Guidance of cell migration by EGF receptor signaling during Drosophila oogenesis

Border cell migration: a model system for live imaging and genetic analysis of collective cell movement

Border cell migration in the Drosophila ovary has emerged as a genetically tractable model for studying collective cell movement. Over many years border cell migration was exclusively studied in fixed samples due to the inability to culture stage 9 egg chambers in vitro. Although culturing late stage egg chambers was long feasible, stage 9 egg chambers survived only briefly outside the female body. We identified culture conditions that support stage 9 egg chamber development and sustain complete migration of border cells ex vivo. This protocol enables one to compare the dynamics of egg chamber development in wild type and mutant egg chambers using time-lapse microscopy and taking advantage of a multiposition microscope with a motorized imaging stage. In addition, this protocol has been successfully used in combination with fluorescence resonance energy transfer biosensors, photo-activatable proteins, and pharmacological agents and can be used with widefield or confocal microscopes in either an upright or inverted configuration.

Modeling cancers in Drosophila

The basic cellular processes deregulated during carcinogenesis and the vast majority of the genes implicated in cancer appear conserved from humans to flies. This conservation, together with an ever-expanding fly genetic toolbox, has made of Drosophila melanogaster a remarkably profitable model to study many fundamental aspects of carcinogenesis. In particular, Drosophila has played a major role in the identification of genes and pathways implicated in cancer and in disclosing novel functional relationships between cancer genes. It has also proved to be a genetically tractable system where to mimic cancer-like situations and characterize the mode of action of human oncogenes. Here, we outline some advances in the study of cancer, both at the basic and more translational levels, which have benefited from research carried out in flies.

Detection of RTK pathway activation in Drosophila using anti-dpERK immunofluorescence staining

In Drosophila, like in other metazoans, receptor tyrosine kinase (RTK) signaling pathways control diverse cellular processes such as migration, growth, fate determination, and differentiation (Shilo, Development 132:4017-4027, 2005). Activation of RTKs by their extracellular ligands triggers a signal transduction cascade, mediated by the Ras/Raf/MEK cassette, which ultimately leads to dual phosphorylation and activation of the mitogen-activated protein kinase/extracellularly regulated kinase (MAPK/Erk). Once active, MAPK/Erk phosphorylates its cytoplasmic and nuclear substrates, consequently modulating (i.e., stimulating or inhibiting) their biological function (Murphy and Blenis, Trends in Biochemical Sciences 31:268-275, 2006). The currently available antibody specific for the doubly phosphorylated form of MAPK/Erk (dpERK) (Yung et al., FEBS Letters 408:292-296, 1997) provides a valuable readout for RTK signaling: it enables the spatiotemporal detection of RTK pathway activity in the developing organism, in situ (Gabay et al., Development 124:3535-3541, 1997; Gabay et al., Science 277:1103-1106, 1997). Here, we present a detailed protocol for anti-dpERK immunofluorescent staining that can be applied to the analysis of MAPK/Erk signaling in Drosophila embryogenesis.

The Ste20 kinase misshapen is essential for the invasive behaviour of ovarian epithelial cells in Drosophila

Stationary-to-migratory transitions of epithelial cells have a key role in development and tumour progression. Border cell migration is a powerful system in which to investigate this transition in living organisms. Here, we identify the Ste20-like kinase misshapen (msn) as a novel regulator of border-cell migration in Drosophila. Expression of msn in border cells is independent of the transcription factor slow border cells and of inputs from all pathways that are known to control border-cell migration. The msn gene functions to modulate the levels and/or distribution of Drosophila E-cadherin to promote the invasive migratory behaviour of border cells.

Rac1-dependent collective cell migration is required for specification of the anterior-posterior body axis of the mouse

Live imaging and analysis of conditional mutants show that the embryonic organizer that determines the anterior-posterior axis in the mouse embryo moves by Rac1-dependent collective cell migration.

Cell migration and cell rearrangements are critical for establishment of the body plan of vertebrate embryos. The first step in organization of the body plan of the mouse embryo, specification of the anterior-posterior body axis, depends on migration of the anterior visceral endoderm from the distal tip of the embryo to a more proximal region overlying the future head. The anterior visceral endoderm (AVE) is a cluster of extra-embryonic cells that secretes inhibitors of the Wnt and Nodal pathways to inhibit posterior development. Because Rac proteins are crucial regulators of cell migration and mouse Rac1 mutants die early in development, we tested whether Rac1 plays a role in AVE migration. Here we show that Rac1 mutant embryos fail to specify an anterior-posterior axis and, instead, express posterior markers in a ring around the embryonic circumference. Cells that express the molecular markers of the AVE are properly specified in Rac1 mutants but remain at the distal tip of the embryo at the time when migration should take place. Using tissue specific deletions, we show that Rac1 acts autonomously within the visceral endoderm to promote cell migration. High-resolution imaging shows that the leading wild-type AVE cells extend long lamellar protrusions that span several cell diameters and are polarized in the direction of cell movement. These projections are tipped by filopodia-like structures that appear to sample the environment. Wild-type AVE cells display hallmarks of collective cell migration: they retain tight and adherens junctions as they migrate and exchange neighbors within the plane of the visceral endoderm epithelium. Analysis of mutant embryos shows that Rac1 is not required for intercellular signaling, survival, proliferation, or adhesion in the visceral endoderm but is necessary for the ability of visceral endoderm cells to extend projections, change shape, and exchange neighbors. The data show that Rac1-mediated epithelial migration of the AVE is a crucial step in the establishment of the mammalian body plan and suggest that Rac1 is essential for collective migration in mammalian tissues.

The specification of the anterior-posterior body axis of the mouse embryo depends on migration of the anterior visceral endoderm (AVE) to a position that overlies the future head. By high-resolution imaging of intact embryos we show that movement of the AVE is a form of collective cell migration, as the migrating cells retain tight and adherens junctions while they migrate and exchange neighbors within the plane of the visceral endoderm epithelium. Using conditional knockouts, we find that the small GTPase Rac1 is absolutely required for specification of the anterior-posterior axis and acts cell-autonomously within the AVE to allow cells to extend long, dynamic lamellar projections that are required for movement. Rac1-mediated epithelial migration of the AVE is a crucial step in the establishment of the mammalian body plan, and Rac1 may be important for collective migration in general in mammalian tissues, including invading tumor cells.

Cell migration: MIM takes the driver's seat

A recent study reports a novel and conserved function for the I-BAR protein MIM in guiding cell migration: MIM has an anti-endocytic activity that moderates intracellular signalling of guidance cues by sequestration of cortactin.

Hypoxia response pathway in border cell migration

Cell invasion and metastasis mark the most lethal phase of cancer, but little is known about the key molecular events that initiate this crucial turning point. Low oxygen, or hypoxia, is thought to be one trigger for metastasis. Hypoxic conditions within the tumor mass are thought to activate signaling pathways that stimulate invasiveness of cancer cells spreading the disease. However, the molecular basis of this process is not well understood. A recent study used Drosophila ovarian border cell migration to model the type of cell migration that occurs in tumors in response to oxygen deprivation through the activation of the hypoxia response pathway (Doronkin et al. Oncogene. 2009). This model organism approach revealed a highly sophisticated mechanism of control of cell migration that is regulated by multiple genetic inputs tied to the hypoxic response. Genetic manipulations with the components of the HIF-1 (hypoxia-inducible factor 1) pathway were able to either inhibit or block the migration of border cells or cause unprecedented acceleration of their migration. The HIF-1-mediated transcriptional cascade appears to be the major regulator of border cell locomotion. Based on the similarity of the fly and human HIF-1 pathways, this model organism study might lead to improvements in understanding hypoxia-induced metastasizing of human cancers. This article discusses new findings in the context of their relevance to cancer metastasis and speculates on the potential regulatory mechanisms and future research directions.

Light-mediated activation reveals a key role for Rac in collective guidance of cell movement in vivo

The small GTPase Rac induces actin polymerization, membrane ruffling and focal contact formation in cultured single cells but can either repress or stimulate motility in epithelial cells depending on the conditions. The role of Rac in collective epithelial cell movements in vivo, which are important for both morphogenesis and metastasis, is therefore difficult to predict. Recently, photoactivatable analogues of Rac (PA-Rac) have been developed, allowing rapid and reversible activation or inactivation of Rac using light. In cultured single cells, light-activated Rac leads to focal membrane ruffling, protrusion and migration. Here we show that focal activation of Rac is also sufficient to polarize an entire group of cells in vivo, specifically the border cells of the Drosophila ovary. Moreover, activation or inactivation of Rac in one cell of the cluster caused a dramatic response in the other cells, suggesting that the cells sense direction as a group according to relative levels of Rac activity. Communication between cells of the cluster required Jun amino-terminal kinase (JNK) but not guidance receptor signalling. These studies further show that photoactivatable proteins are effective tools in vivo.

Direct detection of guidance receptor activity during border cell migration

Guidance receptor signaling is crucial for steering migrating cells. Despite this, we generally lack direct measurements of such signaling. Border cells in Drosophila migrate as a tightly associated group, but dynamically, with front and rear cells exchanging places. They use the receptor tyrosine kinase (RTK) PDGF/VEGF receptor (PVR) as a guidance receptor, perceiving the attractant Pvf1. Here we determine the spatial distribution of PVR signaling by generating an antibody that specifically detects activated PVR in situ. PVR activity is very low in migrating border cells, due to strong activity of cellular phosphatases. Measurements of signal at the cell cortex show variability but a strong bias for both total active PVR and specific activity of PVR to be elevated at the front versus side of the leading cell, often with several-fold difference in signal levels. This polarized active PVR signal requires the E3 ubiquitin ligase Cbl and the recycling regulator Rab11, indicating a dependency on receptor trafficking. The endogenous ligand gradient contributes to shaping of signaling by increasing the specific activity of PVR toward the source in front cells. Surprisingly, signaling is also elevated at the back versus the side of rear cells. This distally polarized distribution of active PVR is ligand independent. Thus the actual guidance signal transmitted in border cells appears to integrate perceived ligand distribution with cell polarity or cell orientation with respect to the cluster. A general implication is that both group configuration and extrinsic cues can directly modulate guidance receptor signaling during collective cell migration.

Morphogenesis in Kyoto: a confluence of cell and developmental biology

Understanding morphogenesis is the ultimate multidisciplinary (ad)venture. Three-dimensional tissues are generated from the actions of genes, biochemical pathways, and cells that form multicellular networks and interact with their biomechanical environment. A comprehensive explanation of morphogenetic processes must encompass these different levels of analysis. A recent meeting in Kyoto on “Building the Body Plan: How Cell Adhesion, Signaling, and Cytoskeletal Regulation Shape Morphogenesis” highlighted recent advances in tackling this challenging problem.

Increased accuracy of ligand sensing by receptor internalization

Many types of cells can sense external ligand concentrations with cell-surface receptors at extremely high accuracy. Interestingly, ligand-bound receptors are often internalized, a process also known as receptor-mediated endocytosis. While internalization is involved in a vast number of important functions for the life of a cell, it was recently also suggested to increase the accuracy of sensing ligand as the overcounting of the same ligand molecules is reduced. Here we show, by extending simple ligand-receptor models to out-of-equilibrium thermodynamics, that internalization increases the accuracy with which cells can measure ligand concentrations in the external environment. Comparison with experimental rates of real receptors demonstrates that our model has indeed biological significance.

Collective cell migration

For all animals, cell migration is an essential and highly regulated process. Cells migrate to shape tissues, to vascularize tissues, in wound healing, and as part of the immune response. Unfortunately, tumor cells can also become migratory and invade surrounding tissues. Some cells migrate as individuals, but many cell types will, under physiological conditions, migrate collectively in tightly or loosely associated groups. This includes invasive tumor cells. This review discusses different types of collective cell migration, including sheet movement, sprouting and branching, streams, and free groups, and highlights recent findings that provide insight into cells' organization and behavior. Cells performing collective migration share many cell biological characteristics with independently migrating cells but, by affecting one another mechanically and via signaling, these cell groups are subject to additional regulation and constraints. New properties that emerge from this connectivity can contribute to shaping, guiding, and ultimately ensuring tissue function.

Collective cell migration in development

Collective cell migration is a key process during the development of most organisms. It can involve either the migration of closely packed mesenchymal cells that make dynamic contacts with frequently changing neighbour cells, or the migration of epithelial sheets that typically display more stable cell-cell interactions and less frequent changes in neighbours. These collective movements can be controlled by short- or long-range dynamic gradients of extracellular signalling molecules, depending on the number of cells involved and their distance of migration. These gradients are sensed by some or all of the migrating cells and translated into directed migration, which in many settings is further modulated by cell-contact-mediated attractive or repulsive interactions that result in contact-following or contact-inhibition of locomotion, respectively. Studies of collective migration of groups of epithelial cells during development indicate that, in some cases, only leader cells sense and migrate up an external signal gradient, and that adjacent cells follow through strong cell-cell contacts. In this Commentary, I review studies of collective cell migration of differently sized cell populations during the development of several model organisms, and discuss our current understanding of the molecular mechanisms that coordinate this migration.

Adult mouse subventricular zone stem and progenitor cells are sessile and epidermal growth factor receptor negatively regulates neuroblast migration

Background

The adult subventricular zone (SVZ) contains stem and progenitor cells that generate neuroblasts throughout life. Although it is well accepted that SVZ neuroblasts are migratory, recent evidence suggests their progenitor cells may also exhibit motility. Since stem and progenitor cells are proliferative and multipotential, if they were also able to move would have important implications for SVZ neurogenesis and its potential for repair.

Methodology/Principal Findings

We studied whether SVZ stem and/or progenitor cells are motile in transgenic GFP+ slices with two photon time lapse microscopy and post hoc immunohistochemistry. We found that stem and progenitor cells; mGFAP-GFP+ cells, bright nestin-GFP+ cells and Mash1+ cells were stationary in the SVZ and rostral migratory stream (RMS). In our search for motile progenitor cells, we uncovered a population of motile βIII-tubulin+ neuroblasts that expressed low levels of epidermal growth factor receptor (EGFr). This was intriguing since EGFr drives proliferation in the SVZ and affects migration in other systems. Thus we examined the potential role of EGFr in modulating SVZ migration. Interestingly, EGFr^low neuroblasts moved slower and in more tortuous patterns than EGFr-negative neuroblasts. We next questioned whether EGFr stimulation affects SVZ cell migration by imaging Gad65-GFP+ neuroblasts in the presence of transforming growth factor alpha (TGF-α), an EGFr-selective agonist. Indeed, acute exposure to TGF-α decreased the percentage of motile cells by approximately 40%.

Conclusions/Significance

In summary, the present study directly shows that SVZ stem and progenitor cells are static, that EGFr is retained on some neuroblasts, and that EGFr stimulation negatively regulates migration. This result suggests an additional role for EGFr signaling in the SVZ.

Self-organization of engineered epithelial tubules by differential cellular motility

Patterning of developing tissues arises from a number of mechanisms, including cell shape change, cell proliferation, and cell sorting from differential cohesion or tension. Here, we reveal that differences in cell motility can also lead to cell sorting within tissues. Using mosaic engineered mammary epithelial tubules, we found that cells sorted depending on their expression level of the membrane-anchored collagenase matrix metalloproteinase (MMP)-14. These rearrangements were independent of the catalytic activity of MMP14 but absolutely required the hemopexin domain. We describe a signaling cascade downstream of MMP14 through Rho kinase that allows cells to sort within the model tissues. Cell speed and persistence time were enhanced by MMP14 expression, but only the latter motility parameter was required for sorting. These results indicate that differential directional persistence can give rise to patterns within model developing tissues.

Origin and function of fluctuations in cell behaviour and the emergence of patterns

Morphogenesis is the process whereby cells assemble into tissues and organs. Recent studies of this process have revealed heterogeneity of individual cell behaviours that contrasts with the deterministic activity of tissues as a whole. Here we review these observations and suggest that fluctuations and heterogeneities are a central substrate for morphogenesis and that there might exist mechanisms dedicated to the averaging of these fluctuations to ensure robust and reproducible behaviours at the tissue level.

Tube formation in Drosophila egg chambers

The reorganization of epithelial sheets into tubes is a fundamental process in the formation of many organs, such as the lungs, kidneys, gut, and neural tube. This process involves the patterning of distinct cell types and the coordination of those cells during the shape changes and rearrangements that produce the tube. A better understanding of the cellular and genetic mechanisms that regulate tube formation is necessary for tissue engineers to develop functional organs in vitro. The Drosophila egg chamber has emerged as an outstanding model for studying tubulogenesis. Synthesis of the dorsal respiratory appendages by the follicular epithelium resembles primary neurulation in vertebrates. This review summarizes work on the patterning and morphogenesis of the dorsal-appendage tubes and highlights key areas where mathematical modeling could contribute to our understanding of these processes.

Proliferating mesodermal cells in murine embryos exhibiting macrophage and lymphendothelial characteristics

BACKGROUND

The data on the embryonic origin of lymphatic endothelial cells (LECs) from either deep embryonic veins or mesenchymal (or circulating) lymphangioblasts presently available remain inconsistent. In various vertebrates, markers for LECs are first expressed in specific segments of embryonic veins arguing for a venous origin of lymph vessels. Very recently, studies on the mouse have strongly supported this view. However, in the chick, we have observed a dual origin of LECs from veins and from mesodermal lymphangioblasts. Additionally, in murine embryos we have detected mesenchymal cells that co-express LEC markers and the pan-leukocyte marker CD45. Here, we have characterized the mesoderm of murine embryos with LEC markers Prox1, Lyve-1 and LA102 in combination with macrophage markers CD11b and F4/80.

RESULTS

We observed cells co-expressing both types of markers (e.g. Prox1 - Lyve-1 - F4/80 triple-positive) located in the mesoderm, immediately adjacent to, and within lymph vessels. Our proliferation studies with Ki-67 antibodies showed high proliferative capacities of both the Lyve-1-positive LECs of lymph sacs/lymphatic sprouts and the Lyve-1-positive mesenchymal cells.

CONCLUSION

Our data argue for a dual origin of LECs in the mouse, although the primary source of embryonic LECs may reside in specific embryonic veins and mesenchymal lymphangioblasts integrated secondarily into lymph vessels. The impact of a dual source of LECs for ontogenetic, phylogenetic and pathological lymphangiogenesis is discussed.

Awd, the homolog of metastasis suppressor gene Nm23, regulates Drosophila epithelial cell invasion

Border cell migration during Drosophila melanogaster oogenesis is a highly pliable model for studying epithelial to mesenchymal transition and directional cell migration. The process involves delamination of a group of 6 to 10 follicle cells from the epithelium followed by guided migration and invasion through the nurse cell complex toward the oocyte. The guidance cue is mainly provided by the homolog of platelet-derived growth factor/vascular endothelial growth factor family of growth factor, or Pvf, emanating from the oocyte, although Drosophila epidermal growth factor receptor signaling also plays an auxiliary role. Earlier studies implicated a stringent control of the strength of Pvf-mediated signaling since both down-regulation of Pvf and overexpression of active Pvf receptor (Pvr) resulted in stalled border cell migration. Here we show that the metastasis suppressor gene homolog Nm23/awd is a negative regulator of border cell migration. Its down-regulation allows for optimal spatial signaling from two crucial pathways, Pvr and JAK/STAT. Its overexpression in the border cells results in stalled migration and can revert the phenotype of overexpressing constitutive Pvr or dominant-negative dynamin. This is a rare example demonstrating the relevance of a metastasis suppressor gene function utilized in a developmental process involving cell invasion.

Constitutive EGFR signaling in oligodendrocyte progenitors leads to diffuse hyperplasia in postnatal white matter

Gliogenesis requires the careful orchestration of migration, differentiation, and proliferation of progenitors. Signaling through the epidermal growth factor receptor (EGFR) has been implicated in regulating these processes in a variety of cell types, including neural progenitors. By retroviral infection, we constitutively expressed an EGFR-GFP fusion protein in white matter glial progenitors at postnatal day 3 of the rat forebrain in vivo and analyzed the development of these cells over the subsequent 15 weeks. EGFR-GFP+ cells remained proliferative and migratory, gradually populating the brains ipsilateral and contralateral to the side of viral infection, but never differentiated into mature glia. The accumulation of these cells doubled the total cell density in white matter and led to a 10-fold increase in the abundance of glial progenitors, giving rise to a progenitor "hyperplasia." The marker profile of infected cells, NG2+, olig2+, PDGFR-alpha+, nestin+, GFAP-, and CC1-, indicated a close resemblance to oligodendrocyte progenitors. Positive immunostaining for phosphorylated EGFR colocalized with punctate accumulation of EGFR-GFP, indicating that a subset of receptors was engaged in active signaling. Furthermore, EGF was required to observe phospho-tyrosine EGFR immunostaining of glial progenitors in culture. These observations suggest that constitutive EGFR expression can inhibit glial differentiation, but requires ligand as well.

A protocol for culturing Drosophila melanogaster stage 9 egg chambers for live imaging

This protocol describes a method for the dissection of egg chambers from intact Drosophila females and culture conditions that permit live imaging of them, with a particular emphasis on stage 9. This stage of development is characterized by oocyte growth and patterning, outer follicle cell rearrangement and migration of border cells. Although in vitro culture of egg chambers of later developmental stages has long been possible, until recently stage 9 egg chambers could only be kept alive for short periods, did not develop normally, and border cell migration failed entirely. We have established culture conditions that support overall egg chamber development including border cell migration in vitro. This protocol makes possible direct observation of molecular and cellular dynamics in both wild-type and mutant egg chambers, and opens the door to testing of pharmacological inhibitors and the use of biosensors. The entire protocol takes approximately 24 h while the preparation of egg chambers for live imaging requires only 15-20 min.

Drosophila follicle cells: morphogenesis in an eggshell

Epithelial morphogenesis is important for organogenesis and pivotal for carcinogenesis, but mechanisms that control it are poorly understood. The Drosophila follicular epithelium is a genetically tractable model to understand these mechanisms in vivo. This epithelium of follicle cells encases germline cells to create an egg. In this review, we summarize progress toward understanding mechanisms that maintain the epithelium or permit migrations essential for oogenesis. Cell-cell communication is important, but the same signals are used repeatedly to control distinct events. Understanding intrinsic mechanisms that alter responses to developmental signals will be important to understand regulation of cell shape and organization.

Tumbling, an interactive way to move forward

The migration of Drosophila border cells has become a powerful model with which to genetically identify guidance cues that control the directed migration of a group of interconnected cells. During oogenesis, border cells delaminate from an epithelial layer and move collectively toward the oocyte. In vivo observation has been added to the impressive experimental toolkit available to study border cell migration. These studies reveal two previously unknown migratory behaviors: one in which cells within the border cell cluster constantly change their position, and another called "tumbling," by which the entire border cell cluster rotates forward. Unexpectedly, the same receptor tyrosine kinases control these different modes of migration through separate downstream pathways. An early mode is mediated by the actin regulatory proteins ELMO and Mbc and resembles cellular polarization during individual cell migration; whereas during a later phase, communication between cells, facilitated by mitogen-activated protein kinase and phospholipase C-gamma, organizes the polarity of the entire cluster.

Collective guidance of collective cell migration

Some cells migrate and find their way as solitary entities. However, during development of multicellular animals and possibly during tumor dissemination, cells often move as groups, associated tightly or loosely. Recent advances in live imaging have aided examination of such 'multicellular cell biology'. Here, I propose a model for how a group of cells can process and react to guidance information as a unit rather than as a gathering of solitary cells. Signaling pathways and regulatory mechanisms can differ substantially between solitary- and collective-guidance modes; a major difference being that, in collective guidance, similar to in bacterial chemotaxis, the signal need not be localized subcellularly within the responding cell. I suggest that collective-guidance signaling occurs alongside individual cell reactions. Both produce directional migration.

The Par-Tiam1 complex controls persistent migration by stabilizing microtubule-dependent front-rear polarity

BACKGROUND

The establishment and maintenance of cell polarity is crucial for many biological functions and is regulated by conserved protein complexes. The Par polarity complex consisting of Par3, Par6, and PKCzeta, in conjunction with Tiam1-mediated Rac signaling, controls apical-basal cell polarity in contacting epithelial cells. Here we tested the hypothesis that the Par complex, in conjunction with Tiam1, controls "front-rear" polarity during the persistent migration of freely migrating keratinocytes.

RESULTS

Wild-type (WT) epidermal keratinocytes lacking cell-cell contacts are stably front-rear polarized and migrate persistently. In contrast, Tiam1-deficient (Tiam1 KO) and (si)Par3-depleted keratinocytes are generally unpolarized and migrate randomly because front-rear polarity is short lived. Immunoprecipitation experiments show that in migrating keratinocytes, Tiam1 associates with Par3 and PKCzeta. Moreover, Par3, PKCzeta, and Tiam1 proteins are enriched at the leading edges of polarized keratinocytes. Tiam1 KO keratinocytes are impaired in chemotactic migration toward growth factors, whereaes haptotactic migration is similar to WT. Par3 depletion or the blocking of PKCzeta signaling in WT keratinocytes impairs chemotaxis but has no additional effect on Tiam1 KO cells. The migratory and morphological defects in keratinocytes with impaired Par-Tiam1 function closely resemble cells with pharmacologically destabilized microtubules (MTs). Indeed, MTs in Tiam1 KO keratinocytes and WT cells treated with a PKCzeta inhibitor are unstable, thereby negatively influencing directional but not random migration.

CONCLUSIONS

We conclude that the Par-Tiam1 complex stabilizes front-rear polarization of noncontacting migratory cells, thereby stimulating persistent and chemotactic migration, whereas in contacting keratinocytes, the same complex controls the establishment of long-lasting apical-basal polarity. These findings underscore a remarkable flexibility of the Par polarity complex that, depending on the biological context, controls distinct forms of cellular polarity.

Modeling migration and metastasis in Drosophila

Cell motility makes essential contributions to normal embryonic development and homeostasis. It is also thought to contribute in important ways to tumor metastasis. Because of this dual importance, cell migration has been extensively studied. The fruit fly Drosophila melanogaster has served as an important model organism for genetic analysis of many aspects of developmental biology, including cell migration. Here we describe the various types of cell movements that have been studied in detail, which represent models for epithelial-to-mesenchymal transition, transepithelial migration, inflammation, wound healing and invasion. We summarize what has been learned about the molecular control of cell migration from genetic studies in the fly. In addition, we describe recent efforts to model tumor metastasis directly in Drosophila by expressing oncogenes and/or mutating tumor suppressor genes. Together these studies suggest that Drosophila has much to offer as a model for varied aspects of tumor metastasis.

Two distinct modes of guidance signalling during collective migration of border cells

Although directed migration is a feature of both individual cells and cell groups, guided migration has been studied most extensively for single cells in simple environments. Collective guidance of cell groups remains poorly understood, despite its relevance for development and metastasis. Neural crest cells and neuronal precursors migrate as loosely organized streams of individual cells, whereas cells of the fish lateral line, Drosophila tracheal tubes and border-cell clusters migrate as more coherent groups. Here we use Drosophila border cells to examine how collective guidance is performed. We report that border cells migrate in two phases using distinct mechanisms. Genetic analysis combined with live imaging shows that polarized cell behaviour is critical for the initial phase of migration, whereas dynamic collective behaviour dominates later. PDGF- and VEGF-related receptor and epidermal growth factor receptor act in both phases, but use different effector pathways in each. The myoblast city (Mbc, also known as DOCK180) and engulfment and cell motility (ELMO, also known as Ced-12) pathway is required for the early phase, in which guidance depends on subcellular localization of signalling within a leading cell. During the later phase, mitogen-activated protein kinase and phospholipase Cgamma are used redundantly, and we find that the cluster makes use of the difference in signal levels between cells to guide migration. Thus, information processing at the multicellular level is used to guide collective behaviour of a cell group.

The planar polarity pathway promotes coordinated cell migration during Drosophila oogenesis

Cell migration is fundamental in both animal morphogenesis and disease. The migration of individual cells is relatively well-studied; however, in vivo, cells often remain joined by cell-cell junctions and migrate in cohesive groups. How such groups of cells coordinate their migration is poorly understood. The planar polarity pathway coordinates the polarity of non-migrating cells in epithelial sheets and is required for cell rearrangements during vertebrate morphogenesis. It is therefore a good candidate to play a role in the collective migration of groups of cells. Drosophila border cell migration is a well-characterised and genetically tractable model of collective cell migration, during which a group of about six to ten epithelial cells detaches from the anterior end of the developing egg chamber and migrates invasively towards the oocyte. We find that the planar polarity pathway promotes this invasive migration, acting both in the migrating cells themselves and in the non-migratory polar follicle cells that they carry along. Disruption of planar polarity signalling causes abnormalities in actin-rich processes on the cell surface and leads to less-efficient migration. This is apparently due, in part, to a loss of regulation of Rho GTPase activity by the planar polarity receptor Frizzled, which itself becomes localised to the migratory edge of the border cells. We conclude that, during collective cell migration, the planar polarity pathway can mediate communication between motile and non-motile cells, which enhances the efficiency of migration via the modulation of actin dynamics.

Cellular and Molecular Mechanisms of Border Cell Migration Analyzed Using Time-Lapse Live-Cell Imaging

Border cells in the Drosophila ovary originate within an epithelium, detach from it, invade neighboring nurse cells, and migrate as a coherent cluster. This migration has served as a useful genetic model for understanding epithelial cell motility. The prevailing model of growth factor-mediated chemotaxis in general, and of border cells in particular, posits that receptor activation promotes cellular protrusion at the leading edge. Here we report the time-lapse video imaging of border cell migration, allowing us to test this model. Reducing the activities of the guidance receptors EGFR and PVR did not result in the expected inhibition of protrusion, but instead resulted in protrusion in all directions. In contrast, reduction in Notch activity resulted in failure of the cells to detach from the epithelium without affecting direction sensing. These observations provide new insight into the cellular dynamics and molecular mechanisms of cell migration in vivo.

Localized VEGF signaling from ectoderm to mesenchyme cells controls morphogenesis of the sea urchin embryo skeleton

During development, cell migration plays an important role in morphogenetic processes. The construction of the skeleton of the sea urchin embryo by a small number of cells, the primary mesenchyme cells (PMCs), offers a remarkable model to study cell migration and its involvement in morphogenesis. During gastrulation, PMCs migrate and become positioned along the ectodermal wall following a stereotypical pattern that determines skeleton morphology. Previous studies have shown that interactions between ectoderm and PMCs regulate several aspects of skeletal morphogenesis, but little is known at the molecular level. Here we show that VEGF signaling between ectoderm and PMCs is crucial in this process. The VEGF receptor (VEGFR) is expressed exclusively in PMCs, whereas VEGF expression is restricted to two small areas of the ectoderm, in front of the positions where the ventrolateral PMC clusters that initiate skeletogenesis will form. Overexpression of VEGF leads to skeletal abnormalities, whereas inhibition of VEGF/VEGFR signaling results in incorrect positioning of the PMCs, downregulation of PMC-specific genes and loss of skeleton. We present evidence that localized VEGF acts as both a guidance cue and a differentiation signal, providing a crucial link between the positioning and differentiation of the migrating PMCs and leading to morphogenesis of the embryonic skeleton.

The EGFR ligands Spitz and Keren act cooperatively in the Drosophila eye

The EGFR signalling cascade is responsible for coordinating a wide variety of events during Drosophila eye development. It remains something of a mystery how it is that cells are able to interpret the signal so as to choose the appropriate response from the battery of possibilities: division, differentiation, cell shape change and so on. Since the cascade is essentially linear below the receptor, different cellular responses cannot be regulated by alternative signal transduction pathways. The main diversity lies upstream, in the multiple activating ligands. Spitz, Gurken and Vein have been long studied, but little is known about the physiological functions of the fourth ligand, Keren, although various roles have been predicted based on the differences between mutants in the known ligands and those of the receptor. Here, we have isolated a mutant in the keren gene, and demonstrate that Keren does indeed participate in EGFR signalling in the eye, where it acts redundantly with Spitz to control R8 spacing, cell clustering and survival. Thus, specificity cannot be determined by ligand choice, and must instead be a consequence of cell-intrinsic factors, although we speculate that there may be some quantitative differences in signalling elicited by the two ligands.

A sensitized PiggyBac-based screen for regulators of border cell migration in Drosophila

Migration of border cells during Drosophila melanogaster oogenesis is a good model system for investigating the genetic requirements for cell migration in vivo. We present a sensitized loss-of-function screen used to identify new genes required in border cells for their migration. Chromosomes bearing FRTs on all four major autosomal arms were mutagenized by insertions of the transposable element PiggyBac, allowing multiple parallel clonal screens and easy identification of the mutated gene. For border cells, we analyzed homozygous mutant clones positively marked with lacZ and sensitized by expression of dominant-negative PVR, the guidance receptor. We identified new alleles of genes already known to be required for border cell migration, including aop/yan, DIAP1, and taiman as well as a conserved Slbo-regulated enhancer downstream of shg/DE-cadherin. Mutations in genes not previously described to be required in border cells were also uncovered: hrp48, vir, rme-8, kismet, and puckered. puckered was unique in that the migration defects were observed only when PVR signaling was reduced. We present evidence that an excess of JNK signaling is deleterious for migration in the absence of PVR activity at least in part through Fos transcriptional activity and possibly through antagonistic effects on DIAP1.

Apelin and its receptor control heart field formation during zebrafish gastrulation

The vertebrate heart arises during gastrulation as cardiac precursors converge from the lateral plate mesoderm territories toward the embryonic midline and extend rostrally to form bilateral heart fields. G protein-coupled receptors (GPCRs) mediate functions of the nervous and immune systems; however, their roles in gastrulation remain largely unexplored. Here, we show that the zebrafish homologs of the Agtrl1b receptor and its ligand, Apelin, implicated in physiology and angiogenesis, control heart field formation. Zebrafish gastrulae express agtrl1b in the lateral plate mesoderm, while apelin expression is confined to the midline. Reduced or excess Agtrl1b or Apelin function caused deficiency of cardiac precursors and, subsequently, the heart. In Apelin-deficient gastrulae, the cardiac precursors converged inefficiently to the heart fields and showed ectopic distribution, whereas cardiac precursors overexpressing Apelin exhibited abnormal morphology and rostral migration. Our results implicate GPCR signaling in movements of discrete cell populations that establish organ rudiments during vertebrate gastrulation.

Spatially localized Kuzbanian required for specific activation of Notch during border cell migration

The transmembrane receptor Notch is used repeatedly during development for a variety of essential functions. During Drosophila oogenesis, Notch activity is required first to specify particular follicle cell fates, then to promote the differentiation of all follicle cell types, to promote border cell migration, and then to form dorsal appendages, raising the question as to how Notch activity is spatially and temporally regulated. Here we show the Notch activity pattern during oogenesis. Notch activation was found in many follicle cells at stage 6 but then at stage 9 was restricted to migrating border cells, despite uniform expression of Delta. Expression of Kuzbanian (KUZ), a metalloproteinase that can activate Notch as well as cleave other substrates, is enriched in border cells at stage 9; and dominant-negative KUZ caused a strong border cell migration defect, without affecting expression of markers of border cell fate or follicle cell differentiation. Constitutively active Notch rescued the migration defect due to dominant-negative KUZ, and conditional alleles of Delta and Notch also exhibited border cell migration defects. Expression of two different reporters of Notch activity was lost upon expression of dominant-negative KUZ. Taken together these results show that Notch activation and KUZ expression are restricted to border cells at stage 9 of oogenesis and are required for migration, but not differentiation, of these cells. This represents a previously unrecognized mechanism for achieving spatial restriction of Notch signaling.

Endocytosis, endosome trafficking, and the regulation of Drosophila development

Endocytosis and endosome trafficking regulate cell signaling in unexpected ways. Here we review the contribution that Drosophila research has made to this exciting field. In addition to attenuating signaling, endocytosis shapes morphogen gradients, activates ligands, and regulates spatially receptor activation within a single cell. Moreover, some receptors signal from within endosomes, and the ability of a specific type of endosome to form controls the ability of cells to signal. Experiments in Drosophila reveal that through regulation of a variety of cell signaling pathways, endocytosis controls cell patterning and cell fate.

Cooperative activities of drosophila DE-cadherin and DN-cadherin regulate the cell motility process of ommatidial rotation

Ommatidial rotation is a cell motility read-out of planar cell polarity (PCP) signaling in the Drosophila eye. Although the signaling aspects of PCP establishment are beginning to be unraveled, the mechanistic aspects of the associated ommatidial rotation process remain unknown. Here, we demonstrate that the Drosophila DE- and DN-cadherins have opposing effects on rotation. DE-cadherin promotes rotation, as DE-cad mutant ommatidia rotate less than wild type or not at all. By contrast, the two DN-cadherins act to restrict this movement, with ommatidia rotating too fast in the mutants. The opposing effects of DE- and DN-cadherins result in a coordinated cellular movement, enabling ommatidia of the same stage to rotate simultaneously. Genetic interactions, phenotypic analysis and localization studies indicate that EGF-receptor and Frizzled-PCP signaling feed into the regulation of cadherin activity and localization in this context. Thus, DE- and DN-cadherins integrate inputs from at least two signaling pathways, resulting in a coordinated cell movement. A similar input into mammalian E- and N-cadherins might function in the progression of diseases such as metastatic ovarian cancer.

The social lives of migrating cells in Drosophila

Studies of cell migration in Drosophila are yielding insights into the complex interactions migrating cells have with each other and with the cells in their environment. Intriguing links between factors that promote cell migration and those that control cell survival have been reported recently. For example, migrating germ cells compete with the surrounding somatic tissue for the substrate of the lipid phosphate phosphatases encoded by the genes Wunen and Wunen2. Germ cells take up the dephosphorylated lipid and require it for their survival. In addition, the secreted growth factors called PVFs, previously thought to guide the migrations of hemocytes in the embryo, were found to function instead predominantly as survival factors. And in border cells, DIAP1 and Dronc, two proteins known mainly for their ability to regulate cell death, were found to control cell migration.

Endosome dynamics during development

Endocytosis has traditionally been studied in isolated cells. More recently, however, the analysis of protein trafficking in whole organisms has revealed that it plays exciting roles during development. Endocytic trafficking of cell adhesion molecules regulates epithelial polarity and cell migration. Developmental signaling pathways are regulated by the trafficking of receptors and their ligands through the endocytic pathway. Finally, impairment of the endocytic machinery can affect proliferation control and contribute to tumor development.

The Ret(C620R) mutation affects renal and enteric development in a mouse model of Hirschsprung's disease

In rare families RET tyrosine kinase receptor substitutions located in exon 10 (especially at positions 609, 618, and 620) can concomitantly cause the MEN 2A (multiple endocrine neoplasia type 2A) or FMTC (familial medullary thyroid carcinoma) cancer syndromes, and Hirschsprung's disease (HSCR). No animal model mimicking the co-existence of the MEN 2 pathology and HSCR is available, and the association of these activating mutations with a developmental defect still represents an unresolved problem. The aim of this work was to investigate the significance of the RET(C620R) substitution in the pathogenesis of both gain- and loss-of-function RET-associated diseases. We report the generation of a line of mice carrying the C620R mutation in the Ret gene. Although Ret(C620R) homozygotes display severe defects in kidney organogenesis and enteric nervous system development leading to perinatal lethality. Ret(C620R) heterozygotes recapitulate features characteristic of HSCR including hypoganglionosis of the gastrointestinal tract. Surprisingly, heterozygotes do not show any defects in the thyroid that might be attributable to a gain-of-function mutation. The Ret(C620R) allele is responsible for HSCR and affects the development of kidneys and the enteric nervous system (ENS). These mice represent an interesting model for studying new therapeutic approaches for the treatment of HSCR disease.

Multiple EGFR ligands participate in guiding migrating border cells

Cell migration is an important feature of embryonic development as well as tumor metastasis. Border cells in the Drosophila ovary have emerged as a useful in vivo model for uncovering the molecular mechanisms that control many aspects of cell migration including guidance. It was previously shown that two receptor tyrosine kinases, epidermal growth factor receptor (EGFR) and PDGF- and VEGF-related receptor (PVR), together contribute to border cell migration. Whereas the ligand for PVR, PVF1, is known to guide border cells, it is unclear which of the four activating EGFR ligands function in this process. We developed an assay to detect the ability of secreted factors to reroute migrating border cells in vivo and tested the activity of EGFR ligands compared to PVF1. Two ligands, Keren and Spitz, guided border cells whereas the other ligands, Gurken and Vein, did not. In addition, only Keren and Spitz were expressed at the appropriate stage in the oocyte, the target of border cell migration. Therefore, a complex combination of EGFR and PVR ligands together guide border cells to the oocyte.

Phosphorylation of neurofibromin by PKC is a possible molecular switch in EGF receptor signaling in neural cells

Children with neurofibromatosis (NF1) typically develop central nervous system (CNS) abnormalities, including aberrant proliferation of astrocytes and formation of benign astrocytomas. The NF1 gene encodes neurofibromin, a Ras-GAP, highly expressed in developing neural cells; the mechanism of regulation of neurofibromin as a Ras-GAP, remains however unknown. We now show that, in response to EGF, neurofibromin is in vivo phosphorylated on serine residues by PKC-alpha, in human, rat, and avian CNS cells and cell lines. EGF-induced PKC phosphorylation was prominent in the cysteine/serine-rich domain (CSRD) of neurofibromin, which lies in the N-terminus and upstream of the Ras-GAP domain (GRD), and this modification significantly increased the association of neurofibromin with actin in co-immunoprecipitations. In addition, we show that Ras activation in response to EGF was significantly lowered when C62B cells overexpressed a construct encoding both CSRD + GRD. Moreover, when PKC-alpha was downregulated, the Ras-GAP activity of CSRD + GRD was significantly diminished, whereas overexpressed GRD alone acted as a weaker GAP and in a PKC-independent manner. Most importantly, functional Ras inhibition and EGF signaling shifts were established at the single cell level in C6-derived cell lines stably overexpressing CSRD + GRD, when transient co-overexpression of Ras and PKC-depletion prior to stimulation with EGF-induced mitosis. Taken together, these data provide the first evidence of a functional, allosteric regulation of GRD by CSRD, which requires neurofibromin phosphorylation by PKC and association with the actin cytoskeleton. Our data may suggest a novel mechanism for regulating biological responses to EGF and provide a new aspect for the understanding of the aberrant proliferation seen in the CNS of children with NF1.

Analysis of Cell Migration Using Whole-Genome Expression Profiling of Migratory Cells in the Drosophila Ovary

Cell migration contributes to normal development and homeostasis as well as to pathological processes such as inflammation and tumor metastasis. Previous genetic screens have revealed signaling pathways that govern follicle cell migrations in the Drosophila ovary, but few downstream targets of the critical transcriptional regulators have been identified. To characterize the gene expression profile of two migratory cell populations and identify Slbo targets, we purified border cells and centripetal cells expressing the mouse CD8 antigen and carried out whole-genome microarray analysis. Genes predicted to control actin dynamics and the endocytic and secretory pathways were overrepresented in the migratory cell transcriptome. Mutations in five genes, including ttk, failed to complement previously isolated mutations that cause cell migration defects in mosaic clones. Functional analysis revealed a role for the Notch-activating protease Kuzbanian in border cell migration and identified Tie as a guidance receptor for the border cells.

Regulating the dynamics of EGF receptor signaling in space and time

The epidermal growth factor receptor (EGFR) signaling cascade represents one of the cardinal pathways that transmits information between cells during development in a broad range of multicellular organisms. Most of the elements that constitute the core EGFR signaling module, as well as a variety of negative and positive modulators, have been identified. Although this molecular pathway is utilized multiple times during development, the spatial and temporal features of its signaling can be modified to fit a particular developmental setting. Recent work has unraveled the various mechanisms by which the EGFR pathway can be modulated.

Subventricular zone neuroblasts emigrate toward cortical lesions

Adult subventricular zone (SVZ) neuroblasts migrate in the rostral migratory stream to the olfactory bulbs. Brain lesions generally increase SVZ neurogenesis or gliogenesis and cause SVZ cell emigration to ectopic locations. We showed previously that glia emigrate from the SVZ toward mechanical injuries of the somatosensory cerebral cortex in mice. Here we tested the hypotheses that SVZ neurogenesis increases, that neuroblasts emigrate, and that epidermal growth factor expression increases after cortical injuries. Using immunohistochemistry for phenotypic markers and BrdU, we show that newborn doublecortin-positive SVZ neuroblasts emigrated toward cerebral cortex lesions. However, the number of doublecortin-positive cells in the olfactory bulbs remained constant, suggesting that dorsal emigration was not at the expense of rostral migration. Although newborn neuroblasts emigrated, rates of SVZ neurogenesis did not increase after cortical lesions. Finally, we examined molecules that may regulate emigration and neurogenesis after cortical lesions and found that epidermal growth factor was increased in the SVZ, corpus callosum, and cerebral cortex. These results suggest that after injuries to the cerebral cortex, neuroblasts emigrate from the SVZ, that emigration does not depend either on redirection of SVZ cells or on increased neurogenesis, and that epidermal growth factor may induce SVZ emigration.

Steroid receptor coactivator-3, a homolog of Taiman that controls cell migration in the Drosophila ovary, regulates migration of human ovarian cancer cells

Border cell migration is a process that occurs during Drosophila ovarian development in which cells derived from a simple epithelium migrate and invade neighboring tissue. This process resembles the behavior of cancerous cells that derive from the simple epithelium of the human ovary. One important regulator of border cell migration is Taiman, a homolog of steroid receptor coactivator-3 (SRC-3). Because increasing evidence indicates that similarities exist between the molecular control of migration of border cells and of cancer cells, we investigated whether SRC-3 controls ovarian cancer cell migration. Little or no SRC-3 expression was detected in normal ovarian surface epithelium, ovarian cysts and borderline ovarian tumors that lack stromal invasion. In contrast, SRC-3 was abundantly expressed in high-grade ovarian carcinomas. Inhibiting SRC-3 expression in ovarian cancer cells markedly reduced cell spreading and migration, and altered intracellular localization of focal adhesion kinase. This inhibitory effect on cell migration was independent of the estrogen receptor (ER) status of the cells. These studies reveal a novel role for SRC-3 in ovarian cancer progression by promoting cell migration, independently of its role in estrogen receptor signaling.

Regulators of endocytosis maintain localized receptor tyrosine kinase signaling in guided migration

Guidance receptors detect extracellular cues and instruct migrating cells how to orient in space. Border cells perform a directional invasive migration during Drosophila oogenesis and use two receptor tyrosine kinases (RTKs), EGFR and PVR (PDGF/VEGF Receptor), to read guidance cues. We find that spatial localization of RTK signaling within these migrating cells is actively controlled. Border cells lacking Cbl, an RTK-associated E3 ubiquitin ligase, have delocalized guidance signaling, resulting in severe migration defects. Absence of Sprint, a receptor-recruited, Ras-activated Rab5 guanine exchange factor, gives related defects. In contrast, increasing the level of RTK signaling by receptor overexpression or removing Hrs and thereby decreasing RTK degradation does not perturb migration. Cbl and Sprint both regulate early steps of RTK endocytosis. Thus, a physiological role of RTK endocytosis is to ensure localized intracellular response to guidance cues by stimulating spatial restriction of signaling.

Nerve regeneration and wound healing are stimulated and directed by an endogenous electrical field in vivo

Biological roles for naturally occurring, extracellular physiological electric fields have been proposed over the past century. However, in the molecular era, many biologists presume that electric fields have little physiological relevance because there has been no unequivocal demonstration of their importance at the single-cell level in vivo. We have used an in vivo rat corneal model, which generates its own endogenous electric field and show that nerve sprouting, the direction of nerve growth and the rate of epithelial wound healing are controlled coordinately by the wound-induced electric field.