Pushing off the walls: a mechanism of cell motility in confinement

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.

Geometry-Sensitive Protrusion Growth Directs Confined Cell Migration

The migratory dynamics of cells can be influenced by the complex microenvironment through which they move. It remains unclear how the motility machinery of confined cells responds and adapts to their microenvironment. Here, we propose a biophysical mechanism for a geometry-dependent coupling between cellular protrusions and the nucleus that leads to directed migration. We apply our model to geometry-guided cell migration to obtain insights into the origin of directed migration on asymmetric adhesive micropatterns and the polarization enhancement of cells observed under strong confinement. Remarkably, for cells that can choose between channels of different size, our model predicts an intricate dependence for cellular decision making as a function of the two channel widths, which we confirm experimentally.

Subcellular mechano-regulation of cell migration in confined extracellular microenvironment

Cell migration is a highly coordinated cellular event that determines diverse physiological and pathological processes in which the continuous interaction of a migrating cell with neighboring cells or the extracellular matrix is regulated by the physical setting of the extracellular microenvironment. In confined spaces, cell migration occurs differently compared to unconfined open spaces owing to the additional forces that limit cell motility, which create a driving bias for cells to invade the confined space, resulting in a distinct cell motility process compared to what is expected in open spaces. Moreover, cells in confined environments can be subjected to elevated mechanical compression, which causes physical stimuli and activates the damage repair cycle in the cell, including the DNA in the nucleus. Although cells have a self-restoring system to repair damage from the cell membrane to the genetic components of the nucleus, this process may result in genetic and/or epigenetic alterations that can increase the risk of the progression of diverse diseases, such as cancer and immune disorders. Furthermore, there has been a shift in the paradigm of bioengineering from the development of new biomaterials to controlling biophysical cues and fine-tuning cell behaviors to cure damaged/diseased tissues. The external physical cues perceived by cells are transduced along the mechanosensitive machinery, which is further channeled into the nucleus through subcellular molecular linkages of the nucleoskeleton and cytoskeleton or the biochemical translocation of transcription factors. Thus, external cues can directly or indirectly regulate genetic transcriptional processes and nuclear mechanics, ultimately determining cell fate. In this review, we discuss the importance of the biophysical cues, response mechanisms, and mechanical models of cell migration in confined environments. We also discuss the effect of force-dependent deformation of subcellular components, specifically focusing on subnuclear organelles, such as nuclear membranes and chromosomal organization. This review will provide a biophysical perspective on cancer progression and metastasis as well as abnormal cellular proliferation.

SPAK-dependent cotransporter activity mediates capillary adhesion and pressure during glioblastoma migration in confined spaces

The invasive potential of glioblastoma cells is attributed to large changes in pressure and volume, driven by diverse elements, including the cytoskeleton and ion cotransporters.  However, how the cell actuates changes in pressure and volume in confinement, and how these changes contribute to invasive motion is unclear. Here, we inhibited SPAK activity, with known impacts on the cytoskeleton and cotransporter activity and explored its role on the migration of glioblastoma cells in confining microchannels to model invasive spread through brain tissue. First, we found that confinement altered cell shape, inducing a transition in morphology that resembled droplet interactions with a capillary vessel, from "wetting" (more adherent) at low confinement, to "nonwetting" (less adherent) at high confinement. This transition was marked by a change from negative to positive pressure by the cells to the confining walls, and an increase in migration speed. Second, we found that the SPAK pathway impacted the migration speed in different ways dependent upon the extent of wetting. For nonwetting cells, SPAK inhibition increased cell-surface tension and cotransporter activity. By contrast, for wetting cells, it also reduced myosin II and YAP phosphorylation. In both cases, membrane-to-cortex attachment is dramatically reduced. Thus, our results suggest that SPAK inhibition differentially coordinates cotransporter and cytoskeleton-induced forces, to impact glioblastoma migration depending on the extent of confinement.

Current reversal in polar flock at order-disorder interface

We studied a system of polar self-propelled particles (SPPs) on a thin rectangular channel designed into three regions of order-disorder-order. The division of the three regions is made on the basis of the noise SPPs experience in the respective regions. The noise in the two wide regions is chosen lower than the critical noise of order-disorder transition and noise in the middle region or interface is higher than the critical noise. This makes the geometry of the system analogous to the Josephson junction (JJ) in solid-state physics. Keeping all other parameters fixed, we study the properties of the moving SPPs in the bulk as well as along the interface for different widths of the junction. On increasing interface width, the system shows an order-to-disorder transition from coherent moving SPPs in the whole system to the interrupted current for large interface width. Surprisingly, inside the interface, we observed the current reversal for intermediate widths of the interface. Such current reversal is due to the strong randomness present inside the interface, which makes the wall of the interface reflecting. Hence, our study gives new interesting collective properties of SPPs at the interface which can be useful to design switching devices using active agents.

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.

The nonlinear motion of cells subject to external forces

To develop a minimal model for a cell moving in a crowded environment such as in tissue, we investigate the response of a liquid drop of active matter moving on a flat rigid substrate to forces applied at its boundaries. We consider two different self-propulsion mechanisms, active stresses and treadmilling polymerisation, and we investigate how the active drop motion is altered by these surface forces. We find a highly non-linear response to forces that we characterise using drop velocity, drop shape, and the traction between the drop and the substrate. Each self-propulsion mechanism gives rise to two main modes of motion: a long thin drop with zero traction in the bulk, mostly occurring under strong stretching forces, and a parabolic drop with finite traction in the bulk, mostly occurring under strong squeezing forces. In each case there is a sharp transition between parabolic, and long thin drops as a function of the applied forces and indications of drop break-up where large forces stretch the drop.

Ameboid cell migration through regular arrays of micropillars under confinement

Migrating cells often encounter a wide variety of topographic features-including the presence of obstacles-when navigating through crowded biological environments. Unraveling the impact of topography and crowding on the dynamics of cells is key to better understand many essential physiological processes such as the immune response. We study the impact of geometrical cues on ameboid migration of HL-60 cells differentiated into neutrophils. A microfluidic device is designed to track the cells in confining geometries between two parallel plates with distance h, in which identical micropillars are arranged in regular pillar forests with pillar spacing e. We observe that the cells are temporarily captured near pillars, with a mean contact time that is independent of h and e. By decreasing the vertical confinement h, we find that the cell velocity is not affected, while the persistence reduces; thus, cells are able to preserve their velocity when highly squeezed but lose the ability to control their direction of motion. At a given h, we show that by decreasing the pillar spacing e in the weak lateral confinement regime, the mean escape time of cells from effective local traps between neighboring pillars grows. This effect, together with the increase of cell-pillar contact frequency, leads to the reduction of diffusion constant D. By disentangling the contributions of these two effects on D in numerical simulations, we verify that the impact of cell-pillar contacts on cell diffusivity is more pronounced at smaller pillar spacing.

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.

Self-organization in amoeboid motility

Amoeboid motility has come to refer to a spectrum of cell migration modes enabling a cell to move in the absence of strong, specific adhesion. To do so, cells have evolved a range of motile surface movements whose physical principles are now coming into view. In response to external cues, many cells—and some single-celled-organisms—have the capacity to turn off their default migration mode. and switch to an amoeboid mode. This implies a restructuring of the migration machinery at the cell scale and suggests a close link between cell polarization and migration mediated by self-organizing mechanisms. Here, I review recent theoretical models with the aim of providing an integrative, physical picture of amoeboid migration.

Actin Turnover Required for Adhesion-Independent Bleb Migration

Cell migration is critical for many vital processes, such as wound healing, as well as harmful processes, such as cancer metastasis. Experiments have highlighted the diversity in migration strategies employed by cells in physiologically relevant environments. In 3D fibrous matrices and confinement between two surfaces, some cells migrate using round membrane protrusions, called blebs. In bleb-based migration, the role of substrate adhesion is thought to be minimal, and it remains unclear if a cell can migrate without any adhesion complexes. We present a 2D computational fluid-structure model of a cell using cycles of bleb expansion and retraction in a channel with several geometries. The cell model consists of a plasma membrane, an underlying actin cortex, and viscous cytoplasm. Cellular structures are immersed in viscous fluid which permeates them, and the fluid equations are solved using the method of regularized Stokeslets. Simulations show that the cell cannot effectively migrate when the actin cortex is modeled as a purely elastic material. We find that cells do migrate in rigid channels if actin turnover is included with a viscoelastic description for the cortex. Our study highlights the non-trivial relationship between cell rheology and its external environment during migration with cytoplasmic streaming.

Organ-Chip Models: Opportunities for Precision Medicine in Pancreatic Cancer

Simple Summary

Among all types of cancer, Pancreatic Ductal Adenocarcinoma (PDAC) has one of the lowest survival rates, partly due to the failure of current chemotherapeutics. This treatment failure can be attributed to the complicated nature of the tumor microenvironment, where the rich fibro-inflammatory responses can hinder drug delivery and efficacy at the tumor site. Moreover, the high molecular variations in PDAC create a large heterogeneity in the tumor microenvironment among patients. Current in vivo and in vitro options for drug testing are mostly ineffective in recapitulating the complex cellular interactions and individual variations in the PDAC tumor microenvironment, and as a result, they fail to provide appropriate models for individualized drug screening. Organ-on-a-chip technology combined with patient-derived organoids may provide the opportunity for developing personalized treatment options in PDAC.

Abstract

Pancreatic Ductal Adenocarcinoma (PDAC) is an expeditiously fatal malignancy with a five-year survival rate of 6–8%. Conventional chemotherapeutics fail in many cases due to inadequate primary response and rapidly developing resistance. This treatment failure is particularly challenging in pancreatic cancer because of the high molecular heterogeneity across tumors. Additionally, a rich fibro-inflammatory component within the tumor microenvironment (TME) limits the delivery and effectiveness of anticancer drugs, further contributing to the lack of response or developing resistance to conventional approaches in this cancer. As a result, there is an urgent need to model pancreatic cancer ex vivo to discover effective drug regimens, including those targeting the components of the TME on an individualized basis. Patient-derived three-dimensional (3D) organoid technology has provided a unique opportunity to study patient-specific cancerous epithelium. Patient-derived organoids cultured with the TME components can more accurately reflect the in vivo tumor environment. Here we present the advances in organoid technology and multicellular platforms that could allow for the development of “organ-on-a-chip” approaches to recapitulate the complex cellular interactions in PDAC tumors. We highlight the current advances of the organ-on-a-chip-based cancer models and discuss their potential for the preclinical selection of individualized treatment in PDAC.

The role of vimentin-nuclear interactions in persistent cell motility through confined spaces

The ability of cells to move through small spaces depends on the mechanical properties of the cellular cytoskeleton and on nuclear deformability. In mammalian cells, the cytoskeleton is composed of three interacting, semi-flexible polymer networks: actin, microtubules, and intermediate filaments (IF). Recent experiments of mouse embryonic fibroblasts with and without vimentin have shown that the IF vimentin plays a role in confined cell motility. Here, we develop a minimal model of a cell moving through a microchannel that incorporates explicit effects of actin and vimentin and implicit effects of microtubules. Specifically, the model consists of a cell with an actomyosin cortex and a deformable cell nucleus and mechanical linkages between the two. By decreasing the amount of vimentin, we find that the cell speed increases for vimentin-null cells compared to cells with vimentin. The loss of vimentin increases nuclear deformation and alters nuclear positioning in the cell. Assuming nuclear positioning is a read-out for cell polarity, we propose a new polarity mechanism which couples cell directional motion with cytoskeletal strength and nuclear positioning and captures the abnormally persistent motion of vimentin-null cells, as observed in experiments. The enhanced persistence indicates that the vimentin-null cells are more controlled by the confinement and so less autonomous, relying more heavily on external cues than their wild-type counterparts. Our modeling results present a quantitative interpretation for recent experiments and have implications for understanding the role of vimentin in the epithelial-mesenchymal transition.

Mechanics of developmental migration

The ability to migrate is a fundamental property of animal cells which is essential for development, homeostasis and disease progression. Migrating cells sense and respond to biochemical and mechanical cues by rapidly modifying their intrinsic repertoire of signalling molecules and by altering their force generating and transducing machinery. We have a wealth of information about the chemical cues and signalling responses that cells use during migration. Our understanding of the role of forces in cell migration is rapidly evolving but is still best understood in the context of cells migrating in 2D and 3D environments in vitro. Advances in live imaging of developing embryos combined with the use of experimental and theoretical tools to quantify and analyse forces in vivo, has begun to shed light on the role of mechanics in driving embryonic cell migration. In this review, we focus on the recent studies uncovering the physical basis of embryonic cell migration in vivo. We look at the physical basis of the classical steps of cell migration such as protrusion formation and cell body translocation and review the recent research on how these processes work in the complex 3D microenvironment of a developing organism.

Calculation of the force field required for nucleus deformation during cell migration through constrictions

During cell migration in confinement, the nucleus has to deform for a cell to pass through small constrictions. Such nuclear deformations require significant forces. A direct experimental measure of the deformation force field is extremely challenging. However, experimental images of nuclear shape are relatively easy to obtain. Therefore, here we present a method to calculate predictions of the deformation force field based purely on analysis of experimental images of nuclei before and after deformation. Such an inverse calculation is technically non-trivial and relies on a mechanical model for the nucleus. Here we compare two simple continuum elastic models of a cell nucleus undergoing deformation. In the first, we treat the nucleus as a homogeneous elastic solid and, in the second, as an elastic shell. For each of these models we calculate the force field required to produce the deformation given by experimental images of nuclei in dendritic cells migrating in microchannels with constrictions of controlled dimensions. These microfabricated channels provide a simplified confined environment mimicking that experienced by cells in tissues. Our calculations predict the forces felt by a deforming nucleus as a migrating cell encounters a constriction. Since a direct experimental measure of the deformation force field is very challenging and has not yet been achieved, our numerical approaches can make important predictions motivating further experiments, even though all the parameters are not yet available. We demonstrate the power of our method by showing how it predicts lateral forces corresponding to actin polymerisation around the nucleus, providing evidence for actin generated forces squeezing the sides of the nucleus as it enters a constriction. In addition, the algorithm we have developed could be adapted to analyse experimental images of deformation in other situations.

Many cell types are able to migrate and squeeze through constrictions that are narrower than the cell’s resting radius. For example, both immune cells and metastatic cancer cells change their shape to migrate through small holes in the complex tissue media they move in. During migration the cell nucleus is more difficult to deform than the cell cytoplasm and therefore significant forces are required for a cell to pass through spaces that are smaller than the resting size of the nucleus. Experimental measurements of these forces are extremely challenging but experimental images of nuclear deformation are regularly obtained in many labs. Therefore we present a computational method to analyse experimental images of nuclear deformation to deduce the forces required to produce such deformations. A mechanical model of the nucleus is necessary for this analysis and here we present two different models. Our computational tool enables us to obtain detailed information about forces causing deformation from microscopy images and consequently provide evidence for actin generated forces squeezing the sides of the nucleus as it enters a constriction.

Ratchetaxis in Channels: Entry Point and Local Asymmetry Set Cell Directions in Confinement

Cell motility is essential in a variety of biological phenomena ranging from early development to organ homeostasis and diseases. This phenomenon has mainly been studied and characterized on flat surfaces in vitro, whereas such conditions are rarely observed in vivo. Recently, cell motion in three-dimensional microfabricated channels was reported to be possible, and it was shown that confined cells push on walls. However, rules setting cell directions in this context have not yet been characterized. Here, we show by using assays that ratchetaxis operates in three-dimensional ratchets in fibroblasts and epithelial cancerous cells. Open ratchets rectify cell motion, whereas closed ratchets impose direct cell migration along channels set by the cell orientation at the channel entry point. We also show that nuclei are pressed in constriction zones through mechanisms involving dynamic asymmetries of focal contacts, stress fibers, and intermediate filaments. Interestingly, cells do not pass these constricting zones when they contain a defective keratin fusion protein implicated in squamous cancer. By combining ratchetaxis with chemical gradients, we finally report that cells are sensitive to local asymmetries in confinement and that topological and chemical cues may be encoded differently by cells. Overall, our ratchet channels could mimic small blood vessels in which cells such as circulating tumor cells are confined; cells can probe local asymmetries that determine their entry into tissues and their subsequent direction. Our results shed light on invasion mechanisms in cancer.

Leukocyte transmigration and longitudinal forward-thrusting force in a microfluidic Transwell device

Transmigration of leukocytes across blood vessels walls is a critical step of the immune response. Transwell assays examine transmigration properties in vitro by counting cells passages through a membrane; however, the difficulty of in situ imaging hampers a clear disentanglement of the roles of adhesion, chemokinesis, and chemotaxis. We used here microfluidic Transwells to image the cells' transition from 2D migration on a surface to 3D migration in a confining microchannel and measure cells longitudinal forward-thrusting force in microchannels. Primary human effector T lymphocytes adhering with integrins LFA-1 (α_Lβ₂) had a marked propensity to transmigrate in Transwells without chemotactic cue. Both adhesion and contractility were important to overcome the critical step of nucleus penetration but were remarkably dispensable for 3D migration in smooth microchannels deprived of topographic features. Transmigration in smooth channels was qualitatively consistent with a propulsion by treadmilling of cell envelope and squeezing of cell trailing edge. Stalling conditions of 3D migration were then assessed by imposing pressure drops across microchannels. Without specific adhesion, the cells slid backward with subnanonewton forces, showing that 3D migration under stress is strongly limited by a lack of adhesion and friction with channels. With specific LFA-1 mediated adhesion, stalling occurred at around 3 and 6 nN in 2 × 4 and 4 × 4 μm² channels, respectively, supporting that stalling of adherent cells was under pressure control rather than force control. The stall pressure of 4 mbar is consistent with the pressure of actin filament polymerization that mediates lamellipod growth. The arrest of adherent cells under stress therefore seems controlled by the compression of the cell leading edge, which perturbs cells front-rear polarization and triggers adhesion failure or polarization reversal. Although stalling assays in microfluidic Transwells do not mimic in vivo transmigration, they provide a powerful tool to scrutinize 2D and 3D migration, barotaxis, and chemotaxis.

Characterization of immune cell migration using microfabrication

The immune system provides our defense against pathogens and aberrant cells, including tumorigenic and infected cells. Motility is one of the fundamental characteristics that enable immune cells to find invading pathogens, control tissue damage, and eliminate primary developing tumors, even in the absence of external treatments. These processes are termed "immune surveillance." Migration disorders of immune cells are related to autoimmune diseases, chronic inflammation, and tumor evasion. It is therefore essential to characterize immune cell motility in different physiologically and pathologically relevant scenarios to understand the regulatory mechanisms of functionality of immune responses. This review is focused on immune cell migration, to define the underlying mechanisms and the corresponding investigative approaches. We highlight the challenges that immune cells encounter in vivo, and the microfabrication methods to mimic particular aspects of their microenvironment. We discuss the advantages and disadvantages of the proposed tools, and provide information on how to access them. Furthermore, we summarize the directional cues that regulate individual immune cell migration, and discuss the behavior of immune cells in a complex environment composed of multiple directional cues.

Human neutrophils swim and phagocytise bacteria

BACKGROUND INFORMATION

Leukocytes migrate in an amoeboid fashion while patrolling our organism in the search for infection or tissue damage. Their capacity to migrate has been proven integrin independent, however, non-specific adhesion or confinement remain a requisite in current models of cell migration. This idea has been challenged twice within the last decade with human neutrophils and effector T lymphocytes, which were shown to migrate in free suspension, a phenomenon termed swimming. While the relevance of leukocyte swimming in vivo remains under judgment, a growing amount of clinical evidence demonstrates that leukocytes are indeed found in liquid-filled body cavities, occasionally with phagocyted pathogens, such as in the amniotic fluid, the cerebrospinal fluid (CSF), or the eye vitreous and aqueous humor.

RESULTS

We studied in vitro swimming of primary human neutrophils in the presence of live bacteria, in 2 and 3 dimensions. We show that swimming neutrophils perform phagocytosis of bacteria in suspension. By micropatterning live bacteria on a substrate with an optical technique, we further prove that they use chemotaxis to swim towards their targets. Moreover, we provide evidence that neutrophil navigation can alternate between adherent and non-adherent modes.

CONCLUSIONS

Our results suggest that human neutrophils do not rely on adhesion to carry out their functions, supporting a versatile phagocytic function adaptable to the various environmental conditions encountered in vivo, as already suggested by clinical data.

SIGNIFICANCE

We verified a claim stated 10 years ago and never reproduced, on the capacity of human neutrophils to swim and perform swimming chemotaxis. We further extended those results to prove that swimming neutrophils can phagocytise bacteria, disregarding adhesion nor confinement as a requisite for accomplishing their function, which differs with current paradigms of leukocyte migration.

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.

Computational estimates of mechanical constraints on cell migration through the extracellular matrix

Cell migration through a three-dimensional (3D) extracellular matrix (ECM) underlies important physiological phenomena and is based on a variety of mechanical strategies depending on the cell type and the properties of the ECM. By using computer simulations of the cell’s mid-plane, we investigate two such migration mechanisms—‘push-pull’ (forming a finger-like protrusion, adhering to an ECM node, and pulling the cell body forward) and ‘rear-squeezing’ (pushing the cell body through the ECM by contracting the cell cortex and ECM at the cell rear). We present a computational model that accounts for both elastic deformation and forces of the ECM, an active cell cortex and nucleus, and for hydrodynamic forces and flow of the extracellular fluid, cytoplasm, and nucleoplasm. We find that relations between three mechanical parameters—the cortex’s contractile force, nuclear elasticity, and ECM rigidity—determine the effectiveness of cell migration through the dense ECM. The cell can migrate persistently even if its cortical contraction cannot deform a near-rigid ECM, but then the contraction of the cortex has to be able to sufficiently deform the nucleus. The cell can also migrate even if it fails to deform a stiff nucleus, but then it has to be able to sufficiently deform the ECM. Simulation results show that nuclear stiffness limits the cell migration more than the ECM rigidity. Simulations show the rear-squeezing mechanism of motility results in more robust migration with larger cell displacements than those with the push-pull mechanism over a range of parameter values. Additionally, results show that the rear-squeezing mechanism is aided by hydrodynamics through a pressure gradient.

Computational simulations of two different mechanisms of 3D cell migration in an extracellular matrix are presented. One mechanism represents a mesenchymal mode, characterized by finger-like actin protrusions, while the second mode is more amoeboid in that rear contraction of the cortex propels the cell forward. In both mechanisms, the cell generates a thin actin protrusion on the cortex that attaches to an ECM node. The cell is then either pulled (mesenchymal) or pushed (amoeboid) forward. Results show both mechanisms result in successful migration over a range of simulated parameter values as long as the contractile tension of the cortex exceeds either the nuclear stiffness or ECM stiffness, but not necessarily both. However, the distance traveled by the amoeboid migration mode is more robust to changes in parameter values, and is larger than in simulations of the mesenchymal mode. Additionally, cells experience a favorable fluid pressure gradient when migrating in the amoeboid mode, and an adverse fluid pressure gradient in the mesenchymal mode.

Amoeboid Swimming Is Propelled by Molecular Paddling in Lymphocytes

Mammalian cells developed two main migration modes. The slow mesenchymatous mode, like crawling of fibroblasts, relies on maturation of adhesion complexes and actin fiber traction, whereas the fast amoeboid mode, observed exclusively for leukocytes and cancer cells, is characterized by weak adhesion, highly dynamic cell shapes, and ubiquitous motility on two-dimensional and in three-dimensional solid matrix. In both cases, interactions with the substrate by adhesion or friction are widely accepted as a prerequisite for mammalian cell motility, which precludes swimming. We show here experimental and computational evidence that leukocytes do swim, and that efficient propulsion is not fueled by waves of cell deformation but by a rearward and inhomogeneous treadmilling of the cell external membrane. Our model consists of a molecular paddling by transmembrane proteins linked to and advected by the actin cortex, whereas freely diffusing transmembrane proteins hinder swimming. Furthermore, continuous paddling is enabled by a combination of external treadmilling and selective recycling by internal vesicular transport of cortex-bound transmembrane proteins. This mechanism explains observations that swimming is five times slower than the retrograde flow of cortex and also that lymphocytes are motile in nonadherent confined environments. Resultantly, the ubiquitous ability of mammalian amoeboid cells to migrate in two dimensions or three dimensions and with or without adhesion can be explained for lymphocytes by a single machinery of heterogeneous membrane treadmilling.

Amoeboid swimming in a compliant channel

Several prokaryotes and eukaryotic cells swim in the presence of deformable and rigid surfaces that form confinement. The most commonly observed examples from biological systems are motility of leukocytes and pathogens present within the blood suspension through a microvascular network, and locomotion of eukaryotic cells such as immune system cells and cancerous cells through interstices between soft interstitial cells and the extracellular matrix within the interstitial tissue. This motivated us to investigate numerically the flow dynamics of amoeboid swimming in a flexible channel. The effects of wall stiffness and channel confinement on the flow dynamics and swimmer motion are studied. The swimmer motion through the flexible channel is substantially decelerated compared to the rigid channel. The strong confinement in the amply flexible channel imprisons the swimmer by severely restricting its forward motion. The swimmer velocity in a stiff channel displays nonmonotonic variation with the confinement while it shows monotonic reduction in a highly flexible channel. The physical rationale behind such distinct velocity behaviour in flexible and rigid channels is illustrated using an instantaneous flow field and flow history displayed by the swimmer. This behavior follows from a subtle interplay between the shape changes exhibited by the swimmer and the wall compliance. This study may aid in understanding the influence of elasticity of the surrounding environment on cell motility in immunological surveillance and invasiveness of cancer cells.

Hydrodynamic effects on the motility of crawling eukaryotic cells

Eukaryotic cell motility is crucial during development, wound healing, the immune response, and cancer metastasis. Some eukaryotic cells can swim, but cells more commonly adhere to and crawl along the extracellular matrix. We study the relationship between hydrodynamics and adhesion that describe whether a cell is swimming, crawling, or combining these motions. Our simple model of a cell, based on the three-sphere swimmer, is capable of both swimming and crawling. As cell-matrix adhesion strength increases, the influence of hydrodynamics on migration diminishes. Cells with significant adhesion can crawl with speeds much larger than their nonadherent, swimming counterparts. We predict that, while most eukaryotic cells are in the strong-adhesion limit, increasing environment viscosity or decreasing cell-matrix adhesion could lead to significant hydrodynamic effects even in crawling cells. Signatures of hydrodynamic effects include a dependence of cell speed on the presence of a nearby substrate or interactions between noncontacting cells. These signatures will be suppressed at large adhesion strengths, but even strongly adherent cells will generate relevant fluid flows that will advect nearby passive particles and swimmers.

Motility and morphodynamics of confined cells

We introduce a minimal hydrodynamic model of polarization, migration, and deformation of a biological cell confined between two parallel surfaces. In our model, the cell is driven out of equilibrium by an active cytsokeleton force that acts on the membrane. The cell cytoplasm, described as a viscous droplet in the Darcy flow regime, contains a diffusive solute that actively transduces the applied cytoskeleton force. While fairly simple and analytically tractable, this quasi-two-dimensional model predicts a range of compelling dynamic behaviours. A linear stability analysis of the system reveals that solute activity first destabilizes a global polarization-translation mode, prompting cell motility through spontaneous symmetry breaking. At higher activity, the system crosses a series of Hopf bifurcations leading to coupled oscillations of droplet shape and solute concentration profiles. At the nonlinear level, we find traveling-wave solutions associated with unique polarized shapes that resemble experimental observations. Altogether, this model offers an analytical paradigm of active deformable systems in which viscous hydrodynamics are coupled to diffusive force transducers.

Regulation of global CD8+ T-cell positioning by the actomyosin cytoskeleton

CD8⁺ T cells have evolved as one of the most motile mammalian cell types, designed to continuously scan peptide-major histocompatibility complexes class I on the surfaces of other cells. Chemoattractants and adhesion molecules direct CD8⁺ T-cell homing to and migration within secondary lymphoid organs, where these cells colocalize with antigen-presenting dendritic cells in confined tissue volumes. CD8⁺ T-cell activation induces a switch to infiltration of non-lymphoid tissue (NLT), which differ in their topology and biophysical properties from lymphoid tissue. Here, we provide a short overview on regulation of organism-wide trafficking patterns during naive T-cell recirculation and their switch to non-lymphoid tissue homing during activation. The migratory lifestyle of CD8⁺ T cells is regulated by their actomyosin cytoskeleton, which translates chemical signals from surface receptors into mechanical work. We explore how properties of the actomyosin cytoskeleton and its regulators affect CD8⁺ T cell function in lymphoid and non-lymphoid tissue, combining recent findings in the field of cell migration and actin network regulation with tissue anatomy. Finally, we hypothesize that under certain conditions, intrinsic regulation of actomyosin dynamics may render NLT CD8⁺ T-cell populations less dependent on input from extrinsic signals during tissue scanning.

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.

Fiber stiffness, pore size and adhesion control migratory phenotype of MDA-MB-231 cells in collagen gels

Cancer cell migration is influenced by cellular phenotype and behavior as well as by the mechanical and chemical properties of the environment. Furthermore, many cancer cells show plasticity of their phenotype and adapt it to the properties of the environment. Here, we study the influence of fiber stiffness, confinement, and adhesion properties on cancer cell migration in porous collagen gels. Collagen gels with soft fibers abrogate migration and promote a round, non-invasive phenotype. Stiffer collagen fibers are inherently more adhesive and lead to the existence of an adhesive phenotype and in general confined migration due to adhesion. Addition of TGF-β lowers adhesion, eliminates the adhesive phenotype and increases the amount of highly motile amoeboid phenotypes. Highest migration speeds and longest displacements are achieved in stiff collagen fibers in pores of about cell size by amoeboid phenotypes. This elucidates the influence of the mechanical properties of collagen gels on phenotype and subsequently migration and shows that stiff fibers, cell sized pores, and low adhesion, are optimal conditions for an amoeboid phenotype and efficient migration.

Mechanisms of 3D cell migration

Cell migration is essential for physiological processes as diverse as development, immune defence and wound healing. It is also a hallmark of cancer malignancy. Thousands of publications have elucidated detailed molecular and biophysical mechanisms of cultured cells migrating on flat, 2D substrates of glass and plastic. However, much less is known about how cells successfully navigate the complex 3D environments of living tissues. In these more complex, native environments, cells use multiple modes of migration, including mesenchymal, amoeboid, lobopodial and collective, and these are governed by the local extracellular microenvironment, specific modalities of Rho GTPase signalling and non-muscle myosin contractility. Migration through 3D environments is challenging because it requires the cell to squeeze through complex or dense extracellular structures. Doing so requires specific cellular adaptations to mechanical features of the extracellular matrix (ECM) or its remodelling. In addition, besides navigating through diverse ECM environments and overcoming extracellular barriers, cells often interact with neighbouring cells and tissues through physical and signalling interactions. Accordingly, cells need to call on an impressively wide diversity of mechanisms to meet these challenges. This Review examines how cells use both classical and novel mechanisms of locomotion as they traverse challenging 3D matrices and cellular environments. It focuses on principles rather than details of migratory mechanisms and draws comparisons between 1D, 2D and 3D migration.

Crawling in a Fluid

There is increasing evidence that mammalian cells not only crawl on substrates but can also swim in fluids. To elucidate the mechanisms of the onset of motility of cells in suspension, a model which couples actin and myosin kinetics to fluid flow is proposed and solved for a spherical shape. The swimming speed is extracted in terms of key parameters. We analytically find super- and subcritical bifurcations from a nonmotile to a motile state and also spontaneous polarity oscillations that arise from a Hopf bifurcation. Relaxing the spherical assumption, the obtained shapes show appealing trends.

Swimming Euglena respond to confinement with a behavioral change enabling effective crawling

Some euglenids, a family of aquatic unicellular organisms, can develop highly concerted, large amplitude peristaltic body deformations. This remarkable behavior has been known for centuries. Yet, its function remains controversial, and is even viewed as a functionless ancestral vestige. Here, by examining swimming Euglena gracilis in environments of controlled crowding and geometry, we show that this behavior is triggered by confinement. Under these conditions, it allows cells to switch from unviable flagellar swimming to a new and highly robust mode of fast crawling, which can deal with extreme geometric confinement and turn both frictional and hydraulic resistance into propulsive forces. To understand how a single cell can control such an adaptable and robust mode of locomotion, we developed a computational model of the motile apparatus of Euglena cells consisting of an active striated cell envelope. Our modeling shows that gait adaptability does not require specific mechanosensitive feedback but instead can be explained by the mechanical self-regulation of an elastic and extended motor system. Our study thus identifies a locomotory function and the operating principles of the adaptable peristaltic body deformation of Euglena cells.

Supracellular migration - beyond collective cell migration

Collective cell migration is a highly complex process in which groups of cells move together. A fundamental question is how cell ensembles can migrate efficiently. In some cases, the group is no more than a collection of individual cells. In others, the group behaves as a supracellular unit, whereby the cell group could be considered as a giant 'supracell', the concept of which was conceived over a century ago. The development of recent tools has provided considerable evidence that cell collectives are highly cooperative, and their migration can better be understood at the tissue level, rather than at the cell level. In this Review, we will define supracellular migration as a type of collective cell migration that operates at a scale higher than the individual cells. We will discuss key concepts of supracellular migration, review recent evidence of collectives exhibiting supracellular features and argue that many seemingly complex collective movements could be better explained by considering the participating cells as supracellular entities.

Modelling actin polymerization: the effect on confined cell migration

The aim of this work is to model cell motility under conditions of mechanical confinement. This cell migration mode may occur in extravasation of tumour and neutrophil-like cells. Cell migration is the result of the complex action of different forces exerted by the interplay between myosin contractility forces and actin processes. Here, we propose and implement a finite element model of the confined migration of a single cell. In this model, we consider the effects of actin and myosin in cell motility. Both filament and globular actin are modelled. We model the cell considering cytoplasm and nucleus with different mechanical properties. The migration speed in the simulation is around 0.1 μm/min, which is in agreement with existing literature. From our simulation, we observe that the nucleus size has an important role in cell migration inside the channel. In the simulation the cell moves further when the nucleus is smaller. However, this speed is less sensitive to nucleus stiffness. The results show that the cell displacement is lower when the nucleus is stiffer. The degree of adhesion between the channel walls and the cell is also very important in confined migration. We observe an increment of cell velocity when the friction coefficient is higher.

Cancer invasion into musculature: Mechanics, molecules and implications

Tumor invasion along structural interphases of surrounding tumor-free tissue represents a key process during tumor progression. Much attention has been devoted to mechanisms of tumor cell migration within extracellular matrix (ECM)-rich connective tissue, however a comprehensive understanding of tumor invasion into tissue of higher structural complexity, such as muscle tissue, is lacking. Muscle invasion in cancer patients is often associated with destructive growth and worsened prognosis. Here, we review biochemical, geometrical and mechanical cues of smooth and skeletal muscle tissues and their relevance for guided invasion of cancer cells. As integrating concept, muscle-organizing ECM-rich surfaces of the epi-, peri- and endomysium provide cleft-like confined spaces along interfaces between dynamic muscle cells, which provide molecular and physical cues that guide migrating cancer cells, forming a possible contribution to cancer progression.

Recent advances in understanding the complexities of metastasis

Tumour metastasis is a dynamic and systemic process. It is no longer seen as a tumour cell-autonomous program but as a multifaceted and complex series of events, which is influenced by the intrinsic cellular mutational burden of cancer cells and the numerous bidirectional interactions between malignant and non-malignant cells and fine-tuned by the various extrinsic cues of the extracellular matrix. In cancer biology, metastasis as a process is one of the most technically challenging aspects of cancer biology to study. As a result, new platforms and technologies are continually being developed to better understand this process. In this review, we discuss some of the recent advances in metastasis and how the information gleaned is re-shaping our understanding of metastatic dissemination.

Recent advances in understanding the complexities of metastasis

Tumour metastasis is a dynamic and systemic process. It is no longer seen as a tumour cell-autonomous program but as a multifaceted and complex series of events, which is influenced by the intrinsic cellular mutational burden of cancer cells and the numerous bidirectional interactions between malignant and non-malignant cells and fine-tuned by the various extrinsic cues of the extracellular matrix. In cancer biology, metastasis as a process is one of the most technically challenging aspects of cancer biology to study. As a result, new platforms and technologies are continually being developed to better understand this process. In this review, we discuss some of the recent advances in metastasis and how the information gleaned is re-shaping our understanding of metastatic dissemination.

Cellular Events of Multinucleated Giant Cells Formation During the Encystation of Entamoeba invadens

Entamoeba histolytica, the causative agent of amoebiasis, does not form cysts in vitro, so reptilian pathogen Entamoeba invadens is used as an Entamoeba encystation model. During the in vitro encystation of E. invadens, a few multinucleated giant cells (MGC) were also appeared in the culture along with cysts. Like the cyst, these MGC's were also formed in the multicellular aggregates found in the encystation culture. Time-lapse live cell imaging revealed that MGC's were the result of repeated cellular fusion with fusion-competent trophozoites as a starting point. The early MGC were non-adherent, and they moved slowly and randomly in the media, but under confinement, MGC became highly motile and directionally persistent. The increased motility resulted in rapid cytoplasmic fissions, which indicated the possibility of continuous cell fusion and division taking place inside the compact multicellular aggregates. Following cell fusion, each nucleus obtained from the fusion-competent trophozoites gave rise to four nuclei with half genomic content. All the haploid nuclei in MGC later aggregated and fused to form a polyploid nucleus. These observations have important implications on Entamoeba biology as they point toward the possibility of E. invadens undergoing sexual or parasexual reproduction.

Directing cell migration on flat substrates and in confinement with microfabrication and microfluidics

Cell motility has been mainly characterized in vitro through the motion of cells on 2D flat Petri dishes, and in Boyden chambers with the passage of cells through sub-cellular sized cavities. These experimental conditions have contributed to understand important features, but these artificial designs can prevent elucidation of mechanisms involved in guiding cell migration in vivo. In this context, microfabrication and microfluidics have provided unprecedented tools to design new assays with local controls in two and three dimensions. Single cells are surrounded by specific environments at a scale where cellular organelles like the nucleus, the cortex, and protrusions can be probed locally in time and in space. Here, we report methods to direct cell motion with emphasis on micro-contact printing for 2D cell migration, and ratchetaxis/chemotaxis in 3D confinements. While sharing similarities, both environments generate distinct experimental issues and questions with potential relevance for in vivo situations.

Integrating Physical and Molecular Insights on Immune Cell Migration

The function of most immune cells depends on their ability to migrate through complex microenvironments, either randomly to patrol for the presence of antigens or directionally to reach their next site of action. The actin cytoskeleton and its partners are key conductors of immune cell migration as they control the intrinsic migratory properties of leukocytes as well as their capacity to respond to cues present in their environment. In this review we focus on the latest discoveries regarding the role of the actomyosin cytoskeleton in optimizing immune cell migration in complex environments, with a special focus on recent insights provided by physical modeling.

A review of microfluidic approaches for investigating cancer extravasation during metastasis

Metastases, or migration of cancers, are common and severe cancer complications. Although the 5-year survival rates of primary tumors have greatly improved, those of metastasis remain below 30%, highlighting the importance of investigating specific mechanisms and therapeutic approaches for metastasis. Microfluidic devices have emerged as a powerful platform for drug target identification and drug response screening and allow incorporation of complex interactions in the metastatic microenvironment as well as manipulation of individual factors. In this work, we review microfluidic devices that have been developed to study cancer cell migration and extravasation in response to mechanical (section ‘Microfluidic investigation of mechanical factors in cancer cell migration’), biochemical (section ‘Microfluidic investigation of biochemical signals in cancer cell invasion’), and cellular (section ‘Microfluidic metastasis-on-a-chip models for investigation of cancer extravasation’) signals. We highlight the device characteristics, discuss the discoveries enabled by these devices, and offer perspectives on future directions for microfluidic investigations of cancer metastasis, with the ultimate aim of identifying the essential factors for a ‘metastasis-on-a-chip’ platform to pursue more efficacious treatment approaches for cancer metastasis.

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.

Reduction in E-cadherin expression fosters migration of Xenopus laevis primordial germ cells

The transition from passive to active migration of primordial germ cells in Xenopus embryos correlates with a reduction in overall adhesion to surrounding endodermal cells as well as with reduced E-cadherin expression. Single cell force spectroscopy, in which cells are brought into brief contact with a gold surface functionalized with E-cadherin constructs, allows for a quantitative estimate of functional E-cadherin molecules on the cell surface. The adhesion force between migratory PGCs and the cadherin-coated surface was almost identical to cells where E-cadherin was knocked down by morpholino oligonucleotides (180 pN). In contrast, non-migratory PGCs display significantly higher adhesion forces (270 pN) on E-cadherin functionalised surfaces. On the basis of these observations, we propose that migration of PGCs in Xenopus embryos is regulated via modulation of E-cadherin expression levels, allowing these cells to move more freely if the level of E-cadherin is reduced.

Comparison of cell migration mechanical strategies in three-dimensional matrices: a computational study

Cell migration on a two-dimensional flat surface has been extensively studied and is generally characterized by a front-protrusion-rear-contraction process. In a three-dimensional (3D) environment, on the other hand, cells adopt multiple migration strategies depending on the cell type and the properties of the extracellular matrix (ECM). By using computer simulations, we find that these migration strategies can be classified by various spatial-temporal dynamics of actin protrusion, actin-myosin contraction and actin-ECM adhesion. We demonstrate that if we include or exclude proteolysis of ECM, and vary adhesion dynamics and spatial distributions of protrusion, contraction and adhesion, our model can reproduce six experimentally observed motility modes: mesenchymal, chimneying, amoeboid, blebbing, finger-like protrusion and rear-squeezing cell locomotory behaviours. We further find that the mode of the cell motility evolves in response to the ECM density and adhesion detachment rate. The model makes non-trivial predictions about cell speed as a function of the adhesion strength, and ECM elasticity and mesh size.

Amoeboid swimming in a channel

Several micro-organisms, such as bacteria, algae, or spermatozoa, use flagellar or ciliary activity to swim in a fluid, while many other micro-organisms instead use ample shape deformation, described as amoeboid, to propel themselves either by crawling on a substrate or swimming. Many eukaryotic cells were believed to require an underlying substratum to migrate (crawl) by using membrane deformation (like blebbing or generation of lamellipodia) but there is now increasing evidence that a large variety of cells (including those of the immune system) can migrate without the assistance of focal adhesion, allowing them to swim as efficiently as they can crawl. This paper details the analysis of amoeboid swimming in a confined fluid by modeling the swimmer as an inextensible membrane deploying local active forces (with zero total force and torque). The swimmer displays a rich behavior: it may settle into a straight trajectory in the channel or navigate from one wall to the other depending on its confinement. The nature of the swimmer is also found to be affected by confinement: the swimmer can behave, on average over one swimming cycle, as a pusher at low confinement, and becomes a puller at higher confinement, or vice versa. The swimmer's nature is thus not an intrinsic property. The scaling of the swimmer velocity V with the force amplitude A is analyzed in detail showing that at small enough A, V∼A(2)/η(2) (where η is the viscosity of the ambient fluid), whereas at large enough A, V is independent of the force and is determined solely by the stroke cycle frequency and the swimmer size. This finding starkly contrasts with models where motion is based on ciliary and flagellar activity, where V∼A/η. To conclude, two definitions of efficiency as put forward in the literature are analyzed with distinct outcomes. We find that one type of efficiency has an optimum as a function of confinement while the other does not. Future perspectives are outlined.

Focal Adhesion-Independent Cell Migration

Cell migration is central to a multitude of physiological processes, including embryonic development, immune surveillance, and wound healing, and deregulated migration is key to cancer dissemination. Decades of investigations have uncovered many of the molecular and physical mechanisms underlying cell migration. Together with protrusion extension and cell body retraction, adhesion to the substrate via specific focal adhesion points has long been considered an essential step in cell migration. Although this is true for cells moving on two-dimensional substrates, recent studies have demonstrated that focal adhesions are not required for cells moving in three dimensions, in which confinement is sufficient to maintain a cell in contact with its substrate. Here, we review the investigations that have led to challenging the requirement of specific adhesions for migration, discuss the physical mechanisms proposed for cell body translocation during focal adhesion-independent migration, and highlight the remaining open questions for the future.

Migration in Confined 3D Environments Is Determined by a Combination of Adhesiveness, Nuclear Volume, Contractility, and Cell Stiffness

In cancer metastasis and other physiological processes, cells migrate through the three-dimensional (3D) extracellular matrix of connective tissue and must overcome the steric hindrance posed by pores that are smaller than the cells. It is currently assumed that low cell stiffness promotes cell migration through confined spaces, but other factors such as adhesion and traction forces may be equally important. To study 3D migration under confinement in a stiff (1.77 MPa) environment, we use soft lithography to fabricate polydimethylsiloxane (PDMS) devices consisting of linear channel segments with 20 μm length, 3.7 μm height, and a decreasing width from 11.2 to 1.7 μm. To study 3D migration in a soft (550 Pa) environment, we use self-assembled collagen networks with an average pore size of 3 μm. We then measure the ability of four different cancer cell lines to migrate through these 3D matrices, and correlate the results with cell physical properties including contractility, adhesiveness, cell stiffness, and nuclear volume. Furthermore, we alter cell adhesion by coating the channel walls with different amounts of adhesion proteins, and we increase cell stiffness by overexpression of the nuclear envelope protein lamin A. Although all cell lines are able to migrate through the smallest 1.7 μm channels, we find significant differences in the migration velocity. Cell migration is impeded in cell lines with larger nuclei, lower adhesiveness, and to a lesser degree also in cells with lower contractility and higher stiffness. Our data show that the ability to overcome the steric hindrance of the matrix cannot be attributed to a single cell property but instead arises from a combination of adhesiveness, nuclear volume, contractility, and cell stiffness.