Nuclear-fallout, a Drosophila protein that cycles from the cytoplasm to the centrosomes, regulates cortical microfilament organization

A geometric-tension-dynamics model of epithelial convergent extension

Convergent extension of epithelial tissue is a key motif of animal morphogenesis. On a coarse scale, cell motion resembles laminar fluid flow; yet in contrast to a fluid, epithelial cells adhere to each other and maintain the tissue layer under actively generated internal tension. To resolve this apparent paradox, we formulate a model in which tissue flow in the tension-dominated regime occurs through adiabatic remodeling of force balance in the network of adherens junctions. We propose that the slow dynamics within the manifold of force-balanced configurations is driven by positive feedback on myosin-generated cytoskeletal tension. Shifting force balance within a tension network causes active cell rearrangements (T1 transitions) resulting in net tissue deformation oriented by initial tension anisotropy. Strikingly, we find that the total extent of tissue deformation depends on the initial cellular packing order. T1s degrade this order so that tissue flow is self-limiting. We explain these findings by showing that coordination of T1s depends on coherence in local tension configurations, quantified by a geometric order parameter in tension space. Our model reproduces the salient tissue- and cell-scale features of germ band elongation during Drosophila gastrulation, in particular the slowdown of tissue flow after approximately twofold elongation concomitant with a loss of order in tension configurations. This suggests local cell geometry contains morphogenetic information and yields experimentally testable predictions. Defining biologically controlled active tension dynamics on the manifold of force-balanced states may provide a general approach to the description of morphogenetic flow.

Kinesin-1 patterns Par-1 and Rho signaling at the cortex of syncytial embryos of Drosophila

The cell cortex of syncytial Drosophila embryos is patterned into cap and intercap regions by centrosomes, specific sets of proteins that are restricted to their respective regions by unknown mechanisms. Here, we found that Kinesin-1 is required for the restriction of plus- and minus-ends of centrosomal and non-centrosomal microtubules to the cap region, marked by EB1 and Patronin/Shot, respectively. Kinesin-1 also directly or indirectly restricts proteins and Rho signaling to the intercap, including the RhoGEF Pebble, Dia, Myosin II, Capping protein-α, and the polarity protein Par-1. Furthermore, we found that Par-1 is required for cap restriction of Patronin/Shot, and vice versa Patronin, for Par-1 enrichment at the intercap. In summary, our data support a model that Kinesin-1 would mediate the restriction of centrosomal and non-centrosomal microtubules to a region close to the centrosomes and exclude Rho signaling and Par-1. In addition, mutual antagonistic interactions would refine and maintain the boundary between cap and intercap and thus generate a distinct cortical pattern.

Reshaping the Syncytial Drosophila Embryo with Cortical Actin Networks: Four Main Steps of Early Development

Drosophila development begins as a syncytium. The large size of the one-cell embryo makes it ideal for studying the structure, regulation, and effects of the cortical actin cytoskeleton. We review four main steps of early development that depend on the actin cortex. At each step, dynamic remodelling of the cortex has specific effects on nuclei within the syncytium. During axial expansion, a cortical actomyosin network assembles and disassembles with the cell cycle, generating cytoplasmic flows that evenly distribute nuclei along the ovoid cell. When nuclei move to the cell periphery, they seed Arp2/3-based actin caps which grow into an array of dome-like compartments that house the nuclei as they divide at the cell cortex. To separate germline nuclei from the soma, posterior germ plasm induces full cleavage of mono-nucleated primordial germ cells from the syncytium. Finally, zygotic gene expression triggers formation of the blastoderm epithelium via cellularization and simultaneous division of ~6000 mono-nucleated cells from a single internal yolk cell. During these steps, the cortex is regulated in space and time, gains domain and sub-domain structure, and undergoes mesoscale interactions that lay a structural foundation of animal development.

Cytoplasmic division cycles without the nucleus and mitotic CDK/cyclin complexes

Cytoplasmic divisions are thought to rely on nuclear divisions and mitotic signals. We demonstrate in Drosophila embryos that cytoplasm can divide repeatedly without nuclei and mitotic CDK/cyclin complexes. Cdk1 normally slows an otherwise faster cytoplasmic division cycle, coupling it with nuclear divisions, and when uncoupled, cytoplasm starts dividing before mitosis. In developing embryos where CDK/cyclin activity can license mitotic microtubule (MT) organizers like the spindle, cytoplasmic divisions can occur without the centrosome, a principal organizer of interphase MTs. However, centrosomes become essential in the absence of CDK/cyclin activity, implying that the cytoplasm can employ either the centrosome-based interphase or CDK/cyclin-dependent mitotic MTs to facilitate its divisions. Finally, we present evidence that autonomous cytoplasmic divisions occur during unperturbed fly embryogenesis and that they may help extrude mitotically stalled nuclei during blastoderm formation. We postulate that cytoplasmic divisions occur in cycles governed by a yet-to-be-uncovered clock mechanism autonomous from CDK/cyclin complexes.

Bitesize bundles F-actin and influences actin remodeling in syncytial Drosophila embryo development

Actin networks undergo rearrangements that influence cell and tissue shape. Actin network assembly and organization is regulated in space and time by a host of actin binding proteins. The Drosophila Synaptotagmin-like protein, Bitesize (Btsz), is known to organize actin at epithelial cell apical junctions in a manner that depends on its interaction with the actin-binding protein, Moesin. Here, we showed that Btsz functions in actin reorganization at earlier, syncytial stages of Drosophila embryo development. Btsz was required for the formation of stable metaphase pseudocleavage furrows that prevented spindle collisions and nuclear fallout prior to cellularization. While previous studies focused on Btsz isoforms containing the Moesin Binding Domain (MBD), we found that isoforms lacking the MBD also function in actin remodeling. Consistent with this, we found that the C-terminal half of BtszB cooperatively binds to and bundles F-actin, suggesting a direct mechanism for Synaptotagmin-like proteins regulating actin organization during animal development.

Membrane-actin interactions in morphogenesis: Lessons learned from Drosophila cellularization

During morphogenesis, changes in the shapes of individual cells are harnessed to mold an entire tissue. These changes in cell shapes require the coupled remodeling of the plasma membrane and underlying actin cytoskeleton. In this review, we highlight cellularization of the Drosophila embryo as a model system to uncover principles of how membrane and actin dynamics are co-regulated in space and time to drive morphogenesis.

Orb-dependent polyadenylation contributes to PLP expression and centrosome scaffold assembly

As the microtubule-organizing centers of most cells, centrosomes engineer the bipolar mitotic spindle required for error-free mitosis. Drosophila Pericentrin-like protein (PLP) directs formation of a pericentriolar material (PCM) scaffold required for PCM organization and microtubule-organizing center function. Here, we investigate the post-transcriptional regulation of Plp mRNA. We identify conserved binding sites for cytoplasmic polyadenylation element binding (CPEB) proteins within the Plp 3'-untranslated region and examine the role of the CPEB ortholog Oo18 RNA-binding protein (Orb) in Plp mRNA regulation. Our data show that Orb interacts biochemically with Plp mRNA to promote polyadenylation and PLP protein expression. Loss of orb, but not orb2, diminishes PLP levels in embryonic extracts. Consequently, PLP localization to centrosomes and its function in PCM scaffolding are compromised in orb mutant embryos, resulting in genomic instability and embryonic lethality. Moreover, we find that PLP overexpression restores centrosome scaffolding and rescues the cell division defects caused by orb depletion. Our data suggest that Orb modulates PLP expression at the level of Plp mRNA polyadenylation and demonstrates that the post-transcriptional regulation of core, conserved centrosomal mRNAs is crucial for centrosome function.

Coordination of Embryogenesis by the Centrosome in Drosophila melanogaster

The first 3 h of Drosophila melanogaster embryo development are exemplified by rapid nuclear divisions within a large syncytium, transforming the zygote to the cellular blastoderm after 13 successive cleavage divisions. As the syncytial embryo develops, it relies on centrosomes and cytoskeletal dynamics to transport nuclei, maintain uniform nuclear distribution throughout cleavage cycles, ensure generation of germ cells, and coordinate cellularization. For the sake of this review, we classify six early embryo stages that rely on processes coordinated by the centrosome and its regulation of the cytoskeleton. The first stage features migration of one of the female pronuclei toward the male pronucleus following maturation of the first embryonic centrosomes. Two subsequent stages distribute the nuclei first axially and then radially in the embryo. The remaining three stages involve centrosome-actin dynamics that control cortical plasma membrane morphogenesis. In this review, we highlight the dynamics of the centrosome and its role in controlling the six stages that culminate in the cellularization of the blastoderm embryo.

Slam protein dictates subcellular localization and translation of its own mRNA

Many mRNAs specifically localize within the cytoplasm and are present in RNA-protein complexes. It is generally assumed that localization and complex formation of these RNAs are controlled by trans-acting proteins encoded by genes different than the RNAs themselves. Here, we analyze slow as molasses (slam) mRNA that prominently colocalizes with its encoded protein at the basal cortical compartment during cellularization. The functional implications of this striking colocalization have been unknown. Here, we show that slam mRNA translation is spatiotemporally controlled. We found that translation was largely restricted to the onset of cellularization when Slam protein levels at the basal domain sharply increase. slam mRNA was translated locally, at least partially, as not yet translated mRNA transiently accumulated at the basal region. Slam RNA accumulated at the basal domain only if Slam protein was present. Furthermore, a slam RNA with impaired localization but full coding capacity was only weakly translated. We detected a biochemical interaction of slam mRNA and protein as demonstrated by specific co-immunoprecipitation from embryonic lysate. The intimate relationship of slam mRNA and protein may constitute a positive feedback loop that facilitates and controls timely and rapid accumulation of Slam protein at the prospective basal region.

Polo kinase mediates the phosphorylation and cellular localization of Nuf/FIP3, a Rab11 effector

Animal cytokinesis involves both actin-myosin-based contraction and vesicle-mediated membrane addition. In many cell types, including early Drosophila embryos, Nuf/FIP3, a Rab11 effector, mediates recycling endosome (RE)-based vesicle delivery to the cytokinesis furrow. Nuf exhibits a cell cycle-regulated concentration at the centrosome that is accompanied by dramatic changes in its phosphorylation state. Here we demonstrate that maximal phosphorylation of Nuf occurs at prophase, when centrosome-associated Nuf disperses throughout the cytoplasm. Accordingly, ectopic Cdk1 activation results in immediate Nuf dispersal from the centrosome. Screening of candidate kinases reveals a specific, dosage-sensitive interaction between Nuf and Polo with respect to Nuf-mediated furrow formation. Inhibiting Polo activity results in Nuf underphosphorylation and prolonged centrosome association. In vitro, Polo directly binds and is required for Nuf phosphorylation at Ser-225 and Thr-227, matching previous in vivo-mapped phosphorylation sites. These results demonstrate a role for Polo kinase in directly mediating Nuf cell cycle-dependent localization.

Nuf and Rip11 requirement for polarity determinant recycling during Drosophila development

A tight relationship between apico-basal polarity and trafficking is essential for epithelial physiology and tissue homeostasis. Recent studies have described how some Rab GTPases, key components of the intracellular traffic machinery, contribute to the establishment of cell polarity in vertebrates. We have demonstrated a novel connection between cell polarity and trafficking: in Drosophila epithelia, the apical determinant aPKC is recycled via Rab11-Nuf-recycling endosomes to maintain cell polarity. Furthermore, the phosphorylation of Nuf by aPKC allows aPKC to control the sub-cellular localization of Nuf and its own membrane accumulation. Here we review these data and show the different contribution of the 2 Drosophila Rab11 adaptor proteins, Nuf and Rip11, to the maintenance of Drosophila embryonic ectoderm polarity.

Proper symmetric and asymmetric endoplasmic reticulum partitioning requires astral microtubules

Mechanisms that regulate partitioning of the endoplasmic reticulum (ER) during cell division are largely unknown. Previous studies have mostly addressed ER partitioning in cultured cells, which may not recapitulate physiological processes that are critical in developing, intact tissues. We have addressed this by analysing ER partitioning in asymmetrically dividing stem cells, in which precise segregation of cellular components is essential for proper development and tissue architecture. We show that in Drosophila neural stem cells, called neuroblasts, the ER asymmetrically partitioned to centrosomes early in mitosis. This correlated closely with the asymmetric nucleation of astral microtubules (MTs) by centrosomes, suggesting that astral MT association may be required for ER partitioning by centrosomes. Consistent with this, the ER also associated with astral MTs in meiotic Drosophila spermatocytes and during syncytial embryonic divisions. Disruption of centrosomes in each of these cell types led to improper ER partitioning, demonstrating the critical role for centrosomes and associated astral MTs in this process. Importantly, we show that the ER also associated with astral MTs in cultured human cells, suggesting that this centrosome/astral MT-based partitioning mechanism is conserved across animal species.

Nuclear fallout provides a new link between aPKC and polarized cell trafficking

Background

Cell polarity, essential for cell physiology and tissue coherence, emerges as a consequence of asymmetric localization of protein complexes and directional trafficking of cellular components. Although molecules required in both processes are well known their relationship is still poorly understood.

Results

Here we show a molecular link between Nuclear Fallout (Nuf), an adaptor of Rab11-GTPase to the microtubule motor proteins during Recycling Endosome (RE) trafficking, and aPKC, a pivotal kinase in the regulation of cell polarity. We demonstrate that aPKC phosphorylates Nuf modifying its subcellular distribution. Accordingly, in aPKC mutants Nuf and Rab11 accumulate apically indicating altered RE delivery. We show that aPKC localization in the apico-lateral cortex is dynamic. When we block exocytosis, by means of exocyst-sec mutants, aPKC accumulates inside the cells. Moreover, apical aPKC concentration is reduced in nuf mutants, suggesting aPKC levels are maintained by recycling.

Conclusions

We demonstrate that active aPKC interacts with Nuf, phosphorylating it and, as a result, modifying its subcellular distribution. We propose a regulatory loop by which Nuf promotes aPKC apical recycling until sufficient levels of active aPKC are reached. Thus, we provide a novel link between cell polarity regulation and traffic control in epithelia.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-016-0253-6) contains supplementary material, which is available to authorized users.

The Seckel syndrome and centrosomal protein Ninein localizes asymmetrically to stem cell centrosomes but is not required for normal development, behavior, or DNA damage response in Drosophila

Ninein associates with the microtubule regulator γ-tubulin, regulates microtubule assembly, and localizes to centrosomes and noncentrosomal microtubule-organizing centers in Drosophila. Ninein localizes to stem cell centrosomes asymmetrically, with a bias for the daughter centrosome. Remarkably, Ninein is dispensable for development, fertility, or viability.

Ninein (Nin) is a centrosomal protein whose gene is mutated in Seckel syndrome (SCKL, MIM 210600), an inherited recessive disease that results in primordial dwarfism, cognitive deficiencies, and increased sensitivity to genotoxic stress. Nin regulates neural stem cell self-renewal, interkinetic nuclear migration, and microtubule assembly in mammals. Nin is evolutionarily conserved, yet its role in cell division and development has not been investigated in a model organism. Here we characterize the single Nin orthologue in Drosophila. Drosophila Nin localizes to the periphery of the centrosome but not at centriolar structures as in mammals. However, Nin shares the property of its mammalian orthologue of promoting microtubule assembly. In neural and germline stem cells, Nin localizes asymmetrically to the younger (daughter) centrosome, yet it is not required for the asymmetric division of stem cells. In wing epithelia and muscle, Nin localizes to noncentrosomal microtubule-organizing centers. Surprisingly, loss of nin expression from a nin mutant does not significantly affect embryonic and brain development, fertility, or locomotor performance of mutant flies or their survival upon exposure to DNA-damaging agents. Although it is not essential, our data suggest that Nin plays a supportive role in centrosomal and extracentrosomal microtubule organization and asymmetric stem cell division.

Interphase centrosome organization by the PLP-Cnn scaffold is required for centrosome function

Cnn and PLP directly interact at two defined sites to coordinate the cell cycle–dependent rearrangement and scaffolding activity of the centrosome to permit normal centrosome organization, cell division, and embryonic viability.

Pericentriolar material (PCM) mediates the microtubule (MT) nucleation and anchoring activity of centrosomes. A scaffold organized by Centrosomin (Cnn) serves to ensure proper PCM architecture and functional changes in centrosome activity with each cell cycle. Here, we investigate the mechanisms that spatially restrict and temporally coordinate centrosome scaffold formation. Focusing on the mitotic-to-interphase transition in Drosophila melanogaster embryos, we show that the elaboration of the interphase Cnn scaffold defines a major structural rearrangement of the centrosome. We identify an unprecedented role for Pericentrin-like protein (PLP), which localizes to the tips of extended Cnn flares, to maintain robust interphase centrosome activity and promote the formation of interphase MT asters required for normal nuclear spacing, centrosome segregation, and compartmentalization of the syncytial embryo. Our data reveal that Cnn and PLP directly interact at two defined sites to coordinate the cell cycle–dependent rearrangement and scaffolding activity of the centrosome to permit normal centrosome organization, cell division, and embryonic viability.

Innexin7a forms junctions that stabilize the basal membrane during cellularization of the blastoderm in Tribolium castaneum

In insects, the fertilized egg undergoes a series of rapid nuclear divisions before the syncytial blastoderm starts to cellularize. Cellularization has been extensively studied in Drosophilamelanogaster, but its thick columnar blastoderm is unusual among insects. We therefore set out to describe cellularization in the beetle Tribolium castaneum, the embryos of which exhibit a thin blastoderm of cuboidal cells, like most insects. Using immunohistochemistry, live imaging and transmission electron microscopy, we describe several striking differences to cellularization in Drosophila, including the formation of junctions between the forming basal membrane and the yolk plasmalemma. To identify the nature of this novel junction, we used the parental RNAi technique for a small-scale screen of junction proteins. We find that maternal knockdown of Triboliuminnexin7a (Tc-inx7a), an ortholog of the Drosophila gap junction gene Innexin 7, leads to failure of cellularization. In Inx7a-depleted eggs, the invaginated plasma membrane retracts when basal cell closure normally begins. Furthermore, transiently expressed tagged Inx7a localizes to the nascent basal membrane of the forming cells in wild-type eggs. We propose that Inx7a forms the newly identified junctions that stabilize the forming basal membrane and enable basal cell closure. We put forward Tribolium as a model for studying a more ancestral mode of cellularization in insects.

Dynamin regulates metaphase furrow formation and plasma membrane compartmentalization in the syncytial Drosophila embryo

The successive nuclear division cycles in the syncytial Drosophila embryo are accompanied by ingression and regression of plasma membrane furrows, which surround individual nuclei at the embryo periphery, playing a central role in embryo compartmentalization prior to cellularization. Here, we demonstrate that cell cycle changes in dynamin localization and activity at the plasma membrane (PM) regulate metaphase furrow formation and PM organization in the syncytial embryo. Dynamin was localized on short PM furrows during interphase, mediating endocytosis of PM components. Dynamin redistributed off ingressed PM furrows in metaphase, correlating with stabilized PM components and the associated actin regulatory machinery on long furrows. Acute inhibition of dynamin in the temperature sensitive shibire mutant embryo resulted in morphogenetic consequences in the syncytial division cycle. These included inhibition of metaphase furrow ingression, randomization of proteins normally polarized to intercap PM and disruption of the diffusion barrier separating PM domains above nuclei. Based on these findings, we propose that cell cycle changes in dynamin orchestrate recruitment of actin regulatory machinery for PM furrow dynamics during the early mitotic cycles in the Drosophila embryo.

Endosomal localization of FIP3/Arfophilin-1 and its involvement in dendritic formation of mouse hippocampal neurons

Endosomal trafficking mediated by Rab11 and Arf6 small GTPases is essential for various neuronal functions. Family of Rab11-interacting protein 3 (FIP3)/Arfophilin-1, also termed Eferin, is a dual effector for Rab11 and Arf6 and implicated in endosomal trafficking during cytokinesis. To understand the neuronal functions of FIP3, we first showed the widespread neuronal expression of FIP3 mRNA in adult mouse brain by in situ hybridization. Immunohistochemical analysis showed the association of FIP3 with a subpopulation of endosomes labeled with EEA1 and syntaxin 12 in hippocampal neurons. Immunoblot analysis showed the progressive increase of FIP3 with a peak around postnatal day 15 during hippocampal development. Furthermore, knockdown of endogenous FIP3 decreased the total dendritic length of cultured hippocampal neurons with a concomitant increase in the number of short (<40μm) primary dendrites. Together, FIP3 is suggested to regulate dendritic formation possibly through Rab11- and Arf6-mediated endosomal trafficking.

Function and dynamics of slam in furrow formation in early Drosophila embryo

The Drosophila embryo undergoes a developmental transition in the blastoderm stage switching from syncytial to cellular development. The cleavage furrow, which encloses nuclei into cells, is a prominent morphological feature of this transition. It is not clear how the pattern of the furrow array is defined and how zygotic genes trigger the formation and invagination of interphase furrows. A key to these questions is provided by the gene slam, which has been previously implicated in controlling furrow invagination. Here we investigate the null phenotype of slam, the dynamics of Slam protein, and its control by the recycling endosome. We find that slam is essential for furrow invagination during cellularisation and together with nullo, for specification of the furrow. During cellularisation, Slam marks first the furrow, which is derived from the metaphase furrow of the previous mitosis. Slightly later, Slam accumulates at new furrows between daughter cells early in interphase. Slam is stably associated with the furrow canal except for the onset of cellularisation as revealed by FRAP experiments. Restriction of Slam to the furrow canal and Slam mobility during cellularisation is controlled by the recycling endosome and centrosomes. We propose a three step model. The retracting metaphase furrow leaves an initial mark. This mark and the border between corresponding daughter nuclei are refined by vesicular transport away from pericentrosomal recycling endosome towards the margins of the somatic buds. Following the onset of zygotic gene expression, Slam and Nullo together stabilise this mark and Slam triggers invagination of the cleavage furrow.

Endocytic transport and cytokinesis: from regulation of the cytoskeleton to midbody inheritance

Abscission is the last step of cytokinesis that leads to the physical separation of two daughter cells. An emerging picture is that abscission is a complex event that relies on changes in both lipid composition and cytoskeletal dynamics. These subcellular processes lead to the establishment of the abscission site and recruitment of the ESCRT-III protein complex to mediate the final separation event. It has become apparent that endocytic transport to the cleavage furrow during late cytokinesis mediates and coordinates lipid and cytoskeleton dynamics, thus playing a key role in abscission. Furthermore, new evidence suggests that endosomes may have additional roles in post-mitotic cellular events such as midbody inheritance and degradation. Here, we highlight recent findings regarding the function of these endosomes in the regulation of cell division.

Polarity sets the stage for cytokinesis

Cell polarity is important for a number of processes, from chemotaxis to embryogenesis. Recent studies suggest a new role for polarity in the orchestration of events during the final cell separation step of cell division called abscission. Abscission shares several features with cell polarization, including rearrangement of phosphatidylinositols, reorganization of microtubules, and trafficking of exocyst-associated membranes. Here we focus on how the canonical pathways for cell polarization and cell migration may play a role in spatiotemporal membrane trafficking events required for the final stages of cytokinesis.

The Drosophila STUbL protein Degringolade limits HES functions during embryogenesis

Degringolade (Dgrn) encodes a Drosophila SUMO-targeted ubiquitin ligase (STUbL) protein similar to that of mammalian RNF4. Dgrn facilitates the ubiquitylation of the HES protein Hairy, which disrupts the repressive activity of Hairy by inhibiting the recruitment of its cofactor Groucho. We show that Hey and all HES family members, except Her, interact with Dgrn and are substrates for its E3 ubiquitin ligase activity. Dgrn displays dynamic subcellular localization, accumulates in the nucleus at times when HES family members are active and limits Hey and HES family activity during sex determination, segmentation and neurogenesis. We show that Dgrn interacts with the Notch signaling pathway by it antagonizing the activity of E(spl)-C proteins. dgrn null mutants are female sterile, producing embryos that arrest development after two or three nuclear divisions. These mutant embryos exhibit fragmented or decondensed nuclei and accumulate higher levels of SUMO-conjugated proteins, suggesting a role for Dgrn in genome stability.

A mitotic kinesin-6, Pav-KLP, mediates interdependent cortical reorganization and spindle dynamics in Drosophila embryos

We investigated the role of Pav-KLP, a kinesin-6, in the coordination of spindle and cortical dynamics during mitosis in Drosophila embryos. In vitro, Pav-KLP behaves as a dimer. In vivo, it localizes to mitotic spindles and furrows. Inhibition of Pav-KLP causes defects in both spindle dynamics and furrow ingression, as well as causing changes in the distribution of actin and vesicles. Thus, Pav-KLP stabilizes the spindle by crosslinking interpolar microtubule bundles and contributes to actin furrow formation possibly by transporting membrane vesicles, actin and/or actin regulatory molecules along astral microtubules. Modeling suggests that furrow ingression during cellularization depends on: (1) a Pav-KLP-dependent force driving an initial slow stage of ingression; and (2) the subsequent Pav-KLP-driven transport of actin- and membrane-containing vesicles to the furrow during a fast stage of ingression. We hypothesize that Pav-KLP is a multifunctional mitotic motor that contributes both to bundling of interpolar microtubules, thus stabilizing the spindle, and to a biphasic mechanism of furrow ingression by pulling down the furrow and transporting vesicles that deliver new material to the descending furrow.

Using total internal reflection fluorescence (TIRF) microscopy to visualize cortical actin and microtubules in the Drosophila syncytial embryo

The Drosophila syncytial embryo is a powerful developmental model system for studying dynamic coordinated cytoskeletal rearrangements. Confocal microscopy has begun to reveal more about the cytoskeletal changes that occur during embryogenesis. Total internal reflection fluorescence (TIRF) microscopy provides a promising new approach for the visualization of cortical events with heightened axial resolution. We have applied TIRF microscopy to the Drosophila embryo to visualize cortical microtubule and actin dynamics in the syncytial blastoderm. Here, we describe the details of this technique, and report qualitative assessments of cortical microtubules and actin in the Drosophila syncytial embryo. In addition, we identified a peak of cortical microtubules during anaphase of each nuclear cycle in the syncytial blastoderm, and using images generated by TIRF microscopy, we quantitatively analyzed microtubule dynamics during this time.

Rab11-FIP3 is a Rab11-binding protein that regulates breast cancer cell motility by modulating the actin cytoskeleton

Cell adhesion and motility are very dynamic processes that require the temporal and spatial coordination of many cellular structures. ADP-ribosylation factor 6 (Arf6) has emerged as master regulator of endocytic membrane traffic and cytoskeletal dynamics during cell movement. Recently, a novel Arf6-binding protein known as FIP3/arfophilin/eferin has been identified. In addition to Arf6, FIP3 also interacts with Rab11, a small monomeric GTPase that regulates endocytic membrane transport. Both Arf6 and Rab11 GTPases have been implicated in regulation of cell motility. Here we test the role of FIP3 in breast carcinoma cell motility. First, we demonstrate that FIP3 is associated with recycling endosomes that are present at the leading edge of motile cells. Second, we show that FIP3 is required for the motility of MDA-MB-231 breast carcinoma cells. Third, we demonstrate that FIP3 regulates Rac1-dependent actin cytoskeleton dynamics and modulates the formation and ruffling of lamellipodia. Finally, we demonstrate that FIP3 regulates the localization of Arf6 at the plasma membrane of MDA-MB-231 cells. Based on our data we propose that FIP3 affects cell motility by regulating Arf6 localization to the plasma membrane of the leading edge, thus regulating polarized Rac1 activation and actin dynamics.

RACK-1 directs dynactin-dependent RAB-11 endosomal recycling during mitosis in Caenorhabditis elegans

Membrane trafficking pathways are necessary for the addition and removal of membrane during cytokinesis. In animal cells, recycling endosomes act as a major source of the additional membranes during furrow progression and abscission. However, the mechanisms and factors that regulate recycling endosomes during the cell cycle remain poorly understood. Here, we show that the Caenorhabditis elegans Receptor of Activated C Kinase 1 (RACK-1) is required for cytokinesis, germline membrane organization, and the recruitment of RAB-11-labeled recycling endosomes to the pericentrosomal region and spindle. RACK-1 is also required for proper chromosome separation and astral microtubule length. RACK-1 localizes to the centrosomes, kinetochores, the midbody, and nuclear envelopes during the cell cycle. We found that RACK-1 directly binds to DNC-2, the C. elegans p50/dynamitin subunit of the dynactin complex. Last, RACK-1 may facilitate the sequestration of recycling endosomes by targeting DNC-2 to centrosomes and the spindle. Our findings suggest a mechanism by which RACK-1 directs the dynactin-dependent redistribution of recycling endosomes during the cell cycle, thus ensuring proper membrane trafficking events during cytokinesis.

Arf GTPase-activating protein ASAP1 interacts with Rab11 effector FIP3 and regulates pericentrosomal localization of transferrin receptor-positive recycling endosome

ADP-ribosylation factors (Arfs) and Arf GTPase-activating proteins (GAPs) are key regulators of membrane trafficking and the actin cytoskeleton. The Arf GAP ASAP1 contains an N-terminal BAR domain, which can induce membrane tubulation. Here, we report that the BAR domain of ASAP1 can also function as a protein binding site. Two-hybrid screening identified FIP3, which is a putative Arf6- and Rab11-effector, as a candidate ASAP1 BAR domain-binding protein. Both coimmunoprecipitation and in vitro pulldown assays confirmed that ASAP1 directly binds to FIP3 through its BAR domain. ASAP1 formed a ternary complex with Rab11 through FIP3. FIP3 binding to the BAR domain stimulated ASAP1 GAP activity against Arf1, but not Arf6. ASAP1 colocalized with FIP3 in the pericentrosomal endocytic recycling compartment. Depletion of ASAP1 or FIP3 by small interfering RNA changed the localization of transferrin receptor, which is a marker of the recycling endosome, in HeLa cells. The depletion also altered the trafficking of endocytosed transferrin. These results support the conclusion that ASAP1, like FIP3, functions as a component of the endocytic recycling compartment.

Nuf, a Rab11 effector, maintains cytokinetic furrow integrity by promoting local actin polymerization

Plasma membrane ingression during cytokinesis involves both actin remodeling and vesicle-mediated membrane addition. Vesicle-based membrane delivery from the recycling endosome (RE) has an essential but ill-defined involvement in cytokinesis. In the Drosophila melanogaster early embryo, Nuf (Nuclear fallout), a Rab11 effector which is essential for RE function, is required for F-actin and membrane integrity during furrow ingression. We find that in nuf mutant embryos, an initial loss of F-actin at the furrow is followed by loss of the associated furrow membrane. Wild-type embryos treated with Latrunculin A or Rho inhibitor display similar defects. Drug- or Rho-GTP–induced increase of actin polymerization or genetically mediated decrease of actin depolymerization suppresses the nuf mutant F-actin and membrane defects. We also find that RhoGEF2 does not properly localize at the furrow in nuf mutant embryos and that RhoGEF2–Rho1 pathway components show strong specific genetic interactions with Nuf. We propose a model in which RE-derived vesicles promote furrow integrity by regulating the rate of actin polymerization through the RhoGEF2–Rho1 pathway.

Sequential Cyk-4 binding to ECT2 and FIP3 regulates cleavage furrow ingression and abscission during cytokinesis

Cytokinesis is a highly regulated and dynamic event that involves the reorganization of the cytoskeleton and membrane compartments. Recently, FIP3 has been implicated in targeting of recycling endosomes to the mid-body of dividing cells and is found required for abscission. Here, we demonstrate that the centralspindlin component Cyk-4 is a FIP3-binding protein. Furthermore, we show that FIP3 binds to Cyk-4 at late telophase and that centralspindlin may be required for FIP3 recruitment to the mid-body. We have mapped the FIP3-binding region on Cyk-4 and show that it overlaps with the ECT2-binding domain. Finally, we demonstrate that FIP3 and ECT2 form mutually exclusive complexes with Cyk-4 and that dissociation of ECT2 from the mid-body at late telophase may be required for the recruitment of FIP3 and recycling endosomes to the cleavage furrow. Thus, we propose that centralspindlin complex not only regulates acto-myosin ring contraction but also endocytic vesicle transport to the cleavage furrow and it does so through sequential interactions with ECT2 and FIP3.

RNAi of mitotic cyclins in Drosophila uncouples the nuclear and centrosome cycle

BACKGROUND

Successful cell duplication requires orderly progression through a succession of dramatic cell-cycle events. Disruption of this precise coupling can compromise genomic integrity. The coordination of cell-cycle events is thought to arise from control by a single master regulator, cyclin:Cdk, whose activity oscillates. However, we still know very little of how individual cell-cycle events are coupled to this oscillator and how the timing of each event is controlled.

RESULTS

We developed an approach with RNA interference (RNAi) and real-time imaging to study cyclin contributions to the rapid syncytial divisions of Drosophila embryos. Simultaneous knockdown of all three mitotic cyclins blocked nuclei from entering mitosis. Despite nuclear arrest, centrosomes and associated myosin cages continued to divide until the midblastula transition. Centrosome division was synchronous throughout the embryo and the period of the uncoupled duplication cycle increased over successive divisions. In contrast to its normal actions, injection of a competitive inhibitor of the anaphase-promoting complex/cyclosome (APC/C) after knockdown of the mitotic cyclins did not interfere with the centrosome-duplication cycles. Finally, we examined how cyclin knockdown affects the onset of cellularization at the midblastula transition and found that nuclear cell-cycle arrest did not advance or delay onset of cellularization.

CONCLUSIONS

We show that knockdown of mitotic cyclins allows centrosomes to duplicate in a cycle that is uncoupled from other cell-cycle events. We suggest that high mitotic cyclin normally ensures that the centrosome cycle remains entrained to the nuclear cycle.

Global analysis of mRNA localization reveals a prominent role in organizing cellular architecture and function

Although subcellular mRNA trafficking has been demonstrated as a mechanism to control protein distribution, it is generally believed that most protein localization occurs subsequent to translation. To address this point, we developed and employed a high-resolution fluorescent in situ hybridization procedure to comprehensively evaluate mRNA localization dynamics during early Drosophila embryogenesis. Surprisingly, of the 3370 genes analyzed, 71% of those expressed encode subcellularly localized mRNAs. Dozens of new and striking localization patterns were observed, implying an equivalent variety of localization mechanisms. Tight correlations between mRNA distribution and subsequent protein localization and function, indicate major roles for mRNA localization in nucleating localized cellular machineries. A searchable web resource documenting mRNA expression and localization dynamics has been established and will serve as an invaluable tool for dissecting localization mechanisms and for predicting gene functions and interactions.

Akt regulates centrosome migration and spindle orientation in the early Drosophila melanogaster embryo

Correct positioning and morphology of the mitotic spindle is achieved through regulating the interaction between microtubules (MTs) and cortical actin. Here we find that, in the Drosophila melanogaster early embryo, reduced levels of the protein kinase Akt result in incomplete centrosome migration around cortical nuclei, bent mitotic spindles, and loss of nuclei into the interior of the embryo. We show that Akt is enriched at the embryonic cortex and is required for phosphorylation of the glycogen synthase kinase-3beta homologue Zeste-white 3 kinase (Zw3) and for the cortical localizations of the adenomatosis polyposis coli (APC)-related protein APC2/E-APC and the MT + Tip protein EB1. We also show that reduced levels of Akt result in mislocalization of APC2 in postcellularized embryonic mitoses and misorientation of epithelial mitotic spindles. Together, our results suggest that Akt regulates a complex containing Zw3, Armadillo, APC2, and EB1 and that this complex has a role in stabilizing MT-cortex interactions, facilitating both centrosome separation and mitotic spindle orientation.

The role of FIP3-dependent endosome transport during cytokinesis

Mitosis is a complex cellular process that is completed by the final abscission step called cytokinesis. The many roles of cytoskeletal proteins in animal cell division have been studied extensively, and are essential for proper daughter cell segregation. Lately, the need for the membrane delivery to the cleavage furrow of dividing cells has been implicated as a necessary and important step in cytokinesis.1-4 Newly published work from several group demonstrate that endosomal membranes are required for cytokinesis and also provide insight into the targeting of these membranes to the cleavage furrow.5-8 Rab11 GTPase and its effector protein FIP3 were shown to play key roles in endosome targeting to the cleavage furrow in a spatially and temporally regulated manner.7-9 Thus, Rab11/FIP3 protein complex have emerged as a regulator of endocytic traffic, essential for the abscission step in cytokinesis.

Rab11 is required for membrane trafficking and actomyosin ring constriction in meiotic cytokinesis of Drosophila males

Rab11 is a small GTPase that regulates several aspects of vesicular trafficking. Here, we show that Rab11 accumulates at the cleavage furrow of Drosophila spermatocytes and that it is essential for cytokinesis. Mutant spermatocytes form regular actomyosin rings, but these rings fail to constrict to completion, leading to cytokinesis failures. rab11 spermatocytes also exhibit an abnormal accumulation of Golgi-derived vesicles at the telophase equator, suggesting a defect in membrane-vesicle fusion. These cytokinesis phenotypes are identical to those elicited by mutations in giotto (gio) and four wheel drive (fwd) that encode a phosphatidylinositol transfer protein and a phosphatidylinositol 4-kinase, respectively. Double mutant analysis and immunostaining for Gio and Rab11 indicated that gio, fwd, and rab11 function in the same cytokinetic pathway, with Gio and Fwd acting upstream of Rab11. We propose that Gio and Fwd mediate Rab11 recruitment at the cleavage furrow and that Rab11 facilitates targeted membrane delivery to the advancing furrow.

The concentration of Nuf, a Rab11 effector, at the microtubule-organizing center is cell cycle regulated, dynein-dependent, and coincides with furrow formation

Animal cytokinesis relies on membrane addition as well as acto-myosin-based constriction. Recycling endosome (RE)-derived vesicles are a key source of this membrane. Rab11, a small GTPase associated with the RE and involved in vesicle targeting, is required for elongation of the cytokinetic furrow. In the early Drosophila embryo, Nuclear-fallout (Nuf), a Rab11 effector, promotes vesicle-mediated membrane delivery and actin organization at the invaginating furrow. Although Rab11 maintains a relatively constant localization at the microtubule-organizing center (MTOC), Nuf is present at the MTOC only during the phases of the cell cycle in which furrow invagination occurs. We demonstrate that Nuf protein levels remain relatively constant throughout the cell cycle, suggesting that Nuf is undergoing cycles of concentration and dispersion from the MTOC. Microtubules, but not microfilaments, are required for proper MTOC localization of Nuf and Rab11. The MTOC localization of Nuf also relies on Dynein. Immunoprecipitation experiments demonstrate that Nuf and Dynein physically interact. In accord with these findings, and in contrast to previous reports, we demonstrate that microtubules are required for proper metaphase furrow formation. We propose that the cell cycle-regulated, Dynein-dependent recruitment of Nuf to the MTOC influences the timing of RE-based vesicle delivery to the invaginating furrows.

Molecular characterization of Rab11-FIP3 binding to ARF GTPases

Rab11-FIP3 is a Rab11-binding protein that has been implicated in regulating cytokinesis in mammalian cells. FIP3 functions by simultaneously interacting with Rab11 as well as Arf GTPases. However, unlike the interaction between Rab11 and FIP3, the structural basis of FIP3 binding to Arf GTPases has not yet been determined. The specificity of interaction between FIP3 and Arf GTPases remains controversial. While it was reported that FIP3 preferentially binds to Arf6 some data suggest that FIP3 can also interact with Arf5 and even possibly Arf4. The Arf-interaction motif on FIP3 also remains to be determined. Finally, the importance of Arf binding to FIP3 in regulating cell division and other cellular functions remains unclear. Here we used a combination of various biochemical techniques to measure the affinity of FIP3 binding to various Arfs and to demonstrate that FIP3 predominantly interacts with Arf6 in vitro and in vivo. In addition, we identified the motifs mediating Arf6 and FIP3 interaction and demonstrated that FIP3 binds to the Arf6 C-terminus rather than switch motifs. Finally we show that FIP3 and Arf6 binding is required for the targeting of Arf6 to the cleavage furrow during cytokinesis. Thus, we propose that FIP3 is a scaffolding protein that, in addition to regulating endosome targeting to the cleavage furrow, also is required for Arf6 recruitment to the midbody during late telophase.

Mouse Rab11-FIP4 regulates proliferation and differentiation of retinal progenitors in a Rab11-independent manner

We identified Rab11-family interacting protein 4 (Rab11-FIP4) as a gene strongly expressed in the developing mouse retina. The major transcript encoding a full-length protein, mRab11-FIP4A, was expressed predominantly in neural tissues; whereas an alternative transcript encoding an N-terminally truncated form of the protein, mRab11-FIP4B, was expressed ubiquitously as a minor form. Gain-of-function of mRab11-FIP4A in retina promoted cell cycle exit and increased subpopulations of retinal cells localized in the inner nuclear layer, such as bipolar cells and Müller glia. Reversal of the phenotype was observed in the loss-of-function experiment. Furthermore, Shh signaling was suggested to be involved in these functions. Analysis using truncation mutants revealed the essential role of the N-terminal region containing a conserved EF-hand motif for the retinal phenotypes induced by the expression of mRab11-FIP4A, whereas binding to Rab11 was dispensable, suggesting the involvement of a novel Rab11-independent mechanism for mRab11-FIP4A action in the regulation of retinal development.

Structural basis for Rab11-mediated recruitment of FIP3 to recycling endosomes

The Rab11 GTPase regulates recycling of internalized plasma membrane receptors and is essential for completion of cytokinesis. A family of Rab11 interacting proteins (FIPs) that conserve a C-terminal Rab-binding domain (RBD) selectively recognize the active form of Rab11. Normal completion of cytokinesis requires a complex between Rab11 and FIP3. Here, we report the crystal structure and mutational analysis of a heterotetrameric complex between constitutively active Rab11 and a FIP3 construct that includes the RBD. Two Rab11 molecules bind to dyad symmetric sites at the C terminus of FIP3, which forms a non-canonical coiled-coiled dimer with a flared C terminus and hook region. The RBD overlaps with the coiled coil and extends through the C-terminal hook. Although FIP3 engages the switch and interswitch regions of Rab11, the mode of interaction differs significantly from that of other Rab-effector complexes. In particular, the switch II region undergoes a large structural rearrangement from an ordered but non-complementary active conformation to a remodeled conformation that facilitates the interaction with FIP3. Finally, we provide evidence that FIP3 can form homo-oligomers in cells, and that a critical determinant of Rab11 binding in vitro is necessary for FIP3 recruitment to recycling endosomes during cytokinesis.

Endosome dynamics during development

Endocytosis has traditionally been studied in isolated cells. More recently, however, the analysis of protein trafficking in whole organisms has revealed that it plays exciting roles during development. Endocytic trafficking of cell adhesion molecules regulates epithelial polarity and cell migration. Developmental signaling pathways are regulated by the trafficking of receptors and their ligands through the endocytic pathway. Finally, impairment of the endocytic machinery can affect proliferation control and contribute to tumor development.

Asymmetric Rab 11 endosomes regulate delta recycling and specify cell fate in the Drosophila nervous system

Drosophila sensory organ precursor (SOP) cells are a well-studied model system for asymmetric cell division. During SOP division, the determinants Numb and Neuralized segregate into the pIIb daughter cell and establish a distinct cell fate by regulating Notch/Delta signaling. Here, we describe a Numb- and Neuralized-independent mechanism that acts redundantly in cell-fate specification. We show that trafficking of the Notch ligand Delta is different in the two daughter cells. In pIIb, Delta passes through the recycling endosome which is marked by Rab 11. In pIIa, however, the recycling endosome does not form because the centrosome fails to recruit Nuclear fallout, a Rab 11 binding partner that is essential for recycling endosome formation. Using a mammalian cell culture system, we demonstrate that recycling endosomes are essential for Delta activity. Our results suggest that cells can regulate signaling pathways and influence their developmental fate by inhibiting the formation of individual endocytic compartments.

Rab11-FIP3 and FIP4 interact with Arf6 and the exocyst to control membrane traffic in cytokinesis

The dual Rab11/Arf binding proteins, family of Rab11-interacting proteins FIP3 and FIP4 function in the delivery of recycling endosomes to the cleavage furrow and are, together with Rab11, essential for completion of abscission, the terminal step of cytokinesis. Here, we report that both FIP3 and FIP4 bind Arf6 in a nucleotide-dependent manner but exhibit differential affinities for Rab11 and Arf6. Both FIP3 and FIP4 can form ternary complexes with Rab11 and Arf6. Arf6 is localised to the furrow and midbody and we show that Arf6-GTP functions to localise FIP3 and FIP4 to midbodies during cytokinesis. Exo70p, a component of the Exocyst complex, also localises to the furrow of dividing cells and interacts with Arf6. We show that depletion of Exo70p leads to cytokinesis failure and an impairment of FIP3 and Rab11 localisation to the furrow and midbody. Moreover, Exo70p co-immunoprecipitates FIP3 and FIP4. Hence, we propose that FIP3 and FIP4 serve to couple Rab11-positive vesicle traffic from recycling endosomes to the cleavage furrow/midbody where they are tethered prior to fusion events via interactions with Arf6 and the Exocyst.

A mutation in dVps28 reveals a link between a subunit of the endosomal sorting complex required for transport-I complex and the actin cytoskeleton in Drosophila

Proteins that constitute the endosomal sorting complex required for transport (ESCRT) are necessary for the sorting of proteins into multivesicular bodies (MVBs) and the budding of several enveloped viruses, including HIV-1. The first of these complexes, ESCRT-I, consists of three proteins: Vps28p, Vps37p, and Vps23p or Tsg101 in mammals. Here, we characterize a mutation in the Drosophila homolog of vps28. The dVps28 gene is essential: homozygous mutants die at the transition from the first to second instar. Removal of maternally contributed dVps28 causes early embryonic lethality. In such embryos lacking dVps28, several processes that require the actin cytoskeleton are perturbed, including axial migration of nuclei, formation of transient furrows during cortical divisions in syncytial embryos, and the subsequent cellularization. Defects in actin cytoskeleton organization also become apparent during sperm individualization in dVps28 mutant testis. Because dVps28 mutant cells contained MVBs, these defects are unlikely to be a secondary consequence of disrupted MVB formation and suggest an interaction between the actin cytoskeleton and endosomal membranes in Drosophila embryos earlier than previously appreciated.

The epsilon-subunit of mitochondrial ATP synthase is required for normal spindle orientation during the Drosophila embryonic divisions

We describe the maternal-effect and zygotic phenotypes of null mutations in the Drosophila gene for the epsilon-subunit of mitochondrial ATP synthase, stunted (sun). Loss of zygotic sun expression leads to a dramatic delay in the growth rate of first instar larvae and ultimately death. Embryos lacking maternally supplied sun (sun embryos) have a sixfold reduction in ATP synthase activity. Cellular analysis of sun embryos shows defects only after the nuclei have migrated to the cortex. During the cortical divisions the actin-based metaphase and cellularization furrows do not form properly, and the nuclei show abnormal spacing and division failures. The most striking abnormality is that nuclei and spindles form lines and clusters, instead of adopting a regular spacing. This is reflected in a failure to properly position neighboring nonsister centrosomes during the telophase-to-interphase transition of the cortical divisions. Our study is consistent with a role for Sun in mitochondrial ATP synthesis and suggests that reduced ATP levels selectively affect molecular motors. As Sun has been identified as the ligand for the Methuselah receptor that regulates aging, Sun may function both within and outside mitochondria.

The FIP3-Rab11 protein complex regulates recycling endosome targeting to the cleavage furrow during late cytokinesis

An integral part of cell division is the separation of daughter cells via cytokinesis. There is now good evidence that the completion of cytokinesis requires coordinated membrane trafficking to deliver new membrane to the tip of the furrow and to complete the abscission. Here we have examined membrane traffic in cytokinesis and describe several novel observations. First, we show that Rab11- and FIP3-containing recycling endosomes accumulate near the cleavage furrow and are required for successful completion of cytokinesis. Second, we demonstrate that the Rab11-FIP3 protein complex is intimately involved in the delivery of endosomes to the cleavage furrow. Significantly, although FIP3 recruitment to endosomes is Rab11 dependent, we find that the targeting of FIP3 to the midbody is independent of Rab11. Third, we show that the Rab11-FIP3 complex is required for a late stage of cytokinesis, possibly abscission. Finally, we demonstrate that localization of FIP3 is subject to substantial spatial and temporal regulation. These data provide the first detailed analysis of recycling endosomes in cell division and provide a new model for membrane traffic to the furrow. We propose that the dynamic Rab11-FIP3 interaction controls the delivery, targeting, and fusion of recycling endosomes with furrow during late cytokinesis and abscission.

Evolution of development in the sea star genus Patiriella: clade-specific alterations in cleavage

Examination of early development in five species of the Patiriella sea star species complex indicates that the ancestral-type radial holoblastic cleavage (Type I) is characteristic of P. regularis and P. exigua, whereas cleavage in species from the calcar clade followed multiple alternatives (Types II-IV) from holoblastic to meroblastic. Considering that invariant radial cleavage is thought to play a role in embryonic axis formation in echinoderms, we documented the details of blastomere formation in Patiriella sp. and followed development of the embryos. In Type II cleavage, the first and second cleavage planes appeared simultaneously at one pole of the embryo, dividing it directly into four equally sized blastomeres. In Type III cleavage, the first and second cleavage planes appeared simultaneously, followed promptly by the third cleavage plane, dividing the embryo directly into eight equally sized blastomeres. In Type IV cleavage, numerous furrows appeared simultaneously at one end of the embryo, dividing it into 32-40 equally sized blastomeres. Confocal sections revealed that embryos with cleavage Types II-IV were initially syncytial. The timing of karyokinesis in embryos with Types II and III cleavage was similar to that seen in clutch mates with Type I cleavage. Karyokinesis in embryos with Type IV cleavage, however, differed in timing compared with Type I clutch mates. Alteration in cleavage was not associated with polarized distribution of maternally provided nutrients. For each cleavage type, development was normal to the competent larval stage. Although variable blastomere configuration in the calcar clade may be linked to possession of a lecithotrophic development, other Patiriella species with this mode of development have typical cleavage. The presence of variable cleavage in all calcar clade species indicates that phylogenetic history has played a role in the distribution of this embryonic trait in Patiriella. The plasticity in early cleavage in these sea stars indicates that this aspect of early development is not constrained against change and that there are many ways to achieve multicellularity.

Rab11-FIP3 localises to a Rab11-positive pericentrosomal compartment during interphase and to the cleavage furrow during cytokinesis

The Rab11-family interacting protein 3 (Rab11-FIP3), also known as Arfophilin and Eferin, is a Rab11 and ADP-ribosylation factor (ARF) binding protein of unknown function. Here, we sought to investigate the subcellular localisation and elucidate the function of Rab11-FIP3 in eukaryotic membrane trafficking. Utilising a polyclonal antibody specific for Rab11-FIP3, we have demonstrated by immunofluorescence microscopy that Rab11-FIP3 colocalises with Rab11 in a distinctive pericentrosomal location in A431 cells. Additionally, we found that Rab11-FIP3 localises to punctate vesicular structures dispersed throughout A431 cells. We have demonstrated that both Rab11 and Rab11-FIP3 localise to the cleavage furrow during cytokinesis, and that Rab11-FIP3 localisation is dependent on both microtubule and actin filament integrity. We show that Rab11-FIP3 does not enter brefeldin A (BFA) induced membrane tubules that are positive for the transferrin receptor (TfnR). Furthermore, we show that expression of an amino-terminally truncated mutant of Rab11-FIP3 (Rab11-FIP3((244-756))) does not inhibit transferrin (Tfn) recycling in HeLa cells. It is likely that Rab11-FIP3 is involved in trafficking events other than Tfn trafficking; these may include the transport of endosomally derived membrane to the cleavage furrow during cytokinesis.

Actin cytoskeleton remodeling during early Drosophila furrow formation requires recycling endosomal components Nuclear-fallout and Rab11

Cytokinesis requires a dramatic remodeling of the cortical cytoskeleton as well as membrane addition. The Drosophila pericentrosomal protein, Nuclear-fallout (Nuf), provides a link between these two processes. In nuf-derived embryos, actin remodeling and membrane recruitment during the initial stages of metaphase and cellular furrow formation are disrupted. Nuf is a homologue of arfophilin-2, an ADP ribosylation factor effector that binds Rab11 and influences recycling endosome (RE) organization. Here, we show that Nuf is an important component of the RE, and that these phenotypes are a consequence of Nuf activities at the RE. Nuf exhibits extensive colocalization with Rab11, a key RE component. GST pull-downs and the presence of a conserved Rab11-binding domain in Nuf demonstrate that Nuf and Rab11 physically associate. In addition, Nuf and Rab11 are mutually required for their localization to the RE. Embryos with reduced levels of Rab11 produce membrane recruitment and actin remodeling defects strikingly similar to nuf-derived embryos. These analyses support a common role for Nuf and Rab11 at the RE in membrane trafficking and actin remodeling during the initial stages of furrow formation.

Trafficking through Rab11 Endosomes Is Required for Cellularization during Drosophila Embryogenesis

BACKGROUND

Embryonic cleavage leads to the formation of an epithelial layer during development. In Drosophila, the process is specialized and called cellularization. The trafficking pathways that underlie this process and that are responsible for the mobilization of membrane pools, however, remain poorly understood.

RESULTS

We provide functional evidence for the role of endocytic trafficking through Rab11 endosomes in remobilizing vesicular membrane pools to ensure lateral membrane growth. Part of the membrane stems from endocytosed apical material. Mutants in the endocytic regulators rab5 and shibire/dynamin inhibit basal-lateral membrane growth, and apical endocytosis is blocked in shibire mutants. In addition, shibire controls vesicular trafficking through Rab11-positive endosomes. In shibire mutants, the transmembrane protein Neurotactin follows the secretory pathway normally but is not properly inserted in the plasma membrane and accumulates instead in Rab11 subapical endosomes. Consistent with a direct role of shibire in vesicular trafficking through Rab11 endosomes, Shibire is enriched in this compartment. Moreover, we show by electron microscopy the large accumulation of intracellular coated pits on subapical endocytic structures in shibire mutants. Finally, we show that Rab11 is essential for membrane growth and invagination during cellularization.

CONCLUSION

Together, the data show that endocytic trafficking is required for basal-lateral membrane growth during cellularization. We identify Rab11 endosomes as key trafficking intermediates that control vesicle exocytosis and membrane growth during cellularization. This pathway may be required in other morphogenetic processes characterized by the growth of a membrane domain.

Arfophilins are dual Arf/Rab 11 binding proteins that regulate recycling endosome distribution and are related to Drosophila nuclear fallout

Arfophilin is an ADP ribosylation factor (Arf) binding protein of unknown function. It is identical to the Rab11 binding protein eferin/Rab11-FIP3, and we show it binds both Arf5 and Rab11. We describe a related protein, arfophilin-2, that interacts with Arf5 in a nucleotide-dependent manner, but not Arf1, 4, or 6 and also binds Rab11. Arfophilin-2 localized to a perinuclear compartment, the centrosomal area, and focal adhesions. The localization of arfophilin-2 to the perinuclear compartment was selectively blocked by overexpression of Arf5-T31N. In contrast, a green fluorescent protein-arfophilin-2 chimera or arfophilin-2 deletions were localized around the centrosome in a region that was also enriched for transferrin receptors and Rab11 but not early endosome markers, suggesting that the distribution of the endosomal recycling compartment was altered. The arfophilins belong to a conserved family that includes Drosophila melanogaster nuclear fallout, a centrosomal protein required for cellularization. Expression of green fluorescent protein-nuclear fallout in HeLa cells resulted in a similar phenotype, indicative of functional homology and thus implicating the arfophilins in mitosis/cytokinesis. We suggest that the novel dual GTPase-binding capacity of the arfophilins could serve as an interface of signals from Rab and Arf GTPases to regulate membrane traffic and integrate distinct signals in the late endosomal recycling compartment.