Division of labor during trunk neural crest development

Transcriptome analysis revealed the mechanism of skeletal muscle growth and development in different hybrid sheep

OBJECTIVE

This study aimed to analyze the molecular mechanism of heterosis in East Friesian sheep×Hu sheep (EH) hybrid sheep and Suffolk×EH (SHE) hybrid sheep (Ovis aries).

METHODS

In this research, the growth performance data of Hu sheep (H), EH and SHE from birth to 8 months of age were analyzed. Three 8-month-old sheep of each of the three strains (9 sheep in total) were chosen and their longissimus dorsi muscles were collected for transcriptome sequencing. We verified the expression of seven differentially expressed genes (DEGs) by real-time quantitative reverse transcription polymerase chain reaction (RT-qPCR).

RESULTS

The results showed: (1) body weight and chest circumference of EH were significantly greater than H (p<0.05), except at 4 months of age. Body weight and chest circumference of SHE was significantly higher than EH (p<0.05), except at 6 months of age. (2) 310 DEGs were screened in the EH and H, gene ontology and Kyoto encyclopedia of genes and genomes showed DEGs were mainly concentrate on the categories of actin cytoskeleton, calcium binding, cGMP-PKG and mitogen-activated protein kinase (MAPK) signaling pathway, which correlating the development of skeletal muscle and energy metabolism. 329 DEGs were screened in the SHE and EH. DEGs were mainly enriched in extracellular matrix-receptor interactions and cell adhesion molecules. (3) Protein-protein interaction screening yielded five (MYL2, TNNI1, TNNI3, MYH11, TNNC1) and three (SOX10, COL2A1, MPZ) pivotal DEGs regulating muscle development in EH and SHE. (4) RT-qPCR test results were consistent with transcriptome sequencing.

CONCLUSION

This study provides candidate genes for improving sheep growth traits. It provides a theoretical basis for analyzing the mechanism of muscle development in crossbred sheep.

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.

Peripheral nerve development in zebrafish requires muscle patterning by tcf15/paraxis

The vertebrate peripheral nervous system (PNS) is an intricate network that conveys sensory and motor information throughout the body. During development, extracellular cues direct the migration of axons and glia through peripheral tissues. Currently, the suite of molecules that govern PNS axon-glial patterning is incompletely understood. To elucidate factors that are critical for peripheral nerve development, we characterized the novel zebrafish mutant, stl159, that exhibits abnormalities in PNS patterning. In these mutants, motor and sensory nerves that develop adjacent to axial muscle fail to extend normally, and neuromasts in the posterior lateral line system, as well as neural crest-derived melanocytes, are incorrectly positioned. The stl159 genetic lesion lies in the basic helix-loop-helix (bHLH) transcription factor tcf15, which has been previously implicated in proper development of axial muscles. We find that targeted loss of tcf15 via CRISPR-Cas9 genome editing results in the PNS patterning abnormalities observed in stl159 mutants. Because tcf15 is expressed in developing muscle prior to nerve extension, rather than in neurons or glia, we predict that tcf15 non-cell-autonomously promotes peripheral nerve patterning in zebrafish through regulation of extracellular patterning cues. Our work underscores the importance of muscle-derived factors in PNS development.

Environmental cues from neural crest derivatives act as metastatic triggers in an embryonic neuroblastoma model

Embryonic malignant transformation is concomitant to organogenesis, often affecting multipotent and migratory progenitors. While lineage relationships between malignant cells and their physiological counterparts are extensively investigated, the contribution of exogenous embryonic signals is not fully known. Neuroblastoma (NB) is a childhood malignancy of the peripheral nervous system arising from the embryonic trunk neural crest (NC) and characterized by heterogeneous and interconvertible tumor cell identities. Here, using experimental models mimicking the embryonic context coupled to proteomic and transcriptomic analyses, we show that signals released by embryonic sympathetic ganglia, including Olfactomedin-1, induce NB cells to shift from a noradrenergic to mesenchymal identity, and to activate a gene program promoting NB metastatic onset and dissemination. From this gene program, we extract a core signature specifically shared by metastatic cancers with NC origin. This reveals non-cell autonomous embryonic contributions regulating the plasticity of NB identities and setting pro-dissemination gene programs common to NC-derived cancers.

Notch controls the cell cycle to define leader versus follower identities during collective cell migration

Coordination of cell proliferation and migration is fundamental for life, and its dysregulation has catastrophic consequences, such as cancer. How cell cycle progression affects migration, and vice versa, remains largely unknown. We address these questions by combining in silico modelling and in vivo experimentation in the zebrafish trunk neural crest (TNC). TNC migrate collectively, forming chains with a leader cell directing the movement of trailing followers. We show that the acquisition of migratory identity is autonomously controlled by Notch signalling in TNC. High Notch activity defines leaders, while low Notch determines followers. Moreover, cell cycle progression is required for TNC migration and is regulated by Notch. Cells with low Notch activity stay longer in G1 and become followers, while leaders with high Notch activity quickly undergo G1/S transition and remain in S-phase longer. In conclusion, TNC migratory identities are defined through the interaction of Notch signalling and cell cycle progression.

The embryonic trunk neural crest microenvironment regulates the plasticity and invasion of human neuroblastoma via TrkB signaling

Mistakes in trunk neural crest (NC) cell migration may lead to birth defects of the sympathetic nervous system (SNS) and neuroblastoma (NB) cancer. Receptor tyrosine kinase B (TrkB) and its ligand BDNF critically regulate NC cell migration during normal SNS development and elevated expression of TrkB is correlated with high-risk NB patients. However, in the absence of a model with in vivo interrogation of human NB cell and gene expression dynamics, the mechanistic role of TrkB in NB disease progression remains unclear. Here, we study the functional relationship between TrkB, cell invasion and plasticity of human NB cells by taking advantage of our validated in vivo chick embryo transplant model. We find that LAN5 (high TrkB) and SHSY5Y (moderate TrkB) human NB cells aggressively invade host embryos and populate typical NC targets, however loss of TrkB function significantly reduces cell invasion. In contrast, NB1643 (low TrkB) cells remain near the transplant site, but over-expression of TrkB leads to significant cell invasion. Invasive NB cells show enhanced expression of genes indicative of the most invasive host NC cells. In contrast, transplanted human NB cells down-regulate known NB tumor initiating and stem cell markers. Human NB cells that remain within the dorsal neural tube transplant also show enhanced expression of cell differentiation genes, resulting in an improved disease outcome as predicted by a computational algorithm. These in vivo data support TrkB as an important biomarker and target to control NB aggressiveness and identify the chick embryonic trunk neural crest microenvironment as a source of signals to drive NB to a less aggressive state, likely acting at the dorsal neural tube.

Development of the sympathetic trunks in human embryos

Although the development of the sympathetic trunks was first described >100 years ago, the topographic aspect of their development has received relatively little attention. We visualised the sympathetic trunks in human embryos of 4.5-10 weeks post-fertilisation, using Amira 3D-reconstruction and Cinema 4D-remodelling software. Scattered, intensely staining neural crest-derived ganglionic cells that soon formed longitudinal columns were first seen laterally to the dorsal aorta in the cervical and upper thoracic regions of Carnegie stage (CS)14 embryos. Nerve fibres extending from the communicating branches with the spinal cord reached the trunks at CS15-16 and became incorporated randomly between ganglionic cells. After CS18, ganglionic cells became organised as irregular agglomerates (ganglia) on a craniocaudally continuous cord of nerve fibres, with dorsally more ganglionic cells and ventrally more fibres. Accordingly, the trunks assumed a "pearls-on-a-string" appearance, but size and distribution of the pearls were markedly heterogeneous. The change in position of the sympathetic trunks from lateral (para-aortic) to dorsolateral (prevertebral or paravertebral) is a criterion to distinguish the "primary" and "secondary" sympathetic trunks. We investigated the position of the trunks at vertebral levels T2, T7, L1 and S1. During CS14, the trunks occupied a para-aortic position, which changed into a prevertebral position in the cervical and upper thoracic regions during CS15, and in the lower thoracic and lumbar regions during CS18 and CS20, respectively. The thoracic sympathetic trunks continued to move further dorsally and attained a paravertebral position at CS23. The sacral trunks retained their para-aortic and prevertebral position, and converged into a single column in front of the coccyx. Based on our present and earlier morphometric measurements and literature data, we argue that differential growth accounts for the regional differences in position of the sympathetic trunks.

AKT Signaling Modifies the Balance between Cell Proliferation and Migration in Neural Crest Cells from Patients Affected with Bosma Arhinia and Microphthalmia Syndrome

Over the recent years, the SMCHD1 (Structural Maintenance of Chromosome flexible Hinge Domain Containing 1) chromatin-associated factor has triggered increasing interest after the identification of variants in three rare and unrelated diseases, type 2 Facio Scapulo Humeral Dystrophy (FSHD2), Bosma Arhinia and Microphthalmia Syndrome (BAMS), and the more recently isolated hypogonadotrophic hypogonadism (IHH) combined pituitary hormone deficiency (CPHD) and septo-optic dysplasia (SOD). However, it remains unclear why certain mutations lead to a specific muscle defect in FSHD while other are associated with severe congenital anomalies. To gain further insights into the specificity of SMCHD1 variants and identify pathways associated with the BAMS phenotype and related neural crest defects, we derived induced pluripotent stem cells from patients carrying a mutation in this gene. We differentiated these cells in neural crest stem cells and analyzed their transcriptome by RNA-Seq. Besides classical differential expression analyses, we analyzed our data using MOGAMUN, an algorithm allowing the extraction of active modules by integrating differential expression data with biological networks. We found that in BAMS neural crest cells, all subnetworks that are associated with differentially expressed genes converge toward a predominant role for AKT signaling in the control of the cell proliferation-migration balance. Our findings provide further insights into the distinct mechanism by which defects in neural crest migration might contribute to the craniofacial anomalies in BAMS.

Development of the Vertebrate Trunk Sensory System: Origins, Specification, Axon Guidance, and Central Connectivity

Crucial to an animal's movement through their environment and to the maintenance of their homeostatic physiology is the integration of sensory information. This is achieved by axons communicating from organs, muscle spindles and skin that connect to the sensory ganglia composing the peripheral nervous system (PNS), enabling organisms to collect an ever-constant flow of sensations and relay it to the spinal cord. The sensory system carries a wide spectrum of sensory modalities - from sharp pain to cool refreshing touch - traveling from the periphery to the spinal cord via the dorsal root ganglia (DRG). This review covers the origins and development of the DRG and the cells that populate it, and focuses on how sensory connectivity to the spinal cord is achieved by the diverse developmental and molecular processes that control axon guidance in the trunk sensory system. We also describe convergences and differences in sensory neuron formation among different vertebrate species to gain insight into underlying developmental mechanisms.

Tracking the movement of individual avian neural crest cells in vitro

The origin, migratory pathways and adult derivatives of neural crest cells (NCCs) are well known. However, less is known about how these cells migrate. In this study, in a laboratory based in a low-resource setting, a hanging drop culture assay was utilised to study the movement of individual avian trunk neural crest cells. Mode of migration by means of lamellipodia and filopodia was studied in live cell cultures with a laser scanning confocal microscope and Airyscan module. Both distance migrated and speed of migration were calculated. NCCs migrated in a chain soon after emerging from the explanted neural tube, but were more dispersed and had random movements when they reached the periphery of the culture. While the distances travelled by these NCCs were less and the cells were slower on gelatine than on other extracellular matrices reported in the literature, the assay afforded detailed observation of actin filament distribution and cytoplasmic protrusions. The study has provided unique evidence of individual NCC movements in vitro, in a simple hanging drop assay optimized for the study of NCCs. The assay could be used for further analysis of the behaviour of NCCs on different extracellular matrices or with targeted action.

Bottom up proteomics identifies neuronal differentiation pathway networks activated by cathepsin inhibition treatment in neuroblastoma cells that are enhanced by concurrent 13-cis retinoic acid treatment

Neuroblastoma is the second most common pediatric cancer involving the peripheral nervous system in which stage IVS metastatic tumors regress due to spontaneous differentiation. 13-cis retinoic acid (13-cis RA) is currently used in the clinic for its differentiation effects and although it improves outcomes, relapse is seen in half of high-risk patients. Combinatorial therapies have been shown to be more effective in oncotherapy and since cathepsin inhibition reduces tumor growth, we explored the potential of coupling 13-cis RA with a cathepsin inhibitor (K777) to enhance therapeutic efficacy against neuroblastoma. Shotgun proteomics was used to identify proteins affected by K777 and dual (13-cis RA/K777) treatment in neuroblastoma SK-N-SH cells. Cathepsin inhibition was more effective in increasing proteins involved in neuronal differentiation and neurite outgrowth than 13-cis RA alone, but the combination of both treatments enhanced the neuronal differentiation effect. SIGNIFICANCE: As neuroblastoma can spontaneously differentiate, determining which proteins are involved in differentiation can guide development of more accurate diagnostic markers and more effective treatments. In this study, we established a differentiation proteomic map of SK-N-SH cells treated with a cathepsin inhibitor (K777) and K777/13-cis RA (dual). Bioinformatic analysis revealed these treatments enhanced neuronal differentiation and axonogenesis pathways. The most affected proteins in these pathways may become valuable biomarkers of efficacy of drugs designed to enhance differentiation of neuroblastoma [1].

The gap junction protein connexin 43 controls multiple aspects of cranial neural crest cell development

Gap junctions are intercellular channels between cells that facilitate cell-cell communication. Connexin 43 (Cx43; also known as GJA1), the predominant gap junction protein in vertebrates, is expressed in premigratory cranial neural crest cells and is maintained throughout the neural crest cell epithelial-to-mesenchymal transition (EMT), but its function in these cells is unknown. To this end, we used a combination of in vivo and ex vivo experiments to assess gap junction formation, and Cx43 function, in chick cranial neural crest cells. Our results demonstrate that gap junctions exist between premigratory and migratory cranial neural crest cells and depend on Cx43 for their function. In the embryo, Cx43 knockdown just prior to EMT delays the emergence of Cx43-depleted neural crest cells from the neural tube, but these cells eventually successfully emigrate and join the migratory stream. This delay can be rescued by introduction of full-length Cx43 into Cx43-depleted cells. Furthermore, Cx43 depletion reduces the size of the premigratory neural crest cell domain through an early effect on neural crest cell specification. Collectively, these data identify new roles for Cx43 in chick cranial neural crest cell development.

Bottom up proteomics reveals novel differentiation proteins in neuroblastoma cells treated with 13-cis retinoic acid

Neuroblastoma, a cancer of the sympathetic nervous system, is the second most common pediatric cancer. A unique feature of neuroblastoma is remission in some patients due to spontaneous differentiation of metastatic tumors. 13-cis retinoic acid (13-cis RA) is currently used in the clinic to treat neuroblastoma due to its differentiation inducing effects. In this study, we used shotgun proteomics to identify proteins affected by 13-cis RA treatment in neuroblastoma SK-N-SH cells. Our results showed that 13-cis RA reduced proteins involved in extracellular matrix synthesis and organization and increased proteins involved in cell adhesion and neurofilament formation. These changes indicate that 13-cis RA induces tumor cell differentiation by decreasing extracellular matrix rigidity and increasing neurite overgrowth. Differentially-affected proteins identified in this study may be novel biomarkers of drug efficacy in the treatment of neuroblastoma. SIGNIFICANCE: As neuroblastoma can spontaneously differentiate, determining which proteins are involved in differentiation can guide development of novel treatments. 13-cis retinoic acid is currently used in the clinic as a differentiation inducer. Here we have established a proteome map of SK-N-SH cells treated with 13-cis retinoic acid. Bioinformatic analysis revealed the involvement of development, differentiation, extracellular matrix assembly, collagen biosynthesis, and neurofilament bundle association. This proteome map provides information as to which proteins are important for differentiation and identifies networks that can be targeted by drugs to treat neuroblastoma [1].

Hijacking of Embryonic Programs by Neural Crest-Derived Neuroblastoma: From Physiological Migration to Metastatic Dissemination

In the developing organism, complex molecular programs orchestrate the generation of cells in adequate numbers, drive them to migrate along the correct pathways towards appropriate territories, eliminate superfluous cells, and induce terminal differentiation of survivors into the appropriate cell-types. Despite strict controls constraining developmental processes, malignancies can emerge in still immature organisms. This is the case of neuroblastoma (NB), a highly heterogeneous disease, predominantly affecting children before the age of 5 years. Highly metastatic forms represent half of the cases and are diagnosed when disseminated foci are detectable. NB arise from a transient population of embryonic cells, the neural crest (NC), and especially NC committed to the establishment of the sympatho-adrenal tissues. The NC is generated at the dorsal edge of the neural tube (NT) of the vertebrate embryo, under the action of NC specifier gene programs. NC cells (NCCs) undergo an epithelial to mesenchymal transition, and engage on a remarkable journey in the developing embryo, contributing to a plethora of cell-types and tissues. Various NCC sub-populations and derived lineages adopt specific migratory behaviors, moving individually as well as collectively, exploiting the different embryonic substrates they encounter along their path. Here we discuss how the specific features of NCC in development are re-iterated during NB metastatic behaviors.

The Neural Crest: A Remarkable Model System for Studying Development and Disease

Neural crest cells are the embryonic precursors of most neurons and all glia of the peripheral nervous system, pigment cells, some endocrine components, and connective tissue of the head, face, neck, and heart. Following induction, crest cells undergo an epithelial to mesenchymal transition that enables them to migrate along specific pathways culminating in their phenotypic differentiation. Researching this unique embryonic population has revealed important understandings of basic biological and developmental principles. These principles are likely to assist in clarifying the etiology and help in finding strategies for the treatment of neural crest diseases, collectively termed neurocristopathies. The progress achieved in neural crest research is made feasible thanks to the continuous development of species-specific in vivo and in vitro paradigms and more recently the possibility to produce neural crest cells and specific derivatives from embryonic or induced pluripotent stem cells. All of the above assist us in elucidating mechanisms that regulate neural crest development using state-of-the art cellular, molecular, and imaging approaches.

Knowledge transfer without knowledge? The case of agentive metaphors in biology

Are scientific metaphors dispensable shortcuts that encapsulate knowledge but can always be translated back? Or do they constitute cases of knowledge transfer, even if seemingly based on scientifically underdeveloped domains? This paper defends the latter view. By drawing on the linguistic theories of metaphors, we assess a variety of agentive metaphors that pervade biology. Intentional metaphors are found unsatisfying because their use is either rigid or too widely flexible. By contrast, rational agent metaphors constitute good scientific metaphors, displaying flexible use and heuristic fruitfulness. Their range of application constantly evolves because they provide guidelines that permit the exploration of the applicability of source domain in a target domain. Their unique heuristic value makes them akin to research programs and allows for knowledge transfer, because they are based on a proper scientific source domain rather than on a folk or underdeveloped one.

Screen for Slit/Robo signaling in trunk neural cells reveals new players

Slits ligands and their Robo receptors are involved in quite disparate cell signaling pathways that include axon guidance, cell proliferation, cell motility and angiogenesis. Neural crest cells emerge by delamination from neural cells in the dorsal neural tube, and give rise to various components of the peripheral nervous system in vertebrates. It is well established that these cells change from a non-migratory to a highly migratory state allowing them to reach distant regions before they differentiate. However, but the mechanism controlling this delamination and subsequent migration are still not fully understood. The repulsive Slit ligand family members, have been classified also as true tumor suppressor molecules. The present study explored in further detail what possible Slit/Robo signals are at play in the trunk neural cells and neural crest cells by carrying out a microarray after Slit2 gain of function in trunk neural tubes. We found that in addition to molecules known to be downstream of Slit/Robo signaling, there were a large set of molecules known to be important in maintaining cells in non-motile, epithelia phenotype. Furthermore, we found new molecules previously not associated with Slit/Robo signaling: cell proliferation markers, Ankyrins and RAB intracellular transporters. Our findings suggest that neural crest cells use and array of different Slit/Robo pathways during their transformation from non-motile to highly motile cells.

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.

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

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

Spatiotemporal expression pattern of Connexin 43 during early chick embryogenesis

During embryogenesis, a single cell develops into new tissues and organs that are made up of a number of different cell types. The assembly of the trigeminal ganglion (cranial nerve V), an important component of the peripheral nervous system, typifies this process. The trigeminal ganglia perform key sensory functions, including sensing pain and touch in the face, and arise from cells of two different progenitor populations, the neural crest and the cranial placodes. One question that remains poorly understood is how these two populations of cells interact with each other during development to form a functional ganglion. Gap junctions are intercellular channels that allow for the passage of small solutes between connected cells and could serve as one potential mechanism by which neural crest and placode cells communicate to create the trigeminal ganglia. To this end, we have generated a comprehensive spatiotemporal expression profile for the gap junction protein Connexin 43, a highly expressed member of the Connexin protein family during development. Our results reveal that Connexin 43 is expressed in the neural folds during neural fold fusion and in premigratory neural crest cells prior to the epithelial-to-mesenchymal transition (EMT), during EMT, and in migratory neural crest cells. During trigeminal gangliogenesis, Connexin 43 is expressed in cranial neural crest cells and the mesenchyme but is strikingly absent in the placode-derived neurons. These data underscore the complexity of bringing two distinct cell populations together to form a new tissue during development and suggest that Connexin 43 may play a key role within neural crest cells during EMT, migration, and trigeminal gangliogenesis.

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

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

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.

PAX3 and PAX7 as upstream regulators of myogenesis

Like other subclasses within the PAX transcription factor family, PAX3 and PAX7 play important roles in the emergence of a number of different tissues during development. PAX3 regulates neural crest and, together with its orthologue PAX7, is also expressed in parts of the central nervous system. In this chapter we will focus on their role in skeletal muscle. Both factors are key regulators of myogenesis where Pax3 plays a major role during early skeletal muscle formation in the embryo while Pax7 predominates during post-natal growth and muscle regeneration in the adult. We review the expression and functions of these factors in the myogenic context. We also discuss mechanistic aspects of PAX3/7 function and modulation of their activity by interaction with other proteins, as well as the post-transcriptional and transcriptional regulation of their expression.

TrkB/BDNF signalling patterns the sympathetic nervous system

The sympathetic nervous system is essential for maintaining mammalian homeostasis. How this intricately connected network, composed of preganglionic neurons that reside in the spinal cord and post-ganglionic neurons that comprise a chain of vertebral sympathetic ganglia, arises developmentally is incompletely understood. This problem is especially complex given the vertebral chain of sympathetic ganglia derive secondarily from the dorsal migration of ‘primary' sympathetic ganglia that are initially located several hundred microns ventrally from their future pre-synaptic partners. Here we report that the dorsal migration of discrete ganglia is not a simple migration of individual cells but a much more carefully choreographed process that is mediated by extensive interactions of pre-and post-ganglionic neurons. Dorsal migration does not occur in the absence of contact with preganglionic axons, and this is mediated by BDNF/TrkB signalling. Thus BDNF released by preganglionic axons acts chemotactically on TrkB-positive sympathetic neurons, to pattern the developing peripheral nervous system.

The signals that pattern the sympathetic nervous system are not fully understood. Here the authors show that the dorsal migration of the primary sympathetic ganglia in chick embryos is orchestrated by BDNF/TrkB signalling and requires contact with preganglionic axons.

Neural crest defects in ephrin-B2 mutant mice are non-autonomous and originate from defects in the vasculature

Ephrin-B2, a member of the Eph/ephrin family of cell signaling molecules, has been implicated in the guidance of cranial and trunk neural crest cells (NCC) and development of the branchial arches(BA), but detailed examination in mice has been hindered by embryonic lethality of Efnb2 null loss of function due to a requirement in angiogenic remodeling. To elucidate the developmental roles for Efnb2, we generated a conditional rescue knock-in allele that allows rescue of ephrin-B2 specifically in the vascular endothelium (VE), but is otherwise ephrin-B2 deficient. Restoration of ephrin-B2 expression specifically to the VE completely circumvents angiogenic phenotypes, indicating that the requirement of ephrin-B2 in angiogenesis is limited to the VE. Surprisingly, we find that expression of ephrin-B2 specifically in the VE is also sufficient for normal NCC migration and that conversely, embryos in which ephrin-B2 is absent specifically from the VE exhibit NCC migration and survival defects. Disruption of vascular development independent of loss of ephrin-B2 function also leads to defects in NCC and BA development. Together, these data indicate that direct ephrin-B2 signaling to NCCs is not required for NCC guidance, which instead depends on proper organization of the embryonic vasculature.

Sox10ER(T2) CreER(T2) mice enable tracing of distinct neural crest cell populations

BACKGROUND

Neural crest cells play an important role in craniofacial morphogenesis and many other developmental processes. The formation of neural crest cells (NCCs) in vivo is a highly dynamic process and remains to be fully understood.

RESULTS

To investigate the spatiotemporal patterning of NCCs in vivo, we have generated Sox10ER(T2) CreER(T2) (SECE) mice, a transgenic line driving inducible Cre expression in NCCs. Inducing Cre activity at different stages triggered reporter expression in distinct NCC populations in SECE; R26R mice. By optimizing the timing and dosage of tamoxifen administration, we controlled Cre expression specifically in cranial NCCs. Using this approach, we demonstrate an important role for PDGFRα in cranial NCCs mitosis within the mandibular processes. Further reducing Cre activity within the cranial NCCs of SECE; R26R embryos revealed that SECE labels preferentially progenitors of medial nasal process (MNP) rather than the lateral nasal process (LNP), before their formation from the frontonasal prominence (FNP).

CONCLUSIONS

Our results indicate that NCCs are formed sequentially from rostral to caudal regions along the neural tube. These findings also suggest that NCCs within the FNP become specified regionally and genetically before they divide into MNP and LNP.

Embryonic Chicken Transplantation is a Promising Model for Studying the Invasive Behavior of Melanoma Cells

Epithelial-to-mesenchymal transition is a hallmark event in the metastatic cascade conferring invasive ability to tumor cells. There are ongoing efforts to replicate the physiological events occurring during mobilization of tumor cells in model systems. However, few systems are able to capture these complex in vivo events. The embryonic chicken transplantation model has emerged as a useful system to assess melanoma cells including functions that are relevant to the metastatic process, namely invasion and plasticity. The chicken embryo represents an accessible and economical 3-dimensional in vivo model for investigating melanoma cell invasion as it exploits the ancestral relationship between melanoma and its precursor neural crest cells. We describe a methodology that enables the interrogation of melanoma cell motility within the developing avian embryo. This model involves the injection of melanoma cells into the neural tube of chicken embryos. Melanoma cells are labeled using fluorescent tracker dye, Vybrant DiO, then cultured as hanging drops for 24 h to aggregate the cells. Groups of approximately 700 cells are placed into the neural tube of chicken embryos prior to the onset of neural crest migration at the hindbrain level (embryonic day 1.5) or trunk level (embryonic day 2.5). Chick embryos are reincubated and analyzed after 48 h for the location of melanoma cells using fluorescent microscopy on whole mounts and cross-sections of the embryos. Using this system, we compared the in vivo invasive behavior of epithelial-like and mesenchymal-like melanoma cells. We report that the developing embryonic microenvironment confers motile abilities to both types of melanoma cells. Hence, the embryonic chicken transplantation model has the potential to become a valuable tool for in vivo melanoma invasion studies. Importantly, it may provide novel insights into and reveal previously unknown mediators of the metastatic steps of invasion and dissemination in melanoma.

Molecular control of the neural crest and peripheral nervous system development

A transient and unique population of multipotent stem cells, known as neural crest cells (NCCs), generate a bewildering array of cell types during vertebrate development. An attractive model among developmental biologists, the study of NCC biology has provided a wealth of knowledge regarding the cellular and molecular mechanisms important for embryogenesis. Studies in numerous species have defined how distinct phases of NCC specification, proliferation, migration, and survival contribute to the formation of multiple functionally distinct organ systems. NCC contributions to the peripheral nervous system (PNS) are well known. Critical developmental processes have been defined that provide outstanding models for understanding how extracellular stimuli, cell-cell interactions, and transcriptional networks cooperate to direct cellular diversification and PNS morphogenesis. Dissecting the complex extracellular and intracellular mechanisms that mediate the formation of the PNS from NCCs may have important therapeutic implications for neurocristopathies, neuropathies, and certain forms of cancer.

Slit molecules prevent entrance of trunk neural crest cells in developing gut

Neural crest cells emerge from the dorsal neural tube early in development and give rise to sensory and sympathetic ganglia, adrenal cells, teeth, melanocytes and especially enteric nervous system. Several inhibitory molecules have been shown to play important roles in neural crest migration, among them are the chemorepulsive Slit1-3. It was known that Slits chemorepellants are expressed at the entry to the gut, and thus could play a role in the differential ability of vagal but not trunk neural crest cells to invade the gut and form enteric ganglia. Especially since trunk neural crest cells express Robo receptor while vagal do not. Thus, although we know that Robo mediates migration along the dorsal pathway in neural crest cells, we do not know if it is responsible in preventing their entry into the gut. The goal of this study was to further corroborate a role for Slit molecules in keeping trunk neural crest cells away from the gut. We observed that when we silenced Robo receptor in trunk neural crest, the sympathoadrenal (somites 18-24) were capable of invading gut mesenchyme in larger proportion than more rostral counterparts. The more rostral trunk neural crest tended not to migrate beyond the ventral aorta, suggesting that there are other repulsive molecules keeping them away from the gut. Interestingly, we also found that when we silenced Robo in sacral neural crest they did not wait for the arrival of vagal crest but entered the gut and migrated rostrally, suggesting that Slit molecules are the ones responsible for keeping them waiting at the hindgut mesenchyme. These combined results confirm that Slit molecules are responsible for keeping the timeliness of colonization of the gut by neural crest cells.

Six1 is a key regulator of the developmental and evolutionary architecture of sensory neurons in craniates

BACKGROUND

Various senses and sensory nerve architectures of animals have evolved during adaptation to exploit diverse environments. In craniates, the trunk sensory system has evolved from simple mechanosensory neurons inside the spinal cord (intramedullary), called Rohon-Beard (RB) cells, to multimodal sensory neurons of dorsal root ganglia (DRG) outside the spinal cord (extramedullary). The fish and amphibian trunk sensory systems switch from RB cells to DRG during development, while amniotes rely exclusively on the DRG system. The mechanisms underlying the ontogenic switching and its link to phylogenetic transition remain unknown.

RESULTS

In Xenopus, Six1 overexpression promoted precocious apoptosis of RB cells and emergence of extramedullary sensory neurons, whereas Six1 knockdown delayed the reduction in RB cell number. Genetic ablation of Six1 and Six4 in mice led to the appearance of intramedullary sensory neuron-like cells as a result of medial migration of neural crest cells into the spinal cord and production of immature DRG neurons and fused DRG. Restoration of SIX1 expression in the neural crest-linage partially rescued the phenotype, indicating the cell autonomous requirements of SIX1 for normal extramedullary sensory neurogenesis. Mouse Six1 enhancer that mediates the expression in DRG neurons activated transcription in Xenopus RB cells earlier than endogenous six1 expression, suggesting earlier onset of mouse SIX1 expression than Xenopus during sensory development.

CONCLUSIONS

The results indicated the critical role of Six1 in transition of RB cells to DRG neurons during Xenopus development and establishment of exclusive DRG system of mice. The study provided evidence that early appearance of SIX1 expression, which correlated with mouse Six1 enhancer, is essential for the formation of DRG-dominant system in mice, suggesting that heterochronic changes in Six1 enhancer sequence play an important role in alteration of trunk sensory architecture and contribute to the evolution of the trunk sensory system.

PleiotRHOpic: Rho pathways are essential for all stages of Neural Crest development

Neural Crest (NC) cells are a multipotent migratory stem cell population unique to vertebrates, which contributes extensively to the formation of a wide array of neural and non-neural structures in the embryo. NC cells originate in the ectoderm at the border of the neural tube, undergo an epithelial-mesenchymal transition and acquire outstanding individual and collective migratory properties that allow them to disseminate and differentiate to different parts of the body. This exquisite capacity to switch from an epithelium to motile cells represents both a puzzling biological issue and an attractive model to address the basic mechanisms of cell migration and their alteration during cancer progression. Here we review how signaling pathways controlled by Rho GTPases, key players in cell adhesion, contraction, migration and polarity, contribute to the control the different phases of NC development.

Characterization of the trunk neural crest in the bamboo shark, Chiloscyllium punctatum

The neural crest is a population of mesenchymal cells that after migrating from the neural tube gives rise to structure and cell types: the jaw, part of the peripheral ganglia, and melanocytes. Although much is known about neural crest development in jawed vertebrates, a clear picture of trunk neural crest development for elasmobranchs is yet to be developed. Here we present a detailed study of trunk neural crest development in the bamboo shark, Chiloscyllium punctatum. Vital labeling with dioctadecyl tetramethylindocarbocyanine perchlorate (DiI) and in situ hybridization using cloned Sox8 and Sox9 probes demonstrated that trunk neural crest cells follow a pattern similar to the migratory paths already described in zebrafish and amphibians. We found shark trunk neural crest along the rostral side of the somites, the ventromedial pathway, the branchial arches, the gut, the sensory ganglia, and the nerves. Interestingly, C. punctatum Sox8 and Sox9 sequences aligned with vertebrate SoxE genes, but appeared to be more ancient than the corresponding vertebrate paralogs. The expression of these two SoxE genes in trunk neural crest cells, especially Sox9, matched the Sox10 migratory patterns observed in teleosts. Also of interest, we observed DiI cells and Sox9 labeling along the lateral line, suggesting that in C. punctatum, glial cells in the lateral line are likely of neural crest origin. Although this has been observed in other vertebrates, we are the first to show that the pattern is present in cartilaginous fishes. These findings demonstrate that trunk neural crest cell development in C. punctatum follows the same highly conserved migratory pattern observed in jawed vertebrates.

Beta-actin is required for proper mouse neural crest ontogeny

The mouse genome consists of six functional actin genes of which the expression patterns are temporally and spatially regulated during development and in the adult organism. Deletion of beta-actin in mouse is lethal during embryonic development, although there is compensatory expression of other actin isoforms. This suggests different isoform specific functions and, more in particular, an important function for beta-actin during early mammalian development. We here report a role for beta-actin during neural crest ontogeny. Although beta-actin null neural crest cells show expression of neural crest markers, less cells delaminate and their migration arrests shortly after. These phenotypes were associated with elevated apoptosis levels in neural crest cells, whereas proliferation levels were unchanged. Specifically the pre-migratory neural crest cells displayed higher levels of apoptosis, suggesting increased apoptosis in the neural tube accounts for the decreased amount of migrating neural crest cells seen in the beta-actin null embryos. These cells additionally displayed a lack of membrane bound N-cadherin and dramatic decrease in cadherin-11 expression which was more pronounced in the pre-migratory neural crest population, potentially indicating linkage between the cadherin-11 expression and apoptosis. By inhibiting ROCK ex vivo, the knockout neural crest cells regained migratory capacity and cadherin-11 expression was upregulated. We conclude that the presence of beta-actin is vital for survival, specifically of pre-migratory neural crest cells, their proper emigration from the neural tube and their subsequent migration. Furthermore, the absence of beta-actin affects cadherin-11 and N-cadherin function, which could partly be alleviated by ROCK inhibition, situating the Rho-ROCK signaling in a feedback loop with cadherin-11.

The distribution of ephrin-B1 and PNA-positive glycoconjugates is correlated with atypical melanoblast migration in Japanese Silky fowl embryos

Melanoblasts are positively stimulated to migrate in the dorsolateral pathway of the avian embryo by ephrins, but are inhibited by PNA-binding glycoconjugates. We analyzed the potential role of these molecules in the Japanese Silky fowl, which displays intense internal pigmentation. The distribution of ephrin ligands was analyzed using Eph receptor-human Fc fusion proteins. Glycoconjugates were labeled using PNA-FITC. In Japanese Silky embryos, ventral areas, including the anterior- and posterior-half somites, expressed ephrin-B1 in a pattern that correlates with the atypical migratory pathways taken by Japanese Silky melanoblasts. White Leghorn embryos displayed little to no ephrin-Bs in the ventral paths. Conversely, PNA-binding barrier tissues, proposed to prevent melanoblasts from migrating ventrally in White Leghorn, are missing or have significant gaps in Japanese Silky embryos. Thus, studies of a naturally occurring pigmentation mutant confirm that a combination of cues regulates melanoblast migration in the chick embryo.

Cranial neural crest cells form corridors prefiguring sensory neuroblast migration

The majority of cranial sensory neurons originate in placodes in the surface ectoderm, migrating to form ganglia that connect to the central nervous system (CNS). Interactions between inward-migrating sensory neuroblasts and emigrant cranial neural crest cells (NCCs) play a role in coordinating this process, but how the relationship between these two cell populations is established is not clear. Here, we demonstrate that NCCs generate corridors delineating the path of migratory neuroblasts between the placode and CNS in both chick and mouse. In vitro analysis shows that NCCs are not essential for neuroblast migration, yet act as a superior substrate to mesoderm, suggesting provision of a corridor through a less-permissive mesodermal territory. Early organisation of NCC corridors occurs prior to sensory neurogenesis and can be recapitulated in vitro; however, NCC extension to the placode requires placodal neurogenesis, demonstrating reciprocal interactions. Together, our data indicate that NCC corridors impose physical organisation for precise ganglion formation and connection to the CNS, providing a local environment to enclose migrating neuroblasts and axonal processes as they migrate through a non-neural territory.

Mmp17b is essential for proper neural crest cell migration in vivo

The extracellular matrix plays a critical role in neural crest (NC) cell migration. In this study, we characterize the contribution of the novel GPI-linked matrix metalloproteinase (MMP) zebrafish mmp17b. Mmp17b is expressed post-gastrulation in the developing NC. Morpholino inactivation of mmp17b function, or chemical inhibition of MMP activity results in aberrant NC cell migration with minimal change in NC proliferation or apoptosis. Intriguingly, a GPI anchored protein with metalloproteinase inhibitor properties, Reversion-inducing-Cysteine-rich protein with Kazal motifs (RECK), which has previously been implicated in NC development, is expressed in close apposition to NC cells expressing mmp17b, raising the possibility that these two gene products interact. Consistent with this possibility, embryos silenced for mmp17b show defective development of the dorsal root ganglia (DRG), a crest-derived structure affected in RECK mutant fish sensory deprived (sdp). Taken together, this study has identified the first pair of MMP, and their putative MMP inhibitor RECK that functions together in NC cell migration.

Collective cell migration of epithelial and mesenchymal cells

Directional cell migration is required for proper embryogenesis, immunity, and healing, and its underpinning regulatory mechanisms are often hijacked during diseases such as chronic inflammations and cancer metastasis. Studies on migratory epithelial tissues have revealed that cells can move as a collective group with shared responsibilities. First thought to be restricted to proper epithelial cell types able to maintain stable cell-cell junctions, the field of collective cell migration is now widening to include cooperative behavior of mesenchymal cells. In this review, we give an overview of the mechanisms driving collective cell migration in epithelial tissues and discuss how mesenchymal cells can cooperate to behave as a collective in the absence of bona fide cell-cell adhesions.

Eif3ba regulates cranial neural crest development by modulating p53 in zebrafish

Congenital diseases caused by abnormal development of the cranial neural crest usually present craniofacial malformations and heart defects while the precise mechanism is not fully understood. Here, we show that the zebrafish eif3ba mutant caused by pseudo-typed retrovirus insertion exhibited a similar phenotype due to the hypogenesis of cranial neural crest cells (NCCs). The derivatives of cranial NCCs, including the NCC-derived cell population of pharyngeal arches, craniofacial cartilage, pigment cells and the myocardium derived from cardiac NCCs, were affected in this mutant. The expression of several neural crest marker genes, including crestin, dlx2a and nrp2b, was specifically reduced in the cranial regions of the eif3ba mutant. Through fluorescence-tracing of the cranial NCC migration marker nrp2b, we observed reduced intensity of NCC-derived cells in the heart. In addition, p53 was markedly up-regulated in the eif3ba mutant embryos, which correlated with pronounced apoptosis in the cranial area as shown by TUNEL staining. These findings suggest a novel function of eif3ba during embryonic development and a novel level of regulation in the process of cranial NCC development, in addition to providing a potential animal model to mimic congenital diseases due to cranial NCC defects. Furthermore, we report the identification of a novel transgenic fish line Et(gata2a:EGFP)pku418 to trace the migration of cranial NCCs (including cardiac NCCs); this may serve as an invaluable tool for investigating the development and dynamics of cranial NCCs during zebrafish embryogenesis.

The neural crest and cancer: a developmental spin on melanoma

Neural crest (NC) cells undergo an epithelial to mesenchymal transition (EMT) in order to exit from the dorsal neural tube. Similarly, ancestrally related melanoma cells employ an EMT-like event during the initial stages of metastasis to dissociate from surrounding keratinocytes. Whether or not the molecular pathogenesis and cellular dynamics of melanoma metastasis resemble the embryonic NC invasion program is unclear. Here, we highlight advances in our understanding of tumor cell behaviors and plasticity, focusing on the relationship between melanoma and the NC invasion programs. We summarize recent discoveries of NC cell guidance and emerging in vivo imaging strategies that permit single cell resolution of fluorescently labeled tumor cells, with a focus on our recently developed in vivo chick embryo transplant model. Crucial to the molecular pathogenesis of metastasis, we highlight advances in gene profiling of small cell numbers, including our novel ability to gather gene expression information during distinct stages of melanoma invasion. Lastly, we present preliminary details of a comparison of specific genetic pathways associated with the early phases of melanoma invasion and known NC induction and migration signals. Our results suggest that malignant melanoma cells hijack portions of the NC program to promote plasticity and facilitate metastasis. In summary, there is considerable power in combining an in vivo model system with molecular analysis of gene expression, within the context of established developmental signaling pathways, to identify and study the molecular mechanisms of metastasis.

The neural crest

The neural crest (NC) is a highly migratory multipotent cell population that forms at the interface between the neuroepithelium and the prospective epidermis of a developing embryo. Following extensive migration throughout the embryo, NC cells eventually settle to differentiate into multiple cell types, ranging from neurons and glial cells of the peripheral nervous system to pigment cells, fibroblasts to smooth muscle cells, and odontoblasts to adipocytes. NC cells migrate in large numbers and their migration is regulated by multiple mechanisms, including chemotaxis, contact-inhibition of locomotion and cell sorting. Here, we provide an overview of NC formation, differentiation and migration, highlighting the molecular mechanisms governing NC migration.

Group choreography: mechanisms orchestrating the collective movement of border cells

Cell movements are essential for animal development and homeostasis but also contribute to disease. Moving cells typically extend protrusions towards a chemoattractant, adhere to the substrate, contract and detach at the rear. It is less clear how cells that migrate in interconnected groups in vivo coordinate their behaviour and navigate through natural environments. The border cells of the Drosophila melanogaster ovary have emerged as an excellent model for the study of collective cell movement, aided by innovative genetic, live imaging, and photomanipulation techniques. Here we provide an overview of the molecular choreography of border cells and its more general implications.

Emergence of embryonic pattern through contact inhibition of locomotion

The pioneering cell biologist Michael Abercrombie first described the process of contact inhibition of locomotion more than 50 years ago when migrating fibroblasts were observed to rapidly change direction and migrate away upon collision. Since then, we have gleaned little understanding of how contact inhibition is regulated and only lately observed its occurrence in vivo. We recently revealed that Drosophila macrophages (haemocytes) require contact inhibition for their uniform embryonic dispersal. Here, to investigate the role that contact inhibition plays in the patterning of haemocyte movements, we have mathematically analysed and simulated their contact repulsion dynamics. Our data reveal that the final pattern of haemocyte distribution, and the details and timing of its formation, can be explained by contact inhibition dynamics within the geometry of the Drosophila embryo. This has implications for morphogenesis in general as it suggests that patterns can emerge, irrespective of external cues, when cells interact through simple rules of contact repulsion.

Expression and function of cell adhesion molecules during neural crest migration

Neural crest cells are highly migratory cells that give rise to many derivatives including peripheral ganglia, craniofacial structures and melanocytes. Neural crest cells migrate along defined pathways to their target sites, interacting with each other and their environment as they migrate. Cell adhesion molecules are critical during this process. In this review we discuss the expression and function of cell adhesion molecules during the process of neural crest migration, in particular cadherins, integrins, members of the immunoglobulin superfamily of cell adhesion molecules, and the proteolytic enzymes that cleave these cell adhesion molecules. The expression and function of these cell adhesion molecules and proteases are compared across neural crest emigrating from different axial levels, and across different species of vertebrates.

Multiscale mechanisms of cell migration during development: theory and experiment

Long-distance cell migration is an important feature of embryonic development, adult morphogenesis and cancer, yet the mechanisms that drive subpopulations of cells to distinct targets are poorly understood. Here, we use the embryonic neural crest (NC) in tandem with theoretical studies to evaluate model mechanisms of long-distance cell migration. We find that a simple chemotaxis model is insufficient to explain our experimental data. Instead, model simulations predict that NC cell migration requires leading cells to respond to long-range guidance signals and trailing cells to short-range cues in order to maintain a directed, multicellular stream. Experiments confirm differences in leading versus trailing NC cell subpopulations, manifested in unique cell orientation and gene expression patterns that respond to non-linear tissue growth of the migratory domain. Ablation experiments that delete the trailing NC cell subpopulation reveal that leading NC cells distribute all along the migratory pathway and develop a leading/trailing cellular orientation and gene expression profile that is predicted by model simulations. Transplantation experiments and model predictions that move trailing NC cells to the migratory front, or vice versa, reveal that cells adopt a gene expression profile and cell behaviors corresponding to the new position within the migratory stream. These results offer a mechanistic model in which leading cells create and respond to a cell-induced chemotactic gradient and transmit guidance information to trailing cells that use short-range signals to move in a directional manner.

Analysis of trunk neural crest cell migration using a modified Zigmond chamber assay

Neural crest cells (NCCs) are a transient population of cells present in vertebrate development that emigrate from the dorsal neural tube (NT) after undergoing an epithelial-mesenchymal transition. Following EMT, NCCs migrate large distances along stereotypic pathways until they reach their targets. NCCs differentiate into a vast array of cell types including neurons, glia, melanocytes, and chromaffin cells. The ability of NCCs to reach and recognize their proper target locations is foundational for the appropriate formation of all structures containing trunk NCC-derived components. Elucidating the mechanisms of guidance for trunk NCC migration has therefore been a matter of great significance. Numerous molecules have been demonstrated to guide NCC migration. For instance, trunk NCCs are known to be repelled by negative guidance cues such as Semaphorin, Ephrin, and Slit ligands. However, not until recently have any chemoattractants of trunk NCCs been identified. Conventional in vitro approaches to studying the chemotactic behavior of adherent cells work best with immortalized, homogenously distributed cells, but are more challenging to apply to certain primary stem cell cultures that initially lack a homogenous distribution and rapidly differentiate (such as NCCs). One approach to homogenize the distribution of trunk NCCs for chemotaxis studies is to isolate trunk NCCs from primary NT explant cultures, then lift and replate them to be almost 100% confluent. However, this plating approach requires substantial amounts of time and effort to explant enough cells, is harsh, and distributes trunk NCCs in a dissimilar manner to that found in in vivo conditions. Here, we report an in vitro approach that is able to evaluate chemotaxis and other migratory responses of trunk NCCs without requiring a homogenous cell distribution. This technique utilizes time-lapse imaging of primary, unperturbed trunk NCCs inside a modified Zigmond chamber (a standard Zigmond chamber is described elsewhere). By exposing trunk NCCs at the periphery of the culture to a chemotactant gradient that is perpendicular to their predicted natural directionality, alterations in migratory polarity induced by the applied chemotactant gradient can be detected. This technique is inexpensive, requires the culturing of only two NT explants per replicate treatment, avoids harsh cell lifting (such as trypsinization), leaves trunk NCCs in a more similar distribution to in vivo conditions, cuts down the amount of time between explantation and experimentation (which likely reduces the risk of differentiation), and allows time-lapse evaluation of numerous migratory characteristics.

Neural crest delamination and migration: from epithelium-to-mesenchyme transition to collective cell migration

After induction and specification in the ectoderm, at the border of the neural plate, the neural crest (NC) population leaves its original territory through a delamination process. Soon afterwards, the NC cells migrate throughout the embryo and colonize a myriad of tissues and organs where they settle and differentiate. The delamination involves a partial or complete epithelium-to-mesenchyme transition (EMT) regulated by a complex network of transcription factors including several proto-oncogenes. Studying the relationship between these genes at the time of emigration, and their individual or collective impact on cell behavior, provides valuable information about their role in EMT in other contexts such as cancer metastasis. During migration, NC cells are exposed to large number of positive and negative regulators that control where they go by generating permissive and restricted areas and by modulating their motility and directionality. In addition, as most NC cells migrate collectively, cell-cell interactions play a crucial role in polarizing the cells and interpreting external cues. Cell cooperation eventually generates an overall polarity to the population, leading to directional collective cell migration. This review will summarize our current knowledge on delamination, EMT and migration of NC cells using key examples from chicken, Xenopus, zebrafish and mouse embryos. Given the similarities between neural crest migration and cancer invasion, these cells may represent a useful model for understanding the mechanisms of metastasis.

Computational modelling of cell chain migration reveals mechanisms that sustain follow-the-leader behaviour

Follow-the-leader chain migration is a striking cell migratory behaviour observed during vertebrate development, adult neurogenesis and cancer metastasis. Although cell–cell contact and extracellular matrix (ECM) cues have been proposed to promote this phenomenon, mechanisms that underlie chain migration persistence remain unclear. Here, we developed a quantitative agent-based modelling framework to test mechanistic hypotheses of chain migration persistence. We defined chain migration and its persistence based on evidence from the highly migratory neural crest model system, where cells within a chain extend and retract filopodia in short-lived cell contacts and move together as a collective. In our agent-based simulations, we began with a set of agents arranged as a chain and systematically probed the influence of model parameters to identify factors critical to the maintenance of the chain migration pattern. We discovered that chain migration persistence requires a high degree of directional bias in both lead and follower cells towards the target. Chain migration persistence was also promoted when lead cells maintained cell contact with followers, but not vice-versa. Finally, providing a path of least resistance in the ECM was not sufficient alone to drive chain persistence. Our results indicate that chain migration persistence depends on the interplay of directional cell movement and biased cell–cell contact.