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Accurate positioning of the nucleus is essential. Microtubules and their associated motors are important players in this process. Although nuclear migration inDrosophilaoocytes is controlled by microtubule, a role for microtubule-associated molecular motors in nuclear positioning has yet to be reported. We characterize novel landmarks that allow a precise description of the pre-migratory stages. Using these newly defined stages, we report that, prior to migration, the nucleus moves from the oocyte anterior side toward the center and concomitantly the centrosomes cluster at the posterior of the nucleus. In absence of Kinesin-1, centrosome clustering is impaired and the nucleus fails to position and migrate properly. The maintenance of a high level of Polo-kinase at centrosomes prevents centrosome clustering and impairs nuclear positioning. In absence of Kinesin-1, SPD2 an essential component of the pericentriolar material is increased at the centrosomes, suggesting that Kinesin-1 associated defects result from a failure to reduce centrosome activity. Consistently, depleting centrosomes rescues the nuclear migration defects induced by Kinesin-1 inactivation. Our results suggest that Kinesin-1 controls nuclear migration in the oocyte by modulating centrosome activity.Summary statementIn this study, we identified a crucial role of Kinesin-1 in centrosome clustering required for nuclear positioning and migration in theDrosophilaoocyte.

Barrier epithelial organs face the constant challenge of sealing the interior body from the external environment while simultaneously replacing the cells that contact this environment. These replacement cells—the progeny of basal stem cells—are born without apical, barrier-forming structures such as a protective, lumen-facing membrane and occluding junctions. How stem cell progeny acquire these structures to become part of the barrier is unknown. Here we use Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM), Correlative Light-Electron Microscopy (CLEM), and volumetric imaging of live and fixed organs to investigate progenitor integration in the intestinal epithelium of adult Drosophila. We find that stem cell daughters gestate their future lumenal-apical membrane beneath a transient, basal niche formed by an umbrella-shaped occluding junction that shelters the growing cell and adheres it to mature neighbor cells. The umbrella junction both targets formation of a deep, microvilli-lined, apical invagination and closes it off from the contents of the gut lumen. When the growing cell is sufficiently mature, the umbrella junction retracts to expose this Pre-Assembled Apical Compartment (PAAC) to the gut lumen, thus incorporating the new cell into the intestinal barrier. When we block umbrella junctions, stem cell daughters grow and attempt to differentiate but fail to integrate; when we block cell growth, no umbrella junctions form and daughters arrest in early differentiation. Thus, stem cell progeny build new barrier structures in the shelter of a transient niche, where they are protected from lumenal insults until they are prepared to withstand them. By coordinating this dynamic junctional niche with progenitor cell differentiation, a physiologically active epithelial organ incorporates new cells while upholding integrity of its barrier.

In cell biology, detection of protein subcellular localizations is often achieved by optical microscopy techniques and more rarely by electron microscopy (EM) despite the greater resolution offered by EM. One of the possible reasons was that protein detection by EM required specific antibodies whereas this need could be circumvented by using fluorescently-tagged proteins in optical microscopy approaches. Recently, the description of a genetically encodable EM tag, the engineered ascorbate peroxidase (APEX), whose activity can be monitored by electron-dense DAB precipitates, has widened the possibilities of specific protein detection in EM. However, this technique still requires the generation of new molecular constructions. Thus, we decided to develop a versatile method that would take advantage of the numerous GFP-tagged proteins already existing and create a tool combining a nanobody anti-GFP (GBP) with APEX. This GBP-APEX tool allows a simple and efficient detection of any GFP fusion proteins without the needs of specific antibodies nor the generation of additional constructions. We have shown the feasibility and efficiency of this method to detect various proteins in Drosophila ovarian follicles such as nuclear proteins, proteins associated with endocytic vesicles, plasma membranes or nuclear envelopes. Lastly, we expressed this tool in Drosophila with the UAS/GAL4 system that enables spatiotemporal control of the protein detection.

We show that autoinhibition regulates the binding between microtubule nucleating complexes and proteins that tether them to sites of microtubule nucleation. Failure to regulate this binding properly can lead to ectopic cytosolic microtubule nucleation and major defects during cell division., γ-Tubulin ring complexes (γ-TuRCs) nucleate microtubules. They are recruited to centrosomes in dividing cells via binding to N-terminal CM1 domains within γ-TuRC–tethering proteins, including Drosophila Centrosomin (Cnn). Binding promotes microtubule nucleation and is restricted to centrosomes in dividing cells, but the mechanism regulating binding remains unknown. Here, we identify an extreme N-terminal CM1 autoinhibition (CAI) domain found specifically within the centrosomal isoform of Cnn (Cnn-C) that inhibits γ-TuRC binding. Robust binding occurs after removal of the CAI domain or with the addition of phosphomimetic mutations, suggesting that phosphorylation helps relieve inhibition. We show that regulation of Cnn binding to γ-TuRCs is isoform specific and that misregulation of binding can result in ectopic cytosolic microtubules and major defects during cell division. We also find that human CDK5RAP2 is autoinhibited from binding γ-TuRCs, suggesting conservation across species. Overall, our results shed light on how and why CM1 domain binding to γ-TuRCs is regulated.

Neurons contain polarised microtubule arrays essential for neuronal function. How microtubule nucleation and polarity are regulated within neurons remains unclear. We show that γ-tubulin localises asymmetrically to the somatic Golgi withinDrosophilaneurons. Microtubules originate from the Golgi with an initial growth preference towards the axon. Their growing plus ends also turn towards and into the axon, adding to the plus-end-out microtubule pool. Any plus ends that reach a dendrite, however, do not readily enter, maintaining minus-end-out polarity. Both turning towards the axon and exclusion from dendrites depend on Kinesin-2, a plus-end-associated motor that guides growing plus ends along adjacent microtubules. We propose that Kinesin-2 engages with a polarised microtubule network within the soma to guide growing microtubules towards the axon; while at dendrite entry sites engagement with microtubules of opposite polarity generates a backward stalling force that prevents entry into dendrites and thus maintains minus-end-out polarity within proximal dendrites.

International audience; Drosophila Ensconsin (also known as MAP7) controls spindle length, centrosome separation in brain neuroblasts (NBs) and asymmetric transport in oocytes. The control of spindle length by Ensconsin is Kinesin-1 independent but centrosome separation and oocyte transport require targeting of Kinesin-1 to microtubules by Ensconsin. However, the molecular mechanism used for this targeting remains unclear. Ensconsin contains a microtubule (MT)-binding domain (MBD) and a Kinesin-binding domain (KBD). Rescue experiments show that only full-length Ensconsin restores the spindle length phenotype. KBD expression rescues ensc centrosome separation defects in NBs, but not the fast oocyte streaming and the localization of Staufen and Gurken. Interestingly, the KBD can stimulate Kinesin-1 targeting to MTs in vivo and in vitro We propose that a KBD and Kinesin-1 complex is a minimal activation module that increases Kinesin-1 affinity for MTs. Addition of the MBD present in full-length Ensconsin allows this process to occur directly on the MT and triggers higher Kinesin-1 targeting. This dual regulation by Ensconsin is essential for optimal Kinesin-1 targeting to MTs in oocytes, but not in NBs, illustrating the importance of adapting Kinesin-1 recruitment to different biological contexts.

Centrosome separation inDrosophilalarval neuroblasts and asymmetric transport of embryonic determinants in oocytes are both microtubule-dependent processes that require Kinesin-1 activation by Ensconsin/microtubule-associated protein 7 (MAP7). However, the molecular mechanism used by Ensconsin to activate Kinesin-1 remains elusive. Ensconsin/ MAP7 contains an N-terminal microtubule-binding domain (MBD) and a C-terminal Kinesin-binding domain (KBD). Using rescue experiments in live flies, we show that KBD expression alone is sufficient to fully rescue Ensconsin-dependent centrosome separation defects, but not the fast oocyte streaming and the localization patterns of Staufen and Gurken proteins. Interestingly, we show here for the first time that KBD binds and stimulates Kinesin-1 binding to Mtsin vivoandin vitro. We propose that the KBD/Kinesin-1 motor represents a minimal activation module that stimulates Kinesin-1 binding to Mts. Addition of the MBD, present in the full length Ensconsin allows this activation to occur directly on the Mt. Our data also suggest that in a very large cell with a complex microtubule network, but not in smaller cells, this dual activation by Ensconsin is essential for optimal Kinesin-1 targeting to the microtubule cytoskeleton.

Wolbachia are gram-negative, obligate, intracellular bacteria carried by a majority of insect species worldwide. Here we use a Wolbachia-infected Drosophila cell line and genome-wide RNA interference (RNAi) screening to identify host factors that influence Wolbachia titer. By screening an RNAi library targeting 15,699 transcribed host genes, we identified 36 candidate genes that dramatically reduced Wolbachia titer and 41 that increased Wolbachia titer. Host gene knockdowns that reduced Wolbachia titer spanned a broad array of biological pathways including genes that influenced mitochondrial function and lipid metabolism. In addition, knockdown of seven genes in the host ubiquitin and proteolysis pathways significantly reduced Wolbachia titer. To test the in vivo relevance of these results, we found that drug and mutant inhibition of proteolysis reduced levels of Wolbachia in the Drosophila oocyte. The presence of Wolbachia in either cell lines or oocytes dramatically alters the distribution and abundance of ubiquitinated proteins. Functional studies revealed that maintenance of Wolbachia titer relies on an intact host Endoplasmic Reticulum (ER)-associated protein degradation pathway (ERAD). Accordingly, electron microscopy studies demonstrated that Wolbachia is intimately associated with the host ER and dramatically alters the morphology of this organelle. Given Wolbachia lack essential amino acid biosynthetic pathways, the reliance of Wolbachia on high rates of host proteolysis via ubiquitination and the ERAD pathways may be a key mechanism for provisioning Wolbachia with amino acids. In addition, the reliance of Wolbachia on the ERAD pathway and disruption of ER morphology suggests a previously unsuspected mechanism for Wolbachia’s potent ability to prevent RNA virus replication.

International audience; BACKGROUND: The control of apical-basal polarity in epithelial layers is a fundamental event in many processes, ranging from embryonic development to tumor formation. A key feature of polarized epithelial cells is their ability to maintain an asymmetric distribution of specific molecular complexes, including the phosphoinositides PI(4,5)P2 and PI(3,4,5)P3. The spatiotemporal regulation of these phosphoinositides is controlled by the concerted action of phosphoinositide kinases and phosphatases. RESULTS: Using the Drosophila follicular epithelium as a model system in vivo, we show here that PI(4,5)P2 is crucial to maintain apical-basal polarity. PI(4,5)P2 is essentially regulated by the PI4P5 kinase Skittles (SKTL), whereas neither the phosphatase PTEN nor the PI(4,5)P3 kinase DP110 lead to loss of apical-basal polarity. By inactivating SKTL and thereby strongly reducing PI(4,5)P2 levels in a single cell of the epithelium, we observe the disassembly of adherens junctions, actin cytoskeleton reorganization, and apical constriction leading to delamination, a process similar to that observed during epithelial-mesenchymal transition. We provide evidence that PI(4,5)P2 controls the apical targeting of PAR-3/Bazooka to the plasma membrane and that the loss of this polarized distribution is sufficient to induce a similar cell shape change. Finally, we show that PI(4,5)P2 is excluded from the cell apex and that PAR-3 diffuses laterally just prior to the apical constriction in a context of endogenous invagination. CONCLUSIONS: All together, these results indicate that the PIP5 kinase SKTL, by controlling PI(4,5)P2 polarity, regulates PAR-3 localization and thus the size of the apical domain.

Summary In animal cells the centrosome is commonly viewed as the main cellular structure driving microtubule (MT) assembly into the mitotic spindle apparatus. However, additional pathways, such as those mediated by chromatin and augmin, are involved in the establishment of functional spindles. The molecular mechanisms involved in these pathways remain poorly understood, mostly due to limitations inherent to current experimental systems available. To overcome these limitations we have developed six new Drosophila cell lines derived from Drosophila homozygous mutants for DSas-4, a protein essential for centriole biogenesis. These cells lack detectable centrosomal structures, astral MT, with dispersed pericentriolar proteins D-PLP, Centrosomin and γ-tubulin. They show poorly focused spindle poles that reach the plasma membrane. Despite being compromised for functional centrosome, these cells could successfully undergo mitosis. Live-cell imaging analysis of acentriolar spindle assembly revealed that nascent MTs are nucleated from multiple points in the vicinity of chromosomes. These nascent MTs then grow away from kinetochores allowing the expansion of fibers that will be part of the future acentriolar spindle. MT repolymerization assays illustrate that acentriolar spindle assembly occurs “inside-out” from the chromosomes. Colchicine-mediated depolymerization of MTs further revealed the presence of a functional Spindle Assembly Checkpoint (SAC) in the acentriolar cells. Finally, pilot RNAi experiments open the potential use of these cell lines for the molecular dissection of anastral pathways in spindle and centrosome assembly.

International audience; Epithelial remodelling is an essential mechanism for organogenesis, during which cells change shape and position while maintaining contact with each other. Adherens junctions (AJs) mediate stable intercellular cohesion but must be actively reorganised to allow morphogenesis. Vesicle trafficking and the microtubule (MT) cytoskeleton contribute to regulating AJs but their interrelationship remains elusive. We carried out a detailed analysis of the role of MTs in cell remodelling during formation of the tracheal system in the Drosophila embryo. Induction of MT depolymerisation specifically in tracheal cells shows that MTs are essential during a specific time frame of tracheal cell elongation while the branch extends. In the absence of MTs, one tracheal cell per branch overelongates, ultimately leading to branch break. Three-dimensional quantifications revealed that MTs are crucial to sustain E-Cadherin (Shotgun) and Par-3 (Bazooka) levels at AJs. Maintaining E-Cadherin/Par-3 levels at the apical domain requires de novo synthesis rather than internalisation and recycling from and to the apical plasma membrane. However, apical targeting of E-Cadherin and Par-3 requires functional recycling endosomes, suggesting an intermediate role for this compartment in targeting de novo synthesized E-Cadherin to the plasma membrane. We demonstrate that apical enrichment of recycling endosomes is dependent on the MT motor Dynein and essential for the function of this vesicular compartment. In addition, we establish that E-Cadherin dynamics and MT requirement differ in remodelling tracheal cells versus planar epithelial cells. Altogether, our results uncover an MT-Dynein-dependent apical restriction of recycling endosomes that controls adhesion by sustaining Par-3 and E-Cadherin levels at AJs during morphogenesis.

Differentiation of the embryonic termini in Drosophiladepends on signaling by the Tor RTK, which induces terminal gene expression by inactivating at the embryonic poles a uniformly distributed repressor activity that involves the Gro corepressor. Here, we identify a new gene, cic, that acts as a repressor of terminal genes regulated by the Tor pathway. cic also mediates repression along the dorsoventral axis, a process that requires the Dorsal morphogen and Gro, and which is also inhibited by Tor signaling at the termini. cic encodes an HMG-box transcription factor that interacts with Gro in vitro. We present evidence that Tor signaling regulates terminal patterning by inactivating Cic at the embryo poles. cic has been evolutionarily conserved, suggesting that Cic-like proteins may act as repressors regulated by RTK signaling in other organisms.

TBCB is a tubulin-binding cofactor required for tubulin heterodimerization. We show that in Drosophila, TBCB is required to sustain the levels of both α- and β-tubulins and for microtubule network formation in different tissues. Strikingly, TBCB is essential for cell polarity but not for cell division., Microtubules (MTs) are essential for cell division, shape, intracellular transport, and polarity. MT stability is regulated by many factors, including MT-associated proteins and proteins controlling the amount of free tubulin heterodimers available for polymerization. Tubulin-binding cofactors are potential key regulators of free tubulin concentration, since they are required for α-β–tubulin dimerization in vitro. In this paper, we show that mutation of the Drosophila tubulin-binding cofactor B (dTBCB) affects the levels of both α- and β-tubulins and dramatically destabilizes the MT network in different fly tissues. However, we find that dTBCB is dispensable for the early MT-dependent steps of oogenesis, including cell division, and that dTBCB is not required for mitosis in several tissues. In striking contrast, the absence of dTBCB during later stages of oogenesis causes major defects in cell polarity. We show that dTBCB is required for the polarized localization of the axis-determining mRNAs within the oocyte and for the apico-basal polarity of the surrounding follicle cells. These results establish a developmental function for the dTBCB gene that is essential for viability and MT-dependent cell polarity, but not cell division.

International audience; Production of specialized cells from precursors depends on a tightly regulated sequence of proliferation and differentiation steps. In the gonad of Drosophila melanogaster, the daughters of germ line stem cells (GSC) go through precisely four rounds of transit amplification divisions to produce clusters of 16 interconnected germ line cells before entering a stereotypic differentiation cascade. Here we show that animals harbouring a transposon insertion in the center of the complex nucleoporin98-96 (nup98-96) locus had severe defects in the early steps of this developmental program, ultimately leading to germ cell loss and sterility. A phenotypic analysis indicated that flies carrying the transposon insertion, designated nup98-96(2288), had dramatically reduced numbers of germ line cells. In contrast to controls, mutant testes contained many solitary germ line cells that had committed to differentiation as well as abnormally small clusters of two, four or eight differentiating germ line cells. This indicates that mutant GSCs rather differentiated than self-renewed, and that these GSCs and their daughters initiated the differentiation cascade after zero, or less than four rounds of amplification divisions. This phenotype remained unaffected by hyper-activation of signalling pathways that normally result in excessive proliferation of GSCs and their daughters. Expression of wildtype nup98-96 specifically in the germ line cells of mutant animals fully restored development of the GSC lineage, demonstrating that the effect of the mutation is cell-autonomous. Nucleoporins are the structural components of the nucleopore and have also been implicated in transcriptional regulation of specific target genes. The nuclear envelopes of germ cells and general nucleocytoplasmic transport in nup98-96 mutant animals appeared normal, leading us to propose that Drosophila nup98-96 mediates the transport or transcription of targets required for the developmental timing between amplification and differentiation.

This study identifies phosphoinositides as key regulators of spermatid cell polarity. Polarization and elongation of spermatids in Drosophila are regulated through local synthesis of PIP2 by Sktl, which drives polarized localization of the exocyst complex to promote targeted membrane delivery and polarization of the elongating spermatid cysts., During spermiogenesis, Drosophila melanogaster spermatids coordinate their elongation in interconnected cysts that become highly polarized, with nuclei localizing to one end and sperm tail growth occurring at the other. Remarkably little is known about the signals that drive spermatid polarity and elongation. Here we identify phosphoinositides as critical regulators of these processes. Reduction of plasma membrane phosphatidylinositol 4,5-bisphosphate (PIP2) by low-level expression of the PIP2 phosphatase SigD or mutation of the PIP2 biosynthetic enzyme Skittles (Sktl) results in dramatic defects in spermatid cysts, which become bipolar and fail to fully elongate. Defects in polarity are evident from the earliest stages of elongation, indicating that phosphoinositides are required for establishment of polarity. Sktl and PIP2 localize to the growing end of the cysts together with the exocyst complex. Strikingly, the exocyst becomes completely delocalized when PIP2 levels are reduced, and overexpression of Sktl restores exocyst localization and spermatid cyst polarity. Moreover, the exocyst is required for polarity, as partial loss of function of the exocyst subunit Sec8 results in bipolar cysts. Our data are consistent with a mechanism in which localized synthesis of PIP2 recruits the exocyst to promote targeted membrane delivery and polarization of the elongating cysts.

International audience; In vertebrates, stathmins form a family of proteins possessing two tubulin binding repeats (TBRs), which each binds one soluble tubulin heterodimer. The stathmins thus sequester two tubulins in a phosphorylation-dependent manner, providing a link between signal transduction and microtubule dynamics. In Drosophila, we show here that a single stathmin gene (stai) encodes a family of D-stathmin proteins. Two of the D-stathmins are maternally deposited and then restricted to germ cells, and the other two are detected in the nervous system during embryo development. Like in vertebrates, the nervous system-enriched stathmins contain an N-terminal domain involved in subcellular targeting. All the D-stathmins possess a domain containing three or four predicted TBRs, and we demonstrate here, using complementary biochemical and biophysical methods, that all four predicted TBR domains actually bind tubulin. D-stathmins can indeed bind up to four tubulins, the resulting complex being directly visualized by electron microscopy. Phylogenetic analysis shows that the presence of regulated multiple tubulin sites is a conserved characteristic of stathmins in invertebrates and allows us to predict key residues in stathmin for the binding of tubulin. Altogether, our results reveal that the single Drosophila stathmin gene codes for a stathmin family similar to the multigene vertebrate one, but with particular tubulin binding properties.

Microtubules (MTs) are essential for many cell features, such as polarity, motility, shape, and vesicle trafficking. Therefore, in a multicellular organism, their organization differs between cell types and during development; however, the control of this process remains elusive. Here, we show that during Drosophila tracheal morphogenesis, MT reorganization is coupled to relocalization of the microtubule organizing centers (MTOC) components from the centrosome to the apical cell domain from where MTs then grow. We reveal that this process is controlled by the trachealess patterning gene in a two-step mechanism. MTOC components are first released from the centrosome by the activity of the MT-severing protein Spastin, and then anchored apically through the transmembrane protein Piopio. We further show that these changes are essential for tracheal development, thus stressing the functional relevance of MT reorganization for morphogenesis. © 2010 Elsevier Inc., Este trabajo ha sido financiado por las conceciones del CNRS y ANR « Blanche » (subvención Cymempol, Blan06-3-139786). Jordi Casanova ha sido financiado por la Generalitat de Catalunya, el Ministerio de España de Ciencia e Innovación, y su programa Consolider-Ingenio 2010.

International audience; Phosphoinositides have emerged as key regulators of membrane traffic through their control of the localization and activity of several effector proteins. Both Rab5 and phosphatidylinositol (4,5)-bisphosphate [PtdIns(4,5)P(2)] are involved in the early steps of the clathrin-dependent endocytic pathway, but little is known about how their functions are coordinated. We have studied the role of PtdIns(4,5)P(2) and Rab5 in the Drosophila germline during oogenesis. We found that Rab5 is required for the maturation of early endocytic vesicles. We show that PtdIns(4,5)P(2) is required for endocytic-vesicle formation, for Rab5 recruitment to endosomes and, consistently, for endocytosis. Furthermore, we reveal a previously undescribed role of Rab5 in releasing PtdIns(4,5)P(2), PtdIns(4,5)P(2)-binding budding factors and F-actin from early endocytic vesicles. Finally, we show that overexpressing the PtdIns(4,5)P(2)-synthesizing enzyme Skittles leads to an endocytic defect that is similar to that seen in rab5 loss-of-function mutants. Hence, our results argue strongly in favor of the hypothesis that the Rab5-dependant release of PtdIns(4,5)P(2) from endosomes that we discovered in this study is crucial for endocytosis to proceed.

International audience; The attachment of the cytoskeleton to the plasma membrane is crucial in controlling the polarized transport of cell-fate-determining molecules. Attachment involves adaptor molecules, which have the capacity to bind to both the plasma membrane and elements of the cytoskeleton, such as microtubules and actin filaments. Using the Drosophila oocyte as a model system, we show that the type I phosphatidylinositol 4-phosphate 5-kinase (PIP5K), Skittles, is necessary to sustain the organization of microtubules and actin cytoskeleton required for the asymmetric transport of oskar, bicoid and gurken mRNAs and thereby controls the establishment of cell polarity. We show that Skittles function is crucial to synthesize and maintain phosphatidylinositol 4,5 bisphosphate (PIP2) at the plasma membrane in the oocyte. Reduction of Skittles activity impairs activation at the plasma membrane of Moesin, a member of the ERM family known to link the plasma membrane to the actin-based cytoskeleton. Furthermore, we provide evidence that Skittles, by controlling the localization of Bazooka, Par-1 and Lgl, but not Lkb1, to the cell membrane, regulates PAR polarity proteins and the maintenance of specific cortical domains along the anteroposterior axis.

International audience; Monitoring Editor: Vivek Malhotra Axis specification during Drosophila embryonic development requires transfer of maternal components during oogenesis from nurse cells (NC) into the oocyte through cytoplasmic bridges. We found that the asymmetrical distribution of Golgi, between nurse cells and the oocyte, is sustained by an active transport process. We have caracterized actin basket structures that asymmetrically cap the NC side of Ring canals (RC) connecting the oocyte. Our results suggest that these actin baskets structurally support transport mechanisms of RC transit. In addition, our tracking analysis indicates that Golgi units are actively transported to the oocyte rather than diffusing. We observed that RC transit is microtubules-based and mediated at least by Dynein. Finally, we show that actin networks may be involved in RC cross-ing through a Myosin II step process, as well as in dispatching Golgi units inside the oocyte subcompartments.

International audience; Unequal segregation of cell fate determinants at mitosis is a conserved mechanism whereby cell fate diversity can be generated during development. In Drosophila, each sensory organ precursor cell (SOP) divides asymmetrically to produce an anterior pIIb and a posterior pIIa cell. The Par6-aPKC complex localizes at the posterior pole of dividing SOPs and directs the actin-dependent localization of the cell fate determinants Numb, Partner of Numb (Pon) and Neuralized at the opposite pole. The plasma membrane lipid phosphatidylinositol (4,5)-bisphosphate (PIP2) regulates the plasma membrane localization and activity of various proteins, including several actin regulators, thereby modulating actin-based processes. Here, we have examined the distribution of PIP2 and of the PIP2-producing kinase Skittles (Sktl) in mitotic SOPs. Our analysis indicates that both Sktl and PIP2 reporters are uniformly distributed in mitotic SOPs. In the course of this study, we have observed that overexpression of full-length Pon or its localization domain (LD) fused to the Red Fluorescent Protein (RFP::Pon(LD)) results in asymmetric distribution of Sktl and PIP2 reporters in dividing SOPs. Our observation that Pon overexpression alters polar protein distribution is relevant because RFP::Pon(LD) is often used as a polarity marker in dividing progenitors.

International audience; The Drosophila oocyte is a highly polarized cell. Secretion occurs towards restricted neighboring cells and asymmetric transport controls the localization of several mRNAs to distinct cortical compartments. Here, we describe a role for the Drosophila ortholog of the Rab6 GTPase, Drab6, in establishing cell polarity during oogenesis. We found that Drab6 localizes to Golgi and Golgi-derived membranes and interacts with BicD. We also provide evidence that Drab6 and BicD function together to ensure the correct delivery of secretory pathway components, such as the TGFalpha homolog Gurken, to the plasma membrane. Moreover, in the absence of Drab6, osk mRNA localization and the organization of microtubule plus-ends at the posterior of the oocyte were both severely affected. Our results point to a possible connection between Rab protein-mediated secretion, organization of the cytoskeleton and mRNA transport.

Molecular motors transport the axis-determining mRNAs oskar,bicoid and gurken along microtubules (MTs) in the Drosophila oocyte. However, it remains unclear how the underlying MT network is organized and how this transport takes place. We have identified a centriole-containing centrosome close to the oocyte nucleus. Remarkably, the centrosomal components, γ-tubulin and Drosophilapericentrin-like protein also strongly accumulate at the periphery of this nucleus. MT polymerization after cold-induced disassembly in wild type and in gurken mutants suggests that in the oocyte the centrosome-nucleus complex is an active center of MT polymerization. We further report that the MT network comprises two perpendicular MT subsets that undergo dynamic rearrangements during oogenesis. This MT reorganization parallels the successive steps in localization of gurken and oskar mRNAs. We propose that in addition to a highly polarized microtubule scaffold specified by the cortex oocyte, the repositioning of the nucleus and its tightly associated centrosome could control MT reorganization and, hence,oocyte polarization.

Background: The cytoskeleton and associated motors play an important role in the establishment of intracellular polarity. Microtubule-based transport is required in many cell types for the asymmetric localization of mRNAs and organelles. A striking example is the Drosophila oocyte, where microtubule-dependent processes govern the asymmetric positioning of the nucleus and the localization to distinct cortical domains of mRNAs that function as cytoplasmic determinants. A conserved machinery for mRNA localization and nuclear positioning involving cytoplasmic Dynein has been postulated; however, the precise role of plus- and minus end-directed microtubule-based transport in axis formation is not yet understood. Results: Here, we show that mRNA localization and nuclear positioning at mid-oogenesis depend on two motor proteins, cytoplasmic Dynein and Kinesin I. Both of these microtubule motors cooperate in the polar transport of bicoid and gurken mRNAs to their respective cortical domains. In contrast, Kinesin I - mediated transport of oskar to the posterior pole appears to be independent of Dynein. Beside their roles in RNA transport, both motors are involved in nuclear positioning and in exocytosis of Gurken protein. Dynein-Dynactin complexes accumulate at two sites within the oocyte: around the nucleus in a microtubule-independent manner and at the posterior pole through Kinesin-mediated transport. Conclusion: The microtubule motors cytoplasmic Dynein and Kinesin I, by driving transport to opposing microtubule ends, function in concert to establish intracellular polarity within the Drosophila oocyte. Furthermore, Kinesin-dependent localization of Dynein suggests that both motors are components of the same complex and therefore might cooperate in recycling each other to the opposite microtubule pole.

We characterized Drosophila endophilin A (D-endoA), and generated and analysed D-endoA mutants. Like its mammalian homologue, D-endoA exhibits lysophosphatidic acid acyl transferase activity and contains a functional SH3 domain. D-endoA is recruited to the sites of endocytosis, as revealed by immunocytochemistry of the neuromuscular junction (NMJ) of mutant L3 larvae carrying the temperature-sensitive allele of dynamin, shibire. D-endoA null mutants show severe defects in motility and die at the early L2 larval stage. Mutants with reduced D-endoA levels exhibit a range of defects of synaptic vesicle endocytosis, as observed at L3 larvae NMJs using FM1-43 uptake and electron microscopy. NMJs with an almost complete loss of synaptic vesicles did not show an accumulation of intermediates of the budding process, whereas NMJs with only slightly reduced levels of synaptic vesicles showed a striking increase in early-stage, but not late-stage, budding intermediates at the plasma membrane. Together with results of previous studies, these observations indicate that endophilin A is essential for synaptic vesicle endocytosis, being required from the onset of budding until fission.

Stathmin is a ubiquitous regulatory phosphoprotein, the generic element of a family of neural phosphoproteins in vertebrates that possess the capacity to bind tubulin and interfere with microtubule dynamics. Although stathmin and the other proteins of the family have been associated with numerous cell regulations, their biological roles remain elusive, as in particular inactivation of the stathmin gene in the mouse resulted in no clear deleterious phenotype. We identified stathmin phosphoproteins inDrosophila, encoded by a unique gene sharing the intron/exon structure of the vertebrate stathmin andstathmin family genes. They interfere with microtubule assembly in vitro, and in vivo when expressed in HeLa cells. Drosophila stathmin expression is regulated during embryogenesis: it is high in the migrating germ cells and in the central and peripheral nervous systems, a pattern resembling that of mammalian stathmin. Furthermore, RNA interference inactivation ofDrosophila stathmin expression resulted in germ cell migration arrest at stage 14. It also induced important anomalies in nervous system development, such as loss of commissures and longitudinal connectives in the ventral cord, or abnormal chordotonal neuron organization. In conclusion, a single Drosophilagene encodes phosphoproteins homologous to the entire vertebrate stathmin family. We demonstrate for the first time their direct involvement in major biological processes such as development of the reproductive and nervous systems.

In Drosophila, dorsoventral polarity is established by the asymmetric positioning of the oocyte nucleus. In egg chambers mutant for cap ‘n’ collar, the oocyte nucleus migrates correctly from a posterior to an anterior–dorsal position where it remains during stage 9 of oogenesis. However, at the end of stage 9, the nucleus leaves its anterior position and migrates towards the posterior pole. The mislocalisation of the nucleus is accompanied by changes in the microtubule network and a failure to maintain bicoid and oskar mRNAs at the anterior and posterior poles, respectively. gurken mRNA associates with the oocyte nucleus in cap ‘n’ collar mutants and initially the local secretion of Gurken protein activates the Drosophila EGF receptor in the overlying dorsal follicle cells. However, despite the presence of spatially correct Grk signalling during stage 9, eggs laid by cap ‘n’ collar females lack dorsoventral polarity. cap ‘n’ collar mutants, therefore, allow for the study of the influence of Grk signal duration on DV patterning in the follicular epithelium.

In Drosophila, the polarity cues controlling the axis formation of the adult fly are asymmetrically distributed in the oocyte, a process depending on the microtubule (MT) network. In order to identify new genes controlling the oocyte polarity, a genetic screen after mutagenesis has been done in the laboratory. In one of the mutants identified, that we named mercury, the posterior determinant oskar mRNA and the nucleus were strongly mislocalised. In the Drosophila germline, the germ cells are surrounded by a monolayer of somatic epithelial cells called the follicle cells. In mercury mutant follicle cells, the apical localisation of Par3 and aPKC and the lateral localization of α-spectrin were strongly disrupted indicating a loss of polarity. The follicle cells also formed multilayers, a characteristic of mispolarised cells. We next mapped the mutation and identified the mutated gene as the homologous of mammalian Tubulin Binding Cofactor B (TBCB). Tubulin binding cofactors have been demonstrated to be necessary for the formation of the αβ-tubulin heterodimers and are therefore potential regulators of the αβ-tubulin pool and of the microtubule stability. In agreement with this, we found that, in mercury mutant oocytes and follicle cells, the microtubule network was strongly destabilised. As the establishment of polarity is a MT dependant process, the loss of polarity observed in mercury mutant cells is probably a consequence of the destabilization of the MT network. Finally, we found that total α- and β-tubulin protein decreased strongly in the mutant cells.

Nuclear hormone receptors and homeodomain proteins are two classes of transcription factor that regulate major developmental processes. Both depend on interactions with other proteins for specificity and activity. The Drosophila gene fushi tarazu (ftz)y which encodes a homeodomain protein1 (Ftz), is required zygo-tically for the formation of alternate segments in the developing embryo2. Here we show that the orphan nuclear receptor α Ftz-Fl (ref. 3), which is deposited in the egg during oogenesis4, is an obligatory cofactor for Ftz. The two proteins interact specifically and directly, both in vitro and in vivo, through a conserved domain in the Ftz polypeptide. This interaction suggests that other nuclear receptor/homeodomain protein interactions maybe important and common in developing organisms.

THE localization of oskar (osk) RNA to the posterior pole of the developing fruit fly (Drosophila) oocyte induces the assembly of pole plasm, causing development of the abdomen and germ line1,2. Failure to localize oskar RNA results in embryos that lack abdomen and germ cells. Conversely, mis-targeting of oskar RNA to the anterior of the oocyte causes formation of ectopic abdomen and germ cells at the anterior pole3. Maternal mutants that have reduced pole plasm activity produce sterile adults with normal abdominal development, suggesting that germ cells are more sensitive than abdomen to defects in pole plasm assembly4. Thus mutations in genes that reduce oskar RNA localization or activity can be recovered as viable sterile adults. In a screen for mutants defective in germ cell formation, we isolated nine alleles of the tropomyosin II gene5. Here we show that mutations in tropomyosin II (TmII) virtually abolish oskar RNA localization to the posterior pole, suggesting an involvement of the actin network in oskar RNA localization.