A multicomponent assembly pathway contributes to the formation of acentrosomal microtubule arrays in interphase Drosophila cells

A Mechanism for Sustained Cellulose Synthesis during Salt Stress

Abiotic stress, such as salinity, drought, and cold, causes detrimental yield losses for all major plant crop species. Understanding mechanisms that improve plants' ability to produce biomass, which largely is constituted by the plant cell wall, is therefore of upmost importance for agricultural activities. Cellulose is a principal component of the cell wall and is synthesized by microtubule-guided cellulose synthase enzymes at the plasma membrane. Here, we identified two components of the cellulose synthase complex, which we call companion of cellulose synthase (CC) proteins. The cytoplasmic tails of these membrane proteins bind to microtubules and promote microtubule dynamics. This activity supports microtubule organization, cellulose synthase localization at the plasma membrane, and renders seedlings less sensitive to stress. Our findings offer a mechanistic model for how two molecular components, the CC proteins, sustain microtubule organization and cellulose synthase localization and thus aid plant biomass production during salt stress. VIDEO ABSTRACT.

When fate follows age: unequal centrosomes in asymmetric cell division

A strong correlation between centrosome age and fate has been reported in some stem cells and progenitors that divide asymmetrically. In some cases, such stereotyped centrosome behaviour is essential to endow stemness to only one of the two daughters, whereas in other cases causality is still uncertain. Here, we present the different cell types in which correlated centrosome age and fate has been documented, review current knowledge on the underlying molecular mechanisms and discuss possible functional implications of this process.

Drosophila CLIP-190 and mammalian CLIP-170 display reduced microtubule plus end association in the nervous system

Usually, CLIPs promote microtubule growth by binding their plus ends. However, in neurons, mouse CLIP-170 and fly CLIP-190 are weak end binders, instead forming actin/myosin VI–dependent patches in the center of growth cones. Total CLIP-190 loss, even together with four other plus end binders, reveals no role in neuronal MT regulation.

Axons act like cables, electrically wiring the nervous system. Polar bundles of microtubules (MTs) form their backbones and drive their growth. Plus end–tracking proteins (+TIPs) regulate MT growth dynamics and directionality at their plus ends. However, current knowledge about +TIP functions, mostly derived from work in vitro and in nonneuronal cells, may not necessarily apply to the very different context of axonal MTs. For example, the CLIP family of +TIPs are known MT polymerization promoters in nonneuronal cells. However, we show here that neither Drosophila CLIP-190 nor mammalian CLIP-170 is a prominent MT plus end tracker in neurons, which we propose is due to low plus end affinity of the CAP-Gly domain–containing N-terminus and intramolecular inhibition through the C-terminus. Instead, both CLIP-190 and CLIP-170 form F-actin–dependent patches in growth cones, mediated by binding of the coiled-coil domain to myosin-VI. Because our loss-of-function analyses in vivo and in culture failed to reveal axonal roles for CLIP-190, even in double-mutant combinations with four other +TIPs, we propose that CLIP-190 and -170 are not essential axon extension regulators. Our findings demonstrate that +TIP functions known from nonneuronal cells do not necessarily apply to the regulation of the very distinct MT networks in axons.

Cytoplasmic nucleation and atypical branching nucleation generate endoplasmic microtubules in Physcomitrella patens

The mechanism underlying microtubule (MT) generation in plants has been primarily studied using the cortical MT array, in which fixed-angled branching nucleation and katanin-dependent MT severing predominate. However, little is known about MT generation in the endoplasm. Here, we explored the mechanism of endoplasmic MT generation in protonemal cells of Physcomitrella patens. We developed an assay that utilizes flow cell and oblique illumination fluorescence microscopy, which allowed visualization and quantification of individual MT dynamics. MT severing was infrequently observed, and disruption of katanin did not severely affect MT generation. Branching nucleation was observed, but it showed markedly variable branch angles and was occasionally accompanied by the transport of nucleated MTs. Cytoplasmic nucleation at seemingly random locations was most frequently observed and predominated when depolymerized MTs were regrown. The MT nucleator γ-tubulin was detected at the majority of the nucleation sites, at which a single MT was generated in random directions. When γ-tubulin was knocked down, MT generation was significantly delayed in the regrowth assay. However, nucleation occurred at a normal frequency in steady state, suggesting the presence of a γ-tubulin-independent backup mechanism. Thus, endoplasmic MTs in this cell type are generated in a less ordered manner, showing a broader spectrum of nucleation mechanisms in plants.

αTubulin 67C and Ncd are essential for establishing a cortical microtubular network and formation of the Bicoid mRNA gradient in Drosophila

The Bicoid (Bcd) protein gradient in Drosophila serves as a paradigm for gradient formation in textbooks. To explain the generation of the gradient, the ARTS model, which is based on the observation of a bcd mRNA gradient, proposes that the bcd mRNA, localized at the anterior pole at fertilization, migrates along microtubules (MTs) at the cortex to the posterior to form a bcd mRNA gradient which is translated to form a protein gradient. To fulfil the criteria of the ARTS model, an early cortical MT network is thus a prerequisite. We report hitherto undiscovered MT activities in the early embryo important for bcd mRNA transport: (i) an early and omnidirectional MT network exclusively at the anterior cortex of early nuclear cycle embryos showing activity during metaphase and anaphase only, (ii) long MTs up to 50 µm extending into the yolk at blastoderm stage to enable basal-apical transport. The cortical MT network is not anchored to the actin cytoskeleton. The posterior transport of the mRNA via the cortical MT network critically depends on maternally-expressed αTubulin67C and the minus-end motor Ncd. In either mutant, cortical transport of the bcd mRNA does not take place and the mRNA migrates along another yet undisclosed interior MT network, instead. Our data strongly corroborate the ARTS model and explain the occurrence of the bcd mRNA gradient.

Acentrosomal Drosophila epithelial cells exhibit abnormal cell division, leading to cell death and compensatory proliferation

Mitotic spindles are critical for accurate chromosome segregation. Centrosomes, the primary microtubule nucleating centers of animal cells, play key roles in forming and orienting mitotic spindles. However, the survival of Drosophila without centrosomes suggested they are dispensable in somatic cells, challenging the canonical view. We used fly wing disc epithelia as a model to resolve these conflicting hypotheses, revealing that centrosomes play vital roles in spindle assembly, function, and orientation. Many acentrosomal cells exhibit prolonged spindle assembly, chromosome missegregation, DNA damage, misoriented divisions, and eventual apoptosis. We found that multiple mechanisms buffer the effects of centrosome loss, including alternative microtubule nucleation pathways and the spindle assembly checkpoint. Apoptosis of acentrosomal cells is mediated by JNK signaling, which also drives compensatory proliferation to maintain tissue integrity and viability. These data reveal the importance of centrosomes in fly epithelia and demonstrate the robust compensatory mechanisms at the cellular and organismal level.

A molecular mechanism of mitotic centrosome assembly in Drosophila

Centrosomes comprise a pair of centrioles surrounded by pericentriolar material (PCM). The PCM expands dramatically as cells enter mitosis, but it is unclear how this occurs. In this study, we show that the centriole protein Asl initiates the recruitment of DSpd-2 and Cnn to mother centrioles; both proteins then assemble into co-dependent scaffold-like structures that spread outwards from the mother centriole and recruit most, if not all, other PCM components. In the absence of either DSpd-2 or Cnn, mitotic PCM assembly is diminished; in the absence of both proteins, it appears to be abolished. We show that DSpd-2 helps incorporate Cnn into the PCM and that Cnn then helps maintain DSpd-2 within the PCM, creating a positive feedback loop that promotes robust PCM expansion around the mother centriole during mitosis. These observations suggest a surprisingly simple mechanism of mitotic PCM assembly in flies.

Indispensable pre-mitotic endocycles promote aneuploidy in the Drosophila rectum

The endocycle is a modified cell cycle that lacks M phase. Endocycles are well known for enabling continued growth of post-mitotic tissues. By contrast, we discovered pre-mitotic endocycles in precursors of Drosophila rectal papillae (papillar cells). Unlike all known proliferative Drosophila adult precursors, papillar cells endocycle before dividing. Furthermore, unlike diploid mitotic divisions, these polyploid papillar divisions are frequently error prone, suggesting papillar structures may accumulate long-term aneuploidy. Here, we demonstrate an indispensable requirement for pre-mitotic endocycles during papillar development and also demonstrate that such cycles seed papillar aneuploidy. We find blocking pre-mitotic endocycles disrupts papillar morphogenesis and causes organismal lethality under high-salt dietary stress. We further show that pre-mitotic endocycles differ from post-mitotic endocycles, as we find only the M-phase-capable polyploid cells of the papillae and female germline can retain centrioles. In papillae, this centriole retention contributes to aneuploidy, as centrioles amplify during papillar endocycles, causing multipolar anaphase. Such aneuploidy is well tolerated in papillae, as it does not significantly impair cell viability, organ formation or organ function. Together, our results demonstrate that pre-mitotic endocycles can enable specific organ construction and are a mechanism that promotes highly tolerated aneuploidy.

γ-Tubulin Ring Complexes and EB1 play antagonistic roles in microtubule dynamics and spindle positioning

γ-Tubulin is critical for microtubule (MT) assembly and organization. In metazoa, this protein acts in multiprotein complexes called γ-Tubulin Ring Complexes (γ-TuRCs). While the subunits that constitute γ-Tubulin Small Complexes (γ-TuSCs), the core of the MT nucleation machinery, are essential, mutation of γ-TuRC-specific proteins in Drosophila causes sterility and morphological abnormalities via hitherto unidentified mechanisms. Here, we demonstrate a role of γ-TuRCs in controlling spindle orientation independent of MT nucleation activity, both in cultured cells and in vivo, and examine a potential function for γ-TuRCs on astral MTs. γ-TuRCs locate along the length of astral MTs, and depletion of γ-TuRC-specific proteins increases MT dynamics and causes the plus-end tracking protein EB1 to redistribute along MTs. Moreover, suppression of MT dynamics through drug treatment or EB1 down-regulation rescues spindle orientation defects induced by γ-TuRC depletion. Therefore, we propose a role for γ-TuRCs in regulating spindle positioning by controlling the stability of astral MTs.

Stem cell decisions: a twist of fate or a niche market?

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Extrinsic and intrinsic cues that impact on stem cell biology.

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The importance to establish methods that allow to compare spindle orientation measurements.

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Mechanisms of centrosome segregation in asymmetrically dividing cells.

Establishing and maintaining cell fate in the right place at the right time is a key requirement for normal tissue maintenance. Stem cells are at the core of this process. Understanding how stem cells balance self-renewal and production of differentiating cells is key for understanding the defects that underpin many diseases. Both, external cues from the environment and cell intrinsic mechanisms can control the outcome of stem cell division. The role of the orientation of stem cell division has emerged as an important mechanism for specifying cell fate decisions. Although, the alignment of cell divisions can dependent on spatial cues from the environment, maintaining stemness is not always linked to positioning of stem cells in a particular microenvironment or `niche'. Alternate mechanisms that could contribute to cellular memory include differential segregation of centrosomes in asymmetrically dividing cells.

The use of cultured Drosophila cells for studying the microtubule cytoskeleton

Cultured Drosophila cell lines have been developed into a powerful tool for studying a wide variety of cellular processes. Their ability to be easily and cheaply cultured as well as their susceptibility to protein knockdown via double-stranded RNA-mediated interference (RNAi) has made them the model system of choice for many researchers in the fields of cell biology and functional genomics. Here we describe basic techniques for gene knockdown, transgene expression, preparation for fluorescence microscopy, and centrosome enrichment using cultured Drosophila cells with an emphasis on studying the microtubule cytoskeleton.

Translocating myonuclei have distinct leading and lagging edges that require kinesin and dynein

Nuclei are precisely positioned within all cells, and mispositioned nuclei are a hallmark of many muscle diseases. Myonuclear positioning is dependent on Kinesin and Dynein, but interactions between these motor proteins and their mechanisms of action are unclear. We find that in developing Drosophila muscles, Dynein and Kinesin work together to move nuclei in a single direction by two separate mechanisms that are spatially segregated. First, the two motors work together in a sequential pathway that acts from the cell cortex at the muscle poles. This mechanism requires Kinesin-dependent localization of Dynein to cell cortex near the muscle pole. From this location Dynein can pull microtubule minus-ends and the attached myonuclei toward the muscle pole. Second, the motors exert forces directly on individual nuclei independently of the cortical pathway. However, the activities of the two motors on the nucleus are polarized relative to the direction of myonuclear translocation: Kinesin acts at the leading edge of the nucleus, whereas Dynein acts at the lagging edge of the nucleus. Consistent with the activities of Kinesin and Dynein being polarized on the nucleus, nuclei rarely change direction, and those that do, reorient to maintain the same leading edge. Conversely, nuclei in both Kinesin and Dynein mutant embryos change direction more often and do not maintain the same leading edge when changing directions. These data implicate Kinesin and Dynein in two distinct and independently regulated mechanisms of moving myonuclei, which together maximize the ability of myonuclei to achieve their proper localizations within the constraints imposed by embryonic development.

PLP inhibits the activity of interphase centrosomes to ensure their proper segregation in stem cells

PLP levels are regulated by Centrobin and act to suppress mother centrosome maturation by blocking the localization of Polo kinase.

Centrosomes determine the mitotic axis of asymmetrically dividing stem cells. Several studies have shown that the centrosomes of the Drosophila melanogaster central brain neural stem cells are themselves asymmetric, organizing varying levels of pericentriolar material and microtubules. This asymmetry produces one active and one inactive centrosome during interphase. We identify pericentrin-like protein (PLP) as a negative regulator of centrosome maturation and activity. We show that PLP is enriched on the inactive interphase centrosome, where it blocks recruitment of the master regulator of centrosome maturation, Polo kinase. Furthermore, we find that ectopic Centrobin expression influenced PLP levels on the basal centrosome, suggesting it may normally function to regulate PLP. Finally, we conclude that, although asymmetric centrosome maturation is not required for asymmetric cell division, it is required for proper centrosome segregation to the two daughter cells.

Dynamic microtubules produce an asymmetric E-cadherin-Bazooka complex to maintain segment boundaries

Distributing junctional components around the cell periphery is key for epithelial tissue morphogenesis and homeostasis. We discovered that positioning of dynamic microtubules controls the asymmetric accumulation of E-cadherin. Microtubules are oriented preferentially along the dorso-ventral axis in Drosophila melanogaster embryonic epidermal cells, and thus more frequently contact E-cadherin at dorso-ventral cell-cell borders. This inhibits RhoGEF2, reducing membrane recruitment of Rho-kinase, and increasing a specific E-cadherin pool that is mobile when assayed by fluorescence recovery after photobleaching. This mobile E-cadherin is complexed with Bazooka/Par-3, which in turn is required for normal levels of mobile E-cadherin. Mobile E-cadherin-Bazooka prevents formation of multicellular rosette structures and cell motility across the segment border in Drosophila embryos. Altogether, the combined action of dynamic microtubules and Rho signaling determines the level and asymmetric distribution of a mobile E-cadherin-Bazooka complex, which regulates cell behavior during the generation of a patterned epithelium.

A method for reversible drug delivery to internal tissues of Drosophila embryos

Drosophila melanogaster is a powerful model organism to elucidate basic cellular mechanisms of development. Indeed, much of our understanding of genetic pathways comes from work in Drosophila. However, mutations in many critical genes cause early embryonic lethality; thus, it is difficult to study the role of proteins that are required for early fundamental processes during later embryonic stages. We have therefore developed a method to reversibly deliver drugs to internal tissues of stage 15-16 Drosophila embryos using a 1:1 combination of D-limonene and heptane (LH). Specifically, delivery of Nocodazole was shown to be effective as evidenced by the significant decrease in microtubule density seen in muscle cells. Following complete depolymerization of the microtubule cytoskeleton, removing the Nocodazole and washing for 10 min was sufficient for the microtubule network to be re-established, indicating that drug delivery is reversible. Additionally, the morphology of LH-treated embryos resembled that of untreated controls, and embryo viability post-treatment with LH was significantly increased compared with previously reported permeabilization techniques. These advances in embryo permeabilization provide a means to disrupt protein function in vivo with high temporally specificity, bypassing the complications associated with genetic disruptions as they relate to the study of late-stage developmental mechanisms.

Spinodal decomposition and the emergence of dissipative transient periodic spatio-temporal patterns in acentrosomal microtubule multitudes of different morphology

We have studied a spontaneous self-organization dynamics in a closed, dissipative (in terms of guansine 5'-triphosphate energy dissipation), reaction-diffusion system of acentrosomal microtubules (those nucleated and organized in the absence of a microtubule-organizing centre) multitude constituted of straight and curved acentrosomal microtubules, in highly crowded conditions, in vitro. Our data give experimental evidence that cross-diffusion in conjunction with excluded volume is the underlying mechanism on basis of which acentrosomal microtubule multitudes of different morphologies (straight and curved) undergo a spatial-temporal demix. Demix is constituted of a bifurcation process, manifested as a slow isothermal spinodal decomposition, and a dissipative process of transient periodic spatio-temporal pattern formation. While spinodal decomposition is an energy independent process, transient periodic spatio-temporal pattern formation is accompanied by energy dissipative process. Accordingly, we have determined that the critical threshold for slow, isothermal spinodal decomposition is 1.0 ± 0.05 mg/ml of microtubule protein concentration. We also found that periodic spacing of transient periodic spatio-temporal patterns was, in the overall, increasing versus time. For illustration, we found that a periodic spacing of the same pattern was 0.375 ± 0.036 mm, at 36 °C, at 155th min, while it was 0.540 ± 0.041 mm at 31 °C, and at 275th min after microtubule assembly started. The lifetime of transient periodic spatio-temporal patterns spans from half an hour to two hours approximately. The emergence of conditions of macroscopic symmetry breaking (that occur due to cross-diffusion in conjunction with excluded volume) may have more general but critical importance in morphological pattern development in complex, dissipative, but open cellular systems.

A non-canonical mode of microtubule organization operates throughout pre-implantation development in mouse

In dividing animal cells, the centrosome, comprising centrioles and surrounding pericentriolar-material (PCM), is the major interphase microtubule-organizing center (MTOC), arranging a polarized array of microtubules (MTs) that controls cellular architecture. The mouse embryo is a unique setting for investigating the role of centrosomes in MT organization, since the early embryo is acentrosomal, and centrosomes emerge de novo during early cleavages. Here we use embryos from a GFP::CETN2 transgenic mouse to observe the emergence of centrosomes and centrioles in embryos, and show that unfocused acentriolar centrosomes first form in morulae (~16-32-cell stage) and become focused at the blastocyst stage (~64-128 cells) concomitant with the emergence of centrioles. We then used high-resolution microscopy and dynamic tracking of MT growth events in live embryos to examine the impact of centrosome emergence upon interphase MT dynamics. We report that pre-implantation mouse embryos of all stages employ a non-canonical mode of MT organization that generates a complex array of randomly oriented MTs that are preferentially nucleated adjacent to nuclear and plasmalemmal membranes and cell-cell interfaces. Surprisingly, however, cells of the early embryo continue to employ this mode of interphase MT organization even after the emergence of centrosomes. Centrosomes are found at MT-sparse sites and have no detectable impact upon interphase MT dynamics. To our knowledge, the early embryo is unique among proliferating cells in adopting an acentrosomal mode of MT organization despite the presence of centrosomes, revealing that the transition to a canonical mode of interphase MT organization remains incomplete prior to implantation.

Cortical microtubule arrays are initiated from a nonrandom prepattern driven by atypical microtubule initiation

The ordered arrangement of cortical microtubules in growing plant cells is essential for anisotropic cell expansion and, hence, for plant morphogenesis. These arrays are dismantled when the microtubule cytoskeleton is rearranged during mitosis and reassembled following completion of cytokinesis. The reassembly of the cortical array has often been considered as initiating from a state of randomness, from which order arises at least partly through self-organizing mechanisms. However, some studies have shown evidence for ordering at early stages of array assembly. To investigate how cortical arrays are initiated in higher plant cells, we performed live-cell imaging studies of cortical array assembly in tobacco (Nicotiana tabacum) Bright Yellow-2 cells after cytokinesis and drug-induced disassembly. We found that cortical arrays in both cases did not initiate randomly but with a significant overrepresentation of microtubules at diagonal angles with respect to the cell axis, which coincides with the predominant orientation of the microtubules before their disappearance from the cell cortex in preprophase. In Arabidopsis (Arabidopsis thaliana) root cells, recovery from drug-induced disassembly was also nonrandom and correlated with the organization of the previous array, although no diagonal bias was observed in these cells. Surprisingly, during initiation, only about one-half of the new microtubules were nucleated from locations marked by green fluorescent protein-γ-tubulin complex protein2-tagged γ-nucleation complexes (γ-tubulin ring complex), therefore indicating that a large proportion of early polymers was initiated by a noncanonical mechanism not involving γ-tubulin ring complex. Simulation studies indicate that the high rate of noncanonical initiation of new microtubules has the potential to accelerate the rate of array repopulation.

Golgi outposts shape dendrite morphology by functioning as sites of acentrosomal microtubule nucleation in neurons

Microtubule nucleation is essential for proper establishment and maintenance of axons and dendrites. Centrosomes, the primary site of nucleation in most cells, lose their function as microtubule organizing centers during neuronal development. How neurons generate acentrosomal microtubules remains unclear. Drosophila dendritic arborization (da) neurons lack centrosomes and therefore provide a model system to study acentrosomal microtubule nucleation. Here, we investigate the origin of microtubules within the elaborate dendritic arbor of class IV da neurons. Using a combination of in vivo and in vitro techniques, we find that Golgi outposts can directly nucleate microtubules throughout the arbor. This acentrosomal nucleation requires gamma-tubulin and CP309, the Drosophila homolog of AKAP450, and contributes to the complex microtubule organization within the arbor and dendrite branch growth and stability. Together, these results identify a direct mechanism for acentrosomal microtubule nucleation within neurons and reveal a function for Golgi outposts in this process.

Centrobin controls mother-daughter centriole asymmetry in Drosophila neuroblasts

During interphase in Drosophila neuroblasts, the Centrobin (CNB)-positive daughter centriole retains pericentriolar material (PCM) and organizes an aster that is a key determinant of the orientation of cell division. Here we show that daughter centrioles depleted of CNB cannot fulfil this function whereas mother centrioles that carry ectopic CNB can. CNB co-precipitates with a set of centrosomal proteins that include γ-TUB, ANA2, CNN, SAS-4, ASL, DGRIP71, POLO and SAS-6. Following chemical inhibition of POLO or removal of three POLO phosphorylation sites present in CNB, the interphase microtubule aster is lost. These results demonstrate that centriolar CNB localization is both necessary and sufficient to enable centrioles to retain PCM and organize the interphase aster in Drosophila neuroblasts. They also reveal an interphase function for POLO in this process that seems to have co-opted part of the protein network involved in mitotic centrosome maturation.

Subdiffraction-resolution fluorescence microscopy reveals a domain of the centrosome critical for pericentriolar material organization

As the main microtubule-organizing centre in animal cells, the centrosome has a fundamental role in cell function. Surrounding the centrioles, the pericentriolar material (PCM) provides a dynamic platform for nucleating microtubules. Although the importance of the PCM is established, its amorphous electron-dense nature has made it refractory to structural investigation. By using SIM and STORM subdiffraction-resolution microscopies to visualize proteins critical for centrosome maturation, we demonstrate that the PCM is organized into two main structural domains: a layer juxtaposed to the centriole wall, and proteins extending farther away from the centriole organized in a matrix. Analysis of Pericentrin-like protein (PLP) reveals that its carboxy terminus is positioned at the centriole wall, it radiates outwards into the matrix and is organized in clusters having quasi-nine-fold symmetry. By RNA-mediated interference (RNAi), we show that PLP fibrils are required for interphase recruitment and proper mitotic assembly of the PCM matrix.

Tubulin nucleotide status controls Sas-4-dependent pericentriolar material recruitment

Regulated centrosome biogenesis is required for accurate cell division and for maintaining genome integrity¹. Centrosomes consist of a centriole pair surrounded by a protein network known as pericentriolar material (PCM)¹. PCM assembly is a tightly regulated, critical step that determines a centrosome’s size and capability^2–4. Here, we report a role for tubulin in regulating PCM recruitment via the conserved centrosomal protein Sas-4. Tubulin directly binds to Sas-4; together they are components of cytoplasmic complexes of centrosomal proteins^5,6. A Sas-4 mutant, which cannot bind tubulin, enhances centrosomal protein complex formation and has abnormally large centrosomes with excessive activity. These suggest that tubulin negatively regulates PCM recruitment. Whereas tubulin-GTP prevents Sas-4 from forming protein complexes, tubulin-GDP promotes it. Thus, tubulin’s regulation of PCM recruitment depends on its GTP/GDP-bound state. These results identify a role for tubulin in regulating PCM recruitment independent of its well-known role as a building block of microtubules⁷. Based on its guanine bound state, tubulin can act as a molecular switch in PCM recruitment.

COPI-mediated membrane trafficking is required for cytokinesis in Drosophila male meiotic divisions

The coatomer protein complex, COPI, mediates retrograde vesicle transport from the Golgi apparatus to the ER. Here, we investigated the meiotic phenotype of Drosophila spermatocytes expressing dsRNA of 52 genes encoding membrane trafficking-related factors. We identified COPI as an essential factor for male meiosis. In Drosophila male meiotic divisions, COPI is localized in the ER-Golgi intermediate compartment of tER-Golgi units scattered throughout the spermatocyte cytoplasm. Prior to chromosome segregation, the vesicles assemble at the spindle pole periphery through a poleward movement, mediated by minus-ended motor dynein along astral microtubules. At the end of each meiotic division, COPI-containing vesicles are equally partitioned between 2 daughter cells. Our present data strongly suggest that spermatocytes possess a regulatory mechanism, to fulfill equal inheritance of several types of membrane vesicles. Using testis-specific knockdown of COPI subunits or small GTPase Arf, or mutations of the γCOP gene, we examined the role of COPI in male meiosis. COPI depletion resulted in the failure of cytokinesis, through disrupted accumulation of essential proteins and lipid components at the cleavage furrow region. Furthermore, it caused a reduction in the number of overlapping central spindle microtubules, which are essential for cytokinesis. Drosophila spermatocytes construct ER-based intracellular structures associated with astral and spindle microtubules. COPI depletion resulted in severe disruption of these ER-based structures. Thus, we propose that COPI plays an important role in Drosophila male meiosis, not only through vesicle transport to the cleavage furrow region, but also via the formation of ER-based structures.

Microtubules are organized independently of the centrosome in Drosophila neurons

Background

The best-studied arrangement of microtubules is that organized by the centrosome, a cloud of microtubule nucleating and anchoring proteins is clustered around centrioles. However, noncentrosomal microtubule arrays are common in many differentiated cells, including neurons. Although microtubules are not anchored at neuronal centrosomes, it remains unclear whether the centrosome plays a role in organizing neuronal microtubules. We use Drosophila as a model system to determine whether centrosomal microtubule nucleation is important in mature neurons.

Results

In developing and mature neurons, centrioles were not surrounded by the core nucleation protein γ-tubulin. This suggests that the centrioles do not organize functional centrosomes in Drosophila neurons in vivo. Consistent with this idea, centriole position was not correlated with a specific region of the cell body in neurons, and growing microtubules did not cluster around the centriole, even after axon severing when the number of growing plus ends is dramatically increased. To determine whether the centrosome was required for microtubule organization in mature neurons, we used two approaches. First, we used DSas-4 centriole duplication mutants. In these mutants, centrioles were present in many larval sensory neurons, but they were not fully functional. Despite reduced centriole function, microtubule orientation was normal in axons and dendrites. Second, we used laser ablation to eliminate the centriole, and again found that microtubule polarity in axons and dendrites was normal, even 3 days after treatment.

Conclusion

We conclude that the centrosome is not a major site of microtubule nucleation in Drosophila neurons, and is not required for maintenance of neuronal microtubule organization in these cells.

The centrosome and bipolar spindle assembly: does one have anything to do with the other?

In vertebrate somatic cells the centrosome functions as the major microtubule-organizing center (MTOC), which splits and separates to form the poles of the mitotic spindle. However, the role of the centriole-containing centrosome in the formation of bipolar mitotic spindles continues to be controversial. Cells normally containing centrosomes are still able to build bipolar spindles after their centrioles have been removed or ablated. In naturally occurring cellular systems that lack centrioles - such as plant cells and many oocytes - bipolar spindles form in the complete absence of canonical centrosomes. These observations have led to the notion that centrosomes play no role during mitosis. However, recent work has re-examined spindle assembly in the absence of centrosomes, both in cells that naturally lack them, and those that have had them experimentally removed. The results of these studies suggest that an appreciation of microtubule network organization- both before and after nuclear envelope breakdown (NEB) - is the key to understanding the mechanisms that regulate spindle assembly and the generation of bipolarity.

Protein kinase A acts at the basal body of the primary cilium to prevent Gli2 activation and ventralization of the mouse neural tube

Protein kinase A (PKA) is an evolutionarily conserved negative regulator of the hedgehog (Hh) signal transduction pathway. PKA is known to be required for the proteolytic processing event that generates the repressor forms of the Ci and Gli transcription factors that keep target genes off in the absence of Hh. Here, we show that complete loss of PKA activity in the mouse leads to midgestation lethality and a completely ventralized neural tube, demonstrating that PKA is as strong a negative regulator of the sonic hedgehog (Shh) pathway as patched 1 (Ptch1) or suppressor of fused (Sufu). Genetic analysis shows that although PKA is important for production of the repressor form of Gli3, the principal function of PKA in the Shh pathway in neural development is to restrain activation of Gli2. Activation of the Hh pathway in PKA mutants depends on cilia, and the catalytic and regulatory subunits of PKA are localized to a compartment at the base of the primary cilia, just proximal to the basal body. The data show that PKA does not affect cilia length or trafficking of smoothened (Smo) in the cilium. Instead, we find that there is a significant increase in the level of Gli2 at the tips of cilia of PKA-null cells. The data suggest a model in which PKA acts at the base of the cilium after Gli proteins have transited the primary cilium; in this model the sequential movement of Gli proteins between compartments in the cilium and at its base controls accessibility of Gli proteins to PKA, which determines the fates of Gli proteins and the activity of the Shh pathway.

The Protein Phosphatase 2A regulatory subunit Twins stabilizes Plk4 to induce centriole amplification

The PP2A subunit Twins and the SV40 small T antigen, a functional mimic of Twins, counteract Plk4 autophosphorylation, leading to its stabilization and to subsequent centriole amplification.

Centriole duplication is a tightly regulated process that must occur only once per cell cycle; otherwise, supernumerary centrioles can induce aneuploidy and tumorigenesis. Plk4 (Polo-like kinase 4) activity initiates centriole duplication and is regulated by ubiquitin-mediated proteolysis. Throughout interphase, Plk4 autophosphorylation triggers its degradation, thus preventing centriole amplification. However, Plk4 activity is required during mitosis for proper centriole duplication, but the mechanism stabilizing mitotic Plk4 is unknown. In this paper, we show that PP2A (Protein Phosphatase 2A^Twins) counteracts Plk4 autophosphorylation, thus stabilizing Plk4 and promoting centriole duplication. Like Plk4, the protein level of PP2A’s regulatory subunit, Twins (Tws), peaks during mitosis and is required for centriole duplication. However, untimely Tws expression stabilizes Plk4 inappropriately, inducing centriole amplification. Paradoxically, expression of tumor-promoting simian virus 40 small tumor antigen (ST), a reported PP2A inhibitor, promotes centrosome amplification by an unknown mechanism. We demonstrate that ST actually mimics Tws function in stabilizing Plk4 and inducing centriole amplification.

The microtubule lattice and plus-end association of Drosophila Mini spindles is spatially regulated to fine-tune microtubule dynamics

Individual microtubules (MTs) exhibit dynamic instability, a behavior in which they cycle between phases of growth and shrinkage while the total amount of MT polymer remains constant. Dynamic instability is promoted by the conserved XMAP215/Dis1 family of microtubule-associated proteins (MAPs). In this study, we conducted an in vivo structure-function analysis of the Drosophila homologue Mini spindles (Msps). Msps exhibits EB1-dependent and spatially regulated MT localization, targeting to microtubule plus ends in the cell interior and decorating the lattice of growing and shrinking microtubules in the cell periphery. RNA interference rescue experiments revealed that the NH(2)-terminal four TOG domains of Msps function as paired units and were sufficient to promote microtubule dynamics and EB1 comet formation. We also identified TOG5 and novel inter-TOG linker motifs that are required for targeting Msps to the microtubule lattice. These novel microtubule contact sites are necessary for the interplay between the conserved TOG domains and inter-TOG MT binding that underlies the ability of Msps to promote MT dynamic instability.

Using the Drosophila melanogaster D17-c3 cell culture system to study cell motility

Cultured Drosophila melanogaster S2 and S2R+ cell lines have become important tools for uncovering fundamental aspects of cell biology as well as for gene discovery. Despite their utility, these cell lines are nonmotile and cannot build polarized structures or cell-cell contacts. Here we outline a previously isolated, but uncharacterized, Drosophila cell line named Dm-D17-c3 (or D17). These cells spread and migrate in culture, form cell-cell junctions and are susceptible to RNA interference (RNAi). Using this protocol, we describe how investigators, upon receiving cells from the Bloomington stock center, can culture cells and prepare the necessary reagents to plate and image migrating D17 cells; they can then be used to examine intracellular dynamics or observe loss-of-function RNAi phenotypes using an in vitro scratch or wound healing assay. From first thawing frozen ampules of D17 cells, investigators can expect to begin assaying RNAi phenotypes in D17 cells within roughly 2-3 weeks.

50 ways to build a spindle: the complexity of microtubule generation during mitosis

The accurate segregation of duplicated chromosomes, essential for the development and viability of a eukaryotic organism, requires the formation of a robust microtubule (MT)-based spindle apparatus. Entry into mitosis or meiosis precipitates a cascade of signalling events which result in the activation of pathways responsible for a dramatic reorganisation of the MT cytoskeleton: through changes in the properties of MT-associated proteins, local concentrations of free tubulin dimer and through enhanced MT nucleation. The latter is generally thought to be driven by localisation and activation of γ-tubulin-containing complexes (γ-TuSC and γ-TuRC) at specific subcellular locations. For example, upon entering mitosis, animal cells concentrate γ-tubulin at centrosomes to tenfold the normal level during interphase, resulting in an aster-driven search and capture of chromosomes and bipolar mitotic spindle formation. Thus, in these cells, centrosomes have traditionally been perceived as the primary microtubule organising centre during spindle formation. However, studies in meiotic cells, plants and cell-free extracts have revealed the existence of complementary mechanisms of spindle formation, mitotic chromatin, kinetochores and nucleation from existing MTs or the cytoplasm can all contribute to a bipolar spindle apparatus. Here, we outline the individual known mechanisms responsible for spindle formation and formulate ideas regarding the relationship between them in assembling a functional spindle apparatus.

Microtubule tip-interacting proteins: a view from both ends

Microtubule ends serve as sites of tubulin addition and removal, and at the same time play crucial roles in microtubule capture, stabilization and attachment to different cellular structures. Microtubule plus and minus-ends possess distinct structural and dynamic properties, and are recognized, bound and regulated by diverse factors. These include specific capping factors such as γ-tubulin, motors, such as plus-end and minus-end directed kinesins, highly specialized kinetochore-bound microtubule-associated proteins, and comet-making plus-end tracking proteins such as EB1 and its partners. Here, we provide an overview of microtubule tip-interacting proteins and the mechanisms responsible for their association with microtubule ends, and discuss the functional cross-talk between microtubule plus and minus-end binding factors.

Patronin regulates the microtubule network by protecting microtubule minus ends

Tubulin assembles into microtubule polymers that have distinct plus and minus ends. Most microtubule plus ends in living cells are dynamic; the transitions between growth and shrinkage are regulated by assembly-promoting and destabilizing proteins. In contrast, minus ends are generally not dynamic, suggesting their stabilization by some unknown protein. Here, we have identified Patronin (also known as ssp4) as a protein that stabilizes microtubule minus ends in Drosophila S2 cells. In the absence of Patronin, minus ends lose subunits through the actions of the Kinesin-13 microtubule depolymerase, leading to a sparse interphase microtubule array and short, disorganized mitotic spindles. In vitro, the selective binding of purified Patronin to microtubule minus ends is sufficient to protect them against Kinesin-13-induced depolymerization. We propose that Patronin caps and stabilizes microtubule minus ends, an activity that serves a critical role in the organization of the microtubule cytoskeleton.

Plus-end-tracking proteins and their interactions at microtubule ends

Microtubules are cytoskeletal elements that are essential for a large number of intracellular processes, including mitosis, cell differentiation and migration, and vesicle transport. In many cells, the microtubule network is organized in a radial manner, with one end of a microtubule (the minus end) embedded near the nucleus and the other end (the plus end) exploring cytoplasmic space, switching between episodes of growth and shrinkage. Mammalian plus-end-tracking proteins (+TIPs) localize to the ends of growing microtubules and regulate both the dynamic behavior of microtubules as well as the interactions of microtubules with other cellular components. Because of these crucial roles, +TIPs and the mechanisms underlying their association with microtubule ends have been intensively investigated. Results indicate that +TIPs reach microtubule ends by motor-mediated transport or diffusion. Individual +TIP molecules exchange rapidly on microtubule end-binding sites that are formed during microtubule polymerization and that have a slower turnover. Most +TIPs associate with the end-binding (EB) proteins, and appear to require these 'core' +TIPs for localization at microtubule ends. Accumulation of +TIPs may also involve structural features of the microtubule end and interactions with other +TIPs. This complexity makes it difficult to assign discrete roles to specific +TIPs. Given that +TIPs concentrate at microtubule ends and that each +TIP binds in a conformationally distinct manner, I propose that the ends of growing microtubules are 'nano-platforms' for productive interactions between selected proteins and that these interactions might persist and be functional elsewhere in the cytoplasm than at the microtubule end at which they originated.

The gammaTuRC revisited: a comparative analysis of interphase and mitotic human gammaTuRC redefines the set of core components and identifies the novel subunit GCP8

The γ-tubulin complex is a multi-subunit protein complex that nucleates microtubule polymerization. γ-Tubulin complexes are present in all eukaryotes, but size and subunit composition vary. In Drosophila, Xenopus, and humans large γ-tubulin ring complexes (γTuRCs) have been described, which have a characteristic open ring-shaped structure and are composed of a similar set of subunits, named γ-tubulin, GCPs 2-6, and GCP-WD in humans. Despite the identification of these proteins, γTuRC function and regulation remain poorly understood. Here we establish a new method for the purification of native human γTuRC. Using mass spectrometry of whole protein mixtures we compared the composition of γTuRCs from nonsynchronized and mitotic human cells. Based on our analysis we can define core subunits as well as more transient interactors such as the augmin complex, which associates specifically with mitotic γTuRCs. We also identified GCP8/MOZART2 as a novel core subunit that is present in both interphase and mitotic γTuRCs. GCP8 depletion does not affect γTuRC assembly but interferes with γTuRC recruitment and microtubule nucleation at interphase centrosomes without disrupting general centrosome structure. GCP8-depleted cells do not display any obvious mitotic defects, suggesting that GCP8 specifically affects the organization of the interphase microtubule network.

Proteins required for centrosome clustering in cancer cells

Current cancer chemotherapies are limited by the lack of tumor-specific targets, which would allow for selective eradication of malignant cells without affecting healthy tissues. In contrast to normal cells, most tumor cells contain multiple centrosomes, which tend to cause the formation of multipolar mitotic spindles, chromosome segregation defects, and cell death. Nevertheless, many cancer cells divide successfully because they can cluster multiple centrosomes into two spindle poles. Inhibition of this centrosomal clustering, with consequent induction of multipolar spindles and subsequent cell death, would specifically target cancer cells and overcome one limitation of current cancer treatments. We have performed a genome-wide RNA interference screen to identify proteins involved in the prevention of spindle multipolarity in human cancer cells with supernumerary centrosomes. The chromosomal passenger complex, Ndc80 microtubule-kinetochore attachment complex, sister chromatid cohesion, and microtubule formation via the augmin complex were identified as necessary for centrosomal clustering. We show that spindle tension is required to cluster multiple centrosomes into a bipolar spindle array in tumor cells with extra centrosomes. These findings may explain the specificity of drugs that interfere with spindle tension for cancer cells and provide entry points for the development of therapeutics.

Methods for expressing and analyzing GFP-tubulin and GFP-microtubule-associated proteins

Important advances in our understanding of the organization and dynamics of the cytoskeleton have been made by direct observations of fluorescently tagged cytoskeletal proteins in living cells. In early experiments, the cytoskeletal protein of interest was purified, covalently modified with a fluorescent dye, and microinjected into living cells. In the mid-1990s, a powerful new technology arose: Researchers developed methods for expressing chimeric proteins consisting of the gene of interest fused to green fluorescent protein (GFP). This approach has become a standard method for characterizing protein localization and dynamics. More recently, a profusion of "XFP" (spectral variants of GFP) has been developed, allowing researchers straightforwardly to perform experiments ranging from simultaneous co-observation of protein dynamics to fluorescence recovery after photobleaching (FRAP), fluorescence resonance energy transfer (FRET), and subresolution techniques such as stimulated emission-depletion microscopy (STED) and photoactivated localization microscopy (PALM). In this article, the methods used to express and analyze GFP- and/or XFP-tagged tubulin and microtubule-associated proteins (MAPs) are discussed. Although some details may be system-specific, the methods and considerations outlined here can be adapted to a wide variety of proteins and organisms.

A central role for polyamines in microtubule assembly in cells

Owing to preferential electrostatic adsorption of multivalent cations on highly anionic surfaces, natural multivalent polyamines and especially quadrivalent spermine can be considered as potential regulators of the complex dynamical properties of anionic MTs (microtubules). Indeed, the C-terminal tails of tubulin display many negative residues in a row which should enable the formation of a correlated liquid-like phase of multivalent counterions on its surface. Although it is known that polyamine counterions promote MT assembly in vitro, little is known about the relevance of this interaction in vivo. In the present study, we have explored the relationship between polyamine levels and MT assembly in HeLa and epithelial NRK (normal rat kidney) cells using DFMO (α-difluoromethylornithine), an irreversible inhibitor of ornithine decarboxylase, and APCHA [N-(3-aminopropyl)-N-cyclohexylamine], a spermine synthase inhibitor. Under conditions of intracellular polyamine depletion, the MT network is clearly disrupted and the MT mass decreases. Addition of spermine to polyamine-depleted cells reverses this phenotype and rapidly promotes the extensions of the MT network. Finally, we show that polyamine levels modulate the coating of MTs with MAP4 (MT-associated protein 4), an MT-stabilizing protein, and the spatial distribution of EB1 (end-binding protein 1), an MT plus-end-binding protein. In addition, polyamines favour the formation of gap junctions in NRK cells, a process which requires MT extensions at the cell periphery. The present study provides a basis for a better understanding of the role played by polyamines in MT assembly and establishes polyamine metabolism as a potential cellular target for modulating MT functions.

Tao-1 is a negative regulator of microtubule plus-end growth

Microtubule dynamics are dominated by events at microtubule plus ends as they switch between discrete phases of growth and shrinkage. Through their ability to generate force and direct polar cell transport, microtubules help to organise global cell shape and polarity. Conversely, because plus-end binding proteins render the dynamic instability of individual microtubules sensitive to the local intracellular environment, cyto-architecture also affects the overall distribution of microtubules. Despite the importance of plus-end regulation for understanding microtubule cytoskeletal organisation and dynamics, little is known about the signalling mechanisms that trigger changes in their behaviour in space and time. Here, we identify a microtubule-associated kinase, Drosophila Tao-1, as an important regulator of microtubule stability, plus-end dynamics and cell shape. Active Tao-1 kinase leads to the destabilisation of microtubules. Conversely, when Tao-1 function is compromised, rates of cortical-induced microtubule catastrophe are reduced and microtubules contacting the actin cortex continue to elongate, leading to the formation of long microtubule-based protrusions. These data reveal a role for Tao-1 in controlling the dynamic interplay between microtubule plus ends and the actin cortex in the regulation of cell form.

A developmentally regulated two-step process generates a noncentrosomal microtubule network in Drosophila tracheal cells

Microtubules (MTs) are essential for many cell features, such as polarity, motility, shape, and vesicle trafficking. Therefore, in a multicellular organism, their organization differs between cell types and during development; however, the control of this process remains elusive. Here, we show that during Drosophila tracheal morphogenesis, MT reorganization is coupled to relocalization of the microtubule organizing centers (MTOC) components from the centrosome to the apical cell domain from where MTs then grow. We reveal that this process is controlled by the trachealess patterning gene in a two-step mechanism. MTOC components are first released from the centrosome by the activity of the MT-severing protein Spastin, and then anchored apically through the transmembrane protein Piopio. We further show that these changes are essential for tracheal development, thus stressing the functional relevance of MT reorganization for morphogenesis.

Kinesin-1 heavy chain mediates microtubule sliding to drive changes in cell shape

Microtubules are typically observed to buckle and loop during interphase in cultured cells by an unknown mechanism. We show that lateral microtubule movement and looping is a result of microtubules sliding against one another in interphase Drosophila S2 cells. RNAi of the kinesin-1 heavy chain (KHC), but not dynein or the kinesin-1 light chain, eliminates these movements. KHC-dependent microtubule sliding powers the formation of cellular processes filled with parallel microtubule bundles. The growth of these cellular processes is independent of the actin cytoskeleton. We further observe cytoplasmic microtubule sliding in Xenopus and Ptk2 cells, and show that antibody inhibition of KHC in mammalian cells prevents sliding. We therefore propose that, in addition to its well established role in organelle transport, an important universal function of kinesin-1 is to mediate cytoplasmic microtubule-microtubule sliding. This provides the cell with a dedicated mechanism to transport long and short microtubule filaments and drive changes in cell shape.

Reaction-diffusion systems in intracellular molecular transport and control

Chemical reactions make cells work only if the participating chemicals are delivered to desired locations in a timely and precise fashion. Most research to date has focused on active-transport mechanisms, although passive diffusion is often equally rapid and energetically less costly. Capitalizing on these advantages, cells have developed sophisticated reaction-diffusion (RD) systems that control a wide range of cellular functions-from chemotaxis and cell division, through signaling cascades and oscillations, to cell motility. These apparently diverse systems share many common features and are "wired" according to "generic" motifs such as nonlinear kinetics, autocatalysis, and feedback loops. Understanding the operation of these complex (bio)chemical systems requires the analysis of pertinent transport-kinetic equations or, at least on a qualitative level, of the characteristic times of the constituent subprocesses. Therefore, in reviewing the manifestations of cellular RD, we also describe basic theory of reaction-diffusion phenomena.

The spectraplakin Short stop is an actin-microtubule cross-linker that contributes to organization of the microtubule network

The dynamics of actin and microtubules are coordinated in a variety of cellular and morphogenetic processes; however, little is known about the molecules mediating this cytoskeletal cross-talk. We are studying Short stop (Shot), the sole Drosophila spectraplakin, as a model actin-microtubule cross-linking protein. Spectraplakins are an ancient family of giant cytoskeletal proteins that are essential for a diverse set of cellular functions; yet, we know little about the dynamics of spectraplakins and how they bridge actin filaments and microtubules. In this study we describe the intracellular dynamics of Shot and a structure-function analysis of its role as a cytoskeletal cross-linker. We find that Shot interacts with microtubules using two different mechanisms. In the cell interior, Shot binds growing plus ends through an interaction with EB1. In the cell periphery, Shot associates with the microtubule lattice via its GAS2 domain, and this pool of Shot is actively engaged as a cross-linker via its NH(2)-terminal actin-binding calponin homology domains. This cross-linking maintains microtubule organization by resisting forces that produce lateral microtubule movements in the cytoplasm. Our results provide the first description of the dynamics of these important proteins and provide key insight about how they function during cytoskeletal cross-talk.

Self-assembling SAS-6 multimer is a core centriole building block

Centrioles are conserved microtubule-based organelles with 9-fold symmetry that are essential for cilia and mitotic spindle formation. A conserved structure at the onset of centriole assembly is a "cartwheel" with 9-fold radial symmetry and a central tubule in its core. It remains unclear how the cartwheel is formed. The conserved centriole protein, SAS-6, is a cartwheel component that functions early in centriole formation. Here, combining biochemistry and electron microscopy, we characterize SAS-6 and show that it self-assembles into stable tetramers, which serve as building blocks for the central tubule. These results suggest that SAS-6 self-assembly may be an initial step in the formation of the cartwheel that provides the 9-fold symmetry. Electron microscopy of centrosomes identified 25-nm central tubules with repeating subunits and show that SAS-6 concentrates at the core of the cartwheel. Recombinant and native SAS-6 self-oligomerizes into tetramers with approximately 6-nm subunits, and these tetramers are components of the centrosome, suggesting that tetramers are the building blocks of the central tubule. This is further supported by the observation that elevated levels of SAS-6 in Drosophila cells resulted in higher order structures resembling central tubule morphology. Finally, in the presence of embryonic extract, SAS-6 tetramers assembled into high density complexes, providing a starting point for the eventual in vitro reconstruction of centrioles.

Assessment of mitotic spindle phenotypes in Drosophila S2 cells

The Drosophila S2 cell line is popularly used to study mitosis. In this cell line, multiple genes can be easily and efficiently knocked down by RNA interference (RNAi), and the associated mitotic phenotypes can be assessed with high-resolution microscopy after immunofluorescence or in a living cell. However, compared to untransformed cells in wild-type organisms such as yeasts or worms, mitosis in the S2 cell line is more variable and often looks abnormal even in RNAi-untreated cells. Therefore, in order to judge whether a phenotype is derived from RNAi of the target gene or is simply a variation of control cells, it is critical to prepare proper control samples and perform objective imaging and image analysis. Here, we discuss how bona fide mitotic phenotypes associated with RNAi can be identified, avoiding selecting false positives, in S2 cells.

Asymmetric microtubule function is an essential requirement for polarized organization of the Drosophila bristle

While previous studies have shown that microtubules (MTs) are essential for maintaining the highly biased axial growth of the Drosophila bristle, the mechanism for this process has remained vague. We report that the MT minus-end marker, Nod-KHC, accumulates at the bristle tip, suggesting that the MT network in the bristle is organized minus end out. Potential markers for studying the importance of properly polarized MTs to bristle axial growth are Ik2 and Spindle-F (Spn-F), since mutations in spn-F and ik2 affect bristle development. We demonstrate that Spn-F and Ik2 are localized to the bristle tip and that mutations in ik2 and spn-F affect bristle MT and actin organization. Specifically, mutation in ik2 affects polarized bristle MT function. It was previously found that the hook mutant exhibited defects in bristle polarity and that hook is involved in endocytic trafficking. We found that Hook is localized at the bristle tip and that this localization is affected in ik2 mutants, suggesting that the contribution of MTs within the bristle shaft is important for correct endocytic trafficking. Thus, our results show that MTs are organized in a polarized manner within the highly elongated bristle and that this organization is essential for biased bristle axial growth.