Polar transport in the Drosophila oocyte requires Dynein and Kinesin I cooperation

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.     

RNA trafficking in axons

A substantial number of studies over a period of four decades have indicated that axons contain mRNAs and ribosomes, and are metabolically active in synthesizing proteins locally. For the most part, little attention has been paid to these findings until recently when the concept of targeting of specific mRNAs and translation in subcellular domains in polarized cells emerged to contribute to the likelihood and acceptance of mRNA targeting to axons as well. Trans-acting factor proteins bind to cis-acting sequences in the untranslated region of mRNAs integrated in ribonucleoprotein (RNPs) complexes determine its targeting in neurons. In vitro studies in immature axons have shown that molecular motors proteins (kinesins and myosins) associate to RNPs suggesting they would participate in its transport to growth cones. Tau and actin mRNAs are transported as RNPs, and targeted to axons as well as ribosomes. Periaxoplasmic ribosomal plaques (PARPs), which are systematically distributed discrete peripheral ribosome-containing, actin-rich formations in myelinated axons, also are enriched with actin and myosin Va mRNAs and additional regulatory proteins. The localization of mRNAs in PARPs probably means that PARPs are local centers of translational activity, and that these domains are the final destination in the axon compartment for targeted macromolecular traffic originating in the cell body. The role of glial cells as a potentially complementary source of axonal mRNAs and ribosomes is discussed in light of early reports and recent ultrastructural observations related to the possibility of glial-axon trans-endocytosis.     

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.     

Why cells move messages: the biological functions of mRNA localization

RNA localization is a widespread mechanism that allows cells to spatially control protein function by determining their sites of synthesis. In embryos, localized mRNAs are involved in morphogen gradient formation or the asymmetric distribution of cell fate determinants. In somatic cell types, mRNA localization contributes to local assembly of protein complexes or facilitates protein targeting to organelles. Long-distance transport of specific mRNAs in plants allows coordination of developmental processes between different plant organs. In this review, we will discuss the biological significance of different patterns of mRNA localization.     

Cis-acting determinants of asymmetric, cytoplasmic RNA transport

Asymmetric subcellular distribution of RNA is used by many organisms to establish cell polarity, differences in cell fate, or to sequester protein activity. Accurate localization of RNA requires specific sequence and/or structural elements in the localized RNA, as well as proteins that recognize these elements and link the RNA to the appropriate molecular motors. Recent advances in biochemistry, molecular biology, and cell imaging have enabled the identification of many RNA localization elements, or "zipcodes," from a variety of systems. This review focuses on the mechanisms by which various zipcodes direct RNA transport and on the known sequence/structural requirements for their recognition by transport complexes. Computational and experimental methods for predicting and identifying zipcodes are also discussed.     

Dynein anchors its mRNA cargo after apical transport in the Drosophila blastoderm embryo

Molecular motors actively transport many types of cargo along the cytoskeleton in a wide range of organisms. One class of cargo is localized mRNAs, which are transported by myosin on actin filaments or by kinesin and dynein on microtubules. How the cargo is kept at its final intracellular destination and whether the motors are recycled after completion of transport are poorly understood. Here, we use a new RNA anchoring assay in living Drosophila blastoderm embryos to show that apical anchoring of mRNA after completion of dynein transport does not depend on actin or on continuous active transport by the motor. Instead, apical anchoring of RNA requires microtubules and involves dynein as a static anchor that remains with the cargo at its final destination. We propose a general principle that could also apply to other dynein cargo and to some other molecular motors, whereby cargo transport and anchoring reside in the same molecule.     

Motor proteins: kinesin can replace Myosin

Directional transport of specific cargos is tuned to specific molecular motors and specific cytoskeletal tracks. Myosin V transports its cargo on actin cables, whereas kinesin or dynein transport their cargo on microtubules. A recent study shows that an engineered kinesin can substitute for myosin V and its cargo-specific transport and subsequent cellular functions.     

Microtubule-Dependent mRNA Transport in Fungi

The localization and local translation of mRNAs constitute an important mechanism to promote the correct subcellular targeting of proteins. mRNA localization is mediated by the active transport of mRNPs, large assemblies consisting of mRNAs and associated factors such as RNA-binding proteins. Molecular motors move mRNPs along the actin or microtubule cytoskeleton for short-distance or long-distance trafficking, respectively. In filamentous fungi, microtubule-based long-distance transport of vesicles, which are involved in membrane and cell wall expansion, supports efficient hyphal growth. Recently, we discovered that the microtubule-mediated transport of mRNAs is essential for the fast polar growth of infectious filaments in the corn pathogen Ustilago maydis. Combining in vivo UV cross-linking and RNA live imaging revealed that the RNA-binding protein Rrm4, which constitutes an integral part of the mRNP transport machinery, mediates the transport of distinct mRNAs encoding polarity factors, protein synthesis factors, and mitochondrial proteins. Moreover, our results indicate that microtubule-dependent mRNA transport is evolutionarily conserved from fungi to higher eukaryotes. This raises the exciting possibility of U. maydis as a model system to uncover basic concepts of long-distance mRNA transport.In order to compartmentalize functions, eukaryotic cells need to sort their proteins to distinct subcellular sites. A widespread mechanism for the spatiotemporal regulation of protein expression is localized translation, i.e., the concerted action of mRNA localization and confined translation. Thereby, the correct subcellular localization of translation products is promoted, and the deleterious mislocalization of proteins is prevented (5, 37).Most commonly, mRNA localization is mediated by active transport along the actin or microtubule cytoskeleton for short-distance or long-distance mRNA transport, respectively. Transported mRNAs contain specific cis-acting sequences that function as zipcodes to determine the correct subcellular destination. These RNA elements are recognized by RNA-binding proteins that combine with accessory factors to form higher-order ribonucleoprotein complexes, designated mRNPs (40, 82). Adaptor proteins are thought to connect mRNPs to molecular motors that actively transport them along the cytoskeleton to their final destination (88, 92). Commonly, premature translation is inhibited during mRNP transport by specific inhibitors. Upon arrival mRNAs are offloaded and kept in place by anchoring factors. The local phosphorylation of RNA-binding proteins then triggers unloading and the release of translational inhibitor (39, 68). When the formation of transport-competent mRNPs fails, mRNAs are translated at wrong locations, leading to the mislocalization of the encoded proteins. An example of the importance of mRNA localization is the local synthesis of morphogens during oogenesis and embryogenesis in Drosophila melanogaster, which determines the two main body axes of developing embryos (55, 59).In fungi, actin-dependent transport was quite extensively studied for Saccharomyces cerevisiae and was recently discovered in filaments of Candida albicans (25, 65, 68). Examples of long-distance mRNA transport along microtubules have so far been reported only for the corn pathogen Ustilago maydis. Study of the role of RNA-binding proteins during filamentous growth and pathogenic development revealed that microtubule-dependent mRNP transport is essential for the fast polar growth of infectious hyphae (6, 7, 26, 45). In this review we will introduce the basic aspects of short- and long-distance mRNA transports in fungal and animal models. In addition, we will shortly address polar growth and microtubule-dependent transport in filamentous fungi. This will be the foundation to present recent advances in the microtubule-dependent transport of mRNAs in U. maydis.     

Neuronal polarity: targeting of microtubule components into axons and dendrites

The functional polarity of nerve cells depends on the outgrowth of both axons and dendrites. These processes, which were distinguished by morphological and physiological criteria, have been shown in recent years to differ in molecular composition, including their cytoskeleton. The asymmetric distribution of cytoskeletal elements and, particularly, the segregation of microtubule-associated proteins by their differential transport, may play an important role in the assembly of distinct microtubules in the two neuronal domains. An additional mechanism to achieve this subcellular localization is the transport of specific mRNAs to allow the local synthesis of specific proteins close to their functional site. This may endow the cell with a rapid mechanism for the regulation of synthesis under special conditions, which may be important during neuronal development and plasticity.     

Direct observation of regulated ribonucleoprotein transport across the nurse cell/oocyte boundary

In Drosophila, the asymmetric localization of specific mRNAs to discrete regions within the developing oocyte determines the embryonic axes. The microtubule motors dynein and kinesin are required for the proper localization of the determinant ribonucleoprotein (RNP) complexes, but the mechanisms that account for RNP transport to and within the oocyte are not well understood. In this work, we focus on the transport of RNA complexes containing bicoid (bcd), an anterior determinant. We show in live egg chambers that, within the nurse cell compartment, dynein actively transports green fluorescent protein-tagged Exuperantia, a cofactor required for bcd RNP localization. Surprisingly, the loss of kinesin I activity elevates RNP motility in nurse cells, whereas disruption of dynein activity inhibits RNP transport. Once RNPs are transferred through the ring canal to the oocyte, they no longer display rapid, linear movements, but they are distributed by cytoplasmic streaming and gradually disassemble. By contrast, bcd mRNA injected into oocytes assembles de novo into RNP particles that exhibit rapid, dynein-dependent transport. We speculate that after delivery to the oocyte, RNP complexes may disassemble and be remodeled with appropriate accessory factors to ensure proper localization.     

Microtubule-dependent mRNA transport in the model microorganism Ustilago maydis

Microtubule-dependent trafficking is essential in moving mRNAs over long distances. This transport mechanism regulates important cellular events such as determining polarity and local protein secretion. Key examples are developmental and neuronal processes studied in Drosophila melanogaster, Xenopus laevis as well as in mammalian cells. A simple eukaryotic system to uncover basic mechanisms was missing. Fungal models are generally well suited for this purpose, since transgenic strains can be generated easily by homologous recombination allowing in vivo studies at native expression levels. Substantial advances in understanding Ustilago maydis showed that this fungus fulfils important criteria to serve as model for microtubule-dependent mRNA trafficking. Here, we summarize progress focusing on target mRNAs, RNA localization elements, RNA-binding proteins, mRNPs, molecular motors and microtubule organization. This serves as the basis to discuss the novel mechanism of mRNP hitchhiking on endosomes as well as an unexpected link to unconventional secretion with its implications for applied sciences.     

Nuclear RNP complex assembly initiates cytoplasmic RNA localization

Cytoplasmic localization of mRNAs is a widespread mechanism for generating cell polarity and can provide the basis for patterning during embryonic development. A prominent example of this is localization of maternal mRNAs in Xenopus oocytes, a process requiring recognition of essential RNA sequences by protein components of the localization machinery. However, it is not yet clear how and when such protein factors associate with localized RNAs to carry out RNA transport. To trace the RNA-protein interactions that mediate RNA localization, we analyzed RNP complexes from the nucleus and cytoplasm. We find that an early step in the localization pathway is recognition of localized RNAs by specific RNA-binding proteins in the nucleus. After transport into the cytoplasm, the RNP complex is remodeled and additional transport factors are recruited. These results suggest that cytoplasmic RNA localization initiates in the nucleus and that binding of specific RNA-binding proteins in the nucleus may act to target RNAs to their appropriate destinations in the cytoplasm.     

Molecular motors in neurons: transport mechanisms and roles in brain function, development, and disease

The kinesin, dynein, and myosin superfamily molecular motors have fundamental roles in neuronal function, plasticity, morphogenesis, and survival by transporting cargos such as synaptic vesicle precursors, neurotransmitter and neurotrophic factor receptors, and mRNAs within axons, dendrites, and synapses. Recent studies have begun to clarify the mechanisms of cargo selection and directional transport in subcellular compartments. Furthermore, molecular genetics has revealed unexpected roles for molecular motors in brain wiring, neuronal survival, neuronal plasticity, higher brain function, and control of central nervous system and peripheral nervous system development. Finally, it is also evident that molecular motors are critically involved in neuronal disease pathogenesis. Thus, molecular motor research is becoming an exciting frontier of neuroscience.