Asymmetric activity of NetrinB controls laterality of the Drosophila brain

Left-Right (LR) asymmetry of the nervous system is widespread across animals and is thought to be important for cognition and behaviour. But in contrast to visceral organ asymmetry, the genetic basis and function of brain laterality remain only poorly characterized. In this study, we performed RNAi screening to identify genes controlling brain asymmetry in Drosophila. We found that the conserved NetrinB (NetB) pathway is required for a small group of bilateral neurons to project asymmetrically into a pair of neuropils (Asymmetrical Bodies, AB) in the central brain in both sexes. While neurons project unilaterally into the right AB in wild-type flies, netB mutants show a bilateral projection phenotype and hence lose asymmetry. Developmental time course analysis reveals an initially bilateral connectivity, eventually resolving into a right asymmetrical circuit during metamorphosis, with the NetB pathway being required just prior symmetry breaking. We show using unilateral clonal analysis that netB activity is required specifically on the right side for neurons to innervate the right AB. We finally show that loss of NetB pathway activity leads to specific alteration of long-term memory, providing a functional link between asymmetrical circuitry determined by NetB and animal cognitive functions.

Biological effects of the loss of homochirality in a multicellular organism

Homochirality is a fundamental feature of all known forms of life, maintaining biomolecules (amino-acids, proteins, sugars, nucleic acids) in one specific chiral form. While this condition is central to biology, the mechanisms by which the adverse accumulation of non-L-α-amino-acids in proteins lead to pathophysiological consequences remain poorly understood. To address how heterochirality build-up impacts organism's health, we use chiral-selective in vivo assays to detect protein-bound non-L-α-amino acids (focusing on aspartate) and assess their functional significance in Drosophila. We find that altering the in vivo chiral balance creates a 'heterochirality syndrome' with impaired caspase activity, increased tumour formation, and premature death. Our work shows that preservation of homochirality is a key component of protein function that is essential to maintain homeostasis across the cell, tissue and organ level.

A dynamic and mosaic basement membrane controls cell intercalation in Drosophila ovaries

Basement membranes (BM) are extracellular matrices assembled into complex and highly organized networks essential for organ morphogenesis and function. However, little is known about the tissue origin of BM components and their dynamics in vivo Here, we unravel the assembly and role of the BM main component, Collagen type IV (ColIV), in Drosophila ovarian stalk morphogenesis. Stalks are short strings of cells assembled through cell intercalation that link adjacent follicles and maintain ovarian integrity. We show that stalk ColIV has multiple origins and is assembled following a regulated pattern leading to a unique BM organisation. Absence of ColIV leads to follicle fusion, as observed upon ablation of stalk cells. ColIV and integrins are both required to trigger cell intercalation and maintain mechanically strong cell-cell attachment within the stalk. These results show how the dynamic assembly of a mosaic BM controls complex tissue morphogenesis and integrity.

The Drosophila actin nucleator DAAM is essential for left-right asymmetry

Left-Right (LR) asymmetry is essential for organ positioning, shape and function. Myosin 1D (Myo1D) has emerged as an evolutionary conserved chirality determinant in both Drosophila and vertebrates. However, the molecular interplay between Myo1D and the actin cytoskeleton underlying symmetry breaking remains poorly understood. To address this question, we performed a dual genetic screen to identify new cytoskeletal factors involved in LR asymmetry. We identified the conserved actin nucleator DAAM as an essential factor required for both dextral and sinistral development. In the absence of DAAM, organs lose their LR asymmetry, while its overexpression enhances Myo1D-induced de novo LR asymmetry. These results show that DAAM is a limiting, LR-specific actin nucleator connecting up Myo1D with a dedicated F-actin network important for symmetry breaking.

Although our body looks symmetrical when viewed from the outside, it is in fact highly asymmetrical when we consider the shape and implantation of organs. For example, our heart is on the left side of the thorax, while the liver is on the right. In addition, our heart is made up of two distinct parts, the right heart and the left heart, which play different roles for blood circulation. These asymmetries, called left-right asymmetries, play a fundamental role in the morphogenesis and function of visceral organs and the brain. Aberrant LR asymmetry in human results in severe anatomical defects leading to embryonic lethality, spontaneous abortion and a number of congenital disorders. Our recent work has identified a particular myosin (Myo1D) as a major player in asymmetry in Drosophila and vertebrates. Myosins are proteins that can interact with the skeleton of cells (called the cytoskeleton) to transport other proteins, contract the cells, allow them to move, etc. In this work, we were able to identify all the genes of the cytoskeleton involved with myosin in left-right asymmetry, in particular a so-called 'nucleator' gene because it is capable of forming new parts of the cytoskeleton necessary for setting up asymmetries.

Molecular to organismal chirality is induced by the conserved myosin 1D

The emergence of asymmetry from an initially symmetrical state is a universal transition in nature. Living organisms show asymmetries at the molecular, cellular, tissular, and organismal level. However, whether and how multilevel asymmetries are related remains unclear. In this study, we show that Drosophila myosin 1D (Myo1D) and myosin 1C (Myo1C) are sufficient to generate de novo directional twisting of cells, single organs, or the whole body in opposite directions. Directionality lies in the myosins' motor domain and is swappable between Myo1D and Myo1C. In addition, Myo1D drives gliding of actin filaments in circular, counterclockwise paths in vitro. Altogether, our results reveal the molecular motor Myo1D as a chiral determinant that is sufficient to break symmetry at all biological scales through chiral interaction with the actin cytoskeleton.

A standardized nomenclature and atlas of the male terminalia of Drosophila melanogaster

Animal terminalia represent some of the most diverse and rapidly evolving structures in the animal kingdom, and for this reason have been a mainstay in the taxonomic description of species. The terminalia of Drosophila melanogaster, with its wide range of experimental tools, have recently become the focus of increased interest in the fields of development, evolution, and behavior. However, studies from different disciplines have often used discrepant terminologies for the same anatomical structures. Consequently, the terminology of genital parts has become a barrier to integrating results from different fields, rendering it difficult to determine what parts are being referenced. We formed a consortium of researchers studying the genitalia of D. melanogaster to help establish a set of naming conventions. Here, we present a detailed visual anatomy of male genital parts, including a list of synonymous terms, and suggest practices to avoid confusion when referring to anatomical parts in future studies. The goal of this effort is to facilitate interdisciplinary communication and help newcomers orient themselves within the exciting field of Drosophila genitalia.

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.

Myosin1D is an evolutionarily conserved regulator of animal left-right asymmetry

The establishment of left-right (LR) asymmetry is fundamental to animal development, but the identification of a unifying mechanism establishing laterality across different phyla has remained elusive. A cilia-driven, directional fluid flow is important for symmetry breaking in numerous vertebrates, including zebrafish. Alternatively, LR asymmetry can be established independently of cilia, notably through the intrinsic chirality of the acto-myosin cytoskeleton. Here, we show that Myosin1D (Myo1D), a previously identified regulator of Drosophila LR asymmetry, is essential for the formation and function of the zebrafish LR organizer (LRO), Kupffer's vesicle (KV). Myo1D controls the orientation of LRO cilia and interacts functionally with the planar cell polarity (PCP) pathway component VanGogh-like2 (Vangl2), to shape a productive LRO flow. Our findings identify Myo1D as an evolutionarily conserved regulator of animal LR asymmetry, and show that functional interactions between Myo1D and PCP are central to the establishment of animal LR asymmetry.

A Conserved Role of the Unconventional Myosin 1d in Laterality Determination

Anatomical and functional asymmetries are widespread in the animal kingdom [1, 2]. In vertebrates, many visceral organs are asymmetrically placed [3]. In snails, shells and inner organs coil asymmetrically, and in Drosophila, genitalia and hindgut undergo a chiral rotation during development. The evolutionary origin of these asymmetries remains an open question [1]. Nodal signaling is widely used [4], and many, but not all, vertebrates use cilia for symmetry breaking [5]. In Drosophila, which lacks both cilia and Nodal, the unconventional myosin ID (myo1d) gene controls dextral rotation of chiral organs [6, 7]. Here, we studied the role of myo1d in left-right (LR) axis formation in Xenopus. Morpholino oligomer-mediated myo1d downregulation affected organ placement in >50% of morphant tadpoles. Induction of the left-asymmetric Nodal cascade was aberrant in >70% of cases. Expression of the flow-target gene dand5 was compromised, as was flow itself, due to shorter, fewer, and non-polarized cilia at the LR organizer. Additional phenotypes pinpointed Wnt/planar cell polarity signaling and suggested that myo1d, like in Drosophila [8], acted in the context of the planar cell polarity pathway. Indeed, convergent extension of gastrula explant cultures was inhibited in myo1d morphants, and the ATF2 reporter gene for non-canonical Wnt signaling was downregulated. Finally, genetic interference experiments demonstrated a functional interaction between the core planar cell polarity signaling gene vangl2 and myo1d in LR axis formation. Thus, our data identified myo1d as a common denominator of arthropod and chordate asymmetry, in agreement with a monophyletic origin of animal asymmetry.

Polycomb and Hox Genes Control JNK-Induced Remodeling of the Segment Boundary during Drosophila Morphogenesis

In segmented tissues, anterior and posterior compartments represent independent morphogenetic domains, which are made of distinct lineages separated by boundaries. During dorsal closure of the Drosophila embryo, specific "mixer cells" (MCs) are reprogrammed in a JNK-dependent manner to express the posterior determinant engrailed (en) and cross the segment boundary. Here, we show that JNK signaling induces de novo expression of en in the MCs through repression of Polycomb (Pc) and release of the en locus from the silencing PcG bodies. Whereas reprogramming occurs in MCs from all thoracic and abdominal segments, cell mixing is restricted to the central abdominal region. We demonstrate that this spatial control of MC remodeling depends on the antagonist activity of the Hox genes abdominal-A and Abdominal-B. Together, these results reveal an essential JNK/en/Pc/Hox gene regulatory network important in controlling both the plasticity of segment boundaries and developmental reprogramming.

Signalling crosstalk at the leading edge controls tissue closure dynamics in the Drosophila embryo

Tissue morphogenesis relies on proper differentiation of morphogenetic domains, adopting specific cell behaviours. Yet, how signalling pathways interact to determine and coordinate these domains remains poorly understood. Dorsal closure (DC) of the Drosophila embryo represents a powerful model to study epithelial cell sheet sealing. In this process, JNK (JUN N-terminal Kinase) signalling controls leading edge (LE) differentiation generating local forces and cell shape changes essential for DC. The LE represents a key morphogenetic domain in which, in addition to JNK, a number of signalling pathways converges and interacts (anterior/posterior -AP- determination; segmentation genes, such as Wnt/Wingless; TGFβ/Decapentaplegic). To better characterize properties of the LE morphogenetic domain, we sought out new JNK target genes through a genomic approach: 25 were identified of which 8 are specifically expressed in the LE, similarly to decapentaplegic or puckered. Quantitative in situ gene profiling of this new set of LE genes reveals complex patterning of the LE along the AP axis, involving a three-way interplay between the JNK pathway, segmentation and HOX genes. Patterning of the LE into discrete domains appears essential for coordination of tissue sealing dynamics. Loss of anterior or posterior HOX gene function leads to strongly delayed and asymmetric DC, due to incorrect zipping in their respective functional domain. Therefore, in addition to significantly increasing the number of JNK target genes identified so far, our results reveal that the LE is a highly heterogeneous morphogenetic organizer, sculpted through crosstalk between JNK, segmental and AP signalling. This fine-tuning regulatory mechanism is essential to coordinate morphogenesis and dynamics of tissue sealing.

Companion Blood Cells Control Ovarian Stem Cell Niche Microenvironment and Homeostasis

The extracellular matrix plays an essential role for stem cell differentiation and niche homeostasis. Yet, the origin and mechanism of assembly of the stem cell niche microenvironment remain poorly characterized. Here, we uncover an association between the niche and blood cells, leading to the formation of the Drosophila ovarian germline stem cell niche basement membrane. We identify a distinct pool of plasmatocytes tightly associated with the developing ovaries from larval stages onward. Expressing tagged collagen IV tissue specifically, we show that the germline stem cell niche basement membrane is produced by these "companion plasmatocytes" in the larval gonad and persists throughout adulthood, including the reproductive period. Eliminating companion plasmatocytes or specifically blocking their collagen IV expression during larval stages results in abnormal adult niches with excess stem cells, a phenotype due to aberrant BMP signaling. Thus, local interactions between the niche and blood cells during gonad development are essential for adult germline stem cell niche microenvironment assembly and homeostasis.

The Atypical Cadherin Dachsous Controls Left-Right Asymmetry in Drosophila

Left-right (LR) asymmetry is essential for organ development and function in metazoans, but how initial LR cue is relayed to tissues still remains unclear. Here, we propose a mechanism by which the Drosophila LR determinant Myosin ID (MyoID) transfers LR information to neighboring cells through the planar cell polarity (PCP) atypical cadherin Dachsous (Ds). Molecular interaction between MyoID and Ds in a specific LR organizer controls dextral cell polarity of adjoining hindgut progenitors and is required for organ looping in adults. Loss of Ds blocks hindgut tissue polarization and looping, indicating that Ds is a crucial factor for both LR cue transmission and asymmetric morphogenesis. We further show that the Ds/Fat and Frizzled PCP pathways are required for the spreading of LR asymmetry throughout the hindgut progenitor tissue. These results identify a direct functional coupling between the LR determinant MyoID and PCP, essential for non-autonomous propagation of early LR asymmetry.

Diversity and convergence in the mechanisms establishing L/R asymmetry in metazoa

Differentiating left and right hand sides during embryogenesis represents a major event in body patterning. Left-Right (L/R) asymmetry in bilateria is essential for handed positioning, morphogenesis and ultimately the function of organs (including the brain), with defective L/R asymmetry leading to severe pathologies in human. How and when symmetry is initially broken during embryogenesis remains debated and is a major focus in the field. Work done over the past 20 years, in both vertebrate and invertebrate models, has revealed a number of distinct pathways and mechanisms important for establishing L/R asymmetry and for spreading it to tissues and organs. In this review, we summarize our current knowledge and discuss the diversity of L/R patterning from cells to organs during evolution.

Starvation induces FoxO-dependent mitotic-to-endocycle switch pausing during Drosophila oogenesis

When exposed to nutrient challenge, organisms have to adapt their physiology in order to balance reproduction with adult fitness. In mammals, ovarian follicles enter a massive growth phase during which they become highly dependent on gonadotrophic factors and nutrients. Somatic tissues play a crucial role in integrating these signals, controlling ovarian follicle atresia and eventually leading to the selection of a single follicle for ovulation. We used Drosophila follicles as a model to study the effect of starvation on follicle maturation. Upon starvation, Drosophila vitellogenic follicles adopt an 'atresia-like' behavior, in which some slow down their development whereas others enter degeneration. The mitotic-to-endocycle (M/E) transition is a critical step during Drosophila oogenesis, allowing the entry of egg chambers into vitellogenesis. Here, we describe a specific and transient phase during M/E switching that is paused upon starvation. The Insulin pathway induces the pausing of the M/E switch, blocking the entry of egg chambers into vitellogenesis. Pausing of the M/E switch involves a previously unknown crosstalk between FoxO, Cut and Notch that ensures full reversion of the process and rapid resumption of oogenesis upon refeeding. Our work reveals a novel genetic mechanism controlling the extent of the M/E switch upon starvation, thus integrating metabolic cues with development, growth and reproduction.

The myosin ID pathway and left-right asymmetry in Drosophila

Drosophila is a classical model to study body patterning, however left-right (L/R) asymmetry had remained unexplored, until recently. The discovery of the conserved myosin ID gene as a major determinant of L/R asymmetry has revealed a novel L/R pathway involving the actin cytoskeleton and the adherens junction. In this process, the HOX gene Abdominal-B plays a major role through the control of myosin ID expression and therefore symmetry breaking. In this review, we present organs and markers showing L/R asymmetry in Drosophila and discuss our current understanding of the underlying molecular genetic mechanisms. Drosophila represents a valuable model system revealing novel strategies to establish L/R asymmetry in invertebrates and providing an evolutionary perspective to the problem of laterality in bilateria.

In vivo characterization of dynein-driven nanovectors using Drosophila oocytes

Molecular motors transport various cargoes including vesicles, proteins and mRNAs, to distinct intracellular compartments. A significant challenge in the field of nanotechnology is to improve drug nuclear delivery by engineering nanocarriers transported by cytoskeletal motors. However, suitable in vivo models to assay transport and delivery efficiency remain very limited. Here, we develop a fast and genetically tractable assay to test the efficiency and dynamics of fluospheres (FS) using microinjection into Drosophila oocytes coupled with time-lapse microscopy. We designed dynein motor driven FS using a collection of dynein light chain 8 (LC8) peptide binding motifs as molecular linkers and characterized in real time the efficiency of the FS movement according to its linker’s sequence. Results show that the conserved LC8 binding motif allows fast perinuclear nanoparticle's accumulation in a microtubule and dynein dependent mechanism. These data reveal the Drosophila oocyte as a new valuable tool for the design of motor driven nanovectors.

Drosophila apc regulates delamination of invasive epithelial clusters

Border Cells in the Drosophila ovaries are a useful genetic model for understanding the molecular events underlying epithelial cell motility. During stage 9 of egg chamber development they detach from neighboring stretched cells and migrate between the nurse cells to reach the oocyte. RNAi screening allowed us to identify the dapc1 gene as being critical in this process. Clonal and live analysis showed a requirement of dapc1 in both outer border cells and contacting stretched cells for delamination. This mutant phenotype was rescued by dapc1 or dapc2 expression. Loss of dapc1 function was associated with an abnormal lasting accumulation of β-catenin/Armadillo and E-cadherin at the boundary between migrating border and stretched cells. Moreover, β-catenin/armadillo or E-cadherin downregulation rescued the dapc1 loss of function phenotype. Altogether these results indicate that Drosophila Apc1 is required for dynamic remodeling of β-catenin/Armadillo and E-cadherin adhesive complexes between outer border cells and stretched cells regulating proper delamination and invasion of migrating epithelial clusters.

DE-Cadherin regulates unconventional Myosin ID and Myosin IC in Drosophila left-right asymmetry establishment

In bilateria, positioning and looping of visceral organs requires proper left-right (L/R) asymmetry establishment. Recent work in Drosophila has identified a novel situs inversus gene encoding the unconventional type ID myosin (MyoID). In myoID mutant flies, the L/R axis is inverted, causing reversed looping of organs, such as the gut, spermiduct and genitalia. We have previously shown that MyoID interacts physically with β-Catenin, suggesting a role of the adherens junction in Drosophila L/R asymmetry. Here, we show that DE-Cadherin co-immunoprecipitates with MyoID and is required for MyoID L/R activity. We further demonstrate that MyoIC, a closely related unconventional type I myosin, can antagonize MyoID L/R activity by preventing its binding to adherens junction components, both in vitro and in vivo. Interestingly, DE-Cadherin inhibits MyoIC, providing a protective mechanism to MyoID function. Conditional genetic experiments indicate that DE-Cadherin, MyoIC and MyoID show temporal synchronicity for their function in L/R asymmetry. These data suggest that following MyoID recruitment by β-Catenin at the adherens junction, DE-Cadherin has a twofold effect on Drosophila L/R asymmetry by promoting MyoID activity and repressing that of MyoIC. Interestingly, the product of the vertebrate situs inversus gene inversin also physically interacts with β-Catenin, suggesting that the adherens junction might serve as a conserved platform for determinants to establish L/R asymmetry both in vertebrates and invertebrates.

Asymmetric localisation of cytokine mRNA is essential for JAK/STAT activation during cell invasiveness

The transition from immotile epithelial cells to migrating cells occurs in all organisms during normal embryonic development, as well as during tumour metastasis. During Drosophila oogenesis, border cells (BCs) are recruited and delaminate from the follicular epithelium. This process is triggered by the polar cells (PCs), which secrete the cytokine Unpaired (Upd) and activate the JAK/STAT pathway in neighbouring cells, turning them into invasive BCs. Interestingly, either a decrease or an increase in BC number alters migration, indicating that mechanisms controlling the level of JAK/STAT signalling are crucial in this process. Here, we show that PCs have a highly stable and polarised network of microtubules along which upd transcripts are asymmetrically transported in a Dynein-dependent manner. We demonstrate that in the absence of upd mRNA localisation the ligand is no longer efficiently secreted, leading to a loss of signalling strength as well as recruitment and migration defects. These findings reveal a novel post-transcriptional regulatory mechanism of JAK/STAT signalling in the control of epithelial cell invasiveness.

Coupling of apoptosis and L/R patterning controls stepwise organ looping

Handed asymmetry in organ shape and positioning is a common feature among bilateria, yet little is known about the morphogenetic mechanisms underlying left-right (LR) organogenesis. We utilize the directional 360° clockwise rotation of genitalia in Drosophila to study LR-dependent organ looping. Using time-lapse imaging, we show that rotation of genitalia by 360° results from an additive process involving two ring-shaped domains, each undergoing 180° rotation. Our results show that the direction of rotation for each ring is autonomous and strictly depends on the LR determinant myosin ID (MyoID). Specific inactivation of MyoID in one domain causes rings to rotate in opposite directions and thereby cancels out the overall movement. We further reveal a specific pattern of apoptosis at the ring boundaries and show that local cell death is required for the movement of each domain, acting as a brake-releaser. These data indicate that organ looping can proceed through an incremental mechanism coupling LR determination and apoptosis. Furthermore, they suggest a model for the stepwise evolution of genitalia posture in Diptera, through the emergence and duplication of a 180° LR module.

Cell migration: MIM takes the driver's seat

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

The Drosophila serine protease homologue Scarface regulates JNK signalling in a negative-feedback loop during epithelial morphogenesis

In Drosophila melanogaster, dorsal closure is a model of tissue morphogenesis leading to the dorsal migration and sealing of the embryonic ectoderm. The activation of the JNK signal transduction pathway, specifically in the leading edge cells, is essential to this process. In a genome-wide microarray screen, we identified new JNK target genes during dorsal closure. One of them is the gene scarface (scaf), which belongs to the large family of trypsin-like serine proteases. Some proteins of this family, like Scaf, bear an inactive catalytic site, representing a subgroup of serine protease homologues (SPH) whose functions are poorly understood. Here, we show that scaf is a general transcriptional target of the JNK pathway coding for a secreted SPH. scaf loss-of-function induces defects in JNK-controlled morphogenetic events such as embryonic dorsal closure and adult male terminalia rotation. Live imaging of the latter process reveals that, like for dorsal closure, JNK directs the dorsal fusion of two epithelial layers in the pupal genital disc. Genetic data show that scaf loss-of-function mimics JNK over-activity. Moreover, scaf ectopic expression aggravates the effect of the JNK negative regulator puc on male genitalia rotation. We finally demonstrate that scaf acts as an antagonist by negatively regulating JNK activity. Overall, our results identify the SPH-encoding gene scaf as a new transcriptional target of JNK signalling and reveal the first secreted regulator of the JNK pathway acting in a negative-feedback loop during epithelial morphogenesis.

JNK signalling controls remodelling of the segment boundary through cell reprogramming during Drosophila morphogenesis

Segments are fundamental units in animal development which are made of distinct cell lineages separated by boundaries. Although boundaries show limited plasticity during their formation for sharpening, cell lineages make compartments that become tightly restricted as development goes on. Here, we characterize a unique case of breaking of the segment boundary in late drosophila embryos. During dorsal closure, specific cells from anterior compartments cross the segment boundary and enter the adjacent posterior compartments. This cell mixing behaviour is driven by an anterior-to-posterior reprogramming mechanism involving de novo expression of the homeodomain protein Engrailed. Mixing is accompanied by stereotyped local cell intercalation, converting the segment boundary into a relaxation compartment important for tension-release during morphogenesis. This process of lineage switching and cell remodelling is controlled by JNK signalling. Our results reveal plasticity of segment boundaries during late morphogenesis and a role for JNK-dependent developmental reprogramming in this process.

JNK signalling influences intracellular trafficking during Drosophila morphogenesis through regulation of the novel target gene Rab30

JNK-mediated closure of the Drosophila dorsal epidermis during embryogenesis is a well-characterised model for morphogenesis. However, little is known about how JNK signalling modifies particular cellular behaviours such as intracellular transport. Here we demonstrate that the gene encoding the small GTPase Rab30 is a new JNK transcriptional target whose function is required during embryonic and adult morphogenesis including JNK-dependent dorsal closure, embryonic head involution and thorax closure. Using immuno-fluorescence and live imaging, we show that EGFP-Rab30 localises to trans-Golgi in addition to small unidentified vesicles, and moves in a microtubule-dependent, polarised dorso-ventral manner in the leading edge during dorsal closure. We propose that JNK activity upregulates genes involved in intracellular transport in order to provide an increased level of trafficking activity in cells undergoing complex morphogenetic arrangements such as dorsal closure.

Drosophila RalA is essential for the maintenance of Jak/Stat signalling in ovarian follicles

Small GTPases of the Ras-like (Ral) family are crucial for signalling functions in both normal and cancer cells; however, their role in a developing organism is poorly understood. Here, we identify the Drosophila Ral homologue RalA as a new key regulator of polar-cell differentiation during oogenesis. Polar cells have a crucial role in patterning the egg chamber and in recruiting border cells, which undergo collective and guided migration. We show that RalA function is essential for the maintenance of anterior and posterior polar-cell fate and survival. RalA is required cell autonomously to control the expression of polar-cell-specific markers, including the Jak/Stat ligand Unpaired. The loss of RalA also causes a cell non-autonomous phenotype owing to reduced Jak/Stat signalling in neighbouring follicle cells. As a result, border-cell assembly and migration as well as the polarization of the oocyte are defective. Thus, RalA is required in organizing centres to control proper patterning and migration in vivo.

Left-right asymmetry in Drosophila

Seminal studies of left-right (L/R) patterning in vertebrate models have led to the discovery of roles for the nodal pathway, ion flows and cilia in this process. Although the molecular mechanisms underlying L/R asymmetries seen in protostomes are less well understood, recent work using Drosophila melanogaster as a novel genetic model system to study this process has identified a number of mutations affecting directional organ looping. The genetic analysis of this, the most evolutionary conserved feature of L/R patterning, revealed the existence of a L/R pathway that involves the actin cytoskeleton and an associated type I myosin. In this review, we describe this work in the context of Drosophila development, and discuss the implications of these results for our understanding of L/R patterning in general.

The endocytic control of JAK/STAT signalling in Drosophila

Domeless (Dome) is an IL-6-related cytokine receptor that activates a conserved JAK/STAT signalling pathway during Drosophila development. Despite good knowledge of the signal transduction pathway in several models, the role of receptor endocytosis in JAK/STAT activation remains poorly understood. Using both in vivo genetic analysis and cell culture assays, we show that ligand binding of Unpaired 1 (Upd1) induces clathrin-dependent endocytosis of receptor-ligand complexes and their subsequent trafficking through the endosomal compartment towards the lysosome. Surprisingly, blocking trafficking in distinct endosomal compartments using mutants affecting either Clathrin heavy chain, rab5, Hrs or deep orange led to an inhibition of the JAK/STAT pathway, whereas this pathway was unchanged when rab11 was affected. This suggests that internalization and trafficking are both required for JAK/STAT activity. The requirement for clathrin-dependent endocytosis to activate JAK/STAT signalling suggests a model in which the signalling `on' state relies not only on ligand binding to the receptor at the cell surface, but also on the recruitment of the complex into endocytic vesicles on their way to lysozomes. Selective activation of the pool of receptors marked for degradation thus provides a way to tightly control JAK/STAT activity.

Strategies to establish left/right asymmetry in vertebrates and invertebrates

Left/right (L/R) asymmetry is essential during embryonic development for organ positioning, looping and handed morphogenesis. A major goal in the field is to understand how embryos initially determine their left and right hand sides, a process known as symmetry breaking. A number of recent studies on several vertebrate and invertebrate model organisms have provided a more complex view on how L/R asymmetry is established, revealing an apparent partition between deuterostomes and protostomes. In deuterostomes, nodal cilia represent a conserved symmetry-breaking process; nevertheless, growing evidence shows the existence of pre-cilia L/R asymmetries involving active ion flows. In protostomes like snails and Drosophila, symmetry breaking relies on different mechanisms, involving, in particular, the actin cytoskeleton and associated molecular motors.

Left-right asymmetry: class I myosins show the direction

Myosins are actin-based molecular motors that are found in almost all eukaryotes. Phylogenetic analysis allows the discrimination of 37 different types of myosins, most with unknown functions. Recent work in Drosophila has revealed a crucial role for type ID unconventional myosin in left-right asymmetry. Mutations in Myosin ID completely reverse the left-right axis (situs inversus), a phenotype that is dependent on an intact actin cytoskeleton. How this myosin might orient the left-right axis has began to be elucidated by showing that it interacts directly with beta-catenin, suggesting that myosin ID interacts with the adherens junction to control the direction of organ looping. This is the first demonstration of a role of a myosin in body patterning.

Type ID unconventional myosin controls left-right asymmetry in Drosophila

Breaking left-right symmetry in Bilateria embryos is a major event in body plan organization that leads to polarized adult morphology, directional organ looping, and heart and brain function. However, the molecular nature of the determinant(s) responsible for the invariant orientation of the left-right axis (situs choice) remains largely unknown. Mutations producing a complete reversal of left-right asymmetry (situs inversus) are instrumental for identifying mechanisms controlling handedness, yet only one such mutation has been found in mice (inversin) and snails. Here we identify the conserved type ID unconventional myosin 31DF gene (Myo31DF) as a unique situs inversus locus in Drosophila. Myo31DF mutations reverse the dextral looping of genitalia, a prominent left-right marker in adult flies. Genetic mosaic analysis pinpoints the A8 segment of the genital disc as a left-right organizer and reveals an anterior-posterior compartmentalization of Myo31DF function that directs dextral development and represses a sinistral default state. As expected of a determinant, Myo31DF has a trigger-like function and is expressed symmetrically in the organizer, and its symmetrical overexpression does not impair left-right asymmetry. Thus Myo31DF is a dextral gene with actin-based motor activity controlling situs choice. Like mouse inversin, Myo31DF interacts and colocalizes with beta-catenin, suggesting that situs inversus genes can direct left-right development through the adherens junction.

An unconventional myosin in Drosophila reverses the default handedness in visceral organs

The internal organs of animals often have left-right asymmetry. Although the formation of the anterior-posterior and dorsal-ventral axes in Drosophila is well understood, left-right asymmetry has not been extensively studied. Here we find that the handedness of the embryonic gut and the adult gut and testes is reversed (not randomized) in viable and fertile homozygous Myo31DF mutants. Myo31DF encodes an unconventional myosin, Drosophila MyoIA (also referred to as MyoID in mammals; refs 3, 4), and is the first actin-based motor protein to be implicated in left-right patterning. We find that Myo31DF is required in the hindgut epithelium for normal embryonic handedness. Disruption of actin filaments in the hindgut epithelium randomizes the handedness of the embryonic gut, suggesting that Myo31DF function requires the actin cytoskeleton. Consistent with this, we find that Myo31DF colocalizes with the cytoskeleton. Overexpression of Myo61F, another myosin I (ref. 4), reverses the handedness of the embryonic gut, and its knockdown also causes a left-right patterning defect. These two unconventional myosin I proteins may have antagonistic functions in left-right patterning. We suggest that the actin cytoskeleton and myosin I proteins may be crucial for generating left-right asymmetry in invertebrates.

Antagonistic actions of ecdysone and insulins determine final size in Drosophila

All animals coordinate growth and maturation to reach their final size and shape. In insects, insulin family molecules control growth and metabolism, whereas pulses of the steroid 20-hydroxyecdysone (20E) initiate major developmental transitions. We show that 20E signaling also negatively controls animal growth rates by impeding general insulin signaling involving localization of the transcription factor dFOXO and transcription of the translation inhibitor 4E-BP. We also demonstrate that the larval fat body, equivalent to the vertebrate liver, is a key relay element for ecdysone-dependent growth inhibition. Hence, ecdysone counteracts the growth-promoting action of insulins, thus forming a humoral regulatory loop that determines organismal size.

Dynamics of the basement membrane in invasive epithelial clusters in Drosophila

The basement membrane (BM) represents a barrier to cell migration, which has to be degraded to promote invasion. However, the role and behaviour of the BM during the development of pre-invasive cells is only poorly understood. Drosophila border cells (BCs) provide an attractive genetic model in which to study the cellular mechanisms underlying the migration of mixed cohorts of epithelial cells. BCs are made of two different epithelial cell types appearing sequentially during oogenesis: the polar cells and the outer BCs. Here, we show that the pre-invasive polar cells undergo an unusual and asymmetrical apical capping with major basement membrane proteins, including the two Drosophila Collagen IV alpha chains, Laminin A and Perlecan. Capping of polar cells proceeds through a novel, basal-to-apical transcytosis mechanism that involves the small GTPase Drab5. Apical capping is transient and is followed by rapid shedding prior to the initiation of BC migration, suggesting that the apical cap blocks migration. Consistently, non-migratory polar cells remain capped. We further show that JAK/STAT signalling and recruitment of outer BCs are required for correct shedding and migration. The dynamics of the BM represents a marker of migratory BC, revealing a novel developmentally regulated behaviour of BM coupled to epithelial cell invasiveness.

Tissue- and developmental stage-specific changes in the subcellular localization of the 26S proteasome in the ovary of Drosophila melanogaster

The intracellular localization of the 26S proteasome in the different ovarian cell types of Drosophila melanogaster was studied by means of immunofluorescence staining and laser scanning microscopy, with the use of antibodies specific for regulatory complex subunits or the catalytic core of the 26S proteasome. During the previtellogenic phase of oogenesis (stages 1-6), strong cytoplasmic staining was observed in the nurse cells and follicular epithelial cells, but the proteasome was not detected in the nuclei of these cell types. The subcellular distribution of the 26S proteasome was completely different in the oocyte. Besides a constant, very faint cytoplasmic staining, there was a gradual nuclear accumulation of proteasomes during the previtellogenic phase of oogenesis. A characteristic subcellular redistribution of the 26S proteasome occurred in the ovarian cells during the vitellogenic phase of oogenesis. There was a gradual decline in the concentration of the 26S proteasome in the nucleus of the oocyte, and in the stage 10 oocyte the proteasome could barely be detected in the nucleus. This was accompanied by a massive nuclear accumulation of proteasomes in the follicular epithelial cells. These results demonstrate that the subcellular distribution of the 26S proteasome in higher eukaryotes is strictly tissue- and developmental stage-specific.

Mechanism of inhibition of the Drosophila and mammalian EGF receptors by the transmembrane protein Kekkon 1

The transmembrane protein Kekkon 1 (Kek1) has previously been shown to act in a negative feedback loop to downregulate the Drosophila Epidermal Growth Factor Receptor (DER) during oogenesis. We show that this protein plays a similar role in other DER-mediated developmental processes. Structure-function analysis reveals that the extracellular Leucine-Rich Repeat (LRR) domains of Kek1 are critical for its function through direct association with DER, whereas its cytoplasmic domain is required for apical subcellular localization. In addition, the use of chimeric proteins between Kek1 extracellular and transmembrane domains fused to DER intracellular domain indicates that Kek1 forms an heterodimer with DER in vivo. To characterize more precisely the mechanism underlying the Kek1/DER interaction, we used mammalian ErbB/EGFR cell-based assays. We show that Kek1 is capable of physically interacting with each of the known members of the mammalian ErbB receptor family and that the Kek1/EGFR interaction inhibits growth factor binding, receptor autophosphorylation and Erk1/2 activation in response to EGF. Finally, in vivo experiments show that Kek1 expression potently suppresses the growth of mouse mammary tumor cells derived from aberrant ErbB receptors activation, but does not interfere with the growth of tumor cells derived from activated Ras. Our results underscore the possibility that Kek1 may be used experimentally to inhibit ErbB receptors and point to the possibility that, as yet uncharacterized, mammalian transmembrane LRR proteins might act as modulators of growth factor signalling.

Drosophila morphogenesis: the Newtonian revolution

Recent quantitative modeling of dorsal closure in the fruitfly Drosophila has revealed how multiple forces drive sealing of the two symmetrical epithelial sheets. A predictive model based on the new data allows gene function to be linked to the forces that drive tissue movement.

The retinoic-like juvenile hormone controls the looping of left-right asymmetric organs in Drosophila

In vertebrate development, the establishment of left-right asymmetry is essential for sidedness and the directional looping of organs like the heart. Both the nodal pathway and retinoic acid play major and conserved regulatory roles in these processes. We carried out a novel screen in Drosophila to identify mutants that specifically affect the looping of left-right asymmetric organs. We report the isolation of spin, a novel mutant in which the looping of the genitalia and spermiduct are incomplete; under-rotation of the genitalia indicates that spin controls looping morphogenesis but not direction, thus uncoupling left-right asymmetry and looping morphogenesis. spin is a novel, rotation-specific allele of the fasciclin2 (Fas2) gene, which encodes a cell-adhesion protein involved in several aspects of neurogenesis. In spin mutants, the synapses connecting specific neurosecretory cells to the corpora allata are affected. The corpus allatum is part of the ring gland and is involved in the control of juvenile hormone titers during development. Our genetic and pharmacological results indicate that Fas2(spin) rotation defects are linked to an abnormal endocrine function and an elevated level of juvenile hormone. As juvenile hormone is an insect sesquiterpenoid related to retinoic acid, these results establish a new genetic model for studying organ looping and demonstrate an evolutionarily conserved role for terpenoids in this process.

The Drosophila cytokine receptor Domeless controls border cell migration and epithelial polarization during oogenesis

In mammals, the JAK/STAT (Janus Kinase/Signal Transducer and Activator of Transcription) signaling pathway is activated in response to cytokines and growth factors to control blood cell development, proliferation and cell determination. In Drosophila, a conserved JAK/STAT signaling pathway controls segmentation in embryos, as well as blood cell development and other processes in larvae and adults. During embryogenesis, transduction of the Unpaired [Upd; also known as Outstretched (Os)] ligand through the JAK/STAT pathway requires Domeless, a putative membrane protein with distant homology to vertebrate type I cytokine receptors. We have isolated domeless (dome) in a screen to identify genes essential in epithelial morphogenesis during oogenesis. The level of dome activity is critical for proper border cell migration and is controlled in part through a negative feedback loop. In addition to its essential role in border cells, we show that dome is required in the germarium for the polarization of follicle cells during encapsulation of germline cells. In this process, dome controls the expression of the apical determinant Crumbs. In contrast to the ligand Upd, whose expression is limited to a pair of polar cells at both ends of the egg chamber, dome is expressed in all germline and follicle cells. However, the Dome protein is specifically localized at apicolateral membranes and undergoes ligand-dependent internalization in the follicle cells. dome mutations interact genetically with JAK/STAT pathway genes in border cell migration and abolish the nuclear translocation of Stat92E in vivo. We also show that dome functions downstream of upd and that both the extracellular and intracellular domains of Dome are required for JAK/STAT signaling. Altogether, our data indicate that Dome is an essential receptor molecule for Upd and JAK/STAT signaling during oogenesis.

Structural and enzymatic characterization of Drosophila Dm2-MMP, a membrane-bound matrix metalloproteinase with tissue-specific expression

We report the isolation and characterization of a cDNA encoding Dm2-MMP, the second matrix metalloproteinase (MMP) identified in the Drosophila melanogaster genome. The cloned cDNA codes for a polypeptide of 758 residues that displays a domain organization similar to that of other MMPs, including signal peptide, propeptide, catalytic, and hemopexin domains. However, the structure of Dm2-MMP is unique because of the presence of an insertion of 214 amino acids between the catalytic and hemopexin domains that is not present in any of the previously described MMPs. Dm2-MMP also contains a C-terminal extension predicted to form a cleavable glycosylphosphatidylinositol anchor site. Western blot and immunofluorescence analysis of S2 cells transfected with the isolated cDNA confirmed that Dm2-MMP is localized at the cell surface. Production of the catalytic domain of Dm2-MMP in Escherichia coli and analysis of its enzymatic activity revealed that this proteinase cleaves several synthetic peptides used for analysis of vertebrate MMPs. This proteolytic activity was abolished by MMP inhibitors such as BB-94, confirming that the isolated cDNA codes for an enzymatically active metalloproteinase. Reverse transcription-PCR analysis showed that Dm2-MMP is expressed at low levels in all of the developmental stages of Drosophila as well as in adult flies. However, detailed in situ hybridization at the larval stage revealed a strong tissue-specific expression in discrete regions of the brain and eye imaginal discs. According to these results, we propose that Dm2-MMP plays both general proteolytic functions during Drosophila development and in adult tissues and specific roles in eye development and neural tissues through the degradation and remodeling of the extracellular matrix.

A P-insertion screen identifying novel X-linked essential genes in Drosophila

The recent determination and annotation of the entire euchromatic sequence of the Drosophila melanogaster genome predicted the existence of about 13600 different genes (Science 287 (2000) 2185; http://www.fruitfly.org/annot/index.html). In parallel, the Berkeley Drosophila Genome Project (BDGP) has undertaken systematic P-insertion screens, to isolate new lethals and misexpressing lines. To date, however, the genes of the X chromosome have been under-represented in the screens performed. In order both to characterize several X-linked genes of prime interest to our laboratories and contribute to the collection of lethal P-insertions available to the community, we performed a P-insertion mutagenesis of the X chromosome. Using the PlacW and PGawB P-elements as mutagens, we generated two complementary sets of enhancer-trap lines, l(1)(T)PL and l(1)(T)PG, respectively, which both contain a reporter gene whose developmental expression can be monitored when driven by nearby enhancer sequences. We report here the characterization of 260 new insertions, mapping to 133 different genes or predicted CGs. Of these, 83 correspond to genes for which no lethal mutation had yet been reported. For 64 of those, we could confirm that lethality was solely due to the P-element insertion. The primary molecular data, reporter gene expression patterns (observed in embryos, third instar larvae and adult ovaries) and proposed CG assignment for each strain can be accessed and updated on our website at the following address: http://www-cbd.ups-tlse.fr:8080/screen.