Par3 controls neural crest migration by promoting microtubule catastrophe during contact inhibition of locomotion

The Rho GEF Trio functions in contact inhibition of locomotion of neural crest cells by interacting with Ptk7

Neural crest (NC) cells are highly migratory cells that contribute to a wide range of vertebrate tissues and must respond to a variety of external signals to precisely control directed cell migration. The RhoGEF Trio is particularly well suited to relay signals to the cytoskeleton because it contains two GEF domains that activate Rac1 and RhoA, respectively. Previously, we have shown that Trio is dynamically localized in Xenopus NC cells and required for their migration. However, how its distinct enzymatic functions are spatially controlled remains unclear. Here, we show that Trio is required for contact inhibition of locomotion (CIL), a phenomenon whereby NC cells change their polarity and directionality upon cell-cell contact. At cell-cell contacts, Trio interacts with Ptk7, a regulator of planar cell polarity that we have recently shown to be required for CIL. Our data suggest that Ptk7 inhibits the Rac1 activity of Trio, thereby limiting Trio activity to the activation of RhoA and promoting CIL.

Interaction of Gαi2 with EB1 controls microtubule dynamics and Rac1 activity in Xenopus neural crest cell migration

Cell migration is crucial in embryonic development, tissue repair and cancer metastasis, driven by the actin and tubulin cytoskeletons that control cell shape, polarity, adhesion and movement in response to various cues. Although heterotrimeric G proteins are known to be involved in cell migration, the specific mechanisms, especially during development, remain elusive. This study examines the role of Gαi2, a heterotrimeric G-protein subunit, in cranial neural crest (NC) cell migration during Xenopus embryonic development. Our research reveals that Gαi2 interacts directly with the microtubule-associated protein EB1, regulating microtubule dynamics. We show that Gαi2 knockdown stabilizes microtubules, disrupts cell polarity and morphology, increases Rac1-GTP at the leading edge and cell-cell contacts, and impairs actin localization and focal adhesion disassembly. Additionally, RhoA-GTP is reduced at cell-cell contacts and concentrated at the leading edge in Gαi2 knockdown cells, providing evidence of a role for Gαi2 in polarity. Treatment with nocodazole, a microtubule-depolymerizing agent, reduces Rac1 activity, restoring cranial NC cell morphology, actin distribution and overall migration. Our findings highlight a crucial role for Gαi2 in cranial NC cell migration by modulating microtubule dynamics through EB1 and Rac1 activity.

Collective mechanics of small migrating cell groups

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

Polarity and migration of cranial and cardiac neural crest cells: underlying molecular mechanisms and disease implications

The Neural Crest cells are multipotent progenitor cells formed at the neural plate border that differentiate and give rise to a wide range of cell types and organs. Directional migration of NC cells and their correct positioning at target sites are essential during embryonic development, and defects in these processes results in congenital diseases. The NC migration begins with the epithelial-mesenchymal transition and extracellular matrix remodeling. The main cellular mechanisms that sustain this migration include contact inhibition of locomotion, co-attraction, chemotaxis and mechanical cues from the surrounding environment, all regulated by proteins that orchestrate cell polarity and motility. In this review we highlight the molecular mechanisms involved in neural crest cell migration and polarity, focusing on the role of small GTPases, Heterotrimeric G proteins and planar cell polarity complex. Here, we also discuss different congenital diseases caused by altered NC cell migration.

Regulation of epithelial cell jamming transition by cytoskeleton and cell-cell interactions

Multicellular systems, such as epithelial cell collectives, undergo transitions similar to those in inert physical systems like sand piles and foams. To remodel or maintain tissue organization during development or disease, these collectives transition between fluid-like and solid-like states, undergoing jamming or unjamming transitions. While these transitions share principles with physical systems, understanding their regulation and implications in cell biology is challenging. Although cell jamming and unjamming follow physics principles described by the jamming diagram, they are fundamentally biological processes. In this review, we explore how cellular processes and interactions regulate jamming and unjamming transitions. We begin with an overview of how these transitions control tissue remodeling in epithelial model systems and describe recent findings of the physical principles governing tissue solidification and fluidization. We then explore the mechanistic pathways that modulate the jamming phase diagram axes, focusing on the regulation of cell fluctuations and geometric compatibility. Drawing upon seminal works in cell biology, we discuss the roles of cytoskeleton and cell-cell adhesion in controlling cell motility and geometry. This comprehensive view illustrates the molecular control of cell jamming and unjamming, crucial for tissue remodeling in various biological contexts.

Centriole Translational Planar Polarity in Monociliated Epithelia

Ciliated epithelia are widespread in animals and play crucial roles in many developmental and physiological processes. Epithelia composed of multi-ciliated cells allow for directional fluid flow in the trachea, oviduct and brain cavities. Monociliated epithelia play crucial roles in vertebrate embryos, from the establishment of left-right asymmetry to the control of axis curvature via cerebrospinal flow motility in zebrafish. Cilia also have a central role in the motility and feeding of free-swimming larvae in a variety of marine organisms. These diverse functions rely on the coordinated orientation (rotational polarity) and asymmetric localization (translational polarity) of cilia and of their centriole-derived basal bodies across the epithelium, both being forms of planar cell polarity (PCP). Here, we review our current knowledge on the mechanisms of the translational polarity of basal bodies in vertebrate monociliated epithelia from the molecule to the whole organism. We highlight the importance of live imaging for understanding the dynamics of centriole polarization. We review the roles of core PCP pathways and of apicobasal polarity proteins, such as Par3, whose central function in this process has been recently uncovered. Finally, we emphasize the importance of the coordination between polarity proteins, the cytoskeleton and the basal body itself in this highly dynamic process.

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.

The RhoGEF Trio is transported by microtubules and affects microtubule stability in migrating neural crest cells

Directed cell migration requires a local fine-tuning of Rho GTPase activity to control protrusion formation, cell-cell contraction, and turnover of cellular adhesions. The Rho guanine nucleotide exchange factor (GEF) TRIO is ideally suited to control RhoGTPase activity because it combines two distinct catalytic domains to control Rac1 and RhoA activity in one molecule. However, at the cellular level, this molecular feature also requires a tight spatiotemporal control of TRIO activity. Here, we analyze the dynamic localization of Trio in Xenopus cranial neural crest (NC) cells, where we have recently shown that Trio is required for protrusion formation and migration. Using live cell imaging, we find that the GEF2 domain, but not the GEF1 domain of Trio, dynamically colocalizes with EB3 at microtubule plus-ends. Microtubule-mediated transport of Trio appears to be relevant for its function in NC migration, as a mutant GEF2 construct lacking the SxIP motif responsible for microtubule plus-end localization was significantly impaired in its ability to rescue the Trio loss-of-function phenotype compared to wild-type GEF2. Furthermore, by analyzing microtubule dynamics in migrating NC cells, we observed that loss of Trio function stabilized microtubules at cell-cell contact sites compared to controls, whereas they were destabilized at the leading edge of NC cells. Our data suggest that Trio is transported by microtubules to distinct subcellular locations where it has different functions in controlling microtubule stability, cell morphology, and cell-cell interaction during directed NC migration.

Pioneer statoacoustic neurons guide neuroblast behaviour during otic ganglion assembly

Cranial ganglia are aggregates of sensory neurons that mediate distinct types of sensation. The statoacoustic ganglion (SAG) develops into several lobes that are spatially arranged to connect appropriately with hair cells of the inner ear. To investigate the cellular behaviours involved in the 3D organization of the SAG, we use high-resolution confocal imaging of single-cell, labelled zebrafish neuroblasts (NBs), photoconversion, photoablation, and genetic perturbations. We show that otic NBs delaminate out of the otic epithelium in an epithelial-mesenchymal transition-like manner, rearranging apical polarity and primary cilia proteins. We also show that, once delaminated, NBs require RhoGTPases in order to perform active migration. Furthermore, tracking of recently delaminated NBs revealed their directed migration and coalescence around a small population of pioneer SAG neurons. These pioneer SAG neurons, not from otic placode origin, populate the coalescence region before otic neurogenesis begins and their ablation disrupts delaminated NB migratory pathways, consequentially affecting SAG shape. Altogether, this work shows for the first time the role of pioneer SAG neurons in orchestrating SAG development.

Tumour follower cells: A novel driver of leader cells in collective invasion (Review)

Collective cellular invasion in malignant tumours is typically characterized by the cooperative migration of multiple cells in close proximity to each other. Follower cells are led away from the tumour by specialized leader cells, and both cell populations play a crucial role in collective invasion. Follower cells form the main body of the migration system and depend on intercellular contact for migration, whereas leader cells indicate the direction for the entire cell population. Although collective invasion can occur in epithelial and non‑epithelial malignant neoplasms, such as medulloblastoma and rhabdomyosarcoma, the present review mainly provided an extensive analysis of epithelial tumours. In the present review, the cooperative mechanisms of contact inhibition locomotion between follower and leader cells, where follower cells coordinate and direct collective movement through physical (mechanical) and chemical (signalling) interactions, is summarised. In addition, the molecular mechanisms of follower cell invasion and metastasis during remodelling and degradation of the extracellular matrix and how chemotaxis and lateral inhibition mediate follower cell behaviour were analysed. It was also demonstrated that follower cells exhibit genetic and metabolic heterogeneity during invasion, unlike leader cells.

Mcrs1 is required for branchial arch and cranial cartilage development

Mcrs1 is a multifunctional protein that is critical for many cellular processes in a wide range of cell types. Previously, we showed that Mcrs1 binds to the Six1 transcription factor and reduces the ability of the Six1-Eya1 complex to upregulate transcription, and that Mcrs1 loss-of-function leads to the expansion of several neural plate genes, reduction of neural border and pre-placodal ectoderm (PPR) genes, and pleiotropic effects on various neural crest (NC) genes. Because the affected embryonic structures give rise to several of the cranial tissues affected in Branchio-otic/Branchio-oto-renal (BOR) syndrome, herein we tested whether these gene expression changes subsequently alter the development of the proximate precursors of BOR affected structures - the otic vesicles (OV) and branchial arches (BA). We found that Mcrs1 is required for the expression of several OV genes involved in inner ear formation, patterning and otic capsule cartilage formation. Mcrs1 knockdown also reduced the expression domains of many genes expressed in the larval BA, derived from either NC or PPR, except for emx2, which was expanded. Reduced Mcrs1 also diminished the length of the expression domain of tbx1 in BA1 and BA2 and interfered with cranial NC migration from the dorsal neural tube; this subsequently resulted in defects in the morphology of lower jaw cartilages derived from BA1 and BA2, including the infrarostral, Meckel's, and ceratohyal as well as the otic capsule. These results demonstrate that Mcrs1 plays an important role in processes that lead to the formation of craniofacial cartilages and its loss results in phenotypes consistent with reduced Six1 activity associated with BOR.

A JAM-A-tetraspanin-αvβ5 integrin complex regulates contact inhibition of locomotion

Contact inhibition of locomotion (CIL) is a process that regulates cell motility upon collision with other cells. Improper regulation of CIL has been implicated in cancer cell dissemination. Here, we identify the cell adhesion molecule JAM-A as a central regulator of CIL in tumor cells. JAM-A is part of a multimolecular signaling complex in which tetraspanins CD9 and CD81 link JAM-A to αvβ5 integrin. JAM-A binds Csk and inhibits the activity of αvβ5 integrin-associated Src. Loss of JAM-A results in increased activities of downstream effectors of Src, including Erk1/2, Abi1, and paxillin, as well as increased activity of Rac1 at cell-cell contact sites. As a consequence, JAM-A-depleted cells show increased motility, have a higher cell-matrix turnover, and fail to halt migration when colliding with other cells. We also find that proper regulation of CIL depends on αvβ5 integrin engagement. Our findings identify a molecular mechanism that regulates CIL in tumor cells and have implications on tumor cell dissemination.

Otic Neurogenesis in Xenopus laevis: Proliferation, Differentiation, and the Role of Eya1

Using immunostaining and confocal microscopy, we here provide the first detailed description of otic neurogenesis in Xenopus laevis. We show that the otic vesicle comprises a pseudostratified epithelium with apicobasal polarity (apical enrichment of Par3, aPKC, phosphorylated Myosin light chain, N-cadherin) and interkinetic nuclear migration (apical localization of mitotic, pH3-positive cells). A Sox3-immunopositive neurosensory area in the ventromedial otic vesicle gives rise to neuroblasts, which delaminate through breaches in the basal lamina between stages 26/27 and 39. Delaminated cells congregate to form the vestibulocochlear ganglion, whose peripheral cells continue to proliferate (as judged by EdU incorporation), while central cells differentiate into Islet1/2-immunopositive neurons from stage 29 on and send out neurites at stage 31. The central part of the neurosensory area retains Sox3 but stops proliferating from stage 33, forming the first sensory areas (utricular/saccular maculae). The phosphatase and transcriptional coactivator Eya1 has previously been shown to play a central role for otic neurogenesis but the underlying mechanism is poorly understood. Using an antibody specifically raised against Xenopus Eya1, we characterize the subcellular localization of Eya1 proteins, their levels of expression as well as their distribution in relation to progenitor and neuronal differentiation markers during otic neurogenesis. We show that Eya1 protein localizes to both nuclei and cytoplasm in the otic epithelium, with levels of nuclear Eya1 declining in differentiating (Islet1/2+) vestibulocochlear ganglion neurons and in the developing sensory areas. Morpholino-based knockdown of Eya1 leads to reduction of proliferating, Sox3- and Islet1/2-immunopositive cells, redistribution of cell polarity proteins and loss of N-cadherin suggesting that Eya1 is required for maintenance of epithelial cells with apicobasal polarity, progenitor proliferation and neuronal differentiation during otic neurogenesis.

Collective cell migration driven by filopodia-New insights from the social behavior of myotubes

Collective migration is a key process that is critical during development, as well as in physiological and pathophysiological processes including tissue repair, wound healing and cancer. Studies in genetic model organisms have made important contributions to our current understanding of the mechanisms that shape cells into different tissues during morphogenesis. Recent advances in high-resolution and live-cell-imaging techniques provided new insights into the social behavior of cells based on careful visual observations within the context of a living tissue. In this review, we will compare Drosophila testis nascent myotube migration with established in vivo model systems, elucidate similarities, new features and principles in collective cell migration.

Ptk7 Is Dynamically Localized at Neural Crest Cell-Cell Contact Sites and Functions in Contact Inhibition of Locomotion

Neural crest (NC) cells are highly migratory cells that contribute to various vertebrate tissues, and whose migratory behaviors resemble cancer cell migration and invasion. Information exchange via dynamic NC cell-cell contact is one mechanism by which the directionality of migrating NC cells is controlled. One transmembrane protein that is most likely involved in this process is protein tyrosine kinase 7 (PTK7), an evolutionary conserved Wnt co-receptor that is expressed in cranial NC cells and several tumor cells. In Xenopus, Ptk7 is required for NC migration. In this study, we show that the Ptk7 protein is dynamically localized at cell-cell contact zones of migrating Xenopus NC cells and required for contact inhibition of locomotion (CIL). Using deletion constructs of Ptk7, we determined that the extracellular immunoglobulin domains of Ptk7 are important for its transient accumulation and that they mediate homophilic binding. Conversely, we found that ectopic expression of Ptk7 in non-NC cells was able to prevent NC cell invasion. However, deletion of the extracellular domains of Ptk7 abolished this effect. Thus, Ptk7 is sufficient at protecting non-NC tissue from NC cell invasion, suggesting a common role of PTK7 in contact inhibition, cell invasion, and tissue integrity.

Planar polarization of cilia in the zebrafish floor-plate involves Par3-mediated posterior localization of highly motile basal bodies

Epithelial cilia, whether motile or primary, often display an off-center planar localization within the apical cell surface. This form of planar cell polarity (PCP) involves the asymmetric positioning of the ciliary basal body (BB). Using the monociliated epithelium of the embryonic zebrafish floor-plate, we investigated the dynamics and mechanisms of BB polarization by live imaging. BBs were highly motile, making back-and-forth movements along the antero-posterior (AP) axis and contacting both the anterior and posterior membranes. Contacts exclusively occurred at junctional Par3 patches and were often preceded by membrane digitations extending towards the BB, suggesting focused cortical pulling forces. Accordingly, BBs and Par3 patches were linked by dynamic microtubules. Later, BBs became less motile and eventually settled at posterior apical junctions enriched in Par3. BB posterior positioning followed Par3 posterior enrichment and was impaired upon Par3 depletion or disorganization of Par3 patches. In the PCP mutant vangl2, BBs were still motile but displayed poorly oriented membrane contacts that correlated with Par3 patch fragmentation and lateral spreading. Thus, we propose an unexpected function for posterior Par3 enrichment in controlling BB positioning downstream of the PCP pathway.

Trio cooperates with Myh9 to regulate neural crest-derived craniofacial development

Trio is a unique member of the Rho-GEF family that has three catalytic domains and is vital for various cellular processes in both physiological and developmental settings. TRIO mutations in humans are involved in craniofacial abnormalities, in which patients present with mandibular retrusion. However, little is known about the molecular mechanisms of Trio in neural crest cell (NCC)-derived craniofacial development, and there is still a lack of direct evidence to assign a functional role to Trio in NCC-induced craniofacial abnormalities. Methods: In vivo, we used zebrafish and NCC-specific knockout mouse models to investigate the phenotype and dynamics of NCC development in Trio morphants. In vitro, iTRAQ, GST pull-down assays, and proximity ligation assay (PLA) were used to explore the role of Trio and its potential downstream mediators in NCC migration and differentiation. Results: In zebrafish and mouse models, disruption of Trio elicited a migration deficit and impaired the differentiation of NCC derivatives, leading to craniofacial growth deficiency and mandibular retrusion. Moreover, Trio positively regulated Myh9 expression and directly interacted with Myh9 to coregulate downstream cellular signaling in NCCs. We further demonstrated that disruption of Trio or Myh9 inhibited Rac1 and Cdc42 activity, specifically affecting the nuclear export of β-catenin and NCC polarization. Remarkably, craniofacial abnormalities caused by trio deficiency in zebrafish could be partially rescued by the injection of mRNA encoding myh9, ca-Rac1, or ca-Cdc42. Conclusions: Here, we identified that Trio, interacting mostly with Myh9, acts as a key regulator of NCC migration and differentiation during craniofacial development. Our results indicate that trio morphant zebrafish and Wnt1-cre;Trio^fl/fl mice offer potential model systems to facilitate the study of the pathogenic mechanisms of Trio mutations causing craniofacial abnormalities.

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

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

Syndecan-4/PAR-3 signaling regulates focal adhesion dynamics in mesenchymal cells

Background

Syndecans regulate cell migration thus having key roles in scarring and wound healing processes. Our previous results have shown that Thy-1/CD90 can engage both αvβ3 integrin and Syndecan-4 expressed on the surface of astrocytes to induce cell migration. Despite a well-described role of Syndecan-4 during cell movement, information is scarce regarding specific Syndecan-4 partners involved in Thy-1/CD90-stimulated cell migration.

Methods

Mass spectrometry (MS) analysis of complexes precipitated with the Syndecan-4 cytoplasmic tail peptide was used to identify potential Syndecan-4-binding partners. The interactions found by MS were validated by immunoprecipitation and proximity ligation assays. The conducted research employed an array of genetic, biochemical and pharmacological approaches, including: PAR-3, Syndecan-4 and Tiam1 silencing, active Rac1 GEFs affinity precipitation, and video microscopy.

Results

We identified PAR-3 as a Syndecan-4-binding protein. Its interaction depended on the carboxy-terminal EFYA sequence present on Syndecan-4. In astrocytes where PAR-3 expression was reduced, Thy-1-induced cell migration and focal adhesion disassembly was impaired. This effect was associated with a sustained Focal Adhesion Kinase activation in the siRNA-PAR-3 treated cells. Our data also show that Thy-1/CD90 activates Tiam1, a PAR-3 effector. Additionally, we found that after Syndecan-4 silencing, Tiam1 activation was decreased and it was no longer recruited to the membrane. Syndecan-4/PAR-3 interaction and the alteration in focal adhesion dynamics were validated in mouse embryonic fibroblast (MEF) cells, thereby identifying this novel Syndecan-4/PAR-3 signaling complex as a general mechanism for mesenchymal cell migration involved in Thy-1/CD90 stimulation.

Conclusions

The newly identified Syndecan-4/PAR-3 signaling complex participates in Thy-1/CD90-induced focal adhesion disassembly in mesenchymal cells. The mechanism involves focal adhesion kinase dephosphorylation and Tiam1 activation downstream of Syndecan-4/PAR-3 signaling complex formation. Additionally, PAR-3 is defined here as a novel adhesome-associated component with an essential role in focal adhesion disassembly during polarized cell migration. These novel findings uncover signaling mechanisms regulating cell migration, thereby opening up new avenues for future research on Syndecan-4/PAR-3 signaling in processes such as wound healing and scarring.

Graphical abstract

The Rho guanine nucleotide exchange factor Trio is required for neural crest cell migration and interacts with Dishevelled

Directional migration during embryogenesis and tumor progression faces the challenge that numerous external signals need to converge to precisely control cell movement. The Rho guanine exchange factor (GEF) Trio is especially well suited to relay signals, as it features distinct catalytic domains to activate Rho GTPases. Here, we show that Trio is required for Xenopus cranial neural crest (NC) cell migration and cartilage formation. Trio cell-autonomously controls protrusion formation of NC cells and Trio morphant NC cells show a blebbing phenotype. Interestingly, the Trio GEF2 domain is sufficient to rescue protrusion formation and migration of Trio morphant NC cells. We show that this domain interacts with the DEP/C-terminus of Dishevelled (DVL). DVL - but not a deletion construct lacking the DEP domain - is able to rescue protrusion formation and migration of Trio morphant NC cells. This is likely mediated by activation of Rac1, as we find that DVL rescues Rac1 activity in Trio morphant embryos. Thus, our data provide evidence for a novel signaling pathway, whereby Trio controls protrusion formation of cranial NC cells by interacting with DVL to activate Rac1.

MMP14 is required for delamination of chick neural crest cells independently of its catalytic activity

Matrix metalloproteinases have a broad spectrum of substrates ranging from extracellular matrix components and adhesion molecules to chemokines and growth factors. Despite being mostly secreted, MMPs have been detected in the cytosol, the mitochondria or the nucleus. Although most of the attention is focused on their role in matrix remodeling, the diversity of their substrates and their complex trafficking open the possibility for non-canonical functions. Yet in vivo examples and experimental demonstration of the physiological relevance of such activities are rare. Here, we have used chick neural crest (NC) cells, a highly migratory stem cell population likened to invasive cancer cells, as a model for physiological epithelial-mesenchymal transition (EMT). We demonstrate that MMP14 is required for NC delamination. Interestingly, this role is independent of its cytoplasmic tail and of its catalytic activity. Our in vivo data indicate that, in addition to being a late pro-invasive factor, MMP14 is also likely to be an early player, owing to its role in EMT.

Neural crest development: insights from the zebrafish

Our understanding of the neural crest, a key vertebrate innovation, is built upon studies of multiple model organisms. Early research on neural crest cells (NCCs) was dominated by analyses of accessible amphibian and avian embryos, with mouse genetics providing complementary insights in more recent years. The zebrafish model is a relative newcomer to the field, yet it offers unparalleled advantages for the study of NCCs. Specifically, zebrafish provide powerful genetic and transgenic tools, coupled with rapidly developing transparent embryos that are ideal for high-resolution real-time imaging of the dynamic process of neural crest development. While the broad principles of neural crest development are largely conserved across vertebrate species, there are critical differences in anatomy, morphogenesis, and genetics that must be considered before information from one model is extrapolated to another. Here, our goal is to provide the reader with a helpful primer specific to neural crest development in the zebrafish model. We focus largely on the earliest events-specification, delamination, and migration-discussing what is known about zebrafish NCC development and how it differs from NCC development in non-teleost species, as well as highlighting current gaps in knowledge.

Microtubules: Evolving roles and critical cellular interactions

Microtubules are cytoskeletal elements known as drivers of directed cell migration, vesicle and organelle trafficking, and mitosis. In this review, we discuss new research in the lens that has shed light into further roles for stable microtubules in the process of development and morphogenesis. In the lens, as well as other systems, distinct roles for characteristically dynamic microtubules and stabilized populations are coming to light. Understanding the mechanisms of microtubule stabilization and the associated microtubule post-translational modifications is an evolving field of study. Appropriate cellular homeostasis relies on not only one cytoskeletal element, but also rather an interaction between cytoskeletal proteins as well as other cellular regulators. Microtubules are key integrators with actin and intermediate filaments, as well as cell–cell junctional proteins and other cellular regulators including myosin and RhoGTPases to maintain this balance.

Impact statement

The role of microtubules in cellular functioning is constantly expanding. In this review, we examine new and exciting fields of discovery for microtubule’s involvement in morphogenesis, highlight our evolving understanding of differential roles for stabilized versus dynamic subpopulations, and further understanding of microtubules as a cellular integrator.

Early expression of Tubulin Beta-III in avian cranial neural crest cells

Neural crest cells are a transient stem-like cell population that forms in the dorsal neural tube of vertebrate embryos and then migrates to various locations to differentiate into diverse derivatives such as craniofacial bone, cartilage, and the enteric and peripheral nervous systems. The current dogma of neural crest cell development suggests that there is a specific hierarchical gene regulatory network (GRN) that controls the induction, specification, and differentiation of these cells at specific developmental times. Our lab has identified that a marker of differentiated neurons, Tubulin Beta-III (TUBB3), is expressed in premigratory neural crest cells. TUBB3 has previously been identified as a major constituent of microtubules and is required for the proper guidance and maintenance of axons during development. Using the model organism, Gallus gallus, we have characterized the spatiotemporal localization of TUBB3 in early stages of development. Here we show TUBB3 is expressed in the developing neural plate, is upregulated in the pre-migratory cranial neural crest prior to cell delamination and migration, and it is maintained or upregulated in neurons in later developmental stages. We believe that TUBB3 likely has a role in early neural crest formation and migration separate from its role in neurogenesis.

In vivo topology converts competition for cell-matrix adhesion into directional migration

When migrating in vivo, cells are exposed to numerous conflicting signals: chemokines, repellents, extracellular matrix, growth factors. The roles of several of these molecules have been studied individually in vitro or in vivo, but we have yet to understand how cells integrate them. To start addressing this question, we used the cephalic neural crest as a model system and looked at the roles of its best examples of positive and negative signals: stromal-cell derived factor 1 (Sdf1/Cxcl12) and class3-Semaphorins. Here we show that Sdf1 and Sema3A antagonistically control cell-matrix adhesion via opposite effects on Rac1 activity at the single cell level. Directional migration at the population level emerges as a result of global Semaphorin-dependent confinement and broad activation of adhesion by Sdf1 in the context of a biased Fibronectin distribution. These results indicate that uneven in vivo topology renders the need for precise distribution of secreted signals mostly dispensable.

Evolution of the Rho guanine nucleotide exchange factors Kalirin and Trio and their gene expression in Xenopus development

Guanine nucleotide exchange factors (GEFs) activate Rho GTPases by accelerating their GDP/GTP exchange. Trio and its paralog Kalirin (Kalrn) are unique members of the Rho-GEFs that harbor three catalytic domains: two functional GEF domains and a serine/threonine kinase domain. The N-terminal GEF domain activates Rac1 and RhoG GTPases, while the C-terminal GEF domain acts specifically on RhoA. Trio and Kalrn have an evolutionary conserved function in morphogenetic processes including neuronal development. De novo mutations in TRIO have lately been identified in patients with intellectual disability, suggesting that this protein family plays an important role in development and disease. Phylogenetic and domain analysis revealed that a Kalrn/Trio ancestor originated in Prebilateria and duplicated in Urbilateria to yield Kalrn and Trio. Only few taxa outside the vertebrates retained both of these highly conserved proteins. To obtain first insights into their redundant or distinct functions in a vertebrate model system, we show for the first time a detailed comparative analysis of trio and kalrn expression in Xenopus laevis development. The mRNAs are maternally transcribed and expression increases starting with neurula stages. Trio and kalrn are detected in mesoderm/somites and different neuronal populations in the neural plate/tube and later also in the brain. However, only trio is expressed in migrating neural crest cells, while kalrn expression is detected in the cranial nerves, suggesting distinct functions. Thus, our expression analysis provides a good basis for further functional studies.

Par3 is essential for the establishment of planar cell polarity of inner ear hair cells

In the inner ear sensory epithelia, stereociliary hair bundles atop sensory hair cells are mechanosensory apparatus with planar polarized structure and orientation. This is established during development by the concerted action of tissue-level, intercellular planar cell polarity (PCP) signaling and a hair cell-intrinsic, microtubule-mediated machinery. However, how various polarity signals are integrated during hair bundle morphogenesis is poorly understood. Here, we show that the conserved cell polarity protein Par3 is essential for planar polarization of hair cells. Par3 deletion in the inner ear disrupted cochlear outgrowth, hair bundle orientation, kinocilium positioning, and basal body planar polarity, accompanied by defects in the organization and cortical attachment of hair cell microtubules. Genetic mosaic analysis revealed that Par3 functions both cell-autonomously and cell-nonautonomously to regulate kinocilium positioning and hair bundle orientation. At the tissue level, intercellular PCP signaling regulates the asymmetric localization of Par3, which in turn maintains the asymmetric localization of the core PCP protein Vangl2. Mechanistically, Par3 interacts with and regulates the localization of Tiam1 and Trio, which are guanine nucleotide exchange factors (GEFs) for Rac, thereby stimulating Rac-Pak signaling. Finally, constitutively active Rac1 rescued the PCP defects in Par3-deficient cochleae. Thus, a Par3-GEF-Rac axis mediates both tissue-level and hair cell-intrinsic PCP signaling.

Family-based exome sequencing and case-control analysis implicate CEP41 as an ASD gene

Autism Spectrum Disorder (ASD) is a complex neurodevelopmental disorder with a strong genetic component. Although next-generation sequencing (NGS) technologies have been successfully applied to gene identification in de novo ASD, the genetic architecture of familial ASD remains largely unexplored. Our approach, which leverages the high specificity and sensitivity of NGS technology, has focused on rare variants in familial autism. We used NGS exome sequencing in 26 families with distantly related affected individuals to identify genes with private gene disrupting and missense variants of interest (VOI). We found that the genes carrying VOIs were enriched for biological processes related to cell projection organization and neuron development, which is consistent with the neurodevelopmental hypothesis of ASD. For a subset of genes carrying VOIs, we then used targeted NGS sequencing and gene-based variant burden case-control analysis to test for association with ASD. Missense variants in one gene, CEP41, associated significantly with ASD (p = 6.185^e-05). Homozygous gene-disrupting variants in CEP41 were initially found to be responsible for recessive Joubert syndrome. Using a zebrafish model, we evaluated the mechanism by which the CEP41 variants might contribute to ASD. We found that CEP41 missense variants affect development of the axonal tract, cranial neural crest migration and social behavior phenotype. Our work demonstrates the involvement of CEP41 heterozygous missense variants in ASD and that biological processes involved in cell projection organization and neuron development are enriched in ASD families we have studied.

In Vivo and In Vitro Quantitative Analysis of Neural Crest Cell Migration

The neural crest is an embryonic cell population induced at the border of the neural plate from where it delaminates and migrates long distances across the embryo. Due to its extraordinary migratory capabilities, the neural crest has become a powerful system to study cellular and molecular aspects of collective and single cell migration both in vivo and in vitro. Here we provide detailed protocols used to perform quantitative analysis of molecular and cellular aspects of Xenopus laevis neural crest cell migration, both in vivo and in vitro.

Supracellular contraction at the rear of neural crest cell groups drives collective chemotaxis

Collective cell chemotaxis, the directed migration of cell groups along gradients of soluble chemical cues, underlies various developmental and pathological processes. We use neural crest cells, a migratory embryonic stem cell population whose behavior has been likened to malignant invasion, to study collective chemotaxis in vivo. Studying Xenopus and zebrafish, we have shown that the neural crest exhibits a tensile actomyosin ring at the edge of the migratory cell group that contracts in a supracellular fashion. This contractility is polarized during collective cell chemotaxis: It is inhibited at the front but persists at the rear of the cell cluster. The differential contractility drives directed collective cell migration ex vivo and in vivo through the intercalation of rear cells. Thus, in neural crest cells, collective chemotaxis works by rear-wheel drive.

Par3 interacts with Prickle3 to generate apical PCP complexes in the vertebrate neural plate

Vertebrate neural tube formation depends on the coordinated orientation of cells in the tissue known as planar cell polarity (PCP). In the Xenopus neural plate, PCP is marked by the enrichment of the conserved proteins Prickle3 and Vangl2 at anterior cell boundaries. Here we show that the apical determinant Par3 is also planar polarized in the neuroepithelium, suggesting a role for Par3 in PCP. Consistent with this hypothesis, interference with Par3 activity inhibited asymmetric distribution of PCP junctional complexes and caused neural tube defects. Importantly, Par3 physically associated with Prickle3 and promoted its apical localization, whereas overexpression of a Prickle3-binding Par3 fragment disrupted PCP in the neural plate. We also adapted proximity biotinylation assay for use in Xenopus embryos and show that Par3 functions by enhancing the formation of the anterior apical PCP complex. These findings describe a mechanistic link between the apical localization of PCP components and morphogenetic movements underlying neurulation.

Gap junction protein Connexin-43 is a direct transcriptional regulator of N-cadherin in vivo

Connexins are the primary components of gap junctions, providing direct links between cells under many physiological processes. Here, we demonstrate that in addition to this canonical role, Connexins act as transcriptional regulators. We show that Connexin 43 (Cx43) controls neural crest cell migration in vivo by directly regulating N-cadherin transcription. This activity requires interaction between Cx43 carboxy tail and the basic transcription factor-3, which drives the translocation of Cx43 tail to the nucleus. Once in the nucleus they form a complex with PolII which directly binds to the N-cadherin promoter. We found that this mechanism is conserved between amphibian and mammalian cells. Given the strong evolutionary conservation of connexins across vertebrates, this may reflect a common mechanism of gene regulation by a protein whose function was previously ascribed only to gap junctional communication.

Connexins are components of gap junctions that link cells and allow intercellular communication. Here, the authors show that the Connexin 43 carboxy tail interacts with basic transcription factor-3, leading to nuclear translocation and direct regulation of N-cadherin expression and neural crest migration.

From Neural Crest Development to Cancer and Vice Versa: How p75NTR and (Pro)neurotrophins Could Act on Cell Migration and Invasion?

The p75 neurotrophin receptor (p75^NTR), also known as low-affinity nerve growth factor, belongs to the tumor necrosis factor family of receptors. p75^NTR is widely expressed in the nervous system during the development, as well as, in the neural crest population, since p75^NTR has been described as ubiquitously expressed and considered as a neural crest marker. Neural crest cells (NCCs) constitute an transient population accurately migrating and invading, with precision, defined sites of the embryo. During migration, NCCs are guided along distinct migratory pathways by specialized molecules present in the extracellular matrix or on the surfaces of those cells. Two main processes direct NCC migration during the development: (1) an epithelial-to-mesenchymal transition and (2) a process known as contact inhibition of locomotion. In adults, p75^NTR remains expressed by NCCs and has been identified in an increasing number of cancer cells. Nonetheless, the regulation of the expression of p75^NTR and the underlying mechanisms in stem cell biology or cancer cells have not yet been sufficiently addressed. The main objective of this review is therefore to analyze elements of our actual knowledge regarding p75^NTR roles during the development (mainly focusing on neural crest development) and see how we can transpose that information from development to cancer (and vice versa) to better understand the link between p75^NTR and cell migration and invasion. In this review, we successively analyzed the molecular mechanisms of p75^NTR when it interacts with several coreceptors and/or effectors. We then analyzed which signaling pathways are the most activated or linked to NCC migration during the development. Regarding cancer, we analyzed the described molecular pathways underlying cancer cell migration when p75^NTR was correlated to cancer cell migration and invasion. From those diverse sources of information, we finally summarized potential molecular mechanisms underlying p75^NTR activation in cell migration and invasion that could lead to new research areas to develop new therapeutic protocols.

Fibronectin amyloid-like aggregation alters its extracellular matrix incorporation and promotes a single and sparsed cell migration

Fibronectin (Fn) is an extracellular matrix (ECM) multifunctional glycoprotein essential for regulating cells behaviors. Within ECM, Fn is found as polymerized fibrils. Apart from fibrils, Fn could also form other kind of supramolecular assemblies such as aggregates. To gain insight into the impact of Fn aggregates on cell behavior, we generated several Fn oligomeric assemblies. These assemblies displayed various amyloid-like properties but were not cytotoxic. In presence of the more amyloid-like structured assemblies of Fn, the cell-ECM networks were altered and the cell shapes shifted toward extended mesenchymal morphologies. Additionnaly, the Fn amyloid-like aggregates promoted a single-cell and sparsed migration of SKOV3 cancer cells, which was associated with a relocalization of αv integrins from plasma membrane to perinuclear vesicles. These data pointed out that the features of supramolecular Fn assemblies could represent a higher level of fine-tuning cell phenotype, and especially migration of cancer cells.

Ric-8A, a GEF for heterotrimeric G-proteins, controls cranial neural crest cell polarity during migration

The neural crest (NC) is a transient embryonic cell population that migrates extensively during development. Ric-8A, a guanine nucleotide exchange factor (GEF) for different Gα subunits regulates cranial NC (CNC) cell migration in Xenopus through a mechanism that still remains to be elucidated. To properly migrate, CNC cells establish an axis of polarization and undergo morphological changes to generate protrusions at the leading edge and retraction of the cell rear. Here, we aim to study the role of Ric-8A in cell polarity during CNC cell migration by examining whether its signaling affects the localization of GTPase activity in Xenopus CNC using GTPase-based probes in live cells and aPKC and Par3 as polarity markers. We show that the levels of Ric-8A are critical during migration and affect the localization of polarity markers and the subcellular localization of GTPase activity, suggesting that Ric-8A, probably through heterotrimeric G-protein signaling, regulates cell polarity during CNC migration.

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

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

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

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

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

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.

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.

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.

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.

Microtubules in 3D cell motility

Three-dimensional (3D) cell motility underlies essential processes, such as embryonic development, tissue repair and immune surveillance, and is involved in cancer progression. Although the cytoskeleton is a well-studied regulator of cell migration, most of what we know about its functions originates from studies conducted in two-dimensional (2D) cultures. This research established that the microtubule network mediates polarized trafficking and signaling that are crucial for cell shape and movement in 2D. In parallel, developments in light microscopy and 3D cell culture systems progressively allowed to investigate cytoskeletal functions in more physiologically relevant settings. Interestingly, several studies have demonstrated that microtubule involvement in cell morphogenesis and motility can differ in 2D and 3D environments. In this Commentary, we discuss these differences and their relevance for the understanding the role of microtubules in cell migration in vivo. We also provide an overview of microtubule functions that were shown to control cell shape and motility in 3D matrices and discuss how they can be investigated further by using physiologically relevant models.

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.

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.

In vivo confinement promotes collective migration of neural crest cells

Collective cell migration is fundamental throughout development and in many diseases. Spatial confinement using micropatterns has been shown to promote collective cell migration in vitro, but its effect in vivo remains unclear. Combining computational and experimental approaches, we show that the in vivo collective migration of neural crest cells (NCCs) depends on such confinement. We demonstrate that confinement may be imposed by the spatiotemporal distribution of a nonpermissive substrate provided by versican, an extracellular matrix molecule previously proposed to have contrasting roles: barrier or promoter of NCC migration. We resolve the controversy by demonstrating that versican works as an inhibitor of NCC migration and also acts as a guiding cue by forming exclusionary boundaries. Our model predicts an optimal number of cells in a given confinement width to allow for directional migration. This optimum coincides with the width of neural crest migratory streams analyzed across different species, proposing an explanation for the highly conserved nature of NCC streams during development.

Leader Cells Define Directionality of Trunk, but Not Cranial, Neural Crest Cell Migration

Collective cell migration is fundamental for life and a hallmark of cancer. Neural crest (NC) cells migrate collectively, but the mechanisms governing this process remain controversial. Previous analyses in Xenopus indicate that cranial NC (CNC) cells are a homogeneous population relying on cell-cell interactions for directional migration, while chick embryo analyses suggest a heterogeneous population with leader cells instructing directionality. Our data in chick and zebrafish embryos show that CNC cells do not require leader cells for migration and all cells present similar migratory capacities. In contrast, laser ablation of trunk NC (TNC) cells shows that leader cells direct movement and cell-cell contacts are required for migration. Moreover, leader and follower identities are acquired before the initiation of migration and remain fixed thereafter. Thus, two distinct mechanisms establish the directionality of CNC cells and TNC cells. This implies the existence of multiple molecular mechanisms for collective cell migration.

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CNC rely on cell-cell interactions to migrate directionally

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Leader cells dictate directionality to followers in the trunk NC population

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Leader and follower identities are acquired before the initiation of migration

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Leader and follower identities are non-interchangeable during migration

Molecular basis of contact inhibition of locomotion

Contact inhibition of locomotion (CIL) is a complex process, whereby cells undergoing a collision with another cell cease their migration towards the colliding cell. CIL has been identified in numerous cells during development including embryonic fibroblasts, neural crest cells and haemocytes and is the driving force behind a range of phenomenon including collective cell migration and dispersion. The loss of normal CIL behaviour towards healthy tissue has long been implicated in the invasion of cancer cells. CIL is a multi-step process that is driven by the tight coordination of molecular machinery. In this review, we shall breakdown CIL into distinct steps and highlight the key molecular mechanisms and components that are involved in driving each step of this process.

The front and rear of collective cell migration

Collective cell migration has a key role during morphogenesis and during wound healing and tissue renewal in the adult, and it is involved in cancer spreading. In addition to displaying a coordinated migratory behaviour, collectively migrating cells move more efficiently than if they migrated separately, which indicates that a cellular interplay occurs during collective cell migration. In recent years, evidence has accumulated confirming the importance of such intercellular communication and exploring the molecular mechanisms involved. These mechanisms are based both on direct physical interactions, which coordinate the cellular responses, and on the collective cell behaviour that generates an optimal environment for efficient directed migration. The recent studies have described how leader cells at the front of cell groups drive migration and have highlighted the importance of follower cells and cell-cell communication, both between followers and between follower and leader cells, to improve the efficiency of collective movement.