Mechanisms driving neural crest induction and migration in the

SPECIAL FOCUS: RECENT ADVANCES IN MOLECULAR AND CELLULAR MECHANISMS GOVERNING NEURAL CREST CELL MIGRATION, REVIEW

Cell Adhesion & Migration 4:4, 595-608; October/November/December 2010; © 2010 Landes Bioscience

Mechanisms driving neural crest induction and migration in the zebrafish and Xenopus laevis Michael W. Klymkowsky,1 Christy Cortez Rossi2,† and Kristin Bruk Artinger2,* Department of Molecular, Cellular and Developmental Biology; University of Colorado Boulder; Boulder, CO USA; 2Department of Craniofacial Biology; University of Colorado Denver; School of Dental Medicine; Aurora, CO USA

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Present Address: M.I.N.D. Institute; University of California; Davis, CA USA

Key words: evolution, neural crest, mesoderm, induction, migration

The neural crest is an evolutionary adaptation, with roots in the formation of mesoderm. Modification of neural crest behavior has been critical for the evolutionary diversification of the vertebrates and defects in neural crest underlie a range of human birth defects. There has been a tremendous increase in our knowledge of the molecular, cellular and inductive interactions that converge on defining the neural crest and determining its behavior. While there is a temptation to look for simple models to explain neural crest behavior, the reality is that the system is complex in its circuitry. In this review, our goal is to identify the broad features of neural crest origins (developmentally) and migration (cellularly) using data from the zebrafish (teleost) and Xenopus laevis (tetrapod amphibian) in order to illuminate where general mechanisms appear to be in play and, equally importantly, where disparities in experimental results suggest areas of profitable study.

Introduction “Ectoderm and endoderm are primary germ layers; they were the first to appear in animal evolution and are the earliest to form embryologically, being present in the unfertilized egg. Mesoderm is a secondary germ layer; it is not preformed in the vertebrate egg but arises following inductive interactions between ectoderm and endoderm. Like mesoderm, the neural crest arises very early in development and gives rise to very divergent cell and tissue types.” –B.K. Hall1 To understand how a tissue such as the neural crest forms and functions, it helps to understand where it comes from, in both evolutionary and embryological terms. Mesoderm has been termed a “secondary” germ layer, since it arises early in embryogenesis through inductive interactions between maternal ectodermal and endodermal determinants.2,3 In that light, neural crest might well be considered a “tertiary” germ layer, since its induction depends upon mesoderm, which acts on ectoderm to induce the *Correspondence to: Kristin Bruk Artinger; Email: [email protected] Submitted: 05/03/10; Accepted: 07/09/10 Previously published online: www.landesbioscience.com/journals/celladhesion/article/12962 DOI: 10.4161/cam.4.4.12962

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differentiation of the neuroectoderm (the nascent nervous system) and neural crest from the embryonic epidermis. The interaction with mesoderm not only induces the neuroectoderm, but also serves to pattern it, with signals from the axial mesoderm/notochord acting to specify the ventral-dorsal axis of the neural tube, while signals from dorsolateral mesoderm are involved in the induction and patterning of the neural crest. Later in development, the neural crest region abuts regions of lateral dorsal mesoderm, which secretes signaling agonists and antagonists as well as extracellular matrix materials through which the neural crest cells migrate toward their final destinations within the later stage embryo. The nascent epidermis and neuroectoderm-neural tube are characterized by differences in bone morphogenic protein (BMP) (throughout, genes names are indicated by the use of lower case italics, proteins are capitalized and non-italic) signaling. The BMPs are members of the transforming growth factor-b (TGFb) family of secreted signaling molecules, and act through surface receptors and SMAD-type transcriptional regulator proteins.4-6 BMP signaling is high in the epidermis and low in the neuroectoderm; the neural crest forms in the region of intermediate BMP signaling, which also corresponds to the most dorsolateral aspect of the neuroectoderm. In addition to BMP secretion, BMP signaling is controlled by secreted antagonists7 as well as intracellular regulators.8 The simple intermediate-gradient model of neural crest induction implies that neural crest induction occurs after neuroectodermal induction, but there is clear evidence that neural crest induction begins during gastrulation, concomitantly with neural induction.9 Evolutionary Considerations This review focuses on two model organisms, the teleost fish Dario rerio (zebrafish) and the amphibian tetrapod Xenopus laevis. Much of the pioneering work on neural crest was carried out in other experimental systems, particularly the chick and chick/ quail chimeras, pioneered by Nicole Le Dourain,10 and a number of more recent studies have attempted to place the origins of the neural crest in an evolutionary context through studies of organisms such as the lamprey11-13 and the cephalochordate Amphioxus.14,15 Given the length of evolutionary time (in the hundreds of millions of years) since these various organisms shared

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from those involved in maintaining and/or “propagating” their effects. Rather than a “clean” linear pathway, it is common (and perhaps universal) to find that the pathways leading to a differentiated phenotype involve a number of interactions that are either co-dependent or required to maintain a (perhaps only slightly) earlier step in the regulatory cascade; feedback and feedforward systems are rampant in embryological processes. As noted previously,26 “Every neural crest specifier examined thus far also appears necessary and/or sufficient for the expression of the other specifiers in Xenopus.” Because of this “backward” connectedness, it can be difficult (and sometimes meaningless) to identify unambiguously who comes first during a process, such as neural crest induction. This is complicated by the fact that many studies of gene expression during development rely primarily on in situ hybridization visualization of mRNA to determine the order in which genes are expressed. Unfortunately, in situ hybridization does not reveal the on-set of gene expression (it requires the accumulation of sufficient RNA for detection) or when the level of gene product reaches a concentration sufficient to produce physiological effects. So the question becomes, what gene is required to be expressed first? This may be Figure 1. Displayed is an evolutionary lineage linking the major model systems used in the study of the neural crest (zebrafish, chick, xenopus and difficult to discern since maintenance and co-dependent mouse). It indicates the distinct eveolutionary lineages of each. Figure adapted effects may be so closely linked to the “initial” step as to from various sources, and based on Romer.249 be functionally indistinguishable. Moreover, as most investigators who have examined the global effects of manipua common ancestor (Fig. 1), it is not surprising that there have lating gene expression can attest, generally the expression been substantial changes in the molecular and cellular underpin- levels of many hundreds of genes change in response to experinings of what appears, superficially, to be the same process—that mental/genetic perturbations. While this has the advantage of is, neural crest formation, migration and differentiation. Such producing (at least for awhile) a steady stream of “new” and “eardrift can occur in even closely related species and can be mecha- liest” regulators, it tends to obscure the interconnectedness of the nistically significant.16,17 An obvious example, with respect to the process under study. Keeping such issues in mind, Sauka-Spengler and Bronnerneural crest, is the “swapping” of expression patterns between the paralogous zinc-finger transcription factors slug/snail2 and snail1 Fraser27 have reviewed the hierarchy of genes involved in what in different species.18 This means that the organismic context of they call the neural crest gene regulatory network (reviewed in a particular study is critical, since results from one organism may ref. 26). Their “unified” network includes inducers (BMPs, Wnts or may not be applicable to others; in this light, it is surpris- and fibroblast growth factors (FGFs)), neural plate border speciing that in many studies the species examined is not identified fiers (zic1, msx1, msx2, dlx3, dlx5, pax3 and pax7), neural crest explicitly in the abstract! This caveat is worth taking seriously, specifiers (snail1, slug/snail2, sox8, sox9, sox10, foxd3, ap-2, twist, given that mammals are derived from synapsids, rather than c-myc and id family members) and effector genes specifically archosaurs, like birds (Fig. 1),19 and there can be apparently dra- expressed in each of the various cell types into which neural crest matic differences in the function of orthologous gene products. cells differentiate (see below); these include the neurons and glia For example, while slug/snail2 appears to be required for neural of the peripheral nervous system, cartilage, bone, connective tiscrest formation in Xenopus and chick,20-22 this does not appear to sue, pigment cells and sympatho-adrenal cells.10,28 Additionally, be the case in the mouse.23 in species that have unpaired fins, such as amphibian tadpoles and fish (e.g., the zebrafish), neural crest cells contribute to the General Models of Neural Crest Induction, fin mesenchyme.29-31 Specification and Migration Neural crest derivatives occupy various positions along an organism’s rostral-caudal axis and are categorized as cranial, When we think about the mechanisms (or perhaps better, the vagal, trunk and sacral.32 The cranial neural crest forms first and circuitry) of developmental processes, it is worth remembering appears to be specified during gastrulation (at least in Xenopus that they are a cascade, with beginnings either in earlier asymme- and likely in zebrafish). More posterior neural crest form later and tries or asymmetries that arise through stochastic processes.24,25 perhaps are subject to different inductive and migratory signals, This makes it critical that we distinguish between cellular and in analogy to the mechanisms involved in the origin of trunk and molecular events that lead to the initiation of inductive effects tailbud dermomyotome.33,34 Following their initial specification,

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neural crest cells migrate into various regions of the embryo; their fate appears to be determined in part based on their axial location.35 While it is obvious (see below) that neural crest cells are migratory, it is interesting that, at least in the chick trunk, inert latex beads and non-migratory cells inserted into the neural crest region “migrate” in a manner similar to neural crest cells, suggesting that at least some of the mechanisms driving neural crest cell migration are passive.36,37 Apparently, inert objects (and cells) can be “carried along” by the neural crest stream. While there are general mechanism common to both Xenopus and zebrafish neural crest development, we have separated the sections to focus on them separately since it is these differences that will help shed light the process. Where they are similar, we will make a note as such. Neural Crest Induction and Migration in Xenopus Perhaps the most dramatic dichotomy reported in neural crest induction involves the role of mesoderm. Ragland and Raible38 reported that signals from mesoderm “are dispensable for zebrafish neural crest induction,” although not for their normal migration (see below). This was a surprising finding, given the report by Bonstein et al.39 that paraxial mesoderm, which corresponds to the dorsal lateral marginal zone of the late blastula/early gastrula stage embryo, is required for neural crest induction in Xenopus. This discrepancy may represent a species difference; however, we side with the notion that the mesoderm is generally important for neural crest specification. There are several reasons for our interpretation. First, the mesoderm and neural crest are in very close proximity during development, and it is likely that interactions between them continue throughout neural crest development, including induction and migration. In addition, several factors that are expressed in neural crest cells have been co-opted from the mesoderm, which suggests they may similarly respond to inducer genes. We look forward to analysis of the role of genes such as slug/snail2, the snail1s and twists for their effects on mesoderm formation in zebrafish. Inductive interactions between the ectoderm and neural plate underlie neural crest formation. In studies carried out in the axolotl, the juxtaposition of neural plate and epidermis gives rise to pigment cells (a neural crest derivative) and both neural plate and epidermal cells can produce neural crest cells.40 More recent experiments revealed the upregulation of molecular markers of neural crest, such as slug/snail2,41 and the appearance of RohonBeard sensory neurons, a cell type that forms at the neural plate border and may be related to neural crest cells and the induction of XBlimp-1(PRDM1) in such explants in Xenopus.42 This raises another issue, namely the rather “mixed up” nature of the early Xenopus fate map; a point that was made explicitly by Wardle and Smith,43 who noted the presence of “rogue” cells in late blastula/early gastrula stage embryos that expressed gene markers outside of their expected expression domain. This type of behavior is illustrated in Figure 2; an early gastrula stage embryo has been stained in situ for endodermin mRNA, a marker of endoderm; not only are rogue cells found outside of the main endodermal domain, but within the endodermal domain there


Figure 2. An in situ hybridization of a gastrula state embryo (stage 11) stained in situ for the endodermal marker endodermin mRNA, reveals two types of behavior—there are cells ectopically expressing the gene (small arrows), while within the domain where the majority of cells express the gene, there are cells that do not express it (large arrow). This suggests that over time gene expression is refined (Zhang C and Klymkowsky MW, previously unpublished observations).

are cells that fail to express endodermin (we might term them recalcitrant cells); they are presumably expressing markers for other developmental fates. As development proceeds into the late gastrula/early neurula stage, the frequency of these rogue cells decreases.43 It would be extremely useful if the pattern of gene expression could be followed in real time by combining real-time imaging with molecular methods like the brainbow technique.44-46 What is clear is that there is substantial mixing of cell fates at early stages. In two recent elegant papers,47,48 the investigators injected their experimental reagents into the dorsal-animal blastomere of an 8-cell embryo to target neural crest, and to avoid influencing mesodermal tissues. As can be seen from the fate map (Fig. 3 illustrates the 8/16-, 32-cell and late blastula/early gastrula fate maps, reviewed in refs. 9, 49–52), this is practically difficult if not impossible, and such experiments need to be carefully controlled by looking explicitly for effects on mesodermallyderived tissues; the myotome is a good choice since it is easy to visualize and the expression of muscle markers begins early.53 In the absence of explicit evidence, we cannot exclude developmentally significant cross talk between tissues, particularly given the increasing evidence for such cross-talk, mediated by secreted and extracellular matrix factors (see below). Developmental Cascades: Mesoderm and Neural Crest Formation In Xenopus, and a range of other organisms, there is a maternally-supplied animal-vegetal axis that underlies ectoderm and endodermal germ layer specification (Fig. 4). In Xenopus, signals from endoderm, activated by the maternally supplied and vegetally localized T-box transcription factor VegT act, together with F-type Sox transcription factors to turn on and maintain


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Figure 3. Left: Depicted are the major contributions of the animal A1 and A2 blastomeres (which correspond to the dorsal-animal blastomere of the 16 cell embryo and the C2 and C3 blastomeres of the 32 cell embryo (images and descriptions modified from XenBase and based on the work of Moody). Center: The fate map of the late blastula/early gastrula stage embryo as revised by Kourakis and Smith. Right: Mapping studies done by Steventon et al. on cell fates in the gastrula stage embryo.

Figure 4. A simplified diagram of the inductive interactions during neuroectoderm/neural crest induction. Dashed arrows indicate inductive interactions. We have indicated where a number of the genes that are the primary focus of this review are thought to act. That said, and as noted in the text, it is often the case that genes expression is co-dependent, so that simple linear pathways should be considered skeptically.

the expression of Nodal-type TGFb family secreted proteins, while animally localized B1-Sox proteins, as well as other factors, such as Ectodermin,54 Coco55,56 and Xema,57 all targets of Sox3 regulation, act to restrict the extent of Nodal signaling.3 This VegT/Nodal cascade leads to the zygotic expression of a number of genes, among which are those that encode the T-box transcription factors Eomesodermin,58 Brachyury (there are in fact three distinct brachyury genes in Xenopus),59-61 the zygotic splice variant of VegT, known as Antipodean,62,63 and the T-box transcription factors Tbx6 and Tbx6-related.64-67 Recent studies indicate that downregulation of zygotic VegT/Antipodean is

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critical for paraxial mesoderm patterning; 68 in our ongoing studies, we find that inhibiting the expression of antipodean has little effect on neural crest, whereas inhibiting the early embryonic expression of antipodean and xbra together inhibits neural crest and myotome formation.69 In a similar vein, morpholino-based inhibition of slug/snail2, snail1 or twist expression leads to the inhibition of antipodean and xbra expression, myotome and neural crest formation.22,69,70


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Defining Mesoderm-Derived Signals Involved in Neural Crest Induction Given the clear role of mesodermally derived signals in neural crest induction in Xenopus, a number of studies have examined the nature of these signals. These studies must be considered in light of the fact that the situation in the late blastula/early gastrula embryo is dynamic and self-regulating.71,72 More to the point, during neural crest specification (early during gastrulation) a plethora of secreted factors are expressed. In the dorsal region these include the BMP antagonists Chordin,73 Noggin,74 Follistatin;75 the Wnt antagonists Frzb-1,76 Dickkopf,77 and Crescent;78 the anti-dorsalizing morphogenic protein (ADMP),79 BMP2,80 and IGFBP5,81 and the multifunctional antagonists Cerberus (anti-Wnt, BMP, Nodal) 82 and Shisa-1 and Shisa-2 (anti-FGF, anti-Wnt).83,84 Similarly, the ventral organizing center has been found to secrete the anti-chordin Xolliod,85-87 the anti-Wnt and anti-Xolloid Sizzled,88 BMPs,89-92 the BMP signaling modulator Crossveinless-2 (CV2),93 and the BMP antagonist Twisted Gastrulation (Tsg).94,95 In zebrafish, BMP and Wnt requirements for neural crest induction is thought to be conserved (see below), although the details of the directness of these interactions remains to be tested. Later in development, other secreted inhibitors have been found to be involved in neural crest differentiation, e.g., the Wnt antagonist WIF-1,96 and the BMP antagonist Gremlin.97 Such secreted factors initially attract interest because manipulating their expression levels produces interesting effects on embryogenesis, and generally a specific signaling pathway is identified as responsible, but this often turns out to be over-simplistic. Take for example the Frizzled-like Wnt inhibitors; these proteins have been found to display a range of effects in various systems, from extending the diffusive reach of Wnt signaling98 to antagonizing each other’s activities.99 In an example particularly relevant to Xenopus, the Frizzled-related protein Sizzled was originally identified as a ventrally expressed Wnt inhibitor but later found to act in a Wnt-independent manner,100 through its effects on the secreted metalloproteinase Tolloid-related,101 which itself is an an inhibitor of Chordin, a dorsally secreted BMP inhibitor. This activity appears to be evolutionarily conserved, at least in the zebrafish.102 Interactions with proteinases may be a common feature of this class of protein, since Frizzled-related protein 2 enhances the activity of the procollagen C proteinase;103 Sizzled and related proteins may regulate the activity of as yet unidentified extracellular proteins. Inductive Interactions Early studies indicated that Wnts (Wnt1, Wnt3a, Wnt7a) induced increased expression of neural crest markers in Xenopus embryos and ectodermal (animal cap) explants and that Wnt signaling was required for neural crest formation.104-106 The frizzled-7 and -3 Wnt receptors,107,108 the Kremen and Kermit receptor associated proteins,109,110 and the splicing factor/transcription co-regulatory/splicing factor Skip111 have all been implicated in neural crest specification. In their studies of mesoderm-neural crest


inductive interactions Monsoro-Burq and colleagues112,113 found a dependence upon FGF-mediated process; they proposed that FGF8 and Wnt signaling acted in parallel, and mediate expression of msx1 and pax3, which in turn activate slug/snail2 expression. They proposed a role for FGF in neural crest induction.114 FGF signaling can modulate Wnt signaling by effects on the LEF/TCF-binding co-repressor Groucho-related 4.115 Two essential targets of Wnt-induced neural crest specification are ap2a116 and gbx2, which encodes a homeobox transcription factor.117 Subsequently, Hong et al.118 reported that FGF acts by inducing Wnt8 in paraxial mesoderm, which in turn acts to induce neural crest, while Wu et al.119 reported that Wnt signaling could induce neural crest even when FGF signaling was blocked. A recent analysis of mesoderm-neural crest interactions9 confirmed and extended Bonstein’s39 original finding that dorsal lateral mesoderm was necessary and sufficient to induce neural crest from animal cap (ectoderm) explants. An analysis of their observation reveals a number of unanswered questions, associated with the dynamic and co-dependent nature of the embryonic system. They found that active canonical (b-catenin-dependent) Wnt signaling was required for the animal cap to respond to mesodermal signals and express neural crest markers (e.g., snail2/slug). That said, inhibition of canonical Wnt signaling appears to be required for neural induction.120 In the embryo, both the requirement for Wnt signaling (for neural crest induction) and its inhibition (in neuroectoderm) are likely to be influenced by the presence of the B1-type Sox proteins, Sox2 and Sox3 (Fig. 5). Sox3 is expressed maternally, and Sox2 and Sox3 are expressed zygotically in the nascent neuroectoderm and both can inhibit canonical Wnt signaling, in part through interactions with b-catenin;121-123 they could play a key role in establishing a Wnt responsive threshold within the epidermal-crest-neuroectodern domain. At the same time, mesodermal secretion of the Wnt inhibitor Dickkopf1 has been found to inhibit the formation of neural crest at the anterior margin of the neural plate.124 Mayor et al.125 found that the zinc-finger transcription factor slug/snail2 is expressed in dorsal mesendoderm, where it acts to repress BMP-4 activity. Our own studies indicate that slug/ snail2 and snail1 are required for both mesoderm and neural crest formation.20,22,69 Since the Snail proteins appear to function exclusively as transcriptional repressors,126,127 the ability of Slug/ Snail2 to regulate its own expression125 is presumably indirect. We have found that the secreted signaling antagonists cerberus and sizzled are immediate-early targets of repression by Slug/ Snail2.69,70 ceberus is expressed in anterior endomesoderm while sizzled is expressed in the ventral region of the embryo; both are involved the interaction network that regulates embryonic patterning.128 BMP4 can induce neural crest cells in naïve chick neuroepithelium129 and in Xenopus.130,131 To examine the role of BMP signaling in neural crest formation, Steventon et al.9 reduced the level of the BMP antagonist Chordin by anti-chordin morpholino injection in ectoderm/dorsomarginal zone explant studies; this led to the loss of snail2/slug in the neural crest; it is not clear whether the expression of earlier crest markers, such as pax3 and zic1132 were affected. This is worth knowing since


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as noggin, dickkopf, cerberus, etc.,) within the dorsolateral marginal zone? 71 Once We Are Neural Crest, We Have to Migrate

Figure 5. Displays the pattern of expression of B1-type Sox-type transcription factors in the early Xenopus embryo. Top: RT-PCR analysis at various embryonic stage; Center: in situ analysis at blastula stages; Bottom: in situ hybridization at early neurual stages (stage 13 and 15) for Sox1 (A and B), Sox2 (C and D) and Sox3 (E and F). Red arrow points to region of reduced Sox expression within the nascent neural plate (Grammer T, Zhang C, Klymkowsky MW, previously unpublished observations).

the accessory protein Irig3 is required for the expression of the neural crest markers slug/snail2, sox9 and foxd3, but not pax3 and zic1.133 Blocking chordin expression (but not Wnt signaling) eliminated the expression of the neural plate marker sox2,9 although whether the level of sox2 (or sox3) expression was altered in Wnt-antagonist expressing explants was not reported (see above). Both chordin and wnt8 are expressed in the dorsal marginal zone of the gastrula stage embryo, although whether they are expressed in the same cells is not clear. Their expression in the dorsal marginal zone is lower than that in their maximum expression regions, the dorsal lip and the ventral marginal zones, respectively. More important is the answer to the question, what is the effect of blocking chordin expression on the embryo’s self-regulating BMP gradient, the expression of other agonists (such as wnt8 and bmp4) and antagonists (such

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The neural crest is the classic example of an epithelial-mesenchymal transition (EMT).134 This process involves a coordinated set of changes in gene expression and cellular behavior. In the trunk crest, it appears that BMP signaling acts to induce wnt1 expression in somites, which in turn acts to initiate crest migration.135 Migration begins with the disassembly of the cell-cell adherence junction complex, and the acquisition of a mobile phenotype; cells are maintained in an undifferentiated state in part through the action of the transcription factor Hairy2.136,137 Within the embryo, neural crest cells migrate through spaces between the basal laminae that define the basal surface of the epidermis and the developing dermomyotome.138,139 These spaces are occupied by extracellular matrix rich in laminin and fibronectin. Neural crest cells fail to enter regions rich in chondroitin sulphate.138,140 Cranial neural crest migrates out in “streams” and the ephrin ligand receptor tyrosine kinase system plays an important role in keeping these streams distinct.141,142 In the chick, neural crest cells migrate “principally in loose clusters, with a few single cells in the lead. The cells in these groups display leadingto-trailing edge adhesions and form tongues or streams of cells directed away from the neural tube”.143 Migrating neural crest cells are “elongated and aligned parallel to the direction of migration. Nearly all protrusive activity occurs at their ventral, leading edges.”144,145 Neural crest cells, like other normal, that is, non-transformed, substrate-adherent eukaryotic cells, display contact inhibition.146 The molecular mechanisms of contact inhibition involve noncononical Wnt signaling, Syndecan-4/Rac1, RhoA, and the PCP pathway.147 The contact inhibition between neural crest cells serves to “channel” motility forward along the cell’s migratory path.148,149 The end result is that normal cells do not pile up, but rather tend to form sheets and, in the neural crest, streams. Cadherin-cadherin interactions are generally involved in these cell-cell interactions. Cadherins interact with cytoplasmic catenins, including b-catenin, plakoglobin (g-catenin) and p120cas, which in turn can interact with and regulate transcription factors. Various signaling pathways, most notably the canonical Wnt signaling, act in part by modulating the stability of these catenins. Some years ago, we referred to these interactions as the body language of cells.150 While inert latex beads migrate when implanted into the region of the neural crest,36,37 it is clear that neural crest cell migration depends upon cellular properties, including interactions with integrins151 and extracellular matrix proteins.152 The onset of migration is associated with many changes in cytoskeletal organization and composition; as an example, keratin-type intermediate filaments are replaced by vimentin-type filaments. In Xenopus, vimentin-positive neural crest derived cells are most prominent in the neural crest-derived and dorsal fin mesenchymal cells.153 The initial steps of this process appear to be mediated by transcription factors, such as Slug/Snail2, Snail1 and Twist,


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which have been found to suppress expression of E-cadherin.154-156 Analysis of the slug/snail2 promoter indicates that it is regulated by canonical Wnt signals.157 In addition, these proteins are potent suppressors of apoptosis.158,159 In Xenopus, disruption of slug/ snail2, snail1 or twist expression leads to a dramatic increase in apoptosis.22,70,160 In slug/snail2 morpholino-injected embryos, the increase in apoptosis can be inhibited by injection of RNA encoding anti-apoptotic factors (Bcl2 or Bclx L); this also rescues neural crest formation, through NFkB-mediated regulation of slug/snail2, snail2 and twist RNA levels.22 In mammalian cells, Slug/Snail2 acts to negatively regulate the pro-apoptotic protein Puma;161 it is an immediate early regulator of the pro-apoptotic caspase, Caspase-9 in Xenopus.22,160 The predominant cadherin in the early Xenopus embryo is C-cadherin;162 beginning in the late blastula an E-cadherin-like molecule is expressed in the developing epidermis and is absent from the developing neural plate.163,164 Beginning at gastrulation, cadherin-11 is expressed in a Wnt-dependent manner;165 later it is expressed in migrating neural crest cells,166 together with the protocadherin PCNS167 and N-cadherin.168 Functional studies of cadherin-11 indicate that its overexpression, or the overexpression of a mutant polypeptide missing its cytoplasmic b-catenin binding domain, blocks cranial neural crest migration and leads to their neural differentiation.169 In contrast, a mutant form of cadherin-11 missing its extracellular domain induced the premature migration of neural crest cells, and led to a decrease in the levels of twist expression.169 Such a version of cadherin-11 (DN-cadherin-11) would be expected to act as an inhibitor of b-catenin-mediated Wnt signaling,170 and so it is not completely surprising that the overexpression of b-catenin (which mimics the effects of a canonical Wnt signal) can rescue twist expression in mutant cadherin-11 expressing embryos.169 In any case, this provides further evidence for the role of Wnt signaling in migrating neural crest. During migration, the extracellular domain of cadherin-11 is cleaved by the ADAM metalloproteinase, to produce an extracellular polypeptide and a cellular polypeptide, similar to DN-cadherin-11;171 the extracellular fragment promotes neural crest cell migration. It is possible that it acts by inhibiting cadherin-11-mediated cell-cell interactions. The membraneassociated, cytoplasmic region of cadherin-11 interacts with the guanine nucleotide exchange factor Trio and stimulates filopodia formation and migration.172 The neural crest expresses the unconventional myosin, myosin-X; morpholino-induced decreased expression of myosin-X leads to loss of filopodial formation, actin cytoskeleton disorganization and migration defects.173,174 Such filopodia interactions are important for contact inhibition of migration between neural crest cells, a behavior mediated by syndecan-4/rac 1 and non-canonical Wnt signaling.149,175 Loss of function studies have implicated Wnt11r in crest migration.176 A component of the non-canonical Wnt signaling pathway PTK7 is required for normal neural crest migration,177 while pescadillo, which appears to be regulated by non-canonical Wnt4 signaling, is also involved in crest migration.178


Neural Crest Induction and Migration in Zebrafish: Extrinsic Signals Involved in Neural Crest Induction in Zebrafish When the ectoderm segregates into neural and non-neural ectoderm, neural tissue lies dorsally while epidermal ectoderm is located laterally and ventrally; the neural plate border (from which the neural crest arises) lies at the junction between these two tissues. Neural crest cells appear after the neural and non-neural ectoderm have segregated from one another but before the neural tube has closed. Rohon-Beard sensory neurons, mechanosensory primary neurons and cranial placodes, which form neurons and glia of the cranial ganglia, arise from the anterior neural plate border.26 During the stages when the neural plate border is beginning to form, a gradient of BMP activity is established in the ectoderm (see above). Similar to Xenopus, neural crest cells (as well as Rohon-Beard sensory neurons) are not specified or are shifted ventrally in BMP pathway zebrafish mutants.179,180 Wnt signaling is required for neural crest induction in zebrafish. Activation of a heat shock version of a mutant tcf3, which does not bind betacatenin and so acts as a dominant antagonist of canonical Wnt signaling, leads to the absence of foxd3 expressing neural crest cells.181 However, wnt8a and 8b are required for early neural crest induction, but later markers of neural crest cells either do not require Wnt signaling or there is some recovery of the later neural crest markers, such as sox10 and mitfa.181 Notch/Delta is another juxtacrine signaling system by which cells within an equivalence domain diversify their cell types.182,183 Early during neurulation in the zebrafish, single cells or small clusters of cells express high levels of deltaA at the neural plate border.184 A dominant negative mutation in deltaA also leads to the formation of supernumerary Rohon-Beard sensory neurons with a concomitant decrease in neural crest-derived pigment cells and cells of the dorsal root ganglia.185,186 Similarly a mutation of mindbomb (mib), which encodes a RING ubiquitin ligase involved in Notch/Delta signaling, leads to supernumerary Rohon-Beard sensory neurons and a decrease in neural crestderived pigment cells.187-189 These data suggest that Notch/Delta signaling functions to specify a Rohon-Beard sensory neuron at the expense of other neural crest derived cell types, presumably from a common progenitor. Intrinsic Signals Involved in Specification Early experiments in chick190,191 and the mouse192,193 revealed that injecting a single cell with a lineage tracer dye prior to the onset of migration resulted in several types of labeled neural crest derivatives at later developmental stages. In zebrafish, single cell labeling of early neural crest cells results in labeling of more than one, and often all combinations of neural crest derivatives, similar to what was previously seen in chick. At the same time labeling of trunk neural crest cells just after they segregated from the neural tube rarely gives rise to multiple neural crest derivatives194 suggesting that neural crest cells may adopt their fates earlier in zebrafish than in other species, a conclusion supported by the observation that labeling cells at gastrulation generally results in


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endomesoderm of late blastula/early gastrula stage embryos, and appears to regulate expression of the secreted BMP-Nodal-Wnt inhibitor Cerberus;200 expression of a “dominant-negative” form of XBlimp1 leads to the loss of embryonic head. That said, homologs of prdm1a are not expressed at the border of the neural plate in amphioxus, chick or the mouse;201 (D. Meulemans-Medieros, personal communication) and so prdm1 does not appear to be part of the “universal” neural crest gene regulatory network. We suspect that another PRDM paralog has taken over this function. The formation of neural crest in the apparent absence of mesoderm suggests that the zebrafish neural crest gene regulatory network is different from the the “universal network” (see above). That said, it includes secreted inducers (BMPs, FGFs and Wnts). In zebrafish the established neural plate border specifiers include msxb, msxc, msxe and prdm1a.202,203 Knockdown of msxb, msxc and msxe together leads to the loss of slug/snail2 and sox10, expression, but leaves expression of foxd3, dlx3b and dlx4b intact; they are required for Rohon-Beard sensory neurons and placode fate but not neural crest cell directly.202,204,205 Other genes in this part of the network have not yet been Figure 6. DiI fate map of cells at the neural plate border. Preliminary fate maps of neural plate border progenitors from mid- to the end of gastrulation between 70 and 90% tested for their role in neural crest development. epiboly in which 5–10 cells were labeled with DiI. Rohon-Beard sensory neurons (yellow Expression analysis of zic2a and pax3 with prdm1a circle) and neural crest cells can share a common precursor domain (yellow star), while suggest that these genes are expressed more highly neural crest cells can arise from a separate precursor (aqua triangle, melanocyte; small in the neural plate than in the neural plate border square, glial cell; double square, melanocyte or glial cell; heart, fin mesenchyme; star, during gastrulation (Law and Artinger, unpubmultiple neural crest types). 70, 75, and 90% epiboly regions are depicted on embryos pictured in reference 215. lished). This would be distinctly different from the case of the other vertebrates reported to date; it suggests that in the zebrafish pax gene expression is required more for neural plate than neural plate border fate only a single type of neural crest derivative (Rossi and Artinger, specification. As for the neural crest specifiers, snail2, sox9, sox10, in preparation) (Fig. 6). This implies that intrinsic information, foxd3 and ap-2 family members have been all shown to be imporcaptured before the onset of migration, is likely to determine the tant in zebrafish neural crest development.206-211 Knockdown behavior of the bulk of neural crest cells. of ap-2a and ap-2c in zebrafish prevents neural crest induction, PRDM1a (also known as narrowminded/uboot in zebrafish with no effect on the neural plate border specifiers, but loss of the and BLIMP1 in other species) appears to be one such “informa- expression of foxd3 and snail2.207,208 Interestingly, these embryos tional” molecule. prdm1a encodes a zing-finger type transcription have reduced numbers of Rohon-Beard sensory neurons and factor.195,196 In zebrafish, prdm1a expression begins well before reduced expression of prdm1a, suggesting that ap-2 genes may that of other neural crest specifiers197,198 and homozygous mutant be more of a neural plate border specifier in zebrafish. The effecprdm1a embryos show a decrease in foxd3 expression in trunk tor genes involved in neural crest differentiation will not be dispre-migratory neural crest cells; it leads to the failure to form cussed here.27 Rohon-Beard sensory neurons and produces significantly fewer trunk neural crest cells, fewer pigment cells, a reduction in fin General and Molecular Mechanisms mesenchyme, loss of ceratobranchial cartilage and fewer neurons of Trunk Neural Crest Migration in Zebrafish per ganglion in the dorsal root ganglia.197-199 This original mutation removes the encoded protein’s putative SET domain and During neurulation, the neural plate morphs into a cylindrithus does not contain a DNA binding region. Comparison of cal tube. In most species, this occurs when the neural plate phenotypes of prdm1a mutant embryos with that of morpholino border regions elevate to form neural folds, which then con“knock-down” embryos suggests that the mutant is not an amor- verge toward the midline and eventually fuse together to phic (or null) allele. The Xenopus homolog of prdm1a, known as form the dorsal aspect of the neural tube. In teleost fish, xblimp1, is expressed at the neural plate border and in the anterior however, there are no neural folds.212 Rather, the neural keel

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(analogous to the neural tube) forms through the ventral thickening of the ectoderm, a process known as secondary neurulation.213,214 Neural crest cells migrate from their position in or immediately adjacent to the dorsal region of the neural tube to various positions within the embryo. The eventual location of each neural crest cell depends on the type of neural crest derivative that it will become. For example, pigment cells are located throughout the epidermis,215 cells in the dorsal root ganglia are lateral to the neural tube,216 and caudal fin mesenchyme is located within the mesenchymal region of the tail. Depending on the species of teleost, neural crest cell migration begins either during or after neural tube closure.28 Trunk neural crest cells migrate along two routes: ventromedially through the somites and dorsolaterally between the epidermis and the somites. Those crest cells that migrate along the medial pathway give rise to more ventral derivatives, such as dorsal root ganglia, sympathetic ganglia, enteric neurons and glia; those that migrate more dorsally give rise to pigment cells (melanocytes). Inductive interactions with the migratory environment, particularly the somites, is thought to determine/influence to cell fate. The regulatory ability of neural crest cells was demonstrated by ablation studies; cells can regulate to fill in the gap of missing neural crest derivatives, even if they would not normally do so.194 In most species, neural crest cells travel through the anterior half of each somite; their migration along the caudal region of the somite is inhibited by the expression of specific inhibitors, such as Ephrin-B1 and T-cadherin.217-219 In the zebrafish, migration occurs through the middle of the medial surface of each somite,220,221 whether the same “molecular migration signposts” are involved remains to be established. Because crest cells migrate along the middle of the somites in zebrafish, and given that a large portion of the somite forms after zebrafish neural crest cells have started migrating, it has been suggested that the sclerotome is not required for their migration. Instead, it appears that myotome, specifically the slow twitch muscle fibers, plays the role of sclerotome. Mutations that reduce sonic hedgehog signaling (syu, smu), reduce the amount of slow twitch muscle and disrupt neural crest cell migration. Replacing the precursors of slow twitch muscle rescues these defects, suggesting that slow twitch muscle is required for zebrafish neural crest migration. Mutations that affect the formation of mesoderm (spt, ntl) and the segmentation of somites (bea, fss), disrupt neural crest migration, but not its induction.221 The lateral surface of the somite induces contact inhibition of migrating neural crest cells, blocking their invasion into the somite and channeling the movement148 (see above). Neural crest cells change direction when confronted with another neural crest cell along its migratory route leading to directed (forward) cell migration in the absence of an attractant. Syndecan-4 morphants have normal neural crest induction but no neural crest cell migration. A series of elegant experiments suggest that Syn4 inhibits Rac while Wnt/Dsh promotes RhoA activity which coordinates the directionality of migration.147 Members of these pathways localize to the site of cell-cell contact and thus control the directed cell protrusions required for migration in a directed fashion. Cadherin 11 is localized to the leading edges of the


migrating cells where it presumably mediates interactions with epithelial cells via the regulation of Rac, RhoA and cdc42.165 General and Molecular Mechanisms of Cranial Neural Crest Migration in Zebrafish Cranial neural crest cells migrate in an anterior stream toward the eye and three streams, with the midbrain and anteriormost hindrain rhombomere (R) 1/2 cells emigrate first, followed by the emigration of the more caudal R4 and R6/7 cells. The segmental fate of the cranial neural crest is dictated by the premigratory position of neural crest cells along the antero-posterior axis of the embryo. Neural crest cells in the pharyngeal arches give rise to the ventral pharyngeal skeletal elements. Specifically, pharyngeal arch one gives rise to the Meckel’s and palatoquadrate cartilages, arch two forms the basihyal, ceratohyal and hyosymplectic cartilages and arches three through seven give rise to the ceratobranchial cartilages.222,223 The dorsal craniofacial cartilages make up the neurocranium or “braincase.” The anterior neurocranium is composed of two rod-like structures, termed trabeculae, which fuse anteriorly and connect to the ethmoid plate. The anterior neurocranium is derived from the anteriormost cranial neural crest, which migrate out from the dorsal neural tube and reach the dorsal anterior aspect of the head following a path along the optic stalks, medial to the eyes.224-226 Migration in the zebrafish cranial region begins at 12 h post fertilization (hpf) with the anterior most cells delaminating and migrating and by 15–16 hpf the cells move into the pharyngeal arches, as shown by Schilling.227 Interestingly fate-mapping data indicates that neural crest cells are segmentally restricted, meaning there is very little mixing between pharyngeal arch compartments.228 This is in contrast to the chick embryo, where there is substantial mixing during migration between hindbrain compartments.229 Zebrafish neural crest cells also migrate in an anterior stream around the eye, which is important for development of the neurocranium.225,226 The microRNA Mirn140 modulates the attraction of cranial neural crest cells via platelet derived growth factor (pdgfaa) signaling during zebrafish neurocranium development. In embryos lacking Mirn140, cranial neural crest cells accumulate around the optic stalk near the attractive Pdgfaa signal and fail to migrate further to the oral ectoderm, resulting in loss of the ethmoid plate and in palatal clefting.230 The eye is important for organizing neural crest cell migration in the zebrafish. In wild-type embryos, neural crest cells migrate anteriorly around the eye and are not detected on the eye surface until 23–24 hpf. Soon after, however, cranial neural crest cells will cover the entire surface of the eye. chokh mutant embryos fail to develop eyes and cranial neural crest cells fail to migrate anteriorly and are disorganized in a region posterior to the normal eye location. Analysis of craniofacial cartilages in chokh mutant larvae shows loss of the ethmoid plate and fusion of the trabeculae. chokh mutant larvae do however have normal viscerocranium development.226 Chemokines are small, secreted chemoattractants that have been well studied for their roles in regulating cell migration during embryogenesis, immune response and cancer.231 The


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chemokine Stromal cell-derived factor (Sdf1), binds to the receptors Cxcr4 and Cxcr7.232,233 cxcr4 has also been shown to be expressed in migrating neural crest cells in the mouse embryo234 while sdf1 is expressed within the neural crest cells migratory path. In the chick, an sdf1 expression domain is associated with the branchial arch region.235 In the mouse Sdf1/Cxcr4 signaling has been found to mediate the positioning of dorsal root ganglia.234 In the zebrafish, Sdf1a signaling has been implicated in melanophore patterning; melanocytes are attracted to the site of the implantation of an sdf1 coated bead.236 There are two cxcr4 genes in zebrafish, cxcr4a and cxcr4b; only cxcr4a is expressed within neural crest cells.237 Loss of Cxcr4a, but not Cxcr7b, results in aberrant cranial neural crest cell migration in the anterior stream, resulting in neurocranial defects and cranial ganglia dismorphogenesis.238 Gain of function of either Sdf1b or Cxcr4a causes aberrant cranial neural crest cell migration and results in ectopic craniofacial cartilages. This suggests that Sdf1b signaling from the pharyngeal arch endoderm and optic stalk to Cxcr4a-expressing cranial neural crest cells is important for both the proper condensation of these cells into the pharyngeal arches and the subsequent patterning and morphogenesis of neural crest-derived tissues, such as proper positioning of the partially neural crest-derived trigeminal sensory ganglia.239 Interestingly, sdf1 signaling also plays a role in neural crest migration in Xenopus (R. Mayor, personal communication). Although cxcr4a is expressed by all arch neural crest cells and sdf1b by the endodermal pouches surrounding the arches in which they are migrating, there is no significant defect of cells derived from the pharyngeal arches, suggesting multiple redundant mechanisms for neural crest migration, especially in the cranial region. Ephrins and Ephrin receptors have been implicated in regulating the migration of cranial neural crest cells in other vertebrates and are thought to function by altering cell adhesion, likely through interactions with integrins. In the mouse, Ephrin-B2 is important for crest cell migration into branchial arch 2, while Ephrin-B1is required for multiple steps during crest migration; mutant mice exhibit a cleft palate.240,241 To date, no such phenotype has been observed in either zebrafish or Xenopus. The Neuropilin family has also been shown to be required for neural crest migration. Knockdown of Neuropilin-1 function in References 1.

Hall BK. The neural crest in development and evolution. New York: Springer-Verlag 1999. 2. Niehrs C. On growth and form: a Cartesian coordinate system of Wnt and BMP signaling specifies bilaterian body axes. Development 2010; 137:845-57. 3. Zhang C, Klymkowsky MW. The Sox axis, Nodal signaling and germ layer specification. Differentiation 2007; 75:536-45. 4. Miyazono K, Kamiya Y, Morikawa M. Bone morphogenetic protein receptors and signal transduction. J Biochem 2010; 147:35-51. 5. Umulis D, O’Connor MB, Blair SS. The extracellular regulation of bone morphogenetic protein signaling. Development 2009; 136:3715-28. 6. Yamamoto Y, Oelgeschlager M. Regulation of bone morphogenetic proteins in early embryonic development. Naturwissenschaften 2004; 91:519-34.

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the chick results in a reduction in the number of crest cells that reach the branchial arches.242 Signaling through Neuropilin-2 by Semaphorin 3F has been implicated for proper cranial neural crest cell migration and condensation of the trigeminal sensory ganglia in mouse. Interestingly, while cranial neural crest cell migration is affected in these mutants, no skeletal defects were apparent.243 Studies in zebrafish implicate sema3F and sema3G in cranial neural crest migration. sema3F and 3G are expressed in neural crest-free regions, while and nrp2a and nrp2b are expressed in cranial neural crest cells.244,245 In zebrafish, mutations in pbx4 is suppressed following knockdown of sema3F/3G and overexpression of sema3Gb permits crest cells to migrate in normally crest cell-free zones.246 Other genes, such as Disc1, have more recently been show to be involved in cranial neural crest cells migration.247 Summary While it is common to treat neural crest as a conserved feature of vertebrate development, it is clear that different molecules and mechanisms have come to assume distinct functions in different lineages (e.g., the different roles of snail-family proteins in zebrafish, Xenopus and mouse). Similarly, while there are parallels between neural crest and cancer metastasis, the differences are likely to be profound.248 What is clear is that multiple redundant mechanisms are at play and required for the proper induction and migration of neural crest. We propose that species differences in neural crest specification are of interest as they are likely to provide insights into a range of processes involved in evolutionary, developmental and pathogenic processes. Acknowledgements

We thank Doris Wedlich and Eugenia Olesnicky Killian for enthusiastic discussions, Dan Medeiros for comments and sparking a new collaboration and Jianli Shi and Courtney Severson for interesting experimental data. Financial Support

HD050698 and DE017699 to K.B.A. and GM084133 to M.W.K.

7. Walsh DW, Godson C, Brazil DP, Martin F. Extracellular BMP-antagonist regulation in development and disease: tied up in knots. Trends Cell Biol 2010;20:244-56. 8. Hariharan R, Pillai MR. Structure-function relationship of inhibitory Smads: Structural flexibility contributes to functional divergence. Proteins 2008; 71: 1853-62. 9. Steventon B, Araya C, Linker C, Kuriyama S, Mayor R. Differential requirements of BMP and Wnt signalling during gastrulation and neurulation define two steps in neural crest induction. Development 2009; 136:771-9. 10. Le Douarin NM. The neural crest. New York: Cambridge Univ Press 1982. 11. Sauka-Spengler T, Meulemans D, Jones M, BronnerFraser M. Ancient evolutionary origin of the neural crest gene regulatory network. Dev Cell 2007; 13: 405-20.


12. Nikitina N, Bronner-Fraser M, Sauka-Spengler T. The sea lamprey Petromyzon marinus: a model for evolutionary and developmental biology. Cold Spring Harb Protoc: Cold Spring Harb. Press 2009. 13. Nikitina NV, Bronner-Fraser M. Gene regulatory networks that control the specification of neural-crest cells in the lamprey. Biochim Biophys Acta 2009; 1789: 274-8. 14. Meulemans D, Bronner-Fraser M. Amphioxus and lamprey AP-2 genes: implications for neural crest evolution and migration patterns. Development 2002; 129:4953-62. 15. Yu JK. The evolutionary origin of the vertebrate neural crest and its developmental gene regulatory network— insights from amphioxus. Zoology 2010; 113:1-9. 16. Nahmad M, Glass L, Abouheif E. The dynamics of developmental system drift in the gene network underlying wing polyphenism in ants: a mathematical model. Evol Dev 2008; 10:360-74.

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17. True JR, Haag ES. Developmental system drift and flexibility in evolutionary trajectories. Evol Dev 2001; 3:109-19. 18. Locascio A, Manzanares M, Blanco MJ, Nieto MA. Modularity and reshuffling of Snail and Slug expression during vertebrate evolution. Proc Natl Acad Sci USA 2002; 99:16841-6. 19. Angielczyk KD. Dimetrodon Is not a dinosaur: using tree thinking to understand the ancient relatives of mammals and their evolution. Evo Dev Outread 2009; 2:257-77. 20. Carl TF, Dufton C, Hanken J, Klymkowsky MW. Inhibition of neural crest migration in Xenopus using antisense slug RNA. Dev Biol 1999; 213:101-15. 21. Nieto MA, Sargent MG, Wilkinson DG, Cooke J. Control of cell behavior during vertebrate development by Slug, a zinc finger gene. Science 1994; 264:835-9. 22. Zhang C, Carl TF, Trudeau ED, Simmet T, Klymkowsky MW. An NFkappaB and slug regulatory loop active in early vertebrate mesoderm. PLoS ONE 2006; 1:106. 23. Murray SA, Gridley T. Snail family genes are required for left-right asymmetry determination, but not neural crest formation, in mice. Procs Natl Acad Sci USA 2006; 103:10300-4. 24. Blewitt ME, Chong S, Whitelaw E. How the mouse got its spots. Trends Genet 2004; 20:550-4. 25. Elowitz MB, Levine AJ, Siggia ED, Swain PS. Stochastic gene expression in a single cell. Science 2002; 297:1183-6. 26. Meulemans D, Bronner-Fraser M. Gene-regulatory interactions in neural crest evolution and development. Dev Cell 2004; 7:291-9. 27. Sauka-Spengler T, Bronner-Fraser M. A gene regulatory network orchestrates neural crest formation. Nat Rev Mol Cell Biol 2008; 557-68. 28. Knecht AK, Bronner-Fraser M. Induction of the neural crest: a multigene process. Nat Rev Genet 2002; 3: 453-61. 29. DuShane GP. Neural fold derivatives in the amphibia: Pigment cells, spinal ganglia and Rohon-Beard cells. J Comp Neurol 1938:485-503. 30. Smith JC. Hedgehog, the floor plate and the zone of polarizing activity. Cell 1994; 76:193-6. 31. Smith M, Hickman A, Amaze D, Lumsden A, Thorogood P. Trunk neural crest origin of caudal fin mesenchyme in the zebrafish Brachydanio rerio. P Roy Soc Lond B Bio 1994; 256:137-45. 32. Bolande RP. Neurocristopathy: its growth and development in 20 years. Pediatr Pathol Lab Med 1997; 17: 1-25. 33. Gont LK, Steinbeisser H, Blumberg B, de Robertis EM. Tail formation as a continuation of gastrulation: the multiple cell populations of the Xenopus tailbud derive from the late blastopore lip. Development 1993; 119:991-1004. 34. Handrigan GR. Concordia discors: duality in the origin of the vertebrate tail. J Anat 2003; 202:255-67. 35. Bronner-Fraser M, Fraser SE. Cell lineage analysis of the avian neural crest. Development 1991; 2:17-22. 36. Bronner-Fraser M. Distribution of latex beads and retinal pigment epithelial cells along the ventral neural crest pathway. Dev Biol 1982; 91:50-63. 37. Jesuthasan S. Neural crest cell migration in the zebrafish can be mimicked by inert objects: Mechanism and implication of latex bead movement in embryos. J Exp Biol 1997; 277:425-34. 38. Ragland JW, Raible DW. Signals derived from the underlying mesoderm are dispensable for zebrafish neural crest induction. Dev Biol 2004; 276:16-30. 39. Bonstein L, Elias S, Frank D. Paraxial-fated mesoderm is required for neural crest induction in Xenopus embryos. Dev Biol 1998; 193:156-68. 40. Moury JD, Jacobson AG. The origins of neural crest cells in the axolotl. Dev Biol 1990; 141:243-53. 41. Mancilla A, Mayor R. Neural crest formation in Xenopus laevis: mechanisms of Xslug induction. Dev Biol 1996; 177:580-9.


42. Cortez-Rossi C, Hernandez-Lagunas L, Zhang C, Choi IF, Kwok L, Klymkowsky MW, et al. Rohon-Beard sensory neurons are induced by BMP4 expressing nonneuronal ectoderm in Xenopus laevis. Dev Biol 2008; 314:351-61. 43. Wardle FC, Smith JC. Refinement of gene expression patterns in the early Xenopus embryo. Development 2004; 131:4687-96. 44. Kirby BB, Takada N, Latimer AJ, Shin J, Carney TJ, Kelsh RN, et al. In vivo time-lapse imaging shows dynamic oligodendrocyte progenitor behavior during zebrafish development. Nat Neurosci 2006; 9:1506-11. 45. Keller PJ, Schmidt AD, Wittbrodt J, Stelzer EH. Reconstruction of zebrafish early embryonic development by scanned light sheet microscopy. Science 2008; 322:1065-9. 46. Livet J, Weissman TA, Kang H, Draft RW, Lu J, Bennis RA, et al. Transgenic strategies for combinatorial expression of fluorescent proteins in the nervous system. Nature 2007; 450:56-62. 47. Bajpai R, Chen DA, Rada-Iglesias A, Zhang J, Xiong Y, Helms J, et al. CHD7 cooperates with PBAF to control multipotent neural crest formation. Nature 2010; 463:958-62. 48. Nichane M, Ren X, Bellefroid EJ. Self-regulation of Stat3 activity coordinates cell cycle progression and neural crest specification. EMBO J 2010; 29:55-67. 49. Kourakis MJ, Smith WC. Did the first chordates organize without the organizer? Trends Genet 2005; 21:506-10. 50. Kumano G, Smith WC. Revisions to the Xenopus gastrula fate map: Implications for mesoderm induction and patterning. Dev Dyn 2002; 225:409-21. 51. Moody SA. Fates of the blastomeres of the 16-cell stage Xenopus embryo. Dev Biol 1987; 119:560-78. 52. Moody SA. Fates of the blastomeres of the 32-cell-stage Xenopus embryo. Dev Biol 1987; 122:300-19. 53. Cary RB, Klymkowsky MW. Desmin organization during the differentiation of the dorsal myotome in Xenopus laevis. Differentiation 1994; 56:31-8. 54. Dupont S, Zacchigna L, Cordenonsi M, Soligo S, Adorno M, Rugge M, et al. Germ-layer specification and control of cell growth by Ectodermin, a Smad4 ubiquitin ligase. Cell 2005; 121:87-99. 55. Vonica A, Brivanlou AH. The left-right axis is regulated by the interplay of Coco, Xnr1 and derriere in Xenopus embryos. Dev Biol 2007; 303:281-94. 56. Bell E, Munoz-Sanjuan I, Altmann CR, Vonica A, Brivanlou AH. Cell fate specification and competence by Coco, a maternal BMP, TGFbeta and Wnt inhibitor. Development 2003; 130:1381-9. 57. Suri C, Haremaki T, Weinstein DC. Xema, a foxi-class gene expressed in the gastrula stage Xenopus ectoderm, is required for the suppression of mesendoderm. Development 2005; 132:2733-42. 58. Ryan K, Butler K, Bellefroid E, Gurdon JB. Xenopus Eomesodermin is expressed in neural differentiation. Mech Dev 1998; 75:167-70. 59. Cunliffe V, Smith JC. Ectopic mesoderm formation in Xenopus embryos caused by widespread expression of a Brachyury homologue. Nature 1992; 358:427-30. 60. Latinkic BV, Umbhauer M, Neal KA, Lerchner W, Smith JC, Cunliffe V. The Xenopus Brachyury promoter is activated by FGF and low concentrations of activin and suppressed by high concentrations of activin and by paired-type homeodomain proteins [published erratum appears in Genes Dev 1998;12:1240]. Genes Dev 1997; 11:3265-76. 61. Hayata T, Eisaki A, Kuroda H, Asashima M. Expression of Brachyury-like T-box transcription factor, Xbra3 in Xenopus embryo. Dev Genes Evol 1999; 209:560-3. 62. Stennard F, Zorn AM, Ryan K, Garrett N, Gurdon JB. Differential expression of VegT and Antipodean protein isoforms in Xenopus. Mech Dev 1999; 86:87-98. 63. Stennard F, Carnac G, Gurdon JB. The Xenopus T-box gene, Antipodean, encodes a vegetally localised maternal mRNA and can trigger mesoderm formation. Development 1996; 122:4179-88.


64. Callery EM, Thomsen GH, Smith JC. A divergent Tbx6-related gene and Tbx6 are both required for neural crest and intermediate mesoderm development in Xenopus. Dev Biol 2010; 340:75-87. 65. Lou X, Fang P, Li S, Hu RY, Kuerner KM, Steinbeisser H, et al. Xenopus Tbx6 mediates posterior patterning via activation of Wnt and FGF signalling. Cell Res 2006; 16:771-9. 66. Uchiyama H, Kobayashi T, Yamashita A, Ohno S, Yabe S. Cloning and characterization of the T-box gene Tbx6 in Xenopus laevis. Dev Growth Differ 2001; 43: 657-69. 67. Yabe S, Tazumi S, Yokoyama J, Uchiyama H. Xtbx6r, a novel T-box gene expressed in the paraxial mesoderm, has anterior neural-inducing activity. Int J Dev Biol 2006; 50:681-9. 68. Fukuda M, Takahashi S, Haramoto Y, Onuma Y, Kim YJ, Yeo CY, et al. Zygotic VegT is required for Xenopus paraxial mesoderm formation and is regulated by Nodal signaling and Eomesodermin. Int J Dev Biol 2010; 54:81-92. 69. Shi J, Severson C, Wedlich D, Klymkowsky MW. Mesodermal Slug/Snail2 regulates neural crest induction. In preparation. 70. Zhang C, Klymkowsky MW. Unexpected functional redundancy between Twist and Slug (Snail2) and their feedback regulation of NFkappaB via Nodal and Cerberus. Dev Biol 2009; 331:340-9. 71. De Robertis EM. Spemann’s organizer and the selfregulation of embryonic fields. Mech Dev 2009; 126:925-41. 72. Plouhinec JL, De Robertis EM. Systems biology of the self-regulating morphogenetic gradient of the Xenopus Gastrula. Cold Spring Harb Perspect Biol 2009; 1:001701. 73. Sasai Y, Lu B, Steinbeisser H, Geissert D, Gont LK, De Robertis EM. Xenopus chordin: a novel dorsalizing factor activated by organizer-specific homeobox genes. Cell 1994; 79:779-90. 74. Smith WC, Harland RM. Expression cloning of noggin, a new dorsalizing factor localized to the Spemann organizer in Xenopus embryos. Cell 1992; 70:829-40. 75. Tashiro K, Yamada R, Asano M, Hashimoto M, Muramatsu M, Shiokawa K. Expression of mRNA for activin-binding protein (follistatin) during early embryonic development of Xenopus laevis. Biochem Biophys Res Commun 1991; 174:1022-7. 76. Leyns L, Bouwmeester T, Kim SH, Piccolo S, De Robertis EM. Frzb-1 is a secreted antagonist of Wnt signaling expressed in the Spemann organizer. Cell 1997; 88:747-56. 77. Glinka A, Wu W, Delius H, Monaghan AP, Blumenstock C, Niehrs C. Dickkopf-1 is a member of a new family of secreted proteins and functions in head induction. Nature 1998; 391:357-62. 78. Shibata M, Ono H, Hikasa H, Shinga J, Taira M. Xenopus crescent encoding a Frizzled-like domain is expressed in the Spemann organizer and pronephros. Mech Dev 2000; 96:243-6. 79. Moos M Jr, Wang S, Krinks M. Anti-dorsalizing morphogenetic protein is a novel TGF-beta homolog expressed in the Spemann organizer. Development 1995; 121:4293-301. 80. Clement JH, Fettes P, Knochel S, Lef J, Knochel W. Bone morphogenetic protein 2 in the early development of Xenopus laevis. Mech Dev 1995; 52:357-70. 81. Pera EM, Wessely O, Li SY, De Robertis EM. Neural and head induction by insulin-like growth factor signals. Dev Cell 2001; 1:655-65. 82. Bouwmeester T, Kim S, Sasai Y, Lu B, De Robertis EM. Cerberus is a head-inducing secreted factor expressed in the anterior endoderm of Spemann’s organizer. Nature 1996; 382:595-601. 83. Nagano T, Takehara S, Takahashi M, Aizawa S, Yamamoto A. Shisa2 promotes the maturation of somitic precursors and transition to the segmental fate in Xenopus embryos. Development 2006; 133: 4643-54.

605

84. Yamamoto A, Nagano T, Takehara S, Hibi M, Aizawa S. Shisa promotes head formation through the inhibition of receptor protein maturation for the caudalizing factors, Wnt and FGF. Cell 2005; 120:223-35. 85. Goodman SA, Albano R, Wardle FC, Matthews G, Tannahill D, Dale L. BMP1-related metalloproteinases promote the development of ventral mesoderm in early Xenopus embryos. Dev Biol 1998; 195:144-57. 86. Lin JJ, Maeda R, Ong RC, Kim J, Lee LM, Kung H, et al. XBMP-1B (Xtld), a Xenopus homolog of dorsoventral polarity gene in Drosophila, modifies tissue phenotypes of ventral explants. Dev Growth Differ 1997; 39:43-51. 87. Piccolo S, Agius E, Lu B, Goodman S, Dale L, De Robertis EM. Cleavage of Chordin by Xolloid metalloprotease suggests a role for proteolytic processing in the regulation of Spemann organizer activity. Cell 1997; 91:407-16. 88. Salic AN, Kroll KL, Evans LM, Kirschner MW. Sizzled: a secreted Xwnt8 antagonist expressed in the ventral marginal zone of Xenopus embryos. Development 1997; 124:4739-48. 89. Dale L, Howes G, Price BM, Smith JC. Bone morphogenetic protein 4: a ventralizing factor in early Xenopus development. Development 1992; 115:573-85. 90. Jones CM, Dale L, Hogan BL, Wright CV, Smith JC. Bone morphogenetic protein-4 (BMP-4) acts during gastrula stages to cause ventralization of Xenopus embryos. Development 1996; 122:1545-54. 91. Wang S, Krinks M, Kleinwaks L, Moos M Jr. A novel Xenopus homologue of bone morphogenetic protein-7 (BMP-7). Genes Funct 1997; 1:259-71. 92. Suzuki A, Kaneko E, Ueno N, Hemmati BA. Regulation of epidermal induction by BMP2 and BMP7 signaling. Dev Biol 1997; 189:112-22. 93. Ambrosio AL, Taelman VF, Lee HX, Metzinger CA, Coffinier C, De Robertis EM. Crossveinless-2 Is a BMP feedback inhibitor that binds Chordin/BMP to regulate Xenopus embryonic patterning. Dev Cell 2008; 15:248-60. 94. Blitz IL, Cho KW, Chang C. Twisted gastrulation lossof-function analyses support its role as a BMP inhibitor during early Xenopus embryogenesis. Development 2003; 130:4975-88. 95. Oelgeschlager M, Larrain J, Geissert D, De Robertis EM. The evolutionarily conserved BMP-binding protein Twisted gastrulation promotes BMP signalling. Nature 2000; 405:757-63. 96. Hsieh JC, Kodjabachian L, Rebbert ML, Rattner A, Smallwood PM, Samos CH, et al. A new secreted protein that binds to Wnt proteins and inhibits their activities. Nature 1999; 398:431-6. 97. Hsu DR, Economides AN, Wang X, Eimon PM, Harland RM. The Xenopus dorsalizing factor Gremlin identifies a novel family of secreted proteins that antagonize BMP activities. Mol Cell 1998; 1: 673-83. 98. Mii Y, Taira M. Secreted Frizzled-related proteins enhance the diffusion of Wnt ligands and expand their signalling range. Development 2009; 136:4083-8. 99. Bovolenta P, Esteve P, Ruiz JM, Cisneros E, Lopez-Rios J. Beyond Wnt inhibition: new functions of secreted Frizzled-related proteins in development and disease. J Cell Sci 2008; 121:737-46. 100. Collavin L, Kirschner MW. The secreted Frizzledrelated protein Sizzled functions as a negative feedback regulator of extreme ventral mesoderm. Development 2003; 130:805-16. 101. Lee HX, Ambrosio AL, Reversade B, De Robertis EM. Embryonic dorsal-ventral signaling: secreted frizzledrelated proteins as inhibitors of tolloid proteinases. Cell 2006; 124:147-59. 102. Muraoka O, Shimizu T, Yabe T, Nojima H, Bae YK, Hashimoto H, et al. Sizzled controls dorso-ventral polarity by repressing cleavage of the Chordin protein. Nat Cell Biol 2006; 8:329-38.

606

103. Kobayashi K, Luo M, Zhang Y, Wilkes DC, Ge G, Grieskamp T, et al. Secreted Frizzled-related protein 2 is a procollagen C proteinase enhancer with a role in fibrosis associated with myocardial infarction. Nat Cell Biol 2009; 11:46-55. 104. Chang C, Hemmati BA. Neural crest induction by Xwnt7B in Xenopus. Dev Biol 1998; 194:129-34. 105. LaBonne C, Bronner-Fraser M. Neural crest induction in Xenopus: evidence for a two-signal model. Development 1998; 125:2403-14. 106. Saint-Jeannet JP, He X, Varmus HE, Dawid IB. Regulation of dorsal fate in the neuraxis by Wnt-1 and Wnt-3a. Proc Natl Acad Sci USA 1997; 94:13713-8. 107. Deardorff MA, Tan C, Saint-Jeannet JP, Klein PS. A role for frizzled 3 in neural crest development. Development 2001; 128:3655-63. 108. Abu-Elmagd M, Garcia-Morales C, Wheeler GN. Frizzled7 mediates canonical Wnt signaling in neural crest induction. Dev Biol 2006; 298:285-98. 109. Tan C, Deardorff MA, Saint-Jeannet JP, Yang J, Arzoumanian A, Klein PS. Kermit, a frizzled interacting protein, regulates frizzled 3 signaling in neural crest development. Development 2001; 128:3665-74. 110. Hassler C, Cruciat CM, Huang YL, Kuriyama S, Mayor R, Niehrs C. Kremen is required for neural crest induction in Xenopus and promotes LRP6-mediated Wnt signaling. Development 2007; 134:4255-63. 111. Wang Y, Fu Y, Gao L, Zhu G, Liang J, Gao C, et al. Xenopus skip modulates Wnt/beta-catenin signaling and functions in neural crest induction. J Biol Chem 2010; 285:10890-901. 112. Monsoro-Burq AH, Wang E, Harland R. Msx1 and Pax3 cooperate to mediate FGF8 and WNT signals during Xenopus neural crest induction. Dev Cell 2005; 8:167-78. 113. Monsoro-Burq AH, Fletcher RB, Harland RM. Neural crest induction by paraxial mesoderm in Xenopus embryos requires FGF signals. Development 2003; 130:3111-24. 114. Villanueva S, Glavic A, Ruiz P, Mayor R. Posteriorization by FGF, Wnt and retinoic acid is required for neural crest induction. Dev Biol 2002; 241:289-301. 115. Burks PJ, Isaacs HV, Pownall ME. FGF signalling modulates transcriptional repression by Xenopus grouchorelated-4. Biol Cell 2009; 101:301-8. 116. Luo T, Matsuo-Takasaki M, Thomas ML, Weeks DL, Sargent TD. Transcription factor AP-2 is an essential and direct regulator of epidermal development in Xenopus. Dev Biol 2002; 245:136-44. 117. Li B, Kuriyama S, Moreno M, Mayor R. The posteriorizing gene Gbx2 is a direct target of Wnt signalling and the earliest factor in neural crest induction. Development 2009; 136:3267-78. 118. Hong CS, Park BY, Saint-Jeannet JP. Fgf8a induces neural crest indirectly through the activation of Wnt8 in the paraxial mesoderm. Development 2008; 135:3903-10. 119. Wu J, Yang J, Klein PS. Neural crest induction by the canonical Wnt pathway can be dissociated from anterior-posterior neural patterning in Xenopus. Dev Biol 2005; 279:220-32. 120. Heeg-Truesdell E, LaBonne C. Neural induction in Xenopus requires inhibition of Wnt-beta-catenin signaling. Dev Biol 2006; 298:71-86. 121. Zhang C, Basta T, Hernandez-Lagunas L, Simpson P, Stemple DL, Artinger KB, et al. Repression of nodal expression by maternal B1-type SOXs regulates germ layer formation in Xenopus and zebrafish. Dev Biol 2004; 273:23-37. 122. Zhang C, Basta T, Jensen ED, Klymkowsky MW. The beta-catenin/VegT-regulated early zygotic gene Xnr5 is a direct target of SOX3 regulation. Development 2003; 130:5609-24.


123. Zorn AM, Barish GD, Williams BO, Lavender P, Klymkowsky MW, Varmus HE. Regulation of Wnt signaling by Sox proteins: XSox17 alpha/beta and XSox3 physically interact with beta-catenin. Mol Cell 1999; 4:487-98. 124. Carmona-Fontaine C, Acuna G, Ellwanger K, Niehrs C, Mayor R. Neural crests are actively precluded from the anterior neural fold by a novel inhibitory mechanism dependent on Dickkopf1 secreted by the prechordal mesoderm. Dev Biol 2007; 309:208-21. 125. Mayor R, Guerrero N, Young RM, Gomez-Skarmeta JL, Cuellar C. A novel function for the Xslug gene: control of dorsal mesendoderm development by repressing BMP-4. Mech Dev 2000; 97:47-56. 126. Savagner P. Leaving the neighborhood: molecular mechanisms involved during epithelial-mesenchymal transition. Bioessays 2001; 23:912-23. 127. LaBonne C, Bronner-Fraser M. Snail-related transcriptional repressors are required in Xenopus for both the induction of the neural crest and its subsequent migration. Dev Biol 2000; 221:195-205. 128. Plouhinec JL, De Robertis EM. Systems biology of embryonic morphogens. Mol Biosyst 2007; 3:454-7. 129. Garcia-Castro MI, Marcelle C, Bronner-Fraser M. Ectodermal Wnt Function as a neural crest inducer. Science 2002. 130. Marchant L, Linker C, Ruiz P, Guerrero N, Mayor R. The inductive properties of mesoderm suggest that the neural crest cells are specified by a BMP gradient. Dev Biol 1998; 198:319-29. 131. Tribulo C, Aybar MJ, Nguyen VH, Mullins MC, Mayor R. Regulation of Msx genes by a Bmp gradient is essential for neural crest specification. Development 2003; 130:6441-52. 132. Sato T, Sasai N, Sasai Y. Neural crest determination by co-activation of Pax3 and Zic1 genes in Xenopus ectoderm. Development 2005; 132:2355-63. 133. Zhao H, Tanegashima K, Ro H, Dawid IB. Lrig3 regulates neural crest formation in Xenopus by modulating Fgf and Wnt signaling pathways. Development 2008; 135:1283-93. 134. Hay ED. An overview of epithelio-mesenchymal transformation. Acta Anat (Basel) 1995; 154:8-20. 135. Shoval I, Ludwig A, Kalcheim C. Antagonistic roles of full-length N-cadherin and its soluble BMP cleavage product in neural crest delamination. Development 2007; 134:491-501. 136. Nagatomo K, Hashimoto C. Xenopus hairy2 functions in neural crest formation by maintaining cells in a mitotic and undifferentiated state. Dev Dyn 2007; 236:1475-83. 137. Nichane M, de Croze N, Ren X, Souopgui J, MonsoroBurq AH, Bellefroid EJ. Hairy2-Id3 interactions play an essential role in Xenopus neural crest progenitor specification. Dev Biol 2008; 322:355-67. 138. Ebendal T. Extracellular matrix fibrils and cell contacts in the chick embryo. Possible roles in orientation of cell migration and axon extension. Cell Tissue Res 1977; 175:439-58. 139. Ebendal T. The relative roles of contact inhibition and contact guidance in orientation of axons extending on aligned collagen fibrils in vitro. Exp Cell Res 1976; 98:159-69. 140. Erickson CA. Control of pathfinding by the avian trunk neural crest. Development 1988; 103:63-80. 141. Robinson V, Smith A, Flenniken AM, Wilkinson DG. Roles of Eph receptors and ephrins in neural crest pathfinding. Cell Tissue Res 1997; 290:265-74. 142. Smith A, Robinson V, Patel K, Wilkinson DG. The EphA4 and EphB1 receptor tyrosine kinases and ephrin-B2 ligand regulate targeted migration of branchial neural crest cells. Curr Biol 1997; 7:561-70. 143. Davis EM, Trinkaus JP. Significance of cell-to cell contacts for the directional movement of neural crest cells within a hydrated collagen lattice. J Embryol Exp Morphol 1981; 63:29-51.

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144. Keller RE, Spieth J. Neural crest cell behavior in white and dark larvae of Ambystoma mexicanum: time-lapse cinemicrographic analysis of pigment cell movement in vivo and in culture. J Exp Zool 1984; 229:109-26. 145. Spieth J, Keller RE. Neural crest cell behavior in white and dark larvae of Ambystoma mexicanum: differences in cell morphology, arrangement and extracellular matrix as related to migration. J Exp Zool 1984; 229:91-107. 146. Gooday D, Thorogood P. Contact behaviour exhibited by migrating neural crest cells in confrontation culture with somitic cells. Cell Tissue Res 1985; 241:165-9. 147. Matthews HK, Marchant L, Carmona-Fontaine C, Kuriyama S, Larrain J, Holt MR, et al. Directional migration of neural crest cells in vivo is regulated by Syndecan-4/Rac1 and non-canonical Wnt signaling/ RhoA. Development 2008; 135:1771-80. 148. Jesuthasan S. Contact inhibition/collapse and pathfinding of neural crest cells in the zebrafish trunk. Development 1996; 122:381-9. 149. Carmona-Fontaine C, Matthews HK, Kuriyama S, Moreno M, Dunn GA, Parsons M, et al. Contact inhibition of locomotion in vivo controls neural crest directional migration. Nature 2008; 456:957-61. 150. Klymkowsky MW, Parr B. A glimpse into the body language of cells: the intimate connection between cell adhesion and gene expression. Cell 1995; 83:5-8. 151. Alfandari D, Cousin H, Gaultier A, Hoffstrom BG, DeSimone DW. Integrin alpha5beta1 supports the migration of Xenopus cranial neural crest on fibronectin. Dev Biol 2003; 260:449-64. 152. Guiral EC, Faas L, Pownall ME. Neural crest migration requires the activity of the extracellular sulphatases XtSulf1 and XtSulf2. Dev Biol 2010. 153. Dent JA, Polson AG, Klymkowsky MW. A wholemount immunocytochemical analysis of the expression of the intermediate filament protein vimentin in Xenopus. Development 1989; 105:61-74. 154. Bolos V, Peinado H, Perez-Moreno MA, Fraga MF, Esteller M, Cano A. The transcription factor Slug represses E-cadherin expression and induces epithelial to mesenchymal transitions: a comparison with Snail and E47 repressors. J Cell Sci 2003; 116:499-511. 155. Hajra KM, Chen DY, Fearon ER. The SLUG zincfinger protein represses E-cadherin in breast cancer. Cancer Res 2002; 62:1613-8. 156. Yang J, Mani SA, Donaher JL, Ramaswamy S, Itzykson RA, Come C, et al. Twist, a master regulator of morphogenesis, plays an essential role in tumor metastasis. Cell 2004; 117:927-39. 157. Vallin J, Thuret R, Giacomello E, Faraldo MM, Thiery JP, Broders F. Cloning and characterization of three Xenopus Slug promoters reveal direct regulation by Lef/beta-catenin signaling. J Biol Chem 2001; 287:30350-8. 158. Inukai T, Inoue A, Kurosawa H, Goi K, Shinjyo T, Ozawa K, et al. SLUG, a ces-1-related zinc finger transcription factor gene with antiapoptotic activity, is a downstream target of the E2A-HLF oncoprotein. Mol Cell 1999; 4:343-52. 159. Maestro R, Dei Tos AP, Hamamori Y, Krasnokutsky S, Sartorelli V, Kedes L, et al. Twist is a potential oncogene that inhibits apoptosis. Genes Dev 1999; 13:2207-17. 160. Tribulo C, Aybar MJ, Sanchez SS, Mayor R. A balance between the anti-apoptotic activity of Slug and the apoptotic activity of msx1 is required for the proper development of the neural crest. Dev Biol 2004; 275:325-42. 161. Wu WS, Heinrichs S, Xu D, Garrison SP, Zambetti GP, Adams JM, et al. Slug antagonizes p53-mediated apoptosis of hematopoietic progenitors by repressing puma. Cell 2005; 123:641-53. 162. Choi YS, Sehgal R, McCrea P, Gumbiner B. A cadherin-like protein in eggs and cleaving embryos of Xenopus laevis is expressed in oocytes in response to progesterone. J Cell Biol 1990; 110:1575-82.


163. Choi YS, Gumbiner B. Expression of cell adhesion molecule E-cadherin in Xenopus embryos begins at gastrulation and predominates in the ectoderm. J Cell Biol 1989; 108:2449-58. 164. Levi G, Gumbiner B, Thiery JP. The distribution of E-cadherin during Xenopus laevis development. Development 1991; 111:159-69. 165. Hadeball B, Borchers A, Wedlich D. Xenopus cadherin-11 (Xcadherin-11) expression requires the Wg/ Wnt signal. Mech Dev 1998; 72:101-13. 166. Vallin J, Girault JM, Thiery JP, Broders F. Xenopus cadherin-11 is expressed in different populations of migrating neural crest cells. Mech Dev 1998; 75:183-6. 167. Rangarajan J, Luo T, Sargent TD. PCNS: a novel protocadherin required for cranial neural crest migration and somite morphogenesis in Xenopus. Dev Biol 2006; 295:206-18. 168. Gessert S, Kuhl M. Comparative gene expression analysis and fate mapping studies suggest an early segregation of cardiogenic lineages in Xenopus laevis. Dev Biol 2009; 334:395-408. 169. Borchers A, David R, Wedlich D. Xenopus cadherin-11 restrains cranial neural crest migration and influences neural crest specification. Development 2001; 128:3049-60. 170. Heasman J, Crawford A, Goldstone K, Garner-Hamrick P, Gumbiner B, McCrea P, et al. Overexpression of cadherins and underexpression of b-catenin inhibit dorsal mesoderm induction in early Xenopus embryos. Cell 1994; 79:791-803. 171. McCusker C, Cousin H, Neuner R, Alfandari D. Extracellular cleavage of cadherin-11 by ADAM metalloproteases is essential for Xenopus cranial neural crest cell migration. Mol Biol Cell 2009; 20:78-89. 172. Kashef J, Kohler A, Kuriyama S, Alfandari D, Mayor R, Wedlich D. Cadherin-11 regulates protrusive activity in Xenopus cranial neural crest cells upstream of Trio and the small GTPases. Genes Dev 2009; 23:1393-8. 173. Hwang YS, Luo T, Xu Y, Sargent TD. Myosin-X is required for cranial neural crest cell migration in Xenopus laevis. Dev Dyn 2009; 238:2522-9. 174. Nie S, Kee Y, Bronner-Fraser M. Myosin-X is critical for migratory ability of Xenopus cranial neural crest cells. Dev Biol 2009; 335:132-42. 175. De Calisto J, Araya C, Marchant L, Riaz CF, Mayor R. Essential role of non-canonical Wnt signalling in neural crest migration. Development 2005; 132:2587-97. 176. Matthews HK, Broders-Bondon F, Thiery JP, Mayor R. Wnt11r is required for cranial neural crest migration. Dev Dyn 2008; 237:3404-9. 177. Shnitsar I, Borchers A. PTK7 recruits dsh to regulate neural crest migration. Development 2008; 135: 4015-24. 178. Gessert S, Maurus D, Rossner A, Kuhl M. Pescadillo is required for Xenopus laevis eye development and neural crest migration. Dev Biol 2007; 310:99-112. 179. Neave B, Holder N, Patient R. A graded response to BMP-4 spatially coordinates patterning of the mesoderm and ectoderm in the zebrafish. Mech Dev 1997; 62:183-95. 180. Nguyen VH, Schmid B, Trout J, Connors SA, Ekker M, Mullins MC. Ventral and lateral regions of the zebrafish gastrula, including the neural crest progenitors, are established by a bmp2b/swirl pathway of genes. Dev Biol 1998; 199:93-110. 181. Lewis JL, Bonner J, Modrell M, Ragland JW, Moon RT, Dorsky RI, et al. Reiterated Wnt signaling during zebrafish neural crest development. Development 2004; 131:1299-308. 182. Chitnis AB. Primary neurogenesis in Xenopus embryos regulated by a homologue of the Drosophila neurogenic gene Delta. Nature 1995; 375:761-6. 183. Chitnis AB. The role of Notch in lateral inhibition and cell fate specification. Mol Cell Neurosci 1995; 6: 311-21.


184. Appel B, Fritz A, Westerfield M, Grunwald DJ, Eisen JS, Riley BB. Delta-mediated specification of midline cell fates in zebrafish embryos. Curr Biol 1999; 9: 247-56. 185. Cornell RA, Eisen JS. Delta/Notch signaling promotes formation of zebrafish neural crest by repressing Neurogenin 1 function. Development 2002; 129:2639-48. 186. Cornell RA, Eisen JS. Delta signaling mediates segregation of neural crest and spinal sensory neurons from zebrafish lateral neural plate. Development 2000; 127:2873-82. 187. Itoh M, Kim CH, Palardy G, Oda T, Jiang YJ, Maust D, et al. Mind bomb is a ubiquitin ligase that is essential for efficient activation of Notch signaling by Delta. Dev Cell 2003; 4:67-82. 188. Jiang YJ, Brand M, Heisenberg CP, Beuchle D, Furutani-Seiki M, Kelsh RN, et al. Mutations affecting neurogenesis and brain morphology in the zebrafish, Danio rerio. Development 1996; 123:205-16. 189. Schier AF, Neuhauss SC, Harvey M, Malicki J, SolnicaKrezel L, Stainier DY, et al. Mutations affecting the development of the embryonic zebrafish brain. Development 1996; 165-78. 190. Bronner-Fraser M, Fraser S. Developmental potential of avian trunk neural crest cells in situ. Neuron 1989; 3:755-66. 191. Bronner-Fraser M, Fraser SE. Cell lineage analysis reveals multipotency of some avian neural crest cells. Nature 1988; 335:161-4. 192. Serbedzija GN, Bronner-Fraser M, Fraser SE. Developmental potential of trunk neural crest cells in the mouse. Development 1994; 120:1709-18. 193. Serbedzija GN, Bronner-Fraser M, Fraser SE. Vital dye analysis of cranial neural crest cell migration in the mouse embryo. Development 1992; 116:297-307. 194. Raible DW, Eisen JS. Restriction of neural crest cell fate in the trunk of the embryonic zebrafish. Development 1994; 120:495-503. 195. Bikoff EK, Morgan MA, Robertson EJ. An expanding job description for Blimp-1/PRDM1. Curr Opin Genet Dev 2009; 19:379-85. 196. John SA, Garrett-Sinha LA. Blimp1: a conserved transcriptional repressor critical for differentiation of many tissues. Exp Cell Res 2009; 315:1077-84. 197. Artinger KB, Chitnis AB, Mercola M, Driever W. Zebrafish narrowminded suggests a genetic link between formation of neural crest and primary sensory neurons. Development 1999; 126:3969-79. 198. Hernandez-Lagunas L, Choi IF, Kaji T, Simpson P, Hershey C, Zhou Y, et al. Zebrafish narrowminded disrupts the transcription factor prdm1 and is required for neural crest and sensory neuron specification. Dev Biol 2005; 278:347-57. 199. Birkholz DA, Killian EC, George KM, Artinger KB. Prdm1a is necessary for posterior pharyngeal arch development in zebrafish. Dev Dyn 2009; 238: 2575-87. 200. de Souza FS, Gawantka V, Gomez AP, Delius H, Ang SL, Niehrs C. The zinc finger gene Xblimp1 controls anterior endomesodermal cell fate in Spemann’s organizer. EMBO J 1999; 18:6062-72. 201. Vincent SD, Dunn NR, Sciammas R, Shapiro-Shalef M, Davis MM, Calame K, et al. The zinc finger transcriptional repressor Blimp1/Prdm1 is dispensable for early axis formation but is required for specification of primordial germ cells in the mouse. Development 2005; 132:1315-25. 202. Phillips BT, Kwon HJ, Melton C, Houghtaling P, Fritz A, Riley BB. Zebrafish msxB, msxC and msxE function together to refine the neural-nonneural border and regulate cranial placodes and neural crest development. Dev Biol 2006; 294:376-90. 203. Rossi CC, Kaji T, Artinger KB. Transcriptional control of Rohon-Beard sensory neuron development at the neural plate border. Dev Dyn 2009; 238:931-43.

607

204. Kaji T, Artinger KB. dlx3b and dlx4b function in the development of Rohon-Beard sensory neurons and trigeminal placode in the zebrafish neurula. Dev Biol 2004; 276:523-40. 205. Esterberg R, Fritz A. dlx3b/4b are required for the formation of the preplacodal region and otic placode through local modulation of BMP activity. Dev Biol 2009; 325:189-99. 206. Carney TJ, Dutton KA, Greenhill E, Delfino-Machin M, Dufourcq P, Blader P, et al. A direct role for Sox10 in specification of neural crest-derived sensory neurons. Development 2006; 133:4619-30. 207. Li W, Cornell RA. Redundant activities of Tfap2a and Tfap2c are required for neural crest induction and development of other non-neural ectoderm derivatives in zebrafish embryos. Dev Biol 2007; 304:338-54. 208. O’Brien EK, d’Alencon C, Bonde G, Li W, Schoenebeck J, Allende ML, et al. Transcription factor Ap-2alpha is necessary for development of embryonic melanophores, autonomic neurons and pharyngeal skeleton in zebrafish. Dev Biol 2004; 265:246-61. 209. Stewart RA, Arduini BL, Berghmans S, George RE, Kanki JP, Henion PD, et al. Zebrafish foxd3 is selectively required for neural crest specification, migration and survival. Dev Biol 2006; 292:174-88. 210. Thisse C, Thisse B, Postlethwait JH. Expression of snail2, a second member of the zebrafish snail family, in cephalic mesendoderm and presumptive neural crest of wild-type and spadetail mutant embryos. Dev Biol 1995; 172:86-99. 211. Yan YL, Willoughby J, Liu D, Crump JG, Wilson C, Miller CT, et al. A pair of Sox: distinct and overlapping functions of zebrafish sox9 co-orthologs in craniofacial and pectoral fin development. Development 2005; 132:1069-83. 212. Papan C, Campos-Ortega JA. Region-specific cell clones in the developing spinal cord of the zebrafish. Dev Genes Evol 1999; 209:135-44. 213. Baker CV, Bronner-Fraser M. The origins of the neural crest. Part I: embryonic induction. Mech Dev 1997; 69:3-11. 214. Baker CV, Bronner-Fraser M. The origins of the neural crest. Part II: an evolutionary perspective. Mech Dev 1997; 69:13-29. 215. Kimmel CB, Ballard WW, Kimmel SR, Ullmann B, Schilling TF. Stages of embryonic development of the zebrafish. Dev Dyn 1995; 203:253-310. 216. Ungos JM, Karlstrom RO, Raible DW. Hedgehog signaling is directly required for the development of zebrafish dorsal root ganglia neurons. Development 2003; 130:5351-62. 217. Krull CE, Kulesa PM. Embryonic explant and slice preparations for studies of cell migration and axon guidance. Curr Top Dev Biol 1998; 36:145-59. 218. Krull CE, Lansford R, Gale NW, Collazo A, Marcelle C, Yancopoulos GD, et al. Interactions of Eph-related receptors and ligands confer rostrocaudal pattern to trunk neural crest migration. Curr Biol 1997; 7: 571-80.

608

219. Ranscht B, Bronner FM. T-cadherin expression alternates with migrating neural crest cells in the trunk of the avian embryo. Development 1991; 111:15-22. 220. Honjo Y, Eisen JS. Slow muscle regulates the pattern of trunk neural crest migration in zebrafish. Development 2005; 132:4461-70. 221. Raible DW, Wood A, Hodsdon W, Henion PD, Weston JA, Eisen JS. Segregation and early dispersal of neural crest cells in the embryonic zebrafish. Dev Dyn 1992; 195:29-42. 222. Piotrowski T, Schilling TF, Brand M, Jiang YJ, Heisenberg CP, Beuchle D, et al. Jaw and branchial arch mutants in zebrafish II: anterior arches and cartilage differentiation. Development 1996; 123:345-56. 223. Schilling TF, Piotrowski T, Grandel H, Brand M, Heisenberg CP, Jiang YJ, et al. Jaw and branchial arch mutants in zebrafish I: branchial arches. Development 1996; 123:329-44. 224. Wada N, Javidan Y, Nelson S, Carney TJ, Kelsh RN, Schilling TF. Hedgehog signaling is required for cranial neural crest morphogenesis and chondrogenesis at the midline in the zebrafish skull. Development 2005; 132:3977-88. 225. Eberhart JK, Swartz ME, Crump JG, Kimmel CB. Early Hedgehog signaling from neural to oral epithelium organizes anterior craniofacial development. Development 2006; 133:1069-77. 226. Langenberg T, Kahana A, Wszalek JA, Halloran MC. The eye organizes neural crest cell migration. Dev Dyn 2008; 237:1645-52. 227. Schilling TF. Genetic analysis of craniofacial development in the vertebrate embryo. Bioessays 1997; 19:459-68. 228. Schilling TF, Kimmel CB. Segment and cell type lineage restrictions during pharyngeal arch development in the zebrafish embryo. Development 1994; 120:483-94. 229. Kulesa PM, Fraser SE. Neural crest cell dynamics revealed by time-lapse video microscopy of whole embryo chick explant cultures. Dev Biol 1998; 204:327-44. 230. Eberhart JK, He X, Swartz ME, Yan YL, Song H, Boling TC, et al. MicroRNA Mirn140 modulates Pdgf signaling during palatogenesis. Nat Genet 2008; 40:290-8. 231. Kucia M, Jankowski K, Reca R, Wysoczynski M, Bandura L, Allendorf DJ, et al. CXCR4-SDF-1 signalling, locomotion, chemotaxis and adhesion. J Mol Histol 2004; 35:233-45. 232. Balabanian K, Lagane B, Infantino S, Chow KY, Harriague J, Moepps B, et al. The chemokine SDF-1/ CXCL12 binds to and signals through the orphan receptor RDC1 in T lymphocytes. J Biol Chem 2005; 280:35760-6. 233. Bleul CC, Farzan M, Choe H, Parolin C, Clark-Lewis I, Sodroski J, et al. The lymphocyte chemoattractant SDF-1 is a ligand for LESTR/fusin and blocks HIV-1 entry. Nature 1996; 382:829-33.


234. Belmadani A, Tran PB, Ren D, Assimacopoulos S, Grove EA, Miller RJ. The chemokine stromal cellderived factor-1 regulates the migration of sensory neuron progenitors. J Neurosci 2005; 25:3995-4003. 235. Rehimi R, Khalida N, Yusuf F, Dai F, MorosanPuopolo G, Brand-Saberi B. Stromal-derived factor-1 (SDF-1) expression during early chick development. Int J Dev Biol 2008; 52:87-92. 236. Svetic V, Hollway GE, Elworthy S, Chipperfield TR, Davison C, Adams RJ, et al. Sdf1a patterns zebrafish melanophores and links the somite and melanophore pattern defects in choker mutants. Development 2007; 134:1011-22. 237. Chong SW, Emelyanov A, Gong Z, Korzh V. Expression pattern of two zebrafish genes, cxcr4a and cxcr4b. Mech Dev 2001; 109:347-54. 238. Olesnicky Killian EC, Birkholz DA, Artinger KB. A role for chemokine signaling in neural crest cell migration and craniofacial development. Dev Biol 2009; 333:161-72. 239. Knaut H, Werz C, Geisler R, Nusslein-Volhard C. A zebrafish homologue of the chemokine receptor Cxcr4 is a germ-cell guidance receptor. Nature 2003; 421:279-82. 240. Davy A, Soriano P. Ephrin signaling in vivo: look both ways. Dev Dyn 2005; 232:1-10. 241. Davy A, Soriano P. Ephrin-B2 forward signaling regulates somite patterning and neural crest cell development. Dev Biol 2007; 304:182-93. 242. McLennan R, Kulesa PM. In vivo analysis reveals a critical role for neuropilin-1 in cranial neural crest cell migration in chick. Dev Biol 2007; 301:227-39. 243. Gammill LS, Gonzalez C, Gu C, Bronner-Fraser M. Guidance of trunk neural crest migration requires neuropilin 2/semaphorin 3F signaling. Development 2006; 133:99-106. 244. Roffers-Agarwal J, Gammill LS. Neuropilin receptors guide distinct phases of sensory and motor neuronal segmentation. Development 2009; 136:1879-88. 245. Schwarz Q, Maden CH, Davidson K, Ruhrberg C. Neuropilin-mediated neural crest cell guidance is essential to organise sensory neurons into segmented dorsal root ganglia. Development 2009; 136:1785-9. 246. Yu HH, Moens CB. Semaphorin signaling guides cranial neural crest cell migration in zebrafish. Dev Biol 2005; 280:373-85. 247. Drerup CM, Wiora HM, Topczewski J, Morris JA. Disc1 regulates foxd3 and sox10 expression, affecting neural crest migration and differentiation. Development 2009; 136:2623-32. 248. Klymkowsky MW, Savagner P. Epithelial-mesenchymal transition: a cancer researcher’s conceptual friend and foe. Am J Pathol 2009; 174:1588-93. 249. Romer AS. The Vertebrate Body. Philadelphia: Saunders WB 1964.

Volume 4 Issue 4