Contact inhibition of locomotion in vivo controls neural crest directional migration

Neural crest delamination and migration: Looking forward to the next 150 years

Neural crest (NC) cells were described for the first time in 1868 by Wilhelm His. Since then, this amazing population of migratory stem cells has been intensively studied. It took a century to fully unravel their incredible abilities to contribute to nearly every organ of the body. Yet, our understanding of the cell and molecular mechanisms controlling their migration is far from complete. In this review, we summarize the current knowledge on epithelial-mesenchymal transition and collective behavior of NC cells and propose further stops at which the NC train might be calling in the near future.

Neural crest and the patterning of vertebrate craniofacial muscles

Patterning of craniofacial muscles overtly begins with the activation of lineage-specific markers at precise, evolutionarily conserved locations within prechordal, lateral, and both unsegmented and somitic paraxial mesoderm populations. Although these initial programming events occur without influence of neural crest cells, the subsequent movements and differentiation stages of most head muscles are neural crest-dependent. Incorporating both descriptive and experimental studies, this review examines each stage of myogenesis up through the formation of attachments to their skeletal partners. We present the similarities among developing muscle groups, including comparisons with trunk myogenesis, but emphasize the morphogenetic processes that are unique to each group and sometimes subsets of muscles within a group. These groups include branchial (pharyngeal) arches, which encompass both those with clear homologues in all vertebrate classes and those unique to one, for example, mammalian facial muscles, and also extraocular, laryngeal, tongue, and neck muscles. The presence of several distinct processes underlying neural crest:myoblast/myocyte interactions and behaviors is not surprising, given the wide range of both quantitative and qualitative variations in craniofacial muscle organization achieved during vertebrate evolution.

Requirement of zebrafish pcdh10a and pcdh10b in melanocyte precursor migration

Melanocytes derive from neural crest cells, which are a highly migratory population of cells that play an important role in pigmentation of the skin and epidermal appendages. In most vertebrates, melanocyte precursor cells migrate solely along the dorsolateral pathway to populate the skin. However, zebrafish melanocyte precursors also migrate along the ventromedial pathway, in route to the yolk, where they interact with other neural crest derivative populations. Here, we demonstrate the requirement for zebrafish paralogs pcdh10a and pcdh10b in zebrafish melanocyte precursor migration. pcdh10a and pcdh10b are expressed in a subset of melanocyte precursor and somatic cells respectively, and knockdown and TALEN mediated gene disruption of pcdh10a results in aberrant migration of melanocyte precursors resulting in fully melanized melanocytes that differentiate precociously in the ventromedial pathway. Live cell imaging analysis demonstrates that loss of pchd10a results in a reduction of directed cell migration of melanocyte precursors, caused by both increased adhesion and a loss of cell-cell contact with other migratory neural crest cells. Also, we determined that the paralog pcdh10b is upregulated and can compensate for the genetic loss of pcdh10a. Disruption of pcdh10b alone by CRISPR mutagenesis results in somite defects, while the loss of both paralogs results in enhanced migratory melanocyte precursor phenotype and embryonic lethality. These results reveal a novel role for pcdh10a and pcdh10b in zebrafish melanocyte precursor migration and suggest that pcdh10 paralogs potentially interact for proper transient migration along the ventromedial pathway.

Neuregulin-1 is a chemoattractant and chemokinetic molecule for trunk neural crest cells

BACKGROUND

Trunk neural crest cells migrate rapidly along characteristic pathways within the developing vertebrate embryo. Proper trunk neural crest cell migration is necessary for the morphogenesis of much of the peripheral nervous system, melanocytes, and the adrenal medulla. Numerous molecules help guide trunk neural crest cell migration throughout the early embryo.

RESULTS

The trophic factor NRG1 is a chemoattractant through in vitro chemotaxis assays and in vivo silencing via a DN-erbB receptor. Interestingly, we also observed changes in migratory responses consistent with a chemokinetic effect of NRG1 in trunk neural crest velocity.

CONCLUSIONS

NRG1 is a trunk neural crest cell chemoattractant and chemokinetic molecule. Developmental Dynamics 247:888-902, 2018.. © 2018 Wiley Periodicals, Inc.

Glycogen synthase kinase 3 controls migration of the neural crest lineage in mouse and Xenopus

Neural crest migration is critical to its physiological function. Mechanisms controlling mammalian neural crest migration are comparatively unknown, due to difficulties accessing this cell population in vivo. Here we report requirements of glycogen synthase kinase 3 (GSK3) in regulating the neural crest in Xenopus and mouse models. We demonstrate that GSK3 is tyrosine phosphorylated (pY) in mouse neural crest cells and that loss of GSK3 leads to increased pFAK and misregulation of Rac1 and lamellipodin, key regulators of cell migration. Genetic reduction of GSK3 results in failure of migration. We find that pY-GSK3 phosphorylation depends on anaplastic lymphoma kinase (ALK), a protein associated with neuroblastoma. Consistent with this, neuroblastoma cells with increased ALK activity express high levels of pY-GSK3, and blockade of GSK3 or ALK can affect migration of these cells. Altogether, this work identifies a role for GSK3 in cell migration during neural crest development and cancer.

Defects in neural crest development cause neurocristopathies and cancer, but what regulates this is unclear. Here, the authors show that glycogen synthase kinase 3 (GSK3) regulates migration of neural crest cells, as shown on genetic deletion of GSK3 in the mouse, and that this acts via anaplastic lymphoma kinase.

Chick cranial neural crest cells use progressive polarity refinement, not contact inhibition of locomotion, to guide their migration

To move directionally, cells can bias the generation of protrusions or select among randomly generated protrusions. Here we use 3D two-photon imaging of chick branchial arch 2 directed neural crest cells to probe how these mechanisms contribute to directed movement, whether a subset or the majority of cells polarize during movement, and how the different classes of protrusions relate to one another. We find that, in contrast to Xenopus, cells throughout the stream are morphologically polarized along the direction of overall stream movement and do not exhibit contact inhibition of locomotion. Instead chick neural crest cells display a progressive sharpening of the morphological polarity program. Neural crest cells have weak spatial biases in filopodia generation and lifetime. Local bursts of filopodial generation precede the generation of larger protrusions. These larger protrusions are more spatially biased than the filopodia, and the subset of protrusions that are productive for motility are the most polarized of all. Orientation rather than position is the best correlate of the protrusions that are selected for cell guidance. This progressive polarity refinement strategy may enable neural crest cells to efficiently explore their environment and migrate accurately in the face of noisy guidance cues.

Cranial Neural Crest Explants

The cranial neural crest (CNC) explant assay was originally designed to assess the basic requirements for CNC migration in vitro. This protocol describes the key parameters of CNC explants in Xenopus laevis, with a focus on how to extirpate CNC cells and assay their migration in vitro. The protocol can be adapted according to the needs of the experimenter, some examples of which are discussed here.

Dynamics of 3D carcinoma cell invasion into aligned collagen

Carcinoma cells frequently expand and invade from a confined lesion, or multicellular clusters, into and through the stroma on the path to metastasis, often with an efficiency dictated by the architecture and composition of the microenvironment. Specifically, in desmoplastic carcinomas such as those of the breast, aligned collagen tracks provide contact guidance cues for directed cancer cell invasion. Yet, the evolving dynamics of this process of invasion remains poorly understood, in part due to difficulties in continuously capturing both spatial and temporal heterogeneity and progression to invasion in experimental systems. Therefore, to study the local invasion process from cell dense clusters into aligned collagen architectures found in solid tumors, we developed a novel engineered 3D invasion platform that integrates an aligned collagen matrix with a cell dense tumor-like plug. Using multiphoton microscopy and quantitative analysis of cell motility, we track the invasion of cancer cells from cell-dense bulk clusters into the pre-aligned 3D matrix, and define the temporal evolution of the advancing invasion fronts over several days. This enables us to identify and probe cell dynamics in key regions of interest: behind, at, and beyond the edge of the invading lesion at distinct time points. Analysis of single cell migration identifies significant spatial heterogeneity in migration behavior between cells in the highly cell-dense region behind the leading edge of the invasion front and cells at and beyond the leading edge. Moreover, temporal variations in motility and directionality are also observed between cells within the cell-dense tumor-like plug and the leading invasive edge as its boundary extends into the anisotropic collagen over time. Furthermore, experimental results combined with mathematical modeling demonstrate that in addition to contact guidance, physical crowding of cells is a key regulating factor orchestrating variability in single cell migration during invasion into anisotropic ECM. Thus, our novel platform enables us to capture spatio-temporal dynamics of cell behavior behind, at, and beyond the invasive front and reveals heterogeneous, local interactions that lead to the emergence and maintenance of the advancing front.

Glucose-6-phosphate dehydrogenase is indispensable in embryonic development by modulation of epithelial-mesenchymal transition via the NOX/Smad3/miR-200b axis

Glucose-6-phosphate dehydrogenase (G6PD) is a housekeeping enzyme involved in the pentose phosphate shunt for producing nicotinamide adenine dinucleotide phosphate (NADPH). Severe G6PD deficiency leads to embryonic lethality, but the underlying mechanism is unclear. In the current study, the effects of G6PD on epithelial-mesenchymal transition (EMT), especially during embryonic development, were investigated. The knockdown of G6PD induced morphological changes, accompanied by the suppression of epithelial markers, E-cadherin and β-catenin, in A549 and MDCK cells. Such modulation of EMT was corroborated by the enhancement of migration ability in G6PD-knockdown A549 cells. Zebrafish embryos with g6pd knockdown exhibited downregulation of the E-cadherin/β-catenin adhesion molecules and impaired embryonic development through reduction in epiboly rate and increase in cell shedding at the embryo surface. The dysregulation in zebrafish embryonic development caused by g6pd knockdown could be rescued through human G6PD or CDH1 (E-cadherin gene) cRNA coinjection. The Smad3/miR-200b axis was dysregulated upon G6PD knockdown, and the reconstitution of SMAD3 in G6PD-knockdown A549 cells restored the expression of E-cadherin/β-catenin. The inhibition of NADPH oxidase (NOX) activation through the loss of p22_phox signaling was involved in the dysregulation of the Smad3/miR-200b axis upon G6PD knockdown. The reconstitution of G6PD led to the recovery of the regulation of NOX/Smad3/miR-200b signaling and increased the expression of E-cadherin/β-catenin in G6PD-knockdown cells. Thus, these results suggest that in the EMT process, G6PD plays an important regulatory role as an integral component of the NOX/Smad3/miR-200b axis.

TGF-β Family Signaling in Epithelial Differentiation and Epithelial-Mesenchymal Transition

Epithelia exist in the animal body since the onset of embryonic development; they generate tissue barriers and specify organs and glands. Through epithelial-mesenchymal transitions (EMTs), epithelia generate mesenchymal cells that form new tissues and promote healing or disease manifestation when epithelial homeostasis is challenged physiologically or pathologically. Transforming growth factor-βs (TGF-βs), activins, bone morphogenetic proteins (BMPs), and growth and differentiation factors (GDFs) have been implicated in the regulation of epithelial differentiation. These TGF-β family ligands are expressed and secreted at sites where the epithelium interacts with the mesenchyme and provide paracrine queues from the mesenchyme to the neighboring epithelium, helping the specification of differentiated epithelial cell types within an organ. TGF-β ligands signal via Smads and cooperating kinase pathways and control the expression or activities of key transcription factors that promote either epithelial differentiation or mesenchymal transitions. In this review, we discuss evidence that illustrates how TGF-β family ligands contribute to epithelial differentiation and induce mesenchymal transitions, by focusing on the embryonic ectoderm and tissues that form the external mammalian body lining.

Nanomechanical Force Mapping of Restricted Cell-To-Cell Collisions Oscillating between Contraction and Relaxation

Contact-mediated cell migration strongly determines the invasiveness of the corresponding cells, collective migration, and morphogenesis. The quantitative study of cellular response upon contact relies on cell-to-cell collision, which rarely occurs in conventional cell culture. Herein, we developed a strategy to activate a robust cell-to-cell collision within smooth muscle cell pairs. Nanomechanical traction force mapping reveals that the collision process is promoted by the oscillatory modulations between contraction and relaxation and orientated by the filopodial bridge composed of nanosized contractile machinery. This strategy can enhance the occurrence of cell-to-cell collision, which renders it advantageous over traditional methods that utilize micropatterned coating to confine cell pairs. Furthermore, modulation of the balance between cell tugging force and traction force can determine the repolarization of cells and thus the direction of cell migration. Overall, our approach could help to reveal the mechanistic contribution in cell motility and provide insights in tissue engineering.

Epithelial Mesenchymal Transition in Embryonic Development, Tissue Repair and Cancer: A Comprehensive Overview

The epithelial mesenchymal transition (EMT) plays a central role in both normal physiological events (e.g., embryonic development) and abnormal pathological events (e.g., tumor formation and metastasis). The processes that occur in embryonic development are often reactivated under pathological conditions such as oncogenesis. Therefore, defining the regulatory networks (both gene and protein levels) involved in the EMT during embryonic development will be fundamental in understanding the regulatory networks involved in tumor development, as well as metastasis. There are many molecules, factors, mediators and signaling pathways that are involved in the EMT process. Although the EMT is a very old topic with numerous publications, recent new technologies and discoveries give this research area some new perspective and direction. It is now clear that these important processes are controlled by a network of transcriptional and translational regulators in addition to post-transcriptional and post-translational modifications that amplify the initial signals. In this review article, we will discuss some key concepts, historical findings, as well as some recent progresses in the EMT research field.

Robo signaling regulates the production of cranial neural crest cells

Slit/Robo signaling plays an important role in the guidance of developing neurons in developing embryos. However, it remains obscure whether and how Slit/Robo signaling is involved in the production of cranial neural crest cells. In this study, we examined Robo1 deficient mice to reveal developmental defects of mouse cranial frontal and parietal bones, which are derivatives of cranial neural crest cells. Therefore, we determined the production of HNK1+ cranial neural crest cells in early chick embryo development after knock-down (KD) of Robo1 expression. Detection of markers for pre-migratory and migratory neural crest cells, PAX7 and AP-2α, showed that production of both was affected by Robo1 KD. In addition, we found that the transcription factor slug is responsible for the aberrant delamination/EMT of cranial neural crest cells induced by Robo1 KD, which also led to elevated expression of E- and N-Cadherin. N-Cadherin expression was enhanced when blocking FGF signaling with dominant-negative FGFR1 in half of the neural tube. Taken together, we show that Slit/Robo signaling influences the delamination/EMT of cranial neural crest cells, which is required for cranial bone development.

Neural crest and cancer: Divergent travelers on similar paths

Neural crest cells are multipotent progenitors that dynamically interpret diverse microenvironments to migrate significant distances as a loosely associated collective and contribute to many tissues in the developing vertebrate embryo. Uncovering details of neural crest migration has helped to inform a general understanding of collective cell migration, including that which occurs during cancer metastasis. Here, we discuss several commonalities and differences of neural crest and cancer cell migration and behavior. First, we focus on some of the molecular pathways required for the initial specification and potency of neural crest cells and the roles of many of these pathways in cancer progression. We also describe epithelial-to-mesenchymal transition, which plays a critical role in initiating both neural crest migration and cancer metastasis. Finally, we evaluate studies that demonstrate myriad forms of cell-cell and cell-environment communication during neural crest and cancer collective migration to highlight the remarkable similarities in their molecular and cell biological regulation.

Cadherins function during the collective cell migration of Xenopus Cranial Neural Crest cells: revisiting the role of E-cadherin

Collective cell migration is a process whereby cells move while keeping contact with other cells. The Xenopus Cranial Neural Crest (CNC) is a population of cells that emerge during early embryogenesis and undergo extensive migration from the dorsal to ventral part of the embryo's head. These cells migrate collectively and require cadherin mediated cell-cell contact. In this review, we will describe the key features of Xenopus CNC migration including the key molecules driving their migration. We will also review the role of the various cadherins during Xenopus CNC emergence and migration. Lastly, we will discuss the recent and seemingly controversial findings showing that E-cadherin presence is essential for CNC migration.

Multidisciplinary approaches to understanding collective cell migration in developmental biology

Mathematical models are becoming increasingly integrated with experimental efforts in the study of biological systems. Collective cell migration in developmental biology is a particularly fruitful application area for the development of theoretical models to predict the behaviour of complex multicellular systems with many interacting parts. In this context, mathematical models provide a tool to assess the consistency of experimental observations with testable mechanistic hypotheses. In this review, we showcase examples from recent years of multidisciplinary investigations of neural crest cell migration. The neural crest model system has been used to study how collective migration of cell populations is shaped by cell–cell interactions, cell–environmental interactions and heterogeneity between cells. The wide range of emergent behaviours exhibited by neural crest cells in different embryonal locations and in different organisms helps us chart out the spectrum of collective cell migration. At the same time, this diversity in migratory characteristics highlights the need to reconcile or unify the array of currently hypothesized mechanisms through the next generation of experimental data and generalized theoretical descriptions.

Cdc42 regulates the cellular localization of Cdc42ep1 in controlling neural crest cell migration

The member of Rho family of small GTPases Cdc42 plays important and conserved roles in cell polarity and motility. The Cdc42ep family proteins have been identified to bind to Cdc42, yet how they interact with Cdc42 to regulate cell migration remains to be elucidated. In this study, we focus on Cdc42ep1, which is expressed predominantly in the highly migratory neural crest cells in frog embryos. Through morpholino-mediated knockdown, we show that Cdc42ep1 is required for the migration of cranial neural crest cells. Loss of Cdc42ep1 leads to rounder cell shapes and the formation of membrane blebs, consistent with the observed disruption in actin organization and focal adhesion alignment. As a result, Cdc42ep1 is critical for neural crest cells to apply traction forces at the correct place to migrate efficiently. We further show that Cdc42ep1 is localized to two areas in neural crest cells: in membrane protrusions together with Cdc42 and in perinuclear patches where Cdc42 is absent. Cdc42 directly interacts with Cdc42ep1 (through the CRIB domain) and changes in Cdc42 level shift the distribution of Cdc42ep1 between these two subcellular locations, controlling the formation of membrane protrusions and directionality of migration as a consequence. These results suggest that Cdc42ep1 elaborates Cdc42 activity in neural crest cells to promote their efficient migration.

Asymmetric localization of DLC1 defines avian trunk neural crest polarity for directional delamination and migration

Following epithelial-mesenchymal transition, acquisition of avian trunk neural crest cell (NCC) polarity is prerequisite for directional delamination and migration, which in turn is essential for peripheral nervous system development. However, how this cell polarization is established and regulated remains unknown. Here we demonstrate that, using the RHOA biosensor in vivo and in vitro, the initiation of NCC polarization is accompanied by highly activated RHOA in the cytoplasm at the cell rear and its fluctuating activity at the front edge. This differential RHOA activity determines polarized NC morphology and motility, and is regulated by the asymmetrically localized RhoGAP Deleted in liver cancer (DLC1) in the cytoplasm at the cell front. Importantly, the association of DLC1 with NEDD9 is crucial for its asymmetric localization and differential RHOA activity. Moreover, NC specifiers, SOX9 and SOX10, regulate NEDD9 and DLC1 expression, respectively. These results present a SOX9/SOX10-NEDD9/DLC1-RHOA regulatory axis to govern NCC migratory polarization.

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

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

Microenvironment-Driven Shift of Cohesion/Detachment Balance within Tumors Induces a Switch toward Metastasis in Neuroblastoma

Neuroblastoma (NB) is a childhood cancer arising from sympatho-adrenal neural crest cells. Disseminated forms have high frequency of multiple tumoral foci whose etiology remains unknown; NB embryonic origin limits investigations in patients and current models. We developed an avian embryonic model driving human NB tumorigenesis in tissues homologous to patients. We found that aggressive NBs display a metastatic mode, secondary dissemination via peripheral nerves and aorta. Through tumor transcriptional profiling, we found that NB dissemination is induced by the shutdown of a pro-cohesion autocrine signal, SEMA3C, which constrains the tumoral mass. Lowering SEMA3C levels shifts the balance toward detachment, triggering NB cells to collectively evade the tumor. Together with patient cohort analysis, this identifies a microenvironment-driven pro-metastatic switch for NB.

Planar cell polarity in moving cells: think globally, act locally

The planar cell polarity (PCP) pathway is best known for its role in polarizing epithelial cells within the plane of a tissue but it also plays a role in a range of cell migration events during development. The mechanism by which the PCP pathway polarizes stationary epithelial cells is well characterized, but how PCP signaling functions to regulate more dynamic cell behaviors during directed cell migration is much less understood. Here, we review recent discoveries regarding the localization of PCP proteins in migrating cells and their impact on the cell biology of collective and individual cell migratory behaviors.

Cadherin-11 promotes neural crest cell spreading by reducing intracellular tension-Mapping adhesion and mechanics in neural crest explants by atomic force microscopy

During development cranial neural crest cells (NCCs) display a striking transition from collective to single-cell migration, but the mechanisms enabling individual NCCs to separate from the neural crest tissue are still incompletely understood. In this study we have employed atomic force microscopy (AFM) to investigate potential adhesive and mechanical changes associated with the dissociation of individual cells from cohesive Xenopus NCC explants at early stages of migration. AFM-based single-cell force spectroscopy (SCFS) revealed a uniform distribution of cell-cell adhesion forces within NCC explants, including semi-detached leader cells in the process of delaminating from the explant edge. This suggested that dissociation from the cell sheet may not require prior weakening of cell-cell contacts. However, mapping NCC sheet elasticity by AFM microbead indentation demonstrated strongly reduced cell stiffness in semi-detached leader cells compared to neighbouring cells in the NCC sheet periphery. Reduced leader cell stiffness coincided with enhanced cell spreading and high substrate traction, indicating a possible mechano-regulation of leader cell delamination. In support, AFM elasticity measurements of individual NCCs in optical side view mode demonstrated that reducing cell tension by inhibiting actomyosin contractility induces rapid spreading, possibly maximizing cell-substrate interactions as a result. Depletion of cadherin-11, a classical cadherin with an essential role in NCC migration and substrate adhesion, prevented the tension reduction necessary for NCC spreading, both in individual cells and at the edge of explanted sheets. In contrast, overexpression of cadherin-11 accelerated spreading of both individual cells and delaminating leader cells. As cadherin-11 expression increases strongly during NCC migration, this suggests an important role of cadherin-11 in regulating NCC elasticity and spreading at later stages of NCC migration. We therefore propose a model in which high tension at the NCC sheet periphery prevents premature NCC spreading and delamination during early stages of migration, while a cadherin-11-dependent local decrease in cell tension promotes leader cell spreading and delamination at later stages of migration.

Chase-and-run dynamics in cell motility and the molecular rupture of interacting active elastic dimers

Cell migration in morphogenesis and cancer metastasis typically involves interplay between different cell types. We construct and study a minimal, one-dimensional model composed of two different motile cells with each cell represented as an active elastic dimer. The interaction between the two cells via cadherins is modeled as a spring that can rupture beyond a threshold force as it undergoes dynamic loading from the interacting motile cells. We obtain a phase diagram consisting of chase-and-run dynamics and clumping dynamics as a function of the stiffness of the interaction spring and the threshold force and, therefore, posit that active rupture, or rupture via active forces, is a mechanosensitive means to regulate dynamics between cells. Since the parameters in the model differentiate between N- and E-cadherins, we make predictions for the interactions between a placodelike cell and a neural crestlike cell in a microchannel as well as discuss how our results inform chase-and-run dynamics found in a group of placode cells interacting with a group of neural crest cells. In particular, an argument was made in the latter case that the feedback between cadherins and cell-substrate interaction via integrins was necessary to obtain the chase-and-run behavior. Based on our two-cell results, we argue that this feedback accentuates, but is not necessary for, the chase-and-run behavior.

Role of the extracellular matrix in cancer-associated epithelial to mesenchymal transition phenomenon

The epithelial to mesenchymal transition (EMT) program is a crucial component in the processes of morphogenesis and embryonic development. The transition of epithelial to mesenchymal phenotype is associated with numerous structural and functional changes, including loss of cell polarity and tight cell-cell junctions, the acquisition of invasive abilities, and the expression of mesenchymal proteins. The switch between the two phenotypes is involved in human pathology and is crucial for cancer progression. Extracellular matrices (ECMs) are multi-component networks that surround cells in tissues. These networks are obligatory for cell survival, growth, and differentiation as well as tissue organization. Indeed, the ECM suprastructure, in addition to its supportive role, can process and deliver a plethora of signals to cells, which ultimately regulate their behavior. Importantly, the ECM derived signals are critically involved in the process of EMT during tumorigenesis. This review discusses the multilayer interaction between the ECM and the EMT process, focusing on contributions of discrete mediators, a strategy that may identify novel potential target molecules. Developmental Dynamics 247:368-381, 2018. © 2017 Wiley Periodicals, Inc.

Connecting individual to collective cell migration

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

Mechanisms of collective cell movement lacking a leading or free front edge in vivo

Collective cell movement is one of the strategies for achieving the complex shapes of tissues and organs. In this process, multiple cells within a group held together by cell-cell adhesion acquire mobility and move together in the same direction. In some well-studied models of collective cell movement, the mobility depends strongly on traction generated at the leading edge by cells located at the front. However, recent advances in live-imaging techniques have led to the discovery of other types of collective cell movement lacking a leading edge or even a free edge at the front, in a diverse array of morphological events, including tubule elongation, epithelial sheet extension, and tissue rotation. We herein review some of the developmental events that are organized by collective cell movement and attempt to elucidate the underlying cellular and molecular mechanisms, which include membrane protrusions, guidance cues, cell intercalation, and planer cell polarity, or chirality pathways.

Actomyosin contractility and collective migration: may the force be with you

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Collective migration relies on the ability of a multicellular co-ordinated unit to efficiently respond to physical changes in their surrounding matrix.

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Conversely, migrating cohorts physically alter their microenvironment using mechanical forces.

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During collective migration, actomyosin contractility acts as a central hub coordinating mechanosensing and mechanotransduction responses.

Collective cell migration is essential during physiological processes such as development or wound healing and in pathological conditions such as cancer dissemination. Cells migrating within multicellular tissues experiment different forces which play an intricate role during tissue formation, development and maintenance. How cells are able to respond to these forces depends largely on how they interact with the extracellular matrix. In this review, we focus on mechanics and mechanotransduction in collective migration. In particular, we discuss current knowledge on how cells integrate mechanical signals during collective migration and we highlight actomyosin contractility as a central hub coordinating mechanosensing and mechanotransduction responses.

Collective motion of cells crawling on a substrate: roles of cell shape and contact inhibition

Contact inhibition plays a crucial role in cell motility, wound healing, and tumour formation. By mimicking the mechanical motion of cells crawling on a substrate, we constructed a minimal model of migrating cells that naturally gives rise to contact inhibition of locomotion (CIL). The model cell consists of two disks, a front disk (a pseudopod) and a back disk (cell body), which are connected by a finite extensible spring. Despite the simplicity of the model, the collective behaviour of the cells is highly non-trivial and depends on both the shape of the cells and whether CIL is enabled. Cells with a small front disk (i.e., a narrow pseudopod) form immobile colonies. In contrast, cells with a large front disk (e.g., a lamellipodium) exhibit coherent migration without any explicit alignment mechanism in the model. This result suggests that crawling cells often exhibit broad fronts because this helps facilitate alignment. After increasing the density, the cells develop density waves that propagate against the direction of cell migration and finally stop at higher densities.

Collective cell migration: a physics perspective

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

Regulators of Metastasis Modulate the Migratory Response to Cell Contact under Spatial Confinement

The breast tumor microenvironment (TMEN) is a unique niche where protein fibers help to promote invasion and metastasis. Cells migrating along these fibers are constantly interacting with each other. How cells respond to these interactions has important implications. Cancer cells that circumnavigate or slide around other cells on protein fibers take a less tortuous path out of the primary tumor; conversely, cells that turn back upon encountering other cells invade less efficiently. The contact response of migrating cancer cells in a fibrillar TMEN is poorly understood. Here, using high-aspect ratio micropatterns as a model fibrillar platform, we show that metastatic cells overcome spatial constraints to slide effectively on narrow fiber-like dimensions, whereas nontransformed MCF-10A mammary epithelial cells require much wider micropatterns to achieve moderate levels of sliding. Downregulating the cell-cell adhesion protein, E-cadherin, enables MCF-10A cells to slide on narrower micropatterns; meanwhile, introducing exogenous E-cadherin in metastatic MDA-MB-231 cells increases the micropattern dimension at which they slide. We propose the characteristic fibrillar dimension (CFD) at which effective sliding is achieved as a metric of sliding ability under spatial confinement. Using this metric, we show that metastasis-promoting genetic perturbations enhance cell sliding and reduce CFD. Activation of ErbB2 combined with downregulation of the tumor suppressor and cell polarity regulator, PARD3, reduced the CFD, in agreement with their cooperative role in inducing metastasis in vivo. The CFD was further reduced by a combination of ErbB2 activation and transforming growth factor β stimulation, which is known to enhance invasive behavior. These findings demonstrate that sliding is a quantitative property and a decrease in CFD is an effective metric to understand how multiple genetic hits interact to change cell behavior in fibrillar environments. This quantitative framework sheds insights into how genetic perturbations conspire with fibrillar maturation in the TMEN to drive the invasive behavior of cancer cells.

How inhibitory cues can both constrain and promote cell migration

Collective cell migration is a common feature in both embryogenesis and metastasis. By coupling studies of neural crest migration in vivo and in vitro with mathematical modeling, Szabó et al. (2016, J. Cell Biol., http://dx.doi.org/10.1083/jcb.201602083) demonstrate that the proteoglycan versican forms a physical boundary that constrains neural crest cells to discrete streams, in turn facilitating their migration.

Xenopus as a model organism to study heterotrimeric G-protein pathway during collective cell migration of neural crest

Collective cell migration is essential in many fundamental aspects of normal development, like morphogenesis, organ formation, wound healing, and immune responses, as well as in the etiology of severe pathologies, like cancer metastasis. In spite of the huge amount of data accumulated on cell migration, such a complex process involves many molecular actors, some of which still remain to be functionally characterized. One of these signals is the heterotrimeric G-protein pathway that has been studied mainly in gastrulation movements. Recently we have reported that Ric-8A, a GEF for Gα proteins, plays an important role in neural crest migration in Xenopus development. Xenopus neural crest cells, a highly migratory embryonic cell population induced at the border of the neural plate that migrates extensively in order to differentiate in other tissues during development, have become a good model to understand the dynamics that regulate cell migration. In this review, we aim to provide sufficient evidence supporting how useful Xenopus model with its different tools, such as explants and transplants, paired with improved in vivo imaging techniques, will allow us to tackle the multiple signaling mechanisms involved in neural crest cell migration.

PDGF controls contact inhibition of locomotion by regulating N-cadherin during neural crest migration

A fundamental property of neural crest (NC) migration is contact inhibition of locomotion (CIL), a process by which cells change their direction of migration upon cell contact. CIL has been proven to be essential for NC migration in amphibians and zebrafish by controlling cell polarity in a cell contact-dependent manner. Cell contact during CIL requires the participation of the cell adhesion molecule N-cadherin, which starts to be expressed by NC cells as a consequence of the switch between E- and N-cadherins during epithelial-to-mesenchymal transition (EMT). However, the mechanism that controls the upregulation of N-cadherin remains unknown. Here, we show that platelet-derived growth factor receptor alpha (PDGFRα) and its ligand platelet-derived growth factor A (PDGF-A) are co-expressed in migrating cranial NC. Inhibition of PDGF-A/PDGFRα blocks NC migration by inhibiting N-cadherin and, consequently, impairing CIL. Moreover, we identify phosphatidylinositol-3-kinase (PI3K)/AKT as a downstream effector of the PDGFRα cellular response during CIL. Our results lead us to propose PDGF-A/PDGFRα signalling as a tissue-autonomous regulator of CIL by controlling N-cadherin upregulation during EMT. Finally, we show that once NC cells have undergone EMT, the same PDGF-A/PDGFRα works as an NC chemoattractant, guiding their directional migration.

Summary: PDGF-A and its receptor control Xenopus neural crest migration by promoting EMT and contact inhibition of locomotion, acting via N-cadherin regulation at early stages of development and working as chemoattractant later.

Mouth development

A mouth is present in all animals, and comprises an opening from the outside into the oral cavity and the beginnings of the digestive tract to allow eating. This review focuses on the earliest steps in mouth formation. In the first half, we conclude that the mouth arose once during evolution. In all animals, the mouth forms from ectoderm and endoderm. A direct association of oral ectoderm and digestive endoderm is present even in triploblastic animals, and in chordates, this region is known as the extreme anterior domain (EAD). Further support for a single origin of the mouth is a conserved set of genes that form a 'mouth gene program' including foxA and otx2. In the second half of this review, we discuss steps involved in vertebrate mouth formation, using the frog Xenopus as a model. The vertebrate mouth derives from oral ectoderm from the anterior neural ridge, pharyngeal endoderm and cranial neural crest (NC). Vertebrates form a mouth by breaking through the body covering in a precise sequence including specification of EAD ectoderm and endoderm as well as NC, formation of a 'pre-mouth array,' basement membrane dissolution, stomodeum formation, and buccopharyngeal membrane perforation. In Xenopus, the EAD is also a craniofacial organizer that guides NC, while reciprocally, the NC signals to the EAD to elicit its morphogenesis into a pre-mouth array. Human mouth anomalies are prevalent and are affected by genetic and environmental factors, with understanding guided in part by use of animal models. WIREs Dev Biol 2017, 6:e275. doi: 10.1002/wdev.275 For further resources related to this article, please visit the WIREs website.

Mechanisms of Cell Polarization

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

Tuning Collective Cell Migration by Cell-Cell Junction Regulation

Collective cell migration critically depends on cell-cell interactions coupled to a dynamic actin cytoskeleton. Important cell-cell adhesion receptor systems implicated in controlling collective movements include cadherins, immunoglobulin superfamily members (L1CAM, NCAM, ALCAM), Ephrin/Eph receptors, Slit/Robo, connexins and integrins, and an adaptive array of intracellular adapter and signaling proteins. Depending on molecular composition and signaling context, cell-cell junctions adapt their shape and stability, and this gradual junction plasticity enables different types of collective cell movements such as epithelial sheet and cluster migration, branching morphogenesis and sprouting, collective network migration, as well as coordinated individual-cell migration and streaming. Thereby, plasticity of cell-cell junction composition and turnover defines the type of collective movements in epithelial, mesenchymal, neuronal, and immune cells, and defines migration coordination, anchorage, and cell dissociation. We here review cell-cell adhesion systems and their functions in different types of collective cell migration as key regulators of collective plasticity.

Phosphorylation of Rab-coupling protein by LMTK3 controls Rab14-dependent EphA2 trafficking to promote cell:cell repulsion

The Rab GTPase effector, Rab-coupling protein (RCP) is known to promote invasive behaviour in vitro by controlling integrin and receptor tyrosine kinase (RTK) trafficking, but how RCP influences metastasis in vivo is unclear. Here we identify an RTK of the Eph family, EphA2, to be a cargo of an RCP-regulated endocytic pathway which controls cell:cell repulsion and metastasis in vivo. Phosphorylation of RCP at Ser435 by Lemur tyrosine kinase-3 (LMTK3) and of EphA2 at Ser897 by Akt are both necessary to promote Rab14-dependent (and Rab11-independent) trafficking of EphA2 which generates cell:cell repulsion events that drive tumour cells apart. Genetic disruption of RCP or EphA2 opposes cell:cell repulsion and metastasis in an autochthonous mouse model of pancreatic adenocarcinoma-whereas conditional knockout of another RCP cargo, α5 integrin, does not suppress pancreatic cancer metastasis-indicating a role for RCP-dependent trafficking of an Eph receptor to drive tumour dissemination in vivo.

A mechanically active heterotypic E-cadherin/N-cadherin adhesion enables fibroblasts to drive cancer cell invasion

Cancer-associated fibroblasts (CAFs) promote tumour invasion and metastasis. We show that CAFs exert a physical force on cancer cells that enables their collective invasion. Force transmission is mediated by a heterophilic adhesion involving N-cadherin at the CAF membrane and E-cadherin at the cancer cell membrane. This adhesion is mechanically active; when subjected to force it triggers β-catenin recruitment and adhesion reinforcement dependent on α-catenin/vinculin interaction. Impairment of E-cadherin/N-cadherin adhesion abrogates the ability of CAFs to guide collective cell migration and blocks cancer cell invasion. N-cadherin also mediates repolarization of the CAFs away from the cancer cells. In parallel, nectins and afadin are recruited to the cancer cell/CAF interface and CAF repolarization is afadin dependent. Heterotypic junctions between CAFs and cancer cells are observed in patient-derived material. Together, our findings show that a mechanically active heterophilic adhesion between CAFs and cancer cells enables cooperative tumour invasion.

Physical models of collective cell motility: from cell to tissue

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

Enteric nervous system development: A crest cell's journey from neural tube to colon

The enteric nervous system (ENS) is comprised of a network of neurons and glial cells that are responsible for coordinating many aspects of gastrointestinal (GI) function. These cells arise from the neural crest, migrate to the gut, and then continue their journey to colonize the entire length of the GI tract. Our understanding of the molecular and cellular events that regulate these processes has advanced significantly over the past several decades, in large part facilitated by the use of rodents, avians, and zebrafish as model systems to dissect the signals and pathways involved. These studies have highlighted the highly dynamic nature of ENS development and the importance of carefully balancing migration, proliferation, and differentiation of enteric neural crest-derived cells (ENCCs). Proliferation, in particular, is critically important as it drives cell density and speed of migration, both of which are important for ensuring complete colonization of the gut. However, proliferation must be tempered by differentiation among cells that have reached their final destination and are ready to send axonal extensions, connect to effector cells, and begin to produce neurotransmitters or other signals. Abnormalities in the normal processes guiding ENCC development can lead to failure of ENS formation, as occurs in Hirschsprung disease, in which the distal intestine remains aganglionic. This review summarizes our current understanding of the factors involved in early development of the ENS and discusses areas in need of further investigation.

Emergent structures and dynamics of cell colonies by contact inhibition of locomotion

Cells in tissues can organize into a broad spectrum of structures according to their function. Drastic changes of organization, such as epithelial-mesenchymal transitions or the formation of spheroidal aggregates, are often associated either to tissue morphogenesis or to cancer progression. Here, we study the organization of cell colonies by means of simulations of self-propelled particles with generic cell-like interactions. The interplay between cell softness, cell-cell adhesion, and contact inhibition of locomotion (CIL) yields structures and collective dynamics observed in several existing tissue phenotypes. These include regular distributions of cells, dynamic cell clusters, gel-like networks, collectively migrating monolayers, and 3D aggregates. We give analytical predictions for transitions between noncohesive, cohesive, and 3D cell arrangements. We explicitly show how CIL yields an effective repulsion that promotes cell dispersal, thereby hindering the formation of cohesive tissues. Yet, in continuous monolayers, CIL leads to collective cell motion, ensures tensile intercellular stresses, and opposes cell extrusion. Thus, our work highlights the prominent role of CIL in determining the emergent structures and dynamics of cell colonies.

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

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

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

Modelling collective cell migration of neural crest

Collective cell migration has emerged in the recent decade as an important phenomenon in cell and developmental biology and can be defined as the coordinated and cooperative movement of groups of cells. Most studies concentrate on tightly connected epithelial tissues, even though collective migration does not require a constant physical contact. Movement of mesenchymal cells is more independent, making their emergent collective behaviour less intuitive and therefore lending importance to computational modelling. Here we focus on such modelling efforts that aim to understand the collective migration of neural crest cells, a mesenchymal embryonic population that migrates large distances as a group during early vertebrate development. By comparing different models of neural crest migration, we emphasize the similarity and complementary nature of these approaches and suggest a future direction for the field. The principles derived from neural crest modelling could aid understanding the collective migration of other mesenchymal cell types.

Mechanisms and in vivo functions of contact inhibition of locomotion

Contact inhibition of locomotion (CIL) is a process whereby a cell ceases motility or changes its trajectory upon collision with another cell. CIL was initially characterized more than half a century ago and became a widely studied model system to understand how cells migrate and dynamically interact. Although CIL fell from interest for several decades, the scientific community has recently rediscovered this process. We are now beginning to understand the precise steps of this complex behaviour and to elucidate its regulatory components, including receptors, polarity proteins and cytoskeletal elements. Furthermore, this process is no longer just in vitro phenomenology; we now know from several different in vivo models that CIL is essential for embryogenesis and in governing behaviours such as cell dispersion, boundary formation and collective cell migration. In addition, changes in CIL responses have been associated with other physiological processes, such as cancer cell dissemination during metastasis.

Collective cell migration in development

During embryonic development, tissues undergo major rearrangements that lead to germ layer positioning, patterning, and organ morphogenesis. Often these morphogenetic movements are accomplished by the coordinated and cooperative migration of the constituent cells, referred to as collective cell migration. The molecular and biomechanical mechanisms underlying collective migration of developing tissues have been investigated in a variety of models, including border cell migration, tracheal branching, blood vessel sprouting, and the migration of the lateral line primordium, neural crest cells, or head mesendoderm. Here we review recent advances in understanding collective migration in these developmental models, focusing on the interaction between cells and guidance cues presented by the microenvironment and on the role of cell–cell adhesion in mechanical and behavioral coupling of cells within the collective.

Formation of a "Pre-mouth Array" from the Extreme Anterior Domain Is Directed by Neural Crest and Wnt/PCP Signaling

The mouth arises from the extreme anterior domain (EAD), a region where the ectoderm and endoderm are directly juxtaposed. Here, we identify a "pre-mouth array" in Xenopus that forms soon after the cranial neural crest has migrated to lie on either side of the EAD. Initially, EAD ectoderm comprises a wide and short epithelial mass that becomes narrow and tall with cells and nuclei changing shape, a characteristic of convergent extension. The resulting two rows of cells-the pre-mouth array-later split down the midline to surround the mouth opening. Neural crest is essential for convergent extension and likely signals to the EAD through the Wnt/planar cell polarity (PCP) pathway. Fzl7 receptor is locally required in EAD ectoderm, while Wnt11 ligand is required more globally. Indeed, heterologous cells expressing Wnt11 can elicit EAD convergent extension. The study reveals a precise cellular mechanism that positions and contributes to the future mouth.