Cell-cell communication and axis specification in the Drosophila oocyte

A maternal dorsoventral prepattern revealed by an asymmetric distribution of ventralizing molecules before fertilization in Xenopus laevis

The establishment of the embryonic dorsoventral axis in Xenopus occurs when the radial symmetry around the egg's animal-vegetal axis is broken to give rise to the typical symmetry of Bilaterians. We have previously shown that the Notch1 protein is ventrally enriched during early embryogenesis in Xenopus laevis and zebrafish and exerts ventralizing activity through β-Catenin destabilization and the positive regulation of ventral center genes in X. laevis. These findings led us to further investigate when these asymmetries arise. In this work, we show that the asymmetrical distribution of Notch1 protein and mRNA precedes cortical rotation and even fertilization in X. laevis. Moreover, we found that in unfertilized eggs transcripts encoded by the ventralizing gene bmp4 are also asymmetrically distributed in the animal hemisphere and notch1 transcripts accumulate consistently on the same side of the eccentric maturation point. Strikingly, a Notch1 asymmetry orthogonal to the animal-vegetal axis appears during X. laevis oogenesis. Thus, we show for the first time a maternal bias in the distribution of molecules that are later involved in ventral patterning during embryonic axialization, strongly supporting the hypothesis of a dorsoventral prepattern or intrinsic bilaterality of Xenopus eggs before fertilization.

Follicle cell contact maintains main body axis polarity in the Drosophila melanogaster oocyte

In Drosophila melanogaster, the anterior-posterior body axis is maternally established and governed by differential localization of partitioning defective (Par) proteins within the oocyte. At mid-oogenesis, Par-1 accumulates at the oocyte posterior end, while Par-3/Bazooka is excluded there but maintains its localization along the remaining oocyte cortex. Past studies have proposed the need for somatic cells at the posterior end to initiate oocyte polarization by providing a trigger signal. To date, neither the molecular identity nor the nature of the signal is known. Here, we provide evidence that mechanical contact of posterior follicle cells (PFCs) with the oocyte cortex causes the posterior exclusion of Bazooka and maintains oocyte polarity. We show that Bazooka prematurely accumulates exclusively where posterior follicle cells have been mechanically detached or ablated. Furthermore, we provide evidence that PFC contact maintains Par-1 and oskar mRNA localization and microtubule cytoskeleton polarity in the oocyte. Our observations suggest that cell-cell contact mechanics modulates Par protein binding sites at the oocyte cortex.

Eya-controlled affinity between cell lineages drives tissue self-organization during Drosophila oogenesis

Cooperative morphogenesis of cell lineages underlies the development of functional units and organs. To study mechanisms driving the coordination of lineages, we investigated soma-germline interactions during oogenesis. From invertebrates to vertebrates, oocytes develop as part of a germline cyst that consists of the oocyte itself and so-called nurse cells, which feed the oocyte and are eventually removed. The enveloping somatic cells specialize to facilitate either oocyte maturation or nurse cell removal, which makes it essential to establish the right match between germline and somatic cells. We uncover that the transcriptional regulator Eya, expressed in the somatic lineage, controls bilateral cell-cell affinity between germline and somatic cells in Drosophila oogenesis. Employing functional studies and mathematical modelling, we show that differential affinity and the resulting forces drive somatic cell redistribution over the germline surface and control oocyte growth to match oocyte and nurse cells with their respective somatic cells. Thus, our data demonstrate that differential affinity between cell lineages is sufficient to drive the complex assembly of inter-lineage functional units and underlies tissue self-organization during Drosophila oogenesis.

Cell-to-Cell Interactions during Early Drosophila Oogenesis: An Ultrastructural Analysis

Drosophila oogenesis requires the subsequent growth of distinct egg chambers each containing a group of sixteen germline cells surrounded by a simple epithelium of follicle cells. The oocyte occupies a posterior position within the germ cells, thus giving a distinct asymmetry to the egg chamber. Although this disposition is critical for the formation of the anterior–posterior axis of the embryo, the interplay between somatic and germ cells during the early stages of oogenesis remains an open question. We uncover by stage 2, when the egg chambers leaved the germarium, some unique spatial interactions between the posterior follicle cells and the oocyte. These interactions are restricted to the surface of the oocyte over the centriole cluster that formed during early oogenesis. Moreover, the posterior follicle cells in front of the oocyte display a convoluted apical membrane with extensive contacts, whereas the other follicle cells have a flat apical surface without obvious surface protrusions. In addition, the germ cells located at the posterior end of the egg chamber have very elongated protrusions that come into contact with each other or with facing follicle cells. These observations point to distinct polarization events during early oogenesis supporting previous molecular data of an inherent asymmetry between the anterior and the posterior regions of the egg chambers.

Oncogenic Notch Triggers Neoplastic Tumorigenesis in a Transition-Zone-like Tissue Microenvironment

During the initial stages of tumorigenesis, the tissue microenvironment where the pro-tumor cells reside plays a crucial role in determining the fate of these cells. Transition zones, where two types of epithelial cells meet, are high-risk sites for carcinogenesis, but the underlying mechanism remains largely unclear. Here, we show that persistent upregulation of Notch signaling induces neoplastic tumorigenesis in a transition zone between the salivary gland imaginal ring cells and the giant cells in Drosophila larvae. In this region, local endogenous JAK-STAT and JNK signaling creates a tissue microenvironment that is susceptible to oncogenic-Notch-induced tumorigenesis, whereas the rest of the salivary gland imaginal ring is refractory to Notch-induced tumor transformation. JNK signaling activates a matrix metalloprotease (MMP1) to promote Notch-induced tumorigenesis at the transition zone. These findings illustrate the significance of local endogenous inflammatory signaling in primary tumor formation.

Centrosomal and Non-Centrosomal Microtubule-Organizing Centers (MTOCs) in Drosophila melanogaster

The centrosome is the best-understood microtubule-organizing center (MTOC) and is essential in particular cell types and at specific stages during Drosophila development. The centrosome is not required zygotically for mitosis or to achieve full animal development. Nevertheless, centrosomes are essential maternally during cleavage cycles in the early embryo, for male meiotic divisions, for efficient division of epithelial cells in the imaginal wing disc, and for cilium/flagellum assembly in sensory neurons and spermatozoa. Importantly, asymmetric and polarized division of stem cells is regulated by centrosomes and by the asymmetric regulation of their microtubule (MT) assembly activity. More recently, the components and functions of a variety of non-centrosomal microtubule-organizing centers (ncMTOCs) have begun to be elucidated. Throughout Drosophila development, a wide variety of unique ncMTOCs form in epithelial and non-epithelial cell types at an assortment of subcellular locations. Some of these cell types also utilize the centrosomal MTOC, while others rely exclusively on ncMTOCs. The impressive variety of ncMTOCs being discovered provides novel insight into the diverse functions of MTOCs in cells and tissues. This review highlights our current knowledge of the composition, assembly, and functional roles of centrosomal and non-centrosomal MTOCs in Drosophila.

Nucleus positioning within Drosophila egg chamber

Both types of Drosophila egg chamber germ cells, i.e. oocyte and nurse cells, have to control their nucleus positions in order to produce a viable gamete. Interestingly, while actin microfilaments are crucial to position the nuclei in nurse cells, these are the microtubules that are important for oocyte nucleus to migrate and adopt the correct position. In this review, we discuss the mechanisms underlying these positioning processes in the two cell types with respect to the organization and dynamics of the actin and microtubule skeleton. In the nurse cells it is essential to keep firmly the nuclei in a central position to prevent them from obstructing the ring canals when the cytoplasmic content of the cells is dumped into the oocyte cells toward the end of oogenesis. This is achieved by the assembly of thick filopodia-like actin cables anchored to the plasma membrane, which grow inwardly and eventually encase tightly the nuclei in a cage-like structure. In the oocyte, the migration at an early stage of oogenesis of the nucleus from a posterior location to an anchorage site at an asymmetric anterior position, is an essential step in the setting up of the dorsoventral polarity axis of the future embryo. This process is controlled by an interplay between MT networks that just start to be untangled. Although both mechanisms have evolved to fulfill cell-type specific cell processes in the context of fly oogenesis, interesting parallels can be drawn with other nuclear positioning mechanisms in the mouse oocyte and the developing muscle respectively.

Somatic insulin signaling regulates a germline starvation response in Drosophila egg chambers

Egg chambers from starved Drosophila females contain large aggregates of processing (P) bodies and cortically enriched microtubules. As this response to starvation is rapidly reversed upon re-feeding females or culturing egg chambers with exogenous bovine insulin, we examined the role of endogenous insulin signaling in mediating the starvation response. We found that systemic Drosophila insulin-like peptides (dILPs) activate the insulin pathway in follicle cells, which then regulate both microtubule and P body organization in the underlying germline cells. This organization is modulated by the motor proteins Dynein and Kinesin. Dynein activity is required for microtubule and P body organization during starvation, while Kinesin activity is required during nutrient-rich conditions. Blocking the ability of egg chambers to form P body aggregates in response to starvation correlated with reduced progeny survival. These data suggest a potential mechanism to maximize fecundity even during periods of poor nutrient availability, by mounting a protective response in immature egg chambers.

Positional correlative anatomy of invertebrate model organisms increases efficiency of TEM data production

Transmission electron microscopy (TEM) is an important tool for studies in cell biology, and is essential to address research questions from bacteria to animals. Recent technological innovations have advanced the entire field of TEM, yet classical techniques still prevail for most present-day studies. Indeed, the majority of cell and developmental biology studies that use TEM do not require cutting-edge methodologies, but rather fast and efficient data generation. Although access to state-of-the-art equipment is frequently problematic, standard TEM microscopes are typically available, even in modest research facilities. However, a major unmet need in standard TEM is the ability to quickly prepare and orient a sample to identify a region of interest. Here, I provide a detailed step-by-step method for a positional correlative anatomy approach to flat-embedded samples. These modifications make the TEM preparation and analytic procedures faster and more straightforward, supporting a higher sampling rate. To illustrate the modified procedures, I provide numerous examples addressing research questions in Caenorhabditis elegans and Drosophila. This method can be equally applied to address questions of cell and developmental biology in other small multicellular model organisms.

Regulation of broad by the Notch pathway affects timing of follicle cell development

During Drosophila oogenesis, activation of Notch signaling in the follicular epithelium (FE) around stage 6 of oogenesis is essential for entry into the endocycle and a series of other changes such as cell differentiation and migration of subsets of the follicle cells. Notch induces the expression of zinc finger protein Hindsight and suppresses homeodomain protein Cut to regulate the mitotic/endocycle (ME) switch. Here we report that broad (br), encoding a small group of zinc-finger transcription factors resulting from alternative splicing, is a transcriptional target of Notch nuclear effector Suppressor of Hairless (Su(H)). The early pattern of Br in the FE, uniformly expressed except in the polar cells, is established by Notch signaling around stage 6, through the binding of Su(H) to the br early enhancer (brE) region. Mutation of the Su(H) binding site leads to a significant reduction of brE reporter expression in follicle cells undergoing the endocycle. Chromatin immunoprecipitation results further confirm Su(H) binding to the br early enhancer. Consistent with its expression in follicle cells during midoogenesis, loss of br function results in a delayed entry into the endocycle. Our findings suggest an important role of br in the timing of follicle cell development, and its transcriptional regulation by the Notch pathway.

Phenotypic rescue of a Drosophila model of mitochondrial ANT1 disease

A point mutation in the Drosophila gene that codes for the major adult isoform of adenine nuclear translocase (ANT) represents a model for human diseases that are associated with ANT insufficiency [stress-sensitive B¹ (sesB¹)]. We characterized the organismal, bioenergetic and molecular phenotype of sesB¹ flies then tested strategies to compensate the mutant phenotype. In addition to developmental delay and mechanical-stress-induced seizures, sesB¹ flies have an impaired response to sound, defective male courtship, female sterility and curtailed lifespan. These phenotypes, excluding the latter two, are shared with the mitoribosomal protein S12 mutant, tko^25t. Mitochondria from sesB¹ adults showed a decreased respiratory control ratio and downregulation of cytochrome oxidase. sesB¹ adults exhibited ATP depletion, lactate accumulation and changes in gene expression that were consistent with a metabolic shift towards glycolysis, characterized by activation of lactate dehydrogenase and anaplerotic pathways. Females also showed downregulation of many genes that are required for oogenesis, and their eggs, although fertilized, failed to develop to the larval stages. The sesB¹ phenotypes of developmental delay and mechanical-stress-induced seizures were alleviated by an altered mitochondrial DNA background. Female sterility was substantially rescued by somatic expression of alternative oxidase (AOX) from the sea squirt Ciona intestinalis, whereas AOX did not alleviate developmental delay. Our findings illustrate the potential of different therapeutic strategies for ANT-linked diseases, based on alleviating metabolic stress.

Genetics and mechanisms of ovarian cancer: parallels between Drosophila and humans

Considering the degree of detail available at the genetic and cellular levels, the Drosophila ovary stands out as a powerful system to identify new players in the regulation of key aspects of cancer progression. In this review, we will comment on how the use of the Drosophila ovary has helped to elucidate some of the molecular bases of ovarian malignancies and to identify and characterize critical tumour suppressor genes and oncogenes with an impact in human pathologies.

The lack of autophagy triggers precocious activation of Notch signaling during Drosophila oogenesis

BACKGROUND

The proper balance of autophagy, a lysosome-mediated degradation process, is indispensable for oogenesis in Drosophila. We recently demonstrated that egg development depends on autophagy in the somatic follicle cells (FC), but not in the germline cells (GCs). However, the lack of autophagy only affects oogenesis when FCs are autophagy-deficient but GCs are wild type, indicating that a dysfunctional signaling between soma and germline may be responsible for the oogenesis defects. Thus, autophagy could play an essential role in modulating signal transduction pathways during egg development.

RESULTS

Here, we provide further evidence for the necessity of autophagy during oogenesis and demonstrate that autophagy is especially required in subsets of FCs. Generation of autophagy-deficient FCs leads to a wide range of phenotypes that are similar to mutants with defects in the classical cell-cell signaling pathways in the ovary. Interestingly, we observe that loss of autophagy leads to a precocious activation of the Notch pathway in the FCs as monitored by the expression of Cut and Hindsight, two downstream effectors of Notch signaling.

CONCLUSION

Our findings point to an unexpected function for autophagy in the modulation of the Notch signaling pathway during Drosophila oogenesis and suggest a function for autophagy in proper receptor activation. Egg development is affected by an imbalance of autophagy between signal sending (germline) and signal receiving cell (FC), thus the lack of autophagy in the germline is likely to decrease the amount of active ligand and accordingly compensates for increased signaling in autophagy-defective follicle cells.

Efficient EGFR signaling and dorsal-ventral axis patterning requires syntaxin dependent Gurken trafficking

Vesicle trafficking plays a crucial role in the establishment of cell polarity in various cellular contexts, including axis-pattern formation in the developing egg chamber of Drosophila. The EGFR ligand, Gurken (Grk), is first localized at the posterior of young oocytes for anterior-posterior axis formation and later in the dorsal anterior region for induction of the dorsal-ventral (DV) axis, but regulation of Grk localization by membrane trafficking in the oocyte remains poorly understood. Here, we report that Syntaxin 1A (Syx1A) is required for efficient trafficking of Grk protein for DV patterning. We show that Syx1A is associated with the Golgi membrane and is required for the transportation of Grk-containing vesicles along the microtubules to their dorsal anterior destination in the oocyte. Our studies reveal that the Syx1A dependent trafficking of Grk protein is required for efficient EGFR signaling during DV patterning.

Drosophila egg chamber elongation: insights into how tissues and organs are shaped

As tissues and organs are formed, they acquire a specific shape that plays an integral role in their ability to function properly. A relatively simple system that has been used to examine how tissues and organs are shaped is the formation of an elongated Drosophila egg. While it has been known for some time that Drosophila egg elongation requires interactions between a polarized intracellular basal actin network and a polarized extracellular network of basal lamina proteins, how these interactions contribute to egg elongation remained unclear. Recent studies using live imaging have revealed two novel processes, global tissue rotation and oscillating basal actomyosin contractions, which have provided significant insight into how the two polarized protein networks cooperate to produce an elongated egg. This review summarizes the proteins involved in Drosophila egg elongation and how this recent work has contributed to our current understanding of how egg elongation is achieved.

A very simple mode of follicular cell diversification in Euborellia fulviceps (Dermaptera, Anisolabididae) involves actively migrating cells

The ovaries of Euborellia fulviceps are composed of five elongated ovarioles of meroistic-polytrophic type. The individual ovariole has three discernible regions: the terminal filament, germarium, and vitellarium. The terminal filament is a stalk of flattened, disc-shaped somatic cells. In the germarium, germline cells in subsequent stages of differentiation are located, and the vitellarium comprises numerous ovarian follicles arranged linearly. The individual ovarian follicles within the vitellarium are separated by prominent interfollicular stalks. The follicles are composed by two germline cells only: an oocyte and a single, polyploid nurse cell, which are surrounded by a monolayer of somatic follicular cells (FCs). During subsequent stages of oogenesis, initially uniform follicular epithelium begins to diversify into morphologically and physiologically distinct subpopulations. In E. fulviceps, the FC diversification mode is rather simple and leads to the formation of only three different FC subpopulations: (1) cuboidal FCs covering the oocyte, (2) stretched FCs surrounding the nurse cell and (3) FCs actively migrating between oocyte and a nurse cell. We found that FCs from the latter subpopulation send long and thin filopodium-like and microtubule-rich processes penetrating between the oocyte and nurse cell membranes. This suggests that, in E. fulviceps, cells from at least one FCs subpopulation show the ability to change position within an ovarian follicle by means of active migration.

At the crossroads of differentiation and proliferation: precise control of cell-cycle changes by multiple signaling pathways in Drosophila follicle cells

Here, we discuss the findings to date about genes and pathways required for regulation of somatic follicle-cell proliferation and differentiation during Drosophila oogenesis and demonstrate how loss of these genes contributes to the tumorigenic potential of mutant cells. Follicle cells undergo cell-fate determination through stepwise activation of multiple signaling pathways, including the Notch, Hedgehog, Wingless, janus kinase/STAT, and JNK pathways. In addition, changes in DNA replication and cellular growth depend on the spatial and temporal activation of the mitotic cycle-endocycle and endocycle-gene amplification cell-cycle switches and insulin-dependent monitoring of cellular health; systemic loss of these pathways contributes to loss of controlled cellular proliferation, loss of differentiation/growth, and aberrant cell polarity in follicle cells. We also highlight the effects of the neoplastic and Hippo pathways on the cell cycle and cellular proliferation in promoting normal development and conclude that lack of coordination of multiple signaling pathways promotes conditions favorable for tumorigenesis.

Regulation of somatic myosin activity by protein phosphatase 1β controls Drosophila oocyte polarization

The Drosophila body axes are established in the oocyte during oogenesis. Oocyte polarization is initiated by Gurken, which signals from the germline through the epidermal growth factor receptor (Egfr) to the posterior follicle cells (PFCs). In response the PFCs generate an unidentified polarizing signal that regulates oocyte polarity. We have identified a loss-of-function mutation of flapwing, which encodes the catalytic subunit of protein phosphatase 1β (PP1β) that disrupts oocyte polarization. We show that PP1β, by regulating myosin activity, controls the generation of the polarizing signal. Excessive myosin activity in the PFCs causes oocyte mispolarization and defective Notch signaling and endocytosis in the PFCs. The integrated activation of JAK/STAT and Egfr signaling results in the sensitivity of PFCs to defective Notch. Interestingly, our results also demonstrate a role of PP1β in generating the polarizing signal independently of Notch, indicating a direct involvement of somatic myosin activity in axis formation.

Microfluidic devices for studying heterotypic cell-cell interactions and tissue specimen cultures under controlled microenvironments

Microfluidic devices allow for precise control of the cellular and noncellular microenvironment at physiologically relevant length- and time-scales. These devices have been shown to mimic the complex in vivo microenvironment better than conventional in vitro assays, and allow real-time monitoring of homotypic or heterotypic cellular interactions. Microfluidic culture platforms enable new assay designs for culturing multiple different cell populations and∕or tissue specimens under controlled user-defined conditions. Applications include fundamental studies of cell population behaviors, high-throughput drug screening, and tissue engineering. In this review, we summarize recent developments in this field along with studies of heterotypic cell-cell interactions and tissue specimen culture in microfluidic devices from our own laboratory.

The microRNA pathway regulates the temporal pattern of Notch signaling in Drosophila follicle cells

Multicellular development requires the correct spatial and temporal regulation of cell division and differentiation. These processes are frequently coordinated by the activities of various signaling pathways such as Notch signaling. From a screen for modifiers of Notch signaling in Drosophila we have identified the RNA helicase Belle, a recently described component of the RNA interference pathway, as an important regulator of the timing of Notch activity in follicle cells. We found that loss of Belle delays activation of Notch signaling, which results in delayed follicle cell differentiation and defects in the cell cycle. Because mutations in well-characterized microRNA components phenocopied the Notch defects observed in belle mutants, Belle might be functioning in the microRNA pathway in follicle cells. The effect of loss of microRNAs on Notch signaling occurs upstream of Notch cleavage, as expression of the constitutively active intracellular domain of Notch in microRNA-defective cells restored proper activation of Notch. Furthermore, we present evidence that the Notch ligand Delta is an important target of microRNA regulation in follicle cells and regulates the timing of Notch activation through cis inhibition of Notch. Here we have uncovered a complex regulatory process in which the microRNA pathway promotes Notch activation by repressing Delta-mediated inhibition of Notch in follicle cells.

Autophagy in Drosophila ovaries is induced by starvation and is required for oogenesis

Autophagy, an evolutionarily conserved lysosome-mediated degradation, promotes cell survival under starvation and is controlled by insulin/target of rapamycin (TOR) signaling. In Drosophila, nutrient depletion induces autophagy in the fat body. Interestingly, nutrient availability and insulin/TOR signaling also influence the size and structure of Drosophila ovaries, however, the role of nutrient signaling and autophagy during this process remains to be elucidated. Here, we show that starvation induces autophagy in germline cells (GCs) and in follicle cells (FCs) in Drosophila ovaries. This process is mediated by the ATG machinery and involves the upregulation of Atg genes. We further demonstrate that insulin/TOR signaling controls autophagy in FCs and GCs. The analysis of chimeric females reveals that autophagy in FCs, but not in GCs, is required for egg development. Strikingly, when animals lack Atg gene function in both cell types, ovaries develop normally, suggesting that the incompatibility between autophagy-competent GCs and autophagy-deficient FCs leads to defective egg development. As egg morphogenesis depends on a tightly linked signaling between FCs and GCs, we propose a model in which autophagy is required for the communication between these two cell types. Our data establish an important function for autophagy during oogenesis and contributes to the understanding of the role of autophagy in animal development.

Symmetry breaking during Drosophila oogenesis

The orthogonal axes of Drosophila are established during oogenesis through a hierarchical series of symmetry-breaking steps, most of which can be traced back to asymmetries inherent in the architecture of the ovary. Oogenesis begins with the formation of a germline cyst of 16 cells connected by ring canals. Two of these 16 cells have four ring canals, whereas the others have fewer. The first symmetry-breaking step is the selection of one of these two cells to become the oocyte. Subsequently, the germline cyst becomes surrounded by somatic follicle cells to generate individual egg chambers. The second symmetry-breaking step is the posterior positioning of the oocyte within the egg chamber, a process mediated by adhesive interactions with a special group of somatic cells. Posterior oocyte positioning is accompanied by a par gene-dependent repolarization of the microtubule network, which establishes the posterior cortex of the oocyte. The next two steps of symmetry breaking occur during midoogenesis after the volume of the oocyte has increased about 10-fold. First, a signal from the oocyte specifies posterior follicle cells, polarizing a symmetric prepattern present within the follicular epithelium. Second, the posterior follicle cells send a signal back to the oocyte, which leads to a second repolarization of the oocyte microtubule network and the asymmetric migration of the oocyte nucleus. This process again requires the par genes. The repolarization of the microtubule network results in the transport of bicoid and oskar mRNAs, the anterior and posterior determinants, respectively, of the embryonic axis, to opposite poles of the oocyte. The asymmetric positioning of the oocyte nucleus defines a cortical region of the oocyte where gurken mRNA is localized, thus breaking the dorsal-ventral symmetry of the egg and embryo.

poly is required for nurse-cell chromosome dispersal and oocyte polarity in Drosophila

During Drosophila oogenesis, nurse cells undergo changes in chromosomal morphology, first from the polytenic form to a transient condensed phase known as the five-blob configuration, then into a diffuse polytenic-polyploid state for the remainder of oogenesis. The mechanism by which nurse-cell chromosome dispersal is regulated remains elusive. Mutations in several genes, including the heterogeneous ribonucleoprotein genes squid (sqd) and hrb27C, the alternative splicing factor gene poly U binding factor 68 kDa (pUf68, also known as half-pint), and the germ-line-specific gene ovarian tumor (otu), that produce defects in nurse-cell chromosome dispersal also produce defects in oocyte polarity, suggesting a link between these two processes. Here, we characterize a novel gene named poly, which, when mutated in the germ line, disrupts nurse-cell chromosome dispersal, as well as localization of anteroposterior and dorsoventral determinants in the oocyte. We also show that poly interacts genetically with hrb27C and otu. We conclude that poly is required for nurse-cell chromosome dispersal and oocyte polarization in the Drosophila germ-line. In addition, our interaction data suggest that poly is probably a member of the characterized mRNP complex that mediates both processes.

Kibra is a regulator of the Salvador/Warts/Hippo signaling network

The Salvador (Sav)/Warts (Wts)/Hippo (Hpo) (SWH) network controls tissue growth by inhibiting cell proliferation and promoting apoptosis. The core of the pathway consists of a MST and LATS family kinase cascade that ultimately phosphorylates and inactivates the YAP/Yorkie (Yki) transcription coactivator. The FERM domain proteins Merlin (Mer) and Expanded (Ex) represent one mode of upstream regulation controlling pathway activity. Here, we identify Kibra as a member of the SWH network. Kibra, which colocalizes and associates with Mer and Ex, also promotes the Mer/Ex association. Furthermore, the Kibra/Mer association is conserved in human cells. Finally, Kibra complexes with Wts and kibra depletion in tissue culture cells induces a marked reduction in Yki phosphorylation without affecting the Yki/Wts interaction. We suggest that Kibra is part of an apical scaffold that promotes SWH pathway activity.

Notch signaling and developmental cell-cycle arrest in Drosophila polar follicle cells

Temporal and spatial regulation of cell division is critical for proper development of multicellular organisms. An important aspect of this regulation is cell-cycle arrest, which in many cell types is coupled with differentiated status. Here we report that the polar cells--a group of follicle cells differentiated early during Drosophila oogenesis--are arrested at G2 phase and can serve as a model cell type for investigation of developmental regulation of cell-cycle arrest. On examining the effects of String, a mitosis-promoting phosphatase Cdc25 homolog, and Notch signaling in polar cells, we found that misexpression of String can trigger mitosis in existing polar cells to induce extra polar cells. Normally, differentiation of the polar cells requires Notch signaling. We found that the Notch-induced extra polar cells arise through recruitment of the neighboring cells rather than promotion of proliferation, and they are also arrested at G2 phase. Notch signaling is probably involved in down-regulating String in polar cells, thus inducing the G2 cell-cycle arrest.

Role of Scrib and Dlg in anterior-posterior patterning of the follicular epithelium during Drosophila oogenesis

Background

Proper patterning of the follicle cell epithelium over the egg chamber is essential for the Drosophila egg development. Differentiation of the epithelium into several distinct cell types along the anterior-posterior axis requires coordinated activities of multiple signaling pathways. Previously, we reported that lethal(2)giant larvae (lgl), a Drosophila tumor suppressor gene, is required in the follicle cells for the posterior follicle cell (PFC) fate induction at mid-oogenesis. Here we explore the role of another two tumor suppressor genes, scribble (scrib) and discs large (dlg), in the epithelial patterning.

Results

We found that removal of scrib or dlg function from the follicle cells at posterior terminal of the egg chamber causes a complete loss of the PFC fate. Aberrant specification and differentiation of the PFCs in the mosaic clones can be ascribed to defects in coordinated activation of the EGFR, JAK and Notch signaling pathways in the multilayered cells. Meanwhile, the clonal analysis revealed that loss-of-function mutations in scrib/dlg at the anterior domains result in a partially penetrant phenotype of defective induction of the stretched and centripetal cell fate, whereas specification of the border cell fate can still occur in the most anterior region of the mutant clones. Further, we showed that scrib genetically interacts with dlg in regulating posterior patterning of the epithelium.

Conclusion

In this study we provide evidence that scrib and dlg function differentially in anterior and posterior patterning of the follicular epithelium at oogenesis. Further genetic analysis indicates that scrib and dlg act in a common pathway to regulate PFC fate induction. This study may open another window for elucidating role of scrib/dlg in controlling epithelial polarity and cell proliferation during development.

The conserved WW-domain binding sites in Dystroglycan C-terminus are essential but partially redundant for Dystroglycan function

Background

Dystroglycan (Dg) is a transmembrane protein that is a part of the Dystrophin Glycoprotein Complex (DGC) which connects the extracellular matrix to the actin cytoskeleton. The C-terminal end of Dg contains a number of putative SH3, SH2 and WW domain binding sites. The most C-terminal PPXY motif has been established as a binding site for Dystrophin (Dys) WW-domain. However, our previous studies indicate that both Dystroglycan PPXY motives, WWbsI and WWbsII can bind Dystrophin protein in vitro.

Results

We now find that both WW binding sites are important for maintaining full Dg function in the establishment of oocyte polarity in Drosophila. If either WW binding site is mutated, the Dg protein can still be active. However, simultaneous mutations in both WW binding sites abolish the Dg activities in both overexpression and loss-of-function oocyte polarity assays in vivo. Additionally, sequence comparisons of WW binding sites in 12 species of Drosophila, as well as in humans, reveal a high level of conservation. This preservation throughout evolution supports the idea that both WW binding sites are functionally required.

Conclusion

Based on the obtained results we propose that the presence of the two WW binding sites in Dystroglycan secures the essential interaction between Dg and Dys and might further provide additional regulation for the cytoskeletal interactions of this complex.

The hippo pathway promotes Notch signaling in regulation of cell differentiation, proliferation, and oocyte polarity

Specification of the anterior-posterior axis in Drosophila oocytes requires proper communication between the germ-line cells and the somatically derived follicular epithelial cells. Multiple signaling pathways, including Notch, contribute to oocyte polarity formation by controlling the temporal and spatial pattern of follicle cell differentiation and proliferation. Here we show that the newly identified Hippo tumor-suppressor pathway plays a crucial role in the posterior follicle cells in the regulation of oocyte polarity. Disruption of the Hippo pathway, including major components Hippo, Salvador, and Warts, results in aberrant follicle-cell differentiation and proliferation and dramatic disruption of the oocyte anterior-posterior axis. These phenotypes are related to defective Notch signaling in follicle cells, because misexpression of a constitutively active form of Notch alleviates the oocyte polarity defects. We also find that follicle cells defective in Hippo signaling accumulate the Notch receptor and display defects in endocytosis markers. Our findings suggest that the interaction between Hippo and classic developmental pathways such as Notch is critical to spatial and temporal regulation of differentiation and proliferation and is essential for development of the body axes in Drosophila.