Wnt/beta-catenin and Fgf signaling control collective cell migration by restricting chemokine receptor expression

Cell traction in collective cell migration and morphogenesis: the chase and run mechanism

Directional collective cell migration plays an important role in development, physiology, and disease. An increasing number of studies revealed key aspects of how cells coordinate their movement through distances surpassing several cell diameters. While physical modeling and measurements of forces during collective cell movements helped to reveal key mechanisms, most of these studies focus on tightly connected epithelial cultures. Less is known about collective migration of mesenchymal cells. A typical example of such behavior is the migration of the neural crest cells, which migrate large distances as a group. A recent study revealed that this persistent migration is aided by the interaction between the neural crest and the neighboring placode cells, whereby neural crest chase the placodes via chemotaxis, but upon contact both populations undergo contact inhibition of locomotion and a rapid reorganization of cellular traction. The resulting asymmetric traction field of the placodes forces them to run away from the chasers. We argue that this chase and run interaction may not be specific only to the neural crest system, but could serve as the underlying mechanism for several morphogenetic processes involving collective cell migration.

Phenomenological approaches to collective behavior in epithelial cell migration

Collective cell migration in epithelial tissues resembles fluid-like behavior in time-lapse recordings. In the last years, hydrodynamic velocity fields in living matter have been studied intensely. The emergent properties were remarkably similar to phenomena known from active soft matter systems. Here, we review migration experiments of large cellular ensembles as well as of mesoscopic cohorts in micro-structured environments. Concepts such as diffusion, velocity correlations, swirl strength and polarization are metrics to quantify the cellular dynamics both in experiments as well as in computational simulations. We discuss challenges relating collective migration to single cell and oligocellular behavior as well as linking the phenotypic parameters to the underlying cytoskeleton dynamics and signaling networks. This article is part of a Special Issue entitled: Mechanobiology.

LSD1 is Required for Hair Cell Regeneration in Zebrafish

Lysine-specific demethylase 1 (LSD1/KDM1A) plays an important role in complex cellular processes such as differentiation, proliferation, apoptosis, and cell cycle progression. It has recently been demonstrated that during development, downregulation of LSD1 inhibits cell proliferation, modulates the expression of cell cycle regulators, and reduces hair cell formation in the zebrafish lateral line, which suggests that LSD1-mediated epigenetic regulation plays a key role in the development of hair cells. However, the role of LSD1 in hair cell regeneration after hair cell loss remains poorly understood. Here, we demonstrate the effect of LSD1 on hair cell regeneration following neomycin-induced hair cell loss. We show that the LSD1 inhibitor trans-2-phenylcyclopropylamine (2-PCPA) significantly decreases the regeneration of hair cells in zebrafish after neomycin damage. In addition, immunofluorescent staining demonstrates that 2-PCPA administration suppresses supporting cell proliferation and alters cell cycle progression. Finally, in situ hybridization shows that 2-PCPA significantly downregulates the expression of genes related to Wnt/β-catenin and Fgf activation. Altogether, our data suggest that downregulation of LSD1 significantly decreases hair cell regeneration after neomycin-induced hair cell loss through inactivation of the Wnt/β-catenin and Fgf signaling pathways. Thus, LSD1 plays a critical role in hair cell regeneration and might represent a novel biomarker and potential therapeutic approach for the treatment of hearing loss.

Control of Wnt5b secretion by Wntless modulates chondrogenic cell proliferation through fine-tuning fgf3 expression

Wnts and Fgfs regulate various tissues development in vertebrates. However, how regional Wnt or Fgf activities are established and how they interact in any given developmental event is elusive. Here, we investigated the Wnt-mediated craniofacial cartilage development in zebrafish and found that fgf3 expression in the pharyngeal pouches is differentially reduced along the anteroposterior axis in wnt5b mutants and wntless (wls) morphants, but its expression is normal in wnt9a and wnt11 morphants. Introducing fgf3 mRNAs rescued the cartilage defects in Wnt5b- and Wls-deficient larvae. In wls morphants, endogenous Wls expression is not detectable but maternally deposited Wls is present in eggs, which might account for the lack of axis defects in wls morphants. Secretion of endogenous Wnt5b but not Wnt11 was affected in the pharyngeal tissue of Wls morphants, indicating that Wls is not involved in every Wnt secretion event. Furthermore, cell proliferation but not apoptosis in the developing jaw was affected in Wnt5b- and Wls-deficient embryos. Therefore, Wnt5b requires Wls for its secretion and regulates the proliferation of chondrogenic cells through fine-tuning the expression of fgf3 during jaw cartilage development.

Heparan Sulfate Proteoglycans Regulate Fgf Signaling and Cell Polarity during Collective Cell Migration

Collective cell migration is a highly regulated morphogenetic movement during embryonic development and cancer invasion that involves the precise orchestration and integration of cell-autonomous mechanisms and environmental signals. Coordinated lateral line primordium migration is controlled by the regulation of chemokine receptors via compartmentalized Wnt/β-catenin and fibroblast growth factor (Fgf) signaling. Analysis of mutations in two exostosin glycosyltransferase genes (extl3 and ext2) revealed that loss of heparan sulfate (HS) chains results in a failure of collective cell migration due to enhanced Fgf ligand diffusion and loss of Fgf signal transduction. Consequently, Wnt/β-catenin signaling is activated ectopically, resulting in the subsequent loss of the chemokine receptor cxcr7b. Disruption of HS proteoglycan (HSPG) function induces extensive, random filopodia formation, demonstrating that HSPGs are involved in maintaining cell polarity in collectively migrating cells. The HSPGs themselves are regulated by the Wnt/β-catenin and Fgf pathways and thus are integral components of the regulatory network that coordinates collective cell migration with organ specification and morphogenesis.

TGFβ1a regulates zebrafish posterior lateral line formation via Smad5 mediated pathway

The zebrafish sensory posterior lateral line (pLL) has become an attractive model for studying collective cell migration and cell morphogenesis. Recent studies have indicated that chemokine, Wnt/β-catenin, Fgf, and Delta-Notch signaling pathways participate in regulating pLL development. However, it remains unclear whether TGFβ signaling pathway is involved in pLL development. Here we report a critical role of TGFβ1 in regulating morphogenesis of the pLL primordium (pLLP). The tgfβ1a gene is abundantly expressed in the lateral line primordium. Knockdown or knockout of tgfβ1a leads to a reduction of neuromast number, an increase of inter-neuromast distance, and a reduced number of hair cells. The aberrant morphogenesis in embryos depleted of tgfβ1a correlates with the reduced expression of atoh1a, deltaA, and n-cadherin/cdh2, which are known important regulators of the pLLP morphogenesis. Like tgfβ1a depletion, knockdown of smad5 that expresses in the pLLP, affects pLLP development whereas overexpression of a constitutive active Smad5 isoform rescues the defects in embryos depleted of tgfβ1a, indicating that Smad5 mediates tgfβ1a function in pLLP development. Therefore, TGFβ/Smad5 signaling plays an important role in the zebrafish lateral line formation.

Identification of a Ly-6 superfamily gene expressed in lateral line neuromasts in zebrafish

Lymphocyte antigen-6 (Ly-6) superfamily members have been identified in zebrafish, but the expression and function of these Ly-6 genes remain largely unknown. Posterior lateral line (pLL) system is produced by migrating pLL primordium (pLLp). Chemokine signaling, Notch, Wnt, and fibroblast growth factor (FGF) signaling regulate migration of pLLp cells and formation of neuromasts. However, the mechanism of neuromast deposition remains to be explored. Identification of novel genes expressed in pLLp will certainly help the study of such a process. Here we identified a Ly-6 gene called neuromast-expressed gpi-anchored lymphocyte antigen-6 (negaly6), which was specifically expressed in neuromast. Quantitative real-time PCR (qRT-PCR) analysis showed that negaly6 started to be expressed at 24 hpf, and whole-mount in situ hybridization analysis indicated that negaly6 was highly expressed in the trailing zone of pLLp and mature neuromast. Furthermore, negaly6 expression was inhibited by FGF signaling antagonist but not by Wnt signaling agonist or antagonist. Collectively, these data indicate that negaly6 may be associated with the regulation of neuromast deposition via FGF signaling pathway.

Cranial placodes: models for exploring the multi-facets of cell adhesion in epithelial rearrangement, collective migration and neuronal movements

Key to morphogenesis is the orchestration of cell movements in the embryo, which requires fine-tuned adhesive interactions between cells and their close environment. The neural crest paradigm has provided important insights into how adhesion dynamics control epithelium-to-mesenchyme transition and mesenchymal cell migration. Much less is known about cranial placodes, patches of ectodermal cells that generate essential parts of vertebrate sensory organs and ganglia. In this review, we summarise the known functions of adhesion molecules in cranial placode morphogenesis, and discuss potential novel implications of adhesive interactions in this crucial developmental process. The great repertoire of placodal cell behaviours offers new avenues for exploring the multiple roles of adhesion complexes in epithelial remodelling, collective migration and neuronal movements.

Itch is required for lateral line development in zebrafish

The zebrafish posterior lateral line is formed during early development by the deposition of neuromasts from a migrating primordium. The molecular mechanisms regulating the regional organization and migration of the primordium involve interactions between Fgf and Wnt/β-catenin signaling and the establishment of specific cxcr4b and cxcr7b cytokine receptor expression domains. Itch has been identified as a regulator in several different signaling pathways, including Wnt and Cxcr4 signaling. We identified two homologous itch genes in zebrafish, itcha and itchb, with generalized expression patterns. By reducing itchb expression in particular upon morpholino knockdown, we demonstrated the importance of Itch in regulating lateral line development by perturbing the patterns of cxcr4b and cxcr7b expression. Itch knockdown results in a failure to down-regulate Wnt signaling and overexpression of cxcr4b in the primordium, slowing migration of the posterior lateral line primordium and resulting in abnormal development of the lateral line.

Chemokine signaling in development and disease

Chemokines are a group of small, secreted molecules that signal through G protein-coupled receptors to promote cell survival and proliferation and to provide directional guidance to migrating cells. CXCL12 is one of the most evolutionary conserved chemokines and signals through the chemokine receptor CXCR4 to guide cell migration during embryogenesis, immune cell trafficking and cancer metastasis. Here and in the accompanying poster, we provide an overview of chemokine signaling, focusing on CXCL12, and we highlight some of the different chemokine-dependent strategies used to guide migrating cells.

Mechanisms of endothelial cell migration

Cell migration plays a central role in a variety of physiological and pathological processes during our whole life. Cellular movement is a complex, tightly regulated multistep process. Although the principle mechanisms of migration follow a defined general motility cycle, the cell type and the context of moving influences the detailed mode of migration. Endothelial cells migrate during vasculogenesis and angiogenesis but also in a damaged vessel to restore vessel integrity. Depending on the situation they migrate individually, in chains or sheets and complex signaling, intercellular signals as well as environmental cues modulate the process. Here, the different modes of cell migration, the peculiarities of endothelial cell migration and specific guidance molecules controlling this process will be reviewed.

There and back again: development and regeneration of the zebrafish lateral line system

The zebrafish lateral line is a sensory system used to detect changes in water flow. It is comprised of clusters of mechanosensory hair cells called neuromasts. The lateral line is initially established by a migratory group of cells, called a primordium, that deposits neuromasts at stereotyped locations along the surface of the fish. Wnt, FGF, and Notch signaling are all important regulators of various aspects of lateral line development, from primordium migration to hair cell specification. As zebrafish age, the organization of the lateral line becomes more complex in order to accommodate the fish's increased size. This expansion is regulated by many of the same factors involved in the initial development. Furthermore, unlike mammalian hair cells, lateral line hair cells have the capacity to regenerate after damage. New hair cells arise from the proliferation and differentiation of surrounding support cells, and the molecular and cellular pathways regulating this are beginning to be elucidated. All in all, the zebrafish lateral line has proven to be an excellent model in which to study a diverse array of processes, including collective cell migration, cell polarity, cell fate, and regeneration.

The contribution of CXCL12-expressing radial glia cells to neuro-vascular patterning during human cerebral cortex development

This study was conducted on human developing brain by laser confocal and transmission electron microscopy (TEM) to make a detailed analysis of important features of blood-brain barrier (BBB) microvessels and possible control mechanisms of vessel growth and differentiation during cerebral cortex vascularization. The BBB status of cortex microvessels was examined at a defined stage of cortex development, at the end of neuroblast waves of migration, and before cortex lamination, with BBB-endothelial cell markers, namely tight junction (TJ) proteins (occludin and claudin-5) and influx and efflux transporters (Glut-1 and P-glycoprotein), the latter supporting evidence for functional effectiveness of the fetal BBB. According to the well-known roles of astroglia cells on microvessel growth and differentiation, the early composition of astroglia/endothelial cell relationships was analyzed by detecting the appropriate astroglia, endothelial, and pericyte markers. GFAP, chemokine CXCL12, and connexin 43 (Cx43) were utilized as markers of radial glia cells, CD105 (endoglin) as a marker of angiogenically activated endothelial cells (ECs), and proteoglycan NG2 as a marker of immature pericytes. Immunolabeling for CXCL12 showed the highest level of the ligand in radial glial (RG) fibers in contact with the growing cortex microvessels. These specialized contacts, recognizable on both perforating radial vessels and growing collaterals, appeared as CXCL12-reactive en passant, symmetrical and asymmetrical, vessel-specific RG fiber swellings. At the highest confocal resolution, these RG varicosities showed a CXCL12-reactive dot-like content whose microvesicular nature was confirmed by ultrastructural observations. A further analysis of RG varicosities reveals colocalization of CXCL12 with Cx43, which is possibly implicated in vessel-specific chemokine signaling.

Tbx1 controls the morphogenesis of pharyngeal pouch epithelia through mesodermal Wnt11r and Fgf8a

The pharyngeal pouches are a segmental series of epithelial structures that organize the embryonic vertebrate face. In mice and zebrafish that carry mutations in homologs of the DiGeorge syndrome gene TBX1, a lack of pouches correlates with severe craniofacial defects, yet how Tbx1 controls pouch development remains unclear. Using mutant and transgenic rescue experiments in zebrafish, we show that Tbx1 functions in the mesoderm to promote the morphogenesis of pouch-forming endoderm through wnt11r and fgf8a expression. Consistently, compound losses of wnt11r and fgf8a phenocopy tbx1 mutant pouch defects, and mesoderm-specific restoration of Wnt11r and Fgf8a rescues tbx1 mutant pouches. Time-lapse imaging further reveals that Fgf8a acts as a Wnt11r-dependent guidance cue for migrating pouch cells. We therefore propose a two-step model in which Tbx1 coordinates the Wnt-dependent epithelial destabilization of pouch-forming cells with their collective migration towards Fgf8a-expressing mesodermal guideposts.

Sensory hair cell regeneration in the zebrafish lateral line

BACKGROUND

Damage or destruction of sensory hair cells in the inner ear leads to hearing or balance deficits that can be debilitating, especially in older adults. Unfortunately, the damage is permanent, as regeneration of the inner ear sensory epithelia does not occur in mammals.

RESULTS

Zebrafish and other non-mammalian vertebrates have the remarkable ability to regenerate sensory hair cells and understanding the molecular and cellular basis for this regenerative ability will hopefully aid us in designing therapies to induce regeneration in mammals. Zebrafish not only possess hair cells in the ear but also in the sensory lateral line system. Hair cells in both organs are functionally analogous to hair cells in the inner ear of mammals. The lateral line is a mechanosensory system found in most aquatic vertebrates that detects water motion and aids in predator avoidance, prey capture, schooling, and mating. Although hair cell regeneration occurs in both the ear and lateral line, most research to date has focused on the lateral line due to its relatively simple structure and accessibility.

CONCLUSIONS

Here we review the recent discoveries made during the characterization of hair cell regeneration in zebrafish.

Leading and trailing cells cooperate in collective migration of the zebrafish posterior lateral line primordium

Collective migration of cells in the zebrafish posterior lateral line primordium (PLLp) along a path defined by Cxcl12a expression depends on Cxcr4b receptors in leading cells and on Cxcr7b in trailing cells. Cxcr7b-mediated degradation of Cxcl12a by trailing cells generates a local gradient of Cxcl12a that guides PLLp migration. Agent-based computer models were built to explore how a polarized response to Cxcl12a, mediated by Cxcr4b in leading cells and prevented by Cxcr7b in trailing cells, determines unidirectional migration of the PLLp. These chemokine signaling-based models effectively recapitulate many behaviors of the PLLp and provide potential explanations for the characteristic behaviors that emerge when the PLLp is severed by laser to generate leading and trailing fragments. As predicted by our models, the bilateral stretching of the leading fragment is lost when chemokine signaling is blocked in the PLLp. However, movement of the trailing fragment toward the leading cells, which was also thought to be chemokine dependent, persists. This suggested that a chemokine-independent mechanism, not accounted for in our models, is responsible for this behavior. Further investigation of trailing cell behavior shows that their movement toward leading cells depends on FGF signaling and it can be re-oriented by exogenous FGF sources. Together, our observations reveal the simple yet elegant manner in which leading and trailing cells coordinate migration; while leading cells steer PLLp migration by following chemokine cues, cells further back play follow-the-leader as they migrate toward FGFs produced by leading cells.

Kremen1 restricts Dkk activity during posterior lateral line development in zebrafish

Canonical Wnt signaling plays crucial roles during development and disease. How Wnt signaling is modulated in different in vivo contexts is currently not well understood. Here, we investigate the modulation of Wnt signaling in the posterior lateral line primordium (pLLP), a cohort of ~100 cells that collectively migrate along the trunk of the zebrafish embryo. The pLLP comprises proliferative progenitor cells and organized epithelial cells that will form the mechanosensory organs of the posterior lateral line. Wnt signaling is active in the leading progenitor zone of the pLLP and restricted from the trailing zone through expression of the secreted Wnt inhibitors dkk1b and dkk2. We have identified a zebrafish strain, krm1(nl10), which carries a mutation in the kremen1 gene, a non-obligate co-receptor for the Dkk family of proteins. Previous studies have shown that Kremen1 inhibits Wnt signaling by facilitating internalization of the Kremen1-Dkk-Lrp5/6 complex. Surprisingly, we found that disruption of Kremen1 in the pLLP exhibited molecular and cellular phenotypes associated with a decrease rather than overactivation of Wnt signaling. Transplantation of wild-type cells into the mutant primordia failed to rescue the krm1(nl10) phenotype, thus revealing that the effects of Kremen1 loss are non-cell-autonomous. Finally, ectopic expression of Dkk1b-mTangerine protein revealed larger spread of the fusion protein in the mutant primordia compared with the wild type. Based on our data, we propose a novel mechanism in which Kremen1 modulates Wnt activity by restricting the range of secreted Dkk proteins during collective cell migration in the pLLP.

sox21a directs lateral line patterning by modulating FGF signaling

The development of organs composed by repeated functional units is, in many cases, accomplished by the transition of cells from a progenitor to a differentiation domain, triggering a reiterated developmental program. Yet, how these discrete fields are formed during development is still a largely unresolved question. The posterior lateral line (pLL), a sensory organ present in fish and amphibians, develops from a primordium that migrates along the flanks of the animal periodically depositing neuromasts, the pLL functional units. In zebrafish (Danio rerio), the developmental program of the pLL is triggered by the transit of progenitor cells from a Wnt to a Fgf signaling domain. It has been proposed that these two fields are defined by the antagonistic activity of these two signaling pathways, but how they are formed and maintained is still an open question in the development of the pLL. In this work, we show that sox21a, an HMG -box transcription factor, is expressed within the Fgf domain. We demonstrate that, while the Fgf signaling pathway do not control sox21a, knockdown of sox21a causes impairment of Fgf signaling, expansion of the Wnt signaling domain and disruption of neuromast development. These results suggest that sox21a is a key player in the pLL primordium patterning, fine-tuning the border of the Fgf and Wnt signaling domains.

Neighborly relations during collective migration

The collective migration of sheets, cohorts, chains or streams of cells contributes to embryogenesis, tissue remodeling and repair as well as to cancer invasion. The functional coordination between neighboring cells is at the heart of collective migration, during which cells migrate with a similar speed in an identical direction. Far from being the result of the simultaneous migration of isolated cells, collective migration relies on the intercellular communication between migrating cells. Although the mechanisms of cell coordination are far from being completely understood, accumulated evidence show that exchange of mechanical and chemical information by direct intercellular contacts and by soluble extracellular signals orchestrate the coordinated behavior of collectively migrating cells.

Ildr1b is essential for semicircular canal development, migration of the posterior lateral line primordium and hearing ability in zebrafish: implications for a role in the recessive hearing impairment DFNB42

Immunoglobulin-like domain containing receptor 1 (ILDR1) is a poorly characterized gene that was first identified in lymphoma cells. Recently, ILDR1 has been found to be responsible for autosomal recessive hearing impairment DFNB42. Patients with ILDR1 mutations cause bilateral non-progressive moderate-to-profound sensorineural hearing impairment. However, the etiology and mechanism of ILDR1-related hearing loss remains to be elucidated. In order to uncover the pathology of DFNB42 deafness, we used the morpholino injection technique to establish an ildr1b-morphant zebrafish model. Ildr1b-morphant zebrafish displayed defective hearing and imbalanced swimming, and developmental delays were seen in the semicircular canals of the inner ear. The gene expression profile and real-time PCR revealed down-regulation of atp1b2b (encoding Na(+)/K(+) transporting, beta 2b polypeptide) in ildr1b-morphant zebrafish. We found that injection of atp1b2b mRNA into ildr1b-knockdown zebrafish could rescue the phenotype of developmental delay of the semicircular canals. Moreover, ildr1b-morphant zebrafish had reduced numbers of lateral line neuromasts due to the disruption of lateral line primordium migration. In situ hybridization showed the involvement of attenuated FGF signaling and the chemokine receptor 4b (cxcr4b) and chemokine receptor 7b (cxcr7b) in posterior lateral line primordium of ildr1b-morphant zebrafish. We concluded that Ildr1b is crucial for the development of the inner ear and the lateral line system. This study provides the first evidence for the mechanism of Ildr1b on hearing in vivo and sheds light on the pathology of DFNB42.

FGF-2 released from degenerating neurons exerts microglial-induced neuroprotection via FGFR3-ERK signaling pathway

Background

The accumulation of activated microglia is a hallmark of various neurodegenerative diseases. Microglia may have both protective and toxic effects on neurons through the production of various soluble factors, such as chemokines. Indeed, various chemokines mediate the rapid and accurate migration of microglia to lesions. In the zebra fish, another well-known cellular migrating factor is fibroblast growth factor-2 (FGF-2). Although FGF-2 does exist in the mammalian central nervous system (CNS), it is unclear whether FGF-2 influences microglial function.

Methods

The extent of FGF-2 release was determined by ELISA, and the expression of its receptors was examined by immunocytochemistry. The effect of several drug treatments on a neuron and microglia co-culture system was estimated by immunocytochemistry, and the neuronal survival rate was quantified. Microglial phagocytosis was evaluated by immunocytochemistry and quantification, and microglial migration was estimated by fluorescence-activated cell sorting (FACS). Molecular biological analyses, such as Western blotting and promoter assay, were performed to clarify the FGF-2 downstream signaling pathway in microglia.

Results

Fibroblast growth factor-2 is secreted by neurons when damaged by glutamate or oligomeric amyloid β 1-42. FGF-2 enhances microglial migration and phagocytosis of neuronal debris, and is neuroprotective against glutamate toxicity through FGFR3-extracellular signal-regulated kinase (ERK) signaling pathway, which is directly controlled by Wnt signaling in microglia.

Conclusions

FGF-2 secreted from degenerating neurons may act as a ‘help-me’ signal toward microglia by inducing migration and phagocytosis of unwanted debris.

ErbB expressing Schwann cells control lateral line progenitor cells via non-cell-autonomous regulation of Wnt/β-catenin

Proper orchestration of quiescence and activation of progenitor cells is crucial during embryonic development and adult homeostasis. We took advantage of the zebrafish sensory lateral line to define niche-progenitor interactions to understand how integration of diverse signaling pathways spatially and temporally regulates the coordination of these processes. Our previous studies demonstrated that Schwann cells play a crucial role in negatively regulating lateral line progenitor proliferation. Here we demonstrate that ErbB/Neuregulin signaling is not only required for Schwann cell migration but that it plays a continued role in postmigratory Schwann cells. ErbB expressing Schwann cells inhibit lateral line progenitor proliferation and differentiation through non-cell-autonomous inhibition of Wnt/β-catenin signaling. Subsequent activation of Fgf signaling controls sensory organ differentiation, but not progenitor proliferation. In addition to the lateral line, these findings have important implications for understanding how niche-progenitor cells segregate interactions during development, and how they may go wrong in disease states. DOI: http://dx.doi.org/10.7554/eLife.01832.001.

Quantitative cell polarity imaging defines leader-to-follower transitions during collective migration and the key role of microtubule-dependent adherens junction formation

The directed migration of cell collectives drives the formation of complex organ systems. A characteristic feature of many migrating collectives is a ‘tissue-scale’ polarity, whereby ‘leader’ cells at the edge of the tissue guide trailing ‘followers’ that become assembled into polarised epithelial tissues en route. Here, we combine quantitative imaging and perturbation approaches to investigate epithelial cell state transitions during collective migration and organogenesis, using the zebrafish lateral line primordium as an in vivo model. A readout of three-dimensional cell polarity, based on centrosomal-nucleus axes, allows the transition from migrating leaders to assembled followers to be quantitatively resolved for the first time in vivo. Using live reporters and a novel fluorescent protein timer approach, we investigate changes in cell-cell adhesion underlying this transition by monitoring cadherin receptor localisation and stability. This reveals that while cadherin 2 is expressed across the entire tissue, functional apical junctions are first assembled in the transition zone and become progressively more stable across the leader-follower axis of the tissue. Perturbation experiments demonstrate that the formation of these apical adherens junctions requires dynamic microtubules. However, once stabilised, adherens junction maintenance is microtubule independent. Combined, these data identify a mechanism for regulating leader-to-follower transitions within migrating collectives, based on the relocation and stabilisation of cadherins, and reveal a key role for dynamic microtubules in this process.

Zebrafish yap1 plays a role in differentiation of hair cells in posterior lateral line

The evolutionarily conserved Hippo signaling pathway controls organ size by regulating cell proliferation and apoptosis and this process involves Yap1. The zebrafish Yap1 acts during neural differentiation, but its function is not fully understood. The detailed analysis of yap1 expression in proliferative regions, revealed it in the otic placode that gives rise to the lateral line system affected by the morpholino-mediated knockdown of Yap1. The comparative microarray analysis of transcriptome of Yap1-deficient embryos demonstrated changes in expression of many genes, including the Wnt signaling pathway and, in particular, prox1a known for its role in development of mechanoreceptors in the lateral line. The knockdown of Yap1 causes a deficiency of differentiation of mechanoreceptors, and this defect can be rescued by prox1a mRNA. Our studies revealed a role of Yap1 in regulation of Wnt signaling pathway and its target Prox1a during differentiation of mechanosensory cells.

The development of lateral line placodes: taking a broader view

The lateral line system of anamniote vertebrates enables the detection of local water movement and weak bioelectric fields. Ancestrally, it comprises neuromasts - small sense organs containing mechanosensory hair cells - distributed in characteristic lines over the head and trunk, flanked on the head by fields of electroreceptive ampullary organs, innervated by afferent neurons projecting respectively to the medial and dorsal octavolateral nuclei in the hindbrain. Given the independent loss of the electrosensory system in multiple lineages, the development and evolution of the mechanosensory and electrosensory components of the lateral line must be dissociable. Nevertheless, the entire system arises from a series of cranial lateral line placodes, which exhibit two modes of sensory organ formation: elongation to form sensory ridges that fragment (with neuromasts differentiating in the center of the ridge, and ampullary organs on the flanks), or migration as collectives of cells, depositing sense organs in their wake. Intensive study of the migrating posterior lateral line placode in zebrafish has yielded a wealth of information concerning the molecular control of migration and neuromast formation in this migrating placode, in this cypriniform teleost species. However, our mechanistic understanding of neuromast and ampullary organ formation by elongating lateral line placodes, and even of other zebrafish lateral line placodes, is sparse or non-existent. Here, we attempt to highlight the diversity of lateral line development and the limits of the current research focus on the zebrafish posterior lateral line placode. We hope this will stimulate a broader approach to this fascinating sensory system.

The Nogo-C2/Nogo receptor complex regulates the morphogenesis of zebrafish lateral line primordium through modulating the expression of dkk1b, a Wnt signal inhibitor

The fish lateral line (LL) is a mechanosensory system closely related to the hearing system of higher vertebrates, and it is composed of several neuromasts located on the surface of the fish. These neuromasts can detect changes in external water flow, to assist fish in maintaining a stationary position in a stream. In the present study, we identified a novel function of Nogo/Nogo receptor signaling in the formation of zebrafish neuromasts. Nogo signaling in zebrafish, like that in mammals, involves three ligands and four receptors, as well as three co-receptors (TROY, p75, and LINGO-1). We first demonstrated that Nogo-C2, NgRH1a, p75, and TROY are able to form a Nogo-C2 complex, and that disintegration of this complex causes defective neuromast formation in zebrafish. Time-lapse recording of the CldnB::lynEGFP transgenic line revealed that functional obstruction of the Nogo-C2 complex causes disordered morphogenesis, and reduces rosette formation in the posterior LL (PLL) primordium during migration. Consistent with these findings, hair-cell progenitors were lost from the PLL primordium in p75, TROY, and Nogo-C2/NgRH1a morphants. Notably, the expression levels of pea3, a downstream marker of Fgf signaling, and dkk1b, a Wnt signaling inhibitor, were both decreased in p75, TROY, and Nogo-C2/NgRH1a morphants; moreover, dkk1b mRNA injection could rescue the defects in neuromast formation resulting from knockdown of p75 or TROY. We thus suggest that a novel Nogo-C2 complex, consisting of Nogo-C2, NgRH1a, p75, and TROY, regulates Fgf signaling and dkk1b expression, thereby ensuring stable organization of the PLL primordium.

FGFR1-WNT-TGF-β signaling in prostate cancer mouse models recapitulates human reactive stroma

The reactive stroma surrounding tumor lesions performs critical roles ranging from supporting tumor cell proliferation to inducing tumorigenesis and metastasis. Therefore, it is critical to understand the cellular components and signaling control mechanisms that underlie the etiology of reactive stroma. Previous studies have individually implicated fibroblast growth factor receptor 1 (FGFR1) and canonical WNT/β-catenin signaling in prostate cancer progression and the initiation and maintenance of a reactive stroma; however, both pathways are frequently found to be coactivated in cancer tissue. Using autochthonous transgenic mouse models for inducible FGFR1 (JOCK1) and prostate-specific and ubiquitously expressed inducible β-catenin (Pro-Cat and Ubi-Cat, respectively) and bigenic crosses between these lines (Pro-Cat × JOCK1 and Ubi-Cat × JOCK1), we describe WNT-induced synergistic acceleration of FGFR1-driven adenocarcinoma, associated with a pronounced fibroblastic reactive stroma activation surrounding prostatic intraepithelial neoplasia (mPIN) lesions found both in in situ and reconstitution assays. Both mouse and human reactive stroma exhibited increased transforming growth factor-β (TGF-β) signaling adjacent to pathologic lesions likely contributing to invasion. Furthermore, elevated stromal TGF-β signaling was associated with higher Gleason scores in archived human biopsies, mirroring murine patterns. Our findings establish the importance of the FGFR1-WNT-TGF-β signaling axes as driving forces behind reactive stroma in aggressive prostate adenocarcinomas, deepening their relevance as therapeutic targets.

Histone deacetylase activity is required for embryonic posterior lateral line development

OBJECTIVES

The posterior lateral line (PLL) system in zebrafish has recently become a model for investigating tissue morphogenesis. PLL primordium periodically deposits neuromasts as it migrates along the horizontal myoseptum from head to tail of the embryonic fish, and this migration requires activity of various molecular mechanisms. Histone deacetylases (HDACs) have been implicated in numerous biological processes of development, by regulating gene transcription, but their roles in regulating PLL during embryonic development have up to now remained unexplored.

MATERIAL AND METHODS

In this study, we used HDAC inhibitors to investigate the role of HDACs in early development of the zebrafish PLL sensory system. We further investigated development of the PLL by cell-specific immunostaining and in situ hybridization.

RESULTS

Our analysis showed that HDACs were involved in zebrafish PLL development as pharmacological inhibition of HDACs resulted in its defective formation. We observed that migration of PLL primordium was altered and accompanied by disrupted development of PLL neuromasts in HDAC inhibitor-treated embryos. In these, positions of PLL neuromasts were affected. In particular, the first PLL neuromast was displaced posteriorly in a treatment dose-dependent manner. Primordium cell proliferation was reduced upon HDAC inhibition. Finally, we showed that inhibition of HDAC function reduced numbers of hair cells in PLL neuromasts of HDAC inhibitor-treated embryos.

CONCLUSION

Here, we have revealed a novel role for HDACs in orchestrating PLL morphogenesis. Our results suggest that HDAC activity is necessary for control of cell proliferation and migration of PLL primordium and hair cell differentiation during early stages of PLL development in zebrafish.

Cell migration

Cell migration is fundamental to establishing and maintaining the proper organization of multicellular organisms. Morphogenesis can be viewed as a consequence, in part, of cell locomotion, from large-scale migrations of epithelial sheets during gastrulation, to the movement of individual cells during development of the nervous system. In an adult organism, cell migration is essential for proper immune response, wound repair, and tissue homeostasis, while aberrant cell migration is found in various pathologies. Indeed, as our knowledge of migration increases, we can look forward to, for example, abating the spread of highly malignant cancer cells, retarding the invasion of white cells in the inflammatory process, or enhancing the healing of wounds. This article is organized in two main sections. The first section is devoted to the single-cell migrating in isolation such as occurs when leukocytes migrate during the immune response or when fibroblasts squeeze through connective tissue. The second section is devoted to cells collectively migrating as part of multicellular clusters or sheets. This second type of migration is prevalent in development, wound healing, and in some forms of cancer metastasis.

The role of Wnt/β-catenin signaling in proliferation and regeneration of the developing basilar papilla and lateral line

Canonical Wnt/β-catenin signaling has been implicated in multiple developmental events including the regulation of proliferation, cell fate, and differentiation. In the inner ear, Wnt/β-catenin signaling is required from the earliest stages of otic placode specification through the formation of the mature cochlea. Within the avian inner ear, the basilar papilla (BP), many Wnt pathway components are expressed throughout development. Here, using reporter constructs for Wnt/β-catenin signaling, we show that this pathway is active throughout the BP (E6-E14) in both hair cells (HCs) and supporting cells. To characterize the role of Wnt/β-catenin activity in developing HCs, we performed gain- and loss-of-function experiments in vitro and in vivo in the chick BP and zebrafish lateral line systems, respectively. Pharmacological inhibition of Wnt signaling in the BP and lateral line neuromasts during the periods of proliferation and HC differentiation resulted in reduced proliferation and decreased HC formation. Conversely, pharmacological activation of this pathway significantly increased the number of HCs in the lateral line and BP. Results demonstrated that this increase was the result of up-regulated cell proliferation within the Sox2-positive cells of the prosensory domains. Furthermore, Wnt/β-catenin activation resulted in enhanced HC regeneration in the zebrafish lateral line following aminoglycoside-induced HC loss. Combined, our data suggest that Wnt/β-catenin signaling specifies the number of cells within the prosensory domain and subsequently the number of HCs. This ability to induce proliferation suggests that the modulation of Wnt/β-catenin signaling could play an important role in therapeutic HC regeneration.

The association between serum Dickkopf-1 levels and esophageal squamous cell carcinoma

Serum Dickkopf-1(DKK-1) is elevated in many malignancies and is an important indicator of malignant potential. However, its significance in esophageal squamous cell cancer (ESCC) has not yet been clarified. We hypothesized a role for DKK-1 in patients with ESCC. The study consisted of 90 ESCC patients and 85 healthy controls. After diagnosis, the level of DKK-1 was measured in the serum samples by enzyme-linked immunosorbent assay and the levels of DKK-1 were much higher in the ESCC patients than in the healthy control group (p < 0.0001). For serum DKK-1, the sensitivity and specificity of the assay were 70 and 80%, respectively. The preoperative serum DKK-1 level was elevated in the ESCC patients. Although serum DKK-1 is not a specific biomarker for ESCC, it might be a useful marker for the diagnosis and treatment of ESCC.

Specification of leading and trailing cell features during collective migration in the Drosophila trachea

The role of tip and rear cells in collective migration is still a matter of debate and their differences at the cytoskeletal level are poorly understood. Here, we analysed these issues in the Drosophila trachea, an organ that develops from the collective migration of clusters of cells that respond to Branchless (Bnl), a fibroblast growth factor (FGF) homologue expressed in surrounding tissues. We track individual cells in the migratory cluster and characterise their features and unveil two prototypical types of cytoskeletal organisation that account for tip and rear cells respectively. Indeed, once the former are specified, they remain as such throughout migration. Furthermore, we show that FGF signalling in a single tip cell can trigger the migration of the cells in the branch. Finally, we found specific Rac activation at the tip cells and analysed how FGF-independent cell features, such as adhesion and motility, act on coupling the behaviour of trailing and tip cells. Thus, the combined effect of FGF promoting leading cell behaviour and the modulation of cell properties in a cluster can account for the wide range of migratory events driven by FGF.

Gβ1 controls collective cell migration by regulating the protrusive activity of leader cells in the posterior lateral line primordium

Collective cell migration is critical for normal development, tissue repair and cancer metastasis. Migration of the posterior lateral line primordium (pLLP) generates the zebrafish sensory organs (neuromasts, NMs). This migration is promoted by the leader cells at the leading edge of the pLLP, which express the G protein-coupled chemokine receptor Cxcr4b and respond to the chemokine Cxcl12a. However, the mechanism by which Cxc112a/Cxcr4b signaling regulates pLLP migration remains unclear. Here we report that signal transduction by the heterotrimeric G protein subunit Gβ1 is essential for proper pLLP migration. Although both Gβ1 and Gβ4 are expressed in the pLLP and NMs, depletion of Gβ1 but not Gβ4 resulted in an arrest of pLLP migration. In embryos deficient for Gβ1, the pLLP cells migrated in an uncoordinated fashion and were unable to extend protrusions at the leading front, phenocopying those in embryos deficient for Cxcl12a or Cxcr4b. A transplantation assay showed that, like Cxcr4b, Gβ1 is required only in the leader cells of the pLLP. Analysis of F-actin dynamics in the pLLP revealed that whereas wild-type leader cells display extensive actin polymerization in the direction of pLLP migration, counterparts defective for Gβ1, Cxcr4b or Cxcl12a do not. Finally, synergy experiments revealed that Gβ1 and Cxcr4b interact genetically in regulating pLLP migration. Collectively, our data indicate that Gβ1 controls migration of the pLLP, likely by acting downstream of the Cxcl12a/Cxcr4b signaling. This study also provides compelling evidence for functional specificity among Gβ isoforms in vivo.

Collective cell migration of epithelial and mesenchymal cells

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

A Hox gene controls lateral line cell migration by regulating chemokine receptor expression downstream of Wnt signaling

The posterior lateral line primordium in zebrafish provides an amenable model to study mechanisms of collective cell migration. The directed migration of the cell cluster along the path of Sdf1a chemokine requires two receptors, Cxcr4b and Cxcr7b, which are expressed in the leading and trailing part of the primordium, respectively. The polarized expression of receptors is regulated by Wnt signaling, but downstream players mediating this control remain to be found. Here, we show that the Hox homeobox gene Hoxb8a is a critical component that acts downstream of the Wnt pathway to coordinate the expression of both chemokine receptors. We find that Hoxb8a is expressed in the leading part of the primordium and is required for the correct speed and extent of migration. Hoxb8a expression is dependent upon Wnt activity and needed both for cxcr4b expression and to repress and thus restrict cxcr7b expression to the trailing zone of the primordium. In the absence of Wnt activity, overexpressed Hoxb8a is able to repress cxcr7b but not up-regulate cxcr4b expression. Together with results from expressing dominant activator and repressor constructs, these findings suggest that Hoxb8a is induced by and cooperates with Wnt signaling to up-regulate cxcr4b, and acts through multiple mechanisms to repress cxcr7b expression.

Quantification of shape and cell polarity reveals a novel mechanism underlying malformations resulting from related FGF mutations during facial morphogenesis

Fibroblast growth factor (FGF) signaling mutations are a frequent contributor to craniofacial malformations including midfacial anomalies and craniosynostosis. FGF signaling has been shown to control cellular mechanisms that contribute to facial morphogenesis and growth such as proliferation, survival, migration and differentiation. We hypothesized that FGF signaling not only controls the magnitude of growth during facial morphogenesis but also regulates the direction of growth via cell polarity. To test this idea, we infected migrating neural crest cells of chicken embryos with  replication-competent avian sarcoma virus expressing either FgfR2(C278F), a receptor mutation found in Crouzon syndrome or the ligand Fgf8. Treated embryos exhibited craniofacial malformations resembling facial dysmorphologies in craniosynostosis syndrome. Consistent with our hypothesis, ectopic activation of FGF signaling resulted in decreased cell proliferation, increased expression of the Sprouty class of FGF signaling inhibitors, and repressed phosphorylation of ERK/MAPK. Furthermore, quantification of cell polarity in facial mesenchymal cells showed that while orientation of the Golgi body matches the direction of facial prominence outgrowth in normal cells, in FGF-treated embryos this direction is randomized, consistent with aberrant growth that we observed. Together, these data demonstrate that FGF signaling regulates cell proliferation and cell polarity and that these cell processes contribute to facial morphogenesis.

Wnt/Dkk negative feedback regulates sensory organ size in zebrafish

Correct organ size must involve a balance between promotion and inhibition of cell proliferation. A mathematical model has been proposed in which an organ is assumed to produce its own growth activator as well as a growth inhibitor [1], but there is as yet no molecular evidence to support this model [2]. The mechanosensory organs of the fish lateral line system (neuromasts) are composed of a core of sensory hair cells surrounded by nonsensory support cells. Sensory cells are constantly replaced and are regenerated from surrounding nonsensory cells [3], while each organ retains the same size throughout life. Moreover, neuromasts also bud off new neuromasts, which stop growing when they reach the same size [4, 5]. Here, we show that the size of neuromasts is controlled by a balance between growth-promoting Wnt signaling activity in proliferation-competent cells and Wnt-inhibiting Dkk activity produced by differentiated sensory cells. This negative feedback loop from Dkk (secreted by differentiated cells) on Wnt-dependent cell proliferation (in surrounding cells) also acts during regeneration to achieve size constancy. This study establishes Wnt/Dkk as a novel mechanism to determine the final size of an organ.

Activation of canonical Wnt/β-catenin signaling stimulates proliferation in neuromasts in the zebrafish posterior lateral line

BACKGROUND

The posterior lateral line in zebrafish develops from a migrating primordium that deposits clusters of cells that differentiate into neuromasts at regular intervals along the trunk. The deposition of these neuromasts is known to be coordinated by Wnt and FGF signals that control the proliferation, migration, and organization of the primordium. However, little is known about the control of proliferation in the neuromasts following their deposition.

RESULTS

We show that pharmacological activation of the Wnt/β-catenin signaling pathway with 1-azakenpaullone upregulates proliferation in neuromasts post-deposition. This results in increased size of the neuromasts and overproduction of sensory hair cells. We also show that activation of Wnt signaling returns already quiescent supporting cells to a proliferative state in mature neuromasts. Additionally, activation of Wnt signaling increases the number of supporting cells that return to the cell cycle in response to hair cell damage and the number of regenerated hair cells. Finally, we show that inhibition of Wnt signaling by overexpression of dkk1b suppresses proliferation during both differentiation and regeneration.

CONCLUSIONS

These data suggest that Wnt/β-catenin signaling is both necessary and sufficient for the control of proliferation of lateral line progenitors during development, ongoing growth of the neuromasts, and hair cell regeneration.

EpCAM is an endoderm-specific Wnt derepressor that licenses hepatic development

Mechanisms underlying cell-type-specific response to morphogens or signaling molecules during embryonic development are poorly understood. To learn how response to the liver-inductive Wnt2bb signal is achieved, we identify an endoderm-enriched, single transmembrane protein, epithelial-cell-adhesion-molecule (EpCAM), as an endoderm-specific Wnt derepressor in zebrafish. hi2151/epcam mutants exhibit defective liver development similar to prt/wnt2bb mutants. EpCAM directly binds to Kremen1 and disrupts the Kremen1-Dickkopf2 (Dkk2) interaction, which prevents Kremen1-Dkk2-mediated removal of Lipoprotein-receptor-related protein 6 (Lrp6) from the cell surface. These data lead to a model in which EpCAM derepresses Lrp6 and cooperates with Wnt ligand to activate Wnt signaling through stabilizing membrane Lrp6 and allowing Lrp6 clustering into active signalosomes. Thus, EpCAM cell autonomously licenses and cooperatively activates Wnt2bb signaling in endodermal cells. Our results identify EpCAM as the key molecule and its functional mechanism to confer endodermal cells the competence to respond to the liver-inductive Wnt2bb signal.

Lef1 regulates Dusp6 to influence neuromast formation and spacing in the zebrafish posterior lateral line primordium

The posterior lateral line primordium (PLLp) migrates caudally and periodically deposits neuromasts. Coupled, but mutually inhibitory, Wnt-FGF signaling systems regulate proto-neuromast formation in the PLLp: FGF ligands expressed in response to Wnt signaling activate FGF receptors and initiate proto-neuromast formation. FGF receptor signaling, in turn, inhibits Wnt signaling. However, mechanisms that determine periodic neuromast formation and deposition in the PLLp remain poorly understood. Previous studies showed that neuromasts are deposited closer together and the PLLp terminates prematurely in lef1-deficient zebrafish embryos. It was suggested that this results from reduced proliferation in the leading domain of the PLLp and/or premature incorporation of progenitors into proto-neuromasts. We found that rspo3 knockdown reduces proliferation in a manner similar to that seen in lef1 morphants. However, it does not cause closer neuromast deposition or premature termination of the PLLp, suggesting that such changes in lef1-deficient embryos are not linked to changes in proliferation. Instead, we suggest that they are related to the role of Lef1 in regulating the balance of Wnt and FGF functions in the PLLp. Lef1 determines expression of the FGF signaling inhibitor Dusp6 in leading cells and regulates incorporation of cells into neuromasts; reduction of Dusp6 in leading cells in lef1-deficient embryos allows new proto-neuromasts to form closer to the leading edge. This is associated with progressively slower PLLp migration, reduced spacing between deposited neuromasts and premature termination of the PLLp system.

Wnt signaling during cochlear development

Wnt signaling is a hallmark of all embryonic development with multiple roles at multiple developmental time points. Wnt signaling is also important in the development of several organs, one of which is the inner ear, where it participates in otic specification, the formation of vestibular structures, and the development of the cochlea. In particular, we focus on Wnt signaling in the auditory organ, the cochlea. Attempting to dissect the multiple Wnt signaling pathways in the mammalian cochlea is a challenging task due to limited expression data, particularly at proliferating stages. To offer predictions about Wnt activity, we compare cochlear development with that of other biological systems such as Xenopus retina, brain, cancer cells and osteoblasts. Wnts are likely to regulate development through crosstalk with other signaling pathways, particularly Notch and FGF, leading to changes in the expression of Sox2 and proneural (pro-hair cell) genes. In this review we have consolidated the known signaling pathways in the cochlea with known developmental roles of Wnts from other systems to generate a potential timeline of cochlear development.

Innervation is required for sense organ development in the lateral line system of adult zebrafish

Superficial mechanosensory organs (neuromasts) distributed over the head and body of fishes and amphibians form the "lateral line" system. During zebrafish adulthood, each neuromast of the body (posterior lateral line system, or PLL) produces "accessory" neuromasts that remain tightly clustered, thereby increasing the total number of PLL neuromasts by a factor of more than 10. This expansion is achieved by a budding process and is accompanied by branches of the afferent nerve that innervates the founder neuromast. Here we show that innervation is essential for the budding process, in complete contrast with the development of the embryonic PLL, where innervation is entirely dispensable. To obtain insight into the molecular mechanisms that underlie the budding process, we focused on the terminal system that develops at the posterior tip of the body and on the caudal fin. In this subset of PLL neuromasts, bud neuromasts form in a reproducible sequence over a few days, much faster than for other PLL neuromasts. We show that wingless/int (Wnt) signaling takes place during, and is required for, the budding process. We also show that the Wnt activator R-spondin is expressed by the axons that innervate budding neuromasts. We propose that the axon triggers Wnt signaling, which itself is involved in the proliferative phase that leads to bud formation. Finally, we show that innervation is required not only for budding, but also for long-term maintenance of all PLL neuromasts.

Fgf signaling governs cell fate in the zebrafish pineal complex

Left-right (L-R) asymmetries in neuroanatomy exist throughout the animal kingdom, with implications for function and behavior. The molecular mechanisms that control formation of such asymmetries are beginning to be understood. Significant progress has been made by studying the zebrafish parapineal organ, a group of neurons on the left side of the epithalamus. Parapineal cells arise from the medially located pineal complex anlage and migrate to the left side of the brain. We have found that Fgf8a regulates a fate decision among anterior pineal complex progenitors that occurs just prior to the initiation of leftward migration. Cell fate analysis shows that in the absence of Fgf8a a subset of cells in the anterior pineal complex anlage differentiate as cone photoreceptors rather than parapineal neurons. Fgf8a acts permissively to promote parapineal fate in conjunction with the transcription factor Tbx2b, but might also block cone photoreceptor fate. We conclude that this subset of anterior pineal complex precursors, which normally become parapineal cells, are bipotential and require Fgf8a to maintain parapineal identity and/or prevent cone identity.

A systematic survey of expression and function of zebrafish frizzled genes

Wnt signaling is crucial for the regulation of numerous processes in development. Consistent with this, the gene families for both the ligands (Wnts) and receptors (Frizzleds) are very large. Surprisingly, while we have a reasonable understanding of the Wnt ligands likely to mediate specific Wnt-dependent processes, the corresponding receptors usually remain to be elucidated. Taking advantage of the zebrafish model's excellent genomic and genetic properties, we undertook a comprehensive analysis of the expression patterns of frizzled (fzd) genes in zebrafish. To explore their functions, we focused on testing their requirement in several developmental events known to be regulated by Wnt signaling, convergent extension movements of gastrulation, neural crest induction, and melanocyte specification. We found fourteen distinct fzd genes in the zebrafish genome. Systematic analysis of their expression patterns between 1-somite and 30 hours post-fertilization revealed complex, dynamic and overlapping expression patterns. This analysis demonstrated that only fzd3a, fzd9b, and fzd10 are expressed in the dorsal neural tube at stages corresponding to the timing of melanocyte specification. Surprisingly, however, morpholino knockdown of these, alone or in combination, gave no indication of reduction of melanocytes, suggesting the important involvement of untested fzds or another type of Wnt receptor in this process. Likewise, we found only fzd7b and fzd10 expressed at the border of the neural plate at stages appropriate for neural crest induction. However, neural crest markers were not reduced by knockdown of these receptors. Instead, these morpholino knockdown studies showed that fzd7a and fzd7b work co-operatively to regulate convergent extension movement during gastrulation. Furthermore, we show that the two fzd7 genes function together with fzd10 to regulate epiboly movements and mesoderm differentiation.