Chase-and-run between adjacent cell populations promotes directional collective migration

Development of the zebrafish anterior lateral line system is influenced by underlying cranial neural crest

The mechanosensory lateral line system of aquatic vertebrates comprises a superficial network of distributed sensory organs, the neuromasts, which are arranged over the head and trunk and innervated by lateral line nerves to allow detection of changes in water flow and pressure. While the well-studied zebrafish posterior lateral line has emerged as a powerful model to study collective cell migration, far less is known about development of the anterior lateral line, which produces the supraorbital and infraorbital lines around the eye, as well as mandibular and opercular lines over the jaw and cheek. Here we show that normal development of the zebrafish anterior lateral line system from cranial placodes is dependent on another vertebrate-specific cell type, the cranial neural crest. We find that cranial neural crest and anterior lateral lines develop in close proximity, with absence of neural crest cells leading to major disruptions in the overlying anterior lateral line system. Specifically, in the absence of neural crest neither supraorbital nor infraorbital lateral lines fully extend, such that the most anterior cranial regions remain devoid of neuromasts, while supernumerary ectopic neuromasts form in the posterior supraorbital region. Both neural crest and cranial placodes contribute neurons to the lateral line ganglia that innervate the neuromasts and in the absence of neural crest these ganglia, as well as the lateral line afferent nerves, are disrupted. Finally, we establish that as ontogeny proceeds, the most anterior supraorbital neuromasts come to lie within neural crest-derived frontal and nasal bones in the developing cranium. These are the same anterior supraorbital neuromasts that are absent or mislocated in specimens lacking neural crest cells. Together, our results establish that cranial neural crest and cranial placode derivatives function in concert over the course of ontogeny to build the complex cranial lateral line system.

N-cadherin facilitates trigeminal sensory neuron outgrowth and target tissue innervation

The trigeminal ganglion emerges from the condensation of two distinct precursor cell populations, cranial placodes and neural crest. While its dual cellular origin is well understood, the molecules underlying its formation remain relatively obscure. Trigeminal ganglion assembly is mediated, in part, by neural cadherin (N-cadherin), which is initially expressed by placodal neurons and is required for their proper coalescence with neural crest cells. Axon outgrowth first occurs from placodal neurons, but as gangliogenesis proceeds, neural crest cells also differentiate into N-cadherin-expressing neurons, and both extend axons toward targets. However, the role of N-cadherin in axon outgrowth and target innervation has not been explored. Our data show that N-cadherin knockdown in chick trigeminal placode cells decreases trigeminal ganglion size, nerve growth and target innervation in vivo, and reduces neurite complexity of neural crest-derived neurons in vitro. Furthermore, blocking N-cadherin-mediated adhesion prevents axon extension in most placodal neurons in vitro. Collectively, these findings reveal cell- and non-cell autonomous functions for N-cadherin, highlighting its crucial role in mediating reciprocal interactions between neural crest- and placode-derived neurons throughout trigeminal ganglion development.

N-cadherin facilitates trigeminal sensory neuron outgrowth and target tissue innervation

The trigeminal ganglion emerges from the condensation of two distinct precursor cell populations, cranial placodes and neural crest. While its dual cellular origin is well understood, the molecules underlying its formation remain relatively obscure. Trigeminal ganglion assembly is mediated, in part, by neural cadherin (N-cadherin), which is initially expressed by placodal neurons and required for their proper coalescence with neural crest cells. Axon outgrowth first occurs from placodal neurons, but as gangliogenesis proceeds, neural crest cells also differentiate into N-cadherin-expressing neurons, and both extend axons toward targets. However, the role of N-cadherin in axon outgrowth and target innervation has not been explored. Our data show that N-cadherin knockdown in chick trigeminal placode cells decreases trigeminal ganglion size, nerve growth, and target innervation in vivo , and reduces neurite complexity of neural crest-derived neurons in vitro. Furthermore, blocking N-cadherin-mediated adhesion prevents axon extension in most placodal neurons in vitro . Collectively, these findings reveal cell- and non-cell autonomous functions for N-cadherin, highlighting its critical role in mediating reciprocal interactions between neural crest- and placode-derived neurons throughout trigeminal ganglion development.

Identification of CCL3 as a Schwann cell chemotactic factor essential for nerve regeneration

Peripheral nerves regenerate following injury, in contrast to those of the central nervous system. This involves the collective migration of Schwann cell (SC) cords, which transport regrowing axons across the wound site. The SC cords migrate along a newly formed vasculature, which bridges the wound site in response to vascular endothelial growth factor, secreted by hypoxic macrophages. However, the directional signals by which SC cords navigate the long distances across the wound, in the absence of those that guide axons during development, remain unknown. Here, we identify CCL3 as the SC chemotactic factor, secreted by hypoxic macrophages, responsible for this process. We show that CCL3 promotes collective SC migration and axonal regrowth in vivo and, using genetic mouse models and widely used CCL3 inhibitors, that CCL3 is essential for effective nerve regeneration. These findings have therapeutic implications for both promoting nerve repair and inhibiting the aberrant nerve growth associated with trauma and disease.

P-cadherin-dependent adhesions are required for single lumen formation and HGF-mediated cell protrusions during epithelial morphogenesis

During epithelial morphogenesis, in vivo, epithelial cells form cysts enclosing a single, hollow lumen and extend protrusions as a precursor for tubulogenesis. Cell-cell adhesions (e.g., cadherins) contribute to successful execution of these processes; while there are many different cadherins, one less studied cadherin in epithelial morphogenesis is P-cadherin (CDH3). Here, we investigated the role of CDH3 on successful lumen formation and cell protrusions, using three-dimensional cultures of Madin-Darby canine kidney (MDCK) and CDH3 knockout cell lines. We show that depletion of CDH3 leads to perturbations of hollow lumen formation associated with defects in cell protrusions and tubulogenesis, mediated by Rho/ROCK pathway. CDH3 knockout cells exert lower forces on the surrounding environment compared to wild-type cells, suggesting CDH3 acts as a mechanosensor for stable cell protrusion establishment. Together, our data suggest that CDH3 has an essential function during epithelial morphogenesis by contributing to lumen formation and cell protrusions.

Development of the zebrafish anterior lateral line system is influenced by underlying cranial neural crest

The mechanosensory lateral line system of aquatic vertebrates comprises a superficial network of distributed sensory organs, the neuromasts, which are arranged over the head and trunk and innervated by lateral line nerves to allow detection of changes in water flow and pressure. While the well-studied zebrafish posterior lateral line has emerged as a powerful model to study collective cell migration, far less is known about development of the anterior lateral line, which produces the supraorbital and infraorbital lines around the eye, as well as mandibular and opercular lines over the jaw and cheek. Here we show that normal development of the zebrafish anterior lateral line system from cranial placodes is dependent on another vertebrate-specific cell type, the cranial neural crest. We find that cranial neural crest and anterior lateral lines develop in close proximity, with absence of neural crest cells leading to major disruptions in the overlying anterior lateral line system. Specifically, in the absence of neural crest neither supraorbital nor infraorbital lateral lines fully extend, such that the most anterior cranial regions remain devoid of neuromasts, while supernumerary ectopic neuromasts form in the posterior supraorbital region. Both neural crest and cranial placodes contribute neurons to the lateral line ganglia that innervate the neuromasts and in the absence of neural crest these ganglia, as well as the lateral line afferent nerves, are disrupted. Finally, we establish that as ontogeny proceeds, the most anterior supraorbital neuromasts come to lie within neural crest-derived frontal and nasal bones in the developing cranium. These are the same anterior supraorbital neuromasts that are absent or mislocated in specimens lacking neural crest cells. Together, our results establish that cranial neural crest and cranial placode derivatives function in concert over the course of ontogeny to build the complex cranial lateral line system.

Highlights

The anterior lateral line and cranial neural crest develop in close proximityAbsence of neural crest disrupts anterior lateral line developmentAbsence of neural crest disrupts lateral line ganglion morphology and innervationEarly interactions of neural crest and placodes prefigure later anatomical interactions.

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.

Model supports asymmetric regulation across the intercellular junction for collective cell polarization

Symmetry breaking, which is ubiquitous in biological cells, functionally enables directed cell movement and organized embryogenesis. Prior to movement, cells break symmetry to form a well-defined cell front and rear in a process called polarization. In developing and regenerating tissues, collective cell movement requires the coordination of the polarity of the migration machineries of neighboring cells. Though several works shed light on the molecular basis of polarity, fewer studies have focused on the regulation across the cell-cell junction required for collective polarization, thus limiting our ability to connect tissue-level dynamics to subcellular interactions. Here, we investigated how polarity signals are communicated from one cell to its neighbor to ensure coordinated front-to-rear symmetry breaking with the same orientation across the group. In a theoretical setting, we systematically searched a variety of intercellular interactions and identified that co-alignment arrangement of the polarity axes in groups of two and four cells can only be achieved with strong asymmetric regulation of Rho GTPases or enhanced assembly of complementary F-actin structures across the junction. Our results held if we further assumed the presence of an external stimulus, intrinsic cell-to-cell variability, or larger groups. The results underline the potential of using quantitative models to probe the molecular interactions required for macroscopic biological phenomena. Lastly, we posit that asymmetric regulation is achieved through junction proteins and predict that in the absence of cytoplasmic tails of such linker proteins, the likeliness of doublet co-polarity is greatly diminished.

Tlx3 mediates neuronal differentiation and proper condensation of the developing trigeminal ganglion

The trigeminal ganglion, the largest of the vertebrate cranial ganglia, is comprised of sensory neurons that relay sensations of pain, touch, and temperature to the brain. These neurons are derived from two embryonic cell types, the neural crest and ectodermal placodes, whose interactions are critical for proper ganglion formation. While the T-cell leukemia homeobox 3 (Tlx3) gene is known to be expressed in placodally-derived sensory neurons and necessary for their differentiation, little was known about Tlx3 expression and/or function in the neural crest-derived component of the developing trigeminal ganglion. By combining lineage labeling with in situ hybridization in the chick embryo, we show that neural crest-derived cells that contribute to the cranial trigeminal ganglion express Tlx3 at a time point that coincides with the onset of ganglion condensation. Importantly, loss of Tlx3 function in vivo diminishes the overall size and abundance of neurons within the trigeminal ganglion. Conversely, ectopic expression of Tlx3 in migrating cranial neural crest results in their premature neuronal differentiation. Taken together, our results demonstrate a critical role for Tlx3 in neural crest-derived cells during chick trigeminal gangliogenesis.

Pattern dynamics of the nonreciprocal Swift-Hohenberg model

We investigate the pattern dynamics of the one-dimensional nonreciprocal Swift-Hohenberg model. Characteristic spatiotemporal patterns such as disordered, aligned, swap, chiral-swap, and chiral phases emerge depending on the parameters. We classify the characteristic spatiotemporal patterns obtained in numerical simulation by focusing on the spatiotemporal Fourier spectrum of the order parameters. We derive a reduced dynamical system by using the spatial Fourier series expansion. We analyze the bifurcation structure around the fixed points corresponding to the aligned and chiral phases, and explain the transitions between them. The disordered phase is destabilized either to the aligned phase by the Turing bifurcation or to the chiral phase by the wave bifurcation, while the aligned phase and the chiral phase are connected by the pitchfork bifurcation.

Fibroblast growth factor 8 promotes in vitro neurite outgrowth of placode-derived petrosal and nodose ganglia to varying degrees

Fibroblast growth factors (FGFs) are required for the specification and formation of the epibranchial placodes, which give rise to the distal part of the cranial sensory ganglia. However, it remains unclear whether FGFs play a role in regulating the neurite outgrowth of the epibranchial placode-derived ganglia during further development. Previous studies have shown that Fibroblast growth factor 8 (FGF8) promotes neurite outgrowth from the statoacoustic ganglion in vitro. However, these studies did not distinguish between the neural crest- and placode-derived components of the sensory ganglia. In this study, we focused on the petrosal and nodose ganglia as representatives of the epibranchial ganglia and investigated their axonal outgrowth under the influence of FGF8 signaling protein in vitro. To precisely isolate the placode-derived ganglion part, we labeled the placode and its derivatives with enhanced green fluorescent protein (EGFP) through electroporation. The isolated ganglia were then collected for qRT-PCR assay and cultured in a collagen gel with and without FGF8 protein. Our findings revealed that both placode-derived petrosal and nodose ganglia expressed FGFR1 and FGFR2. In culture, FGF8 exerted a neural trophic effect on the axon outgrowth of both ganglia. While the expression levels of FGFR1/2 were similar between the two ganglia, the petrosal ganglion exhibited greater sensitivity to FGF8 compared to the nodose ganglion. This indicates that the placode-derived ganglia have differential responsiveness to FGF8 signaling during axonal extension. Thus, FGF8 is not only required for the early development of the epibranchial placode, as shown in previous studies, but also promotes neurite outgrowth of placode-derived ganglia.

Variations in non-local interaction range lead to emergent chase-and-run in heterogeneous populations

In a chase-and-run dynamic, the interaction between two individuals is such that one moves towards the other (the chaser), while the other moves away (the runner). Examples can be found in both interacting cells and animals. Here, we investigate the behaviours that can emerge at a population level, for a heterogeneous group that contains subpopulations of chasers and runners. We show that a wide variety of patterns can form, from stationary patterns to oscillatory and population-level chase-and-run, where the latter describes a synchronized collective movement of the two populations. We investigate the conditions under which different behaviours arise, specifically focusing on the interaction ranges: the distances over which cells or organisms can sense one another's presence. We find that when the interaction range of the chaser is sufficiently larger than that of the runner-or when the interaction range of the chase is sufficiently larger than that of the run-population-level chase-and-run emerges in a robust manner. We discuss the results in the context of phenomena observed in cellular and ecological systems, with particular attention to the dynamics observed experimentally within populations of neural crest and placode cells.

Modelling variability and heterogeneity of EMT scenarios highlights nuclear positioning and protrusions as main drivers of extrusion

Epithelial-Mesenchymal Transition (EMT) is a key process in physiological and pathological settings. EMT is often presented as a linear sequence with (i) disassembly of cell-cell junctions, (ii) loss of epithelial polarity and (iii) reorganization of the cytoskeleton leading to basal extrusion from the epithelium. Once out, cells can adopt a migratory phenotype with a front-rear polarity. While this sequence can occur, in vivo observations have challenged it. It is now accepted that multiple EMT scenarios coexist in heterogeneous cell populations. However, the relative importance of each step as well as that of variability and heterogeneity on the efficiency of cell extrusion has not been assessed. Here we used computational modelling to simulate multiple EMT-like scenarios and confronted these data to the EMT of neural crest cells. Overall, our data point to a key role of nuclear positioning and protrusive activity to generate timely basal extrusion.

Impact of non-reciprocal interactions on colloidal self-assembly with tunable anisotropy

Non-reciprocal (NR) effective interactions violating Newton's third law occur in many biological systems, but can also be engineered in synthetic, colloidal systems. Recent research has shown that such NR interactions can have tremendous effects on the overall collective behavior and pattern formation, but can also influence aggregation processes on the particle scale. Here, we focus on the impact of non-reciprocity on the self-assembly of a colloidal system (originally passive) with anisotropic interactions whose character is tunable by external fields. In the absence of non-reciprocity, that is, under equilibrium conditions, the colloids form square-like and hexagonal aggregates with extremely long lifetimes yet no large-scale phase separation [Kogler et al., Soft Matter 11, 7356 (2015)], indicating kinetic trapping. Here, we study, based on Brownian dynamics simulations in 2D, an NR version of this model consisting of two species with reciprocal isotropic, but NR anisotropic interactions. We find that NR induces an effective propulsion of particle pairs and small aggregates ("active colloidal molecules") forming at the initial stages of self-assembly, an indication of the NR-induced non-equilibrium. The shape and stability of these initial clusters strongly depend on the degree of anisotropy. At longer times, we find, for weak NR interactions, large (even system-spanning) clusters where single particles can escape and enter at the boundaries, in stark contrast to the small rigid aggregates appearing at the same time in the passive case. In this sense, weak NR shortcuts the aggregation. Increasing the degree of NR (and thus, propulsion), we even observe large-scale phase separation if the interactions are weakly anisotropic. In contrast, systems with strong NR and anisotropy remain essentially disordered. Overall, the NR interactions are shown to destabilize the rigid aggregates interrupting self-assembly and phase separation in the passive case, thereby helping the system to overcome kinetic barriers.

Perspectives in collective cell migration - moving forward

Collective cell migration, where cells move as a cohesive unit, is a vital process underlying morphogenesis and cancer metastasis. Thanks to recent advances in imaging and modelling, we are beginning to understand the intricate relationship between a cell and its microenvironment and how this shapes cell polarity, metabolism and modes of migration. The use of biophysical and mathematical models offers a fresh perspective on how cells migrate collectively, either flowing in a fluid-like state or transitioning to more static states. Continuing to unite researchers in biology, physics and mathematics will enable us to decode more complex biological behaviours that underly collective cell migration; only then can we understand how this coordinated movement of cells influences the formation and organisation of tissues and directs the spread of metastatic cancer. In this Perspective, we highlight exciting discoveries, emerging themes and common challenges that have arisen in recent years, and possible ways forward to bridge the gaps in our current understanding of collective cell migration.

Emergent Chirality and Hyperuniformity in an Active Mixture with Nonreciprocal Interactions

We investigate collective dynamics in a binary mixture of programmable robots in experiments and simulations. While robots of the same species align their motion direction, interaction between species is distinctly nonreciprocal: species A aligns with B and species B antialigns with A. This nonreciprocal interaction gives rise to the emergence of collective chiral motion that can be stabilized by limiting the robot angular speed to be below a threshold. Within the chiral phase, increasing the robot density or extending the range of local repulsive interactions can drive the system through an absorbing-active transition. At the transition point, the robots exhibit a remarkable capacity for self-organization, forming disordered hyperuniform states.

Collective durotaxis along a self-generated mobile stiffness gradient in vivo

A crucial aspect of tissue self-organization during morphogenesis, wound healing, and cancer invasion is directed migration of cell collectives. The majority of in vivo directed migration has been guided by chemotaxis, whereby cells follow a chemical gradient. In certain situations, migrating cell collectives can also self-generate the stiffness gradient in the surrounding tissue, which can have a feedback effect on the directionality of the migration. The phenomenon has been observed during collective durotaxis in vivo. Along the biointerface between neighbouring tissues, heterotypic cell-cell interactions are the main cause of this self-generated stiffness gradient. The physical processes in charge of tissue self-organization along the biointerface, which are related to the interplay between cell signalling and the formation of heterotypic cell-cell adhesion contacts, are less well-developed than the biological mechanisms of the cellular interactions. This complex phenomenon is discussed here in the model system, such as collective migration of neural crest cells between ectodermal placode and mesoderm subpopulations within Xenopus embryos by pointing to the role of the dynamics along the biointerface between adjacent cell subpopulations on the subpopulation stiffness.

Steroid hormone signaling synchronizes cell migration machinery, adhesion and polarity to direct collective movement

Migratory cells - either individually or in cohesive groups - are critical for spatiotemporally regulated processes such as embryonic development and wound healing. Their dysregulation is the underlying cause of formidable health problems such as congenital abnormalities and metastatic cancers. Border cell behavior during Drosophila oogenesis provides an effective model to study temporally regulated, collective cell migration in vivo. Developmental timing in flies is primarily controlled by the steroid hormone ecdysone, which acts through a well-conserved, nuclear hormone receptor complex. Ecdysone signaling determines the timing of border cell migration, but the molecular mechanisms governing this remain obscure. We found that border cell clusters expressing a dominant-negative form of ecdysone receptor extended ineffective protrusions. Additionally, these clusters had aberrant spatial distributions of E-cadherin (E-cad), apical domain markers and activated myosin that did not overlap. Remediating their expression or activity individually in clusters mutant for ecdysone signaling did not restore proper migration. We propose that ecdysone signaling synchronizes the functional distribution of E-cadherin, atypical protein kinase C (aPKC), Discs large (Dlg1) and activated myosin post-transcriptionally to coordinate adhesion, polarity and contractility and temporally control collective cell migration.

A non local model for cell migration in response to mechanical stimuli

Cell migration is one of the most studied phenomena in biology since it plays a fundamental role in many physiological and pathological processes such as morphogenesis, wound healing and tumorigenesis. In recent years, researchers have performed experiments showing that cells can migrate in response to mechanical stimuli of the substrate they adhere to. Motion towards regions of the substrate with higher stiffness is called durotaxis, while motion guided by the stress or the deformation of the substrate itself is called tensotaxis. Unlike chemotaxis (i.e. the motion in response to a chemical stimulus), these migratory processes are not yet fully understood from a biological point of view. In this respect, we present a mathematical model of single-cell migration in response to mechanical stimuli, in order to simulate these two processes. Specifically, the cell moves by changing its direction of polarization and its motility according to material properties of the substrate (e.g., stiffness) or in response to proper scalar measures of the substrate strain or stress. The equations of motion of the cell are non-local integro-differential equations, with the addition of a stochastic term to account for random Brownian motion. The mechanical stimulus to be integrated in the equations of motion is defined according to experimental measurements found in literature, in the case of durotaxis. Conversely, in the case of tensotaxis, substrate strain and stress are given by the solution of the mechanical problem, assuming that the extracellular matrix behaves as a hyperelastic Yeoh's solid. In both cases, the proposed model is validated through numerical simulations that qualitatively reproduce different experimental scenarios.

Irreversibility across a Nonreciprocal PT-Symmetry-Breaking Phase Transition

Nonreciprocal interactions are commonplace in continuum-level descriptions of both biological and synthetic active matter, yet studies addressing their implications for time reversibility have so far been limited to microscopic models. Here, we derive a general expression for the average rate of informational entropy production in the most generic mixture of conserved phase fields with nonreciprocal couplings and additive conservative noise. For the particular case of a binary system with Cahn-Hilliard dynamics augmented by nonreciprocal cross-diffusion terms, we observe a nontrivial scaling of the entropy production rate across a parity-time symmetry breaking phase transition. We derive a closed-form analytic expression in the weak-noise regime for the entropy production rate due to the emergence of a macroscopic dynamic phase, showing it can be written in terms of the global polar order parameter, a measure of parity-time symmetry breaking.

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.

Exosome-shuttled miR-126 mediates ethanol-induced disruption of neural crest cell-placode cell interaction by targeting SDF1

During embryonic development, 2 populations of multipotent stem cells, cranial neural crest cells (NCCs) and epibranchial placode cells (PCs), are anatomically adjacent to each other. The coordinated migration of NCCs and PCs plays a major role in the morphogenesis of craniofacial skeletons and cranial nerves. It is known that ethanol-induced dysfunction of NCCs and PCs is a key contributor to the defects of craniofacial skeletons and cranial nerves implicated in fetal alcohol spectrum disorder (FASD). However, how ethanol disrupts the coordinated interaction between NCCs and PCs was not elucidated. To fill in this gap, we established a well-designed cell coculture system to investigate the reciprocal interaction between human NCCs (hNCCs) and human PCs (hPCs), and also monitored the migration behavior of NCCs and PCs in zebrafish embryos. We found that ethanol exposure resulted in a disruption of coordinated hNCCs-hPCs interaction, as well as in zebrafish embryos. Treating hNCCs-hPCs with exosomes derived from ethanol-exposed hNCCs (ExoEtOH) mimicked ethanol-induced impairment of hNCCs-hPCs interaction. We also observed that SDF1, a chemoattractant, was downregulated in ethanol-treated hPCs and zebrafish embryos. Meanwhile, miR-126 level in ExoEtOH was significantly higher than that in control exosomes (ExoCon). We further validated that ExoEtOH-encapsulated miR-126 from hNCCs can be transferred to hPCs to suppress SDF1 expression in hPCs. Knockdown of SDF1 replicated ethanol-induced abnormalities either in vitro or in zebrafish embryos. On the contrary, overexpression of SDF1 or inhibiting miR-126 strongly rescued ethanol-induced impairment of hNCCs-hPCs interaction and developmental defects.

Expect the unexpected: conventional and unconventional roles for cadherins in collective cell migration

Migrating cell collectives navigate complex tissue environments both during normal development and in pathological contexts such as tumor invasion and metastasis. To do this, cells in collectives must stay together but also communicate information across the group. The cadherin superfamily of proteins mediates junctional adhesions between cells, but also serve many essential functions in collective cell migration. Besides keeping migrating cell collectives cohesive, cadherins help follower cells maintain their attachment to leader cells, transfer information about front-rear polarity among the cohort, sense and respond to changes in the tissue environment, and promote intracellular signaling, in addition to other cellular behaviors. In this review, we highlight recent studies that reveal diverse but critical roles for both classical and atypical cadherins in collective cell migration, specifically focusing on four in vivo model systems in development: the Drosophila border cells, zebrafish mesendodermal cells, Drosophila follicle rotation, and Xenopus neural crest cells.

Cell-matrix and cell-cell interaction mechanics in guiding migration

Physical properties of tissue are increasingly recognised as major regulatory cues affecting cell behaviours, particularly cell migration. While these properties of the extracellular matrix have been extensively discussed, the contribution from the cellular components that make up the tissue are still poorly appreciated. In this mini-review, we will discuss two major physical components: stiffness and topology with a stronger focus on cell-cell interactions and how these can impact cell migration.

Dimethyladenosine Transferase 1 Homolog Promotes Human Gastric Carcinoma Cell Proliferation and Inhibits Apoptosis via the AKT Pathway

BACKGROUND/AIMS

Our previous work identified the dimethyladenosine transferase 1 homolog as a novel prognostic factor for detecting human gastric carcinoma with high sensitivity and specificity. The high expression of dimethyladenosine transferase 1 is closely associated with the occurrence and progression of gastric carcinoma. However, the underlying mechanism of dimethyladenosine transferase 1 for the occurrence and development of gastric carcinoma is not well elucidated yet.

MATERIALS AND METHODS

In our present study, the biological role of dimethyladenosine transferase 1 on cell proliferation, apoptosis, and cell cycle progression in human gastric carcinoma cells was investigated through in vitro and in vivo assays by the overexpression and knockdown of dimethyladenosine transferase 1 2-way authentication method.

RESULTS

We found that the overexpression of dimethyladenosine transferase 1 significantly promotes cell proliferation (P < .001) and inhibition of cell apoptosis (P < .01) in SGC-7901 cells. However, the in vivo experiment results of the knockdown dimethyladenosine transferase 1 using small interfering RNAs in the MKN-45 are just the opposite. Reverse-transcriptase polymerase chain reaction and western blotting analysis revealed that overexpressed dimethyladenosine transferase 1 in SGC-7901 cells significantly activated the AKT pathway compared to control cells. In contrast, we found that apoptosis genes such as Caspase-3 and Caspase-9 were downregulated in those cells. The xenograft nude mice model exhibited increased tumor growth (P < .01) and weight loss (P < .01), with the overexpression of dimethyladenosine transferase 1 homolog in the SGC-7901 cells. These results have been further confirmed through backward verification in dimethyladenosine transferase 1 knockdown cells.

CONCLUSIONS

Taken together, our results indicated that the dimethyladenosine transferase 1 plays a crucial role in stimulating cancer cell proliferation and contributes to apoptosis resistance in human gastric carcinoma. Meanwhile, it provides a potential therapeutic target for gastric carcinoma treatment and is worthy of further studies.

Paracrine regulation of neural crest EMT by placodal MMP28

Epithelial-mesenchymal transition (EMT) is an early event in cell dissemination from epithelial tissues. EMT endows cells with migratory, and sometimes invasive, capabilities and is thus a key process in embryo morphogenesis and cancer progression. So far, matrix metalloproteinases (MMPs) have not been considered as key players in EMT but rather studied for their role in matrix remodelling in later events such as cell migration per se. Here, we used Xenopus neural crest cells to assess the role of MMP28 in EMT and migration in vivo. We show that a catalytically active MMP28, expressed by neighbouring placodal cells, is required for neural crest EMT and cell migration. We provide strong evidence indicating that MMP28 is imported in the nucleus of neural crest cells where it is required for normal Twist expression. Our data demonstrate that MMP28 can act as an upstream regulator of EMT in vivo raising the possibility that other MMPs might have similar early roles in various EMT-related contexts such as cancer, fibrosis, and wound healing.

Droplet duos on water display pairing, autonomous motion, and periodic eruption

Under non-equilibrium conditions, liquid droplets dynamically couple with their milieu through the continuous flux of matter and energy, forming active systems capable of self-organizing functions reminiscent of those of living organisms. Among the various dynamic behaviors demonstrated by cells, the pairing of heterogeneous cell units is necessary to enable collective activity and cell fusion (to reprogram somatic cells). Furthermore, the cyclic occurrence of eruptive events such as necroptosis or explosive cell lysis is necessary to maintain cell functions. However, unlike the self-propulsion behavior of cells, cyclic cellular behavior involving pairing and eruption has not been successfully modeled using artificial systems. Here, we show that a simple droplet system based on quasi-immiscible hydrophobic oils (perfluorodecalin and decane) deposited on water, mimics such complex cellular dynamics. Perfluorodecalin and decane droplet duos form autonomously moving Janus or coaxial structures, depending on their volumes. Notably, the system with a coaxial structure demonstrates cyclic behavior, alternating between autonomous motion and eruption. Despite their complexity, the dynamic behaviors of the system are consistently explained in terms of the spreading properties of perfluorodecalin/decane duplex interfacial films.

Homozygous CDH2 variant may be associated with hypopituitarism without neurological disorders

Context

Congenital hypopituitarism is a genetically heterogeneous condition. Whole exome sequencing (WES) is a promising approach for molecular diagnosis of patients with this condition.

Objectives

The aim of this study is to conduct WES in a patient with congenital hypopituitarism born to consanguineous parents, CDH2 screening in a cohort of patients with congenital hypopituitarism, and functional testing of a novel CDH2 variant.

Design

Genomic DNA from a proband and her consanguineous parents was analyzed by WES. Copy number variants were evaluated. The genetic variants were filtered for population frequency (ExAC, 1000 genomes, gnomAD, and ABraOM), in silico prediction of pathogenicity, and gene expression in the pituitary and/or hypothalamus. Genomic DNA from 145 patients was screened for CDH2 by Sanger sequencing.

Results

One female patient with deficiencies in growth hormone, thyroid-stimulating hormone, adrenocorticotropic hormone, luteinizing hormone, and follicle-stimulating hormone and ectopic posterior pituitary gland contained a rare homozygous c.865G>A (p.Val289Ile) variant in CDH2. To determine whether the p.Val289Ile variant in CDH2 affects cell adhesion properties, we stably transfected L1 fibroblast lines, labeled the cells with lipophilic dyes, and quantified aggregation. Large aggregates formed in cells expressing wildtype CDH2, but aggregation was impaired in cells transfected with variant CDH2 or non-transfected.

Conclusion

A homozygous CDH2 allelic variant was found in one hypopituitarism patient, and the variant impaired cell aggregation function in vitro. No disease-causing variants were found in 145 other patients screened for CDH2 variants. Thus, CDH2 is a candidate gene for hypopituitarism that needs to be tested in different populations.

Significance statement

A female patient with hypopituitarism was born from consanguineous parents and had a homozygous, likely pathogenic, CDH2 variant that impairs cell aggregation in vitro. No other likely pathogenic variants in CDH2 were identified in 145 hypopituitarism patients.

Joint representation: Modeling a phenomenon with multiple biological systems

Biologists often study particular biological systems as models of a phenomenon of interest even if they already know that the phenomenon is produced by diverse mechanisms and hence none of those systems alone can sufficiently represent it. To understand this modeling practice, the present paper provides an account of how multiple model systems can be used to study a phenomenon that is produced by diverse mechanisms. Even if generalizability of results from a single model system is significantly limited, generalizations concerning specific aspects of mechanisms often hold across certain ranges of biological systems, which enables multiple model systems to jointly represent such a phenomenon. Comparing mechanisms that operate in different biological systems as examples of the same phenomenon also facilitates characterization and investigation of individual mechanisms. I also compare my account with two existing accounts of the use of multiple model systems and argue that my account is distinct from and complementary to them.

Neural crest E-cadherin loss drives cleft lip/palate by epigenetic modulation via pro-inflammatory gene-environment interaction

Gene-environment interactions are believed to play a role in multifactorial phenotypes, although poorly described mechanistically. Cleft lip/palate (CLP), the most common craniofacial malformation, has been associated with both genetic and environmental factors, with little gene-environment interaction experimentally demonstrated. Here, we study CLP families harbouring CDH1/E-Cadherin variants with incomplete penetrance and we explore the association of pro-inflammatory conditions to CLP. By studying neural crest (NC) from mouse, Xenopus and humans, we show that CLP can be explained by a 2-hit model, where NC migration is impaired by a combination of genetic (CDH1 loss-of-function) and environmental (pro-inflammatory activation) factors, leading to CLP. Finally, using in vivo targeted methylation assays, we demonstrate that CDH1 hypermethylation is the major target of the pro-inflammatory response, and a direct regulator of E-cadherin levels and NC migration. These results unveil a gene-environment interaction during craniofacial development and provide a 2-hit mechanism to explain cleft lip/palate aetiology.

Colec12 and Trail signaling confine cranial neural crest cell trajectories and promote collective cell migration

BACKGROUND

Collective and discrete neural crest cell (NCC) migratory streams are crucial to vertebrate head patterning. However, the factors that confine NCC trajectories and promote collective cell migration remain unclear.

RESULTS

Computational simulations predicted that confinement is required only along the initial one-third of the cranial NCC migratory pathway. This guided our study of Colec12 (Collectin-12, a transmembrane scavenger receptor C-type lectin) and Trail (tumor necrosis factor-related apoptosis-inducing ligand, CD253) which we show expressed in chick cranial NCC-free zones. NCC trajectories are confined by Colec12 or Trail protein stripes in vitro and show significant and distinct changes in cell morphology and dynamic migratory characteristics when cocultured with either protein. Gain- or loss-of-function of either factor or in combination enhanced NCC confinement or diverted cell trajectories as observed in vivo with three-dimensional confocal microscopy, respectively, resulting in disrupted collective migration.

CONCLUSIONS

These data provide evidence for Colec12 and Trail as novel NCC microenvironmental factors playing a role to confine cranial NCC trajectories and promote collective cell migration.

DUX4 double whammy: The transcription factor that causes a rare muscular dystrophy also kills the precursors of the human nose

SMCHD1 mutations cause congenital arhinia (absent nose) and a muscular dystrophy called FSHD2. In FSHD2, loss of SMCHD1 repressive activity causes expression of double homeobox 4 (DUX4) in muscle tissue, where it is toxic. Studies of arhinia patients suggest a primary defect in nasal placode cells (human nose progenitors). Here, we show that upon SMCHD1 ablation, DUX4 becomes derepressed in H9 human embryonic stem cells (hESCs) as they differentiate toward a placode cell fate, triggering cell death. Arhinia and FSHD2 patient-derived induced pluripotent stem cells (iPSCs) express DUX4 when converted to placode cells and demonstrate variable degrees of cell death, suggesting an environmental disease modifier. HSV-1 may be one such modifier as herpesvirus infection amplifies DUX4 expression in SMCHD1 KO hESC and patient iPSC. These studies suggest that arhinia, like FSHD2, is due to compromised SMCHD1 repressive activity in a cell-specific context and provide evidence for an environmental modifier.

Gibbs process distinguishes survival and reveals contact-inhibition genes in Glioblastoma multiforme

Tumor growth is a spatiotemporal birth-and-death process with loss of heterotypic contact-inhibition of locomotion (CIL) of tumor cells promoting invasion and metastasis. Therefore, representing tumor cells as two-dimensional points, we can expect the tumor tissues in histology slides to reflect realizations of spatial birth-and-death process which can be mathematically modeled to reveal molecular mechanisms of CIL, provided the mathematics models the inhibitory interactions. Gibbs process as an inhibitory point process is a natural choice since it is an equilibrium process of the spatial birth-and-death process. That is if the tumor cells maintain homotypic contact inhibition, the spatial distributions of tumor cells will result in Gibbs hard core process over long time scales. In order to verify if this is the case, we applied the Gibbs process to 411 TCGA Glioblastoma multiforme patient images. Our imaging dataset included all cases for which diagnostic slide images were available. The model revealed two groups of patients, one of which - the "Gibbs group," showed the convergence of the Gibbs process with significant survival difference. Further smoothing the discretized (and noisy) inhibition metric, for both increasing and randomized survival time, we found a significant association of the patients in the Gibbs group with increasing survival time. The mean inhibition metric also revealed the point at which the homotypic CIL establishes in tumor cells. Besides, RNAseq analysis between patients with loss of heterotypic CIL and intact homotypic CIL in the Gibbs group unveiled cell movement gene signatures and differences in Actin cytoskeleton and RhoA signaling pathways as key molecular alterations. These genes and pathways have established roles in CIL. Taken together, our integrated analysis of patient images and RNAseq data provides for the first time a mathematical basis for CIL in tumors, explains survival as well as uncovers the underlying molecular landscape for this key tumor invasion and metastatic phenomenon.

Neural crest cells development and neuroblastoma progression: Role of Wnt signaling

Neuroblastoma (NB) is one of the most common heterogeneous extracranial cancers in infancy that arises from neural crest (NC) cells of the sympathetic nervous system. The Wnt signaling pathway, both canonical and noncanonical pathway, is a highly conserved signaling pathway that regulates the development and differentiation of the NC cells during embryogenesis. Reports suggest that aberrant activation of Wnt ligands/receptors in Wnt signaling pathways promote progression and relapse of NB. Wnt signaling pathways regulate NC induction and migration in a similar manner; it regulates proliferation and metastasis of NB. Inhibiting the Wnt signaling pathway or its ligands/receptors induces apoptosis and abrogates proliferation and tumorigenicity in all major types of NB cells. Here, we comprehensively discuss the Wnt signaling pathway and its mechanisms in regulating the development of NC and NB pathogenesis. This review highlights the implications of aberrant Wnt signaling in the context of etiology, progression, and relapse of NB. We have also described emerging strategies for Wnt-based therapies against the progression of NB that will provide new insights into the development of Wnt-based therapeutic strategies for NB.

Characteristic tetraspanin expression patterns mark various tissues during early Xenopus development

The tetraspanins (Tspans) constitute a family of cell surface proteins with four transmembrane domains. Tspans have been found on the plasma membrane and on exosomes of various organelles. Reports on the function of Tspans during the early development of Xenopus have mainly focused on the expression of uroplakins in gametes. Although the roles of extracellular vesicles (EVs) including exosomes have been actively analyzed in cancer research, the contribution of EVs to early development is not well understood. This is because the diffusivity of EVs is not compatible with a very strict developmental process. In this study, we analyzed members of the Tspan family in early development of Xenopus. Expression was prominent in specific organs such as the notochord, eye, cranial neural crest cells (CNCs), trunk neural crest cells, placodes, and somites. We overexpressed several combinations of Tspans in CNCs in vitro and in vivo. Changing the partner changed the distribution of fluorescent-labeled Tspans. Therefore, it is suggested that expression of multiple Tspans in a particular tissue might produce heterogeneity of intercellular communication, which has not yet been recognized.

Quantitative Experimental Embryology: A Modern Classical Approach

Experimental Embryology is often referred to as a classical approach of developmental biology that has been to some extent replaced by the introduction of molecular biology and genetic techniques to the field. Inspired by the combination of this approach with advanced techniques to uncover core principles of neural crest development by the laboratory of Roberto Mayor, we review key quantitative examples of experimental embryology from recent work in a broad range of developmental biology questions. We propose that quantitative experimental embryology offers essential ways to explore the reaction of cells and tissues to targeted cell addition, removal, and confinement. In doing so, it is an essential methodology to uncover principles of development that remain elusive such as pattern regulation, scaling, and self-organisation.

The cerebrofacial metameric syndromes: An embryological review and proposal of a novel classification scheme

The cerebrofacial metameric syndromes are a group of congenital syndromes that result in vascular malformations throughout specific anatomical distributions of the brain, cranium and face. Multiple reports of patients with high-flow or low-flow vascular malformations following a metameric distribution have supported this idea. There has been much advancement in understanding of segmental organization and cell migration since the concept of metameric vascular syndromes was first proposed. We aim to give an updated review of these embryological considerations and then propose a more detailed classification system for these syndromes, predominately incorporating the contribution of neural crest cells and somitomeres to the pharyngeal arches.

Emerging concepts on the mechanical interplay between migrating cells and microenvironment in vivo

During embryogenesis, tissues develop into elaborate collectives through a myriad of active mechanisms, with cell migration being one of the most common. As cells migrate, they squeeze through crowded microenvironments to reach the positions where they ultimately execute their function. Much of our knowledge of cell migration has been based on cells' ability to navigate in vitro and how cells respond to the mechanical properties of the extracellular matrix (ECM). These simplified and largely passive surroundings contrast with the complexity of the tissue environments in vivo, where different cells and ECM make up the milieu cells migrate in. Due to this complexity, comparatively little is known about how the physical interactions between migrating cells and their tissue environment instruct cell movement in vivo. Work in different model organisms has been instrumental in addressing this question. Here, we explore various examples of cell migration in vivo and describe how the physical interplay between migrating cells and the neighboring microenvironment controls cell behavior. Understanding this mechanical cooperation in vivo will provide key insights into organ development, regeneration, and disease.

Pairing-induced motion of source and inert particles driven by surface tension

We experimentally and theoretically investigate systems with a pair of source and inert particles that interact through a concentration field. The experimental system comprises a camphor disk as the source particle and a metal washer as the inert particle. Both are floated on an aqueous solution of glycerol at various concentrations, where the glycerol modifies the viscosity of the aqueous phase. The particles form a pair owing to the attractive lateral capillary force. As the camphor disk spreads surface-active molecules at the aqueous surface, the camphor disk and metal washer move together, driven by the surface tension gradient. The washer is situated in the front of the camphor disk, keeping the distance constant during their motion, which we call a pairing-induced motion. The pairing-induced motion exhibited a transition between circular and straight motions as the glycerol concentration in the aqueous phase changed. Numerical calculations using a model that considers forces caused by the surface tension gradient and lateral capillary interaction reproduced the observed transition in the pairing-induced motion. Moreover, this transition agrees with the result of the linear stability analysis on the reduced dynamical system obtained by the expansion with respect to the particle velocity. Our results reveal that the effect of the particle velocity cannot be overlooked to describe the interaction through the concentration field.

Neural crest mechanosensors: Seeing old proteins in a new light

Mechanical forces exerted on neural crest cells control their collective migration and differentiation. This perspective discusses our current understanding of neural crest mechanotransduction during cell migration and differentiation. Additionally, we describe proteins that have mechanosensitive functions in other systems, such as mechanosensitive G-protein-coupled receptors, mechanosensitive ion channels, cell-cell adhesion, and cell-matrix-interacting proteins, and highlight that these same proteins have in the past been studied in neural crest development from a purely signaling point of view. We propose that future studies elucidate the mechanosensitive functions these receptors may play in neural crest development and integrate this with their known molecular role.

Neural crest cell-placodal neuron interactions are mediated by Cadherin-7 and N-cadherin during early chick trigeminal ganglion assembly

Background: Arising at distinct positions in the head, the cranial ganglia are crucial for integrating various sensory inputs. The largest of these ganglia is the trigeminal ganglion, which relays pain, touch and temperature information through its three primary nerve branches to the central nervous system. The trigeminal ganglion and its nerves are composed of derivatives of two critical embryonic cell types, neural crest cells and placode cells, that migrate from different anatomical locations, coalesce together, and differentiate to form trigeminal sensory neurons and supporting glia. While the dual cellular origin of the trigeminal ganglion has been known for over 60 years, molecules expressed by neural crest cells and placode cells that regulate initial ganglion assembly remain obscure. Prior studies revealed the importance of cell surface cadherin proteins during early trigeminal gangliogenesis, with Cadherin-7 and neural cadherin (N-cadherin) expressed in neural crest cells and placode cells, respectively. Although cadherins typically interact in a homophilic ( i.e., like) fashion, the presence of different cadherins on these intermingling cell populations raises the question as to whether heterophilic cadherin interactions may also be occurring during initial trigeminal ganglion formation, which was the aim of this study. Methods: To assess potential interactions between Cadherin-7 and N-cadherin, we used biochemistry and innovative imaging assays conducted in vitro and in vivo, including in the forming chick trigeminal ganglion. Results: Our data revealed a physical interaction between Cadherin-7 and N-cadherin. Conclusions: These studies identify a new molecular basis by which neural crest cells and placode cells can aggregate in vivo to build the trigeminal ganglion during embryogenesis.

Neural crest cell-placodal neuron interactions are mediated by Cadherin-7 and N-cadherin during early chick trigeminal ganglion assembly

Background: Arising at distinct positions in the head, the cranial ganglia are crucial for integrating various sensory inputs. The largest of these ganglia is the trigeminal ganglion, which relays pain, touch and temperature information through its three primary nerve branches to the central nervous system. The trigeminal ganglion and its nerves are composed of derivatives of two critical embryonic cell types, neural crest cells and placode cells, that migrate from different anatomical locations, coalesce together, and differentiate to form trigeminal sensory neurons and supporting glia. While the dual cellular origin of the trigeminal ganglion has been known for over 60 years, molecules expressed by neural crest cells and placode cells that regulate initial ganglion assembly remain obscure. Prior studies revealed the importance of cell surface cadherin proteins during early trigeminal gangliogenesis, with Cadherin-7 and neural cadherin (N-cadherin) expressed in neural crest cells and placode cells, respectively. Although cadherins typically interact in a homophilic ( i.e., like) fashion, the presence of different cadherins on these intermingling cell populations raises the question as to whether heterophilic cadherin interactions may also be occurring during initial trigeminal ganglion formation, which was the aim of this study. Methods: To assess potential interactions between Cadherin-7 and N-cadherin, we used biochemistry and innovative imaging assays conducted in vitro and in vivo, including in the forming chick trigeminal ganglion. Results: Our data revealed a physical interaction between Cadherin-7 and N-cadherin. Conclusions: These studies identify a new molecular basis by which neural crest cells and placode cells can aggregate in vivo to build the trigeminal ganglion during embryogenesis.

Neural crest cell-placodal neuron interactions are mediated by Cadherin-7 and N-cadherin during early chick trigeminal ganglion assembly

Background: Arising at distinct positions in the head, the cranial ganglia are crucial for integrating various sensory inputs. The largest of these ganglia is the trigeminal ganglion, which relays pain, touch and temperature information through its three primary nerve branches to the central nervous system. The trigeminal ganglion and its nerves are composed of derivatives of two critical embryonic cell types, neural crest cells and placode cells, that migrate from different anatomical locations, coalesce together, and differentiate to form trigeminal sensory neurons and supporting glia. While the dual cellular origin of the trigeminal ganglion has been known for over 60 years, molecules expressed by neural crest cells and placode cells that regulate initial ganglion assembly remain obscure. Prior studies revealed the importance of cell surface cadherin proteins during early trigeminal gangliogenesis, with Cadherin-7 and neural cadherin (N-cadherin) expressed in neural crest cells and placode cells, respectively. Although cadherins typically interact in a homophilic ( i.e., like) fashion, the presence of different cadherins expressed in neural crest cells and placode cells raises the question as to whether heterophilic cadherin interactions may also be occurring. Given this, the aim of the study was to understand whether Cadherin-7 and N-cadherin were interacting during initial trigeminal ganglion formation. Methods: To assess potential interactions between Cadherin-7 and N-cadherin, we used biochemistry and innovative imaging assays conducted in vitro and in vivo, including in the forming chick trigeminal ganglion. Results: Our data revealed a physical interaction between Cadherin-7 and N-cadherin. Conclusions: These studies identify a new molecular basis by which neural crest cells and placode cells can aggregate in vivo to build the trigeminal ganglion during embryogenesis.

Cellular plasticity in the neural crest and cancer

In vertebrates, neural crest cells (NCCs) are a multipotent embryonic population generating both neural/neuronal and mesenchymal derivatives, and thus the neural crest (NC) is often referred to as the fourth germ layer. NC development is a dynamic process, where NCCs possess substantial plasticity in transcriptional and epigenomic profiles. Recent technical advances in single-cell and low-input sequencing have empowered fine-resolution characterisation of NC development. In this review, we summarise the latest models underlying NC-plasticity acquirement and cell-fate restriction, outline the connections between NC plasticity and NC-derived cancer and envision the new opportunities in studying NC plasticity and its link to cancer.

BMP-gated cell-cycle progression drives anoikis during mesenchymal collective migration

Tissue homeostasis involves the elimination of abnormal cells to avoid compromised patterning and function. Although quality control through cell competition is well studied in epithelial tissues, it is unknown if and how homeostasis is regulated in mesenchymal collectives. Here, we demonstrate that collectively migrating Drosophila muscle precursors utilize both fibroblast growth factor (FGF) and bone morphogenetic protein (BMP) signaling to promote homeostasis via anoikis, a form of cell death in response to substrate de-adhesion. Cell-cycle-regulated expression of the cell death gene head involution defective is responsible for caudal visceral mesoderm (CVM) anoikis. The secreted BMP ligand drives cell-cycle progression via a visceral mesoderm-specific cdc25/string enhancer to synchronize collective proliferation, as well as apoptosis of cells that have lost access to substrate-derived FGF. Perturbation of BMP-dependent cell-cycle progression is sufficient to confer anoikis resistance to mismigrating cells and thus facilitate invasion of other tissues. This BMP-gated cell-cycle checkpoint defines a quality control mechanism during mesenchymal collective migration.

Active nonreciprocal attraction between motile particles in an elastic medium

We show from experiments and simulations on vibration-activated granular matter that self-propelled polar rods in an elastic medium on a substrate turn and move towards each other. We account for this effective attraction through a coarse-grained theory of a motile particle as a moving point-force density that creates elastic strains in the medium that reorient other particles. Our measurements confirm qualitatively the predicted features of the distortions created by the rods, including the |x|^{-1/2} tail of the trailing displacement field and nonreciprocal sensing and pursuit. A discrepancy between the magnitudes of displacements along and transverse to the direction of motion remains. Our theory should be of relevance to the interaction of motile cells in the extracellular matrix or in a supported layer of gel or tissue.

Directing with restraint: Mechanisms of protrusion restriction in collective cell migrations

Cell migration is necessary for morphogenesis, tissue homeostasis, wound healing and immune response. It is also involved in diseases. In particular, cell migration is inherent in metastasis. Cells can migrate individually or in groups. To migrate efficiently, cells need to be able to organize into a leading front that protrudes by forming membrane extensions and a trailing edge that contracts. This organization is scaled up at the group level during collective cell movements. If a cell or a group of cells is unable to limit its leading edge and hence to restrict the formation of protrusions to the front, directional movements are impaired or abrogated. Here we summarize our current understanding of the mechanisms restricting protrusion formation in collective cell migration. We focus on three in vivo examples: the neural crest cell migration, the rotatory migration of follicle cells around the Drosophila egg chamber and the border cell migration during oogenesis.

The mechanical forces that shape our senses

Developing organs are shaped, in part, by physical interaction with their environment in the embryo. In recent years, technical advances in live-cell imaging and material science have greatly expanded our understanding of the mechanical forces driving organ formation. Here, we provide a broad overview of the types of forces generated during embryonic development and then focus on a subset of organs underlying our senses: the eyes, inner ears, nose and skin. The epithelia in these organs emerge from a common origin: the ectoderm germ layer; yet, they arrive at unique and complex forms over developmental time. We discuss exciting recent animal studies that show a crucial role for mechanical forces in, for example, the thickening of sensory placodes, the coiling of the cochlea and the lengthening of hair. Finally, we discuss how microfabricated organoid systems can now provide unprecedented insights into the physical principles of human development.

Mechanical transmission enables EMT cancer cells to drive epithelial cancer cell migration to guide tumor spheroid disaggregation

Cell phenotype heterogeneity within tumor tissue, especially which due to the emergence of epithelial-mesenchymal transition (EMT) in cancer cells, is associated with cancer invasion and metastasis. However, our understanding of the cellular mechanism(s) underlying the cooperation between EMT cell and epithelial cancer cell migration remains incomplete. Herein, heterotypic tumor spheroids containing both epithelial and EMT cancer cells were generated in vitro. We observed that EMT cells dominated the peripheral region of the self-organized heterotypic tumor spheroid. Furthermore, our results demonstrated that EMT cells could serve as leader cells to improve the collective migration efficiency of epithelial cancer cells and promote dispersion and invasion of the tumor spheroids, which was regulated by the force transition between EMT cells and epithelial cancer cells. Mechanistically, our data further suggest that force transmission is mediated by heterophilic N-cadherin/E-cadherin adhesion complexes between EMT and epithelial cancer cells. Impairment of N-cadherin/E-cadherin adhesion complex formation abrogated the ability of EMT cells to guide epithelial cancer cell migration and blocked the dispersion of tumor spheroids. Together, our data provide new insight into the mechanical interaction between epithelial and EMT cancer cells through heterophilic cadherin adhesion, which enables cooperative tumor cell migration, highlighting the role of EMT cells in tumor invasion.

Intertissue mechanical interactions shape the olfactory circuit in zebrafish

While the chemical signals guiding neuronal migration and axon elongation have been extensively studied, the influence of mechanical cues on these processes remains poorly studied in vivo. Here, we investigate how mechanical forces exerted by surrounding tissues steer neuronal movements and axon extension during the morphogenesis of the olfactory placode in zebrafish. We mainly focus on the mechanical contribution of the adjacent eye tissue, which develops underneath the placode through extensive evagination and invagination movements. Using quantitative analysis of cell movements and biomechanical manipulations, we show that the developing eye exerts lateral traction forces on the olfactory placode through extracellular matrix, mediating proper morphogenetic movements and axon extension within the placode. Our data shed new light on the key participation of intertissue mechanical interactions in the sculpting of neuronal circuits.