Mechanisms and in vivo functions of contact inhibition of locomotion

Neural crest cells utilize primary cilia to regulate ventral forebrain morphogenesis via Hedgehog-dependent regulation of oriented cell division

Development of the brain directly influences the development of the face via both physical growth and Sonic hedgehog (SHH) activity; however, little is known about how neural crest cells (NCCs), the mesenchymal population that comprise the developing facial prominences, influence the development of the brain. We utilized the conditional ciliary mutant Wnt1-Cre;Kif3a^fl/fl to demonstrate that loss of primary cilia on NCCs resulted in a widened ventral forebrain. We found that neuroectodermal Shh expression, dorsal/ventral patterning, and amount of proliferation in the ventral neuroectoderm was not changed in Wnt1-Cre;Kif3a^fl/fl mutants; however, tissue polarity and directional cell division were disrupted. Furthermore, NCCs of Wnt1-Cre;Kif3a^fl/fl mutants failed to respond to a SHH signal emanating from the ventral forebrain. We were able to recapitulate the ventral forebrain phenotype by removing Smoothened from NCCs (Wnt1-Cre;Smo^fl/fl) indicating that changes in the ventral forebrain were mediated through a Hedgehog-dependent mechanism. Together, these data suggest a novel, cilia-dependent mechanism for NCCs during forebrain development.

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.

Cadherin-mediated cell-cell interactions in normal and cancer cells

Adherens junctions (AJs) are molecular complexes that mediate cell-cell adhesive interactions and play pivotal roles in maintenance of tissue organization in adult organisms and at various stages of development. AJs consist of cadherin adhesion receptors, providing homophilic ligation with cadherins on adjacent cells, and members of the catenin protein family: p120, β- and α-catenin. α-catenin's linkage with the actin cytoskeleton defines the linear or punctate organization of AJs in different cell types. Myosin II-dependent tension drives vinculin recruitment by α-catenin and stabilizes the linkage of the cadherin/catenin complex to F-actin. Neoplastic transformation leads to prominent changes in the organization, regulation and stability of AJs. Epithelial-mesenchymal transition (EMT) whereby epithelial cells lose stable cell-cell adhesion, and reorganize their cytoskeleton to acquire migratory activity, plays the central role in cancer cell invasion and metastasis. Recent data demonstrated that a partial EMT resulting in a hybrid epithelial/mesenchymal phenotype with retention of E-cadherin is essential for cancer cell dissemination. E-cadherin and E-cadherin-based AJs are required for collective invasion and migration, survival in circulation, and metastatic outgrowth.

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.

Optogenetic activation of Plexin-B1 reveals contact repulsion between osteoclasts and osteoblasts

During bone remodelling, osteoclasts induce chemotaxis of osteoblasts and yet maintain spatial segregation. We show that osteoclasts express the repulsive guidance factor Semaphorin 4D and induce contact inhibition of locomotion (CIL) in osteoblasts through its receptor Plexin-B1. To examine causality and elucidate how localized Plexin-B1 stimulation may spatiotemporally coordinate its downstream targets in guiding cell migration, we develop an optogenetic tool for Plexin-B1 designated optoPlexin. Precise optoPlexin activation at the leading edge of migrating osteoblasts readily induces local retraction and, unexpectedly, distal protrusions to steer cells away. These morphological changes are accompanied by reorganization of Myosin II, PIP₃, adhesion and active Cdc42. We attribute the resultant repolarization to RhoA/ROCK-mediated redistribution of β-Pix, which activates Cdc42 and promotes protrusion. Thus, our data demonstrate a causal role of Plexin-B1 for CIL in osteoblasts and reveals a previously unknown effect of Semaphorin signalling on spatial distribution of an activator of cell 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.

Mechanisms of Cell Polarization

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

Physical models of collective cell motility: from cell to tissue

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