Accepted Manuscript Title: Chemotaxis during neural crest migration Author: Adam Shellard Roberto Mayor PII: DOI: Reference:
S1084-9521(16)30031-3 http://dx.doi.org/doi:10.1016/j.semcdb.2016.01.031 YSCDB 1942
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Seminars in Cell & Developmental Biology
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21-12-2015 22-1-2016
Please cite this article as: Shellard Adam, Mayor Roberto.Chemotaxis during neural crest migration.Seminars in Cell and Developmental Biology http://dx.doi.org/10.1016/j.semcdb.2016.01.031 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Chemotaxis during neural crest migration Running title: Chemotaxis in neural crest Adam Shellard, Roberto Mayor*
[email protected] Department of Cell and Developmental Biology, University College London, Gower Street, London WC1E 6BT, UK. *
Corresponding author.
Abstract Chemotaxis refers to the directional migration of cells towards external, soluble factors along their gradients. It is a process that is used by many different cell types during development for tissue organisation and the formation of embryonic structures, as well as disease like cancer metastasis. The neural crest (NC) is a multipotent, highly migratory cell population that contribute to a range of tissues. It has been hypothesised that NC migration, at least in part, is reliant on chemotactic signals. This review will explore the current evidence for proposed chemoattractants of NC cells, and outline mechanisms for the chemotactic response of the NC to them. Keywords: chemotaxis; neural crest; collective migration; SDF1/CXCL12; VEGF; C3/C3aR
1. Chemotaxis 1.1 Definition Cell migration is fundamental to many processes in development and disease, including embryonic morphogenesis, wound healing and the immune response [1]. This often involves cells responding to specific signals that guide their movement, either from mechanical stimuli, molecules bound to the extracellular matrix or soluble external factors [2‐8]. Cell migration in response to gradients of the latter, called chemotaxis, has been widely studied and it is a well‐established mechanism that provides directionality and persistence to migrating cells [7, 9, 10]. The chemotactic response of cells, in part, involves the polymerisation of actin at the leading edge and the accompanying formation of protrusions, and myosin‐II‐mediated contraction at the rear [11]. 1.2 Criteria to define a chemoattractant The first description of chemotaxis was made by Engelmann and Pfeffer in bacteria over a century ago [12, 13]. Since then, repulsive [14, 15] and attractive cues have been found for a variety of processes [1, 9]. However, most factors are multifunctional on cell behaviour, which makes definitive demonstration of chemoattractant behaviour in vivo difficult. Nevertheless, some attributes of chemoattractants may be summarised as follows. Chemoattractants are generally transcribed, translated and secreted by the target tissue itself to where the responsive cells are migrating. These responding cells are required to express a receptor for the chemoattractant when temporally appropriate. Loss of the chemoattractant or its receptor should lead to failure of cells reaching the target region; instead, non‐directional migration can be expected. In vitro, localised chemoattractants should cause chemotaxis and in vivo, cells should be diverted from their normal path by ectopic, localised sources of chemoattractant. Chemotaxis should be rescued by an exogenous ligand when the endogenous chemoattractant is lost, if placed into the region the cells would normal migrate toward. Chemotaxis requires that cells migrate up a concentration gradient of a soluble factor, so sufficient and consistent changes in the chemoattractant’s concentration should be found to give rise to a detectable gradient. This last point is perhaps the most difficult to demonstrate due to technical limitations and that in some cases the gradient is generated in situ by the migrating cell [16]. Nonetheless, a fulfilment of these criteria is important to show that not only are the cells capable of being
chemotactic towards the factor, but also that chemotaxis is actually happening in vivo. Altered migration in response to the external factor would otherwise demonstrate chemokinesis, the process by which factors simply promote or support migration, rather than providing directionality to the movement as in the case of chemotaxis, as seen in various cell types in physiology and throughout development [9, 11]. 2. The neural crest 2.1 Neural crest formation The neural crest (NC) is a transient cell population exclusively found in vertebrates. It is initially induced at the neural plate border as a result of the interaction between the ectodermal neural plate and the epidermis [17]. Changes in the structure of the neural plate cells cause fusion of the neural folds, resulting in the formation of a closed neural tube and of NC on its dorsolateral aspect on each side [17, 18]. Both the prospective neural plate and the prospective epidermis contribute to the NC [19, 20]. After induction, NC cells undergo an epithelial‐to‐mesenchymal transition (EMT) [21], in which cells acquire motility, epithelial polarity is lost and there is a switch from more adhesive to weaker cadherin expression. These and the accompanying cytoskeletal changes mean that the NC cells leave the neuroepithelium of the dorsal neural tube and become highly migratory [18]. 2.2 Neural crest derivatives The NC are multipotent stem cells, able to differentiate into many cell types and extensively contribute numerous tissues (Fig. 1A) [22]. NC cells receive inductive signals from the neural tube, paraxial mesoderm and the overlying ectoderm as they migrate [23]. Their specification is a multistep process; their fate is based on these paracrine signals, as well as the time at which they migrate, their origin and the stream in which they are found [23‐27]. The cranial NC contributes to the craniofacial mesenchyme, which includes cartilage, bone, teeth, cranial neurons, glia and connective tissue. Cardiac NC contributes to the cardiovascular system, developing into melanocytes, cartilage, connective tissue and pharyngeal arch neurons. Trunk NC gives rise to melanocytes, glia and neurons of the peripheral nervous system and epinephrine‐producing cells of the adrenal gland. The vagal and sacral NC develops into the ganglia of the enteric nervous system and sympathetic ganglia.
3. Neural crest migration 3.1 Neural crest streams After undergoing EMT, the NC becomes a highly migratory cell population, often likened to invasive cancers [18, 28, 29]. NC cell migration has been studied in a variety of vertebrate animal models, including Xenopus, zebrafish, chick, mouse [30] and even non‐classical model organisms such as lamprey [31], hagfish [32] and turtle [33, 34]. The NC migrate ventrally down the embryo, initially as a continuous wave away from the neural tube, but quickly splitting into discrete streams along stereotypical pathways to various sites (Fig. 1A). The cranial NC migrates along dorsolateral routes between the ectoderm and underlying paraxial mesoderm [35, 36]. In chick and mouse, early trunk NC migrates ventrolaterally through the anterior sclerotome [37‐40]. Trunk NC migrating later, which will become melanocytes, follow the dorsolateral path between the dermomyotome and dorsal ectoderm, with their migration affected by the structure of the somites [41]. However in zebrafish and Xenopus, melanocytes use both ventromedial and dorsolateral pathways [42, 43]. The cranial NC divide into three streams that invade the segmented branchial arches (BAs), due to, at least in part, the repulsive signals of ephrins and class 3 semaphorins (Fig. 1B). Eph/ephrin signalling prevents NC cells from invading non‐NC tissue and the caudal half of somites, thereby restricting them to the rostral half of somites in chick embryos [44, 45]. Likewise, class 3 semaphorins contribute to NC segregation in the head, trunk and caudal regions of the sclerotome [46‐51] by acting through plexin‐neuropilin complexes expressed by the NC [47‐49, 51]. The mixing of NC from different streams is also prohibited because NC belonging to different streams express complementary Eph receptors and ephrin ligands [35]. 3.2 Collective migration NC displays a range of migratory behaviours depending on species and location within the embryo. Some exhibit a more individual migratory behaviour [52], whereas most of NC cells migrate together, either as chains, groups or even single sheets, in spite of the fact that NC go through EMT [1, 29, 53‐55]. For example, cephalic NC maintain short and long‐range cell‐ cell interactions during migration both in vitro [56] and in vivo [57‐59]. This kind of
movement has been called collective cell migration, which can be defined as the coordinated migration of cells as tight clusters or loose groups (as in the case of NC), where cooperation between cells contributes to their overall directionality [18, 53, 60‐63]. Overall directionality during collective cell migration is higher than during single cell migration, indicating that intercellular interactions promote the directionality of migrating NC [57‐59]. Unlike epithelial cells, which move slowly and have tightly formed intercellular adhesions, the collective mass of the mesenchymal NC is a cohesive unit linked by transient contacts, such as N‐Cadherin adhesions [64‐67]. N‐Cadherin dynamics is regulated by lysophosphatidic acid receptor 2, prompting N‐cadherin endocytosis which leads to an increase in tissue plasticity [68]. This plasticity allows NC to migrate under physical constrains without abolishing cell cooperation [68]. Moreover, semaphorin and ephrin inhibitory signals ensure NC remain in streams (see Section 3.1), and short‐range chemotaxis (see Section 5.2) promotes collectiveness of the group. 3.3 Directional migration The importance of directional migration for the NC lies with the fact that they must reach and populate specific target regions. Directional migration requires cell polarisation, in order to specify a front that has localised actin polymerisation and a rear that is able to contract [11, 69]. Contact inhibition of locomotion (CIL), the process by which contacting cells collapse their protrusions at the site of contact and change their direction of migration [70, 71], is a mechanism that is able to polarize cells in a contact‐dependent manner [70, 71]. NC exhibits CIL in vitro and in vivo [72]. The Rho GTPases, Rac, Cdc42 and Rho, are important for cell polarisation and cell migration [73]. Non‐canonical (PCP) Wnt signalling is necessary for CIL in NC, by activating RhoA at sites of intercellular contact, which in turn suppresses the generation and maintenance of lamellipodia through its target ROCK [72, 74]. The proteoglycan syndecan‐4, expressed by NC, cooperates with non‐canonical Wnt and N‐ Cadherin signalling to inhibit Rac activity at the cell‐cell contact [74‐76]. Together, mutually exclusive zones of Rac1 and RhoA activity are generated in NC cells, meaning that protrusions are formed only at sites where there is no NC‐NC contact. Most NC cells migrating in vivo maintain close proximity and move in compact groups. Therefore, the polarity required for directional migration is established because at the free edge of the cell cluster, due to the lack of NC‐NC contact, cells become polarized and generate protrusions
away from the group [77] (Fig. 1B, purple protrusions). Hence, directional migration is an emergent property of NC cells that depends on cell‐cell interactions [78‐80]. Importantly, it has been shown that the pre‐established polarisation of NC arising from cell‐cell contacts allows NC cells to respond to external chemoattractants more efficiently as a collective than as individual cells [81]. Consequently, NC chemotaxis becomes more efficient as cell density increases [81]. This collective interpretation of a chemotactic gradient is referred to as collective chemotaxis, and it has been supported by mathematical modelling of collective cell migration [82]. However, CIL alone is not sufficient to explain directional migration, as it would promote cell dispersion on its own. Significant evidence supports the presence and requirement of chemoattractants for NC migration in vitro and in vivo. 4 Neural crest long‐range chemoattractants Various chemoattractants have been proposed for the NC, including SDF‐1/CXCL12 [81, 83‐ 85], FGF [86‐88], VEGF [89‐92], PDGF [93‐95], SCF [96], NT‐3 [97], GDNF [98‐100], NRG1 [101] and TGFβ [102]. However, whether chemotaxis mediates the long‐range directional migration of NC in vivo has not been conclusively demonstrated. Chemoattractants do not seem necessary for directional migration in vitro and in silico, where it has been suggested to be a self‐organising property of the NC [81, 82, 103] as discussed in Section 3.3. Furthermore, many NC cells begin migration prior to full development of the target tissue and it is unclear how different NC subpopulations would be able to share common migratory routes and invade different target regions using a limited number of chemoattractants. Conversely, some factors fill many of the criteria discussed in Section 1.2, including appropriate expression patterns and chemotactic behaviour of NC toward them. Here we will examine the current evidence of the four most studied potential chemoattractants, which have the most convincing data. 4.1 Stromal cell‐derived factor 1 (SDF‐1) SDF‐1 (also named CXCL12) regulates many directional migration events during embryonic development, including migration of the zebrafish posterior lateral line primordium (PLLp), primordial germ cells and various NC‐derived cells [84, 104‐108]. In many model organisms, SDF‐1 is expressed along the path taken by NC cells [83, 84, 101, 109‐111] that express the corresponding receptor, CXCR4 [83, 84, 111‐114]. In some of these cases, chemotactic
activity of the NC to SDF‐1 has not been properly tested, and how chemotaxis would be achieved in chick, where SDF‐1 is not found as a gradient, is unclear [109, 110]. But there are some examples of chemotaxis to SDF‐1 that are supported by experimental evidence. For example, CXCR4‐expressing NC are chemotactic to SDF‐1 in vitro [84, 115] and SDF‐1 misexpression diverts these NC cells away from their normal path, causing major defects such as cardiovascular abnormalities in many organisms [109, 111, 112, 115‐120], although mice NC behave rather differently in that SDF‐1 and CXCR4 mutants display only mild abnormalities [121, 122]. Perturbed SDF‐1/CXCR4 signalling disrupts NC cell migration [83, 85, 111, 112], and some of the downstream components of this pathway have been identified. For example, the GEF Ric‐8A is required for NC chemotaxis to SDF‐1 in vitro [123], but its mechanism of action is unclear. The regulation of the CXCR4 receptor has also been shown to be important for NC migration, as the transcription factor HIF‐1α controls chemotaxis to SDF‐1 by regulating CXCR4 expression [124]. In Xenopus, cell‐cell interactions are essential for the collective chemotaxis of NC cells toward placodal‐produced SDF‐1 [81]. SDF‐1 is only able to stabilize cell polarity in cells already polarized by cell‐cell contacts, and therefore cannot attract non‐polarized individual NC cells [81]. Mathematical modelling has shown that cell contact enhances the chemotactic response [125], consistent with the experimental evidence that SDF‐1 stabilises and amplifies cell protrusions promoted by cell contact [81], similar to the chemotactic response of Drosophila border cells to EGFR and PVR [126]. One major long‐standing question is how NC segregates into different regions to colonize and differentiate into distinct tissues and organs. It has been proposed that different NC subpopulations express different receptors [127]. Indeed, it has been shown that differential response to SDF‐1 and neuregulin by distinct NC subpopulations determines whether these cells will migrate into the sympathetic ganglia or the dorsal root ganglia [101, 111]. 4.2 Vascular endothelial growth factor (VEGF) By the onset of NC migration, VEGF is expressed in the head ectoderm of avian embryos, specifically overlaying the dorsolateral migratory path of the rhombomeric 4 (r4) cranial NC, which expresses its canonical receptor, VEGFR2, and co‐receptor, neuropilin‐1 [89, 128,
129]. VEGF expression later extends to the second branchial arch (BA2), and seems to be reduced in the on‐route ectoderm [89]. During the initial stages of migration, VEGF is uniformly expressed in the overlying ectoderm, rather than as a gradient [89]. Nonetheless, both VEGFR2 and neuropilin‐1 receptors are required for VEGF‐mediated migration to BA2 [90, 92]. In vitro, cranial NC are attracted to BA2 and VEGF [89] and in vivo, r4 NC can be diverted from their normal path by ectopic VEGF [89, 91]. Perturbed VEGF/VEGFR2/neuropilin‐1 signalling does not affect directional migration toward the BA2 entrance, but prevents invasion of BA2 at later stages [89, 92, 129]. It is not clear how VEGF can control directional NC migration, as no VEGF gradient has been demonstrated so far. A mathematical model of NC migration has proposed that the VEGF signal is diluted through the proliferation of NC cells which self‐generate a VEGF gradient by the endocytosis of the ligand (Fig. 2A) [130]. This model posits that only leader cells respond to VEGF, whereas trailing cells respond to a second, unknown signal produced by leader cells [130]. However, a recent publication suggests that trailing cells can indeed respond to VEGF [91]. Moreover, there are key assumptions that are still awaiting experimental evidence: the consumption of VEGFA, the short‐range signals transmitted from leader to follower cells, and the exclusive response of leader cells to VEGFA. It is unlikely that the NC self‐generate a gradient in mice, because murine NC express VEGFA themselves [131]. 4.3 Fibroblast growth factor (FGF) FGF8 is expressed in the pharyngeal arch ectoderm and endoderm during NC migration through the arches [132, 133] and it is not expressed by the NC [134]. Its expression is partly dependent on Notch in mouse, and on the presence of the NC cells themselves in chick [86, 135, 136]. Migration of different NC populations to their targets is dependent on FGF8 [133‐ 135, 137‐139]. However, there is varying evidence of chemotaxis between different NC subpopulations and species. In some cases, the NC have been shown to express FGF8’s cognate receptors, FGFR1 and FGFR3, and there is evidence that NC can be diverted from their usual paths by ectopic FGF8 beads [86, 87]. For other cases, there is only evidence that FGF8 is important for NC migration, but not for chemotaxis [137‐139]. Species differences in NC migration can be illustrated in cardiac development, where NC chemotaxis to FGF8 is critical for heart development in chick and mouse [133, 140, 141], unlike in zebrafish where FGF signalling is redundant for NC contribution to the heart [139].
FGF2 has also been proposed as a chemoattractant for NC. FGF2 is locally expressed and under the control of FGF8 in the mandibular mesenchyme [88]. Mesencephalic mouse NC cells express FGFR1 and FGFR3, but although these NC are chemotactic to FGF in vitro, there are no functional studies of FGF2 chemotaxis in vivo [88]. 4.4 Platelet‐derived growth factor (PDGF) PDGFRα is expressed in the migrating NC of many species [33, 94, 142‐145] and in non‐ neuronal derivatives of the cranial NC [140, 144, 146]. PDGFRα protein also localises to NC, although its expression is not exclusive to NC and NC‐derived tissues [95]. Patch heterozygotes, in which PDGFRα is deleted, have defects in pigment cells derived from NC [147]. Patch homozygotes have abnormalities suggestive of defective cardiac NC [148, 149] and PDGFRα mutants exhibit cleft palate, which results from failed NC development [145, 148, 150]. PDGFRα’s cognate ligands, PDGFA and PDGFC, are found in the ectoderm, otic vesicle and pharyngeal endoderm [94, 143, 146, 151, 152], which are NC targets. In mouse, both PDGFRα and PDGFRβ are required for the normal migration of cardiac NC [153]. Although some NC derivatives are capable of chemotaxis to PDGFA in vitro [146], which ligand is required for signalling through PDGFRβ, and whether it acts chemotactically on NC cells in vivo, is unknown. Exogenously implanted PDGF‐AA is able to attract PDGFRα‐ expressing NC in vivo [93‐95]. In zebrafish, it appears that PDGF‐AA pre‐localised to where the PDGFRα‐expressing NC cells migrate [94]. Interestingly, the expression pattern of a PDGFRα negative regulator, Mirn140, is identical to PDGFRα, and it has been proposed that this mechanism of PDGFR signalling modulation mediates the chemotaxis of cranial NC to the oral ectoderm, since overexpression of Mirn140 phenocopies PDGFRα mutants [94]. In conclusion, although there is some evidence that suggest that SDF‐1, VEGF, PDGF and FGF could work as NC chemoattractants, none of these molecules have been shown to be present in a gradient along the NC migratory pathways. Instead of precluding these molecules to be classified as NC chemoattractant, the mechanism to sense a chemoattractant could be more complex than simply reading a long range gradient.
5 Neural crest short‐range chemoattractants 5.1 Chase and run Many examples of paracrine chemotaxis, to enhance migration and for cell guidance have been described in development and cancer [54, 56, 105]. Xenopus and zebrafish NC cells, which express CXCR4, also undergo paracrine chemotaxis in response to SDF‐1 secreted by placodal cells in vitro [154]. Cranial placodes are thickened regions of ectoderm that contribute to the development of cranial sensory structures [155]. Reciprocal interactions between the NC and placodal cells are required for normal morphogenesis of both populations [155]. Mechanistically, contact inhibition of locomotion (CIL) generates polarised NC [72] whose protrusions are stabilised by SDF‐1/CXCR4 which enhances and maintains the polarity [81]. Upon contact with NC cells a transient but functional N‐ Cadherin‐based adhesion complex is formed between NC and placodal cells [154]. Migratory NC explants normally generate traction forces around the edge [156], but at the point of N‐ Cadherin engagement focal adhesions and protrusions are downregulated as CIL is induced [154]. Consequently, NC repolarise and separate from the placodal cells, whilst loss of focal adhesions and collapse of protrusions in the rear of the placode cluster causes the placodal cells to move away from the NC. This process has been termed ‘chase and run’ in which NC chase placodal cells by short‐range chemotaxis, whereas the placode runs away from NC by CIL (Fig. 2B). The bidirectional interactions between NC and placodal cells coordinate highly efficient directional migration of both populations towards lateral and ventral regions. 5.2 Co‐attraction Short‐range chemotaxis may also maintain the cohesion of groups of cells during migration, as suggested in cancer [157, 158] and demonstrated in Dictyostelium [11]. Despite having weak cell adhesion complexes, most NC cells migrate collectively rather than as individuals [29, 54, 58]. Short‐range chemotaxis is used to maintain collectiveness in NC groups during directional migration. NC cells produce the complement factor C3a, and express its receptor, C3aR [159]. Therefore, high levels of C3a are found where NC cells are abundant, and cells that lose contact with their neighbours are able to migrate back to the group, following this chemotactic gradient. Mechanistically, C3a signalling leads to Rac1 activation which is sufficient to polarise escaping NC back to the group (Fig. 2C) [159]. This mechanism of short‐
range chemotaxis is termed co‐attraction. Co‐attraction counterbalances CIL, which is required for directional migration but promotes cell dispersion [72, 159]. Accordingly, inhibition of C3 or its receptor reduces collectiveness, as cells are forced apart by CIL [159]. C3a and C3aR have also been found in cephalic NC cells in mouse (Lambris and Mayor, unpublished) and chick (Bronner and Mayor, unpublished), and in the mesoderm of Xenopus embryos [160]. NC cell migration in zebrafish and avian embryos also suggest a co‐attractive behaviour, although the molecular mechanisms have not yet been described. The importance of short‐range chemotaxis to hold groups of cells together is supported by mathematical models, where co‐attraction and CIL are necessary and sufficient for generating directional migration of groups in confined streams [82, 159]. 6 Concluding remarks Various molecules have been proposed as chemoattractants for the NC, some of which have very strong evidence, such as SDF‐1 and VEGF [81, 89]. However, some aspects of the criteria required to unequivocally demonstrate chemoattractant activity are still lacking. For example, convincing graded expression patterns have not been shown for any factors, and there is not enough experimental evidence for various aspects of the proposed model of self‐generated chemotactic gradients. Nonetheless, novel concepts have emerged from studies of NC chemotaxis, including collectiveness maintained by chemotaxis (co‐attraction) [159], directional migration of distinct cell populations based on short‐range chemotaxis (‘chase and run’) [154], collective chemotaxis [81], and explanations of how differential response to chemotactic cues may achieve tissue organisation during development [101, 111]. Thanks to improved in vivo imaging techniques and the development of genetic models, the future holds better prospects of further assessing NC cell chemotaxis and dissecting the molecular mechanisms involved [161, 162]. Acknowledgements We thank Isabel Bahm and Andras Szabo for comments on the manuscript. Work in R.M. lab is supported by grants from MRC (M010465 and J000655), BBSRC (M008517) and Wellcome Trust. A.S. is a recipient of a Wellcome Trust PhD fellowship.
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Figure Captions
Figure 1. (A) Migration routes of the NC (green) in a representative vertebrate embryo. D, diencephalon; M, mesencephalon; R, rhombomere; OV, otic vesicle; BA, branchial arch; red squares, somites. Below, examples of some of the cell types to which NC differentiate. (B) Representation of NC migrating in a cephalic stream. NC cells migrate in distinct streams, mostly as a collective. Lateral migration is restricted by inhibitory signals at the borders (blue). Directional migration is an emergent property from CIL, whereby Rho (orange) is upregulated at sites of N‐Cadherin‐based contact (red) between cells; only leaders can generate Rac‐dependent protrusions (purple). This leads to a polarised group of NC cells. Migration is inefficient by individual cells because polarity is not generated by CIL, a process dependent on cell interactions.
Figure 2: Mechanisms of chemotaxis. (A) Proposed model for a NC self‐generated gradient of VEGF (pink) from an initially uniform expression of VEGF in the overlying ectoderm. VEGF is consumed by NC, potentially by its endocytosis when bound to VEGFR2/neuropilin‐1 (purple). Leaders are able to respond to unconsumed VEGF in front, relaying a signal to its followers. (B) NC undergo short‐range chemotaxis to placodal cells (blue) via placodal‐ secreted SDF‐1 (yellow) which binds CXCR4 (olive) on NC. CIL through PCP signalling and a transient N‐Cadherin adhesion mediates repulsion between NC and placodes, leading to the placode moving away (run) from the NC, while the NC still follow (chase) the placode due to chemotaxis. This is referred to as ‘chase and run’. (C) NC co‐attraction. NC co‐expresses the chemoattractant C3a and its cognate receptor C3aR. NC‐produced C3a binds to C3aR on NC cells, causing activation of Rac. In this manner, C3a promotes cohesion of the NC cluster; cells that move away by CIL return to the high concentration of C3a present in the cluster.