusions, cell polarization, and crawling

Cell crawling requires the generation of intracellular forces by the cytoskeleton and their transmission to an extracellular substrate through specific adhesion molecules. Crawling cells show many features of excitable systems, such as spontaneous symmetry breaking and crawling in the absence of external cues, and periodic and propagating waves of activity. Mechanical instabilities in the active cytoskeleton network and feedback loops in the biochemical network of activators and repressors of cytoskeleton dynamics have been invoked to explain these dynamical features. Here, I show that the interplay between the dynamics of cell-substrate adhesion and linear cellular mechanics is sufficient to reproduce many nonlinear dynamical patterns observed in spreading and crawling cells. Using an analytical formalism of the molecular clutch model of cell adhesion, regulated by local mechanical forces, I show that cellular traction forces exhibit stick-slip dynamics resulting in periodic waves of protrusion/retraction and propagating waves along the cell edge. This can explain spontaneous symmetry breaking and polarization of spreading cells, leading to steady crawling or bipedal motion, and bistability, where persistent cell motion requires a sufficiently strong transient external stimulus. The model also highlights the role of membrane tension in providing the long-range mechanical communication across the cell required for symmetry breaking.

Parallelized Manipulation of Adherent Living Cells by Magnetic Nanoparticles-Mediated Forces

The remote actuation of cellular processes such as migration or neuronal outgrowth is a challenge for future therapeutic applications in regenerative medicine. Among the different methods that have been proposed, the use of magnetic nanoparticles appears to be promising, since magnetic fields can act at a distance without interactions with the surrounding biological system. To control biological processes at a subcellular spatial resolution, magnetic nanoparticles can be used either to induce biochemical reactions locally or to apply forces on different elements of the cell. Here, we show that cell migration and neurite outgrowth can be directed by the forces produced by a switchable parallelized array of micro-magnetic pillars, following the passive uptake of nanoparticles. Using live cell imaging, we first demonstrate that adherent cell migration can be biased toward magnetic pillars and that cells can be reversibly trapped onto these pillars. Second, using differentiated neuronal cells we were able to induce events of neurite outgrowth in the direction of the pillars without impending cell viability. Our results show that the range of forces applied needs to be adapted precisely to the cellular process under consideration. We propose that cellular actuation is the result of the force on the plasma membrane caused by magnetically filled endo-compartments, which exert a pulling force on the cell periphery.

Real-time nanodiamond thermometry probing in vivo thermogenic responses

Real-time temperature monitoring inside living organisms provides a direct measure of their biological activities. However, it is challenging to reduce the size of biocompatible thermometers down to submicrometers, despite their potential applications for the thermal imaging of subtissue structures with single-cell resolution. Here, using quantum nanothermometers based on optically accessible electron spins in nanodiamonds, we demonstrate in vivo real-time temperature monitoring inside Caenorhabditis elegans worms. We developed a microscope system that integrates a quick-docking sample chamber, particle tracking, and an error correction filter for temperature monitoring of mobile nanodiamonds inside live adult worms with a precision of ±0.22°C. With this system, we determined temperature increases based on the worms' thermogenic responses during the chemical stimuli of mitochondrial uncouplers. Our technique demonstrates the submicrometer localization of temperature information in living animals and direct identification of their pharmacological thermogenesis, which may allow for quantification of their biological activities based on temperature.

Cell shape, and not 2D migration, predicts extracellular matrix-driven 3D cell invasion in breast cancer

Metastasis, the leading cause of death in cancer patients, requires the invasion of tumor cells through the stroma in response to migratory cues, in part provided by the extracellular matrix (ECM). Recent advances in proteomics have led to the identification of hundreds of ECM proteins, which are more abundant in tumors relative to healthy tissue. Our goal was to develop a pipeline to easily predict which ECM proteins are more likely to have an effect on cancer invasion and metastasis. We evaluated the effect of four ECM proteins upregulated in breast tumor tissue in multiple human breast cancer cell lines in three assays. There was no linear relationship between cell adhesion to ECM proteins and ECM-driven 2D cell migration speed, persistence, or 3D invasion. We then used classifiers and partial-least squares regression analysis to identify which metrics best predicted ECM-driven 2D migration and 3D invasion responses. We find that ECM-driven 2D cell migration speed or persistence did not predict 3D invasion in response to the same cue. However, cell adhesion, and in particular cell elongation and shape irregularity, accurately predicted the magnitude of ECM-driven 2D migration and 3D invasion. Our models successfully predicted the effect of novel ECM proteins in a cell-line specific manner. Overall, our studies identify the cell morphological features that determine 3D invasion responses to individual ECM proteins. This platform will help provide insight into the functional role of ECM proteins abundant in tumor tissue and help prioritize strategies for targeting tumor-ECM interactions to treat metastasis.

Role of the nuclear membrane protein Emerin in front-rear polarity of the nucleus

Cell polarity refers to the intrinsic asymmetry of cells, including the orientation of the cytoskeleton. It affects cell shape and structure as well as the distribution of proteins and organelles. In migratory cells, front-rear polarity is essential and dictates movement direction. While the link between the cytoskeleton and nucleus is well-studied, we aim to investigate if front-rear polarity can be transmitted to the nucleus. We show that the knock-down of emerin, an integral protein of the nuclear envelope, abolishes preferential localization of several nuclear proteins. We propose that the frontally biased localization of the endoplasmic reticulum, through which emerin reaches the nuclear envelope, is sufficient to generate its observed bias. In primary emerin-deficient myoblasts, its expression partially rescues the polarity of the nucleus. Our results demonstrate that front-rear cell polarity is transmitted to the nucleus and that emerin is an important determinant of nuclear polarity.

During cell migration, cells are polarized with distinct front vs. rear regions but whether and how polarity is transmitted to the nucleus is unclear. Here the authors show that frontally-biased endoplasmic reticulum and the nuclear membrane protein Emerin contribute to front-rear nuclear cell polarity.

Quasi-periodic migration of single cells on short microlanes

Cell migration on microlanes represents a suitable and simple platform for the exploration of the molecular mechanisms underlying cell cytoskeleton dynamics. Here, we report on the quasi-periodic movement of cells confined in stripe-shaped microlanes. We observe persistent polarized cell shapes and directed pole-to-pole motion within the microlanes. Cells depolarize at one end of a given microlane, followed by delayed repolarization towards the opposite end. We analyze cell motility via the spatial velocity distribution, the velocity frequency spectrum and the reversal time as a measure for depolarization and spontaneous repolarization of cells at the microlane ends. The frequent encounters of a boundary in the stripe geometry provides a robust framework for quantitative investigations of the cytoskeleton protrusion and repolarization dynamics. In a first advance to rigorously test physical models of cell migration, we find that the statistics of the cell migration is recapitulated by a Cellular Potts model with a minimal description of cytoskeleton dynamics. Using LifeAct-GFP transfected cells and microlanes with differently shaped ends, we show that the local deformation of the leading cell edge in response to the tip geometry can locally either amplify or quench actin polymerization, while leaving the average reversal times unaffected.

Area and Geometry Dependence of Cell Migration in Asymmetric Two-State Micropatterns

Microstructured surfaces provide a unique framework to probe cell migration and cytoskeletal dynamics in a standardized manner. Here, we report on the steady-state occupancy probability of cells in asymmetric two-state microstructures that consist of two fibronectin-coated adhesion sites connected by a thin guidance cue. In these dumbbell-like structures, cells transition between the two sites in a repeated and stochastic manner, and average dwell times in the respective microenvironments are determined from the cell trajectories. We study the dynamics of human breast carcinoma cells (MDA-MB-231) in these microstructures as a function of area, shape, and orientation of the adhesion sites. On square adhesive sites with different areas, we find that the occupancy probability ratio is directly proportional to the ratio of corresponding adhesion site areas. These asymmetries are well captured by a simple model for the stochastic nonlinear dynamics of the cells, which reveals generic features of the motion. Sites of equal area but different shape lead to equal occupancy if shapes are isotropic (e.g., squared or circular). In contrast, an asymmetry in the occupancy is induced by anisotropic shapes like rhombi, triangles, or rectangles that enable motion in the direction perpendicular to the transition axis. Analysis of the two-dimensional motion of cells between two rectangles with orthogonal orientation suggests that cellular transition rates depend on the cell polarization induced by anisotropic micropatterns. Taken together, our results illustrate how two-state micropatterns provide a dynamic migration assay with distinct dwell times and relative cell occupancy as readouts, which may be useful to probe cell-microenvironment interactions.

Golgi-associated microtubules are fast cargo tracks and required for persistent cell migration

Microtubules derived from the Golgi (Golgi MTs) have been implicated to play critical roles in persistent cell migration, but the underlying mechanisms remain elusive, partially due to the lack of direct observation of Golgi MT-dependent vesicular trafficking. Here, using super-resolution stochastic optical reconstruction microscopy (STORM), we discovered that post-Golgi cargos are more enriched on Golgi MTs and also surprisingly move much faster than on non-Golgi MTs. We found that, compared to non-Golgi MTs, Golgi MTs are morphologically more polarized toward the cell leading edge with significantly fewer inter-MT intersections. In addition, Golgi MTs are more stable and contain fewer lattice repair sites than non-Golgi MTs. Our STORM/live-cell imaging demonstrates that cargos frequently pause at the sites of both MT intersections and MT defects. Furthermore, by optogenetic maneuvering of cell direction, we demonstrate that Golgi MTs are essential for persistent cell migration but not for cells to change direction. Together, our study unveils the role of Golgi MTs in serving as a group of "fast tracks" for anterograde trafficking of post-Golgi cargos.

Active gel segment behaving as an active particle

We reduce a one-dimensional model of an active segment (AS), which is used, for instance, in the description of contraction-driven cell motility, to a zero-dimensional model of an active particle (AP) characterized by two internal degrees of freedom: position and polarity. Both models give rise to hysteretic force-velocity relations showing that an active agent can support two opposite polarities under the same external force and that it can maintain the same polarity while being dragged by external forces with opposite orientations. This double bistability results in a rich dynamic repertoire which we illustrate by studying static, stalled, motile, and periodically repolarizing regimes displayed by an active agent confined in a viscoelastic environment. We show that the AS and AP models can be calibrated to generate quantitatively similar dynamic responses.

Stick-slip dynamics of cell adhesion triggers spontaneous symmetry breaking and directional migration of mesenchymal cells on one-dimensional lines

Directional cell motility relies on the ability of single cells to establish a front-rear polarity and can occur in the absence of external cues. The initiation of migration has often been attributed to the spontaneous polarization of cytoskeleton components, while the spatiotemporal evolution of cell-substrate interaction forces has yet to be resolved. Here, we establish a one-dimensional microfabricated migration assay that mimics the complex in vivo fibrillar environment while being compatible with high-resolution force measurements, quantitative microscopy, and optogenetics. Quantification of morphometric and mechanical parameters of NIH-3T3 fibroblasts and RPE1 epithelial cells reveals a generic stick-slip behavior initiated by contractility-dependent stochastic detachment of adhesive contacts at one side of the cell, which is sufficient to trigger cell motility in 1D in the absence of pre-established polarity. A theoretical model validates the crucial role of adhesion dynamics, proposing that front-rear polarity can emerge independently of a complex self-polarizing system.

Inducible microRNA-200c decreases motility of breast cancer cells and reduces filamin A

Cancer progression and metastases are frequently related to changes of cell motility. Amongst others, the microRNA-200c (miR-200c) was shown to maintain the epithelial state of cells and to hamper migration. Here, we describe two miR-200c inducible breast cancer cell lines, derived from miR-200c knock-out MCF7 cells as well as from the miR-200c-negative MDA-MB-231 cells and report on the emerging phenotypic effects after miR-200s induction. The induction of miR-200c expression seems to effect a rapid reduction of cell motility, as determined by 1D microlane migration assays. Sustained expression of miR200c leads to a changed morphology and reveals a novel mechanism by which miR-200c interferes with cytoskeletal components. We find that filamin A expression is attenuated by miRNA-200c induced downregulation of the transcription factors c-Jun and MRTF/SRF. This potentially novel pathway that is independent of the prominent ZEB axis could lead to a broader understanding of the role that miR200c plays in cancer metastasis.

Osmotic-engine-driven liposomes in microfluidic channels

Self-propelled underwater microrobots that locomote without external sources of energy have potential application as drug carriers and probes in narrow spaces. In this study, we focused on an osmotic engine model, which is a migration mechanism, and applied it as a negative chemotaxis mechanism to induce liposome displacement. First, we confirmed the osmotic flow across the lipid bilayer and calculated the osmotic flow velocity to be 8.5 fL min-1 μm-2 when a salt concentration difference was applied to the lipid bilayer. Next, we designed and fabricated a microchannel that can trap a giant liposome and apply a salt concentration difference to the front and rear of the liposome. Then, we demonstrated the movement of the liposome by flowing it to the microchannel. The liposome successfully moved in the direction of the lower ion concentration at a speed of 0.6 μm min-1 owing to the osmotic pressure difference. Finally, we visualized the inner flow in the liposome by encapsulating microbeads in the liposome and observed the movement of the microbeads to verify that an osmotic flow was generated on the liposome. As a result, we observed the circulation of the microbeads in the liposome when the concentration difference was applied to the front and rear of the liposome, suggesting that the movement of the liposome was driven by the osmotic flow generated by the osmotic pressure difference. These results indicate that the osmotic-pressure-based migration mechanism has the potential to be utilized as the actuator of molecular robots.

A Toolbox for Organelle Mechanobiology Research-Current Needs and Challenges

Mechanobiology studies from the last decades have brought significant insights into many domains of biological research, from development to cellular signaling. However, mechano-regulation of subcellular components, especially membranous organelles, are only beginning to be unraveled. In this paper, we take mitochondrial mechanobiology as an example to discuss recent advances and current technical challenges in this field. In addition, we discuss the needs for future toolbox development for mechanobiological research of intracellular organelles.

Substrate-rigidity dependent migration of an idealized twitching bacterium

Mechanical properties of the extracellular matrix are important determinants of cellular migration in diverse processes, such as immune response, wound healing, and cancer metastasis. Moreover, recent studies indicate that even bacterial surface colonization can depend on the mechanics of the substrate. Here, we focus on physical mechanisms that can give rise to substrate-rigidity dependent migration. We study a "twitcher", a cell driven by extension-retraction cycles, to idealize bacteria and perhaps eukaryotic cells that employ a slip-stick mode of motion. The twitcher is asymmetric and always pulls itself forward at its front. Analytical calculations show that the migration speed of a twitcher depends non-linearly on substrate rigidity. For soft substrates, deformations do not lead to build-up of significant force and the migration speed is therefore determined by stochastic adhesion unbinding. For rigid substrates, forced adhesion rupture determines the migration speed. Depending on the force-sensitivity of front and rear adhesions, forced bond rupture implies an increase or a decrease of the migration speed. A requirement for the occurrence of rigidity-dependent stick-slip migration is a "sticky" substrate, with binding rates being an order of magnitude larger than unbinding rates in absence of force. Computer simulations show that small stall forces of the driving machinery lead to a reduced movement on high rigidities, regardless of force-sensitivities of bonds. The simulations also confirm the occurrence of rigidity-dependent migration speed in a generic model for slip-stick migration of cells on a sticky substrate.

Actin stress fiber dynamics in laterally confined cells

Multiple cellular processes are affected by spatial constraints from the extracellular matrix and neighboring cells. In vitro experiments using defined micro-patterning allow for in-depth analysis and a better understanding of how these constraints impact cellular behavior and functioning. Herein we focused on the analysis of actin cytoskeleton dynamics as a major determinant of mechanotransduction mechanisms in cells. We seeded primary human umbilical vein endothelial cells onto stripe-like cell-adhesive micro-patterns with varying widths and then monitored and quantified the dynamic reorganization of actin stress fibers, including fiber velocities, orientation and density, within these live cells using the cell permeable F-actin marker SiR-actin. Although characteristic parameters describing the overall stress fiber architecture (average orientation and density) were nearly constant throughout the observation time interval of 60 min, we observed permanent transport and turnover of individual actin stress fibers. Stress fibers were more strongly oriented along stripe direction with decreasing stripe width, (5° on 20 μm patterns and 10° on 40 μm patterns), together with an overall narrowing of the distribution of fiber orientation. Fiber dynamics was characterized by a directed movement from the cell edges towards the cell center, where fiber dissolution frequently took place. By kymograph analysis, we found median fiber velocities in the range of 0.2 μm/min with a weak dependence on pattern width. Taken together, these data suggest that cell geometry determines actin fiber orientation, while it also affects actin fiber transport and turnover.

The matrix environmental and cell mechanical properties regulate cell migration and contribute to the invasive phenotype of cancer cells

The minimal structural unit of a solid tumor is a single cell or a cellular compartment such as the nucleus. A closer look inside the cells reveals that there are functional compartments or even structural domains determining the overall properties of a cell such as the mechanical phenotype. The mechanical interaction of these living cells leads to the complex organization such as compartments, tissues and organs of organisms including mammals. In contrast to passive non-living materials, living cells actively respond to the mechanical perturbations occurring in their microenvironment during diseases such as fibrosis and cancer. The transformation of single cancer cells in highly aggressive and hence malignant cancer cells during malignant cancer progression encompasses the basement membrane crossing, the invasion of connective tissue, the stroma microenvironments and transbarrier migration, which all require the immediate interaction of the aggressive and invasive cancer cells with the surrounding extracellular matrix environment including normal embedded neighboring cells. All these steps of the metastatic pathway seem to involve mechanical interactions between cancer cells and their microenvironment. The pathology of cancer due to a broad heterogeneity of cancer types is still not fully understood. Hence it is necessary to reveal the signaling pathways such as mechanotransduction pathways that seem to be commonly involved in the development and establishment of the metastatic and mechanical phenotype in several carcinoma cells. We still do not know whether there exist distinct metastatic genes regulating the progression of tumors. These metastatic genes may then be activated either during the progression of cancer by themselves on their migration path or in earlier stages of oncogenesis through activated oncogenes or inactivated tumor suppressor genes, both of which promote the metastatic phenotype. In more detail, the adhesion of cancer cells to their surrounding stroma induces the generation of intracellular contraction forces that deform their microenvironments by alignment of fibers. The amplitude of these forces can adapt to the mechanical properties of the microenvironment. Moreover, the adhesion strength of cancer cells seems to determine whether a cancer cell is able to migrate through connective tissue or across barriers such as the basement membrane or endothelial cell linings of blood or lymph vessels in order to metastasize. In turn, exposure of adherent cancer cells to physical forces, such as shear flow in vessels or compression forces around tumors, reinforces cell adhesion, regulates cell contractility and restructures the ordering of the local stroma matrix that leads subsequently to secretion of crosslinking proteins or matrix degrading enzymes. Hence invasive cancer cells alter the mechanical properties of their microenvironment. From a mechanobiological point-of-view, the recognized physical signals are transduced into biochemical signaling events that guide cellular responses such as cancer progression after the malignant transition of cancer cells from an epithelial and non-motile phenotype to a mesenchymal and motile (invasive) phenotype providing cellular motility. This transition can also be described as the physical attempt to relate this cancer cell transitional behavior to a T1 phase transition such as the jamming to unjamming transition. During the invasion of cancer cells, cell adaptation occurs to mechanical alterations of the local stroma, such as enhanced stroma upon fibrosis, and therefore we need to uncover underlying mechano-coupling and mechano-regulating functional processes that reinforce the invasion of cancer cells. Moreover, these mechanisms may also be responsible for the awakening of dormant residual cancer cells within the microenvironment. Physicists were initially tempted to consider the steps of the cancer metastasis cascade as single events caused by a single mechanical alteration of the overall properties of the cancer cell. However, this general and simple view has been challenged by the finding that several mechanical properties of cancer cells and their microenvironment influence each other and continuously contribute to tumor growth and cancer progression. In addition, basement membrane crossing, cell invasion and transbarrier migration during cancer progression is explained in physical terms by applying physical principles on living cells regardless of their complexity and individual differences of cancer types. As a novel approach, the impact of the individual microenvironment surrounding cancer cells is also included. Moreover, new theories and models are still needed to understand why certain cancers are malignant and aggressive, while others stay still benign. However, due to the broad variety of cancer types, there may be various pathways solely suitable for specific cancer types and distinct steps in the process of cancer progression. In this review, physical concepts and hypotheses of cancer initiation and progression including cancer cell basement membrane crossing, invasion and transbarrier migration are presented and discussed from a biophysical point-of-view. In addition, the crosstalk between cancer cells and a chronically altered microenvironment, such as fibrosis, is discussed including the basic physical concepts of fibrosis and the cellular responses to mechanical stress caused by the mechanically altered microenvironment. Here, is highlighted how biophysical approaches, both experimentally and theoretically, have an impact on classical hallmarks of cancer and fibrosis and how they contribute to the understanding of the regulation of cancer and its progression by sensing and responding to the physical environmental properties through mechanotransduction processes. Finally, this review discusses various physical models of cell migration such as blebbing, nuclear piston, protrusive force and unjamming transition migration modes and how they contribute to cancer progression. Moreover, these cellular migration modes are influenced by microenvironmental perturbances such as fibrosis that can induce mechanical alterations in cancer cells, which in turn may impact the environment. Hence, the classical hallmarks of cancer need to be refined by including biomechanical properties of cells, cell clusters and tissues and their microenvironment to understand mechano-regulatory processes within cancer cells and the entire organism.

Reconstitution of cell migration at a glance

Single cells migrate in a myriad of physiological contexts, such as tissue patrolling by immune cells, and during neurogenesis and tissue remodeling, as well as in metastasis, the spread of cancer cells. To understand the basic principles of single-cell migration, a reductionist approach can be taken. This aims to control and deconstruct the complexity of different cellular microenvironments into simpler elementary constrains that can be recombined together. This approach is the cell microenvironment equivalent of in vitro reconstituted systems that combine elementary molecular players to understand cellular functions. In this Cell Science at a Glance article and accompanying poster, we present selected experimental setups that mimic different events that cells undergo during migration in vivo These include polydimethylsiloxane (PDMS) devices to deform whole cells or organelles, micro patterning, nano-fabricated structures like grooves, and compartmentalized collagen chambers with chemical gradients. We also outline the main contribution of each technique to the understanding of different aspects of single-cell migration.

Optogenetic dissection of Rac1 and Cdc42 gradient shaping

During cell migration, Rho GTPases spontaneously form spatial gradients that define the front and back of cells. At the front, active Cdc42 forms a steep gradient whereas active Rac1 forms a more extended pattern peaking a few microns away. What are the mechanisms shaping these gradients, and what is the functional role of the shape of these gradients? Here we report, using a combination of optogenetics and micropatterning, that Cdc42 and Rac1 gradients are set by spatial patterns of activators and deactivators and not directly by transport mechanisms. Cdc42 simply follows the distribution of Guanine nucleotide Exchange Factors, whereas Rac1 shaping requires the activity of a GTPase-Activating Protein, β2-chimaerin, which is sharply localized at the tip of the cell through feedbacks from Cdc42 and Rac1. Functionally, the spatial extent of Rho GTPases gradients governs cell migration, a sharp Cdc42 gradient maximizes directionality while an extended Rac1 gradient controls the speed.

A steep gradient of Cdc42 is at the front of migrating cells, whereas the active Rac1 gradient is graded. Here the authors show that Cdc42 gradients follow the distribution of GEFs and govern direction of migration, while Rac1 gradients require the activity of the GAP β2-chimaerin and control cell speed.

Light-Guided Motility of a Minimal Synthetic Cell

Cell motility is an important but complex process; as cells move, new adhesions form at the front and adhesions disassemble at the back. To replicate this dynamic and spatiotemporally controlled asymmetry of adhesions and achieve motility in a minimal synthetic cell, we controlled the adhesion of a model giant unilamellar vesicle (GUV) to the substrate with light. For this purpose, we immobilized the proteins iLID and Micro, which interact under blue light and dissociate from each other in the dark, on a substrate and a GUV, respectively. Under blue light, the protein interaction leads to adhesion of the vesicle to the substrate, which is reversible in the dark. The high spatiotemporal control provided by light, allowed partly illuminating the GUV and generating an asymmetry in adhesions. Consequently, the GUV moves into the illuminated area, a process that can be repeated over multiple cycles. Thus, our system reproduces the dynamic spatiotemporal distribution of adhesions and establishes mimetic motility of a synthetic cell.