models, cell movement is encouraged by cell-cell contact, which may be manifested as nudging from behind (Davis and Trinkaus, 1981) or the detachment of cells at the front of a migratory cell sheet (Carmona-Fontaine et al., 2008) to create space. These cell behaviors alone may not produce directional movement of a multicellular stream, but when local inhibitory signals restrict cell movements the result can be long-distance directed cell movement.By contrast, other models suggest that cells respond to chemotactic signals that drive the directional migration of individual cells (Dormann and Weijer, 2003;Richardson and Lehmann, 2010;Tarbashevich and Raz, 2010;Roussos et al., 2011;Cai et al., 2012) or cell clusters (Valentin et al., 2007;Aman and Piotrowski, 2010;Streichan et al., 2011). In these models, cells may respond directly to a chemotactic signal or receive guidance from neighboring cells.As long-distance cell migration is a major aspect of embryonic development (Dormann and Weijer, 2003;Richardson and Lehmann, 2010;Tarbashevich and Raz, 2010;Kulesa and Gammill, 2010), adult morphogenesis (Hatten and Roussel, 2011), tissue repair (Burns and Steinberg, 2011) and cancer metastasis (Roussos et al., 2011;Friedl and Gilmour, 2009), the examination of this phenomenon could have significant implications for better understanding birth defects and disease. Yet, even with multiscale data collected from different model systems and emerging computational models, the cellular and molecular mechanisms of long distance cell migration are still unclear. This is due in part to a disconnect between theory and experiment that limits the testing of various hypotheses parametrised by biological data. Thus, what is needed is a fully integrative experimental-modeling approach that can reject certain hypotheses in favor of others and elucidate multiscale mechanisms of cell migration.Here, we examine how a subpopulation of embryonic cells travel long distances and respond to tissue growth to accurately reach a target. We study this question using the neural crest (NC) as our model experimental system. NC cells exit the dorsal neural tube (NT) and travel long distances throughout the developing embryo along stereotypical pathways rich in microenvironmental signals, mesoderm and extracellular matrix (Noden and Trainor, 2005;Perris and Perissinotto, 2000). The NC cell population is crucial for proper development of the face, heart and peripheral nervous systems, and is the cellular origin of the highly aggressive cancers, melanoma and neuroblastoma (Trainor, 2005;Sauka-Spengler and Bronner-Fraser, 2008;Gammill and Roffers-Agarwal, 2010;Kasemeier-Kulesa et al., 2008;Jiang et al., 2011 SUMMARYLong-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....

Cot-based cloning and sequencing (CBCS) is a powerful tool for isolating and characterizing the various repetitive components of any genome, combining the established principles of DNA reassociation kinetics with high-throughput sequencing. CBCS was used to generate sequence libraries representing the high, middle, and low-copy fractions of the chicken genome. Sequencing high-copy DNA of chicken to about 2.7× coverage of its estimated sequence complexity led to the initial identification of several new repeat families, which were then used for a survey of the newly released first draft of the complete chicken genome. The analysis provided insight into the diversity and biology of known repeat structures such as CR1 and CNM, for which only limited sequence data had previously been available. Cot sequence data also resulted in the identification of four novel repeats (Birddawg, Hitchcock, Kronos, and Soprano), two new subfamilies of CR1 repeats, and many elements absent from the chicken genome assembly. Multiple autonomous elements were found for a novel Mariner-like transposon, Galluhop, in addition to nonautonomous deletion derivatives. Phylogenetic analysis of the high-copy repeats CR1, Galluhop, and Birddawg provided insight into two distinct genome dispersion strategies. This study also exemplifies the power of the CBCS method to create representative databases for the repetitive fractions of genomes for which only limited sequence data is available.

Neural crest (NC) cell migration is crucial to the formation of peripheral tissues during vertebrate development. However, how NC cells respond to different microenvironments to maintain persistence of direction and cohesion in multicellular streams remains unclear. To address this, we profiled eight subregions of a typical cranial NC cell migratory stream. Hierarchical clustering showed significant differences in the expression profiles of the lead three subregions compared with newly emerged cells. Multiplexed imaging of mRNA expression using fluorescent hybridization chain reaction (HCR) quantitatively confirmed the expression profiles of lead cells. Computational modeling predicted that a small fraction of lead cells that detect directional information is optimal for successful stream migration. Single-cell profiling then revealed a unique molecular signature that is consistent and stable over time in a subset of lead cells within the most advanced portion of the migratory front, which we term trailblazers. Model simulations that forced a lead cell behavior in the trailing subpopulation predicted cell bunching near the migratory domain entrance. Misexpression of the trailblazer molecular signature by perturbation of two upstream transcription factors agreed with the in silico prediction and showed alterations to NC cell migration distance and stream shape. These data are the first to characterize the molecular diversity within an NC cell migratory stream and offer insights into how molecular patterns are transduced into cell behaviors.

SUMMARY Cancer cells must regulate plasticity and invasion to survive and metastasize. However, the identification of targetable mechanisms to inhibit metastasis has been slow. Signaling programs that drive stem and progenitor cells during normal development offer an inroad to discover mechanisms common to metastasis. Using a chick embryo transplant model, we have compared molecular signaling programs of melanoma and their embryonic progenitors, the neural crest. We report that malignant melanoma cells hijack portions of the embryonic neural crest invasion program. Genes associated with neural crest induction, delamination, and migration are dynamically regulated by melanoma cells exposed to an embryonic neural crest microenvironment. Specifically, we demonstrate that metastatic melanoma cells exploit neural crest-related receptor tyrosine kinases to increase plasticity and facilitate invasion while primary melanocytes may actively suppress these responses under the same microenvironmental conditions. We conclude that aberrant regulation of neural crest developmental genes promotes plasticity and invasiveness in malignant melanoma.

Embryonic neural crest cells travel in discrete streams to precise locations throughout the head and body. We previously showed that cranial neural crest cells respond chemotactically to vascular endothelial growth factor (VEGF) and that cells within the migratory front have distinct behaviors and gene expression. We proposed a cell-induced gradient model in which lead neural crest cells read out directional information from a chemoattractant profile and instruct trailers to follow. In this study, we show that migrating chick neural crest cells do not display distinct lead and trailer gene expression profiles in culture. However, exposure to VEGF in vitro results in the upregulation of a small subset of genes associated with an in vivo lead cell signature. Timed addition and removal of VEGF in culture reveals the changes in neural crest cell gene expression are rapid. A computational model incorporating an integrate-and-switch mechanism between cellular phenotypes predicts migration efficiency is influenced by the timescale of cell behavior switching. To test the model hypothesis that neural crest cellular phenotypes respond to changes in the VEGF chemoattractant profile, we presented ectopic sources of VEGF to the trailer neural crest cell subpopulation and show diverted cell trajectories and stream alterations consistent with model predictions. Gene profiling of trailer cells that diverted and encountered VEGF revealed upregulation of a subset of 'lead' genes. Injection of neuropilin1 (Np1)-Fc into the trailer subpopulation or electroporation of VEGF morpholino to reduce VEGF signaling failed to alter trailer neural crest cell trajectories, suggesting trailers do not require VEGF to maintain coordinated migration. These results indicate that VEGF is one of the signals that establishes lead cell identity and its chemoattractant profile is critical to neural crest cell migration.