Drosophila cytoplasmic dynein, a microtubule motor that is asymmetrically localized in the oocyte

The Shot CH1 domain recognises a distinct form of F-actin during Drosophila oocyte determination

In Drosophila, only one cell in a multicellular female germline cyst is specified as an oocyte and a similar process occurs in mammals. The symmetry-breaking cue for oocyte selection is provided by the fusome, a tubular structure connecting all cells in the cyst. The Drosophila spectraplakin Shot localises to the fusome and translates its asymmetry into a polarised microtubule network that is essential for oocyte specification, but how Shot recognises the fusome is unclear. Here, we demonstrate that the actin-binding domain (ABD) of Shot is necessary and sufficient to localise Shot to the fusome and mediates Shot function in oocyte specification together with the microtubule-binding domains. The calponin homology domain 1 of the Shot ABD recognises fusomal F-actin and requires calponin homology domain 2 to distinguish it from other forms of F-actin in the cyst. By contrast, the ABDs of utrophin, Fimbrin, Filamin, Lifeact and F-tractin do not recognise fusomal F-actin. We therefore propose that Shot propagates fusome asymmetry by recognising a specific conformational state of F-actin on the fusome.

The dynamic duo of microtubule polymerase Mini spindles/XMAP215 and cytoplasmic dynein is essential for maintaining Drosophila oocyte fate

In many species, only one oocyte is specified among a group of interconnected germline sister cells. In Drosophila melanogaster, 16 interconnected cells form a germline cyst, where one cell differentiates into an oocyte, while the rest become nurse cells that supply the oocyte with mRNAs, proteins, and organelles through intercellular cytoplasmic bridges named ring canals via microtubule-based transport. In this study, we find that a microtubule polymerase Mini spindles (Msps), the Drosophila homolog of XMAP215, is essential for maintenance of the oocyte specification. mRNA encoding Msps is transported and concentrated in the oocyte by dynein-dependent transport along microtubules. Translated Msps stimulates microtubule polymerization in the oocyte, causing more microtubule plus ends to grow from the oocyte through the ring canals into nurse cells, further enhancing nurse cell-to-oocyte transport by dynein. Knockdown of msps blocks the oocyte growth and causes gradual loss of oocyte determinants. Thus, the Msps-dynein duo creates a positive feedback loop, ensuring oocyte fate maintenance by promoting high microtubule polymerization activity in the oocyte, and enhancing dynein-dependent nurse cell-to-oocyte transport.

Drosophila oocyte specification is maintained by the dynamic duo of microtubule polymerase Mini spindles/XMAP215 and dynein

In many species, only one oocyte is specified among a group of interconnected germline sister cells. In Drosophila melanogaster , 16-cell interconnected cells form a germline cyst, where one cell differentiates into an oocyte, while the rest become nurse cells that supply the oocyte with mRNAs, proteins, and organelles through intercellular cytoplasmic bridges named ring canals via microtubule-based transport. In this study, we find that a microtubule polymerase Mini spindles (Msps), the Drosophila homolog of XMAP215, is essential for the oocyte fate determination. mRNA encoding Msps is concentrated in the oocyte by dynein-dependent transport along microtubules. Translated Msps stimulates microtubule polymerization in the oocyte, causing more microtubule plus ends to grow from the oocyte through the ring canals into nurse cells, further enhancing nurse cell-to-oocyte transport by dynein. Knockdown of msps blocks the oocyte growth and causes gradual loss of oocyte determinants. Thus, the Msps-dynein duo creates a positive feedback loop, enhancing dynein-dependent nurse cell-to-oocyte transport and transforming a small stochastic difference in microtubule polarity among sister cells into a clear oocyte fate determination.

Significance statement

Oocyte determination in Drosophila melanogaster provides a valuable model for studying cell fate specification. We describe the crucial role of the duo of microtubule polymerase Mini spindles (Msps) and cytoplasmic dynein in this process. We show that Msps is essential for oocyte fate determination. Msps concentration in the oocyte is achieved through dynein-dependent transport of msps mRNA along microtubules. Translated Msps stimulates microtubule polymerization in the oocyte, further enhancing nurse cell-to-oocyte transport by dynein. This creates a positive feedback loop that transforms a small stochastic difference in microtubule polarity among sister cells into a clear oocyte fate determination. Our findings provide important insights into the mechanisms of oocyte specification and have implications for understanding the development of multicellular organisms.

In vivo proximity biotin ligation identifies the interactome of Egalitarian, a Dynein cargo adaptor

Numerous motors of the Kinesin family contribute to plus-end-directed microtubule transport. However, almost all transport towards the minus-end of microtubules involves Dynein. Understanding the mechanism by which Dynein transports this vast diversity of cargo is the focus of intense research. In selected cases, adaptors that link a particular cargo with Dynein have been identified. However, the sheer diversity of cargo suggests that additional adaptors must exist. We used the Drosophila egg chamber as a model to address this issue. Within egg chambers, Egalitarian is required for linking mRNA with Dynein. However, in the absence of Egalitarian, Dynein transport into the oocyte is severely compromised. This suggests that additional cargoes might be linked to Dynein in an Egalitarian-dependent manner. We therefore used proximity biotin ligation to define the interactome of Egalitarian. This approach yielded several novel interacting partners, including P body components and proteins that associate with Dynein in mammalian cells. We also devised and validated a nanobody-based proximity biotinylation strategy that can be used to define the interactome of any GFP-tagged protein.

The careful control of Polo kinase by APC/C-Ube2C ensures the intercellular transport of germline centrosomes during Drosophila oogenesis

A feature of metazoan reproduction is the elimination of maternal centrosomes from the oocyte. In animals that form syncytial cysts during oogenesis, including Drosophila and human, all centrosomes within the cyst migrate to the oocyte where they are subsequently degenerated. The importance and the underlying mechanism of this event remain unclear. Here, we show that, during early Drosophila oogenesis, control of the Anaphase Promoting Complex/Cyclosome (APC/C), the ubiquitin ligase complex essential for cell cycle control, ensures proper transport of centrosomes into the oocyte through the regulation of Polo/Plk1 kinase, a critical regulator of the integrity and activity of the centrosome. We show that novel mutations in the APC/C-specific E2, Vihar/Ube2c, that affect its inhibitory regulation on APC/C cause precocious Polo degradation and impedes centrosome transport, through destabilization of centrosomes. The failure of centrosome migration correlates with weakened microtubule polarization in the cyst and allows ectopic microtubule nucleation in nurse cells, leading to the loss of oocyte identity. These results suggest a role for centrosome migration in oocyte fate maintenance through the concentration and confinement of microtubule nucleation activity into the oocyte. Considering the conserved roles of APC/C and Polo throughout the animal kingdom, our findings may be translated into other animals.

Optimal RNA binding by Egalitarian, a Dynein cargo adaptor, is critical for maintaining oocyte fate in Drosophila

The Dynein motor is responsible for the localization of numerous mRNAs within Drosophila oocytes and embryos. The RNA binding protein, Egalitarian (Egl), is thought to link these various RNA cargoes with Dynein. Although numerous studies have shown that Egl is able to specifically associate with these RNAs, the nature of these interactions has remained elusive. Egl contains a central RNA binding domain that shares limited homology with an exonuclease, yet Egl binds to RNA without degrading it. Mutations have been identified within Egl that disrupt its association with its protein interaction partners, BicaudalD (BicD) and Dynein light chain (Dlc), but no mutants have been described that are specifically defective for RNA binding. In this report, we identified a series of positively charged residues within Egl that are required for RNA binding. Using corresponding RNA binding mutants, we demonstrate that specific RNA cargoes are more reliant on maximal Egl RNA biding activity for their correct localization in comparison to others. We also demonstrate that specification and maintenance of oocyte fate requires maximal Egl RNA binding activity. Even a subtle reduction in Egl's RNA binding activity completely disrupts this process. Our results show that efficient RNA localization at the earliest stages of oogenesis is required for specification of the oocyte and restriction of meiosis to a single cell.

Regulation of apical constriction via microtubule- and Rab11-dependent apical transport during tissue invagination

The formation of an epithelial tube is a fundamental process for organogenesis. During Drosophila embryonic salivary gland (SG) invagination, Folded gastrulation (Fog)-dependent Rho-associated kinase (Rok) promotes contractile apical myosin formation to drive apical constriction. Microtubules (MTs) are also crucial for this process and are required for forming and maintaining apicomedial myosin. However, the underlying mechanism that coordinates actomyosin and MT networks still remains elusive. Here, we show that MT-dependent intracellular trafficking regulates apical constriction during SG invagination. Key components involved in protein trafficking, such as Rab11 and Nuclear fallout (Nuf), are apically enriched near the SG invagination pit in a MT-dependent manner. Disruption of the MT networks or knockdown of Rab11 impairs apicomedial myosin formation and apical constriction. We show that MTs and Rab11 are required for apical enrichment of the Fog ligand and the continuous distribution of the apical determinant protein Crumbs (Crb) and the key adherens junction protein E-Cadherin (E-Cad) along junctions. Targeted knockdown of crb or E-Cad in the SG disrupts apical myosin networks and results in apical constriction defects. Our data suggest a role of MT- and Rab11-dependent intracellular trafficking in regulating actomyosin networks and cell junctions to coordinate cell behaviors during tubular organ formation.

Patterning the Drosophila embryo: A paradigm for RNA-based developmental genetic regulation

Embryonic anterior–posterior patterning is established in Drosophila melanogaster by maternally expressed genes. The mRNAs of several of these genes accumulate at either the anterior or posterior pole of the oocyte via a number of mechanisms. Many of these mRNAs are also under elaborate translational regulation. Asymmetric RNA localization coupled with spatially restricted translation ensures that their proteins are restricted to the position necessary for the developmental process that they drive. Bicoid (Bcd), the anterior determinant, and Oskar (Osk), the determinant for primordial germ cells and posterior patterning, have been studied particularly closely. In early embryos an anterior–posterior gradient of Bcd is established, activating transcription of different sets of zygotic genes depending on local Bcd concentration. At the posterior pole, Osk seeds formation of polar granules, ribonucleoprotein complexes that accumulate further mRNAs and proteins involved in posterior patterning and germ cell specification. After fertilization, polar granules associate with posterior nuclei and mature into nuclear germ granules. Osk accumulates in these granules, and either by itself or as part of the granules, stimulates germ cell division.

This article is categorized under:

RNA Export and Localization > RNA Localization

Translation > Translation Regulation

RNA in Disease and Development > RNA in Development

The Egalitarian binding partners Dynein light chain and Bicaudal-D act sequentially to link mRNA to the Dynein motor

A conserved mechanism of polarity establishment is the localization of mRNA to specific cellular regions. Although it is clear that many mRNAs are transported along microtubules, much less is known about the mechanism by which these mRNAs are linked to microtubule motors. The RNA binding protein Egalitarian (Egl) is necessary for localization of several mRNAs in Drosophila oocytes and embryos. Egl also interacts with Dynein light chain (Dlc) and Bicaudal-D (BicD). The role of Dlc and BicD in mRNA localization has remained elusive. Both proteins are required for oocyte specification, as is Egl. Null alleles in these genes result in an oogenesis block. In this report, we used an shRNA-depletion strategy to overcome the oogenesis block. Our findings reveal that the primary function of Dlc is to promote Egl dimerization. Loss of dimerization compromises the ability of Egl to bind RNA. Consequently, Egl is not bound to cargo, and is not able to efficiently associate with BicD and the Dynein motor. Our results therefore identify the key molecular steps required for assembling a localization-competent mRNP.

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.

Recent advances in understanding oogenesis: interactions with the cytoskeleton, microtubule organization, and meiotic spindle assembly in oocytes

Maternal control of development begins with production of the oocyte during oogenesis. All of the factors necessary to complete oocyte maturation, meiosis, fertilization, and early development are produced in the transcriptionally active early oocyte. Active transcription of the maternal genome is a mechanism to ensure that the oocyte and development of the early embryo begin with all of the factors needed for successful embryonic development. To achieve the maximum maternal store, only one functional cell is produced from the meiotic divisions that produce the oocyte. The oocyte receives the bulk of the maternal cytoplasm and thus is significantly larger than its sister cells, the tiny polar bodies, which receive a copy of the maternal genome but essentially none of the maternal cytoplasm. This asymmetric division is accomplished by an enormous cell that is depleted of centrosomes in early oogenesis; thus, meiotic divisions in oocytes are distinct from those of mitotic cells. Therefore, these cells must partition the chromosomes faithfully to ensure euploidy by using mechanisms that do not rely on a conventional centrosome-based mitotic spindle. Several mechanisms that contribute to assembly and maintenance of the meiotic spindle in oocytes have been identified; however, none is fully understood. In recent years, there have been many exciting and significant advances in oogenesis, contributed by studies using a myriad of systems. Regrettably, I cannot adequately cover all of the important advances here and so I apologize to those whose beautiful work has not been included. This review focuses on a few of the most recent studies, conducted by several groups, using invertebrate and vertebrate systems, that have provided mechanistic insight into how microtubule assembly and meiotic spindle morphogenesis are controlled in the absence of centrosomes.

Acquisition of Oocyte Polarity

Acquisition of oocyte polarity involves complex translocation and aggregation of intracellular organelles, RNAs, and proteins, along with strict posttranscriptional regulation. While much is still unknown regarding the formation of the animal-vegetal axis, an early marker of polarity, animal models have contributed to our understanding of these early processes controlling normal oogenesis and embryo development. In recent years, it has become clear that proteins with self-assembling properties are involved in assembling discrete subcellular compartments or domains underlying subcellular asymmetries in the early mitotic and meiotic cells of the female germline. These include asymmetries in duplication of the centrioles and formation of centrosomes and assembly of the organelle and RNA-rich Balbiani body, which plays a critical role in oocyte polarity. Notably, at specific stages of germline development, these transient structures in oocytes are temporally coincident and align with asymmetries in the position and arrangement of nuclear components, such as the nuclear pore and the chromosomal bouquet and the centrioles and cytoskeleton in the cytoplasm. Formation of these critical, transient structures and arrangements involves microtubule pathways, intrinsically disordered proteins (proteins with domains that tend to be fluid or lack a rigid ordered three-dimensional structure ranging from random coils, globular domains, to completely unstructured proteins), and translational repressors and activators. This review aims to examine recent literature and key players in oocyte polarity.

Localised dynactin protects growing microtubules to deliver oskar mRNA to the posterior cortex of the Drosophila oocyte

The localisation of oskar mRNA to the posterior of the Drosophila oocyte defines where the abdomen and germ cells form in the embryo. Kinesin 1 transports oskar mRNA to the oocyte posterior along a polarised microtubule cytoskeleton that grows from non-centrosomal microtubule organising centres (ncMTOCs) along the anterior/lateral cortex. Here, we show that the formation of this polarised microtubule network also requires the posterior regulation of microtubule growth. A missense mutation in the dynactin Arp1 subunit causes most oskar mRNA to localise in the posterior cytoplasm rather than cortically. oskar mRNA transport and anchoring are normal in this mutant, but the microtubules fail to reach the posterior pole. Thus, dynactin acts as an anti-catastrophe factor that extends microtubule growth posteriorly. Kinesin 1 transports dynactin to the oocyte posterior, creating a positive feedback loop that increases the length and persistence of the posterior microtubules that deliver oskar mRNA to the cortex.

Loss of function of the Drosophila Ninein-related centrosomal protein Bsg25D causes mitotic defects and impairs embryonic development

The centrosome-associated proteins Ninein (Nin) and Ninein-like protein (Nlp) play significant roles in microtubule stability, nucleation and anchoring at the centrosome in mammalian cells. Here, we investigate Blastoderm specific gene 25D (Bsg25D), which encodes the only Drosophila protein that is closely related to Nin and Nlp. In early embryos, we find that Bsg25D mRNA and Bsg25D protein are closely associated with centrosomes and astral microtubules. We show that sequences within the coding region and 3′UTR of Bsg25D mRNAs are important for proper localization of this transcript in oogenesis and embryogenesis. Ectopic expression of eGFP-Bsg25D from an unlocalized mRNA disrupts microtubule polarity in mid-oogenesis and compromises the distribution of the axis polarity determinant Gurken. Using total internal reflection fluorescence microscopy, we show that an N-terminal fragment of Bsg25D can bind microtubules in vitro and can move along them, predominantly toward minus-ends. While flies homozygous for a Bsg25D null mutation are viable and fertile, 70% of embryos lacking maternal and zygotic Bsg25D do not hatch and exhibit chromosome segregation defects, as well as detachment of centrosomes from mitotic spindles. We conclude that Bsg25D is a centrosomal protein that, while dispensable for viability, nevertheless helps ensure the integrity of mitotic divisions in Drosophila.

Summary: In humans, mutations in Ninein or Ninein-like protein result in microcephaly and other severe diseases. We show that while flies lacking the Ninein orthologue can survive, many die as embryos with defects in mitosis.

Dynamic microtubule organization and mitochondrial transport are regulated by distinct Kinesin-1 pathways

The microtubule (MT) plus-end motor kinesin heavy chain (Khc) is well known for its role in long distance cargo transport. Recent evidence showed that Khc is also required for the organization of the cellular MT network by mediating MT sliding. We found that mutations in Khc and the gene of its adaptor protein, kinesin light chain (Klc) resulted in identical bristle morphology defects, with the upper part of the bristle being thinner and flatter than normal and failing to taper towards the bristle tip. We demonstrate that bristle mitochondria transport requires Khc but not Klc as a competing force to dynein heavy chain (Dhc). Surprisingly, we demonstrate for the first time that Dhc is the primary motor for both anterograde and retrograde fast mitochondria transport. We found that the upper part of Khc and Klc mutant bristles lacked stable MTs. When following dynamic MT polymerization via the use of GFP-tagged end-binding protein 1 (EB1), it was noted that at Khc and Klc mutant bristle tips, dynamic MTs significantly deviated from the bristle parallel growth axis, relative to wild-type bristles. We also observed that GFP-EB1 failed to concentrate as a focus at the tip of Khc and Klc mutant bristles. We propose that the failure of bristle tapering is due to defects in directing dynamic MTs at the growing tip. Thus, we reveal a new function for Khc and Klc in directing dynamic MTs during polarized cell growth. Moreover, we also demonstrate a novel mode of coordination in mitochondrial transport between Khc and Dhc.

Summary: Highly polarized Drosophila bristle cells reveal that dynamic microtubule organization and mitochondrial transport are regulated by distinct Kinesin-1 pathways, and a novel mode of coordination between Khc and Dhc in mitochondrial transport.

A transport and retention mechanism for the sustained distal localization of Spn-F-IKKε during Drosophila bristle elongation

Stable localization of the signaling complex is essential for the robust morphogenesis of polarized cells. Cell elongation involves molecular signaling centers that coordinately regulate intracellular transport and cytoskeletal structures. In Drosophila bristle elongation, the protein kinase IKKε is activated at the distal tip of the growing bristle and regulates the shuttling movement of recycling endosomes and cytoskeletal organization. However, how the distal tip localization of IKKε is established and maintained during bristle elongation is unknown. Here, we demonstrate that IKKε distal tip localization is regulated by Spindle-F (Spn-F), which is stably retained at the distal tip and functions as an adaptor linking IKKε to cytoplasmic dynein. We found that Javelin-like (Jvl) is a key regulator of Spn-F retention. In jvl mutant bristles, IKKε and Spn-F initially localize to the distal tip but fail to be retained there. In S2 cells, particles that stain positively for Jvl or Spn-F move in a microtubule-dependent manner, whereas Jvl and Spn-F double-positive particles are immobile, indicating that Jvl and Spn-F are transported separately and, upon forming a complex, immobilize each other. These results suggest that polarized transport and selective retention regulate the distal tip localization of the Spn-F–IKKε complex during bristle cell elongation.

Summary: In the Drosophila bristle, the microtubule binding protein Jvl, the adaptor Spn-F and cytoplasmic dynein are required for localised transport and retention of polarised signalling factors.

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.

Dynein associates with oskar mRNPs and is required for their efficient net plus-end localization in Drosophila oocytes

In order for eukaryotic cells to function properly, they must establish polarity. The Drosophila oocyte uses mRNA localization to establish polarity and hence provides a genetically tractable model in which to study this process. The spatial restriction of oskar mRNA and its subsequent protein product is necessary for embryonic patterning. The localization of oskar mRNA requires microtubules and microtubule-based motor proteins. Null mutants in Kinesin heavy chain (Khc), the motor subunit of the plus end-directed Kinesin-1, result in oskar mRNA delocalization. Although the majority of oskar particles are non-motile in khc nulls, a small fraction of particles display active motility. Thus, a motor other than Kinesin-1 could conceivably also participate in oskar mRNA localization. Here we show that Dynein heavy chain (Dhc), the motor subunit of the minus end-directed Dynein complex, extensively co-localizes with Khc and oskar mRNA. In addition, immunoprecipitation of the Dynein complex specifically co-precipitated oskar mRNA and Khc. Lastly, germline-specific depletion of Dhc resulted in oskar mRNA and Khc delocalization. Our results therefore suggest that efficient posterior localization of oskar mRNA requires the concerted activities of both Dynein and Kinesin-1.

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.

A spatio-temporal model of Notch signalling in the zebrafish segmentation clock: conditions for synchronised oscillatory dynamics

In the vertebrate embryo, tissue blocks called somites are laid down in head-to-tail succession, a process known as somitogenesis. Research into somitogenesis has been both experimental and mathematical. For zebrafish, there is experimental evidence for oscillatory gene expression in cells in the presomitic mesoderm (PSM) as well as evidence that Notch signalling synchronises the oscillations in neighbouring PSM cells. A biological mechanism has previously been proposed to explain these phenomena. Here we have converted this mechanism into a mathematical model of partial differential equations in which the nuclear and cytoplasmic diffusion of protein and mRNA molecules is explicitly considered. By performing simulations, we have found ranges of values for the model parameters (such as diffusion and degradation rates) that yield oscillatory dynamics within PSM cells and that enable Notch signalling to synchronise the oscillations in two touching cells. Our model contains a Hill coefficient that measures the co-operativity between two proteins (Her1, Her7) and three genes (her1, her7, deltaC) which they inhibit. This coefficient appears to be bounded below by the requirement for oscillations in individual cells and bounded above by the requirement for synchronisation. Consistent with experimental data and a previous spatially non-explicit mathematical model, we have found that signalling can increase the average level of Her1 protein. Biological pattern formation would be impossible without a certain robustness to variety in cell shape and size; our results possess such robustness. Our spatially-explicit modelling approach, together with new imaging technologies that can measure intracellular protein diffusion rates, is likely to yield significant new insight into somitogenesis and other biological processes.

Characterization and localization of dynein and myosins V and VI in the ovaries of queen bees

The presence of myosin and dynein in the ovaries of both Apis mellifera and Scaptotrigona postica was investigated in extracts and in histological sections. In the ovary extracts, motor proteins, myosins V, VI and dynein were detected by Western blot. In histological sections, they were detected by immunocytochemistry, using a mouse monoclonal antibody against the intermediary chain of dynein and a rabbit polyclonal antibody against the myosin V head domain. The myosin VI tail domain was recognized by a pig polyclonal antibody. The results show that these molecular motors are expressed in the ovaries of both bee species with few differences in location and intensity, in regions where movement of substances is expected during oogenesis. The fact that antibodies against vertebrate proteins recognize proteins of bee species indicates that the specific epitopes are evolutionarily well preserved.

Dlg1 binds GKAP to control dynein association with microtubules, centrosome positioning, and cell polarity

The small GTPase Cdc42 regulates interactions of dynein with microtubules through the polarity protein Dlg1 and the scaffolding protein GKAP.

Centrosome positioning is crucial during cell division, cell differentiation, and for a wide range of cell-polarized functions including migration. In multicellular organisms, centrosome movement across the cytoplasm is thought to result from a balance of forces exerted by the microtubule-associated motor dynein. However, the mechanisms regulating dynein-mediated forces are still unknown. We show here that during wound-induced cell migration, the small G protein Cdc42 acts through the polarity protein Dlg1 to regulate the interaction of dynein with microtubules of the cell front. Dlg1 interacts with dynein via the scaffolding protein GKAP and together, Dlg1, GKAP, and dynein control microtubule dynamics and organization near the cell cortex and promote centrosome positioning. Our results suggest that, by modulating dynein interaction with leading edge microtubules, the evolutionary conserved proteins Dlg1 and GKAP control the forces operating on microtubules and play a fundamental role in centrosome positioning and cell polarity.

The impact on microtubule network of a bracovirus IkappaB-like protein

Polydnavirus-encoded IkappaB-like proteins are similar to insect and mammalian IkappaB, and an immunosuppressive function in the host cells has been inferred to these proteins. Here we show that the expression of one of these IkappaB-like viral genes, the TnBVank1, in the Drosophila germline affects the localization of gurken, bicoid, and oskar mRNAs whose gene products are relevant for proper embryonic patterning. The altered localization of these mRNAs is suggestive of general defects in the intracellular, microtubule-based, trafficking routes. Analysis of microtubule motor proteins components such as the dynein heavy chain and the kinesin heavy chain revealed defects in the polarized microtubule network. Interestingly, the TnBVANK1 viral protein is uniformly distributed over the entire oocyte cortex, and appears to be anchored to the microtubule ends. Our data open up a very interesting issue on novel function(s) played by the ank gene family by interfering with cytoskeleton organization.

Identification of genes differentially expressed by calorie restriction in the rotifer (Brachionus plicatilis)

A monogonont rotifer Brachionus plicatilis has been widely used as a model organism for physiological, ecological studies and for ecotoxicology. Because of the availability of parthenogenetic mode of reproduction as well as its versatility to be used as live food in aquaculture, the population dynamic studies using the rotifer have become more important and acquired the priority over those using other species. Although many studies have been conducted to identify environmental factors that influence rotifer populations, the molecular mechanisms involved still remain to be elucidated. In this study, gene(s) differentially expressed by calorie restriction in the rotifer was analyzed, where a calorie-restricted group was fed 3 h day(-1) and a well-fed group fed ad libitum. A subtracted cDNA library from the calorie-restricted rotifer was constructed using suppression subtractive hybridization (SSH). One hundred sixty-three expressed sequence tags (ESTs) were identified, which included 109 putative genes with a high identity to known genes in the publicly available database as well as 54 unknown ESTs. After assembling, a total of 38 different genes were obtained among 109 ESTs. Further validation of expression by semi-quantitative reverse transcription-PCR showed that 29 out of the 38 genes obtained by SSH were up regulated by calorie restriction.

Altered dynein-dependent transport in piRNA pathway mutants

Maintenance of genome integrity in germ cells is crucial for the success of future generations. In Drosophila, and mammals, transposable element activity in the germline can cause DNA breakage and sterility. Recent studies have shown that proteins involved in piRNA (PIWI-interacting RNA) biogenesis are necessary for retrotransposon silencing in the Drosophila germline. Females mutant for genes in the piRNA biogenesis pathway produce eggs with patterning defects that result from Chk-2 (checkpoint kinase-2) DNA damage checkpoint activation. Here we show that large ribonucleoprotein aggregates form in response to DNA damage checkpoint activation in egg chambers of females defective in piRNA biogenesis. Aggregate formation is specific to piRNA biogenesis mutants, as other mutations that activate the same Chk-2-dependent checkpoint do not cause aggregate formation. These aggregates contain components of the dynein motor machinery, retrotransposon RNA, and protein and axial patterning RNAs. Disruption of the aggregates by colcemid treatment leads to increased retrotransposon RNA levels, indicating that these structures may be the destination of retrotransposon RNA transport and may be degradation or sequestration sites. We propose that aggregate formation is a cellular response to protect germ cells from DNA damage caused by elevated retrotransposon expression.

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.

Mouse dynein axonemal intermediate chain 2: cloning and expression

Ovarian follicular development is a complex process. Investigation of the mechanisms regulating the initiation of follicular growth, and the growth and differentiation of preantral follicles is of great interest. In an effort to clone follicular development-related genes, we selected a partial cDNA fragment by differential display reverse-transcription PCR using total RNA extracted from 5-day-old and 10-day-old mouse ovaries, and its open reading frame was obtained by rapid amplification of cDNA ends. Sequencing showed that the fragment is the mouse dynein axonemal intermediate chain 2 gene (Dnaic2), which has an 87% homology with human DNAI2, a candidate gene for primary ciliary dyskinesia. Northern and western analyses indicate that Dnaic2 produces an approximate 3 kb mRNA that is translated into an approximate 70 kDa protein. The mRNA is predominantly expressed in mouse ovary, testis, and lung. In mouse ovaries, Dnaic2 mRNA was detected at high levels in vivo on day 10, with a subsequent decrease on days 15 and 20, in adult and old ovaries. However, Dnaic2 expression was weak on day 5. Dnaic2 protein was localized on the surface of the oocyte. No obvious fluorescence signal was detected in primordial and primary follicles, while strong signals were detected on the oocyte surface of secondary and antral follicles, in particular for secondary follicles in day 10. These data suggest that Dnaic2 plays a role in ovarian follicular development.

The dynamics of fluorescently labeled endogenous gurken mRNA in Drosophila

During Drosophila oogenesis, the targeted localization of gurken (grk) mRNA leads to the establishment of the axis polarity of the egg. In early stages of oogenesis, grk mRNA is found at the posterior of the oocyte, whereas in the later stages grk mRNA is positioned at the dorsal anterior corner of the oocyte. In order to visualize the real-time localization and anchorage of endogenous grk mRNA in living oocytes, we have utilized the MS2-MCP system. We show that MCP-GFP-tagged endogenous grk mRNA localizes properly within wild-type oocytes and behaves aberrantly in mutant backgrounds. Fluorescence recovery after photobleaching (FRAP) experiments of localized grk mRNA in egg chambers reveal a difference in the dynamics of grk mRNA between young and older egg chambers. grk mRNA particles, as a population, are highly dynamic molecules that steadily lose their dynamic nature as oogenesis progresses. This difference in dynamics is attenuated in K10 and sqd(1) mutants such that mislocalized grk mRNA in older stages is much more dynamic compared with that in wild-type controls. By contrast, in flies with compromised dynein activity, properly localized grk mRNA is much more static. Taken together, we have observed the nature of localized grk mRNA in live oocytes and propose that its maintenance changes from a dynamic to a static process as oogenesis progresses.

Dynein regulates epithelial polarity and the apical localization of stardust A mRNA

Intense investigation has identified an elaborate protein network controlling epithelial polarity. Although precise subcellular targeting of apical and basolateral determinants is required for epithelial architecture, little is known about how the individual determinant proteins become localized within the cell. Through a genetic screen for epithelial defects in the Drosophila follicle cells, we have found that the cytoplasmic Dynein motor is an essential regulator of apico–basal polarity. Our data suggest that Dynein acts through the cytoplasmic scaffolding protein Stardust (Sdt) to localize the transmembrane protein Crumbs, in part through the apical targeting of specific sdt mRNA isoforms. We have mapped the sdt mRNA localization signal to an alternatively spliced coding exon. Intriguingly, the presence or absence of this exon corresponds to a developmental switch in sdt mRNA localization in which apical transcripts are only found during early stages of epithelial development, while unlocalized transcripts predominate in mature epithelia. This work represents the first demonstration that Dynein is required for epithelial polarity and suggests that mRNA localization may have a functional role in the regulation of apico–basal organization. Moreover, we introduce a unique mechanism in which alternative splicing of a coding exon is used to control mRNA localization during development.

Cells within epithelial sheets are highly polarized with distinct apical and basolateral membrane domains. This cellular organization is critical to both epithelial form and function, and a failure to maintain epithelial polarity is often linked to tumor progression. The protein network that establishes and maintains the two membrane domains relies on the precise subcellular localization of its molecular components, but little is known about how these proteins are targeted to their sites of action. We have shown that the localization of the apical determinant protein Stardust depends on the microtubule motor Dynein. While investigating the relationship between Dynein and Stardust, we also made two unexpected observations about stardust mRNA regulation. First, the mechanism by which Dynein localizes Stardust may depend, in part, on the apical targeting of the stardust mRNA. Second, some stardust mRNA is apically localized during early stages of epithelial development, but the selective removal of the apical localization signal leads to the sole production of uniformly localized transcripts in mature epithelial cells. Together, these results introduce roles for Dynein in apico–basal polarity regulation and raise important questions about the role of mRNA localization in the targeting of polarity determinant proteins and epithelial maturation.

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.

Mitch a rapidly evolving component of the Ndc80 kinetochore complex required for correct chromosome segregation in Drosophila

We identified an essential kinetochore protein, Mitch, from a genetic screen in D. melanogaster. Mitch localizes to the kinetochore, and its targeting is independent of microtubules (MTs) and several other known kinetochore components. Animals carrying mutations in mitch die as late third-instar larvae; mitotic neuroblasts in larval brains exhibit high levels of aneuploidy. Analysis of fixed D. melanogaster brains and mitch RNAi in cultured cells, as well as video recordings of cultured mitch mutant neuroblasts, reveal that chromosome alignment in mitch mutants is compromised during spindle formation, with many chromosomes displaying persistent mono-orientation. These misalignments lead to aneuploidy during anaphase. Mutations in mitch also disrupt chromosome behavior during both meiotic divisions in spermatocytes: the entire chromosome complement often moves to only one spindle pole. Mutant mitotic cells exhibit contradictory behavior with respect to the spindle assembly checkpoint (SAC). Anaphase onset is delayed in untreated cells, probably because incorrect kinetochore attachment maintains the SAC. However, mutant brain cells and mitch RNAi cells treated with MT poisons prematurely disjoin their chromatids, and exit mitosis. These data suggest that Mitch participates in SAC signaling that responds specifically to disruptions in spindle microtubule dynamics. The mitch gene corresponds to the transcriptional unit CG7242, and encodes a protein that is a possible ortholog of the Spc24 or Spc25 subunit of the Ndc80 kinetochore complex. Despite the crucial role of Mitch in cell division, the mitch gene has evolved very rapidly among species in the genus Drosophila.

From stem cell to embryo without centrioles

Centrosome asymmetry plays a key role in ensuring the asymmetric division of Drosophila neural stem cells (neuroblasts [NBs]) and male germline stem cells (GSCs) [1–3]. In both cases, one centrosome is anchored close to a specific cortical region during interphase, thus defining the orientation of the spindle during the ensuing mitosis. To test whether asymmetric centrosome behavior is a general feature of stem cells, we have studied female GSCs, which divide asymmetrically, producing another GSC and a cystoblast. The cystoblast then divides and matures into an oocyte, a process in which centrosomes exhibit a series of complex behaviors proposed to play a crucial role in oogenesis [4–6]. We show that the interphase centrosome does not define spindle orientation in female GSCs and that DSas-4 mutant GSCs [7], lacking centrioles and centrosomes, invariably divide asymmetrically to produce cystoblasts that proceed normally through oogenesis—remarkably, oocyte specification, microtubule organization, and mRNA localization are all unperturbed. Mature oocytes can be fertilized, but embryos that cannot support centriole replication arrest very early in development. Thus, centrosomes are dispensable for oogenesis but essential for early embryogenesis. These results reveal that asymmetric centrosome behavior is not an essential feature of stem cell divisions.

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.

How dynein helps the cell find its center: a servomechanical model

Cytoplasmic dynein is the major minus-end-directed microtubule motor protein in interphase cells. In addition to its well-established roles in vesicular transport and chromosome dynamics, cytoplasmic dynein also associates with the cell cortex. From this site, it appears to pull on the cytoplasmic microtubule network, influencing mitotic spindle orientation, nuclear position and other aspects of cell polarity and organization. Recent evidence indicates that the cell has the remarkable ability to calculate is geometric center, and, with the help of dynein, to position the centrosome at this central site. Here, we outline models to account for this behavior.

Milton controls the early acquisition of mitochondria by Drosophila oocytes

Mitochondria in many species enter the young oocyte en mass from interconnected germ cells to generate the large aggregate known as the Balbiani body. Organelles and germ plasm components frequently associate with this structure. Balbiani body mitochondria are thought to populate the germ line, ensuring that their genomes will be inherited preferentially. We find that milton, a gene whose product was previously shown to associate with Kinesin and to mediate axonal transport of mitochondria, is needed to form a normal Balbiani body. In addition, germ cells mutant for some milton or Kinesin heavy chain (Khc) alleles transport mitochondria to the oocyte prematurely and excessively, without disturbing Balbiani body-associated components. Our observations show that the oocyte acquires the majority of its mitochondria by competitive bidirectional transport along microtubules mediated by the Milton adaptor. These experiments provide a molecular explanation for Balbiani body formation and, surprisingly, show that viable fertile offspring can be obtained from eggs in which the normal program of mitochondrial acquisition has been severely perturbed.

Genetic analysis of the cytoplasmic dynein subunit families

Cytoplasmic dyneins, the principal microtubule minus-end-directed motor proteins of the cell, are involved in many essential cellular processes. The major form of this enzyme is a complex of at least six protein subunits, and in mammals all but one of the subunits are encoded by at least two genes. Here we review current knowledge concerning the subunits, their interactions, and their functional roles as derived from biochemical and genetic analyses. We also carried out extensive database searches to look for new genes and to clarify anomalies in the databases. Our analysis documents evolutionary relationships among the dynein subunits of mammals and other model organisms, and sheds new light on the role of this diverse group of proteins, highlighting the existence of two cytoplasmic dynein complexes with distinct cellular roles.

Folding is coupled to dimerization of Tctex-1 dynein light chain

Equilibrium analyses have been performed to elucidate the role of dimerization in folding and stability of dynein light chain Tctex-1. The equilibrium unfolding transition was monitored by intrinsic fluorescence intensity, fluorescence anisotropy, and circular dichroism and was modeled as a two-state mechanism where a folded dimer dissociates to two unfolded monomers without populating thermodynamically stable monomeric or dimeric intermediates. Sedimentation equilibrium and chemical cross-linking experiments performed at increasing concentrations of denaturants show no change in the association state before the unfolding transition and are consistent with the two-state model of dissociation coupled to unfolding. A linear dependence on denaturant concentration is observed by fluorescence intensity and anisotropy before unfolding in the 0-2 M GdnCl, and 0-4 M urea concentration range. This change is not protein concentration-dependent and possibly reflects relief of quenching associated with premelting conformational disorder in the vicinity of Trp 83. The data clearly show that the dissociation-coupled unfolding mechanism of Tctex-1 is different from the three-state denaturation mechanism of its structural homologue light chain LC8. The absence of a stable monomer in Tctex-1 offers insight into its functional differences from LC8.

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.

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.

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.

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.

The Drosophila tctex-1 light chain is dispensable for essential cytoplasmic dynein functions but is required during spermatid differentiation

Variations in subunit composition and modification have been proposed to regulate the multiple functions of cytoplasmic dynein. Here, we examine the role of the Drosophila ortholog of tctex-1, the 14-kDa dynein light chain. We show that the 14-kDa light chain is a bona fide component of Drosophila cytoplasmic dynein and use P element excision to generate flies that completely lack this dynein subunit. Remarkably, the null mutant is viable and the only observed defect is complete male sterility. During spermatid differentiation, the 14-kDa light chain is required for the localization of a nuclear "cap" of cytoplasmic dynein and for proper attachment between the sperm nucleus and flagellar basal body. Our results provide evidence that the function of the 14-kDa light chain in Drosophila is distinct from other dynein subunits and is not required for any essential functions in early development or in the adult organism.

Egalitarian binds dynein light chain to establish oocyte polarity and maintain oocyte fate

In many cell types polarized transport directs the movement of mRNAs and proteins from their site of synthesis to their site of action, thus conferring cell polarity. The cytoplasmic dynein microtubule motor complex is involved in this process. In Drosophila melanogaster, the Egalitarian (Egl) and Bicaudal-D (BicD) proteins are also essential for the transport of macromolecules to the oocyte and to the apical surface of the blastoderm embryo. Hence, Egl and BicD, which have been shown to associate, may be part of a conserved core localization machinery in Drosophila, although a direct association between these molecules and the dynein motor complex has not been shown. Here we report that Egl interacts directly with Drosophila dynein light chain (Dlc), a microtubule motor component, through an Egl domain distinct from that which binds BicD. We propose that the Egl-BicD complex is loaded through Dlc onto the dynein motor complex thereby facilitating transport of cargo. Consistent with this model, point mutations that specifically disrupt Egl-Dlc association also disrupt microtubule-dependant trafficking both to and within the oocyte, resulting in a loss of oocyte fate maintenance and polarity. Our data provide a direct link between a molecule necessary for oocyte specification and the microtubule motor complex, and supports the hypothesis that microtubule-mediated transport is important for preserving oocyte fate.

The origin of dorsoventral polarity in Drosophila

In Drosophila dorsoventral (DV) polarity arises during oogenesis when the oocyte nucleus moves from a central posterior to an asymmetrical anterior position. Nuclear movement is a symmetry-breaking step and establishes orthogonality between the anteroposterior and the DV axes. The asymmetrically anchored nucleus defines a cortical region within the oocyte which accumulates high levels of gurken messenger RNA (mRNA) and protein. Gurken is an ovarian-specific member of the transforming growth factor-alpha (TGF-alpha) family of secreted ligands. Secreted Gurken forms a concentration gradient that results in a dorsal-to-ventral gradient of EGF receptor activation in the follicle cells surrounding the oocyte. This leads to concentration-dependent activation or repression of target genes of the EGF pathway in the follicular epithelium. One outcome of this process is the restriction of pipe expression to a ventral domain that comprises 40% of the egg circumference. Pipe presumably modifies extracellular matrix components that are secreted by the follicle cells and are present at the ventral side of embryo after egg deposition. Here, they activate a proteolytic cascade that generates a gradient of the diffusible ligand, Spätzle. Spätzle activates the Toll receptor at the surface of the embryo that stimulates the nuclear uptake of the transcription factor Dorsal. This leads to a nuclear concentration gradient of Dorsal that specifies the cell types along the DV axis of the embryo.

gamma-Tubulin function during female germ-cell development and oogenesis in Drosophila

A series of unconventional microtubule organizing centers play a fundamental role during egg chamber development in Drosophila. To gain a better understanding of their molecular nature, we have studied the centrosomal component gamma-tubulin during Drosophila oogenesis. We find that although single mutations in either of the two gamma-tubulin genes identified in Drosophila do not affect oogenesis progression the simultaneous depletion of both protein products has severe consequences. The combination of loss-of-function mutant alleles for the two gamma-tubulin genes leads to mitotic defects in female germ cells, resulting in agametic ovaries. A combination of weaker mutant alleles instead allows female germ-cell development to proceed, although the resulting egg chambers display pleiotropic abnormalities, most frequently affecting the number of nurse cells and oocytes per egg chamber. Thus, gamma-tubulin is required for several processes at different stages of germ-cell development and oogenesis, including oocyte determination and differentiation. Our data provide a functional link between a component of the peri-centriolar material, such as gamma-tubulin, and microtubule organization during Drosophila oogenesis. In addition, our results show that gamma-tubulin is required for female germ-cell proliferation and that the two gamma-tubulins present in Drosophila are functionally equivalent during female germ-cell development and oogenesis.

The third P-loop domain in cytoplasmic dynein heavy chain is essential for dynein motor function and ATP-sensitive microtubule binding

Sequence comparisons and structural analyses show that the dynein heavy chain motor subunit is related to the AAA family of chaperone-like ATPases. The core structure of the dynein motor unit derives from the assembly of six AAA domains into a hexameric ring. In dynein, the first four AAA domains contain consensus nucleotide triphosphate-binding motifs, or P-loops. The recent structural models of dynein heavy chain have fostered the hypothesis that the energy derived from hydrolysis at P-loop 1 acts through adjacent P-loop domains to effect changes in the attachment state of the microtubule-binding domain. However, to date, the functional significance of the P-loop domains adjacent to the ATP hydrolytic site has not been demonstrated. Our results provide a mutational analysis of P-loop function within the first and third AAA domains of the Drosophila cytoplasmic dynein heavy chain. Here we report the first evidence that P-loop-3 function is essential for dynein function. Significantly, our results further show that P-loop-3 function is required for the ATP-induced release of the dynein complex from microtubules. Mutation of P-loop-3 blocks ATP-mediated release of dynein from microtubules, but does not appear to block ATP binding and hydrolysis at P-loop 1. Combined with the recent recognition that dynein belongs to the family of AAA ATPases, the observations support current models in which the multiple AAA domains of the dynein heavy chain interact to support the translocation of the dynein motor down the microtubule lattice.