Paracrine signaling through the JAK/STAT pathway activates invasive behavior of ovarian epithelial cells in Drosophila

The pattern of the follicle cell diversification in ovarian follicles of the true fruit flies, Tephritidae

In flies (Diptera), the ovary displays several distinct patterns of the follicular epithelium formation and diversification. Two main patterns have been identified in the true flies or Brachycera, namely the Rhagio type and the Drosophila type. These patterns align with the traditional division of Brachycera into Orthorrhapha and Cyclorrhapha. However, studies of the follicular epithelium morphogenesis in cyclorrhaphans other than Drosophila are scarce. We characterise the developmental changes associated with the emergence of follicle cell (FC) diversity in two cyclorrhaphans belonging to the family Tephritidae (Brachycera, Cyclorrhapha). Our analysis revealed that the diversification of FCs in these species shows characteristics of both the Rhagio and Drosophila types. First, a distinct cluster of FCs, consisting of polar cells and border-like cells, differentiates at the posterior pole of the ovarian follicle. This feature is unique to the Rhagio type and has only been reported in species representing the Orthorrhapha group. Second, morphological criteria have identified a significantly smaller number of subpopulations of FCs than in Drosophila. Furthermore, while the general pattern of FC migration is similar to that of Drosophila, the distinctive migration of the anterior-dorsal FCs is absent. In the studied tephritids, the migration of the anterior polar cell/border cell cluster towards the anterior pole of the oocyte is followed by the posterior migration of the main body cuboidal FCs to cover the expanding oocyte. Finally, during the onset of vitellogenesis, a distinct subset of FCs migrates towards the centre of the ovarian follicle to cover the oocyte's anterior pole. Our study also highlights specific actions of some FCs that accompany the migration process, which has not been previously documented in cyclorrhaphans. These results support the hypothesis that the posterior and centripetal migrations of morphologically unique FC subsets arose in the common ancestor of Cyclorrhapha. These events appear to have occurred fairly recently in the evolutionary timeline of Diptera.

Premature endocycling of Drosophila follicle cells causes pleiotropic defects in oogenesis

Endocycling cells grow and repeatedly duplicate their genome without dividing. Cells switch from mitotic cycles to endocycles in response to developmental signals during the growth of specific tissues in a wide range of organisms. The purpose of switching to endocycles, however, remains unclear in many tissues. Additionally, cells can switch to endocycles in response to conditional signals, which can have beneficial or pathological effects on tissues. However, the impact of these unscheduled endocycles on development is underexplored. Here, we use Drosophila ovarian somatic follicle cells as a model to examine the impact of unscheduled endocycles on tissue growth and function. Follicle cells normally switch to endocycles at mid-oogenesis. Inducing follicle cells to prematurely switch to endocycles resulted in the lethality of the resulting embryos. Analysis of ovaries with premature follicle cell endocycles revealed aberrant follicular epithelial structure and pleiotropic defects in oocyte growth, developmental gene amplification, and the migration of a special set of follicle cells known as border cells. Overall, these findings reveal how unscheduled endocycles can disrupt tissue growth and function to cause aberrant development.

JAK-STAT-dependent contact between follicle cells and the oocyte controls Drosophila anterior-posterior polarity and germline development

The number of embryonic primordial germ cells in Drosophila is determined by the quantity of germ plasm, whose assembly starts in the posterior region of the oocyte during oogenesis. Here, we report that extending JAK-STAT activity in the posterior somatic follicular epithelium leads to an excess of primordial germ cells in the future embryo. We show that JAK-STAT signaling is necessary for the differentiation of approximately 20 specialized follicle cells maintaining tight contact with the oocyte. These cells define, in the underlying posterior oocyte cortex, the anchoring of the germ cell determinant oskar mRNA. We reveal that the apical surface of these posterior anchoring cells extends long filopodia penetrating the oocyte. We identify two JAK-STAT targets in these cells that are each sufficient to extend the zone of contact with the oocyte, thereby leading to production of extra primordial germ cells. JAK-STAT signaling thus determines a fixed number of posterior anchoring cells required for anterior-posterior oocyte polarity and for the development of the future germline.

Premature endocycling of Drosophila follicle cells causes pleiotropic defects in oogenesis

Endocycling cells grow and repeatedly duplicate their genome without dividing. Cells switch from mitotic cycles to endocycles in response to developmental signals during the growth of specific tissues in a wide range of organisms. The purpose of switching to endocycles, however, remains unclear in many tissues. Additionally, cells can switch to endocycles in response to conditional signals, which can have beneficial or pathological effects on tissues. However, the impact of these unscheduled endocycles on development is underexplored. Here, we use Drosophila ovarian somatic follicle cells as a model to examine the impact of unscheduled endocycles on tissue growth and function. Follicle cells normally switch to endocycles at mid-oogenesis. Inducing follicle cells to prematurely switch to endocycles resulted in lethality of the resulting embryos. Analysis of ovaries with premature follicle cell endocycles revealed aberrant follicular epithelial structure and pleiotropic defects in oocyte growth, developmental gene amplification, and the migration of a special set of follicle cells known as border cells. Overall, these findings reveal how unscheduled endocycles can disrupt tissue growth and function to cause aberrant development.

Hyperactive Rac stimulates cannibalism of living target cells and enhances CAR-M-mediated cancer cell killing

The 21kD GTPase Rac is an evolutionarily ancient regulator of cell shape and behavior. Rac2 is predominantly expressed in hematopoietic cells where it is essential for survival and motility. The hyperactivating mutation Rac2E62K also causes human immunodeficiency, although the mechanism remains unexplained. Here, we report that in Drosophila, hyperactivating Rac stimulates ovarian cells to cannibalize neighboring cells, destroying the tissue. We then show that hyperactive Rac2E62K stimulates human HL60-derived macrophage-like cells to engulf and kill living T cell leukemia cells. Primary mouse Rac2+/E62K bone-marrow-derived macrophages also cannibalize primary Rac2+/E62K T cells due to a combination of macrophage hyperactivity and T cell hypersensitivity to engulfment. Additionally, Rac2+/E62K macrophages non-autonomously stimulate wild-type macrophages to engulf T cells. Rac2E62K also enhances engulfment of target cancer cells by chimeric antigen receptor-expressing macrophages (CAR-M) in a CAR-dependent manner. We propose that Rac-mediated cell cannibalism may contribute to Rac2+/E62K human immunodeficiency and enhance CAR-M cancer immunotherapy.

Transcriptome analysis reveals temporally regulated genetic networks during Drosophila border cell collective migration

BACKGROUND

Collective cell migration underlies many essential processes, including sculpting organs during embryogenesis, wound healing in the adult, and metastasis of cancer cells. At mid-oogenesis, Drosophila border cells undergo collective migration. Border cells round up into a small group at the pre-migration stage, detach from the epithelium and undergo a dynamic and highly regulated migration at the mid-migration stage, and stop at the oocyte, their final destination, at the post-migration stage. While specific genes that promote cell signaling, polarization of the cluster, formation of protrusions, and cell-cell adhesion are known to regulate border cell migration, there may be additional genes that promote these distinct active phases of border cell migration. Therefore, we sought to identify genes whose expression patterns changed during border cell migration.

RESULTS

We performed RNA-sequencing on border cells isolated at pre-, mid-, and post-migration stages. We report that 1,729 transcripts, in nine co-expression gene clusters, are temporally and differentially expressed across the three migration stages. Gene ontology analyses and constructed protein-protein interaction networks identified genes expected to function in collective migration, such as regulators of the cytoskeleton, adhesion, and tissue morphogenesis, but also uncovered a notable enrichment of genes involved in immune signaling, ribosome biogenesis, and stress responses. Finally, we validated the in vivo expression and function of a subset of identified genes in border cells.

CONCLUSIONS

Overall, our results identified differentially and temporally expressed genetic networks that may facilitate the efficient development and migration of border cells. The genes identified here represent a wealth of new candidates to investigate the molecular nature of dynamic collective cell migrations in developing tissues.

Nuclear lamin facilitates collective border cell invasion into confined spaces in vivo

Cells migrate collectively through confined environments during development and cancer metastasis. The nucleus, a stiff organelle, impedes single cells from squeezing into narrow channels within artificial environments. However, how nuclei affect collective migration into compact tissues is unknown. Here, we use border cells in the fly ovary to study nuclear dynamics in collective, confined in vivo migration. Border cells delaminate from the follicular epithelium and squeeze into tiny spaces between cells called nurse cells. The lead cell nucleus transiently deforms within the lead cell protrusion, which then widens. The nuclei of follower cells deform less. Depletion of the Drosophila B-type lamin, Lam, compromises nuclear integrity, hinders expansion of leading protrusions, and impedes border cell movement. In wildtype, cortical myosin II accumulates behind the nucleus and pushes it into the protrusion, whereas in Lam-depleted cells, myosin accumulates but does not move the nucleus. These data suggest that the nucleus stabilizes lead cell protrusions, helping to wedge open spaces between nurse cells.

Single-cell RNA sequencing identifies eggplant as a regulator of germ cell development in Drosophila

Drosophila ovarian germline stem cells (GSCs) are a powerful model for stem cell research. In this study, we use single-cell RNA sequencing (scRNA-seq), an RNAi screen and bioinformatic analysis, to identify genes involved in germ cell differentiation, including 34 genes with upregulated expression during early germ cell development and 19 genes that may regulate germ cell differentiation. Among these, a gene we have named eggplant (eggpl) is highly expressed in GSCs and downregulated in early daughter cells. RNAi knockdown of eggpl causes germ cell proliferation and differentiation defects. In flies fed a rich yeast diet, the expression of eggpl is significantly lower and knockdown or knockout of eggpl phenocopies a rich diet. In addition, eggpl knockdown suppresses the reduction in germ cell proliferation caused by inhibition of the insulin effector PI3K. These findings suggest that downregulation of eggpl links nutritional status to germ cell proliferation and differentiation. Collectively, this study provides new insights into the signaling networks that regulate early germ cell development and identifies eggpl as a key player in this process.

Rap1 promotes epithelial integrity and cell viability in a growing tissue

Having intact epithelial tissues is critical for embryonic development and adult homeostasis. How epithelia respond to damaging insults or tissue growth while still maintaining intercellular connections and barrier integrity during development is poorly understood. The conserved small GTPase Rap1 is critical for establishing cell polarity and regulating cadherin-catenin cell junctions. Here, we identified a new role for Rap1 in maintaining epithelial integrity and tissue shape during Drosophila oogenesis. Loss of Rap1 activity disrupted the follicle cell epithelium and the shape of egg chambers during a period of major growth. Rap1 was required for proper E-Cadherin localization in the anterior epithelium and for epithelial cell survival. Both Myo-II and the adherens junction-cytoskeletal linker protein α-Catenin were required for normal egg chamber shape but did not strongly affect cell viability. Blocking the apoptotic cascade failed to rescue the cell shape defects caused by Rap1 inhibition. One consequence of increased cell death caused by Rap1 inhibition was the loss of polar cells and other follicle cells, which later in development led to fewer cells forming a migrating border cell cluster. Our results thus indicate dual roles for Rap1 in maintaining epithelia and cell survival in a growing tissue during development.

[I lead, follow me! How cells coordinate during collective migrations.]

During development and wound healing, cells frequently move in a so-called "collective cell migration" process. The same type of migration is used by some cancer cells during metastasis formation. A powerful model to study collective cell migration is the border cell cluster in Drosophila as it allows the observation and manipulation of a collective cell migration in its normal environment. This review describes the molecular machinery used by the border cells to migrate directionally, focusing on the mechanisms used to detect and reacts to chemoattractants, and to organise the group in leader and follower cells.

Germline protein, Cup, non-cell autonomously limits migratory cell fate in Drosophila oogenesis

Specification of migratory cell fate from a stationary population is complex and indispensable both for metazoan development as well for the progression of the pathological condition like tumor metastasis. Though this cell fate transformation is widely prevalent, the molecular understanding of this phenomenon remains largely elusive. We have employed the model of border cells (BC) in Drosophila oogenesis and identified germline activity of an RNA binding protein, Cup that limits acquisition of migratory cell fate from the neighbouring follicle epithelial cells. As activation of JAK-STAT in the follicle cells is critical for BC specification, our data suggest that Cup, non-cell autonomously restricts the domain of JAK-STAT by activating Notch in the follicle cells. Employing genetics and Delta endocytosis assay, we demonstrate that Cup regulates Delta recycling in the nurse cells through Rab11GTPase thus facilitating Notch activation in the adjacent follicle cells. Since Notch and JAK-STAT are antagonistic, we propose that germline Cup functions through Notch and JAK-STAT to modulate BC fate specification from their static epithelial progenitors.

Tracking Follicle Cell Development

Somatic follicle cells are critical support cells for Drosophila oogenesis, as they provide signals and molecules needed to produce a mature egg. Throughout this process, the follicle cells differentiate into multiple subpopulations and transition between three different cell cycle programs to support nurse cell and oocyte development. The follicle cells are mitotic in early egg chamber development, as they cover the germline cyst. In mid-oogenesis, follicle cells switch from mitosis to endocycling, increasing their ploidy from 2C to 16C. Finally, in late oogenesis, cells transition from endocycling to gene amplification, increasing the copy number of a small subset of genes, including the genes encoding proteins required for egg maturation. In order to explore the genetic regulation of these cell cycle switches and follicle cell development and specification, clonal analysis and the GAL4/UAS system are used frequently to reduce or increase expression of genes of interest. These genetic approaches combined with immunohistochemistry and in situ hybridization are powerful tools for characterizing the mechanisms regulating follicle cell development and the mitosis/endocycle and endocycle/gene amplification transitions. This chapter describes the genetic tools available to manipulate gene expression in follicle cells, as well as the methods and reagents that can be utilized to explore gene expression throughout follicle cell development.

Integrins control epithelial stem cell proliferation in the Drosophila ovary by modulating the Notch pathway

Cell proliferation and differentiation show a remarkable inverse relationship. The temporal coupling between cell cycle withdrawal and differentiation of stem cells (SCs) is crucial for epithelial tissue growth, homeostasis and regeneration. Proliferation vs. differentiation SC decisions are often controlled by the surrounding microenvironment, of which the basement membrane (BM; a specialized form of extracellular matrix surrounding cells and tissues), is one of its main constituents. Years of research have shown that integrin-mediated SC-BM interactions regulate many aspects of SC biology, including the proliferation-to-differentiation switch. However, these studies have also demonstrated that the SC responses to interactions with the BM are extremely diverse and depend on the cell type and state and on the repertoire of BM components and integrins involved. Here, we show that eliminating integrins from the follicle stem cells (FSCs) of the Drosophila ovary and their undifferentiated progeny increases their proliferation capacity. This results in an excess of various differentiated follicle cell types, demonstrating that cell fate determination can occur in the absence of integrins. Because these phenotypes are similar to those found in ovaries with decreased laminin levels, our results point to a role for the integrin-mediated cell-BM interactions in the control of epithelial cell division and subsequent differentiation. Finally, we show that integrins regulate proliferation by restraining the activity of the Notch/Delta pathway during early oogenesis. Our work increases our knowledge of the effects of cell-BM interactions in different SC types and should help improve our understanding of the biology of SCs and exploit their therapeutic potential.

Standardization of Single-Cell RNA-Sequencing Analysis Workflow to Study Drosophila Ovary

Developments in single-cell technology have considerably changed the way we study biology. Significant efforts have been made over the last few years to build comprehensive cell-type-specific transcriptomic atlases for a wide range of tissues in several model organisms in order to discover cell-type-specific markers and drivers of gene expression. One such tissue is the ovary of the fruit-fly Drosophila melanogaster, which is a popular model system with wide-ranging applications in the study of both development and disease. Three independent studies have recently produced comprehensive maps of cell-type-specific gene expression that describe both spatiotemporal regulation of the process of oogenesis and unique transcriptomic profiles of different cell types that constitute the ovary. In this chapter, we outlined the wet-lab protocol that was followed in our recent study for sample preparation and reanalyze the resultant dataset to discuss the benchmarks in data analysis, which are fundamental to comprehensive curation of the single-cell dataset representing the fly ovary.

Single-cell transcriptomics identifies Keap1-Nrf2 regulated collective invasion in a Drosophila tumor model

Apicobasal cell polarity loss is a founding event in epithelial-mesenchymal transition and epithelial tumorigenesis, yet how pathological polarity loss links to plasticity remains largely unknown. To understand the mechanisms and mediators regulating plasticity upon polarity loss, we performed single-cell RNA sequencing of Drosophila ovaries, where inducing polarity-gene l(2)gl-knockdown (Lgl-KD) causes invasive multilayering of the follicular epithelia. Analyzing the integrated Lgl-KD and wildtype transcriptomes, we discovered the cells specific to the various discernible phenotypes and characterized the underlying gene expression. A genetic requirement of Keap1-Nrf2 signaling in promoting multilayer formation of Lgl-KD cells was further identified. Ectopic expression of Keap1 increased the volume of delaminated follicle cells that showed enhanced invasive behavior with significant changes to the cytoskeleton. Overall, our findings describe the comprehensive transcriptome of cells within the follicle cell tumor model at the single-cell resolution and identify a previously unappreciated link between Keap1-Nrf2 signaling and cell plasticity at early tumorigenesis.

Two Rac1 pools integrate the direction and coordination of collective cell migration

Integration of collective cell direction and coordination is believed to ensure collective guidance for efficient movement. Previous studies demonstrated that chemokine receptors PVR and EGFR govern a gradient of Rac1 activity essential for collective guidance of Drosophila border cells, whose mechanistic insight is unknown. By monitoring and manipulating subcellular Rac1 activity, here we reveal two switchable Rac1 pools at border cell protrusions and supracellular cables, two important structures responsible for direction and coordination. Rac1 and Rho1 form a positive feedback loop that guides mechanical coupling at cables to achieve migration coordination. Rac1 cooperates with Cdc42 to control protrusion growth for migration direction, as well as to regulate the protrusion-cable exchange, linking direction and coordination. PVR and EGFR guide correct Rac1 activity distribution at protrusions and cables. Therefore, our studies emphasize the existence of a balance between two Rac1 pools, rather than a Rac1 activity gradient, as an integrator for the direction and coordination of collective cell migration.

Previous studies suggested a chemokine receptor governed gradient of Rac1 activity is essential for collective guidance of Drosophila border cells. Here, Zhou et al. report that two distinct Rac1 pools at protrusions and cables, not Rac1 activity gradient, integrate the direction and coordination for collective guidance.

Live imaging of delamination in Drosophila shows epithelial cell motility and invasiveness are independently regulated

Delaminating cells undergo complex, precisely regulated changes in cell–cell adhesion, motility, polarity, invasiveness, and other cellular properties. Delamination occurs during development and in pathogenic conditions such as cancer metastasis. We analyzed the requirements for epithelial delamination in Drosophila ovary border cells, which detach from the structured epithelial layer and begin to migrate collectively. We used live imaging to examine cellular dynamics, particularly epithelial cells’ acquisition of motility and invasiveness, in delamination-defective mutants during the time period in which delamination occurs in the wild-type ovary. We found that border cells in slow border cells (slbo), a delamination-defective mutant, lacked invasive cellular protrusions but acquired basic cellular motility, while JAK/STAT-inhibited border cells lost both invasiveness and motility. Our results indicate that invasiveness and motility, which are cooperatively required for delamination, are regulated independently. Our reconstruction experiments also showed that motility is not a prerequisite for acquiring invasiveness.

Exploring the recent trends in perturbing the cellular signaling pathways in cancer by natural products

Cancer is commonly thought to be the product of irregular cell division. According to the World Health Organization (WHO), cancer is the major cause of death globally. Nature offers an abundant supply of bioactive compounds with high therapeutic efficacy. Anticancer effects have been studied in a variety of phytochemicals found in nature. When Food and Drug Administration (FDA)-approved anticancer drugs are combined with natural compounds, the effectiveness improves. Several agents have already progressed to clinical trials based on these promising results of natural compounds against various cancer forms. Natural compounds prevent cancer cell proliferation, development, and metastasis by inducing cell cycle arrest, activating intrinsic and extrinsic apoptosis pathways, generating reactive oxygen species (ROS), and down-regulating activated signaling pathways. These natural chemicals are known to affect numerous important cellular signaling pathways, such as NF-B, MAPK, Wnt, Notch, Akt, p53, AR, ER, and many others, to cause cell death signals and induce apoptosis in pre-cancerous or cancer cells without harming normal cells. As a result, non-toxic “natural drugs” taken from nature’s bounty could be effective for the prevention of tumor progression and/or therapy of human malignancies, either alone or in combination with conventional treatments. Natural compounds have also been shown in preclinical studies to improve the sensitivity of resistant cancers to currently available chemotherapy agents. To summarize, preclinical and clinical findings against cancer indicate that natural-sourced compounds have promising anticancer efficacy. The vital purpose of these studies is to target cellular signaling pathways in cancer by natural compounds.

Mind bomb 2 promotes cell migration and epithelial structure by regulating adhesion complexes and the actin cytoskeleton

Cell migration is essential in animal development and co-opted during metastasis and inflammatory diseases. Some cells migrate collectively, which requires them to balance epithelial characteristics such as stable cell-cell adhesions with features of motility like rapid turnover of adhesions and dynamic cytoskeletal structures. How this is regulated is not entirely clear but important to understand. While investigating Drosophila oogenesis, we found that the putative E3 ubiquitin ligase, Mind bomb 2 (Mib2), is required to promote epithelial stability and the collective cell migration of border cells. Through biochemical analysis, we identified components of Mib2 complexes, which include E-cadherin and α- and β-catenins, as well as actin regulators. We also found that three Mib2 interacting proteins, RhoGAP19D, Supervillin, and Myosin heavy chain-like, affect border cell migration. mib2 mutant main body follicle cells have drastically reduced E-cadherin-based adhesion complexes and diminished actin filaments. We conclude that Mib2 acts to stabilize E-cadherin-based adhesion complexes and promote a robust actin cytoskeletal network, which is important for maintenance of epithelial integrity. The interaction with cadherin adhesion complexes and other cytoskeletal regulators contribute to its role in collective cell migration. Since Mib2 is well conserved, it may have similar functional significance in other organisms.

E-cadherin acts as a positive regulator of the JAK-STAT signaling pathway during Drosophila oogenesis

The JAK-STAT pathway is evolutionary conserved. The simplicity of this signaling in Drosophila, due to the limited redundancy between pathway components, makes it an ideal model for investigation. In the Drosophila follicular epithelium, highly stereotyped functions of JAK-STAT signaling have been well characterized, but how signaling activity is regulated precisely to allow the different outcomes is not well understood. In this tissue, the ligand is secreted by the polar cells positioned at each follicle extremity, thus generating a gradient of JAK-STAT activity in adjacent cells. One way to control the delivered quantity of ligand is by regulating the number of polar cells, which is reduced by apoptosis to exactly two at each pole by mid-oogenesis. Hence, JAK-STAT activity is described as symmetrical between follicle anterior and posterior regions. Here, we show that JAK-STAT signaling activity is actually highly dynamic, resulting in asymmetry between poles by mid-oogenesis. Interestingly, we found similar temporal dynamics at follicle poles in the accumulation of the adherens junction E-cadherin protein. Remarkably, E-cadherin and JAK-STAT signaling not only display patterning overlaps but also share functions during oogenesis. In particular, we show that E-cadherin, like JAK-STAT signaling, regulates polar cell apoptosis non-cell-autonomously from follicle cells. Finally, our work reveals that E-cadherin is required for optimal JAK-STAT activity throughout oogenesis and that E-cadherin and Stat92E, the transcription factor of the pathway, form part of a physical complex in follicle cells. Taken together, our study establishes E-cadherin as a new positive regulator of JAK-STAT signaling during oogenesis.

Drosophila CTP synthase regulates collective cell migration by controlling the polarized endocytic cycle

Phosphatidylinositol (PI) 4,5-bisphosphate (PIP2) is involved in many biological functions. However, the mechanisms of PIP2 in collective cell migration remain elusive. This study highlights the regulatory role of cytidine triphosphate synthase (CTPsyn) in collective border cell migration through regulating the asymmetrical distribution of PIP2. We demonstrated that border cell clusters containing mutant CTPsyn cells suppressed migration. CTPsyn was co-enriched with Actin at the leading edge of the Drosophila border cell cluster where PIP2 was enriched, and this enrichment depended on the CTPsyn activity. Genetic interactions of border cell migration were found between CTPsyn mutant and genes in PI biosynthesis. The CTPsyn reduction resulted in loss of the asymmetric activity of endocytosis recycling. Also, genetic interactions were revealed between components of the exocyst complex and CTPsyn mutant, indicating that CTPsyn activity regulates the PIP2-related asymmetrical exocytosis activity. Furthermore, CTPsyn activity is essential for RTK-polarized distribution in the border cell cluster. We propose a model in which CTPsyn activity is required for the asymmetrical generation of PIP2 to enrich RTK signaling through endocytic recycling in collective cell migration.

Spatiotemporal gating of Stat nuclear influx by Drosophila Npas4 in collective cell migration

Collective migration is important to embryonic development and cancer metastasis, but migratory and nonmigratory cell fate discrimination by differential activity of signal pathways remains elusive. In Drosophila oogenesis, Jak/Stat signaling patterns the epithelial cell fates in early egg chambers but later renders motility to clustered border cells. How Jak/Stat signal spatiotemporally switches static epithelia to motile cells is largely unknown. We report that a nuclear protein, Dysfusion, resides on the inner nuclear membrane and interacts with importin α/β and Nup153 to modulate Jak/Stat signal by attenuating Stat nuclear import. Dysfusion is ubiquitously expressed in oogenesis but specifically down-regulated in border cells when migrating. Increase of nuclear Stat by Dysfusion down-regulation triggers invasive cell behavior and maintains persistent motility. Mammalian homolog of Dysfusion (NPAS4) also negatively regulates the nuclear accumulation of STAT3 and cancer cell migration. Thus, our finding demonstrates that Dysfusion-dependent gating mechanism is conserved and may serve as a therapeutic target for Stat-mediated cancer metastasis.

Border cell polarity and collective migration require the spliceosome component Cactin

Border cells are an in vivo model for collective cell migration. Here, we identify the gene cactin as essential for border cell cluster organization, delamination, and migration. In Cactin-depleted cells, the apical proteins aPKC and Crumbs (Crb) become abnormally concentrated, and overall cluster polarity is lost. Apically tethering excess aPKC is sufficient to cause delamination defects, and relocalizing apical aPKC partially rescues delamination. Cactin is conserved from yeast to humans and has been implicated in diverse processes. In border cells, Cactin's evolutionarily conserved spliceosome function is required. Whole transcriptome analysis revealed alterations in isoform expression in Cactin-depleted cells. Mutations in two affected genes, Sec23 and Sec24CD, which traffic Crb to the apical cell surface, partially rescue border cell cluster organization and migration. Overexpression of Rab5 or Rab11, which promote Crb and aPKC recycling, similarly rescues. Thus, a general splicing factor is specifically required for coordination of cluster polarity and migration, and migrating border cells are particularly sensitive to splicing and cell polarity disruptions.

The Blimp-1 transcription factor acts in non-neuronal cells to regulate terminal differentiation of the Drosophila eye

The formation of a functional organ such as the eye requires specification of the correct cell types and their terminal differentiation into cells with the appropriate morphologies and functions. Here, we show that the zinc-finger transcription factor Blimp-1 acts in secondary and tertiary pigment cells in the Drosophila retina to promote the formation of a bi-convex corneal lens with normal refractive power, and in cone cells to enable complete extension of the photoreceptor rhabdomeres. Blimp-1 expression depends on the hormone ecdysone, and loss of ecdysone signaling causes similar differentiation defects. Timely termination of Blimp-1 expression is also important, as its overexpression in the eye has deleterious effects. Our transcriptomic analysis revealed that Blimp-1 regulates the expression of many structural and secreted proteins in the retina. Blimp-1 may function in part by repressing another transcription factor; Slow border cells is highly upregulated in the absence of Blimp-1, and its overexpression reproduces many of the effects of removing Blimp-1. This work provides insight into the transcriptional networks and cellular interactions that produce the structures necessary for visual function.

Intrinsic and damage-induced JAK/STAT signaling regulate developmental timing by the Drosophila prothoracic gland

Development involves tightly paced, reproducible sequences of events, yet it must adjust to conditions external to it, such as resource availability and organismal damage. A major mediator of damage-induced immune responses in vertebrates and insects is JAK/STAT signaling. At the same time, JAK/STAT activation by the Drosophila Upd cytokines is pleiotropically involved in normal development of multiple organs. Whether inflammatory and developmental JAK/STAT roles intersect is unknown. Here, we show that JAK/STAT is active during development of the prothoracic gland (PG), which controls metamorphosis onset through ecdysone production. Reducing JAK/STAT signaling decreased PG size and advanced metamorphosis. Conversely, JAK/STAT hyperactivation by overexpression of pathway components or SUMOylation loss caused PG hypertrophy and metamorphosis delay. Tissue damage and tumors, known to secrete Upd cytokines, also activated JAK/STAT in the PG and delayed metamorphosis, at least in part by inducing expression of the JAK/STAT target Apontic. JAK/STAT damage signaling, therefore, regulates metamorphosis onset by co-opting its developmental role in the PG. Our findings in Drosophila provide insights on how systemic effects of damage and cancer can interfere with hormonally controlled development and developmental transitions.

Summary: Damage signaling from tumors mediated by JAK/STAT-activating Upd cytokines delays the Drosophila larva–pupa transition through co-option of a JAK/STAT developmental role in the prothoracic gland.

Endogenous epitope tagging of a JAK/STAT ligand Unpaired1 in Drosophila

Unpaired1 (Upd1) is a ligand of the Janus kinase/signal transducer and activator of transcription (JAK/STAT) signaling pathway in Drosophila. In this study, using the CRISPR/Cas9 technique, we generate a transgenic fly strain in which a hemagglutinin (HA) epitope tag sequence is inserted into the endogenous locus of the upd1 gene. Anti-HA antibody staining confirms that the distribution of the epitope-tagged Upd1::HA in various tissues is consistent with upd1 expression patterns revealed by previous studies. This transgenic fly strain will be useful in studying the expression, localization, and association partners of Upd1, and thus will contribute to understanding how activation of the JAK/STAT pathway is regulated.

A single-cell atlas reveals unanticipated cell type complexity in Drosophila ovaries

Organ function relies on the spatial organization and functional coordination of numerous cell types. The Drosophila ovary is a widely used model system to study the cellular activities underlying organ function, including stem cell regulation, cell signaling and epithelial morphogenesis. However, the relative paucity of cell type-specific reagents hinders investigation of molecular functions at the appropriate cellular resolution. Here, we used single-cell RNA sequencing to characterize all cell types of the stem cell compartment and early follicles of the Drosophila ovary. We computed transcriptional signatures and identified specific markers for nine states of germ cell differentiation, and 23 somatic cell types and subtypes. We uncovered an unanticipated diversity of escort cells, the somatic cells that directly interact with differentiating germline cysts. Three escort cell subtypes reside in discrete anatomical positions, and express distinct sets of secreted and transmembrane proteins, suggesting that diverse micro-environments support the progressive differentiation of germ cells. Finally, we identified 17 follicle cell subtypes, and characterized their transcriptional profiles. Altogether, we provide a comprehensive resource of gene expression, cell type-specific markers, spatial coordinates and functional predictions for 34 ovarian cell types and subtypes.

Collective metastasis: coordinating the multicellular voyage

The metastatic process is arduous. Cancer cells must escape the confines of the primary tumor, make their way into and travel through the circulation, then survive and proliferate in unfavorable microenvironments. A key question is how cancer cells overcome these multiple barriers to orchestrate distant organ colonization. Accumulating evidence in human patients and animal models supports the hypothesis that clusters of tumor cells can complete the entire metastatic journey in a process referred to as collective metastasis. Here we highlight recent studies unraveling how multicellular coordination, via both physical and biochemical coupling of cells, induces cooperative properties advantageous for the completion of metastasis. We discuss conceptual challenges and unique mechanisms arising from collective dissemination that are distinct from single cell-based metastasis. Finally, we consider how the dissection of molecular transitions regulating collective metastasis could offer potential insight into cancer therapy.

Drosophila USP22/nonstop polarizes the actin cytoskeleton during collective border cell migration

Polarization of the actin cytoskeleton is vital for the collective migration of cells in vivo. During invasive border cell migration in Drosophila, actin polarization is directly controlled by the Hippo signaling complex, which resides at contacts between border cells in the cluster. Here, we identify, in a genetic screen for deubiquitinating enzymes involved in border cell migration, an essential role for nonstop/USP22 in the expression of Hippo pathway components expanded and merlin. Loss of nonstop function consequently leads to a redistribution of F-actin and the polarity determinant Crumbs, loss of polarized actin protrusions, and tumbling of the border cell cluster. Nonstop is a component of the Spt-Ada-Gcn5-acetyltransferase (SAGA) transcriptional coactivator complex, but SAGA’s histone acetyltransferase module, which does not bind to expanded or merlin, is dispensable for migration. Taken together, our results uncover novel roles for SAGA-independent nonstop/USP22 in collective cell migration, which may help guide studies in other systems where USP22 is necessary for cell motility and invasion.

GAGA Regulates Border Cell Migration in Drosophila

Collective cell migration is a complex process that happens during normal development of many multicellular organisms, as well as during oncological transformations. In Drosophila oogenesis, a small set of follicle cells originally located at the anterior tip of each egg chamber become motile and migrate as a cluster through nurse cells toward the oocyte. These specialized cells are referred to as border cells (BCs) and provide a simple and convenient model system to study collective cell migration. The process is known to be complexly regulated at different levels and the product of the slow border cells (slbo) gene, the C/EBP transcription factor, is one of the key elements in this process. However, little is known about the regulation of slbo expression. On the other hand, the ubiquitously expressed transcription factor GAGA, which is encoded by the Trithorax-like (Trl) gene was previously demonstrated to be important for Drosophila oogenesis. Here, we found that Trl mutations cause substantial defects in BC migration. Partially, these defects are explained by the reduced level of slbo expression in BCs. Additionally, a strong genetic interaction between Trl and slbo mutants, along with the presence of putative GAGA binding sites within the slbo promoter and enhancer, suggests the direct regulation of this gene by GAGA. This idea is supported by the reduction in the slbo-Gal4-driven GFP expression within BC clusters in Trl mutant background. However, the inability of slbo overexpression to compensate defects in BC migration caused by Trl mutations suggests that there are other GAGA target genes contributing to this process. Taken together, the results define GAGA as another important regulator of BC migration in Drosophila oogenesis.

Development and Analysis of a Quantitative Mathematical Model of Bistability in the Cross Repression System Between APT and SLBO Within the JAK/STAT Signaling Pathway

Cell migration is a key component in development, homeostasis, immune function, and pathology. It is important to understand the molecular activity that allows some cells to migrate. Drosophila melanogaster is a useful model system because its genes are largely conserved with humans and it is straightforward to study biologically. The well-conserved transcriptional regulator Signal Transducer and Activator of Transcription (STAT) promotes cell migration, but its signaling is modulated by downstream targets Apontic (APT) and Slow Border Cells (SLBO). Inhibition of STAT activity by APT and cross-repression of APT and SLBO determines whether an epithelial cell in the Drosophila egg chamber becomes motile or remains stationary. Through mathematical modeling and analysis, we examine how the interaction of STAT, APT, and SLBO creates bistability in the Janus Kinase (JAK)/STAT signaling pathway. In this paper, we update and analyze earlier models to represent mechanistically the processes of the JAK/STAT pathway. We utilize parameter, bifurcation, and phase portrait analyses, and make reductions to the system to produce a minimal three-variable quantitative model. We analyze the manifold between migratory and stationary steady states in this minimal model and show that when the initial conditions of our model are near this manifold, cell migration can be delayed.

Prostaglandins regulate invasive, collective border cell migration

While prostaglandins (PGs), short-range lipid signals, regulate single cell migration, their roles in collective migration remain unclear. To address this, we use Drosophila border cell migration, an invasive, collective migration that occurs during Stage 9 of oogenesis. Pxt is the Drosophila cyclooxygenase-like enzyme responsible for PG synthesis. Loss of Pxt results in both delayed border cell migration and elongated clusters, whereas somatic Pxt knockdown causes delayed migration and compacted clusters. These findings suggest PGs act in both the border cells and nurse cells, the substrate on which the border cells migrate. As PGs regulate the actin bundler Fascin, and Fascin is required for on-time migration, we assessed whether PGs regulate Fascin to promote border cell migration. Coreduction of Pxt and Fascin results in delayed migration and elongated clusters. The latter may be due to altered cell adhesion, as loss of Pxt or Fascin, or coreduction of both, decreases integrin levels on the border cell membranes. Conversely, integrin localization is unaffected by somatic knockdown of Pxt. Together these data lead to the model that PG signaling controls Fascin in the border cells to promote migration and in the nurse cells to maintain cluster cohesion.

Protein phosphatase 1 activity controls a balance between collective and single cell modes of migration

Collective cell migration is central to many developmental and pathological processes. However, the mechanisms that keep cell collectives together and coordinate movement of multiple cells are poorly understood. Using the Drosophila border cell migration model, we find that Protein phosphatase 1 (Pp1) activity controls collective cell cohesion and migration. Inhibition of Pp1 causes border cells to round up, dissociate, and move as single cells with altered motility. We present evidence that Pp1 promotes proper levels of cadherin-catenin complex proteins at cell-cell junctions within the cluster to keep border cells together. Pp1 further restricts actomyosin contractility to the cluster periphery rather than at individual internal border cell contacts. We show that the myosin phosphatase Pp1 complex, which inhibits non-muscle myosin-II (Myo-II) activity, coordinates border cell shape and cluster cohesion. Given the high conservation of Pp1 complexes, this study identifies Pp1 as a major regulator of collective versus single cell migration.

Clustered cell migration: Modeling the model system of Drosophila border cells

In diverse developmental contexts, certain cells must migrate to fulfill their roles. Many questions remain unanswered about the genetic and physical properties that govern cell migration. While the simplest case of a single cell moving alone has been well-studied, additional complexities arise in considering how cohorts of cells move together. Significant differences exist between models of collectively migrating cells. We explore the experimental model of migratory border cell clusters in Drosophila melanogaster egg chambers, which are amenable to direct observation and precise genetic manipulations. This system involves two special characteristics that are worthy of attention: border cell clusters contain a limited number of both migratory and non-migratory cells that require coordination, and they navigate through a heterogeneous three-dimensional microenvironment. First, we review how clusters of motile border cells are specified and guided in their migration by chemical signals and the physical impact of adjacent tissue interactions. In the second part, we examine questions around the 3D structure of the motile cluster and surrounding microenvironment in understanding the limits to cluster size and speed of movement through the egg chamber. Mathematical models have identified sufficient gene regulatory networks for specification, the key forces that capture emergent behaviors observed in vivo, the minimal regulatory topologies for signaling, and the distribution of key signaling cues that direct cell behaviors. This interdisciplinary approach to studying border cells is likely to reveal governing principles that apply to different types of cell migration events.

Cell interactions in collective cell migration

Collective cell migration is the coordinated movement of a physically connected group of cells and is a prominent driver of development and metastasis. Interactions between cells within migrating collectives, and between migrating cells and other cells in the environment, play key roles in stimulating motility, steering and sometimes promoting cell survival. Similarly, diverse heterotypic interactions and collective behaviors likely contribute to tumor metastasis. Here, we describe a sampling of cells that migrate collectively in vivo, including well-established and newer examples. We focus on the under-appreciated property that many - perhaps most - collectively migrating cells move as cooperating groups of distinct cell types.

Signaling between somatic follicle cells and the germline patterns the egg and embryo of Drosophila

In Drosophila, specification of the embryonic body axes requires signaling between the germline and the somatic follicle cells. These signaling events are necessary to properly localize embryonic patterning determinants in the egg or eggshell during oogenesis. There are three maternal patterning systems that specify the anterior-posterior axis, and one that establishes the dorsal-ventral axis. We will first review oogenesis, focusing on the establishment of the oocyte and nurse cells and patterning of the follicle cells into different subpopulations. We then describe how two coordinated signaling events between the oocyte and follicle cells establish polarity of the oocyte and localize the anterior determinant bicoid, the posterior determinant oskar, and Gurken/epidermal growth factor (EGF), which breaks symmetry to initiate dorsal-ventral axis establishment. Next, we review how dorsal-ventral asymmetry of the follicle cells is transmitted to the embryo. This process also involves Gurken-EGF receptor (EGFR) signaling between the oocyte and follicle cells, leading to ventrally-restricted expression of the sulfotransferase Pipe. These events promote the ventral processing of Spaetzle, a ligand for Toll, which ultimately sets up the embryonic dorsal-ventral axis. We then describe the activation of the terminal patterning system by specialized polar follicle cells. Finally, we present open questions regarding soma-germline signaling during Drosophila oogenesis required for cell identity and embryonic axis formation.

Characteristics of in Vivo Model Systems for Ovarian Cancer Studies

An understanding of the molecular pathogenesis and heterogeneity of ovarian cancer holds promise for the development of early detection strategies and novel, efficient therapies. In this review, we discuss the advantages and limitations of animal models available for basic and preclinical studies. The fruit fly model is suitable mainly for basic research on cellular migration, invasiveness, adhesion, and the epithelial-to-mesenchymal transition. Higher-animal models allow to recapitulate the architecture and microenvironment of the tumor. We discuss a syngeneic mice model and the patient derived xenograft model (PDX), both useful for preclinical studies. Conditional knock-in and knock-out methodology allows to manipulate selected genes at a given time and in a certain tissue. Such models have built our knowledge about tumor-initiating genetic events and cell-of-origin of ovarian cancers; it has been shown that high-grade serous ovarian cancer may be initiated in both the ovarian surface and tubal epithelium. It is postulated that clawed frog models could be developed, enabling studies on tumor immunity and anticancer immune response. In laying hen, ovarian cancer develops spontaneously, which provides the opportunity to study the genetic, biochemical, and environmental risk factors, as well as tumor initiation, progression, and histological origin; this model can also be used for drug testing. The chick embryo chorioallantoic membrane is another attractive model and allows the study of drug response.

Coordination of protrusion dynamics within and between collectively migrating border cells by myosin II

Collective cell migration is emerging as a major driver of embryonic development, organogenesis, tissue homeostasis, and tumor dissemination. In contrast to individually migrating cells, collectively migrating cells maintain cell–cell adhesions and coordinate direction-sensing as they move. While nonmuscle myosin II has been studied extensively in the context of cells migrating individually in vitro, its roles in cells migrating collectively in three-dimensional, native environments are not fully understood. Here we use genetics, Airyscan microscopy, live imaging, optogenetics, and Förster resonance energy transfer to probe the localization, dynamics, and functions of myosin II in migrating border cells of the Drosophila ovary. We find that myosin accumulates transiently at the base of protrusions, where it functions to retract them. E-cadherin and myosin colocalize at border cell-border cell contacts and cooperate to transmit directional information. A phosphomimetic form of myosin is sufficient to convert border cells to a round morphology and blebbing migration mode. Together these studies demonstrate that distinct and dynamic pools of myosin II regulate protrusion dynamics within and between collectively migrating cells and suggest a new model for the role of protrusions in collective direction sensing in vivo.

A Gene Expression Screen in Drosophila melanogaster Identifies Novel JAK/STAT and EGFR Targets During Oogenesis

The Janus Kinase/Signal Transducer and Activator of Transcription (JAK/STAT) and epidermal growth factor receptor (EGFR) signaling pathways are conserved regulators of tissue patterning, morphogenesis, and other cell biological processes. During Drosophila oogenesis, these pathways determine the fates of epithelial follicle cells (FCs). JAK/STAT and EGFR together specify a population of cells called the posterior follicle cells (PFCs), which signal to the oocyte to establish the embryonic axes. In this study, whole genome expression analysis was performed to identify genes activated by JAK/STAT and/or EGFR. We observed that 317 genes were transcriptionally upregulated in egg chambers with ectopic JAK/STAT and EGFR activity in the FCs. The list was enriched for genes encoding extracellular matrix (ECM) components and ECM-associated proteins. We tested 69 candidates for a role in axis establishment using RNAi knockdown in the FCs. We report that the signaling protein Semaphorin 1b becomes enriched in the PFCs in response to JAK/STAT and EGFR. We also identified ADAM metallopeptidase with thrombospondin type 1 motif A (AdamTS-A) as a novel target of JAK/STAT in the FCs that regulates egg chamber shape. AdamTS-A mRNA becomes enriched at the anterior and posterior poles of the egg chamber at stages 6 to 7 and is regulated by JAK/STAT. Altering AdamTS-A expression in the poles or middle of the egg chamber produces rounder egg chambers. We propose that AdamTS-A regulates egg shape by remodeling the basement membrane.

OCT4 accelerates tumorigenesis through activating JAK/STAT signaling in ovarian cancer side population cells

BACKGROUND

Although surgery, chemotherapy, and radiotherapy eliminate clinically apparent ovarian tumor, the 5-year survival rate is no more than 45%. Cancer stem cells (CSCs) have been identified for precaution of tumor metastasis and recurrence in many kinds of cancers including ovarian cancer.

AIM

This study aims to explore the function of OCT4, a CSC marker, in ovarian cancer progression and to investigate its underlying mechanism.

MATERIALS AND METHODS

By Hoechst side population (SP) technique, CSC-like SP cells from human ovarian cancer SKOV3 and A2780 cells were isolated and used for this study. shRNA and lentivirus targeting human OCT4 gene were used to knock down OCT4 in SP cells and upregulate OCT4 in non-SP (NSP) cells stably. Peficitinib was used to inhibit JAK/STAT signaling. Cell counting kit-8, flow cytometry, and in vivo xenograft model were used to evaluate the effects of OCT4/JAK/STAT on the viability, drug resistance, apoptosis, cycle, and tumorigenesis of the SP cells. Immunofluorescence staining was used to detect the location of STAT6.

RESULTS

Results showed that OCT4 was upregulated in the SP of SKOV3 and A2780 cells when compared with the NSP cells. Downregulation of OCT4 inhibited SP cell viability, tumorigenesis, and reduced cell drug resistance and induced a G2/M phase arrest, while upregulation of OCT4 conferred NSP cell malignant features. Besides, OCT4 upregulation in NSP cells increased the phosphorylated levels of proteins in JAK and STAT families, especially in JAK1 and STAT6. Furthermore, the roles of apoptosis inhibition and viability, invasion, and tumorigenesis promotions induced by OCT4 in NSP cells were all abolished when adding peficitinib.

CONCLUSION

Our study demonstrated that OCT4 accelerated ovarian cancer progression through activating JAK/STAT signaling pathway.

Drosophila Jak/STAT Signaling: Regulation and Relevance in Human Cancer and Metastasis

Over the past three-decades, Janus kinase (Jak) and signal transducer and activator of transcription (STAT) signaling has emerged as a paradigm to understand the involvement of signal transduction in development and disease pathology. At the molecular level, cytokines and interleukins steer Jak/STAT signaling to transcriptional regulation of target genes, which are involved in cell differentiation, migration, and proliferation. Jak/STAT signaling is involved in various types of blood cell disorders and cancers in humans, and its activation is associated with carcinomas that are more invasive or likely to become metastatic. Despite immense information regarding Jak/STAT regulation, the signaling network has numerous missing links, which is slowing the progress towards developing drug therapies. In mammals, many components act in this cascade, with substantial cross-talk with other signaling pathways. In Drosophila, there are fewer pathway components, which has enabled significant discoveries regarding well-conserved regulatory mechanisms. Work across species illustrates the relevance of these regulators in humans. In this review, we showcase fundamental Jak/STAT regulation mechanisms in blood cells, stem cells, and cell motility. We examine the functional relevance of key conserved regulators from Drosophila to human cancer stem cells and metastasis. Finally, we spotlight less characterized regulators of Drosophila Jak/STAT signaling, which stand as promising candidates to be investigated in cancer biology. These comparisons illustrate the value of using Drosophila as a model for uncovering the roles of Jak/STAT signaling and the molecular means by which the pathway is controlled.

Cytokine exocytosis and JAK/STAT activation in the Drosophila ovary requires the vesicle trafficking regulator α-Snap

How vesicle trafficking components actively contribute to regulation of paracrine signaling is unclear. We genetically uncovered a requirement for α-soluble NSF attachment protein (α-Snap) in the activation of the Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway during Drosophila egg development. α-Snap, a well-conserved vesicle trafficking regulator, mediates association of N-ethylmaleimide-sensitive factor (NSF) and SNAREs to promote vesicle fusion. Depletion of α-Snap or the SNARE family member Syntaxin1A in epithelia blocks polar cells maintenance and prevents specification of motile border cells. Blocking apoptosis rescues polar cell maintenance in α-Snap-depleted egg chambers, indicating that the lack of border cells in mutants is due to impaired signaling. Genetic experiments implicate α-Snap and NSF in secretion of a STAT-activating cytokine. Live imaging suggests that changes in intracellular Ca2+ are linked to this event. Our data suggest a cell-type specific requirement for particular vesicle trafficking components in regulated exocytosis during development. Given the central role for STAT signaling in immunity, this work may shed light on regulation of cytokine release in humans.

Rap1 GTPase promotes coordinated collective cell migration in vivo

During development and in cancer, cells often move together in small to large collectives. To move as a unit, cells within collectives need to stay coupled together and coordinate their motility. How cell collectives remain interconnected and migratory, especially when moving through in vivo environments, is not well understood. The genetically tractable border cell group undergoes a highly polarized and cohesive cluster-type migration in the Drosophila ovary. Here we report that the small GTPase Rap1, through activation by PDZ-GEF, regulates border cell collective migration. We find that Rap1 maintains cell contacts within the cluster, at least in part by promoting the organized distribution of E-cadherin at specific cell-cell junctions. Rap1 also restricts migratory protrusions to the front of the border cell cluster and promotes the extension of protrusions with normal dynamics. Further, Rap1 is required in the outer migratory border cells but not in the central nonmigratory polar cells. Such cell specificity correlates well with the spatial distribution of the inhibitory Rapgap1 protein, which is higher in polar cells than in border cells. We propose that precisely regulated Rap1 activity reinforces connections between cells and polarizes the cluster, thus facilitating the coordinated collective migration of border cells.

The Drosophila insulin pathway controls Profilin expression and dynamic actin-rich protrusions during collective cell migration

Understanding how different cell types acquire their motile behaviour is central to many normal and pathological processes. Drosophila border cells represent a powerful model for addressing this issue and to specifically decipher the mechanisms controlling collective cell migration. Here, we identify the Drosophila Insulin/Insulin-like growth factor signalling (IIS) pathway as a key regulator in controlling actin dynamics in border cells, independently of its function in growth control. Loss of IIS activity blocks the formation of actin-rich long cellular extensions that are important for the delamination and the migration of the invasive cluster. We show that IIS specifically activates the expression of the actin regulator chickadee, the Drosophila homolog of Profilin, which is essential for promoting the formation of actin extensions and migration through the egg chamber. In this process, the transcription factor FoxO acts as a repressor of chickadee expression. Altogether, these results show that local activation of IIS controls collective cell migration through regulation of actin homeostasis and protrusion dynamics.

Insulin signaling modulates border cell movement in Drosophila oogenesis

As collective cell migration is intimately involved in different aspects of metazoan development, molecular mechanisms underlying this process are being explored in a variety of developmental contexts. Border cell (BC) migration during Drosophila oogenesis has emerged as an excellent genetic model for studying collective cell migration. BCs are of epithelial origin but acquire partial mesenchymal characteristics before migrating as a group towards the oocyte. Here, we report that insulin signaling modulates collective BC movement during Drosophila oogenesis. Supporting the involvement of Insulin pathway, we demonstrate that compromising Insulin-like Receptor (InR) levels in BCs, inhibits their migration. Furthermore, we show that canonical Insulin signaling pathway components participate in this process. Interestingly, visualization of InR-depleted BC clusters, using time-lapse imaging, revealed a delay in detachment of BC clusters from the surrounding anterior follicle cells and altered protrusion dynamics. Lastly, based on genetic interactions between InR, the polarity determinant, par-1 and a regulatory subunit of Drosophila Myosin (spaghetti squash), we propose that Insulin signaling likely influences par-1 activity to engineer border cell detachment and subsequent movement via Drosophila Myosin.

Embryonic hematopoiesis modulates the inflammatory response and larval hematopoiesis in Drosophila

Recent lineage tracing analyses have significantly improved our understanding of immune system development and highlighted the importance of the different hematopoietic waves. The current challenge is to understand whether these waves interact and whether this affects the function of the immune system. Here we report a molecular pathway regulating the immune response and involving the communication between embryonic and larval hematopoietic waves in Drosophila. Down-regulating the transcription factor Gcm specific to embryonic hematopoiesis enhances the larval phenotypes induced by over-expressing the pro-inflammatory Jak/Stat pathway or by wasp infestation. Gcm works by modulating the transduction of the Upd cytokines to the site of larval hematopoiesis and hence the response to chronic (Jak/Stat over-expression) and acute (wasp infestation) immune challenges. Thus, homeostatic interactions control the function of the immune system in physiology and pathology. Our data also indicate that a transiently expressed developmental pathway has a long-lasting effect on the immune response.

Cell motility in cancer invasion and metastasis: insights from simple model organisms

Metastasis remains the greatest challenge in the clinical management of cancer. Cell motility is a fundamental and ancient cellular behaviour that contributes to metastasis and is conserved in simple organisms. In this Review, we evaluate insights relevant to human cancer that are derived from the study of cell motility in non-mammalian model organisms. Dictyostelium discoideum, Caenorhabditis elegans, Drosophila melanogaster and Danio rerio permit direct observation of cells moving in complex native environments and lend themselves to large-scale genetic and pharmacological screening. We highlight insights derived from each of these organisms, including the detailed signalling network that governs chemotaxis towards chemokines; a novel mechanism of basement membrane invasion; the positive role of E-cadherin in collective direction-sensing; the identification and optimization of kinase inhibitors for metastatic thyroid cancer on the basis of work in flies; and the value of zebrafish for live imaging, especially of vascular remodelling and interactions between tumour cells and host tissues. While the motility of tumour cells and certain host cells promotes metastatic spread, the motility of tumour-reactive T cells likely increases their antitumour effects. Therefore, it is important to elucidate the mechanisms underlying all types of cell motility, with the ultimate goal of identifying combination therapies that will increase the motility of beneficial cells and block the spread of harmful cells.

Polar cell fate stimulates Wolbachia intracellular growth

Bacteria are crucial partners in the development and evolution of vertebrates and invertebrates. A large fraction of insects harbor Wolbachia, bacterial endosymbionts that manipulate host reproduction to favor their spreading. Because they are maternally inherited, Wolbachia are under selective pressure to reach the female germline and infect the offspring. However, Wolbachia infection is not limited to the germline. Somatic cell types, including stem cell niches, have higher Wolbachia loads compared with the surrounding tissue. Here, we show a novel Wolbachia tropism to polar cells (PCs), specialized somatic cells in the Drosophila ovary. During oogenesis, all stages of PC development are easily visualized, facilitating the investigation of the kinetics of Wolbachia intracellular growth. Wolbachia accumulation is triggered by particular events of PC morphogenesis, including differentiation from progenitors and between stages 8 and 9 of oogenesis. Moreover, induction of ectopic PC fate is sufficient to promote Wolbachia accumulation. We found that Wolbachia PC tropism is evolutionarily conserved across most Drosophila species, but not in Culex mosquitos. These findings highlight the coordination of endosymbiont tropism with host development and cell differentiation.

Jak-Stat pathway induces Drosophila follicle elongation by a gradient of apical contractility

Tissue elongation and its control by spatiotemporal signals is a major developmental question. Currently, it is thought that Drosophila ovarian follicular epithelium elongation requires the planar polarization of the basal domain cytoskeleton and of the extra-cellular matrix, associated with a dynamic process of rotation around the anteroposterior axis. Here we show, by careful kinetic analysis of fat2 mutants, that neither basal planar polarization nor rotation is required during a first phase of follicle elongation. Conversely, a JAK-STAT signaling gradient from each follicle pole orients early elongation. JAK-STAT controls apical pulsatile contractions, and Myosin II activity inhibition affects both pulses and early elongation. Early elongation is associated with apical constriction at the poles and with oriented cell rearrangements, but without any visible planar cell polarization of the apical domain. Thus, a morphogen gradient can trigger tissue elongation through a control of cell pulsing and without a planar cell polarity requirement.