Jing: a downstream target of slbo required for developmental control of border cell migration

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.

The Drosophila homeodomain transcription factor Homeobrain is involved in the formation of the embryonic protocerebrum and the supraesophageal brain commissure

During the embryonic development of Drosophila melanogaster many transcriptional activators are involved in the formation of the embryonic brain. In our study we show that the transcription factor Homeobrain (Hbn), a member of the 57B homeobox gene cluster, is an additional factor involved in the formation of the embryonic Drosophila brain. Using a Hbn antibody and specific cell type markers a detailed expression analysis during embryonic brain development was conducted. We show that Hbn is expressed in several regions in the protocerebrum, including fibre tract founder cells closely associated with the supraesophageal brain commissure and also in the mushroom bodies. During the formation of the supraesophageal commissure, Hbn and FasII-positive founder cells build an interhemispheric bridge priming the commissure and thereby linking both brain hemispheres. The Hbn expression is restricted to neural but not glial cells in the embryonic brain. In a mutagenesis screen we generated two mutant hbn alleles that both show embryonic lethality. The phenotype of the hbn mutant alleles is characterized by a reduction of the protocerebrum, a loss of the supraesophageal commissure and mushroom body progenitors and also by a dislocation of the optic lobes. Extensive apoptosis correlates with the impaired formation of the embryonic protocerebrum and the supraesophageal commissure. Our results show that Hbn is another important factor for embryonic brain development in Drosophila melanogaster.

Unique Structural Platforms of Suz12 Dictate Distinct Classes of PRC2 for Chromatin Binding

Developmentally regulated accessory subunits dictate PRC2 function. Here, we report the crystal structures of a 120 kDa heterotetrameric complex consisting of Suz12, Rbbp4, Jarid2, and Aebp2 fragments that is minimally active in nucleosome binding and of an inactive binary complex of Suz12 and Rbbp4. Suz12 contains two unique structural platforms that define distinct classes of PRC2 holo complexes for chromatin binding. Aebp2 and Phf19 compete for binding of a non-canonical C2 domain of Suz12; Jarid2 and EPOP occupy an overlapped Suz12 surface required for chromatin association of PRC2. Suz12 and Aebp2 progressively block histone H3K4 binding to Rbbp4, suggesting that Rbbp4 may not be directly involved in PRC2 inhibition by the active H3K4me3 histone mark. Nucleosome binding enabled by Jarid2 and Aebp2 is in part accounted for by the structures, which also reveal that disruption of the Jarid2-Suz12 interaction may underlie the disease mechanism of an oncogenic chromosomal translocation of Suz12.

Circuitous Genetic Regulation Governs a Straightforward Cell Migration

Drosophila border cells undergo a straightforward and stereotypical collective migration during egg development. However, a complex genetic program underlies this process. A variety of approaches, including biochemical, genetic, and imaging strategies have identified many regulatory components, revealing layers of control. This complexity suggests that the active processes of evaluating the environment, remodeling the cytoskeleton, and coordinating movements among cells, demand rapid systems for modulating cell behaviors. Multiple signaling inputs, nodes of integration, and feedback loops act as molecular rheostats to fine-tune gene expression levels and physical responses. Since key genetic regulators of border cell migration have been shown to be required in other types of cell migration, this model system continues to provide an important avenue for genetic discovery.

Tousled-like kinase regulates cytokine-mediated communication between cooperating cell types during collective border cell migration

Tousled-like kinase is required for signaling between polar cells and border cells in the Drosophila ovary, thus controlling their collective migration. Tlk knockdown in polar cells inhibits cytokine expression without affecting polar cell fate or viability. This study shows novel, cell type–specific functions for this ubiquitous nuclear protein.

Collective cell migration is emerging as a major contributor to normal development and disease. Collective movement of border cells in the Drosophila ovary requires cooperation between two distinct cell types: four to six migratory cells surrounding two immotile cells called polar cells. Polar cells secrete a cytokine, Unpaired (Upd), which activates JAK/STAT signaling in neighboring cells, stimulating their motility. Without Upd, migration fails, causing sterility. Ectopic Upd expression is sufficient to stimulate motility in otherwise immobile cells. Thus regulation of Upd is key. Here we report a limited RNAi screen for nuclear proteins required for border cell migration, which revealed that the gene encoding Tousled-like kinase (Tlk) is required in polar cells for Upd expression without affecting polar cell fate. In the absence of Tlk, fewer border cells are recruited and motility is impaired, similar to inhibition of JAK/STAT signaling. We further show that Tlk in polar cells is required for JAK/STAT activation in border cells. Genetic interactions further confirmed Tlk as a new regulator of Upd/JAK/STAT signaling. These findings shed light on the molecular mechanisms regulating the cooperation of motile and nonmotile cells during collective invasion, a phenomenon that may also drive metastatic cancer.

AEBP2 as a transcriptional activator and its role in cell migration

Aebp2 encodes an evolutionarily conserved zinc finger protein that has not been well studied so far, yet recent studies indicated that this gene is closely associated with the Polycomb Repressive Complex 2 (PRC2). Thus, the current study characterized the basic aspects of this gene, including alternative promoters and protein isoforms. According to the results, Aebp2 is controlled through three alternative promoters, deriving three different transcripts encoding the embryonic (32 kDa) and somatic (52 kDa) forms. Chromatin Immuno-Precipitation (ChIP) experiments revealed that AEBP2 binds to its own promoter as well as the promoters of Jarid2 and Snai2. While the embryonic form acts as a transcriptional repressor for Snai2, the somatic form functions as a transcriptional activator for Jarid2, Aebp2 and Snai2. Cell migration assays also demonstrated that the Aebp2 somatic form has an enhancing activity in cell migration. This is consistent with the functional association of Aebp2 with migratory neural crest cells. These results suggest that the two protein isoforms of AEBP2 may have opposite functions for the PcG target genes, and may play significant roles in cell migration during development.

Recruitment and biological consequences of histone modification of H3K27me3 and H3K9me3

Two histone marks, H3K27me3 and H3K9me3, are well known for their repressive roles in the genic and nongenic regions of metazoan genomes. Several protein complexes are known to be responsible for generating these marks, including polycomb repression complex 2 and several H3K9 methylases. Recent studies have shown that the targeting of these histone-modifying complexes within mammalian genomes may be mediated through several DNA-binding proteins, including AEBP2, JARID2, and YY1. In this review, we discuss the potential targeting mechanisms in light of the recent results that have been derived from genome-wide chromatin immunoprecipitation sequencing data and the in vivo functions of these two histone marks in light of the results derived from mouse and human genetic studies.

The zinc finger transcription factor Jing is required for dendrite/axonal targeting in Drosophila antennal lobe development

An important role in olfactory system development is played by transcription factors which act in sensory neurons or in their interneuron targets as cell autonomous regulators of downstream effectors such as cell surface molecules and signalling systems that control neuronal identity and process guidance. Some of these transcriptional regulators have been characterized in detail in the development of the neural elements that innervate the antennal lobe in the olfactory system of Drosophila. Here we identify the zinc finger transcription factor Jing as a cell autonomously acting transcriptional regulator that is required both for dendrite targeting of projection neurons and local interneurons as well as for axonal targeting of olfactory sensory neurons in Drosophila olfactory system development. Immunocytochemical analysis shows that Jing is widely expressed in the neural cells during postembryonic development. MARCM-based clonal analysis of projection neuron and local interneuron lineages reveals a requirement for Jing in dendrite targeting; Jing loss-of-function results in loss of innervation in specific glomeruli, ectopic innervation of inappropriate glomeruli, aberrant profuse dendrite arborisation throughout the antennal lobe, as well as mistargeting to other parts of the CNS. ey-FLP-based MARCM analysis of olfactory sensory neurons reveals an additional requirement for Jing in axonal targeting; mutational inactivation of Jing causes specific mistargeting of some olfactory sensory neuron axons to the DA1 glomerulus, reduction of targeting to other glomeruli, as well as aberrant stalling of axons in the antennal lobe. Taken together, these findings indicate that Jing acts as a key transcriptional control element in wiring of the circuitry in the developing olfactory sensory system in Drosophila.

Group choreography: mechanisms orchestrating the collective movement of border cells

Cell movements are essential for animal development and homeostasis but also contribute to disease. Moving cells typically extend protrusions towards a chemoattractant, adhere to the substrate, contract and detach at the rear. It is less clear how cells that migrate in interconnected groups in vivo coordinate their behaviour and navigate through natural environments. The border cells of the Drosophila melanogaster ovary have emerged as an excellent model for the study of collective cell movement, aided by innovative genetic, live imaging, and photomanipulation techniques. Here we provide an overview of the molecular choreography of border cells and its more general implications.

A core subunit of Polycomb repressive complex 1 is broadly conserved in function but not primary sequence

Polycomb Group (PcG) proteins mediate heritable gene silencing by modifying chromatin structure. An essential PcG complex, PRC1, compacts chromatin and inhibits chromatin remodeling. In Drosophila melanogaster, the intrinsically disordered C-terminal region of PSC (PSC-CTR) mediates these noncovalent effects on chromatin, and is essential for viability. Because the PSC-CTR sequence is poorly conserved, the significance of its effects on chromatin outside of Drosophila was unclear. The absence of folded domains also made it difficult to understand how the sequence of PSC-CTR encodes its function. To determine the mechanistic basis and extent of conservation of PSC-CTR activity, we identified 17 metazoan PSC-CTRs spanning chordates to arthropods, and examined their sequence features and biochemical properties. PSC-CTR sequences are poorly conserved, but are all highly charged and structurally disordered. We show that active PSC-CTRs--which bind DNA tightly and inhibit chromatin remodeling efficiently--are distinguished from less active ones by the absence of extended negatively charged stretches. PSC-CTR activity can be increased by dispersing its contiguous negative charge, confirming the importance of this property. Using the sequence properties defined as important for PSC-CTR activity, we predicted the presence of active PSC-CTRs in additional diverse genomes. Our analysis reveals broad conservation of PSC-CTR activity across metazoans. This conclusion could not have been determined from sequence alignments. We further find that plants that lack active PSC-CTRs instead possess a functionally analogous PcG protein, EMF1. Thus, our study suggests that a disordered domain with dispersed negative charges underlies PRC1 activity, and is conserved across metazoans and plants.

Polycomb repressive complex 2-dependent and -independent functions of Jarid2 in transcriptional regulation in Drosophila

Jarid2 was recently identified as an important component of the mammalian Polycomb repressive complex 2 (PRC2), where it has a major effect on PRC2 recruitment in mouse embryonic stem cells. Although Jarid2 is conserved in Drosophila, it has not previously been implicated in Polycomb (Pc) regulation. Therefore, we purified Drosophila Jarid2 and its associated proteins and found that Jarid2 associates with all of the known canonical PRC2 components, demonstrating a conserved physical interaction with PRC2 in flies and mammals. Furthermore, in vivo studies with Jarid2 mutants in flies demonstrate that among several histone modifications tested, only methylation of histone 3 at K27 (H3K27), the mark implemented by PRC2, was affected. Genome-wide profiling of Jarid2, Su(z)12 (Suppressor of zeste 12), and H3K27me3 occupancy by chromatin immunoprecipitation with sequencing (ChIP-seq) indicates that Jarid2 and Su(z)12 have very similar distribution patterns on chromatin. However, Jarid2 and Su(z)12 occupancy levels at some genes are significantly different, with Jarid2 being present at relatively low levels at many Pc response elements (PREs) of certain Homeobox (Hox) genes, providing a rationale for why Jarid2 was never identified in Pc screens. Gene expression analyses show that Jarid2 and E(z) (Enhancer of zeste, a canonical PRC2 component) are not only required for transcriptional repression but might also function in active transcription. Identification of Jarid2 as a conserved PRC2 interactor in flies provides an opportunity to begin to probe some of its novel functions in Drosophila development.

Aebp2 as an epigenetic regulator for neural crest cells

Aebp2 is a potential targeting protein for the mammalian Polycomb Repression Complex 2 (PRC2). We generated a mutant mouse line disrupting the transcription of Aebp2 to investigate its in vivo roles. Aebp2-mutant homozygotes were embryonic lethal while heterozygotes survived to adulthood with fertility. In developing mouse embryos, Aebp2 is expressed mainly within cells of neural crest origin. In addition, many heterozygotes display a set of phenotypes, enlarged colon and hypopigmentation, similar to those observed in human patients with Hirschsprung's disease and Waardenburg syndrome. These phenotypes are usually caused by the absence of the neural crest-derived ganglia in hindguts and melanocytes. ChIP analyses demonstrated that the majority of the genes involved in the migration and development process of neural crest cells are downstream target genes of AEBP2 and PRC2. Furthermore, expression analyses confirmed that some of these genes are indeed affected in the Aebp2 heterozygotes. Taken together, these results suggest that Aebp2 may regulate the migration and development of the neural crest cells through the PRC2-mediated epigenetic mechanism.

The Drosophila jing gene is a downstream target in the Trachealess/Tango tracheal pathway

Primary branching in the Drosophila trachea is regulated by the Trachealess (Trh) and Tango (Tgo) basic helix-loop-helix-PAS (bHLH-PAS) heterodimers, the POU protein Drifter (Dfr)/Ventral Veinless (Vvl), and the Pointed (Pnt) ETS transcription factor. The jing gene encodes a zinc finger protein also required for tracheal development. Three Trh/Tgo DNA-binding sites, known as CNS midline elements, in 1.5 kb of jing 5′ cis-regulatory sequence (jing1.5) previously suggested a downstream role for jing in the pathway. Here, we show that jing is a direct downstream target of Trh/Tgo and that Vvl and Pnt are also involved in jing tracheal activation. In vivo lacZ enhancer detection assays were used to identify cis-regulatory elements mediating embryonic expression patterns of jing. A 2.8-kb jing enhancer (jing2.8) drove lacZ expression in all tracheal cell lineages, the CNS midline and Engrailed-positive segmental stripes, mimicking endogenous jing expression. A 1.3-kb element within jing2.8 drove expression that was restricted to Engrailed-positive CNS midline cells and segmental ectodermal stripes. Surprisingly, jing1.5-lacZ expression was restricted to tracheal fusion cells despite the presence of consensus DNA-binding sites for bHLH-PAS, ETS, and POU domain transcription factors. Given the absence of Trh/Tgo DNA-binding sites in the jing1.3 enhancer, these results are consistent with previous observations suggesting a combinatorial basis to Trh-/Tgo-mediated transcriptional regulation in the trachea.

Drosophila Jing is part of the breathless fibroblast growth factor receptor positive feedback loop

In the developing Drosophila trachea, extensive cell migration lays the foundation for an elaborate network of tubules to form. This process is controlled by the Drosophila fibroblast growth factor receptor, known as Breathless (Btl), whose expression is activated by the Trachealess (Trh) and Tango (Tgo) basic helix-loop-helix (bHLH)-PAS transcription factors. We previously identified the jing zinc finger transcription factor as a gene sensitive to the dosage of bHLH-PAS transcriptional activity and showed that its mutations interact genetically with those of trh and btl. Here, we demonstrate that jing is required for btl expression in the branching trachea and dominantly interacts with known regulators of btl expression, including the ETS and POU transcription factors, pointed, and drifter/ventral veinless, respectively. Furthermore, the zinc finger-containing C-terminus of Jing associates with a btl tracheal enhancer in a Trh/Tgo-dependent manner in chromatin immunoprecipitation assays in vitro and interferes with btl in vitro and in vivo. Together, our results support a model by which Jing/Trh/Tgo complexes regulate btl transcript levels during primary tracheal branching.

AEBP2 as a potential targeting protein for Polycomb Repression Complex PRC2

AEBP2 is a zinc finger protein that has been shown to interact with the mammalian Polycomb Repression Complex 2 (PRC2). In the current study, we characterized this unknown protein and tested its potential targeting roles for the PRC2. AEBP2 is an evolutionarily well-conserved gene that is found in the animals ranging from flying insects to mammals. The transcription of mammalian AEBP2 is driven by two alternative promoters and produces at least two isoforms of the protein. These isoforms show developmental stage-specific expression patterns: the adult-specific larger form (51 kDa) and the embryo-specific smaller form (32 kDa). The AEBP2 protein binds to a DNA-binding motif with an unusual bipartite structure, CTT(N)15-23cagGCC with lower-case being less critical. A large fraction of AEBP2's target loci also map closely to the known target loci of the PRC2. In fact, many of these loci are co-occupied by the two proteins, AEBP2 and SUZ12. This suggests that AEBP2 is most likely a targeting protein for the mammalian PRC2 complex.

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.

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.

Notch signaling links interactions between the C/EBP homolog slow border cells and the GILZ homolog bunched during cell migration

In the follicle cell (FC) epithelium that surrounds the Drosophila egg, a complex set of cell signals specifies two cell fates that pattern the eggshell: the anterior centripetal FC that produce the operculum and the posterior columnar FC that produce the main body eggshell structure. We have previously shown that the long-range morphogen DPP represses the expression of the bunched (bun) gene in the anterior-most centripetal FC. bun, which encodes a homolog of vertebrate TSC-22/GILZ, in turn represses anterior gene expression and antagonizes Notch signaling to restrict centripetal FC fates in posterior cells. From a screen for novel targets of bun repression we have identified the C/EBP homolog slow border cells (slbo). At stage 10A, slbo expression overlaps bun in anterior FC; by stage 10B they repress each other's expression to establish a sharp slbo/bun expression boundary. The precise position of the slbo/bun expression boundary is sensitive to Notch signaling, which is required for both slbo activation and bun repression. As centripetal migration proceeds from stages 10B-14, slbo represses its own expression and both slbo loss-of-function mutations and overexpression approaches reveal that slbo is required to coordinate centripetal migration with nurse cell dumping. We propose that in anterior FC exposed to a Dpp morphogen gradient, high and low levels of slbo and bun, respectively, are established by modulation of Notch signaling to direct threshold cell fates. Interactions among Notch, slbo and bun resemble a conserved signaling cassette that regulates mammalian adipocyte differentiation.

Glial and neuronal functions of the Drosophila homolog of the human SWI/SNF gene ATR-X (DATR-X) and the jing zinc-finger gene specify the lateral positioning of longitudinal glia and axons

Neuronal-glial communication is essential for constructing the orthogonal axon scaffold in the developing Drosophila central nervous system (CNS). Longitudinal glia (LG) guide extending commissural and longitudinal axons while pioneer and commissural neurons maintain glial survival and positioning. However, the transcriptional regulatory mechanisms controlling these processes are not known. Previous studies showed that the midline function of the jing C2H2-type zinc-finger transcription factor was only partially required for axon scaffold formation in the Drosophila CNS. We therefore screened for gain-of-function enhancers of jing gain of function in the eye and identified the Drosophila homolog of the disease gene of human alpha-thalassemia/mental retardation X-linked (ATR-X) as well as other genes with potential roles in gene expression, translation, synaptic transmission, and cell cycle. jing and DATR-X reporter genes are expressed in both CNS neurons and glia, including the LG. Coexpression of jing and DATR-X in embryonic neurons synergistically affects longitudinal connective formation. During embryogenesis, jing and DATR-X have autonomous and nonautonomous roles in the lateral positioning of LG, neurons, and longitudinal axons as shown by cell-specific knockdown of gene expression. jing and DATR-X are also required autonomously for glial survival. jing and DATR-X mutations show synergistic effects during longitudinal axon formation suggesting that they are functionally related. These observations support a model in which downstream gene expression controlled by a potential DATR-X-Jing complex facilitates cellular positioning and axon guidance, ultimately allowing for proper connectivity in the developing Drosophila CNS.

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.

jing is required for wing development and to establish the proximo-distal axis of the leg in Drosophila melanogaster

The establishment of the proximo-distal (PD) axis in the legs of Drosophila melanogaster requires the expression of a nested set of transcription factors that are activated in discreet domains by secreted signaling molecules. The precise regulation of these transcription factor domains is critical for generating the stereotyped morphological characteristics that exist along the PD axis, such as the positioning of specific bristle types and leg joints. Here we provide evidence that the Zn-finger protein encoded by the gene jing is critical for PD axis formation in the Drosophila legs. Our data suggest that jing represses transcription and that it is necessary to keep the proximal gene homothorax (hth) repressed in the medial domain of the PD axis. We further show that jing is also required for alula and vein development in the adult wing. In the wing, Jing is required to repress another proximal gene, teashirt (tsh), in a small domain that will give rise to the alula. Interestingly, we also demonstrate that two other genes affecting alula development, Alula and elbow, also exhibit tsh derepression in the same region of the wing disc as jing- clones. Finally, we show that jing genetically interacts with several members of the Polycomb (Pc) group of genes during development. Together, our data suggest that jing encodes a transcriptional repressor that may participate in a subset of Pc-dependent activities during Drosophila appendage development.

Differentiation and diversification of follicular cells in polytrophic ovaries of crane flies (Diptera: Nematocera: Tipulomorpha and Trichoceridae)

To gain insight into the evolution of differentiation pathways that are involved in the follicular cells' morphogenesis in dipteran ovaries we have undertaken the comparative morphological analysis of the follicular cell behavior in crane flies, representatives of more ancestral nematocerous flies. This analysis revealed that initially the organization of the follicular epithelium in the species under study shows significant similarities to that reported in the ovaries of true flies (Brachycera), indicating that the ancestors of dipterans must have evolved a common and specific system of the early patterning of their follicular epithelium. On the other hand, in contrast to Drosophila and other advanced brachycerans, the follicular cells in the studied nematoceran ovaries do not exhibit any migratory activity. Instead, they were found to change their relative position but only within the epithelial layer. These "translocations" appeared to depend merely on cell shape changes. Although the "immobility" of the follicular cells in the ovaries of crane flies results in the lower number of their specialized subgroups when compared with the true flies, the functional homology between particular subsets of follicular cells can be postulated. We suggest that the anterior polar cells and the micropyle forming anterior terminal follicular cells in crane fly ovaries have their counterparts in the brachyceran anterior polar cells and border cells, respectively.

Ovarian cancer metastasis: integrating insights from disparate model organisms

Despite considerable efforts to improve early detection, and advances in chemotherapy, metastasis remains a major challenge in the clinical management of ovarian cancer. Studies of new murine models are providing novel insights into the pathophysiology of ovarian cancer, but these models are not readily amenable to genetic screens. Genetic analysis of border-cell migration in the Drosophila melanogaster ovary provides clues that will improve our understanding of ovarian cancer metastasis at the molecular level, and also might lead to potential therapeutic targets.

The jing and ras1 pathways are functionally related during CNS midline and tracheal development

The Drosophila jing gene encodes a zinc finger protein required for the differentiation and survival of embryonic CNS midline and tracheal cells. We show that there is a functional relationship between jing and the Egfr pathway in the developing CNS midline and trachea. jing function is required for Egfr pathway gene expression and MAPK activity in both the CNS midline and trachea. jing over-expression effects phenocopy those of the Egfr pathway and require Egfr pathway function. Activation of the Egfr pathway in loss-of-function jing mutants partially rescues midline cell loss. Egfr pathway genes and jing show dominant genetic interactions in the trachea and CNS midline. Together, these results show that jing regulates signal transduction in developing midline and tracheal cells.

Insect basic leucine zipper proteins and their role in cyclic AMP-dependent regulation of gene expression

The cAMP-protein kinase A (PKA) pathway is an important intracellular signal transduction cascade that can be activated by a large variety of stimuli. Activation or inhibition of this pathway will ultimately affect the transcriptional regulation of various genes through distinct responsive sites. In vertebrates, the best- characterized nuclear targets of PKA are the cyclic AMP response element-binding (CREB) proteins. It is now well established that CREB is not only regulated by PKA, but many other kinases can exert an effect as well. Since CREB-like proteins were also discovered in invertebrates, several studies unraveling their physiological functions in this category of metazoans have been performed. This review will mainly focus on the presence and regulation of CREB proteins in insects. Differences in transcriptional responses to the PKA pathway and other CREB-regulating stimuli between cells, tissues, and even organisms can be partially attributed to the presence of different CREB isoforms. In addition, the regulation of CREB appears to show some important differences between insects and vertebrates. Since CREB is a basic leucine zipper (bZip) protein, other insect members of this important family of transcriptional regulators will be briefly discussed as well.

Differentiation and diversification of the follicular cells in flies: insight from the studies of the lower brachycerans' ovaries

Although all dipteran species have ovaries of the same meroistic-polytrophic type, the structure of individual ovarian follicles (egg chambers) as well as the course of oogenesis in major dipteran taxa are highly diversified and often significantly different from the widely known Drosophila model. In this report we present results of the morphological studies of the ovary structure in the representatives of three families of lower brachycerans (Orthorrhapha) and compare them with the present knowledge of the processes that lead to the formation of a mature egg cell in the model dipteran, the cyclorrhaphan fruit fly, Drosophila melanogaster. The most conspicuous and developmentally significant differences between Drosophila and lower brachycerans were found in the events that accompany the differentiation and diversification of somatic follicular cells. Our observations indicate that the directed migrations of some follicular cells within the egg chamber and the ability of border cells to invade the nurse cell compartment can be considered as evolutionary novelties that evolved in the ancestors of higher brachycerans.

Border-cell migration: the race is on

The conversion of stationary epithelial cells into migratory, invasive cells is important for normal embryonic development and tumour metastasis. Border-cell migration in the ovary of Drosophila melanogaster has emerged as a simple, genetically tractable model for studying this process. Three distinct signals, which are also upregulated in cancer, control border-cell migration, so identifying further genes that are involved in border-cell migration could provide new insights into tumour invasion.

Retinoic acid modulates the radiosensitivity of head-and-neck squamous carcinoma cells grown in collagen gel

PURPOSE

Collagen gels are increasingly regarded as reliable scaffolds for studying cells in vitro, displaying the same three-dimensional network of collagen fibers as encountered in vivo. As a contribution to therapeutic control of head-and-neck cancer, we grew HSCO86 cells in collagen gel and assessed their behavior in the presence of retinoic acid (RA) and radiation.

METHODS AND MATERIALS

The malignant epithelial cell line HSCO86 was isolated from a postirradiation human oropharyngeal squamous carcinoma; it was EGFR-negative by immunocytochemical criteria. The cells were embedded in hydrated collagen I at a density of 10(6) cells/mL, and on Days 8, 10, and 12 of culture, they were treated with 10(-5) M retinoic acid. Radiation was administered using two different schedules: simultaneously with RA in three daily doses totaling 10 Gy, or with a single dose of 8 Gy on Day 29 of culture, after the effects of RA had taken place. Cell proliferation was evaluated by the MTT assay, whereas morphometric characteristics were detected in the cultured gels directly or in the gels after they were fixed and stained with hematoxylin.

RESULTS

Contrary to growth in monolayer, where HSCO86 cells displayed a high proliferation rate, in collagen gel only a tiny fraction of the cells, usually less than 0.02%, survived the environmental stress; these cells spontaneously organized themselves into clonal multicellular spheroids growing up to 0.8 mm in diameter. After exposure to 10(-5) M retinoic acid, cell proliferation first declined and then, about 15 days after treatment, it started to increase to a level far above that in the control group. This surge in proliferation was ascribed to the appearance of numerous fibroblast-like cells at the edge of the spheroids. These cells, called HSCO-F, were the result of epithelial-to-mesenchymal conversion. When the gels were disaggregated by collagenase, and the cells were seeded in monolayer, HSCO-F cells reversed their morphology into parental HSCO86 cells. Treatment of collagen gels with 10 Gy, fractionated in three daily doses, did not substantially affect the growth of HSCO86 spheroids. However, when radiation was given simultaneously with RA, cell growth was significantly inhibited, both in terms of cell proliferation and size of spheroids (p < 0.0001 vs. untreated controls). This synergism applied mainly to parental HSCO86 cells, because no significant damage was induced by radiation on the HSCO-F cells previously generated by treatment with RA.

CONCLUSION

Differences in the radiosensitivity of HSCO86 and HSCO-F cells are surprising in view of their common origin; this suggests a scenario in which, to overcome a microenvironmental stress, head-and-neck carcinoma cells can temporarily shift from an epithelial to a mesenchymal phenotype. In particular, morphologic and functional data suggested that HSCO-F cells were transformed into vascular endothelial cells whose characteristics included the following: (1) distinctive expression of Factor VIII and beta(1)-integrin, not detected in parental HSCO86 cells; (2) active migration in the collagen network by extruded pseudopodia, frequently appearing as colonies of filamentous cells aligned along the radial axis of the spheroids; and (3) efficient contraction of floating collagen gels. The implication of our study is that head-and-neck carcinomas may respond to RA treatment by selecting cell populations both resistant to radiation and capable of migrating inside the connective tissue, mimicking the behavior of vascular capillaries.

Initiating and guiding migration: lessons from border cells

Cell migration occurs in many different contexts. Amoebae and other isolated cells migrate in culture. In animals, 'professional' migratory cells of the immune system constantly survey the body for intruders, whereas other cell types perform specific developmentally regulated migrations. One simple model for the latter type of event is migration of border cells during Drosophila oogenesis. Recent findings have shed light on how border cell fate is induced and on how the migration is guided. This article discusses the implications of these studies and compares (invasive) migration through a tissue with what is known about cells crawling on a flat substratum.

The art and design of genetic screens: Drosophila melanogaster

The success of Drosophila melanogaster as a model organism is largely due to the power of forward genetic screens to identify the genes that are involved in a biological process. Traditional screens, such as the Nobel-prize-winning screen for embryonic-patterning mutants, can only identify the earliest phenotype of a mutation. This review describes the ingenious approaches that have been devised to circumvent this problem: modifier screens, for example, have been invaluable for elucidating signal-transduction pathways, whereas clonal screens now make it possible to screen for almost any phenotype in any cell at any stage of development.

The JAK/STAT pathway is required for border cell migration during Drosophila oogenesis

During Drosophila oogenesis, border cells perform a stereotypic migration. Slbo, a C/EBP transcription factor, is required for this migration. Here we identify Drosophila Stat92E (signal transducer and activator of transcription) in a screen for gain-of-function suppressors of the slbo mutant phenotype. By clonal analysis for Stat92E and hop (Drosophila JAK) mutants we find that the JAK/STAT pathway is required in border cells for their migration. The activating ligand for the pathway, Unpaired, is expressed in polar cells. Polar cells are specialized cells which can induce border cell fate in anterior follicle cells. On its own, ectopic expression of Unpaired can induce ectopic expression of border cell markers including Slbo. However, Stat92E mutant cells still express normal levels of Slbo protein, thus Stat92E must regulate other targets critical for border cell migration.

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

The JAK/STAT signaling pathway, renowned for its effects on cell proliferation and survival, is constitutively active in various human cancers, including ovarian. We have found that JAK and STAT are required to convert the border cells in the Drosophila ovary from stationary, epithelial cells to migratory, invasive cells. The ligand for this pathway, Unpaired (UPD), is expressed by two central cells within the migratory cell cluster. Mutations in upd or jak cause defects in migration and a reduction in the number of cells recruited to the cluster. Ectopic expression of either UPD or JAK is sufficient to induce extra epithelial cells to migrate. Thus, a localized signal activates the JAK/STAT pathway in neighboring epithelial cells, causing them to become invasive.

Guidance of cell migration by the Drosophila PDGF/VEGF receptor

Directed cell migrations are important for development, but the signaling pathways and mechanisms responsible for guiding cell migration in vivo are poorly understood. Migration of border cells during Drosophila oogenesis is a simple and attractive model system in which to address these questions. We demonstrate that PVR, a receptor tyrosine kinase related to mammalian PDGF and VEGF receptors, acts in border cells to guide them to the oocyte. The oocyte is the source of a ligand for PVR, PDGF/VEGF factor 1 (PVF1). Intriguingly, the guidance function of PVR is largely redundant with that of EGFR. We present evidence implicating Rac and the Rac activator Mbc/DOCK180/CED-5 as mediators of the guidance signal.

Cell migration in invertebrates: clues from border and distal tip cells

Recent studies in two invertebrate systems, border cells in Drosophila melanogaster and distal tip cells in Caenorhabditis elegans, have provided important insight into the mechanisms of directed cell migration. These migrating cells are guided by extracellular signals, such as EGF, TGF-beta and netrin. In addition, metalloproteases alter the extracellular matrix of the tissue through which these cells migrate. Along the migratory path, migrating cells respond to changes in guidance signals by altering the expression of receptor signaling pathways. Finally, Dock180, CrkII and the GTPase Rac link the extracellular signals to the cellular machinery that controls cell motility.

Command and control: regulatory pathways controlling invasive behavior of the border cells

The invasiveness of cancer cells resembles the normal behavior of cells that migrate into surrounding tissues during development. For example, the border cells in the Drosophila ovary undergo a partial epithelial to mesenchymal transition and invade the neighboring cluster of germline cells, migrating to the oocyte border. Once there, they provide patterning information to the oocyte and produce an eggshell specialization known as the micropyle. Border cell migration has been subjected to extensive genetic analyses using a variety of screening approaches. Recent findings demonstrate that conversion of the border cells from a stationary group of epithelial cells to invasive cells requires integration of the activities of at least two transcriptional regulatory pathways. One such pathway requires the slbo gene, which encodes Drosophila C/EBP, a basic region/leucine zipper transcriptional activator that is required for elevated expression of a number of downstream targets, including DE-cadherin and focal adhesion kinase (FAK). An independent pathway requires the activity of the ecdysone receptor and a recently identified co-activator for the ecdysone receptor known as Taiman (abbreviated TAI, pronounced ti-maan', meaning too slow). Ecdysone is produced in the Drosophila ovary in response to adequate nutrition and is required for progression of oogenesis through stage 9, when border cell migration occurs. Border cells mutant for tai accumulate abnormally high levels of adhesion complexes at their surfaces, which may account for their inability to migrate. Thus border cell migration requires a differentiation program mediated by the C/EBP pathway, which is required for elevated expression of a number of proteins required for motility. In addition, migration requires a hormonal signal that relays information regarding nutritional status and appears to be required for regulation of the proper localization of some of the C/EBP targets. These findings suggest that steroid hormones can regulate cell motility relatively directly, independent of the effects on proliferation. This may contribute to the metastatic effects of steroid hormones on certain cancers and the inhibition of metastasis by steroid hormone antagonists such as tamoxifen.