Transition from non-motile behaviour to directed migration during early PGC development in zebrafish

Cell migration during morphogenesis

During development, functional structures must form with the correct three-dimensional geometry composed of the correct cell types. In many cases cell types are specified at locations distant to where they will ultimately reside for normal biological function. Although cell migration is crucial for normal development and morphogenesis of animal body plans and organ systems, abnormal cell migration during adult life underlies pathological states such as invasion and metastasis of cancer. In both contexts cells migrate either individually, as loosely associated sheets or as clusters of cells. In this review, we summarize, compare and integrate knowledge gained from several in vivo model systems that have yielded insights into the regulation of morphogenic cell migration, such as the zebrafish lateral line primordium and primordial germ cells, Drosophila border cell clusters, vertebrate neural crest migration and angiogenic sprouts in the post-natal mouse retina. Because of its broad multicontextual and multiphylletic distribution, understanding cell migration in its various manifestations in vivo is likely to provide new insights into both the function and malfunction of key embryonic and postembryonic events. In this review, we will provide a succinct phenotypic description of the many model systems utilized to study cell migration in vivo. More importantly, we will highlight, compare and integrate recent advances in our understanding of how cell migration is regulated in these varied model systems with special emphasis on individual and collective cell movements.

Single-molecule microscopy reveals membrane microdomain organization of cells in a living vertebrate

It has been possible for several years to study the dynamics of fluorescently labeled proteins by single-molecule microscopy, but until now this technology has been applied only to individual cells in culture. In this study, it was extended to stem cells and living vertebrate organisms. As a molecule of interest we used yellow fluorescent protein fused to the human H-Ras membrane anchor, which has been shown to serve as a model for proteins anchored in the plasma membrane. We used a wide-field fluorescence microscopy setup to visualize individual molecules in a zebrafish cell line (ZF4) and in primary embryonic stem cells. A total-internal-reflection microscopy setup was used for imaging in living organisms, in particular in epidermal cells in the skin of 2-day-old zebrafish embryos. Our results demonstrate the occurrence of membrane microdomains in which the diffusion of membrane proteins in a living organism is confined. This membrane organization differed significantly from that observed in cultured cells, illustrating the relevance of performing single-molecule microscopy in living organisms.

Trafficking and cell migration

The migration of single cells and epithelial sheets is of great importance for gastrulation and organ formation in developing embryos and, if misregulated, can have dire consequences e.g. during cancer metastasis. A keystone of cell migration is the regulation of adhesive contacts, which are dynamically assembled and disassembled via endocytosis. Here, we discuss some of the basic concepts about the function of endocytic trafficking during cell migration: transport of integrins from the cell rear to the leading edge in fibroblasts; confinement of signalling to the front of single cells by endocytic transport of growth factors; regulation of movement coherence in multicellular sheets by cadherin turnover; and shaping of extracellular chemokine gradients. Taken together, endocytosis enables migrating cells and tissues to dynamically modulate their adhesion and signalling, allowing them to efficiently migrate through their extracellular environment.

Chemokine signaling in embryonic cell migration: a fisheye view

Chemokines and their receptors were discovered about twenty years ago as mediators of leukocyte traffic. Over the past decade, functional studies of these molecules have revealed their importance for cell migration processes during embryogenesis, which, in addition to providing mechanistic insights into embryonic development, could complement information about chemokine function in the immune system. Here, we review the roles of the chemokine stromal cell-derived factor 1 (SDF-1/CXCL12) and its receptor CXCR4 during zebrafish and mouse embryonic development, and discuss their function in regulating the interactions of cells with their extracellular environment, in directing their migration, and in maintaining their location.

Generating chimeric zebrafish embryos by transplantation

One of the most powerful tools used to gain insight into complex developmental processes is the analysis of chimeric embryos. A chimera is defined as an organism that contains cells from more than one animal; mosaics are one type of chimera in which cells from more than one genotype are mixed, usually wild-type and mutant. In the zebrafish, chimeras can be readily made by transplantation of cells from a donor embryo into a host embryo at the appropriate embryonic stage. Labeled donor cells are generated by injection of a lineage marker, such as a fluorescent dye, into the one-cell stage embryo. Labeled donor cells are removed from donor embryos and introduced into unlabeled host embryos using an oil-controlled glass pipette mounted on either a compound or dissecting microscope. Donor cells can in some cases be targeted to a specific region or tissue of the developing blastula or gastrula stage host embryo by choosing a transplantation site in the host embryo based on well-established fate maps.

The maternal-effect gene cellular island encodes aurora B kinase and is essential for furrow formation in the early zebrafish embryo

Females homozygous for a mutation in cellular island (cei) produce embryos with defects in cytokinesis during early development. Analysis of the cytoskeletal events associated with furrow formation reveal that these defects include a general delay in furrow initiation as well as a complete failure to form furrow-associated structures in distal regions of the blastodisc. A linkage mapping-based candidate gene approach, including transgenic rescue, shows that cei encodes the zebrafish Aurora B kinase homologue. Genetic complementation analysis between the cei mutation and aurB zygotic lethal mutations corroborate gene assignment and reveal a complex nature of the maternal-effect cei allele, which appears to preferentially affect a function important for cytokinesis in the early blastomeres. Surprisingly, in cei mutant embryos a short yet otherwise normal furrow forms in the center of the blastodisc. Furrow formation is absent throughout the width of the blastodisc in cei mutant embryos additionally mutant for futile cycle, which lack a spindle apparatus, showing that the residual furrow signal present in cei mutants is derived from the mitotic spindle. Our analysis suggests that partially redundant signals derived from the spindle and astral apparatus mediate furrow formation in medial and distal regions of the early embryonic blastomeres, respectively, possibly as a spatial specialization to achieve furrow formation in these large cells. In addition, our data also suggest a role for Cei/AurB function in the reorganization of the furrow-associated microtubules in both early cleavage- and somite-stage embryos. In accordance with the requirement for cei/aurB in furrow induction in the early cleavage embryo, germ plasm recruitment to the forming furrow is also affected in embryos lacking normal cei/aurB function.

Germ cell migration in zebrafish is cyclopamine-sensitive but Smoothened-independent

Primordial germ cells (PGCs) are the progenitors of reproductive cells in metazoans and are an important model for the study of cell migration in vivo. Previous reports have suggested that Hedgehog (Hh) protein acts as a chemoattractant for PGC migration in the Drosophila embryo and that downstream signaling proteins such as Patched (Ptc) and Smoothened (Smo) are required for PGC localization to somatic gonadal precursors. Here we interrogate whether Hh signaling is required for PGC migration in vertebrates, using the zebrafish as a model system. We find that cyclopamine, an inhibitor of Hh signaling, causes strong defects in the migration of PGCs in the zebrafish embryo. However, these defects are not due to inhibition of Smoothened (Smo) by cyclopamine; rather, we find that neither maternal nor zygotic Smo is required for PGC migration in the zebrafish embryo. Cyclopamine instead acts independently of Smo to decrease the motility of zebrafish PGCs, in part by dysregulating cell adhesion and uncoupling cell polarization and translocation. These results demonstrate that Hh signaling is not required for zebrafish PGC migration, and underscore the importance of regulated cell-cell adhesion for cell migration in vivo.

Paracrine insulin-like growth factor signaling influences primordial germ cell migration: in vivo evidence from the zebrafish model

IGF signaling has been shown to stimulate migration of multiple cell types in vitro, but few studies have confirmed an equivalent function for IGF signaling in vivo. We recently showed that suppression of IGF receptors in the zebrafish embryo disrupts primordial germ cell (PGC) migration, but the mechanism underlying these effects has not been elucidated. We hypothesized that PGCs are intrinsically dependent upon IGF signaling during the migratory phase of development. To test this hypothesis, we first examined the spatial expression patterns of IGF ligand genes (igf1, igf2a, and igf2b) in the zebrafish embryo. In situ analyses revealed distinct expression patterns for each IGF ligand gene, with igf2b mRNA expressed in a spatial pattern that correlates strongly with PGC migration. To determine whether PGC migration is responsive to IGF signaling in vivo, we synthesized gene hybrid expression constructs that permit conditional overexpression of IGF ligands by PGCs into the PGC microenvironment. Conditional overexpression of IGF ligands consistently disrupted PGC migration, confirming that PGC migration is sensitive to local aberrations in IGF signaling. Finally, we show that conditional suppression of IGF signaling, via PGC-specific overexpression of a mutant IGF-I receptor, disrupts PGC migration, confirming that zebrafish PGCs intrinsically require IGF signaling for directional migration in vivo. Collectively, these studies confirm an in vivo role for IGF signaling in cell migration and identify a candidate ligand gene (igf2b) regulating PGC migration in the zebrafish.

Blebs lead the way: how to migrate without lamellipodia

Blebs are spherical membrane protrusions that are produced by contractions of the actomyosin cortex. Blebs are often considered to be a hallmark of apoptosis; however, blebs are also frequently observed during cytokinesis and during migration in three-dimensional cultures and in vivo. For tumour cells and a number of embryonic cells, blebbing migration seems to be a common alternative to the more extensively studied lamellipodium-based motility. We argue that blebs should be promoted to a more prominent place in the world of cellular protrusions.

Control over the morphology and segregation of Zebrafish germ cell granules during embryonic development

BACKGROUND

Zebrafish germ cells contain granular-like structures, organized around the cell nucleus. These structures share common features with polar granules in Drosophila, germinal granules in Xenopus and chromatoid bodies in mice germ cells, such as the localization of the zebrafish Vasa, Piwi and Nanos proteins, among others. Little is known about the structure of these granules as well as their segregation in mitosis during early germ-cell development.

RESULTS

Using transgenic fish expressing a fluorescently labeled novel component of Zebrafish germ cell granules termed Granulito, we followed the morphology and distribution of the granules. We show that whereas these granules initially exhibit a wide size variation, by the end of the first day of development they become a homogeneous population of medium size granules. We investigated this resizing event and demonstrated the role of microtubules and the minus-end microtubule dependent motor protein Dynein in the process. Last, we show that the function of the germ cell granule resident protein the Tudor domain containing protein-7 (Tdrd7) is required for determination of granule morphology and number.

CONCLUSION

Our results suggest that Zebrafish germ cell granules undergo a transformation process, which involves germ cell specific proteins as well as the microtubular network.

Control of chemokine-guided cell migration by ligand sequestration

Primordial germ cell (PGC) migration in zebrafish is directed by the chemokine SDF-1a that activates its receptor CXCR4b. Little is known about the molecular mechanisms controlling the distribution of this chemoattractant in vivo. We demonstrate that the activity of a second SDF-1/CXCL12 receptor, CXCR7, is crucial for proper migration of PGCs toward their targets. We show that CXCR7 functions primarily in the somatic environment rather than within the migrating cells. In CXCR7 knocked-down embryos, the PGCs exhibit a phenotype that signifies defects in SDF-1a gradient formation as the cells fail to polarize effectively and to migrate toward their targets. Indeed, somatic cells expressing CXCR7 show enhanced internalization of the chemokine suggesting that CXCR7 acts as a sink for SDF-1a, thus allowing the dynamic changes in the transcription of sdf-1a to be mirrored by similar dynamics at the protein level.

Stem cell trafficking in tissue development, growth, and disease

Regulated movement of stem cells is critical for organogenesis during development and for homeostasis and repair in adulthood. Here we analyze the biological significance and molecular mechanisms underlying stem cell trafficking in the generation of the germline, and the generation and regeneration of blood and muscle. Comparison across organisms and lineages reveals remarkable conservation as well as specialization in homing and migration mechanisms used by mature leukocytes, adult and fetal stem cells, and cancer stem cells. In vivo trafficking underpins the successful therapeutic application of hematopoietic stem cells for bone-marrow transplant, and further elucidation of homing and migration pathways in other systems will enable broader application of stem cells for targeted cell therapy and drug delivery.

Control of receptor internalization, signaling level, and precise arrival at the target in guided cell migration

Activation of the chemokine receptor CXCR4 by SDF1 controls a variety of biological processes in development, immune response, and disease [1-5]. The carboxyl-terminal region of CXCR4 is subject to phosphorylation that allows binding of regulatory proteins [5]; this results in downregulation of CXCR4 signaling and receptor internalization [6]. Notably, truncations of this part of CXCR4 have been implicated in WHIM syndrome, a dominantly inherited immunodeficiency disorder [7, 8]. Despite its importance in receptor signaling and the clinical relevance of its regulation, the precise function of regulating signaling level and internalization in controlling cell behavior is not known. Whereas a number of in vitro studies suggested that the carboxyl terminus of CXCR4 positively regulates chemotaxis (e.g., [9]), others reached the opposite conclusion [8, 10, 11]. These conflicting results highlight the importance of investigating this process under physiological conditions in the live animal. In this study, we demonstrate the significance of internalization and of controlling receptor signaling level for SDF-1-guided migration. We found that whereas internalization and the control over signaling intensity are dispensable for cell motility and directional sensing, they are essential for fine-tuning of migration in vivo, allowing precise arrival of zebrafish PGCs at their target, the region where the gonad develops.

A role for RhoA in the two-phase migratory pattern of post-otic neural crest cells

Neural crest (NC) cells have been elegantly traced to follow stereotypical migratory pathways throughout the vertebrate embryo, yet we still lack complete information on individual cell migratory behaviors and how molecular mechanisms direct NC cell guidance. Here, we analyze the spatio-temporal migratory pattern of post-otic NC and the in vivo role of the small Rho GTPase, RhoA, using fluorescent cell labeling, molecular perturbation, and intravital 4D (3D+ time) confocal imaging in the intact chick embryo. We find that the post-otic NC cell migratory pattern is established in two phases with distinct cell migratory behaviors. An initial wide front of lateral-directed NC cells, led by NC from rhombomere 7 (r7), move as a distinct subpopulation. This is followed in time by fewer NC cells that migrate collectively from r7 to r8 in a follow-the-leader manner with extensive cellular extensions between cells. We show that post-otic migratory NC cells express RhoA, using RT-PCR on isolated, flow cytometry sorted NC cells and in neural tube culture explants. When RhoA function is altered by expression of a dominant negative or constitutively active form, or injection of C3, there are two major consequences. RhoA constitutively active expressing NC cells are less directional, slower and form fewer follow-the-leader chain assemblies. NC cells expressing RhoA-DN are less affective in retracting filopodia, migrate slower and also form fewer follow-the-leader chain assemblies. Together, these alterations to NC cell intrinsic signaling and cell-cell contact disrupt the precise spatio-temporal post-otic NC cell migratory pattern.

Spatiotemporal localization of germ plasm RNAs during zebrafish oogenesis

In zebrafish, primordial germ cells (PGCs) are determined by a specialized maternal cytoplasm, the germ plasm, which forms at the distal ends of the cleavage furrows in 4-cell embryos. The germ plasm includes maternal mRNAs from the germline-specific genes such as vasa and nanos1, and vegetally localized dazl RNA is also incorporated into the germ plasm. However, little is known about the distributions and assembly mechanisms of germ plasm components, especially during oogenesis. Here we report that the germ plasm RNAs vasa, nanos1, and dazl co-localize with the mitochondrial cloud (MC) and are transported to the vegetal cortex during early oogenesis. We found that a mitochondrial cloud localization element (MCLE) previously identified in the 3' untranslated region (3'UTR) of Xenopus Xcat2 gene can direct RNA localization to the vegetal cortex via the MC in zebrafish oocytes. In addition, the RNA-binding protein Hermes is a component of the MC in zebrafish oocytes, as is the case in Xenopus. Moreover, we provide evidence that the dazl 3'UTR possesses at least three types of cis-acting elements that direct multiple steps in the localization process: MC localization, anchorage at the vegetal cortex, and localization at the cleavage furrows. Taken together, the data show that the MC functions as a conserved feature that participates in transport of the germ plasm RNAs in Xenopus and zebrafish oocytes. Furthermore, we propose that the germ plasm components are assembled in a stepwise and spatiotemporally-regulated manner during oogenesis and early embryogenesis in zebrafish.

Differential regulation of germline mRNAs in soma and germ cells by zebrafish miR-430

Early in development, primordial germ cells (PGCs) are set aside from somatic cells and acquire a unique gene-expression program . The mechanisms underlying germline-specific gene expression are largely unknown. Nanos expression is required during germline development and is posttranscriptionally restricted to PGCs . Here we report that the microRNA miR-430 targets the 3' untranslated region (UTR) of nanos1 during zebrafish embryogenesis. A miR-430 target site within the nanos1 3' UTR reduces poly(A) tail length, mRNA stability, and translation. Repression is disrupted in maternal-zygotic dicer mutants (MZdicer), which lack mature miRNAs , and is restored by injection of processed miR-430. Although miR-430 represses other genes equally in germline and soma, specific regions in the nanos1 3' UTR compensate for microRNA-mediated repression in PGCs and allow germline-specific expression. We show that the 3' UTR of an additional PGC-specific gene, TDRD7, is also targeted by miR-430. These results indicate that miR-430 targets the 3' UTRs of germline genes and suggest that differential susceptibility to microRNAs contributes to tissue-specific gene expression.

Birth and life of tissue macrophages and their migration in embryogenesis and inflammation in medaka

Macrophages detecting and migrating toward sites of injury and infection represent one of the first steps in an immune response. Here we directly image macrophage birth and migration in vivo in transgenic medaka fish. Macrophages are born as frequently dividing, immotile cells with spherical morphology that differentiate into flat, highly motile cells. They retain mitotic activity while spreading over the entire body. Cells follow restricted paths not only in directed migration, but also during patrolling. Along those paths the macrophages rapidly patrol the tissue and respond to wounding and bacterial infection from long distances. Upon injury they increase their speed and migratory persistence. Specifically targeting PI3-kinase isoforms efficiently blocks the wounding response and results in a distinct inhibition of cell motility and chemotaxis. Our study provides in situ insights into the properties of immature and migratory macrophages and presents a unique model to further test modulating compounds in vivo.

In vivo migration: a germ cell perspective

The basic concepts of the molecular machinery that mediates cell migration have been gleaned from cell culture systems. However, the three-dimensional environment within an organism presents migrating cells with a much greater challenge. They must move between and among other cells while interpreting multiple attractive and repulsive cues to choose their proper path. They must coordinate their cell adhesion with their surroundings and know when to start and stop moving. New insights into the control of these remaining mysteries have emerged from genetic dissection and live imaging of germ cell migration in Drosophila, zebrafish, and mouse embryos. In this review, we first describe germ cell migration in cellular and mechanistic detail in these different model systems. We then compare these systems to highlight the emerging principles. Finally, we contrast the migration of germ cells with that of immune and cancer cells to outline the conserved and different mechanisms.

Modern mosaic analysis in the zebrafish

One of the most powerful tools used to gain insight into complex developmental processes is the analysis of mosaic embryos. A mosaic is defined as an organism that contains cells of more than one genotype, usually wild-type and mutant. It is the interplay between wild-type and mutant cells in the mosaic that reveals information about the normal function of the mutated gene. Mosaic analysis has been utilized extensively in Caenorhabditis elegans, Drosophila, mice, and zebrafish to elucidate when, where, and how a gene acts during development. In the zebrafish, mosaic analysis has been used to dissect a number of different developmental processes, including gastrulation movements, mesoderm and endoderm specification, neuronal patterning and migration, axon pathfinding, angiogenesis, and cardiac, retinal, and neural crest development. Mosaic analysis is a particularly effective method for understanding gene function in the zebrafish, a model organism particularly suited to forward genetic, molecular, and classical embryological approaches. These attributes, when combined with the accessibility and optical clarity of the zebrafish embryo, facilitate the real time observation of individual cell behaviors and interactions within mosaic embryos.

Migration of Zebrafish Primordial Germ Cells: A Role for Myosin Contraction and Cytoplasmic Flow

The molecular and cellular mechanisms governing cell motility and directed migration in response to the chemokine SDF-1 are largely unknown. Here, we demonstrate that zebrafish primordial germ cells whose migration is guided by SDF-1 generate bleb-like protrusions that are powered by cytoplasmic flow. Protrusions are formed at sites of higher levels of free calcium where activation of myosin contraction occurs. Separation of the acto-myosin cortex from the plasma membrane at these sites is followed by a flow of cytoplasm into the forming bleb. We propose that polarized activation of the receptor CXCR4 leads to a rise in free calcium that in turn activates myosin contraction in the part of the cell responding to higher levels of the ligand SDF-1. The biased formation of new protrusions in a particular region of the cell in response to SDF-1 defines the leading edge and the direction of cell migration.

Attraction rules: germ cell migration in zebrafish

The migration of zebrafish primordial germ cell towards the region where the gonad develops is guided by the chemokine SDF-1a. Recent studies show that soon after their specification, the cells undergo a series of morphological alterations before they become motile and are able to respond to attractive cues. As migratory cells, primordial germ cells move towards their target while correcting their path upon exiting a cyclic phase in which morphological cell polarity is lost. In the following stages, the cells gather at specific locations and move as cell clusters towards their final target. In all of these stages, zebrafish germ cells respond as individual cells to alterations in the shape of the sdf-1a expression domain, by directed migration towards their target - the position where the gonad develops.