Search, capture and signal: games microtubules and centrosomes play

Centrosomes and associated proteins in pathogenesis and treatment of breast cancer

Breast cancer is the most prevalent malignancy among women worldwide. Despite significant advances in treatment, it remains one of the leading causes of female mortality. The inability to effectively treat advanced and/or treatment-resistant breast cancer demonstrates the need to develop novel treatment strategies and targeted therapies. Centrosomes and their associated proteins have been shown to play key roles in the pathogenesis of breast cancer and thus represent promising targets for drug and biomarker development. Centrosomes are fundamental cellular structures in the mammalian cell that are responsible for error-free execution of cell division. Centrosome amplification and aberrant expression of its associated proteins such as Polo-like kinases (PLKs), Aurora kinases (AURKs) and Cyclin-dependent kinases (CDKs) have been observed in various cancers, including breast cancer. These aberrations in breast cancer are thought to cause improper chromosomal segregation during mitosis, leading to chromosomal instability and uncontrolled cell division, allowing cancer cells to acquire new genetic changes that result in evasion of cell death and the promotion of tumor formation. Various chemical compounds developed against PLKs and AURKs have shown meaningful antitumorigenic effects in breast cancer cells in vitro and in vivo. The mechanism of action of these inhibitors is likely related to exacerbation of numerical genomic instability, such as aneuploidy or polyploidy. Furthermore, growing evidence demonstrates enhanced antitumorigenic effects when inhibitors specific to centrosome-associated proteins are used in combination with either radiation or chemotherapy drugs in breast cancer. This review focuses on the current knowledge regarding the roles of centrosome and centrosome-associated proteins in breast cancer pathogenesis and their utility as novel targets for breast cancer treatment.

Microtubules: Evolving roles and critical cellular interactions

Microtubules are cytoskeletal elements known as drivers of directed cell migration, vesicle and organelle trafficking, and mitosis. In this review, we discuss new research in the lens that has shed light into further roles for stable microtubules in the process of development and morphogenesis. In the lens, as well as other systems, distinct roles for characteristically dynamic microtubules and stabilized populations are coming to light. Understanding the mechanisms of microtubule stabilization and the associated microtubule post-translational modifications is an evolving field of study. Appropriate cellular homeostasis relies on not only one cytoskeletal element, but also rather an interaction between cytoskeletal proteins as well as other cellular regulators. Microtubules are key integrators with actin and intermediate filaments, as well as cell–cell junctional proteins and other cellular regulators including myosin and RhoGTPases to maintain this balance.

Impact statement

The role of microtubules in cellular functioning is constantly expanding. In this review, we examine new and exciting fields of discovery for microtubule’s involvement in morphogenesis, highlight our evolving understanding of differential roles for stabilized versus dynamic subpopulations, and further understanding of microtubules as a cellular integrator.

The actomyosin network is influenced by NMHC IIA and regulated by CrpF46, which is involved in controlling cell migration

When a cell migrates, the centrosome positions between the nucleus and the leading edge of migration via the microtubule system. The protein Crp^F46 (centrosome-related protein F46) has a known role during mitosis and centrosome duplication. However, how Crp^F46 efficiently regulates centrosome-related cell migration is unclear. Here, we report that knockdown of Crp^F46 resulted in the disruption of microtubule arrangement, with impaired centrosomal reorientation, and slowed down cell migration. In cells that express low levels of Crp^F46, stress fibers were weakened, which could be rescued by recovering Flag-Crp^F46. We also found that Crp^F46 interacted with non-muscle myosin high chain IIA (NMHC IIA) and that its three coiled-coil domains are pivotal for its binding to NMHC IIA. Additionally, analyses of phosphorylation of NMHC IIA and RLC (regulatory light chain) demonstrated that Crp^F46 was associated with myosin IIA during filament formation. Indirect immunofluorescence images indicated that NM IIA filaments were inhibited when Crp^F46 was under-expressed. Thus, Crp^F46 regulates cell migration by centrosomal reorientation and altering the function of the actomyosin network by controlling specific phosphorylation of myosin.

Activation of the motor protein upon attachment: Anchors weigh in on cytoplasmic dynein regulation

Cytoplasmic dynein is the major minus-end-directed motor protein in eukaryotes, and has functions ranging from organelle and vesicle transport to spindle positioning and orientation. The mode of regulation of dynein in the cell remains elusive, but a tantalising possibility is that dynein is maintained in an inhibited, non-motile state until bound to cargo. In vivo, stable attachment of dynein to the cell membrane via anchor proteins enables dynein to produce force by pulling on microtubules and serves to organise the nuclear material. Anchor proteins of dynein assume diverse structures and functions and differ in their interaction with the membrane. In yeast, the anchor protein has come to the fore as one of the key mediators of dynein activity. In other systems, much is yet to be discovered about the anchors, but future work in this area will prove invaluable in understanding dynein regulation in the cell.

Structural and biochemical characterization of the interaction between LGN and Frmpd1

The tetratricopeptide repeat (TPR) motif-containing protein LGN binds multiple targets and regulates their subcellular localizations and functions during both asymmetric and symmetric cell divisions. Here, we characterized the interaction between LGN-TPR motifs and FERM and PDZ domain containing 1 (Frmpd1) and reported the crystal structure of the complex at 2.4Å resolution. A highly conserved fragment at the center of Frmpd1 of ~20 residues was found to be necessary and sufficient to bind to LGN-TPR. This Frmpd1 fragment forms an extended structure and runs along the concave channel of the TPR superhelix in an antiparallel manner in the complex. Structural comparisons and biochemical studies of LGN/Frmpd1 and other known LGN/target interactions demonstrate that the LGN-TPR motifs are versatile and capable of recognizing multiple targets via diverse binding modes. Nevertheless, a conserved "E/QxEx4-5E/D/Qx1-2K/R" motif in LGN/Pins (partner of inscuteable) TPR binding proteins has been identified.

Polar body emission

Generation of a haploid female germ cell, the egg, consists of two rounds of asymmetric cell division (meiosis I and meiosis II), yielding two diminutive and nonviable polar bodies and a large haploid egg. Animal eggs are also unique in the lack of centrioles and therefore form meiotic spindles without the pre-existence of the two dominant microtubule organizing centers (centrosomes) found in mitosis. Meiotic spindle assembly is further complicated by the unique requirement of sister chromatid mono-oriented in meiosis I. Nonetheless, the eggs appear to adopt many of the same proteins and mechanisms described in mitosis, with necessary modifications to accommodate their special needs. Unraveling these special modifications will not only help understanding animal reproduction, but should also enhance our understanding of cell division in general.

Epothilone B inhibits migration of glioblastoma cells by inducing microtubule catastrophes and affecting EB1 accumulation at microtubule plus ends

Invasion of normal brain tissue by tumor cells is a major contributing factor to the recurrence of glioblastoma and its resistance to therapy. Here, we have assessed the efficacy of the microtubule (MT) targeting agent Epothilone B (patupilone) on glioblastoma cell migration, a prerequisite for invasive tumor cell behavior. At non-cytotoxic concentrations, patupilone inhibited glioblastoma cell movement, as shown by transwell cell migration, random motility and spheroid assays. This anti-migratory effect was associated with a reduced accumulation of EB1 and other MT plus end tracking proteins at MT ends and with the induction of MT catastrophes, while the MT growth rate and other MT dynamic instability parameters remained unaltered. An increase in MT catastrophes led to the reduction of the number of MTs reaching the leading edge. Analysis of the effect of patupilone on MT dynamics in a reconstituted in vitro system demonstrated that the induction of MT catastrophes and an alteration of EB1 accumulation at MT plus end are intrinsic properties of patupilone activity. We have thus demonstrated that patupilone antagonizes glioblastoma cell migration by a novel mechanism, which is distinct from suppression of MT dynamic instability. Taken together, our results suggest that EB proteins may represent a new potential target for anti-cancer therapy in highly invasive tumors.

Positioning centrosomes and spindle poles: looking at the periphery to find the centre

Centrosome positioning is tightly controlled throughout the cell cycle and probably shares common regulatory mechanisms with spindle-pole positioning. In this article, we detail the possible mechanisms controlling centrosome and spindle positioning in various organisms both in interphase and mitotic cells, and discuss recent findings showing how microtubule plus-end-associated proteins interact with the cell cortex. We suggest that microtubule plus-end complexes simultaneously regulate microtubule dynamics and microtubule anchoring at the cell periphery to allow proper centrosome and spindle-pole positioning.

Why is the microtubule lattice helical?

Microtubules polymerize from identical tubulin heterodimers, which form a helical lattice pattern that is the microtubule. This pattern always has left-handed chirality, but it is not known why. But as tubulin, similar to other proteins, evolved for a purpose, the question of the title of this artcile appears to be meaningful. In a computer simulation that explores the 'counterfactual biology' of microtubules without helicity, we demonstrate that these have the same mechanical properties as Nature's microtubules with helicity. Thus only a dynamical reason for helicity is left as potential explanation. We find that helicity solves 'the problem of the blind mason', i.e. how to correctly build a structure, guided only by the shape of the bricks. This answer in turn raises some new questions for researchers to address.

FORMIN a link between kinetochores and microtubule ends

The mammalian diaphanous-related (mDia) formin proteins are well known for their actin-nucleation and filament-elongation activities in mediating actin dynamics. They also directly bind to microtubules and regulate microtubule stabilization at the leading edge of the cell during cell migration. Recently, the formin mDia3 was shown to associate with the kinetochore and to contribute to metaphase chromosome alignment, a process in which kinetochores form stable attachments with growing and shrinking microtubules. We suggest that the formin mDia3 could contribute to the regulation of kinetochore-bound microtubule dynamics, in coordination with attachment via its own microtubule-binding activity, as well as via its interaction with the tip-tracker EB1 (end-binding protein 1).

Aurora B regulates formin mDia3 in achieving metaphase chromosome alignment

Proper bipolar attachment of sister kinetochores to the mitotic spindle is critical for accurate chromosome segregation in mitosis. Here we show an essential role of the formin mDia3 in achieving metaphase chromosome alignment. This function is independent of mDia3 actin nucleation activity, but is attributable to EB1-binding by mDia3. Furthermore, the microtubule binding FH2 domain of mDia3 is phosphorylated by Aurora B kinase in vitro, and cells expressing the nonphosphorylatable mDia3 mutant cannot position chromosomes at the metaphase plate. Purified recombinant mDia3 phosphorylated by Aurora B exhibits reduced ability to bind microtubules and stabilize microtubules against cold-induced disassembly in vitro. Cells expressing the phosphomimetic mDia3 mutant do not form stable kinetochore microtubule fibers; despite they are able to congress chromosomes to the metaphase plate. These findings reveal a key role for mDia3 and its regulation by Aurora B phosphorylation in achieving proper stable kinetochore microtubule attachment.

The role of centrosomal Nlp in the control of mitotic progression and tumourigenesis

The human centrosomal ninein-like protein (Nlp) is a new member of the γ-tubulin complexes binding proteins (GTBPs) that is essential for proper execution of various mitotic events. The primary function of Nlp is to promote microtubule nucleation that contributes to centrosome maturation, spindle formation and chromosome segregation. Its subcellular localisation and protein stability are regulated by several crucial mitotic kinases, such as Plk1, Nek2, Cdc2 and Aurora B. Several lines of evidence have linked Nlp to human cancer. Deregulation of Nlp in cell models results in aberrant spindle, chromosomal missegregation and multinulei, and induces chromosomal instability and renders cells tumourigenic. Overexpression of Nlp induces anchorage-independent growth and immortalised primary cell transformation. In addition, we first demonstrate that the expression of Nlp is elevated primarily due to NLP gene amplification in human breast cancer and lung carcinoma. Consistently, transgenic mice overexpressing Nlp display spontaneous tumours in breast, ovary and testicle, and show rapid onset of radiation-induced lymphoma, indicating that Nlp is involved in tumourigenesis. This review summarises our current knowledge of physiological roles of Nlp, with an emphasis on its potentials in tumourigenesis.

Mutation of Ser172 in yeast β tubulin induces defects in microtubule dynamics and cell division

Ser172 of β tubulin is an important residue that is mutated in a human brain disease and phosphorylated by the cyclin-dependent kinase Cdk1 in mammalian cells. To examine the role of this residue, we used the yeast S. cerevisiae as a model and produced two different mutations (S172A and S172E) of the conserved Ser172 in the yeast β tubulin Tub2p. The two mutants showed impaired cell growth on benomyl-containing medium and at cold temperatures, altered microtubule (MT) dynamics, and altered nucleus positioning and segregation. When cytoplasmic MT effectors Dyn1p or Kar9p were deleted in S172A and S172E mutants, cells were viable but presented increased ploidy. Furthermore, the two β tubulin mutations exhibited synthetic lethal interactions with Bik1p, Bim1p or Kar3p, which are effectors of cytoplasmic and spindle MTs. In the absence of Mad2p-dependent spindle checkpoint, both mutations are deleterious. These findings show the importance of Ser172 for the correct function of both cytoplasmic and spindle MTs and for normal cell division.

The Cdc42 GEF Intersectin 2 controls mitotic spindle orientation to form the lumen during epithelial morphogenesis

Epithelial organs are made of tubes and cavities lined by a monolayer of polarized cells that enclose the central lumen. Lumen formation is a crucial step in the formation of epithelial organs. The Rho guanosine triphosphatase (GTPase) Cdc42, which is a master regulator of cell polarity, regulates the formation of the central lumen in epithelial morphogenesis. However, how Cdc42 is regulated during this process is still poorly understood. Guanine nucleotide exchange factors (GEFs) control the activation of small GTPases. Using the three-dimensional Madin-Darby canine kidney model, we have identified a Cdc42-specific GEF, Intersectin 2 (ITSN2), which localizes to the centrosomes and regulates Cdc42 activation during epithelial morphogenesis. Silencing of either Cdc42 or ITSN2 disrupts the correct orientation of the mitotic spindle and normal lumen formation, suggesting a direct relationship between these processes. Furthermore, we demonstrated this direct relationship using LGN, a component of the machinery for mitotic spindle positioning, whose disruption also results in lumen formation defects.

Spatial uncoupling of mitosis and cytokinesis during appressorium-mediated plant infection by the rice blast fungus Magnaporthe oryzae

To infect plants, many pathogenic fungi develop specialized infection structures called appressoria. Here, we report that appressorium development in the rice blast fungus Magnaporthe oryzae involves an unusual cell division, in which nuclear division is spatially uncoupled from the site of cytokinesis and septum formation. The position of the appressorium septum is defined prior to mitosis by formation of a heteromeric septin ring complex, which was visualized by spatial localization of Septin4:green fluorescent protein (GFP) and Septin5:GFP fusion proteins. Mitosis in the fungal germ tube is followed by long-distance nuclear migration and rapid formation of an actomyosin contractile ring in the neck of the developing appressorium, at a position previously marked by the septin complex. By contrast, mutants impaired in appressorium development, such as Deltapmk1 and DeltacpkA regulatory mutants, undergo coupled mitosis and cytokinesis within the germ tube. Perturbation of the spatial control of septation, by conditional mutation of the SEPTATION-ASSOCIATED1 gene of M. oryzae, prevented the fungus from causing rice blast disease. Overexpression of SEP1 did not affect septation during appressorium formation, but instead led to decoupling of nuclear division and cytokinesis in nongerminated conidial cells. When considered together, these results indicate that SEP1 is essential for determining the position and frequency of cell division sites in M. oryzae and demonstrate that differentiation of appressoria requires a cytokinetic event that is distinct from cell divisions within hyphae.

Spectrin mutations that cause spinocerebellar ataxia type 5 impair axonal transport and induce neurodegeneration in Drosophila

How spectrin mutations caused Purkinje cell death becomes clearer following studies that examined the effect of expressing mutant SCA5 in the fly eye. Mutant spectrin causes deficits in synapse formation at the neuromuscular junction and disrupts vesicular trafficking.

Spinocerebellar ataxia type 5 (SCA5) is an autosomal dominant neurodegenerative disorder caused by mutations in the SPBTN2 gene encoding β-III–spectrin. To investigate the molecular basis of SCA5, we established a series of transgenic Drosophila models that express human β-III–spectrin or fly β-spectrin proteins containing SCA5 mutations. Expression of the SCA5 mutant spectrin in the eye causes a progressive neurodegenerative phenotype, and expression in larval neurons results in posterior paralysis, reduced synaptic terminal growth, and axonal transport deficits. These phenotypes are genetically enhanced by both dynein and dynactin loss-of-function mutations. In summary, we demonstrate that SCA5 mutant spectrin causes adult-onset neurodegeneration in the fly eye and disrupts fundamental intracellular transport processes that are likely to contribute to this progressive neurodegenerative disease.

Role of the nuclear migration protein Lis1 in cell morphogenesis in Ustilago maydis

Ustilago maydis is a basidiomycete fungus that exhibits a yeast-like and a filamentous form. Growth of the fungus in the host leads to additional morphological transitions. The different morphologies are characterized by distinct nuclear movements. Dynein and alpha-tubulin are required for nuclear movements and for cell morphogenesis of the yeast-like form. Lis1 is a microtubule plus-end tracking protein (+TIPs) conserved in eukaryotes and required for nuclear migration and spindle positioning. Defects in nuclear migration result in altered cell fate and aberrant development in metazoans, slow growth in fungi and disease in humans (e.g. lissencephaly). Here we investigate the role of the human LIS1 homolog in U. maydis and demonstrate that it is essential for cell viability, not previously seen in other fungi. With a conditional null mutation we show that lis1 is necessary for nuclear migration in the yeast-like cell and during the dimorphic transition. Studies of asynchronous exponentially growing cells and time-lapse microscopy uncovered novel functions of lis1: It is necessary for cell morphogenesis, positioning of the septum and cell wall integrity. lis1-depleted cells exhibit altered axes of growth and loss of cell polarity leading to grossly aberrant cells with clusters of nuclei and morphologically altered buds devoid of nuclei. Altered septum positioning and cell wall deposition contribute to the aberrant morphology. lis1-depleted cells lyse, indicative of altered cell wall properties or composition. We also demonstrate, with indirect immunofluorescence to visualize tubulin, that lis1 is necessary for the normal organization of the microtubule cytoskeleton: lis1-depleted cells contain more and longer microtubules that can form coils perpendicular to the long axis of the cell. We propose that lis1 controls microtubule dynamics and thus the regulated delivery of vesicles to growth sites and other cell domains that govern nuclear movements.

Spindle orientation bias in gut epithelial stem cell compartments is lost in precancerous tissue

The importance of asymmetric divisions for stem cell function and maintenance is well established in the developing nervous system and the skin; however, its role in gut epithelium and its importance for tumorigenesis is still debated. We demonstrate alignment of mitotic spindles perpendicular to the apical surface specifically in the stem cell compartments of mouse and human intestine and colon. This orientation correlates with the asymmetric retention of label-retaining DNA. Both the preference for perpendicular spindle alignment and asymmetric label retention are lost in precancerous tissue heterozygous for the adenomatous polyposis coli tumor suppressor (Apc). This loss correlates with cell shape changes specifically in the stem cell compartment. Our data suggest that loss of asymmetric division in stem cells might contribute to the oncogenic effect of Apc mutations in gut epithelium.

Imaging and analysis of the microtubule cytoskeleton in giardia

Giardia intestinalis, a common parasitic protist, possesses a complex microtubule cytoskeleton critical for cellular function and transitioning between the cyst and trophozoite life cycle stages. The giardial microtubule cytoskeleton is comprised of highly dynamic and stable structures. Novel microtubule structures include the ventral disc that is essential for the parasite's attachment to the intestinal villi to avoid peristalsis. The completed Giardia genome combined with new molecular genetic tools and live imaging will aid in the characterization and analysis of cytoskeletal dynamics in Giardia. Fundamental areas of giardial cytoskeletal biology remain to be explored and knowledge of the molecular mechanisms of cytoskeletal functioning is needed to better understand Giardia's unique biology and pathogenesis.

Positioning cytokinesis

Cytokinesis is the terminal step of the cell cycle during which a mother cell divides into daughter cells. Often, the machinery of cytokinesis is positioned in such a way that daughter cells are born roughly equal in size. However, in many specialized cell types or under certain environmental conditions, the cell division machinery is placed at nonmedial positions to produce daughter cells of different sizes and in many cases of different fates. Here we review the different mechanisms that position the division machinery in prokaryotic and eukaryotic cell types. We also describe cytokinesis-positioning mechanisms that are not adequately explained by studies in model organisms and model cell types.

The role of APC in mitosis and in chromosome instability

The established role of APC in regulating microtubules and actin in polarized epithelia naturally raises the possibility that APC similarly influences the mitotic cytoskeleton. The recent accumulation of experimental evidence in mitotic cells supports this supposition. APC associates with mitotic spindle microtubules, most notably at the plus-ends of microtubules that interact with kinetochores. Genetic experiments implicate APC in the regulation of spindle microtubule dynamics, probably through its interaction with the microtubule plus-end binding protein, EB1. Moreover, functional data show that APC modulates kinetochore-microtubule attachments and is required for the spindle checkpoint to detect transiently misaligned chromosomes. Together this evidence points to a role for APC in maintaining mitotic fidelity. Such a role is particularly significant when considered in the context of the chromosome instability observed in colorectal tumors bearing mutations in APC. The prevalence of APC truncation mutants in colorectal tumors and the ability of these alleles to act dominantly to inhibit the mitotic spindle place chromosome instability at the earliest stage of colorectal cancer progression (i.e., prior to deregulation of beta-catenin). This may contribute to the autosomal dominant predisposition of patients with familial adenomatous polyposis to develop colon cancer. In this chapter, we will review the literature linking APC to regulation of mitotic fidelity and discuss the implications for dividing epithelial cells in the intestine.

Beyond polymer polarity: how the cytoskeleton builds a polarized cell

Cell polarity relies on the asymmetric organization of cellular components and structures. Actin and microtubules are well suited to provide the structural basis for cell polarization because of their inherent structural polarity along the polymer lattices and intrinsic dynamics that allow them to respond rapidly to polarity cues. In general, the actin cytoskeleton drives the symmetry-breaking process that enables the establishment of a polarized distribution of regulatory molecules, whereas microtubules build on this asymmetry and maintain the stability of the polarized organization. Crosstalk coordinates the functions of the two cytoskeletal systems.

Organelle positioning and cell polarity

In spite of conspicuous differences in their polarized architecture, swimming unicellular eukaryotes and migrating cells from metazoa display a conserved hierarchical interlocking of the main cellular compartments, in which the microtubule network has a dominant role. A microtubule array can organize the distribution of endomembranes owing to a cell-wide and polarized extension around a unique nucleus-associated structure. The nucleus-associated structure in animal cells contains a highly conserved organelle, the centriole or basal body. This organelle has a defined polarity that can be transmitted to the cell. Its conservative mode of duplication seems to be a core mechanism for the transmission of polarities through cell division.

Giardia lamblia EB1 is a functional homolog of yeast Bim1p that binds to microtubules

Giardia lamblia, with two nuclei and a distinct polarized morphology, is an interesting organism for investigating how distribution of its microtubule (MT) is controlled during its cell cycle. In this study, we identified the end-binding protein 1 (EB1) of G. lamblia, a well-known microtubule-associated protein that organizes MTs in eukaryotes. Immunofluorescence assays using recombinant EB1 (rEB1)-specific antibodies demonstrated EB1 localization in nuclear membrane as well as in some cytoskeletal structures such as axomenes and median bodies of trophozoites of G. lamblia. Complementation experiments using the BIM1 knock-out mutant of yeast, the yeast homolog of mammalian EB1, showed that giardial EB1 was able to carry out a homologous function in controlling MT dynamics. In addition, rEB1 of G. lamblia co-precipitated with MTs by an in vitro binding assay, thereby demonstrating that G. lamblia EB1 is a MT-associated protein. These results, taken together, suggest that G. lamblia EB1 is a functional homolog of eukaryotic EB1 and is likely to be a determinant for MT distribution.

Src and Wnt signaling regulate dynactin accumulation to the P2-EMS cell border in C. elegans embryos

In many organisms, the dynein-dynactin complex is required for the alignment of the mitotic spindle onto the axis of polarity of a cell undergoing asymmetric cell division. How this complex transduces polarity cues, either intrinsic or extrinsic, and rotationally aligns the spindle accordingly is not well understood. The Caenorhabditis elegans blastomere P2 polarizes the neighboring EMS blastomere, which causes the EMS spindle to rotationally align along the defined axis of polarity via two redundant signaling pathways: Wnt and Src. Here, we describe how components of the dynactin complex became locally enriched at the P2-EMS border prior to and during rotational alignment of their spindles. Wnt and Src signaling were required for both localized dynactin enrichment, and for rotational alignment of the P2 and EMS spindles. Depleting the trimeric G-protein subunit G alpha did not abolish dynactin accumulation to the P2-EMS border, yet both EMS and P2 spindles failed to rotationally align, indicating that G alpha might act to regulate dynein/dynactin motor activity. By RNAi of a weak dnc-1(ts) allele, we showed that dynactin activity was required at least for EMS spindle rotational alignment.

Kinesin-13 regulates flagellar, interphase, and mitotic microtubule dynamics in Giardia intestinalis

Microtubule depolymerization dynamics in the spindle are regulated by kinesin-13, a nonprocessive kinesin motor protein that depolymerizes microtubules at the plus and minus ends. Here we show that a single kinesin-13 homolog regulates flagellar length dynamics, as well as other interphase and mitotic dynamics in Giardia intestinalis, a widespread parasitic diplomonad protist. Both green fluorescent protein-tagged kinesin-13 and EB1 (a plus-end tracking protein) localize to the plus ends of mitotic and interphase microtubules, including a novel localization to the eight flagellar tips, cytoplasmic anterior axonemes, and the median body. The ectopic expression of a kinesin-13 (S280N) rigor mutant construct caused significant elongation of the eight flagella with significant decreases in the median body volume and resulted in mitotic defects. Notably, drugs that disrupt normal interphase and mitotic microtubule dynamics also affected flagellar length in Giardia. Our study extends recent work on interphase and mitotic kinesin-13 functioning in metazoans to include a role in regulating flagellar length dynamics. We suggest that kinesin-13 universally regulates both mitotic and interphase microtubule dynamics in diverse microbial eukaryotes and propose that axonemal microtubules are subject to the same regulation of microtubule dynamics as other dynamic microtubule arrays. Finally, the present study represents the first use of a dominant-negative strategy to disrupt normal protein function in Giardia and provides important insights into giardial microtubule dynamics with relevance to the development of antigiardial compounds that target critical functions of kinesins in the giardial life cycle.

TheDrosophilaCLASP homologue, Mast/Orbit regulates the dynamic behaviour of interphase microtubules by promoting the pause state

An important group of microtubule associated proteins are the plus-end tracking proteins which includes the Mast/Orbit/CLASPs family amongst others. Several of these proteins have important functions during interphase and mitosis in the modulation of the dynamic properties of microtubules, however, the precise mechanism remains to be elucidated. To investigate the role of Mast in the regulation of microtubule behaviour during interphase, we used RNAi in Drosophila S2 culture cells stably expressing GFP-alpha-tubulin and followed the behaviour of microtubules in vivo. Mast depleted cells show a significant reduction of microtubule density and an abnormal interphase microtubule array that rarely reaches the cell cortex. Analysis of the dynamic parameters revealed that in the absence of Mast, microtubules are highly dynamic, constantly growing or shrinking. These alterations are characterized by a severe reduction in the transition frequencies to and from the pause state. Moreover, analysis of de novo microtubule polymerization after cold treatment showed that Mast is not required for nucleation since Mast depleted cells nucleate microtubules soon after return to normal temperature. Taken together these results suggest that Mast plays an essential role in reducing the dynamic behaviour of microtubules by specifically promoting the pause state.

Microtubule plus-end-tracking proteins target gap junctions directly from the cell interior to adherens junctions

Gap junctions are intercellular channels that connect the cytoplasms of adjacent cells. For gap junctions to properly control organ formation and electrical synchronization in the heart and the brain, connexin-based hemichannels must be correctly targeted to cell-cell borders. While it is generally accepted that gap junctions form via lateral diffusion of hemichannels following microtubule-mediated delivery to the plasma membrane, we provide evidence for direct targeting of hemichannels to cell-cell junctions through a pathway that is dependent on microtubules; through the adherens-junction proteins N-cadherin and beta-catenin; through the microtubule plus-end-tracking protein (+TIP) EB1; and through its interacting protein p150(Glued). Based on live cell microscopy that includes fluorescence recovery after photobleaching (FRAP), total internal reflection fluorescence (TIRF), deconvolution, and siRNA knockdown, we propose that preferential tethering of microtubule plus ends at the adherens junction promotes delivery of connexin hemichannels directly to the cell-cell border. These findings support an unanticipated mechanism for protein delivery to points of cell-cell contact.

Generation of noncentrosomal microtubule arrays

In most proliferating and migrating animal cells, the centrosome is the main site for microtubule (MT) nucleation and anchoring, leading to the formation of radial MT arrays in which MT minus ends are anchored at the centrosomes and plus ends extend to the cell periphery. By contrast, in most differentiated animal cell types, including muscle, epithelial and neuronal cells, as well as most fungi and vascular plant cells, MTs are arranged in noncentrosomal arrays that are non-radial. Recent studies suggest that these noncentrosomal MT arrays are generated by a three step process. The initial step involves formation of noncentrosomal MTs by distinct mechanisms depending on cell type: release from the centrosome, catalyzed nucleation at noncentrosomal sites or breakage of pre-existing MTs. The second step involves transport by MT motor proteins or treadmilling to sites of assembly. In the final step, the noncentrosomal MTs are rearranged into cell-type-specific arrays by bundling and/or capture at cortical sites, during which MTs acquire stability. Despite their relative stability, the final noncentrosomal MT arrays may still exhibit dynamic properties and in many cases can be remodeled.

Microtubule dynamics in root hairs of Medicago truncatula

The microtubular cytoskeleton plays an important role in the development of tip-growing plant cells, but knowledge about its dynamics is incomplete. In this study, root hairs of the legume Medicago truncatula have been chosen for a detailed analysis of microtubular cytoskeleton dynamics using GFP-MBD and EB1-YFP as markers and 4D imaging. The microtubular cytoskeleton appears mainly to be composed of bundles which form tracks along which new microtubules polymerise. Polymerisation rates of microtubules are highest in the tip of growing root hairs. Treatment of root hairs with Nod factor and latrunculin B result in a twofold decrease in polymerisation rate. Nonetheless, no direct, physical interaction between the actin filament cytoskeleton and microtubules could be observed. A new picture of how the plant cytoskeleton is organised in apically growing root hairs emerges from these observations, revealing similarities with the organisation in other, non-plant, tip-growing cells.

Role of microtubules in tip growth of fungi

Polarized cell growth is observed ubiquitously in all living organisms. Tip growth of filamentous fungi serves as a typical model for polar growth. It is well known that the actin cytoskeleton plays a central role in cellular growth. In contrast, the role of microtubules in polar growth of fungal tip cells has not been critically addressed. Our recent study, using a green fluorescent protein (GFP)-labeled tubulin-expressing strain of the filamentous fungus Aspergillus nidulans and treatment with an anti-microtubule reagent, revealed that microtubules are essential for rapid hyphal growth. Our results indicated that microtubule organization contributes to continuous tip growth throughout the cell cycle, which in turn enables the maintenance of an appropriate mass of cytoplasm for the multinucleate system. In filamentous fungi, the microtubule is an essential component of the tip growth machinery that enables continuous and rapid growth. Recent research developments are starting to elucidate the components of the tip growth machinery and their functions in many organisms. This recent knowledge, in turn, is starting to enhance the importance of fungal systems as simple model systems to understand the polar growth of cells.

Principles and Implementations of Dissipative (Dynamic) Self-Assembly

Dynamic self-assembly (DySA) processes occurring outside of thermodynamic equilibrium underlie many forms of adaptive and intelligent behaviors in natural systems. Relatively little, however, is known about the principles that govern DySA and the ways in which it can be extended to artificial ensembles. This article discusses recent advances in both the theory and the practice of nonequilibrium self-assembly. It is argued that a union of ideas from thermodynamics and dynamic systems' theory can provide a general description of DySA. In parallel, heuristic design rules can be used to construct DySA systems of increasing complexities based on a variety of suitable interactions/potentials on length scales from nanoscopic to macroscopic. Applications of these rules to magnetohydrodynamic DySA are also discussed.

Midbodies and phragmoplasts: analogous structures involved in cytokinesis

Cytokinesis is an event common to all organisms that involves the precise coordination of independent pathways involved in cell-cycle regulation and microtubule, membrane, actin and organelle dynamics. In animal cells, the spindle midzone/midbody with associated endo-membrane system are required for late cytokinesis events, including furrow ingression and scission. In plants, cytokinesis is mediated by the phragmoplast, an array of microtubules, actin filaments and associated molecules that act as a framework for the future cell wall. In this article (which is part of the Cytokinesis series), we discuss recent studies that highlight the increasing number of similarities in the components and function of the spindle midzone/midbody in animals and the phragmoplast in plants, suggesting that they might be analogous structures.

Cortical capture of microtubules and spindle polarity in budding yeast - where's the catch?

In asymmetric divisions, the mitotic spindle must align according to the cell polarity axis. This is achieved through targeting astral microtubules emanating from each spindle pole to opposite cell cortex compartments. Saccharomyces cerevisiae is a powerful genetic model for dissection of this complex process. Intense research in this yeast has rendered detailed models for a program linking actin organization and spindle orientation along the mother-bud axis. This program requires the separate contributions of Kar9p, a protein guiding microtubules along polarized actin cables, and the polarity determinant Bud6p/Aip3 that marks sites for cortical capture at the bud tip and bud neck. In an added layer of complexity, cyclin-dependent kinase (Cdk) differentially regulates spindle pole function to dictate asymmetric spindle pole fate. Asymmetric contacts established by the spindle poles impart a further layer of extrinsic asymmetry restricting recruitment of Kar9p to the pole destined for the daughter cell. As a result, astral microtubules from a single pole are guided to the bud compartment after spindle assembly. Finally, Cdk might also translocate along astral microtubules in association with Kar9p to modulate microtubule-cortex interactions following spindle alignment. Insertion of the mitotic spindle into the bud neck is driven by the microtubule motor dynein. This process relies on the combined action of microtubule-plus-end-tracking proteins and kinesins that control the cell-cycle-dependent abundance of dynein at microtubule plus ends. Thus, this actin-independent pathway for spindle orientation might also be influenced by Cdk.

Kinesin-1 mediates translocation of the meiotic spindle to the oocyte cortex through KCA-1, a novel cargo adapter

In animals, female meiotic spindles are attached to the egg cortex in a perpendicular orientation at anaphase to allow the selective disposal of three haploid chromosome sets into polar bodies. We have identified a complex of interacting Caenorhabditis elegans proteins that are involved in the earliest step in asymmetric positioning of anastral meiotic spindles, translocation to the cortex. This complex is composed of the kinesin-1 heavy chain orthologue, UNC-116, the kinesin light chain orthologues, KLC-1 and -2, and a novel cargo adaptor, KCA-1. Depletion of any of these subunits by RNA interference resulted in meiosis I metaphase spindles that remained stationary at a position several micrometers from the cell cortex during the time when wild-type spindles translocated to the cortex. After this prolonged stationary period, unc-116(RNAi) spindles moved to the cortex through a partially redundant mechanism that is dependent on the anaphase-promoting complex. This study thus reveals two sequential mechanisms for translocating anastral spindles to the oocyte cortex.

Spindle positioning by cortical pulling forces

Proper spatial control of the cell division plane is essential to any developing organism. In most cell types, the relative size of the two daughter cells is determined by the position of the mitotic spindle within the geometry of the mother cell. We review the underlying mechanisms responsible for positioning of the mitotic spindle, both in cases where the spindle is placed in the center of the cell and in cases where the spindle is placed away from the center of the cell. We discuss the idea that cortical pulling forces are sufficient to provide a general mechanism for spindle positioning within symmetrically and asymmetrically dividing cells.

The Rho GTP exchange factor Lfc promotes spindle assembly in early mitosis

Rho GTPases regulate reorganization of actin and microtubule cytoskeletal structures during both interphase and mitosis. The timing and subcellular compartment in which Rho GTPases are activated is controlled by the large family of Rho GTP exchange factors (RhoGEFs). Here, we show that the microtubule-associated RhoGEF Lfc is required for the formation of the mitotic spindle during prophase/prometaphase. The inability of cells to assemble a functioning spindle after Lfc inhibition resulted in a delay in mitosis and an accumulation of prometaphase cells. Inhibition of Lfc's primary target Rho GTPase during prophase/prometaphase, or expression of a catalytically inactive mutant of Lfc, also prevented normal spindle assembly and resulted in delays in mitotic progression. Coinjection of constitutively active Rho GTPase rescued the spindle defects caused by Lfc inhibition, suggesting the requirement of RhoGTP in regulating spindle assembly. Lastly, we implicate mDia1 as an important effector of Lfc signaling. These findings demonstrate a role for Lfc, Rho, and mDia1 during mitosis.

Spatial re-organisation of cortical microtubules in vivo during polarisation and asymmetric division of Fucus zygotes

Fucus zygotes polarise and germinate a rhizoid before their first asymmetrical division. The role of microtubules (MTs) in orienting the first division plane has been extensively studied by immunofluorescence approaches. In the present study, the re-organisation of MT arrays during the development of Fucus zygotes and embryos was followed in vivo after microinjection of fluorescent tubulin. A dynamic cortical MT array that shows dramatic reorganization during zygote polarization was detected for the first time. Randomly distributed cortical MTs were redistributed to the presumptive rhizoid site by the time of polarisation and well before rhizoid germination. The cortical MT re-organisation occurs independently of centrosome separation and nucleation. By the time of mitosis the cortical array depolymerised to cortical foci in regions from which it also reformed following mitosis, suggesting that it is nucleated from cortical sites. We confirm previous indications from immunodetection studies that centrosomal alignment and nuclear rotation occur via MT connexions to stabilised cortical sites and that definitive alignment is post-metaphasic. Finally, we show that cortical MTs align parallel to the growth axis during rhizoid tip growth and our results suggest that they may be involved in regulating rhizoid growth by shaping the rhizoid and containing turgor pressure.

Microtubule-associated proteins and their essential roles during mitosis

Microtubules play essential roles during mitosis, including chromosome capture, congression, and segregation. In addition, microtubules are also required for successful cytokinesis. At the heart of these processes is the ability of microtubules to do work, a property that derives from their intrinsic dynamic behavior. However, if microtubule dynamics were not properly regulated, it is certain that microtubules alone could not accomplish any of these tasks. In vivo, the regulation of microtubule dynamics is the responsibility of microtubule-associated proteins. Among these, we can distinguish several classes according to their function: (1) promotion and stabilization of microtubule polymerization, (2) destabilization or severance of microtubules, (3) functioning as linkers between various structures, or (4) motility-related functions. Here we discuss how the various properties of microtubule-associated proteins can be used to assemble an efficient mitotic apparatus capable of ensuring the bona fide transmission of the genetic information in animal cells.

Tumor suppressor RASSF1A is a microtubule-binding protein that stabilizes microtubules and induces G2/M arrest

RASSF1A is a putative tumor suppressor gene that is inactivated in a variety of human tumors. Expression of exogenous RASSF1A has been shown to inhibit tumor growth in vitro and in animals. However, the molecular mechanisms by which RASSF1A mediates its tumor suppressive effects remain to be elucidated. Here, we report that RASSF1A is a microtubule-binding protein that interacts with and stabilizes microtubules. We have identified the RASSF1A region harboring a basic domain that appears to mediate the interactions between RASSF1A and microtubules. The basic domain-containing RASSF1C isoform also interacts with and stabilizes microtubules. We further show that in addition to G1 arrest, RASSF1A promotes growth arrest in the G2/M phase of the cell cycle and endogenous RASSF1A also interacts with microtubules. Based on our results, we propose that RASSF1A may mediate its tumor suppressive effects by inducing growth arrest in the G1 and G2/M phases. Together, these results provide important new insights into the molecular mechanisms by which this novel tumor suppressor mediates its biological effects.

Analysis of a spindle pole body mutant reveals a defect in biorientation and illuminates spindle forces

The spindle pole body (SPB) is the microtubule organizing center in Saccharomyces cerevisiae. An essential task of the SPB is to ensure assembly of the bipolar spindle, which requires a proper balancing of forces on the microtubules and chromosomes. The SPB component Spc110p connects the ends of the spindle microtubules to the core of the SPB. We previously reported the isolation of a mutant allele spc110-226 that causes broken spindles and SPB disintegration 30 min after spindle formation. By live cell imaging of mutant cells with green fluorescent protein (GFP)-Tub1p or Spc97p-GFP, we show that spc110-226 mutant cells have early defects in spindle assembly. Short spindles form but do not advance to the 1.5-microm stage and frequently collapse. Kinetochores are not arranged properly in the mutant cells. In 70% of the cells, no stable biorientation occurs and all kinetochores are associated with only one SPB. Examination of the SPB remnants by electron microscopy tomography and fluorescence microscopy revealed that the Spc110-226p/calmodulin complex is stripped off of the central plaque of the SPB and coalesces to from a nucleating structure in the nucleoplasm. The central plaque components Spc42p and Spc29p remain behind in the nuclear envelope. The delamination is likely due to a perturbed interaction between Spc42p and Spc110-226p as detected by fluorescence resonance energy transfer analysis. We suggest that the force exerted on the SPB by biorientation of the chromosomes pulls the Spc110-226p out of the SPB; removal of force exerted by coherence of the sister chromatids reduced fragmentation fourfold. Removal of the forces exerted by the cytoplasmic microtubules had no effect on fragmentation. Our results provide insights into the relative contributions of the kinetochore and cytoplasmic microtubules to the forces involved in formation of a bipolar spindle.

Positioning and capture of cell surface-associated microtubules in epithelial tendon cells that differentiate in primary embryonic Drosophila cell cultures

Using primary embryonic Drosophila cell cultures, we have investigated the assembly of transcellular microtubule bundles in epidermal tendon cells. Muscles attach to the tendon cells of previously undescribed epidermal balls that form shortly after culture initiation. Basal capture of microtubule ends in cultured tendon cells is confined to discrete sites that occupy a relatively small proportion of the basal cell surface. These capturing sites are associated with hemiadherens junctions that link the ends of muscle cells to tendon cell bases. In vivo, muscle attachment and microtubule capture occur across the entire cell base. The cultured tendon cells reveal that the basal ends of their microtubules can be precisely targeted to small, pre-existing, structurally well-defined cortical capturing sites. However, a search and capture targeting procedure, such as that undertaken by kinetochore microtubules, cannot fully account for the precision of microtubule capture and positioning in tendon cells. We propose that cross-linkage of microtubules is also required to zip them into apicobasally oriented alignment, progressing from captured basal plus ends to apical minus ends. This involves repositioning of apical minus ends before they become anchored to an apical set of hemiadherens junctions. The proposal is consistent with our finding that hemiadherens junctions assemble at tendon cell bases before they do so at cell apices in both cultures and embryos. It is argued that control of microtubule positioning in the challenging spatial situations found in vitro involves the same procedures as those that operate in vivo.