Kinesin I-dependent cortical exclusion restricts pole plasm to the oocyte posterior

Kinesin-1 tail autoregulation and microtubule-binding regions function in saltatory transport but not ooplasmic streaming

The N-terminal head domain of kinesin heavy chain (Khc) is well known for generating force for transport along microtubules in cytoplasmic organization processes during metazoan development, but the functions of the C-terminal tail are not clear. To address this, we studied the effects of tail mutations on mitochondria transport, determinant mRNA localization and cytoplasmic streaming in Drosophila. Our results show that two biochemically defined elements of the tail - the ATP-independent microtubule-binding sequence and the IAK autoinhibitory motif - are essential for development and viability. Both elements have positive functions in the axonal transport of mitochondria and determinant mRNA localization in oocytes, processes that are accomplished by biased saltatory movement of individual cargoes. Surprisingly, there were no indications that the IAK autoinhibitory motif acts as a general downregulator of Kinesin-1 in those processes. Time-lapse imaging indicated that neither tail region is needed for fast cytoplasmic streaming in oocytes, which is a non-saltatory bulk transport process driven solely by Kinesin-1. Thus, the Khc tail is not constitutively required for Kinesin-1 activation, force transduction or linkage to cargo. It might instead be crucial for more subtle elements of motor control and coordination in the stop-and-go movements of biased saltatory transport.

piRNAs, transposon silencing, and Drosophila germline development

Transposons are prominent features of most eukaryotic genomes and mobilization of these elements triggers genetic instability. Transposon silencing is particularly critical in the germline, which maintains the heritable genetic complement. Piwi-interacting RNAs (piRNAs) have emerged as central players in transposon silencing and genome maintenance during germline development. In particular, research on Drosophila oogenesis has provided critical insights into piRNA biogenesis and transposon silencing. In this system, the ability to place piRNA mutant phenotypes within a well-defined developmental framework has been instrumental in elucidating the molecular mechanisms underlying the connection between piRNAs and transposon control.

Symmetry breaking during Drosophila oogenesis

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

Sm proteins specify germ cell fate by facilitating oskar mRNA localization

Sm and Sm-like proteins are RNA-binding factors found in all three domains of life. Eukaryotic Sm proteins play essential roles in pre-mRNA splicing, forming the cores of spliceosomal small nuclear ribonucleoproteins (snRNPs). Recently, Sm proteins have been implicated in the specification of germ cells. However, a mechanistic understanding of their involvement in germline specification is lacking and a germline-specific RNA target has not been identified. We demonstrate that Drosophila SmB and SmD3 are specific components of the oskar messenger ribonucleoprotein (mRNP), proper localization of which is required for establishing germline fate and embryonic patterning. Importantly, oskar mRNA is delocalized in females harboring a hypomorphic mutation in SmD3, and embryos from mutant mothers are defective in germline specification. We conclude that Sm proteins function to establish the germline in Drosophila, at least in part by mediating oskar mRNA localization.

Bazooka is required for polarisation of the Drosophila anterior-posterior axis

The Drosophila anterior-posterior (AP) axis is determined by the polarisation of the stage 9 oocyte and the subsequent localisation of bicoid and oskar mRNAs to opposite poles of the cell. Oocyte polarity has been proposed to depend on the same PAR proteins that generate AP polarity in C. elegans, with a complex of Bazooka (Baz; Par-3), Par-6 and aPKC marking the anterior and lateral cortex, and Par-1 defining the posterior. The function of the Baz complex in oocyte polarity has remained unclear, however, because although baz-null mutants block oocyte determination, egg chambers that escape this early arrest usually develop normal polarity at stage 9. Here, we characterise a baz allele that produces a penetrant polarity phenotype at stage 9 without affecting oocyte determination, demonstrating that Baz is essential for axis formation. The dynamics of Baz, Par-6 and Par-1 localisation in the oocyte indicate that the axis is not polarised by a cortical contraction as in C. elegans, and instead suggest that repolarisation of the oocyte is triggered by posterior inactivation of aPKC or activation of Par-1. This initial asymmetry is then reinforced by mutual inhibition between the anterior Baz complex and posterior Par-1 and Lgl. Finally, we show that mutation of the aPKC phosphorylation site in Par-1 results in the uniform cortical localisation of Par-1 and the loss of cortical microtubules. Since non-phosphorylatable Par-1 is epistatic to uninhibitable Baz, Par-1 seems to function downstream of the other PAR proteins to polarize the oocyte microtubule cytoskeleton.

Bazooka regulates microtubule organization and spatial restriction of germ plasm assembly in the Drosophila oocyte

Localization of the germ plasm to the posterior of the Drosophila oocyte is required for anteroposterior patterning and germ cell development during embryogenesis. While mechanisms governing the localization of individual germ plasm components have been elucidated, the process by which germ plasm assembly is restricted to the posterior pole is poorly understood. In this study, we identify a novel allele of bazooka (baz), the Drosophila homolog of Par-3, which has allowed the analysis of baz function throughout oogenesis. We demonstrate that baz is required for spatial restriction of the germ plasm and axis patterning, and we uncover multiple requirements for baz in regulating the organization of the oocyte microtubule cytoskeleton. Our results suggest that distinct cortical domains established by Par proteins polarize the oocyte through differential effects on microtubule organization. We further show that microtubule plus-end enrichment is sufficient to drive germ plasm assembly even at a distance from the oocyte cortex, suggesting that control of microtubule organization is critical not only for the localization of germ plasm components to the posterior of the oocyte but also for the restriction of germ plasm assembly to the posterior pole.

Kinesin-dependent transport results in polarized migration of the nucleus in oocytes and inward movement of yolk granules in meiotic embryos

During female meiosis, meiotic spindles are positioned at the oocyte cortex to allow expulsion of chromosomes into polar bodies. In C. elegans, kinesin-dependent translocation of the entire spindle to the cortex precedes dynein-dependent rotation of one spindle pole toward the cortex. To elucidate the role of kinesin-1 in spindle translocation, we examined the localization of kinesin subunits in meiotic embryos. Surprisingly, kinesin-1 was not associated with the spindle and instead was restricted to the cytoplasm in the middle of the embryo. Yolk granules moved on linear tracks, in a kinesin-dependent manner, away from the cortex, resulting in their concentration in the middle of the embryo where the kinesin was concentrated. These results suggest that cytoplasmic microtubules might be arranged with plus ends extending inward, away from the cortex. This microtubule arrangement would not be consistent with direct transport of the meiotic spindle toward the cortex by kinesin-1. In maturing oocytes, the nucleus underwent kinesin-dependent migration to the future site of spindle attachment at the anterior cortex. Thus the spindle translocation defect observed in kinesin-1 mutants may be a result of failed nuclear migration, which places the spindle too far from the cortex for the spindle translocation mechanism to function.

Lighting up mRNA localization in Drosophila oogenesis

The asymmetric localization of four maternal mRNAs - gurken, bicoid, oskar and nanos - in the Drosophila oocyte is essential for the development of the embryonic body axes. Fluorescent imaging methods are now being used to visualize these mRNAs in living tissue, allowing dynamic analysis of their behaviors throughout the process of localization. This review summarizes recent findings from such studies that provide new insight into the elaborate cellular mechanisms that are used to transport mRNAs to different regions of the oocyte and to maintain their localized distributions during oogenesis.

Dynamic organization and plasticity of sponge bodies

Sponge bodies, cytoplasmic structures containing post-transcriptional regulatory factors, are distributed throughout the nurse cells and oocytes of the Drosophila ovary and share components with P bodies of yeast and mammalian cells. We show that sponge body composition differs between nurse cells and the oocyte, and that the sponge bodies change composition rapidly after entry into the oocyte. We identify conditions that affect sponge body organization. At one extreme, components are distributed relatively uniformly or in small dispersed bodies. At the other extreme, components are present in large reticulated bodies. Both types of sponge bodies allow normal development, but show substantial differences in distribution of Staufen protein and oskar mRNA, whose localization within the oocyte is essential for axial patterning. Based on these and other results we propose a model for the relationship between P bodies and the various cytoplasmic bodies containing P body proteins in the Drosophila ovary.

Myosin-V regulates oskar mRNA localization in the Drosophila oocyte

Intracellular mRNA localization is an effective mechanism for protein targeting leading to functional polarization of the cell. The mechanisms controlling mRNA localization and specifically how the actin and microtubule (MT) cytoskeletons cooperate in this process are not well understood. In Drosophila, Oskar protein accumulation at the posterior pole of the oocyte is required for embryonic development and is achieved by the transport of oskar mRNA and its exclusive translation at the posterior pole. oskar mRNA localization requires the activity of the MT-based motor Kinesin, as well as the formation of a transport-competent ribonucleoprotein (RNP) complex. Here, we show that didum, encoding the Drosophila actin-based motor Myosin-V, is a new posterior group gene that promotes posterior accumulation of Oskar. Myosin-V associates with the oskar mRNA transport complex preferentially at the oocyte cortex, revealing a short-range actomyosin-based mechanism that mediates the local entrapment of oskar at the posterior pole. Our results also show that Myosin-V interacts with Kinesin heavy chain and counterbalances Kinesin function, preventing ectopic accumulation of oskar in the cytoplasm. Our findings reveal that a balance of microtubule- and actin-based motor activities regulates oskar mRNA localization in the Drosophila oocyte.

Kinesin-1 and cytoplasmic dynein act sequentially to move the meiotic spindle to the oocyte cortex in Caenorhabditis elegans

During female meiosis in animals, the meiotic spindle is attached to the egg cortex by one pole during anaphase to allow selective disposal of half the chromosomes in a polar body. In Caenorhabditis elegans, this anaphase spindle position is achieved sequentially through kinesin-1-dependent early translocation followed by anaphase-promoting complex (APC)-dependent spindle rotation. Partial depletion of cytoplasmic dynein heavy chain by RNA interference blocked spindle rotation without affecting early translocation. Dynein depletion also blocked the APC-dependent late translocation that occurs in kinesin-1-depleted embryos. Time-lapse imaging of green fluorescent protein-tagged dynein heavy chain as well as immunofluorescence with dynein-specific antibodies revealed that dynein starts to accumulate at spindle poles just before the initiation of rotation or late translocation. Accumulation of dynein at poles was kinesin-1 independent and APC dependent, just like dynein driven spindle movements. This represents a case of kinesin-1/dynein coordination in which these two motors of opposite polarity act sequentially and independently on a cargo to move it in the same direction.

Microtubules, the ER and Exu: new associations revealed by analysis of mini spindles mutations

During Drosophila oogenesis, organized microtubule networks coordinate the localization of specific RNAs, the positioning of the oocyte nucleus, and ooplasmic streaming events. We used mutations in mini spindles (msps), a microtubule-associated protein, to disrupt microtubule function during mid- and late-oogenesis, and show that msps is required for these microtubule-based events. Since endoplasmic reticulum (ER) organization is influenced by microtubules in other systems, we hypothesized that using msps to alter microtubule dynamics might affect the structure and organization of the ER in nurse cells and the oocyte. ER organization was monitored using GFP-tagged versions of Reticulon-like1 and protein disulfide isomerase. Analyses of living cells indicate microtubule associations mediate the movement of ER components within the oocyte. Surprisingly, the distribution and behavior of tubular ER in the oocyte differs from general ER, suggesting these two compartments of the ER interact differently with microtubules. We find that the morphology of Exu particles is msps-dependent, and that Exu is specifically associated with tubular ER in msps mutants. Our results extend previous descriptions of sponge bodies and the fusome, suggesting both are manifestations of a dynamic structure that interacts with microtubules and persists throughout oogenesis.

The genetics and cell biology of Wolbachia-host interactions

Wolbachia are gram-negative bacteria that are widespread in nature, carried by the majority of insect species as well as some mites, crustaceans, and filarial nematodes. Wolbachia can range from parasitic to symbiotic, depending upon the interaction with the host species. The success of Wolbachia is attributed to efficient maternal transmission and manipulations of host reproduction that favor infected females, such as sperm-egg cytoplasmic incompatibility (CI). Much remains unknown about the mechanistic basis for Wolbachia-host interactions. Here we summarize the current understanding of Wolbachia interaction with insect hosts, with a focus on Drosophila. The areas of discussion include Wolbachia transmission in oogenesis, Wolbachia distribution in spermatogenesis, induction and rescue of the CI phenotype, Wolbachia genomics, and Wolbachia-membrane interactions.

Drosophila ensconsin promotes productive recruitment of Kinesin-1 to microtubules

Ensconsin is a conserved microtubule-associated protein (MAP) that interacts dynamically with microtubules, but its cellular function has remained elusive. We show that Drosophila ensconsin is required for all known kinesin-1-dependent processes in the polarized oocyte without detectable effects on microtubules. ensconsin is also required in neurons. Using a single molecule assay for kinesin-1 motility in Drosophila ovary extract, we show that recruitment to microtubules and subsequent motility is severely impaired without ensconsin. Ensconsin protein is enriched at the oocyte anterior and apically in polarized epithelial cells, although required for localization of posterior determinants. Par-1 is required for ensconsin localization and directly phosphorylates it at conserved sites. Our results reveal an unexpected function of a MAP, promoting productive recruitment of a specific motor to microtubules, and an additional level of kinesin regulation. Furthermore, spatial control of motor recruitment can provide additional regulatory control in Par-1 and microtubule-dependent cell polarity.

Par-1 and Tau regulate the anterior-posterior gradient of microtubules in Drosophila oocytes

The formation of an anterior-posterior (AP) gradient of microtubules in Drosophila oocytes is essential for specification of the AP axis. Proper microtubule organization in the oocyte requires the function of serine/threonine kinase Par-1. The N1S isoform of Par-1 is enriched at the posterior cortex of the oocyte from stage 7 of oogenesis. Here we report that posterior restriction of Par-1 (N1S) kinase activity is critical for microtubule AP gradient formation. Egg chambers with excessive and ectopic Par-1 (N1S) kinase activity in the germline cells display phenotypes similar to those of egg chambers treated with the microtubule-depolymerizing drug colcemid: depolymerization of microtubules in the oocyte and disruption of oocyte nucleus localization. A phosphorylation target of Par-1, the microtubule-associated protein Tau, is also involved in oocyte polarity formation, and overexpression of Tau alleviates the phenotypes caused by ectopic Par-1 (N1S) kinase activity, suggesting that Par-1 regulates oocyte polarity at least partly through Tau. Our findings reveal that maintaining proper levels of Par-1 at correct position in the oocyte is key to oocyte polarity formation and that the conserved role of Par-1 and Tau is crucial for the establishment of an AP gradient of microtubules and for AP axis specification.

A dual role for actin and microtubule cytoskeleton in the transport of Golgi units from the nurse cells to the oocyte across ring canals

Axis specification during Drosophila embryonic development requires transfer of maternal components during oogenesis from nurse cells (NCs) 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 characterized actin basket structures that asymmetrically cap the NC side of Ring canals (RCs) 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 are actively transported to the oocyte rather than diffusing. We observed that RC transit is microtubule-based and mediated at least by dynein. Finally, we show that actin networks may be involved in RC crossing through a myosin II step process, as well as in dispatching Golgi units inside the oocyte subcompartments.

In vivo imaging of oskar mRNA transport reveals the mechanism of posterior localization

oskar mRNA localization to the posterior of the Drosophila oocyte defines where the abdomen and germ cells form in the embryo. Although this localization requires microtubules and the plus end-directed motor, kinesin, its mechanism is controversial and has been proposed to involve active transport to the posterior, diffusion and trapping, or exclusion from the anterior and lateral cortex. By following oskar mRNA particles in living oocytes, we show that the mRNA is actively transported along microtubules in all directions, with a slight bias toward the posterior. This bias is sufficient to localize the mRNA and is reversed in mago, barentsz, and Tropomyosin II mutants, which mislocalize the mRNA anteriorly. Since almost all transport is mediated by kinesin, oskar mRNA localizes by a biased random walk along a weakly polarized cytoskeleton. We also show that each component of the oskar mRNA complex plays a distinct role in particle formation and transport.

Multiple kinesin motors coordinate cytoplasmic RNA transport on a subpopulation of microtubules in Xenopus oocytes

RNA localization is a widely conserved mechanism for generating cellular asymmetry. In Xenopus oocytes, microtubule-dependent transport of RNAs to the vegetal cortex underlies germ layer patterning. Although kinesin motors have been implicated in this process, the apparent polarity of the microtubule cytoskeleton has pointed instead to roles for minus-end-directed motors. To resolve this issue, we have analyzed participation of kinesin motors in vegetal RNA transport and identified a direct role for Xenopus kinesin-1. Moreover, in vivo interference and biochemical experiments reveal a key function for multiple motors, specifically kinesin-1 and kinesin-2, and suggest that these motors may interact during transport. Critically, we have discovered a subpopulation of microtubules with plus ends at the vegetal cortex, supporting roles for these kinesin motors in vegetal RNA transport. These results provide a new mechanistic basis for understanding directed RNA transport within the cytoplasm.

A cellular basis for Wolbachia recruitment to the host germline

Wolbachia are among the most widespread intracellular bacteria, carried by thousands of metazoan species. The success of Wolbachia is due to efficient vertical transmission by the host maternal germline. Some Wolbachia strains concentrate at the posterior of host oocytes, which promotes Wolbachia incorporation into posterior germ cells during embryogenesis. The molecular basis for this localization strategy is unknown. Here we report that the wMel Wolbachia strain relies upon a two-step mechanism for its posterior localization in oogenesis. The microtubule motor protein kinesin-1 transports wMel toward the oocyte posterior, then pole plasm mediates wMel anchorage to the posterior cortex. Trans-infection tests demonstrate that factors intrinsic to Wolbachia are responsible for directing posterior Wolbachia localization in oogenesis. These findings indicate that Wolbachia can direct the cellular machinery of host oocytes to promote germline-based bacterial transmission. This study also suggests parallels between Wolbachia localization mechanisms and those used by other intracellular pathogens.

Microtubule polarity and axis formation in the Drosophila oocyte

The body axes of the fruit fly are established in mid-oogenesis by the localization of three mRNA determinants, bicoid, oskar, and gurken, within the oocyte. General mechanisms of RNA localization and cell polarization, applicable to many cell types, have emerged from investigation of these determinants in Drosophila oogenesis. Localization of these RNAs is dependent on the germline microtubules, which reorganize to form a polarized array at mid-oogenesis in response to a signaling relay between the oocyte and the surrounding somatic follicle cells. Here we describe what is known about this microtubule reorganization and the signaling relay that triggers it. Recent studies have identified a number of ubiquitous RNA binding proteins essential for this process. So far, no targets for any of these proteins have been identified, and future work will be needed to illuminate how they function to reorganize microtubes and whether similar mechanisms also exist in other cell types.

Spindle assembly in the oocytes of mouse and Drosophila--similar solutions to a problem

In the oocytes of many organisms a bipolar spindle is assembled in the absence of centrosomes. In this article we review how this occurs in two model organisms, Drosophila melanogaster and Mus musculus. Common themes include an important role for the chromosomes but paradoxically, organization of a bipolar spindle may not involve kinetochore microtubules. Some comparisons are not yet possible, however, since the same genes have usually not been studied in both systems.

Microtubule anchoring by cortical actin bundles prevents streaming of the oocyte cytoplasm

The localisation of the determinants of the body axis during Drosophila oogenesis is dependent on the microtubule (MT) cytoskeleton. Mutations in the actin binding proteins Profilin, Cappuccino (Capu) and Spire result in premature streaming of the cytoplasm and a reorganisation of the oocyte MT network. As a consequence, the localisation of axis determinants is abolished in these mutants. It is unclear how actin regulates the organisation of the MTs, or what the spatial relationship between these two cytoskeletal elements is. Here, we report a careful analysis of the oocyte cytoskeleton. We identify thick actin bundles at the oocyte cortex, in which the minus ends of the MTs are embedded. Disruption of these bundles results in cortical release of the MT minus ends, and premature onset of cytoplasmic streaming. Thus, our data indicate that the actin bundles anchor the MTs minus ends at the oocyte cortex, and thereby prevent streaming of the cytoplasm. We further show that actin bundle formation requires Profilin but not Capu and Spire. Thus, our results support a model in which Profilin acts in actin bundle nucleation, while Capu and Spire link the bundles to MTs. Finally, our data indicate how cytoplasmic streaming contributes to the reorganisation of the MT cytoskeleton. We show that the release of the MT minus ends from the cortex occurs independently of streaming, while the formation of MT bundles is streaming dependent.

Misregulation of the kinesin-like protein Subito induces meiotic spindle formation in the absence of chromosomes and centrosomes

Bipolar spindles assemble in the absence of centrosomes in the oocytes of many species. In Drosophila melanogaster oocytes, the chromosomes have been proposed to initiate spindle assembly by nucleating or capturing microtubules, although the mechanism is not understood. An important contributor to this process is Subito, which is a kinesin-6 protein that is required for bundling interpolar microtubules located within the central spindle at metaphase I. We have characterized the domains of Subito that regulate its activity and its specificity for antiparallel microtubules. This analysis has revealed that the C-terminal domain may interact independently with microtubules while the motor domain is required for maintaining the interaction with the antiparallel microtubules. Surprisingly, deletion of the N-terminal domain resulted in a Subito protein capable of promoting the assembly of bipolar spindles that do not include centrosomes or chromosomes. Bipolar acentrosomal spindle formation during meiosis in oocytes may be driven by the bundling of antiparallel microtubules. Furthermore, these experiments have revealed evidence of a nuclear- or chromosome-based signal that acts at a distance to activate Subito. Instead of the chromosomes directly capturing microtubules, signals released upon nuclear envelope breakdown may activate proteins like Subito, which in turn bundles together microtubules.

The molecular chaperone Hsp90 is required for mRNA localization in Drosophila melanogaster embryos

Localization of maternal nanos mRNA to the posterior pole is essential for development of both the abdominal segments and primordial germ cells in the Drosophila embryo. Unlike maternal mRNAs such as bicoid and oskar that are localized by directed transport along microtubules, nanos is thought to be trapped as it swirls past the posterior pole during cytoplasmic streaming. Anchoring of nanos depends on integrity of the actin cytoskeleton and the pole plasm; other factors involved specifically in its localization have not been described to date. Here we use genetic approaches to show that the Hsp90 chaperone (encoded by Hsp83 in Drosophila) is a localization factor for two mRNAs, nanos and pgc. Other components of the pole plasm are localized normally when Hsp90 function is partially compromised, suggesting a specific role for the chaperone in localization of nanos and pgc mRNAs. Although the mechanism by which Hsp90 acts is unclear, we find that levels of the LKB1 kinase are reduced in Hsp83 mutant egg chambers and that localization of pgc (but not nos) is rescued upon overexpression of LKB1 in such mutants. These observations suggest that LKB1 is a primary Hsp90 target for pgc localization and that other Hsp90 partners mediate localization of nos.

How does an mRNA find its way? Intracellular localisation of transcripts

The localisation of transcripts to specific regions of the cell probably occurs in all cell types and has many distinct functions that go from the control of body axis formation to learning and memory. mRNAs can be localised by a variety of mechanisms including local protection from degradation, diffusion to a localised anchor, and active transport by motor proteins along the cytoskeleton. In this review, I consider the evidence for each of these mechanisms using a limited, but illustrative, number of examples of localised mRNAs.

An oskar-dependent positive feedback loop maintains the polarity of the Drosophila oocyte

The localization of oskar mRNA to the posterior of the Drosophila oocyte defines the site of assembly of the pole plasm, which contains the abdominal and germline determinants 1, 2, 3. oskar mRNA localization requires the polarization of the microtubule cytoskeleton, which depends on the recruitment of PAR-1 to the posterior cortex in response to a signal from the follicle cells, where it induces an enrichment of microtubule plus ends 4, 5, 6, 7. Here, we show that overexpressed oskar mRNA localizes to the middle of the oocyte, as well as the posterior. This ectopic localization depends on the premature translation of Oskar protein, which recruits PAR-1 and microtubule-plus-end markers to the oocyte center instead of the posterior pole, indicating that Oskar regulates the polarity of the cytoskeleton. Oskar also plays a role in the normal polarization of the oocyte; mutants that disrupt oskar mRNA localization or translation strongly reduce the posterior recruitment of microtubule plus ends. Thus, oskar mRNA localization is required to stabilize and amplify microtubule polarity, generating a positive feedback loop in which Oskar recruits PAR-1 to the posterior to increase the microtubule cytoskeleton's polarization, which in turn directs the localization of more oskar mRNA.

Translocation of mRNAs by molecular motors: think complex?

In recent years, closer inspection of the dynamics of cytoplasmic mRNA transport processes has shed new light on the mechanisms by which transcripts are recognized by motor complexes and deposited at the correct site. Several studies have highlighted the significance of the motile properties of motor complexes in differential transcript localization. In yeast, mRNA cargoes may stimulate either the movement or anchorage of actin-based motors. In higher eukaryotes, emerging evidence suggests that mRNA cargoes can control their sorting by regulating the motility of motor complexes or their choice of subsets of cytoskeletal tracks. The transport machinery that is utilized by differentially localizing mRNAs appears to share some common motors and regulatory factors. A major challenge for the future is therefore to understand how motor complexes decode the information in mRNA sequences.

Pathways for mRNA localization in the cytoplasm

Studies of the intracellular localization of mRNA have clearly demonstrated that certain subsets of mRNA are concentrated in discrete locations within the cytoplasm. Localization is one aspect of the post-transcriptional control of gene expression, and is intertwined with the translation and turnover of mRNA to achieve the goal of local protein production. Different mechanisms have been identified that enable localized mRNAs to target different subcellular compartments, and recent advances in understanding these pathways is reviewed here.

DrosophilaIk2, a member of the IκB kinase family, is required for mRNA localization during oogenesis

In both Drosophila and mammals, IκB kinases (IKKs) regulate the activity of Rel/NF-κB transcription factors by targeting their inhibitory partner proteins, IκBs, for degradation. We identified mutations in ik2, the gene that encodes one of two Drosophila IKKs, and found that the gene is essential for viability. During oogenesis, ik2 is required in an NF-κB-independent process that is essential for the localization of oskar and gurken mRNAs; as a result, females that lack ik2 in the germline produce embryos that are both bicaudal and ventralized. The abnormal RNA localization in ik2 mutant oocytes can be attributed to defects in the organization of microtubule minus-ends. In addition, both mutant oocytes and mutant escaper adults have abnormalities in the organization of the actin cytoskeleton. These data suggest that this IκB kinase has an NF-κB-independent role in mRNA localization and helps to link microtubule minus-ends to the oocyte cortex, a novel function of the IKK family.

spn-F encodes a novel protein that affects oocyte patterning and bristle morphology in Drosophila

The anteroposterior and dorsoventral axes of the Drosophila embryo are established during oogenesis through the activities of Gurken (Grk), a Tgfalpha-like protein, and the Epidermal growth factor receptor (Egfr). spn-F mutant females produce ventralized eggs similar to the phenotype produced by mutations in the grk-Egfr pathway. We found that the ventralization of the eggshell in spn-F mutants is due to defects in the localization and translation of grk mRNA during mid-oogenesis. Analysis of the microtubule network revealed defects in the organization of the microtubules around the oocyte nucleus. In addition, spn-F mutants have defective bristles. We cloned spn-F and found that it encodes a novel coiled-coil protein that localizes to the minus end of microtubules in the oocyte, and this localization requires the microtubule network and a Dynein heavy chain gene. We also show that Spn-F interacts directly with the Dynein light chain Ddlc-1. Our results show that we have identified a novel protein that affects oocyte axis determination and the organization of microtubules during Drosophila oogenesis.

Coordination of microtubule and microfilament dynamics by Drosophila Rho1, Spire and Cappuccino

The actin-nucleation factors Spire and Cappuccino (Capu) regulate the onset of ooplasmic streaming in Drosophila melanogaster. Although this streaming event is microtubule-based, actin assembly is required for its timing. It is not understood how the interaction of microtubules and microfilaments is mediated in this context. Here, we demonstrate that Capu and Spire have microtubule and microfilament crosslinking activity. The spire locus encodes several distinct protein isoforms (SpireA, SpireC and SpireD). SpireD was recently shown to nucleate actin, but the activity of the other isoforms has not been addressed. We find that SpireD does not have crosslinking activity, whereas SpireC is a potent crosslinker. We show that SpireD binds to Capu and inhibits F-actin/microtubule crosslinking, and activated Rho1 abolishes this inhibition, establishing a mechanistic basis for the regulation of Capu and Spire activity. We propose that Rho1, cappuccino and spire are elements of a conserved developmental cassette that is capable of directly mediating crosstalk between microtubules and microfilaments.

BicD-dependent localization processes: from Drosophilia development to human cell biology

Many eukaryotic cells depend on proper cell polarization for their development and physiological function. The establishment of these polarities often involve the subcellular localization of a specific subset of proteins, RNAs and organelles. In Drosophila, the microtubule-dependent BicD (BicaudalD) localization machinery is involved in the proper localization of mRNA during oogenesis and embryogenesis and the proper positioning of the oocyte and photoreceptor nuclei. BicD acts together with the minus-end directed motor dynein as well as Egl and Lis-1. The finding that the mammalian homologs of BicD function in retrograde Golgi-to-ER transport has supported the view that BicD may be part of a repeatedly used and evolutionary conserved localization machinery. In this review we focus on the various processes in which BicD is involved during Drosophilian development and in mammals. In addition, we evaluate the interactions between BicD, the dynein localization machinery and associated factors.

The RNA-binding protein Squid is required for the establishment of anteroposterior polarity in theDrosophilaoocyte

The heterogeneous nuclear ribonucleoprotein (hnRNP) Squid (Sqd) is a highly abundant protein that is expected to bind most cellular RNAs. Nonetheless, Sqd plays a very specific developmental role in dorsoventral (DV) axis formation during Drosophila oogenesis by localizing gurken(grk) RNA. Here, we report that Sqd is also essential for anteroposterior (AP) axis formation. We identified sqd in a screen for modifiers of the Protein Kinase A (PKA) oogenesis polarity phenotype. The AP defects of sqd mutant oocytes resemble those of PKA mutants in several ways. In both cases, the cytoskeletal reorganization at mid-oogenesis, which depends on a signal from the posterior follicle cells, does not produce a correctly polarized microtubule (MT)network. This causes the posterior determinant, oskar (osk)RNA, to localize to central regions of the oocyte, where it is ectopically translated. Additionally, MT-dependent anterior movement of the oocyte nucleus and the grk-dependent specification of posterior follicle cells are unaffected in both mutants. However, in contrast to PKA mutants, sqd mutants do not retain a discrete posterior MT organizing center(MTOC) capable of supporting ectopic posterior localization of bicoid(bcd) RNA. sqd mutants also display several other phenotypes not seen in PKA mutants; these probably result from the disruption of MT polarity in earlier stages of oogenesis. Loss of Sqd does not affect polarity in follicle cells, wings or eyes, indicating a specific role in the determination of MT polarity within the germline.

The centrosome-nucleus complex and microtubule organization in the Drosophila oocyte

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, gamma-tubulin and Drosophila pericentrin-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.

Wolbachia utilizes host microtubules and Dynein for anterior localization in the Drosophila oocyte

To investigate the role of the host cytoskeleton in the maternal transmission of the endoparasitic bacteria Wolbachia, we have characterized their distribution in the female germ line of Drosophila melanogaster. In the germarium, Wolbachia are distributed to all germ cells of the cyst, establishing an early infection in the cell destined to become the oocyte. During mid-oogenesis, Wolbachia exhibit a distinct concentration between the anterior cortex and the nucleus in the oocyte, where many bacteria appear to contact the nuclear envelope. Following programmed rearrangement of the microtubule network, Wolbachia dissociate from this anterior position and become dispersed throughout the oocyte. This localization pattern is distinct from mitochondria and all known axis determinants. Manipulation of microtubules and cytoplasmic Dynein and Dynactin, but not Kinesin-1, disrupts anterior bacterial localization in the oocyte. In live egg chambers, Wolbachia exhibit movement in nurse cells but not in the oocyte, suggesting that the bacteria are anchored by host factors. In addition, we identify mid-oogenesis as a period in the life cycle of Wolbachia in which bacterial replication occurs. Total bacterial counts show that Wolbachia increase at a significantly higher rate in the oocyte than in the average nurse cell, and that normal Wolbachia levels in the oocyte depend on microtubules. These findings demonstrate that Wolbachia utilize the host microtubule network and associated proteins for their subcellular localization in the Drosophila oocyte. These interactions may also play a role in bacterial motility and replication, ultimately leading to the bacteria's efficient maternal transmission.

Intracellular bacteria that cause disease exploit host cells for their survival and reproduction. Here the authors are studying Wolbachia, an intracellular bacteria that infects many insect species and other invertebrates, such as filarial nematodes, the agents of diseases such as African river blindness and elephantiasis. Wolbachia transmission is similar to mitochondrial inheritance, that is, from mother to offspring in the host. However, little is known about the mechanisms of this maternal transmission and the interactions of Wolbachia and its hosts at the cellular level. Here the authors show that in the fruit fly Drosophila melanogaster, Wolbachia bacteria are initially distributed uniformly throughout the female germ line but concentrate during the middle stages of oogenesis in the future oocyte, exhibiting a striking anterior localization that is distinct from mitochondria and all known axis determinants. The authors demonstrate that microtubules and the minus-end-directed motor Dynein are required for this subcellular localization, suggesting that Wolbachia uses the host's microtubule cytoskeleton and transport system to ensure its transmission to the next generation. This microtubule dependence contrasts with the Actin-based motility observed for many other intracellular bacteria and may provide a foundation for further probing of the molecular and cellular basis of Wolbachia replication and transmission.

Dynein and the actin cytoskeleton control kinesin-driven cytoplasmic streaming in Drosophila oocytes

Mass movements of cytoplasm, known as cytoplasmic streaming, occur in some large eukaryotic cells. In Drosophila oocytes there are two forms of microtubule-based streaming. Slow, poorly ordered streaming occurs during stages 8-10A, while pattern formation determinants such as oskar mRNA are being localized and anchored at specific sites on the cortex. Then fast well-ordered streaming begins during stage 10B, just before nurse cell cytoplasm is dumped into the oocyte. We report that the plus-end-directed microtubule motor kinesin-1 is required for all streaming and is constitutively capable of driving fast streaming. Khc mutations that reduce the velocity of kinesin-1 transport in vitro blocked streaming yet still supported posterior localization of oskar mRNA, suggesting that streaming is not essential for the oskar localization mechanism. Inhibitory antibodies indicated that the minus-end-directed motor dynein is required to prevent premature fast streaming, suggesting that slow streaming is the product of a novel dynein-kinesin competition. As F-actin and some associated proteins are also required to prevent premature fast streaming, our observations support a model in which the actin cytoskeleton triggers the shift from slow to fast streaming by inhibiting dynein. This allows a cooperative self-amplifying loop of plus-end-directed organelle motion and parallel microtubule orientation that drives vigorous streaming currents and thorough mixing of oocyte and nurse-cell cytoplasm.

Drosophila Nod protein binds preferentially to the plus ends of microtubules and promotes microtubule polymerization in vitro

Nod, a nonmotile kinesin-like protein, plays a critical role in segregating achiasmate chromosomes during female meiosis. In addition to localizing to oocyte chromosomes, we show that functional full-length Nod-GFP (Nod(FL)-GFP) localizes to the posterior pole of the oocyte at stages 9-10A, as does kinesin heavy chain (KHC), a plus end-directed motor. This posterior localization is abolished in grk mutants that no longer maintain the microtubule (MT) gradient in the oocyte. To test the hypothesis that Nod binds to the plus ends of MTs, we expressed and purified both full-length Nod (Nod(FL)) and a truncated form of Nod containing only the motor-like domain (Nod318) from Escherichia coli and assessed their interactions with MTs in vitro. Both Nod(FL) and Nod318 demonstrate preferential binding to the ends of the MTs, displaying a strong preference for binding to the plus ends. When Nod318-GFP:MT collision complexes were trapped by glutaraldehyde fixation, the preference for binding to plus ends versus minus ends was 17:1. Nod(FL) and Nod318 also promote MT polymerization in vitro in a time-dependent manner. The observation that Nod is preferentially localized to the plus ends of MTs and stimulates MT polymerization suggests a mechanism for its function.

Axes formation and RNA localization

Axes formation in flies and frogs largely depends on RNA localization pathways functioning in the oocytes. It is thought that motors moving along the cytoskeleton enable the selective transport of RNAs to different destinations during oocyte development. Many of the steps in RNA localization are conserved, despite the existence of a variety of mechanisms, including the formation of nuclear ribonucleoprotein complexes, and active transport along microtubules.

Moving messages: the intracellular localization of mRNAs

mRNA localization is a common mechanism for targeting proteins to regions of the cell where they are required. It has an essential role in localizing cytoplasmic determinants, controlling the direction of protein secretion and allowing the local control of protein synthesis in neurons. New methods for in vivo labelling have revealed that several mRNAs are transported by motor proteins, but how most mRNAs are coupled to these proteins remains obscure.

RNA localization mechanisms in oocytes

In many animals, normal development depends on the asymmetric distribution of maternal determinants, including various coding and noncoding RNAs, within the oocyte. The temporal and spatial distribution of localized RNAs is determined by intricate mechanisms that regulate their movement and anchoring. These mechanisms involve cis-acting sequences within the RNA molecules and a multitude of trans-acting factors, as well as a polarized cytoskeleton, molecular motors and specific transporting organelles. The latest studies show that the fates of localized RNAs within the oocyte cytoplasm are predetermined in the nucleus and that nuclear proteins, some of them deposited on RNAs during splicing, together with the components of the RNA-silencing pathway, dictate the proper movement, targeting, anchoring and translatability of localized RNAs.

Drosophila valois encodes a divergent WD protein that is required for Vasa localization and Oskar protein accumulation

valois (vls) was identified as a posterior group gene in the initial screens for Drosophila maternal-effect lethal mutations. Despite its early genetic identification, it has not been characterized at the molecular level until now. We show that vls encodes a divergent WD domain protein and that the three available EMS-induced point mutations cause premature stop codons in the vls ORF. We have generated a null allele that has a stronger phenotype than the EMS mutants. The vlsnull mutant shows that vls+ is required for high levels of Oskar protein to accumulate during oogenesis, for normal posterior localization of Oskar in later stages of oogenesis and for posterior localization of the Vasa protein during the entire process of pole plasm assembly. There is no evidence for vls being dependent on an upstream factor of the posterior pathway, suggesting that Valois protein (Vls) instead acts as a co-factor in the process. Based on the structure of Vls, the function of similar proteins in different systems and our phenotypic analysis, it seems likely that vls may promote posterior patterning by facilitating interactions between different molecules.

Transport of Drosophila fragile X mental retardation protein-containing ribonucleoprotein granules by kinesin-1 and cytoplasmic dynein

Transport and translation of mRNA are tightly coupled to ensure strict temporal and spatial expression of nascent proteins. Fragile X mental retardation protein (FMRP) has been shown to be involved in translational regulation and is found in ribonucleoprotein (RNP) granules that travel along dendrites of neurons. In this study, GFP-tagged Drosophila homologue of FMRP (dFMR) was used to visualize RNP granule movement in Drosophila S2 cells. GFP-dFMR form granules that contain both endogenous dFMR and mRNA. Live fluorescence microscopy revealed that dFMR-containing RNP granules move bidirectionally in thin processes formed by S2 cells in the presence of cytochalasin D. Knocking down the heavy chains of either kinesin-1 (kinesin heavy chain) or cytoplasmic dynein (dynein heavy chain) by RNA interference blocks the movement of the dFMR granules. In contrast, knockdown of kinesin light chain (KLC), which is typically necessary for movement of membrane organelles by kinesin-1, had no effect on the dFMR granule translocation. In immunoprecipitation assays, dFMR associates with both kinesin heavy chain and dynein heavy chain, but not KLC. Based on these findings, we conclude that dFMR-containing RNP granules are moved by both kinesin-1 and cytoplasmic dynein and that KLC is not essential and is likely missing from RNP-transporting kinesin-1.

mRNA localization and the cytoskeleton

mRNA localization is a widespread post-transcriptional mechanism for targeting protein synthesis to specific cellular sites. It is involved in the generation of cell polarity, asymmetric segregation of cell fate determinants and germ cell specification. Actin and microtubule filaments have key functions during RNA localization, especially during transport of mRNAs and anchoring at target sites. Recent advances in understanding the role of motors and filament systems have mainly resulted from the contribution of live imaging of mRNA movement and from the purification of putative localization ribonucleoproteins. There have also been new findings on the role of centrosomes in RNA localization.

[Establishment and expression of embryonic axes: comparisons between different model organisms]

In an accompanying article (C. Sardet et al. m/s 2004; 20 : 414-423) we reviewed determinants of polarity in early development and the mechanisms which regulate their localization and expression. Such determinants have for the moment been identified in only a few species: the insect Drosophila melanogaster, the worm Caenorhabditis elegans, the frog Xenopus laevis and the ascidians Ciona intestinalis and Holocynthia roretzi. Although oogenesis, fertilization, and cell divisions in these embryos differ considerably, with respect to early polarities certain common themes emerge, such as the importance of cortical mRNAs, the PAR polarity proteins, and reorganizations mediated by the cytoskeleton. Here we highlight similarities and differences in axis establishment between these species, describing them in a chronological order from oocyte to gastrula, and add two more classical model organisms, sea urchin and mouse, to complete the comparisons depicted in the form of a Poster which can be downloaded from the site http://biodev.obs-vlfr.fr/biomarcell.

Hrp48, a Drosophila hnRNPA/B homolog, binds and regulates translation of oskar mRNA

Establishment of the Drosophila embryonic axes provides a striking example of RNA localization as an efficient mechanism for protein targeting within a cell. oskar mRNA encodes the posterior determinant and is essential for germline and abdominal development in the embryo. Tight restriction of Oskar activity to the posterior is achieved by mRNA localization-dependent translational control, whereby unlocalized mRNA is translationally repressed and repression is overcome upon mRNA localization. Here we identify the previously reported oskar RNA binding protein p50 as Hrp48, an abundant Drosophila hnRNP. Analysis of three hrp48 mutant alleles reveals that Hrp48 levels are crucial for polarization of the oocyte during mid-oogenesis. Our data also show that Hrp48, which binds to the 5' and 3' regions of oskar mRNA, plays an important role in restricting Oskar activity to the posterior of the oocyte, by repressing oskar mRNA translation during transport.

Xenopus Staufen is a component of a ribonucleoprotein complex containing Vg1 RNA and kinesin

RNA localization is a key mechanism for generating cell and developmental polarity in a wide variety of organisms. We have performed studies to investigate a role for the Xenopus homolog of the double-stranded RNA-binding protein, Staufen, in RNA localization during oogenesis. We have found that Xenopus Staufen (XStau) is present in a ribonucleoprotein complex, and associates with both a kinesin motor protein and vegetally localized RNAs Vg1 and VegT. A functional role for XStau was revealed through expression of a dominant-negative version that blocks localization of Vg1 RNA in vivo. Our results suggest a central role for XStau in RNA localization in Xenopus oocytes, and provide evidence that Staufen is a conserved link between specific mRNAs and the RNA localization machinery.

mRNA localisation gets more complex

Recent advances in techniques for visualising mRNA movement in living cells have led to rapid progress in understanding the mechanism of mRNA localisation in the cytoplasm. There is an emerging consensus that in many cases the mRNA signals that determine intracellular destination are more complex and difficult to define than was first anticipated. Furthermore, the transacting factors that interpret the mRNA signals are numerous and their combinations change during the life of an mRNA, perhaps allowing the selection of many sub-destinations in the cell. Lastly, an emerging theme over the past few years is that many proteins that determine the destinations of mRNAs are recruited on nascent transcripts in the nucleus. They often function in many different processes in the biogenesis of mRNA and probably act in concert to provide specificity.

The Drosophila SDE3 homolog armitage is required for oskar mRNA silencing and embryonic axis specification

Polarization of the microtubule cytoskeleton during early oogenesis is required to specify the posterior of the Drosophila oocyte, which is essential for asymmetric mRNA localization during mid-oogenesis and for embryonic axis specification. The posterior determinant oskar mRNA is translationally silent until mid-oogenesis. We show that mutations in armitage and three components of the RNAi pathway disrupt oskar mRNA translational silencing, polarization of the microtubule cytoskeleton, and posterior localization of oskar mRNA. armitage encodes a homolog of SDE3, a presumptive RNA helicase involved in posttranscriptional gene silencing (RNAi) in Arabidopsis, and is required for RNAi in Drosophila ovaries. Armitage forms an asymmetric network associated with the polarized microtubule cytoskeleton and is concentrated with translationally silent oskar mRNA in the oocyte. We conclude that RNA silencing is essential for establishment of the cytoskeletal polarity that initiates embryonic axis specification and for translational control of oskar mRNA.