Requirement for phosphorylation and localization of the Bicaudal-D protein in Drosophila oocyte differentiation

To be or not to be: orb, the fusome and oocyte specification in Drosophila

In the fruit fly Drosophila melanogaster, two cells in a cyst of 16 interconnected cells have the potential to become the oocyte, but only one of these will assume an oocyte fate as the cysts transition through regions 2a and 2b of the germarium. The mechanism of specification depends on a polarized microtubule network, a dynein dependent Egl:BicD mRNA cargo complex, a special membranous structure called the fusome and its associated proteins, and the translational regulator orb. In this work, we have investigated the role of orb and the fusome in oocyte specification. We show here that specification is a stepwise process. Initially, orb mRNAs accumulate in the two pro-oocytes in close association with the fusome. This association is accompanied by the activation of the orb autoregulatory loop, generating high levels of Orb. Subsequently, orb mRNAs become enriched in only one of the pro-oocytes, the presumptive oocyte, and this is followed, with a delay, by Orb localization to the oocyte. We find that fusome association of orb mRNAs is essential for oocyte specification in the germarium, is mediated by the orb 3' UTR, and requires Orb protein. We also show that the microtubule minus end binding protein Patronin functions downstream of orb in oocyte specification. Finally, in contrast to a previously proposed model for oocyte selection, we find that the choice of which pro-oocyte becomes the oocyte does not seem to be predetermined by the amount of fusome material in these two cells, but instead depends upon a competition for orb gene products.

Role of BicDR in bristle shaft construction and support of BicD functions

Cell polarization requires asymmetric localization of numerous mRNAs, proteins and organelles. The movement of cargo towards the minus end of microtubules mostly depends on cytoplasmic dynein motors. In the dynein-dynactin-Bicaudal-D transport machinery, Bicaudal-D (BicD) links the cargo to the motor. Here, we focus on the role of Drosophila BicD-related (BicDR, CG32137) in the development of the long bristles. Together with BicD, it contributes to the organization and stability of the actin cytoskeleton in the not-yet-chitinized bristle shaft. BicD and BicDR also support the stable expression and distribution of Rab6 and Spn-F in the bristle shaft, including the distal tip localization of Spn-F, pointing to the role of microtubule-dependent vesicle trafficking for bristle construction. BicDR supports the function of BicD, and we discuss the hypothesis whereby BicDR might transport cargo more locally, with BicD transporting cargo over long distances, such as to the distal tip. We also identified embryonic proteins that interact with BicDR and appear to be BicDR cargo. For one of them, EF1γ (also known as eEF1γ), we show that the encoding gene EF1γ interacts with BicD and BicDR in the construction of the bristles.

Role of BicDR in bristle shaft construction, tracheal development, and support of BicD functions

Cell polarization requires asymmetric localization of numerous mRNAs, proteins, and organelles. The movement of cargo towards the minus end of microtubules mostly depends on cytoplasmic dynein motors, which function as multiprotein complexes. In the dynein/dynactin/Bicaudal-D (DDB) transport machinery, Bicaudal-D (BicD) links the cargo to the motor. Here we focus on the role of BicD-related (BicDR) and its contribution to microtubule-dependent transport processes. Drosophila BicDR is required for the normal development of bristles and dorsal trunk tracheae. Together with BicD, it contributes to the organization and stability of the actin cytoskeleton in the not-yet-chitinized bristle shaft and the localization of Spn-F and Rab6 at the distal tip. We show that BicDR supports the function of BicD in bristle development and our results suggest that BicDR transports cargo more locally whereas BicD is more responsible for delivering functional cargo over the long distance to the distal tip. We identified the proteins that interact with BicDR and appear to be BicDR cargo in embryonic tissues. For one of them, EF1γ, we showed that EF1γ genetically interacts with BicD and BicDR in the construction of the bristles.

dTtc1, a conserved tetratricopeptide repeat protein, is required for maturation of Drosophila egg chambers via its role in stabilizing electron transport chain components

We recently identified the Drosophila ortholog of TTC1 (dTtc1) as an interacting partner of Egalitarian, an RNA adaptor of the Dynein motor. In order to better understand the function of this relatively uncharacterized protein, we depleted dTtc1 in the Drosophila female germline. Depletion of dTtc1 resulted in defective oogenesis and no mature eggs were produced. A closer examination revealed that mRNA cargoes normally transported by Dynein were relatively unaffected. However, mitochondria in dTtc1 depleted egg chambers displayed an extremely swollen phenotype. Ultrastructural analysis revealed a lack of cristae. These phenotypes were not observed upon disruption of Dynein. Thus, this function of dTtc1 is likely to be Dynein independent. Consistent with a role for dTtc1 in mitochondrial biology, a published proteomics screen revealed that dTtc1 interacts with numerous components of electron transport chain (ETC) complexes. Our results indicate that the expression level of several of these ETC components was significantly reduced upon depletion of dTtc1. Importantly, this phenotype was completely rescued upon expression of wild-type GFP-dTtc1 in the depleted background. Lastly, we demonstrate that the mitochondrial phenotype caused by a lack of dTtc1 is not restricted to the germline but is also observed in somatic tissues. Our model suggests that dTtc1, likely in combination with cytoplasmic chaperones, is required for stabilizing ETC components.

BICD2 phosphorylation regulates dynein function and centrosome separation in G2 and M

The activity of dynein is regulated by a number of adaptors that mediate its interaction with dynactin, effectively activating the motor complex while also connecting it to different cargos. The regulation of adaptors is consequently central to dynein physiology but remains largely unexplored. We now describe that one of the best-known dynein adaptors, BICD2, is effectively activated through phosphorylation. In G2, phosphorylation of BICD2 by CDK1 promotes its interaction with PLK1. In turn, PLK1 phosphorylation of a single residue in the N-terminus of BICD2 results in a structural change that facilitates the interaction with dynein and dynactin, allowing the formation of active motor complexes. Moreover, modified BICD2 preferentially interacts with the nucleoporin RanBP2 once RanBP2 has been phosphorylated by CDK1. BICD2 phosphorylation is central for dynein recruitment to the nuclear envelope, centrosome tethering to the nucleus and centrosome separation in the G2 and M phases of the cell cycle. This work reveals adaptor activation through phosphorylation as crucial for the spatiotemporal regulation of dynein activity.

Fusome topology and inheritance during insect gametogenesis

From insects to mammals, oocytes and sperm develop within germline cysts comprising cells connected by intercellular bridges (ICBs). In numerous insects, formation of the cyst is accompanied by growth of the fusome-a membranous organelle that permeates the cyst. Fusome composition and function are best understood in Drosophila melanogaster: during oogenesis, the fusome dictates cyst topology and size and facilitates oocyte selection, while during spermatogenesis, the fusome synchronizes the cyst's response to DNA damage. Despite its distinct and sex-specific roles during insect gametogenesis, elucidating fusome growth and inheritance in females and its structure and connectivity in males has remained challenging. Here, we take advantage of advances in three-dimensional (3D) confocal microscopy and computational image processing tools to reconstruct the topology, growth, and distribution of the fusome in both sexes. In females, our experimental findings inform a theoretical model for fusome assembly and inheritance and suggest that oocyte selection proceeds through an 'equivalency with a bias' mechanism. In males, we find that cell divisions can deviate from the maximally branched pattern observed in females, leading to greater topological variability. Our work consolidates existing disjointed experimental observations and contributes a readily generalizable computational approach for quantitative studies of gametogenesis within and across species.

Polarity and axis formation in the Drosophila female germ line

By the time a Drosophila egg is laid, both major body axes have already been defined and it contains all the nutrients needed to develop into a free-living larva in 24 h. By contrast, it takes almost a week to make an egg from a female germline stem cell, during the complex process of oogenesis. This review will discuss key symmetry-breaking steps in Drosophila oogenesis that lead to the polarisation of both body axes: the asymmetric divisions of the germline stem cells; the selection of the oocyte from the 16-cell germline cyst; the positioning of the oocyte at the posterior of the cyst; Gurken signalling from the oocyte to polarise the anterior-posterior axis of the somatic follicle cell epithelium around the developing germline cyst; the signalling back from the posterior follicle cells to polarise the anterior-posterior axis of the oocyte; and the migration of the oocyte nucleus that specifies the dorsal-ventral axis. Since each event creates the preconditions for the next, I will focus on the mechanisms that drive these symmetry-breaking steps, how they are linked and the outstanding questions that remain to be answered.

Subcellular spatial transcriptomics identifies three mechanistically different classes of localizing RNAs

Intracellular RNA localization is a widespread and dynamic phenomenon that compartmentalizes gene expression and contributes to the functional polarization of cells. Thus far, mechanisms of RNA localization identified in Drosophila have been based on a few RNAs in different tissues, and a comprehensive mechanistic analysis of RNA localization in a single tissue is lacking. Here, by subcellular spatial transcriptomics we identify RNAs localized in the apical and basal domains of the columnar follicular epithelium (FE) and we analyze the mechanisms mediating their localization. Whereas the dynein/BicD/Egl machinery controls apical RNA localization, basally-targeted RNAs require kinesin-1 to overcome a default dynein-mediated transport. Moreover, a non-canonical, translation- and dynein-dependent mechanism mediates apical localization of a subgroup of dynein-activating adaptor-encoding RNAs (BicD, Bsg25D, hook). Altogether, our study identifies at least three mechanisms underlying RNA localization in the FE, and suggests a possible link between RNA localization and dynein/dynactin/adaptor complex formation in vivo.

Female meiosis II and pronuclear fusion require the microtubule transport factor Bicaudal D

Bicaudal D (BicD) is a dynein adaptor that transports different cargoes along microtubules. Reducing the activity of BicD specifically in freshly laid Drosophila eggs by acute protein degradation revealed that BicD is needed to produce normal female meiosis II products, to prevent female meiotic products from re-entering the cell cycle, and for pronuclear fusion. Given that BicD is required to localize the spindle assembly checkpoint (SAC) components Mad2 and BubR1 to the female meiotic products, it appears that BicD functions to localize these components to control metaphase arrest of polar bodies. BicD interacts with Clathrin heavy chain (Chc), and both proteins localize to centrosomes, mitotic spindles and the tandem spindles during female meiosis II. Furthermore, BicD is required to localize clathrin and the microtubule-stabilizing factors transforming acidic coiled-coil protein (D-TACC/Tacc) and Mini spindles (Msps) correctly to the meiosis II spindles, suggesting that failure to localize these proteins may perturb SAC function. Furthermore, immediately after the establishment of the female pronucleus, D-TACC and Caenorhabditis elegans BicD, tacc and Chc are also needed for pronuclear fusion, suggesting that the underlying mechanism might be more widely used across species.

Polarity Events in the Drosophila melanogaster Oocyte

Cell polarity is a pre-requirement for many fundamental processes in animal cells, such as asymmetric cell division, axon specification, morphogenesis and epithelial tissue formation. For all these different processes, polarization is established by the same set of proteins, called partitioning defective (Par) proteins. During development in Drosophila melanogaster, decision making on the cellular and organism level is achieved with temporally controlled cell polarization events. The initial polarization of Par proteins occurs as early as in the germline cyst, when one of the 16 cells becomes the oocyte. Another marked event occurs when the anterior–posterior axis of the future organism is defined by Par redistribution in the oocyte, requiring external signaling from somatic cells. Here, we review the current literature on cell polarity events that constitute the oogenesis from the stem cell to the mature egg.

Dynamics of maternal gene expression in Rhodnius prolixus

The study of developmental processes in Rhodnius prolixus has recently advanced with the sequencing of the genome. In this work, we analyze the maternal gene expression driving oogenesis and early embryogenesis in R. prolixus. We examined the transcriptional profile of mRNAs to establish the genes expressed across the ovary, unfertilized eggs and different embryonic stages of R. prolixus until the formation of the germ band anlage (0, 12, 24, and 48 h post egg laying). We identified 81 putative maternal and ovary-related genes and validated their expression by qRT-PCR. We validate the function of the ortholog gene Bicaudal-D (Rp-BicD) by in situ hybridization and parental RNAi. Consistent with a role in oogenesis and early development of R. prolixus, we show that lack of Rp-BicD does not significantly affect oogenesis but impairs the formation of the blastoderm. Based on our findings, we propose three times of action for maternal genes during oogenesis and embryogenesis in R. prolixus.

Symmetry breaking in the female germline cyst

In mammals and flies, only one cell in a multicellular female germline cyst becomes an oocyte, but how symmetry is broken to select the oocyte is unknown. Here, we show that the microtubule (MT) minus end-stabilizing protein Patronin/CAMSAP marks the future Drosophila oocyte and is required for oocyte specification. The spectraplakin Shot recruits Patronin to the fusome, a branched structure extending into all cyst cells. Patronin stabilizes more MTs in the cell with the most fusome material. Our data suggest that this weak asymmetry is amplified by Dynein-dependent transport of Patronin-stabilized MTs. This forms a polarized MT network, along which Dynein transports oocyte determinants into the presumptive oocyte. Thus, Patronin amplifies a weak fusome anisotropy to break symmetry and select one cell to become the oocyte.

Coordinating Proliferation, Polarity, and Cell Fate in the Drosophila Female Germline

Gametes are highly specialized cell types produced by a complex differentiation process. Production of viable oocytes requires a series of precise and coordinated molecular events. Early in their development, germ cells are an interconnected group of mitotically dividing cells. Key regulatory events lead to the specification of mature oocytes and initiate a switch to the meiotic cell cycle program. Though the chromosomal events of meiosis have been extensively studied, it is unclear how other aspects of oocyte specification are temporally coordinated. The fruit fly, Drosophila melanogaster, has long been at the forefront as a model system for genetics and cell biology research. The adult Drosophila ovary continuously produces germ cells throughout the organism’s lifetime, and many of the cellular processes that occur to establish oocyte fate are conserved with mammalian gamete development. Here, we review recent discoveries from Drosophila that advance our understanding of how early germ cells balance mitotic exit with meiotic initiation. We discuss cell cycle control and establishment of cell polarity as major themes in oocyte specification. We also highlight a germline-specific organelle, the fusome, as integral to the coordination of cell division, cell polarity, and cell fate in ovarian germ cells. Finally, we discuss how the molecular controls of the cell cycle might be integrated with cell polarity and cell fate to maintain oocyte production.

The Drosophila CPEB Protein Orb Specifies Oocyte Fate by a 3'UTR-Dependent Autoregulatory Loop

orb encodes one of the two fly CPEB proteins. These widely conserved proteins bind to the 3'UTRs of target messenger RNAs (mRNAs) and activate or repress their translation. We show here that a positive autoregulatory loop driven by the orb gene propels the specification of oocyte identity in Drosophila egg chambers. Oocyte fate specification is mediated by a 3'UTR-dependent mechanism that concentrates orb mRNAs and proteins in one of the two pro-oocytes in the 16-cell germline cyst. When the orb 3'UTR is deleted, orb mRNA and protein fail to localize and all 16 cells become nurse cells. In wild type, the oocyte is specified when orb and other gene products concentrate in a single cell in region 2b of the germarium. A partially functional orb 3'UTR replacement delays oocyte specification until the egg chambers reach stage 2 of oogenesis. Before this point, orb mRNA and protein are unlocalized, as are other markers of oocyte identity, and the oocyte is not specified. After stage 2, ∼50% of the chambers successfully localize orb in a single cell, and this cell assumes oocyte identity. In the remaining chambers, the orb autoregulatory loop is not activated and no oocyte is formed. Finally, maintenance of oocyte identity requires continuous orb activity.

A Developmental Program Truncates Long Transcripts to Temporally Regulate Cell Signaling

Rapid mitotic divisions and a fixed transcription rate limit the maximal length of transcripts in early Drosophila embryos. Previous studies suggested that transcription of long genes is initiated but aborted, as early nuclear divisions have short interphases. Here, we identify long genes that are expressed during short nuclear cycles as truncated transcripts. The RNA binding protein Sex-lethal physically associates with transcripts for these genes and is required to support early termination to specify shorter transcript isoforms in early embryos of both sexes. In addition, one truncated transcript for the gene short-gastrulation encodes a product in embryos that functionally relates to a previously characterized dominant-negative form, which maintains TGF-β signaling in the off-state. In summary, our results reveal a developmental program of short transcripts functioning to help temporally regulate Drosophila embryonic development, keeping cell signaling at early stages to a minimum in order to support its proper initiation at cellularization.

Dynein activators and adaptors at a glance

Cytoplasmic dynein-1 (hereafter dynein) is an essential cellular motor that drives the movement of diverse cargos along the microtubule cytoskeleton, including organelles, vesicles and RNAs. A long-standing question is how a single form of dynein can be adapted to a wide range of cellular functions in both interphase and mitosis. Recent progress has provided new insights - dynein interacts with a group of activating adaptors that provide cargo-specific and/or function-specific regulation of the motor complex. Activating adaptors such as BICD2 and Hook1 enhance the stability of the complex that dynein forms with its required activator dynactin, leading to highly processive motility toward the microtubule minus end. Furthermore, activating adaptors mediate specific interactions of the motor complex with cargos such as Rab6-positive vesicles or ribonucleoprotein particles for BICD2, and signaling endosomes for Hook1. In this Cell Science at a Glance article and accompanying poster, we highlight the conserved structural features found in dynein activators, the effects of these activators on biophysical parameters, such as motor velocity and stall force, and the specific intracellular functions they mediate.

Subcellular Specialization and Organelle Behavior in Germ Cells

Gametes, eggs and sperm, are the highly specialized cell types on which the development of new life solely depends. Although all cells share essential organelles, such as the ER (endoplasmic reticulum), Golgi, mitochondria, and centrosomes, germ cells display unique regulation and behavior of organelles during gametogenesis. These germ cell-specific functions of organelles serve critical roles in successful gamete production. In this chapter, I will review the behaviors and roles of organelles during germ cell differentiation.

Protecting and Diversifying the Germline

Gametogenesis represents the most dramatic cellular differentiation pathways in both female and male flies. At the genome level, meiosis ensures that diploid germ cells become haploid gametes. At the epigenome level, extensive changes are required to turn on and shut off gene expression in a precise spatiotemporally controlled manner. Research applying conventional molecular genetics and cell biology, in combination with rapidly advancing genomic tools have helped us to investigate (1) how germ cells maintain lineage specificity throughout their adult reproductive lifetime; (2) what molecular mechanisms ensure proper oogenesis and spermatogenesis, as well as protect genome integrity of the germline; (3) how signaling pathways contribute to germline-soma communication; and (4) if such communication is important. In this chapter, we highlight recent discoveries that have improved our understanding of these questions. On the other hand, restarting a new life cycle upon fertilization is a unique challenge faced by gametes, raising questions that involve intergenerational and transgenerational epigenetic inheritance. Therefore, we also discuss new developments that link changes during gametogenesis to early embryonic development-a rapidly growing field that promises to bring more understanding to some fundamental questions regarding metazoan development.

RNA localization and transport

RNA localization serves numerous purposes from controlling development and differentiation to supporting the physiological activities of cells and organisms. After a brief introduction into the history of the study of mRNA localization I will focus on animal systems, describing in which cellular compartments and in which cell types mRNA localization was observed and studied. In recent years numerous novel localization patterns have been described, and countless mRNAs have been documented to accumulate in specific subcellular compartments. These fascinating revelations prompted speculations about the purpose of localizing all these mRNAs. In recent years experimental evidence for an unexpected variety of different functions has started to emerge. Aside from focusing on the functional aspects, I will discuss various ways of localizing mRNAs with a focus on the mechanism of active and directed transport on cytoskeletal tracks. Structural studies combined with imaging of transport and biochemical studies have contributed to the enormous recent progress, particularly in understanding how dynein/dynactin/BicD (DDB) dependent transport on microtubules works. This transport process actively localizes diverse cargo in similar ways to the minus end of microtubules and, at least in flies, also individual mRNA molecules. A sophisticated mechanism ensures that cargo loading licenses processive transport.

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

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

Cbp80 is needed for the expression of piRNA components and piRNAs

Cap binding protein 80 (Cbp80) is the larger subunit of the nuclear cap-binding complex (nCBC), which is known to play important roles in nuclear mRNA processing, export, stability and quality control events. Reducing Cbp80 mRNA levels in the female germline revealed that Cbp80 is also involved in defending the germline against transposable elements. Combining such knockdown experiments with large scale sequencing of small RNAs further showed that Cbp80 is involved in the initial biogenesis of piRNAs as well as in the secondary biogenesis pathway, the ping-pong amplification cycle. We further found that Cbp80 knockdown not only led to the upregulation of transposons, but also to delocalization of Piwi, Aub and Ago3, key factors in the piRNA biosynthesis pathway. Furthermore, compared to controls, levels of Piwi and Aub were also reduced upon knock down of Cbp80. On the other hand, with the same treatment we could not detect significant changes in levels or subcellular distribution (nuage localization) of piRNA precursor transcripts. This shows that Cbp80 plays an important role in the production and localization of the protein components of the piRNA pathway and it seems to be less important for the production and export of the piRNA precursor transcripts.

Neuronal Roles of the Bicaudal D Family of Motor Adaptors

All cell types rely on active intracellular cargo transport to shuttle essential cellular components such as proteins, lipids, RNA, and even organelles from the center to the periphery and vice versa. Additionally, several signaling pathways take advantage of intracellular transport to propagate their signals by moving activated receptors and protein effectors to specific locations inside the cell. Neurons particularly, being a very polarized cell type, are highly dependent on molecular motors for the anterograde and retrograde delivery of essential cellular components and signaling molecules. For these reasons, motor adaptor proteins have been extensively investigated in regard to their role in physiology and pathology of the nervous system. In this chapter, we will concentrate on a family of motor adaptor proteins, Bicaudal D (BICD), and their function in the context of the nervous system. BicD was originally described as essential for the correct localization of maternal mRNAs in Drosophila's oocyte and a regulator of the Golgi to ER retrograde transport in mammalian cells. Both mammalian BICD1 and BICD2 are highly expressed in the nervous system during development, and their importance in neuronal homeostasis has been recently under scrutiny. Several mutations in BICD2 have been linked to the development of neuromuscular diseases, and BICD2 knockout (KO) mice display migration defects of the radial cerebellar granule cells. More in line with the overall topic of this book, BICD1 was identified as a novel regulator of neurotrophin (NT) signaling as its deletion leads to defective sorting of ligand-activated NT receptors with dramatic consequences on the NT-mediated signaling pathway.

The mRNA transportome of the BicD/Egl transport machinery

mRNA (mRNA) transport focuses the expression of encoded proteins to specific regions within cells providing them with the means to assume specific functions and even identities. BicD and the mRNA binding protein Egl interact with the microtubule motor dynein to localize mRNAs in Drosophila. Because relatively few mRNA cargos were known, we isolated and identified Egl::GFP associated mRNAs. The top candidates were validated by qPCR, in situ hybridization and genetically by showing that their localization requires BicD. In young embryos these Egl target mRNAs are preferentially localized apically, between the plasma membrane and the blastoderm nuclei, but also in the pole plasm at the posterior pole. Egl targets expressed in the ovary were mostly enriched in the oocyte and some were apically localized in follicle cells. The identification of a large group of novel mRNAs associated with BicD/Egl points to several novel developmental and physiological functions of this dynein dependent localization machinery. The verified dataset also allowed us to develop a tool that predicts conserved A'-form-like stem loops that serve as localization elements in 3′UTRs.

Reduced-representation Phosphosignatures Measured by Quantitative Targeted MS Capture Cellular States and Enable Large-scale Comparison of Drug-induced Phenotypes

Profiling post-translational modifications represents an alternative dimension to gene expression data in characterizing cellular processes. Many cellular responses to drugs are mediated by changes in cellular phosphosignaling. We sought to develop a common platform on which phosphosignaling responses could be profiled across thousands of samples, and created a targeted MS assay that profiles a reduced-representation set of phosphopeptides that we show to be strong indicators of responses to chemical perturbagens.To develop the assay, we investigated the coordinate regulation of phosphosites in samples derived from three cell lines treated with 26 different bioactive small molecules. Phosphopeptide analytes were selected from these discovery studies by clustering and picking 1 to 2 proxy members from each cluster. A quantitative, targeted parallel reaction monitoring assay was developed to directly measure 96 reduced-representation probes. Sample processing for proteolytic digestion, protein quantification, peptide desalting, and phosphopeptide enrichment have been fully automated, making possible the simultaneous processing of 96 samples in only 3 days, with a plate phosphopeptide enrichment variance of 12%. This highly reproducible process allowed ∼95% of the reduced-representation phosphopeptide probes to be detected in ∼200 samples.The performance of the assay was evaluated by measuring the probes in new samples generated under treatment conditions from discovery experiments, recapitulating the observations of deeper experiments using a fraction of the analytical effort. We measured these probes in new experiments varying the treatments, cell types, and timepoints to demonstrate generalizability. We demonstrated that the assay is sensitive to disruptions in common signaling pathways (e.g. MAPK, PI3K/mTOR, and CDK). The high-throughput, reduced-representation phosphoproteomics assay provides a platform for the comparison of perturbations across a range of biological conditions, suitable for profiling thousands of samples. We believe the assay will prove highly useful for classification of known and novel drug and genetic mechanisms through comparison of phosphoproteomic signatures.

Bicaudal D Family of Motor Adaptors: Linking Dynein Motility to Cargo Binding

Transport of different intracellular cargoes along cytoskeleton filaments is essential for the morphogenesis and function of a broad variety of eukaryotic cells. Intracellular transport is mediated by cytoskeletal motors including myosin, kinesin, and dynein, which are typically linked to various cargoes by adaptor proteins. Recent studies suggest that adaptor proteins can also act as essential transport cofactors, which control motor activity and coordination. Characterization of the evolutionary conserved Bicaudal D (BICD) family of dynein adaptor proteins has provided important insights into the fundamental mechanisms governing cargo trafficking. This review highlights the advances in the current understanding of how BICD adaptors regulate microtubule-based transport and how they contribute to developmental processes and human disease.

A Drosophila XPD model links cell cycle coordination with neuro-development and suggests links to cancer

XPD functions in transcription, DNA repair and in cell cycle control. Mutations in human XPD (also known as ERCC2) mainly cause three clinical phenotypes: xeroderma pigmentosum (XP), Cockayne syndrome (XP/CS) and trichothiodystrophy (TTD), and only XP patients have a high predisposition to developing cancer. Hence, we developed a fly model to obtain novel insights into the defects caused by individual hypomorphic alleles identified in human XP-D patients. This model revealed that the mutations that displayed the greatest in vivo UV sensitivity in Drosophila did not correlate with those that led to tumor formation in humans. Immunoprecipitations followed by targeted quantitative MS/MS analysis showed how different xpd mutations affected the formation or stability of different transcription factor IIH (TFIIH) subcomplexes. The XP mutants most clearly linked to high cancer risk, Xpd R683W and R601L, showed a reduced interaction with the core TFIIH and also an abnormal interaction with the Cdk-activating kinase (CAK) complex. Interestingly, these two XP alleles additionally displayed high levels of chromatin loss and free centrosomes during the rapid nuclear division phase of the Drosophila embryo. Finally, the xpd mutations showing defects in the coordination of cell cycle timing during the Drosophila embryonic divisions correlated with those human mutations that cause the neurodevelopmental abnormalities and developmental growth defects observed in XP/CS and TTD patients.

Clathrin heavy chain plays multiple roles in polarizing the Drosophila oocyte downstream of Bic-D

Bicaudal-D (Bic-D), Egalitarian (Egl), microtubules and their motors form a transport machinery that localizes a remarkable diversity of mRNAs to specific cellular regions during oogenesis and embryogenesis. Bic-D family proteins also promote dynein-dependent transport of Golgi vesicles, lipid droplets, synaptic vesicles and nuclei. However, the transport of these different cargoes is still poorly understood. We searched for novel proteins that either mediate Bic-D-dependent transport processes or are transported by them. Clathrin heavy chain (Chc) co-immunopurifies with Bic-D in embryos and ovaries, and a fraction of Chc colocalizes with Bic-D. Both proteins control posterior patterning of the Drosophila oocyte and endocytosis. Although the role of Chc in endocytosis is well established, our results show that Bic-D is also needed for the elevated endocytic activity at the posterior of the oocyte. Apart from affecting endocytosis indirectly by its role in osk mRNA localization, Bic-D is also required to transport Chc mRNA into the oocyte and for transport and proper localization of Chc protein to the oocyte cortex, pointing to an additional, more direct role of Bic-D in the endocytic pathway. Furthermore, similar to Bic-D, Chc also contributes to proper localization of osk mRNA and to oocyte growth. However, in contrast to other endocytic components and factors of the endocytic recycling pathway, such as Rabenosyn-5 (Rbsn-5) and Rab11, Chc is needed during early stages of oogenesis (from stage 6 onwards) to localize osk mRNA correctly. Moreover, we also uncovered a novel, presumably endocytosis-independent, role of Chc in the establishment of microtubule polarity in stage 6 oocytes.

A stem-loop structure directs oskar mRNA to microtubule minus ends

mRNA transport coupled with translational control underlies the intracellular localization of many proteins in eukaryotic cells. This is exemplified in Drosophila, where oskar mRNA transport and translation at the posterior pole of the oocyte direct posterior patterning of the embryo. oskar localization is a multistep process. Within the oocyte, a spliced oskar localization element (SOLE) targets oskar mRNA for plus end-directed transport by kinesin-1 to the posterior pole. However, the signals mediating the initial minus end-directed, dynein-dependent transport of the mRNA from nurse cells into the oocyte have remained unknown. Here, we show that a 67-nt stem-loop in the oskar 3' UTR promotes oskar mRNA delivery to the developing oocyte and that it shares functional features with the fs(1)K10 oocyte localization signal. Thus, two independent cis-acting signals, the oocyte entry signal (OES) and the SOLE, mediate sequential dynein- and kinesin-dependent phases of oskar mRNA transport during oogenesis. The OES also promotes apical localization of injected RNAs in blastoderm stage embryos, another dynein-mediated process. Similarly, when ectopically expressed in polarized cells of the follicular epithelium or salivary glands, reporter RNAs bearing the oskar OES are apically enriched, demonstrating that this element promotes mRNA localization independently of cell type. Our work sheds new light on how oskar mRNA is trafficked during oogenesis and the RNA features that mediate minus end-directed transport.

asunder is required for dynein localization and dorsal fate determination during Drosophila oogenesis

We previously showed that asunder (asun) is a critical regulator of dynein localization during Drosophila spermatogenesis. Because the expression of asun is much higher in Drosophila ovaries and early embryos than in testes, we herein sought to determine whether ASUN plays roles in oogenesis and/or embryogenesis. We characterized the female germline phenotypes of flies homozygous for a null allele of asun (asun(d93)). We find that asun(d93) females lay very few eggs and contain smaller ovaries with a highly disorganized arrangement of ovarioles in comparison to wild-type females. asun(d93) ovaries also contain a significant number of egg chambers with structural defects. A majority of the eggs laid by asun(d93) females are ventralized to varying degrees, from mild to severe; this ventralization phenotype may be secondary to defective localization of gurken transcripts, a dynein-regulated step, within asun(d93) oocytes. We find that dynein localization is aberrant in asun(d93) oocytes, indicating that ASUN is required for this process in both male and female germ cells. In addition to the loss of gurken mRNA localization, asun(d93) ovaries exhibit defects in other dynein-mediated processes such as migration of nurse cell centrosomes into the oocyte during the early mitotic divisions, maintenance of the oocyte nucleus in the anterior-dorsal region of the oocyte in late-stage egg chambers, and coupling between the oocyte nucleus and centrosomes. Taken together, our data indicate that asun is a critical regulator of dynein localization and dynein-mediated processes during Drosophila oogenesis.

Lissencephaly-1 promotes the recruitment of dynein and dynactin to transported mRNAs

Lissencephaly-1 promotes the interaction of dynein with dynactin and facilitates motor complex association with mRNA cargos.

Microtubule-based transport mediates the sorting and dispersal of many cellular components and pathogens. However, the mechanisms by which motor complexes are recruited to and regulated on different cargos remain poorly understood. Here we describe a large-scale biochemical screen for novel factors associated with RNA localization signals mediating minus end–directed mRNA transport during Drosophila development. We identified the protein Lissencephaly-1 (Lis1) and found that minus-end travel distances of localizing transcripts are dramatically reduced in lis1 mutant embryos. Surprisingly, given its well-documented role in regulating dynein mechanochemistry, we uncovered an important requirement for Lis1 in promoting the recruitment of dynein and its accessory complex dynactin to RNA localization complexes. Furthermore, we provide evidence that Lis1 levels regulate the overall association of dynein with dynactin. Our data therefore reveal a critical role for Lis1 within the mRNA localization machinery and suggest a model in which Lis1 facilitates motor complex association with cargos by promoting the interaction of dynein with dynactin.

PRC2 controls Drosophila oocyte cell fate by repressing cell cycle genes

The oocyte is a unique cell type that undergoes extensive chromosome changes on its way to fertilization, but the chromatin determinants of its fate are unknown. Here, we show that Polycomb group (PcG) proteins of the Polycomb repressive complex 2 (PRC2) determine the fate of the oocyte in Drosophila. Mutation of the enzymatic PRC2 subunit Enhancer of zeste (E(z)) in the germline abolishes spatial and temporal control of the cell cycle and induces sterility via transdetermination of the oocyte into a nurse-like cell. This fate switch depends on loss of silencing of two PRC2 target genes, Cyclin E and the cyclin-dependent kinase inhibitor dacapo. By contrast, the PRC1 component Polycomb (Pc) plays no role in this process. Our results demonstrate that PRC2 plays an exquisite role in the determination of the oocyte fate by preventing its switching into an endoreplicative program.

Drosophila sosie functions with β(H)-Spectrin and actin organizers in cell migration, epithelial morphogenesis and cortical stability

Morphogenesis in multicellular organisms requires the careful coordination of cytoskeletal elements, dynamic regulation of cell adhesion and extensive cell migration. sosie (sie) is a novel gene required in various morphogenesis processes in Drosophila oogenesis. Lack of sie interferes with normal egg chamber packaging, maintenance of epithelial integrity and control of follicle cell migration, indicating that sie is involved in controlling epithelial integrity and cell migration. For these functions sie is required both in the germ line and in the soma. Consistent with this, Sosie localizes to plasma membranes in the germ line and in the somatic follicle cells and is predicted to present an EGF-like domain on the extracellular side. Two positively charged residues, C-terminal to the predicted transmembrane domain (on the cytoplasmic side), are required for normal plasma membrane localization of Sosie. Because sie also contributes to normal cortical localization of β_H-Spectrin, it appears that cortical β_H-Spectrin mediates some of the functions of sosie. sie also interacts with the genes coding for the actin organizers Filamin and Profilin and, in the absence of sie function, F-actin is less well organized and nurse cells frequently fuse.

Conservation of the RNA Transport Machineries and Their Coupling to Translation Control across Eukaryotes

Restriction of proteins to discrete subcellular regions is a common mechanism to establish cellular asymmetries and depends on a coordinated program of mRNA localization and translation control. Many processes from the budding of a yeast to the establishment of metazoan embryonic axes and the migration of human neurons, depend on this type of cell polarization. How factors controlling transport and translation assemble to regulate at the same time the movement and translation of transported mRNAs, and whether these mechanisms are conserved across kingdoms is not yet entirely understood. In this review we will focus on some of the best characterized examples of mRNA transport machineries, the "yeast locasome" as an example of RNA transport and translation control in unicellular eukaryotes, and on the Drosophila Bic-D/Egl/Dyn RNA localization machinery as an example of RNA transport in higher eukaryotes. This focus is motivated by the relatively advanced knowledge about the proteins that connect the localizing mRNAs to the transport motors and the many well studied proteins involved in translational control of specific transcripts that are moved by these machineries. We will also discuss whether the core of these RNA transport machineries and factors regulating mRNA localization and translation are conserved across eukaryotes.

Single-molecule assays reveal that RNA localization signals regulate dynein-dynactin copy number on individual transcript cargoes

Subcellular localization of mRNAs by cytoskeletal motors plays critical roles in the spatial control of protein function¹. However, optical limitations of studying mRNA transport in vivo mean that there is little mechanistic insight into how transcripts are packaged and linked to motors, and how the movement of mRNA:motor complexes on the cytoskeleton is orchestrated. Here, we have reconstituted transport of mRNPs containing specific RNAs in vitro. We show directly that mRNAs that are either apically localized or non-localized in Drosophila embryos associate with the dynein motor and move bidirectionally on individual microtubules, with localizing mRNPs exhibiting a strong minus-end-directed bias. Single-molecule fluorescence measurements reveal that RNA localization signals increase the average number of dynein and dynactin components recruited to individual mRNPs. We find that, surprisingly, individual RNA molecules are present in motile mRNPs in vitro and present evidence that this is also the case in vivo. Thus, RNA oligomerization is not obligatory for transport. Our findings lead to a model in which RNA localization signals produce highly polarized distributions of transcript populations through modest changes in motor copy number on single mRNA molecules.

Roles of cytoplasmic RNP granules in intracellular RNA localization and translational control in the Drosophila oocyte

Intracellular mRNA localization and translation are ways to achieve asymmetric protein sorting in polarized cells, and they play fundamental roles in cell-fate decisions and body patterning during animal development. These processes are regulated by the interplay between cis-acting elements and trans-acting RNA-binding proteins that form and occur within a ribonucleoprotein (RNP) complex. Recent studies in the Drosophila oocyte have revealed that RNP complex assembly in the nucleus is critical for the regulation of cytoplasmic mRNA localization and translation. Furthermore, several trans-acting factors promote the reorganization of target mRNAs in the cytoplasm into higher-order RNP granules, which are often visible by light microscopy. Therefore, RNA localization and translation are likely to be coupled within these RNP granules. Notably, diverse cytoplasmic RNP granules observed in different cell types share conserved sets of proteins, suggesting they have fundamental and common cellular functions.

Pabp binds to the osk 3'UTR and specifically contributes to osk mRNA stability and oocyte accumulation

RNA localization is tightly coordinated with RNA stability and translation control. Bicaudal-D (Bic-D), Egalitarian (Egl), microtubules and their motors are part of a Drosophila transport machinery that localizes mRNAs to specific cellular regions during oogenesis and embryogenesis. We identified the Poly(A)-binding protein (Pabp) as a protein that forms an RNA-dependent complex with Bic-D in embryos and ovaries. pabp also interacts genetically with Bic-D and, similar to Bic-D, pabp is essential in the germline for oocyte growth and accumulation of osk mRNA in the oocyte. In the absence of pabp, reduced stability of osk mRNA and possibly also defects in osk mRNA transport prevent normal oocyte localization of osk mRNA. pabp also interacts genetically with osk and lack of one copy of pabp(+) causes osk to become haploinsufficient. Moreover, pointing to a poly(A)-independent role, Pabp binds to A-rich sequences (ARS) in the osk 3'UTR and these turned out to be required in vivo for osk function during early oogenesis. This effect of pabp on osk mRNA is specific for this RNA and other tested mRNAs localizing to the oocyte are less and more indirectly affected by the lack of pabp.

Reversible response of protein localization and microtubule organization to nutrient stress during Drosophila early oogenesis

The maturation of animal oocytes is highly sensitive to nutrient availability. During Drosophila oogenesis, a prominent metabolic checkpoint occurs at the onset of yolk uptake (vitellogenesis): under nutrient stress, egg chambers degenerate by apoptosis. To investigate additional responses to nutrient deprivation, we studied the intercellular transport of cytoplasmic components between nurse cells and the oocyte during previtellogenic stages. Using GFP protein-traps, we showed that Ypsilon Schachtel (Yps), a putative RNA binding protein, moved into the oocyte by both microtubule (MT)-dependent and -independent mechanisms, and was retained in the oocyte in a MT-dependent manner. These data suggest that oocyte enrichment is accomplished by a combination of MT-dependent polarized transport and MT-independent flow coupled with MT-dependent trapping within the oocyte. Under nutrient stress, Yps and other components of the oskar ribonucleoprotein complex accumulated in large processing bodies in nurse cells, accompanied by MT reorganization. This response was detected as early as 2h after starvation, suggesting that young egg chambers rapidly respond to nutrient stress. Moreover, both Yps aggregation and MT reorganization were reversed with re-feeding of females or the addition of exogenous insulin to cultured egg chambers. Our results suggest that egg chambers rapidly mount a stress response by altering intercellular transport upon starvation. This response implies a mechanism for preserving young egg chambers so that egg production can rapidly resume when nutrient availability improves.

Physiological apoptosis of polar cells during Drosophila oogenesis is mediated by Hid-dependent regulation of Diap1

Although much has been learned in recent years about the apoptotic machinery, the mechanisms underlying survival and death choices during development of metazoans remain less clearly understood. During early oogenesis in Drosophila, a small excess in the number of specialized somatic cells, called polar cells (PCs), produced at follicle extremities is reduced to exactly two cells through apoptosis by mid-oogenesis. We have found that PCs destined to die first lose their apical contacts and then round up and shrink progressively until they disappear. Caspases are activated only once the cells have begun to shrink, suggesting that they are implicated in this part of the process, but not in the initial loss of cell polarity. Loss-of-function analyses based on mutant, clonal and RNAi approaches show that among the RHG family of pro-apoptotic factors, Hid is specifically necessary for PC apoptosis, as well as the initiator caspase Dronc and its adaptor Dark/Apaf-1, and likely several effector caspases, in particular Drice. In addition, we show that Hid protein and transcripts accumulate specifically in PCs destined to die, while the anti-apoptotic factor Diap1 is downregulated in these cells in a hid-dependent manner. Therefore, our results implicate the Hid-Diap1 module as an important regulatory point in a developmental case of apoptosis.

Reduced cul-5 activity causes aberrant follicular morphogenesis and germ cell loss in Drosophila oogenesis

Drosophila oogenesis is especially well suited for studying stem cell biology, cellular differentiation, and morphogenesis. The small modifier protein ubiquitin regulates many cellular pathways. Ubiquitin is conjugated to target proteins by a diverse class of enzymes called ubiquitin E3 ligases. Here we characterize the requirement of Cul-5, a key component of a subgroup of Cullin-RING-type ubiquitin E3 ligases, in Drosophila oogenesis. We find that reduced cul-5 activity causes the formation of aberrant follicles that are characterized by excess germ cells. We show that germ line cells overproliferate in cul-5 mutant females, causing the formation of abnormally large germ line cysts. Also, the follicular epithelium that normally encapsulates single germ line cysts develops aberrantly in cul-5 mutant, leading to defects in cyst formation. We additionally found that Cul-5 is required for germ cell maintenance, as germ cells are depleted in a substantial fraction of cul-5 mutant ovaries. All of these cul-5 phenotypes are strongly enhanced by reduced activity of gustavus (gus), which encodes a substrate receptor of Cul-5-based ubiquitin E3 ligases. Taken together, our results implicate Cul-5/Gus ubiquitin E3 ligases in ovarian tissue morphogenesis, germ cell proliferation and maintenance of the ovarian germ cell population.

Mago Nashi, Tsunagi/Y14, and Ranshi form a complex that influences oocyte differentiation in Drosophila melanogaster

During Drosophila melanogaster oogenesis, a germline stem cell divides forming a cyst of 16 interconnected cells. One cell enters the oogenic pathway, and the remaining 15 differentiate as nurse cells. Although directed transport and localization of oocyte differentiation factors within the single cell are indispensible for selection, maintenance, and differentiation of the oocyte, the mechanisms regulating these events are poorly understood. Mago Nashi and Tsunagi/Y14, core components of the exon junction complex (a multiprotein complex assembled on spliced RNAs), are essential for restricting oocyte fate to a single cell and for localization of oskar mRNA. Here we provide evidence that Mago Nashi and Tsunagi/Y14 form an oogenic complex with Ranshi, a protein with a zinc finger-associated domain and zinc finger domains. Genetic analyses of ranshi reveal that (1) 16-cell cysts are formed, (2) two cells retain synaptonemal complexes, (3) all cells have endoreplicated DNA (as observed in nurse cells), and (4) oocyte-specific cytoplasmic markers accumulate and persist within a single cell but are not localized within the posterior pole of the presumptive oocyte. Our results indicate that Ranshi interacts with the exon junction complex to localize components essential for oocyte differentiation within the posterior pole of the presumptive oocyte.

Bicaudal-D and its role in cargo sorting by microtubule-based motors

Many cytoplasmic cargoes are transported along microtubules using dynein or kinesin molecular motors. As the sorting machinery of the cell needs to be tightly controlled, associated factors are employed to either recruit cargoes to motors or to regulate their activities. In the present review, we concentrate on the BicD (Bicaudal-D) protein, which has recently emerged as an essential element for transport of several important cargoes by the minus-end-directed motor cytoplasmic dynein. BicD was proposed to be a linker bridging cargo and dynein, although recent studies suggest that it may also have roles in the regulation of cargo motility. Here we summarize the current knowledge of the role that BicD plays in the transport of diverse cellular constituents. We catalogue the molecular interactions that underpin these functions and also highlight important questions to be addressed in the future.

Egalitarian is a selective RNA-binding protein linking mRNA localization signals to the dynein motor

Cytoplasmic sorting of mRNAs by microtubule-based transport is widespread, yet very little is known at the molecular level about how specific transcripts are linked to motor complexes. In Drosophila, minus-end-directed transport of developmentally important transcripts by the dynein motor is mediated by seemingly divergent mRNA elements. Here we provide evidence that direct recognition of these mRNA localization signals is mediated by the Egalitarian (Egl) protein. Egl and the dynein cofactor Bicaudal-D (BicD) are the only proteins from embryonic extracts that are abundantly and specifically enriched on RNA localization signals from transcripts of gurken, hairy, K10, and the I factor retrotransposon. In vitro assays show that, despite lacking a canonical RNA-binding motif, Egl directly recognizes active localization elements. We also reveal a physical interaction between Egl and a conserved domain for cargo recruitment in BicD and present data suggesting that Egl participates selectively in BicD-mediated transport of mRNA in vivo. Our work leads to the first working model for a complete connection between minus-end-directed mRNA localization signals and microtubules and reveals molecular strategies that are likely to be of general relevance for cargo transport by dynein.

Systematic functional analysis of Bicaudal-D serine phosphorylation and intragenic suppression of a female sterile allele of BicD

Protein phosphorylation is involved in posttranslational control of essentially all biological processes. Using mass spectrometry, recent analyses of whole phosphoproteomes led to the identification of numerous new phosphorylation sites. However, the function of most of these sites remained unknown. We chose the Drosophila Bicaudal-D protein to estimate the importance of individual phosphorylation events. Being involved in different cellular processes, BicD is required for oocyte determination, for RNA transport during oogenesis and embryogenesis, and for photoreceptor nuclei migration in the developing eye. The numerous roles of BicD and the available evidence for functional importance of BicD phosphorylation led us to identify eight phosphorylation sites of BicD, and we tested a total of 14 identified and suspected phosphoserine residues for their functional importance in vivo in flies. Surprisingly, all these serines turned out to be dispensable for providing sufficient basal BicD activity for normal growth and development. However, in a genetically sensitized background where the BicD(A40V) protein variant provides only partial activity, serine 103 substitutions are not neutral anymore, but show surprising differences. The S103D substitution completely inactivates the protein, whereas S103A behaves neutral, and the S103F substitution, isolated in a genetic screen, restores BicD(A40V) function. Our results suggest that many BicD phosphorylation events may either be fortuitous or play a modulating function as shown for Ser(103). Remarkably, amongst the Drosophila serines we found phosphorylated, Ser(103) is the only one that is fully conserved in mammalian BicD.

BicaudalD actively regulates microtubule motor activity in lipid droplet transport

BACKGROUND

A great deal of sub-cellular organelle positioning, and essentially all minus-ended organelle transport, depends on cytoplasmic dynein, but how dynein's function is regulated is not well understood. BicD is established to play a critical role in mediating dynein function-loss of BicD results in improperly localized nuclei, mRNA particles, and a dispersed Golgi apparatus-however exactly what BicD's role is remains unknown. Nonetheless, it is widely believed that BicD may act to tether dynein to cargos. Here we use a combination of biophysical and biochemical studies to investigate BicD's role in lipid droplet transport during Drosophila embryogenesis.

METHODOLOGY/PRINCIPAL FINDINGS

Functional loss of BicD impairs the embryo's ability to control the net direction of droplet transport; the developmentally controlled reversal in transport is eliminated. We find that minimal BicD expression (near-BicD(null)) decreases the average run length of both plus and minus end directed microtubule (MT) based transport. A point mutation affecting the BicD N-terminus has very similar effects on transport during cellularization (phase II), but in phase III (gastrulation) motion actually appears better than in the wild-type.

CONCLUSIONS/SIGNIFICANCE

In contrast to a simple static tethering model of BicD function, or a role only in initial dynein recruitment to the cargo, our data uncovers a new dynamic role for BicD in actively regulating transport. Lipid droplets move bi-directionally, and our investigations demonstrate that BicD plays a critical-and temporally changing-role in balancing the relative contributions of plus-end and minus-end motors to control the net direction of transport. Our results suggest that while BicD might contribute to recruitment of dynein to the cargo it is not absolutely required for such dynein localization, and it clearly contributes to regulation, helping activation/inactivation of the motors.

Mago Nashi and Tsunagi/Y14, respectively, regulate Drosophila germline stem cell differentiation and oocyte specification

A protein complex consisting of Mago Nashi and Tsunagi/Y14 is required to establish the major body axes and for the localization of primordial germ cell determinants during Drosophila melanogaster oogenesis. The Mago Nashi:Tsunagi/Y14 heterodimer also serves as the core of the exon junction complex (EJC), a multiprotein complex assembled on spliced mRNAs. In previous studies, reduced function alleles of mago nashi and tsunagi/Y14 were used to characterize the roles of the genes in oogenesis. Here, we investigated mago nashi and tsunagi/Y14 using null alleles and clonal analysis. Germline clones lacking mago nashi function divide but fail to differentiate. The mago nashi null germline stem cells produce clones over a period of at least 11 days, suggesting that mago nashi is not necessary for stem cell self-renewal. However, germline stem cells lacking tsunagi/Y14 function are indistinguishable from wild type. Additionally, in tsunagi/Y14 null germline cysts, centrosomes and oocyte-specific components fail to concentrate within a single cell and oocyte fate is not restricted to a single cell. Together, our results suggest not only that mago nashi is required for germline stem cell differentiation but that surprisingly mago nashi functions independently of tsunagi/Y14 in this process. On the other hand, Tsunagi/Y14 is essential for restricting oocyte fate to a single cell and may function with mago nashi in this process.

Nuclear migration during retinal development

In this review we focus on the mechanisms, regulation, and cellular consequences of nuclear migration in the developing retina. In the nervous system, nuclear migration is prominent during both proliferative and post-mitotic phases of development. Interkinetic nuclear migration is the process where the nucleus oscillates from the apical to basal surfaces in proliferative neuroepithelia. Proliferative nuclear movement occurs in step with the cell cycle, with M-phase being confined to the apical surface and G1-, S-, and G2-phases occurring at more basal locations. Later, following cell cycle exit, some neuron precursors migrate by nuclear translocation. In this mode of cellular migration, nuclear movement is the driving force for motility. Following discussion of the key components and important regulators for each of these processes, we present an emerging model where interkinetic nuclear migration functions to distinguish cell fates among retinal neuroepithelia.

A Dynein-dependent shortcut rapidly delivers axis determination transcripts into the Drosophila oocyte

The primary axes of Drosophila are set up by the localization of transcripts within the oocyte. These mRNAs originate in the nurse cells, but how they move into the oocyte remains poorly understood. Here, we study the path and mechanism of movement of gurken RNA within the nurse cells and towards and through ring canals connecting them to the oocyte. gurken transcripts, but not control transcripts, recruit the cytoplasmic Dynein-associated co-factors Bicaudal D (BicD) and Egalitarian in the nurse cells. gurken RNA requires BicD and Dynein for its transport towards the ring canals, where it accumulates before moving into the oocyte. Our results suggest that bicoid and oskar transcripts are also delivered to the oocyte by the same mechanism, which is distinct from cytoplasmic flow. We propose that Dynein-mediated transport of specific RNAs along specialized networks of microtubules increases the efficiency of their delivery, over the flow of general cytoplasmic components, into the oocyte.