The role of nucleation in patterning microtubule networks

Thin and stretchable extracellular matrix (ECM) membrane reinforced by nanofiber scaffolds for developing in vitro barrier models

An extracellular matrix (ECM) membrane made up of ECM hydrogels has great potentials to develop a physiologically relevant organ-on-a-chip because of its biochemical and biophysical similarity to in vivo basement membranes (BMs). However, the limited mechanical stability of the ECM hydrogels makes it difficult to utilize the ECM membrane in long-term and dynamic cell/tissue cultures. This study proposes an ultra-thin but robust and transparent ECM membrane reinforced with silk fibroin (SF)/polycaprolactone (PCL) nanofibers, which is achieved by in situ self-assembly throughout a freestanding SF/PCL nanofiber scaffold. The SF/PCL nanofiber-reinforced ECM (NaRE) membrane shows biophysical characteristics reminiscent of native BMs, including small thickness (< 5 μm), high permeability (< 9 × 10−5 cm s-1), and nanofibrillar architecture (~10 to 100 nm). With the BM-like characteristics, the nanofiber reinforcement ensured that the NaRE membrane stably supported the construction of various types of in vitro barrier models, from epithelial or endothelial barrier models to complex co-culture models, even over two weeks of cell culture periods. Furthermore, the stretchability of the NaRE membrane allowed emulating the native organ-like cyclic stretching motions (10 to 15%) and was demonstrated to manipulate the cell and tissue-level functions of the in vitro barrier model.

SPR2 protects minus ends to promote severing and reorientation of plant cortical microtubule arrays

The cortical microtubule arrays of higher plants are organized without centrosomes and feature treadmilling polymers that are dynamic at both ends. The control of polymer end stability is fundamental for the assembly and organization of cytoskeletal arrays, yet relatively little is understood about how microtubule minus ends are controlled in acentrosomal microtubule arrays, and no factors have been identified that act at the treadmilling minus ends in higher plants. Here, we identify Arabidopsis thaliana SPIRAL2 (SPR2) as a protein that tracks minus ends and protects them against subunit loss. SPR2 function is required to facilitate the rapid reorientation of plant cortical arrays as stimulated by light perception, a process that is driven by microtubule severing to create a new population of microtubules. Quantitative live-cell imaging and computer simulations reveal that minus protection by SPR2 acts by an unexpected mechanism to promote the lifetime of potential SPR2 severing sites, increasing the likelihood of severing and thus the rapid amplification of the new microtubule array.

The expression pattern of the C-terminal kinesin gene kifc1 during the spermatogenesis of Sepiella maindroni

In this study, we investigated the gene sequence and characteristic of kifc1 in Sepiella maindroni through PCR and RACE technology. Our research aimed particularly at the spatio-temporal expression pattern of kifc1 in the developmental testis through in situ hybridization. The particular role of kifc1 in the spermatogenesis of S. maindroni was our particular interest. Based on multiple protein sequence alignments of KIFC1 homologues, kifc1 gene from the testis of S. maindroni was identified, which consisted of 2432bp including a 2109 in-frame ORF corresponding to 703 continuous amino acids. The encoded polypeptide shared highest similarity with Octopus tankahkeei. Through the prediction of the secondary and tertiary structures, the motor domain of KIFC1 was conserved at the C-terminal, having putative ATP-binding and microtubule-binding motifs, while the N-terminal was more specific to bind various cargoes for cellular events. The stalk domain connecting between the C-terminal and N-terminal determined the direction of movement. According to RT-PCR results, the kifc1 gene is not tissue-specific, commonly detected in different tissues, for example, the testis, liver, stomach, muscle, caecum and gills. Through an in situ hybridization method, the expression pattern of KIFC1 protein mimics in the spermatogenesis of S. maindroni. During the primary stage of the spermatogenesis, the kifc1 mRNA signal was barely detectable. At the early spermatids, the signal started to be present. With the elongation of spermatids, the signals increased substantially. It peaked and gathered around the acrosome area when the spermatids began to transform to spindle shape. As the spermatids developed into mature sperm, the signal vanished. In summary, the expression of kfic1 at specific stages during spermiogenesis and its distribution shed light on the potential functions of this motor in major cytological transformations. The KIFC1 homologue may provide a direct shaping force to the nucleus or influence the shaping process through indirect regulation.

XMAP215 activity sets spindle length by controlling the total mass of spindle microtubules

Metaphase spindles are microtubule-based structures that use a multitude of proteins to modulate their morphology and function. Today, we understand many details of microtubule assembly, the role of microtubule-associated proteins, and the action of molecular motors. Ultimately, the challenge remains to understand how the collective behaviour of these nanometre-scale processes gives rise to a properly sized spindle on the micrometre scale. By systematically engineering the enzymatic activity of XMAP215, a processive microtubule polymerase, we show that Xenopus laevis spindle length increases linearly with microtubule growth velocity, whereas other parameters of spindle organization, such as microtubule density, lifetime and spindle shape, remain constant. We further show that mass balance can be used to link the global property of spindle size to individual microtubule dynamic parameters. We propose that spindle length is set by a balance of non-uniform nucleation and global microtubule disassembly in a liquid-crystal-like arrangement of microtubules.

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.

Influence of centriole number on mitotic spindle length and symmetry

The functional role of centrioles or basal bodies in mitotic spindle assembly and function is currently unclear. Although supernumerary centrioles have been associated with multipolar spindles in cancer cells, suggesting centriole number might dictate spindle polarity, bipolar spindles are able to assemble in the complete absence of centrioles, suggesting a level of centriole-independence in the spindle assembly pathway. In this report we perturb centriole number using mutations in Chlamydomonas reinhardtii, and measure the response of the mitotic spindle to these perturbations in centriole number. Although altered centriole number increased the frequency of monopolar and multipolar spindles, the majority of spindles remained bipolar regardless of the centriole number. But even when spindles were bipolar, abnormal centriole numbers led to asymmetries in tubulin distribution, half-spindle length and spindle pole focus. Half spindle length correlated directly with number of centrioles at a pole, such that an imbalance in centriole number between the two poles of a bipolar spindle correlated with increased asymmetry between half spindle lengths. These results are consistent with centrioles playing an active role in regulating mitotic spindle length. Mutants with centriole number alteration also show increased cytokinesis defects, but these do not correlate with centriole number in the dividing cell and may therefore reflect downstream consequences of defects in preceding cell divisions.

The Sertoli cell cytoskeleton

The cytoskeleton of terminally differentiated mammalian Sertoli cells is one of the most elaborate of those that have been described for cells in tissues. Actin filaments, intermediate filaments and microtubules have distinct patterns of distribution that change during the cyclic process of spermatogenesis. Each of the three major cytoskeletal elements is either concentrated at or related in part to intercellular junctions. Actin filaments are concentrated in unique structures termed ectoplasmic specializations that function in intercellular adhesion, and at tubulobulbar complexes that are thought to be involved with junction internalization during sperm release and movement of spermatocytes through basal junctions between neighboring Sertoi cells. Intermediate filaments occur in a perinuclear network which has peripheral extensions to desmosome-like junctions with adjacent cells and to small hemidesmosome-like attachments to the basal lamina. Unlike in most other epithelia where the intermediate filaments are of the keratin type, intermediate filaments in mature Sertoli cells are of the vimentin type. The function of intermediate filaments in Sertoli cells in not entirely clear; however, the pattern of filament distribution and the limited experimental data available are consistent with a role in maintaining tissue integrity when the epithelium is mechanically stressed. Microtubules are abundant in Sertoli cells and are predominantly oriented parallel to the long axis of the cell. Microtubules are involved with maintaining the columnar shape of Sertoli cells, with transporting and positioning organelles in the cytoplasm, and with secreting seminiferous tubule fluid. In addition, microtubule-based transport machinery is coupled to intercellular junctions to translocate and position adjacent spermatids in the epithelium. Although the cytoskeleton of Sertoli cells has structural and functional properties common to cells generally, there are a number of properties that are unique and that appear related to processes fundamental to spermatogenesis and to interfacing somatic cells both with similar neighboring somatic cells and with differentiating cells of the germ cell line.

Ethylene receptor ETR2 controls trichome branching by regulating microtubule assembly in Arabidopsis thaliana

The single-celled trichome of Arabidopsis thaliana is a widely used model system for studying cell development. While the pathways that control the later stages of trichome development are well characterized, the early signalling events that co-ordinate these pathways are less well understood. Hormones such as gibberellic acid, salicylic acid, cytokinins, and ethylene are known to affect trichome initiation and development. To understand the role of the plant hormone ethylene in trichome development, an Arabidopsis loss-of-function ethylene receptor mutant, etr2-3, which has completely unbranched trichomes, is analysed in this study. It was hypothesized that ETR2 might affect the assembly of the microtubule cytoskeleton based on analysis of the cytoskeleton in developing trichomes, and exposures to paclitaxol and oryzalin, which respectively act either to stabilize or depolymerize the cytoskeleton. Through epistatic and gene expression analyses it is shown that ETR2 is positioned upstream of CHROMATIN ASSEMBLY FACTOR1 and TRYPTICHON and is independent of the GLABRA2 and GLABRA3 pathways. These results help extend understanding of the early events that control trichome development and identify a signalling pathway through which ethylene affects trichome branching.

Beta-tubulin cofactor D and ARL2 take part in apical junctional complex disassembly and abrogate epithelial structure

In epithelial cells, the apical junctional complex (AJC), composed of tight junctions (TJs) and adherens junctions (AJs), maintains cell-surface polarity by forming a fence that prevents lateral movement and diffusion of proteins and lipids between the apical and basolateral PM and holds the epithelial monolayer intact through cell-cell contacts. Disassembly of this complex is a prime event in development and cell transformation. Maintenance of the AJC has been shown to involve mainly the actin cytoskeleton. Recent findings also point to the involvement of the microtubule (MT) system. Here we show the first evidence that in polarized epithelial MDCK cells, ARF-like protein 2 (ARL2) and beta-tubulin cofactor D, known to be involved in MT dynamics, have a role in disassembly of the AJC followed by cell dissociation from the epithelial monolayer, which is not dependent on MT depolymerization. In addition, we show that beta-tubulin cofactor D is partially localized to the lateral PM through its 15 C-terminal amino acids and intact MTs. ARL2 inhibited beta-tubulin cofactor D-dependent cell dissociation from the monolayer and AJC disassembly. To our knowledge, this is the first evidence that beta-tubulin cofactor D plays a role in cells independent of its presumed role in folding tubulin heterodimers. We conclude that ARL2 and beta-tubulin cofactor D participate in AJC disassembly and epithelial depolarization.

Excessive expression of the plant kinesin TBK5 converts cortical and perinuclear microtubules into a radial array emanating from a single focus

TBK5 is a plant-specific kinesin constantly expressed in tobacco BY-2 cells. An analysis of the distribution of green fluorescent protein-tagged TBK5 (GFP-TBK5) transiently expressed in BY-2 protoplasts revealed that TBK5 could associate with microtubules in vivo. GFP-TBK5 often assembled to form a single particle when accumulated in cells. The particle was located in close proximity to the nucleus, and its formation was accompanied by the development of a radial array of microtubules emanating from it and the loss of cortical microtubules. Microtubule depolymerization by treatment with propyzamide inhibited particle formation and stimulated the formation of dispersed aggregates of GFP-TBK5. Through expression of different TBK5 mutants as GFP fusions, the motor domain, two separated coiled-coil domains and the C-terminal domain of TBK5 were identified as the domains playing essential roles in particle formation. Mutants with putatively non-motile motor domains or lacking the C-terminal domain were localized to cortical and perinuclear microtubules, whereas those lacking either of the coiled-coil domains were preferentially distributed around the nucleus and along perinuclar microtubules. Further, the deletion of one of the coiled-coil domains or the C-terminal domain was sufficient to inhibit the propyzamide-induced formation of dispersed aggregates, whereas the mutation in the motor domain was not. These results led us to propose a model in which the particle is formed through the microtubule-based movement of GFP-TBK5 toward the nucleus and subsequent microtubule-independent aggregation based on coiled-coil interactions. The dramatic microtubule rearrangement would be explained if GFP-TBK5 relocated and gathered newly formed microtubules and/or microtubule-nucleating units.

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.

Non-centrosomal microtubule-organising centres in cold-treated cultured Drosophila cells

In this paper we describe a new type of non-centrosomal microtubule-organising centre (MTOC), which is induced by cold treatment of certain cultured Drosophila cells and allows rapid reassembly of microtubule (MT) arrays. Prolonged cooling of two types of cultured Drosophila cells, muscle cells in primary culture and a wing imaginal disc cell line Cl.8+ results in disassembly of MT arrays and induces the formation of clusters of short MTs that have not been described before. Upon rewarming, the clusters are lost and the MT array is re-established within 1 h. In Cl.8+ cells, gamma-tubulin-containing centrosomes are detected, both in cell extensions and in the expected juxtanuclear position, and gamma-tubulin co-localises with the cold-induced MT clusters. The MT plus-end-binding protein, Drosophila EB1, decorates growing tips of MTs extending from clusters. We conclude that the cold-induced MT clusters represent acentrosomal MTOCs, allowing rapid reassembly of MT arrays following exposure to cold.

Kinetochore fiber formation in animal somatic cells: dueling mechanisms come to a draw

The attachment to and movement of a chromosome on the mitotic spindle are mediated by the formation of a bundle of microtubules (MTs) that tethers the kinetochore on the chromosome to a spindle pole. The origin of these "kinetochore fibers" (K fibers) has been investigated for over 125 years. As noted in 1944 by Schrader [Mitosis, Columbia University Press, New York, 110 pp.], there are three possible ways to form a K fiber: (a) it grows from the pole until it contacts the kinetochore, (b) it grows directly from the kinetochore, or (c) it forms as a result of an interaction between the pole and the chromosome. Since Schrader's time, it has been firmly established that K fibers in centrosome-containing animal somatic cells form as kinetochores capture MTs growing from the spindle pole (route a). It is now similarly clear that in cells lacking centrosomes, including higher plants and many animal oocytes, K fibers "self-assemble" from MTs generated by the chromosomes (route b). Can animal somatic cells form K fibers in the absence of centrosomes by the "self-assembly" pathway? In 2000, the answer to this question was shown to be a resounding "yes." With this result, the next question became whether the presence of a centrosome normally suppresses K fiber self-assembly or if this route works concurrently with centrosome-mediated K-fiber formation. This question, too, has recently been answered: observations on untreated live animal cells expressing green fluorescent protein-tagged tubulin clearly show that kinetochores can nucleate the formation of their associated MTs in a unique manner in the presence of functional centrosomes. The concurrent operation of these two "dueling" routes for forming K fibers in animal cells helps explain why the attachment of kinetochores and the maturation of K fibers occur as quickly as they do on all chromosomes within a cell.

Cytoskeleton: microtubules born on the run

The organization of microtubules into large arrays determines cell morphology and structure. Recent work in the fission yeast describes a novel mechanism for microtubule self-organization in the absence of centrosomes; this mechanism may function in a variety of cell types found in diverse organisms.

The Kinesin Klp2 Mediates Polarization of Interphase Microtubules in Fission Yeast

Fission yeast (Schizosaccharomyces pombe) cells grow longitudinally in a manner dependent on a polarized distribution of their interphase microtubules. We found that this distribution required sliding of microtubules toward the cell center along preexisting microtubules. This sliding was mediated by the minus end-directed kinesin motor Klp2, which helped microtubules to become properly organized with plus ends predominantly oriented toward the cell ends and minus ends toward the cell center. Thus, interphase microtubules in the fission yeast require motor activities for their proper organization.

Aster self-organization at meiosis: a conserved mechanism in insect parthenogenesis?

Unfertilized eggs usually lack maternal centrosomes and cannot develop without sperm contribution. However, several insect species lay eggs that develop to adulthood as unfertilized in the absence of a preexisting centrosome. We report that the oocyte of the parthenogenetic viviparous pea aphid Acyrthosiphon pisum is able to self-organize microtubule-based asters, which in turn interact with the female chromatin to form the first mitotic spindle. This mode of reproduction provides a good system to investigate how the oocyte can assemble new centrosomes and how their number can be exactly monitored. We propose that the cooperative interaction of motor proteins and randomly nucleated surface microtubules could lead to the formation of aster-like structures in the absence of pre-existing centrosomes. Recruitment of material along the microtubules might contribute to the accumulation of pericentriolar material and centriole precursors at the focus of the asters, thus leading to the formation of true centrosomes. The appearance of microtubule asters at the surface of activated oocytes could represent a possible common mechanism for centrosome formation during insect parthenogenesis.

Functional role of centrosomes in spindle assembly and organization

The centrosome is the main MT organizing center in animal cells, and has traditionally been regarded as essential for organization of the bipolar spindle that facilitates chromosome segregation during mitosis. Centrosomes are associated with the poles of the mitotic spindle, and several cell types require these organelles for spindle formation. However, most plant cells and some female meiotic systems get along without this organelle, and centrosome-independent spindle assembly has now been identified within some centrosome containing cells. How can such observations, which point to mutually incompatible conclusions regarding the requirement of centrosomes in spindle formation, be interpreted? With emphasis on the functional role of centrosomes, this article summarizes the current models of spindle formation, and outlines how observations obtained from spindle assembly assays in vitro may reconcile conflicting opinions about the mechanism of spindle assembly. It is further described how Drosophila mutants are used to address the functional interrelationships between individual centrosomal proteins and spindle formation in vivo.

Glutamylated and glycylated tubulin isoforms in the aberrant sperm axoneme of the gall-midge fly, Asphondylia ruebsaameni

The axonemal organization expressed in the sperm flagella of the cecidomyiid dipteran Asphondylia ruebsaameni is unconventional, being characterized by the presence of an exceedingly high number of microtubular doublets and by the absence of both the inner dynein arms and the central pair/radial spoke complex. Consequently, its motility, both in vivo and in vitro, is also peculiar. Using monoclonal antibodies directed against posttranslational modifications, we have analyzed the presence and distribution of glutamylated and glycylated tubulin isoforms in this aberrant axonemal structure, and compared them with those of a reference insect species (Apis mellifera), endowed with a conventional axoneme. Our results have shown that the unorthodox structure and motility of the Asphondylia axoneme are concomitant with: (1). a very low glutamylation extent in the alpha-tubulin subunit, (2). a high level of glutamylation in the beta-subunit, (3). an extremely low total extent of glycylation, with regard to both monoglycylated and polyglycylated sites, either in alpha- or in beta-tubulin, (4). the presence of a strong labeling of glutamylated tubulin isoforms at the proximal end of the axoneme, and (5). a uniform distribution of glutamylated as well as glycylated isoforms along the rest of the axoneme. Thus, our data indicate that tubulin molecular heterogeneity is much lower in the Asphondylia axoneme than in the conventional 9+2 axoneme with regard to both isoform content and isoform distribution along the axoneme.

Ase1p organizes antiparallel microtubule arrays during interphase and mitosis in fission yeast

Proper microtubule organization is essential for cellular processes such as organelle positioning during interphase and spindle formation during mitosis. The fission yeast Schizosaccharomyces pombe presents a good model for understanding microtubule organization. We identify fission yeast ase1p, a member of the conserved ASE1/PRC1/MAP65 family of microtubule bundling proteins, which functions in organizing the spindle midzone during mitosis. Using fluorescence live cell imaging, we show that ase1p localizes to sites of microtubule overlaps associated with microtubule organizing centers at both interphase and mitosis. ase1Delta mutants fail to form overlapping antiparallel microtubule bundles, leading to interphase nuclear positioning defects, and premature mitotic spindle collapse. FRAP analysis revealed that interphase ase1p at overlapping microtubule minus ends is highly dynamic. In contrast, mitotic ase1p at microtubule plus ends at the spindle midzone is more stable. We propose that ase1p functions to organize microtubules into overlapping antiparallel bundles both in interphase and mitosis and that ase1p may be differentially regulated through the cell cycle.

The effect of 6-dimethylaminopurine (6-DMAP) and cycloheximide (CHX) on the development and chromosomal complement of sheep parthenogenetic and nuclear transfer embryos

The effects of activation by 6-dimethylaminopurine (6-DMAP) and cycloheximide (CHX) on the development and chromosomal complement of sheep parthenogenetic and SCNT embryos were investigated. The results revealed that the blastocyst development of parthenogenetic embryos was significantly higher (P < 0.05) in 6-DMAP activated oocytes, compared to those activated with CHX (21.0 +/- 0.9 vs. 14.9 +/- 0.5, respectively). In contrast, the blastocyst frequencies did not significantly differ (P > 0.05) between the two activation treatment groups for SCNT embryos. The 6-DMAP or CHX treatment did not result in any significant difference in the blastocyst total cell number in either parthenote or SCNT embryos. The chromosomal analysis revealed that all the parthenogenetic embryos (100.0%) derived from 6-DMAP treatment, were chromosomally abnormal whereas in CHX-treated embryos, it was significantly lowered (93.6%, P < 0.05). Conversely, the proportions of chromosomally abnormal SCNT embryos did not significantly differ (P > 0.05) among the 6-DMAP and CHX- treated embryo groups (60.0% vs. 56.2%, respectively). This study demonstrated that oocyte activation agents such as DMAP and CHX have differing effects on meiotic or mitotic nuclei. The study also highlighted the feasibility of using bovine X and Y chromosome specific painting probes in sheep embryos.

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.

Microtubule organization and function in epithelial cells

Microtubules are essential for many aspects of polarity in multicellular organisms, ranging from the asymmetric distribution of cell-fate determinants in the one-cell embryo to the transient polarity generated in migrating fibroblasts. Epithelial cells exhibit permanent cell polarity characterized by apical and basolateral surface domains of distinct protein and lipid composition that are segregated by tight junctions. They are also endowed with a microtubule network that reflects the asymmetry of their cell surface: microtubule minus-ends face the apical- and microtubule plus-ends the basal domain. Strikingly, the formation of distinct surface domains during epithelial differentiation is accompanied by the re-organization of microtubules from a uniform array focused at the centrosome to the noncentrosomal network that aligns along the apico-basolateral polarity axis. The significance of this coincidence for epithelial morphogenesis and the signaling mechanisms that drive microtubule repolymerization in developing epithelia remain major unresolved questions that we are only beginning to address. Studies in cultured polarized epithelial cells have established that microtubules serve as tracks that facilitate targeted vesicular transport. Novel findings suggest, moreover, that microtubule-based transport promotes protein sorting, and even the generation of transport carriers in the endo- and exocytic pathways.

Confocal and video imaging of cytoskeleton dynamics in the leech zygote

Ooplasmic segregation in the late interphase zygote of the leech Theromyzon trizonare is accomplished by reorganization of an ectoplasmic cytoskeleton formed by polar rings and meridional bands. The dynamic properties of this cytoskeleton were explored by time-lapse confocal and video microscopy. Cytoskeleton assembly was investigated in zygotes pulse-labeled with microinjected fluorophore-tagged or biotin-tagged dimeric tubulin and G-actin. Cytoskeleton disassembly was studied by comparing the linear dimensions of the cytoskeleton at different time points during late interphase. The relative distributions of F- and-G-actin were determined after microinjection of rhodamine-labeled actin and fluorescein-labeled DNase I. Results showed that labeled precursors were readily incorporated into a network of microtubules or actin filaments. Bipolar translocation of the rings and meridional bands was accompanied by the rapid assembly and disassembly of microtubules and actin filaments. Because labeled microtubules and microfilaments gradually decreased, the rate of cytoskeleton disassembly was greater than the rate of cytoskeleton assembly. Hence, ooplasmic segregation was accompanied by the rapid turnover of cytoskeletal components. Co-distribution of F- and-G-actin during mid and late interphase may favor polymer-monomer interchange. We conclude that cytoskeleton reorganization during foundation of cytoplasmic domains can be conveniently studied in the live leech zygote after microinjection of labeled precursors.

The adenomatous polyposis coli protein is required for the formation of robust spindles formed in CSF Xenopus extracts

Mutations in the adenomatous polyposis coli (APC) protein occur early in colon cancer and correlate with chromosomal instability. Here, we show that depletion of APC from cystostatic factor (CSF) Xenopus extracts leads to a decrease in microtubule density and changes in tubulin distribution in spindles and asters formed in such extracts. Addition of full-length APC protein or a large, N-terminally truncated APC fragment to APC-depleted extracts restored normal spindle morphology and the intact microtubule-binding site of APC was necessary for this rescue. These data indicate that the APC protein plays a role in the formation of spindles that is dependent on its effect on microtubules. Spindles formed in cycled extracts were not sensitive to APC depletion. In CSF extracts, spindles predominantly formed from aster-like intermediates, whereas in cycled extracts chromatin was the major site of initial microtubule polymerization. These data suggest that APC is important for centrosomally driven spindle formation, which was confirmed by our finding that APC depletion reduced the size of asters nucleated from isolated centrosomes. We propose that lack of microtubule binding in cancer-associated mutations of APC may contribute to defects in the assembly of mitotic spindles and lead to missegregation of chromosomes.

Microtubule assembly: catastrophe factors to the rescue

Two new studies have shown how regulation of microtubule dynamics by members of the kinesin superfamily may guard against errors in spindle assembly and chromosome segregation.

Modeling oscillatory microtubule polymerization

Polymerization of microtubules is ubiquitous in biological cells and under certain conditions it becomes oscillatory in time. Here, simple reaction models are analyzed that capture such oscillations as well as the length distribution of microtubules. We assume reaction conditions that are stationary over many oscillation periods, and it is a Hopf bifurcation that leads to a persistent oscillatory microtubule polymerization in these models. Analytical expressions are derived for the threshold of the bifurcation and the oscillation frequency in terms of reaction rates, and typical trends of their parameter dependence are presented. Both, a catastrophe rate that depends on the density of guanosine triphosphate liganded tubulin dimers and a delay reaction, such as the depolymerization of shrinking microtubules or the decay of oligomers, support oscillations. For a tubulin dimer concentration below the threshold, oscillatory microtubule polymerization occurs transiently on the route to a stationary state, as shown by numerical solutions of the model equations. Close to threshold, a so-called amplitude equation is derived and it is shown that the bifurcation to microtubule oscillations is supercritical.

Microtubule organization requires cell cycle-dependent nucleation at dispersed cytoplasmic sites: polar and perinuclear microtubule organizing centers in the plant pathogen Ustilago maydis

Growth of most eukaryotic cells requires directed transport along microtubules (MTs) that are nucleated at nuclear-associated microtubule organizing centers (MTOCs), such as the centrosome and the fungal spindle pole body (SPB). Herein, we show that the pathogenic fungus Ustilago maydis uses different MT nucleation sites to rearrange MTs during the cell cycle. In vivo observation of green fluorescent protein-MTs and MT plus-ends, tagged by a fluorescent EB1 homologue, provided evidence for antipolar MT orientation and dispersed cytoplasmic MT nucleating centers in unbudded cells. On budding gamma-tubulin containing MTOCs formed at the bud neck, and MTs reorganized with >85% of all minus-ends being focused toward the growth region. Experimentally induced lateral budding resulted in MTs that curved out of the bud, again supporting the notion that polar growth requires polar MT nucleation. Depletion or overexpression of Tub2, the gamma-tubulin from U. maydis, affected MT number in interphase cells. The SPB was inactive in G2 phase but continuously recruited gamma-tubulin until it started to nucleate mitotic MTs. Taken together, our data suggest that MT reorganization in U. maydis depends on cell cycle-specific nucleation at dispersed cytoplasmic sites, at a polar MTOC and the SPB.

Centrosomal TACCtics

Although the centrosome was first described over 100 years ago, we still know relatively little of the molecular mechanisms responsible for its functions. Recently, members of a novel family of centrosomal proteins have been identified in a wide variety of organisms. The transforming acidic coiled-coil-containing (TACC) proteins all appear to play important roles in cell division and cellular organisation in both embryonic and somatic systems. These closely related molecules have been implicated in microtubule stabilisation, acentrosomal spindle assembly, translational regulation, haematopoietic development and cancer progression. In this review, I summarise what we already know of this protein family and will use the TACC proteins to illustrate the many facets that centrosomes have developed during the course of evolution.

Centrin-2 is required for centriole duplication in mammalian cells

BACKGROUND

Centrosomes are the favored microtubule-organizing framework of eukaryotic cells. Centrosomes contain a pair of centrioles that normally duplicate once during the cell cycle to give rise to two mitotic spindle poles, each containing one old and one new centriole. However, aside from their role as an anchor point for pericentriolar material and as basal bodies of flagella and cilia, the functional attributes of centrioles remain enigmatic.

RESULTS

Here, using RNA interference, we demonstrate that "knockdown" of centrin-2, a protein of centrioles, results in failure of centriole duplication during the cell cycle in HeLa cells. Following inhibition of centrin-2 synthesis, the preexisting pair of centrioles separate, and functional bipolar spindles form with only one centriole at each spindle pole. Centriole dilution results from the ensuing cell division, and daughter cells are "born" with only a single centriole. Remarkably, these unicentriolar daughter cells may complete a second and even third bipolar mitosis in which spindle microtubules converge onto unusually broad spindle poles and in which cell division results in daughter cells containing either one or no centrioles at all. Cells thus denuded of the mature or both centrioles fail to undergo cytokinesis in subsequent cell cycles, give rise to multinucleate products, and finally die.

CONCLUSIONS

These results demonstrate a requirement for centrin in centriole duplication and demonstrate that centrioles play a role in organizing spindle pole morphology and in the completion of cytokinesis.

Mitosis in primary cultures ofDrosophila melanogasterlarval neuroblasts

Although Drosophila larval neuroblasts are routinely used to define mutations affecting mitosis, the dynamics of karyokinesis in this system remain to be described. Here we outline a simple method for the short-term culturing of neuroblasts, from Drosophila third instar larvae, that allows mitosis to be followed by high-resolution multi-mode light microscopy. At 24°C, spindle formation takes 7±0.5 minutes. Analysis of neuroblasts containing various GFP-tagged proteins (e.g. histone,fizzy, fizzy-related and α-tubulin) reveals that attaching kinetochores exhibit sudden, rapid pole-directed motions and that congressing and metaphase chromosomes do not undergo oscillations. By metaphase, the arms of longer chromosomes can be resolved as two chromatids, and they often extend towards a pole. Anaphase A and B occur concurrently, and during anaphase A chromatids move poleward at 3.2±0.1 μm/minute, whereas during anaphase B the spindle poles separate at 1.6±01 μm/minute. In larger neuroblasts,the spindle undergoes a sudden shift in position during midanaphase, after which the centrally located centrosome preferentially generates a robust aster and stops moving, even while the spindle continues to elongate. Together these two processes contribute to an asymmetric positioning of the spindle midzone,which, in turn, results in an asymmetric cytokinesis. Bipolar spindles form predominately (83%) in association with the separating centrosomes. However,in 17% of the cells, secondary spindles form around chromosomes without respect to centrosome position: in most cases these spindles coalesce with the primary spindle by anaphase, but in a few they remain separate and define additional ectopic poles.

Regulation and subcellular localization of the microtubule-destabilizing stathmin family phosphoproteins in cortical neurons

Stathmin is a ubiquitous cytosolic phosphoprotein, preferentially expressed in the nervous system, and the generic element of a protein family that includes the neural-specific proteins SCG10, SCLIP, and RB3 and its splice variants, RB3' and RB3". All phosphoproteins of the family share with stathmin its tubulin binding and microtubule (MT)-destabilizing activities. To understand better the specific roles of these proteins in neuronal cells, we performed a comparative study of their expression, regulation, and intracellular distribution in embryonic cortical neurons in culture. We found that stathmin is highly expressed ( approximately 0.25% of total proteins) and uniformly present in the various neuronal compartments (cell body, dendrites, axon, growth cones). It appeared mainly unphosphorylated or weakly phosphorylated on one site, and antisera to specific phosphorylated sites (serines 16, 25, or 38) did not reveal a differential regulation of its phosphorylation among neuronal cell compartments. However, they revealed a subpopulation of cells in which stathmin was highly phosphorylated on serine 16, possibly by CaM kinase II also active in a similar subpopulation. The other proteins of the stathmin family are expressed about 100-fold less than stathmin in partially distinct neuronal populations, RB3 being detected in only about 20% of neurons in culture. In contrast to stathmin, they are each mostly concentrated at the Golgi apparatus and are also present along dendrites and axons, including growth cones. Altogether, our results suggest that the different members of the stathmin family have complementary, at least partially distinct functions in neuronal cell regulation, in particular in relation to MT dynamics.

Gamma-tubulin in barley and tobacco: sequence relationship and RNA expression patterns in developing leaves during mitosis and post-mitotic growth

gamma-Tubulin is typically associated with microtubule organising centres, such as the centrosome, and appears to mediate microtubule nucleation. Centrosomes are usually not found in higher plants, but active genes homologous to gamma-tubulin have been identified in the plant kingdom, including the angiosperms Arabidopsis, maize and rice. We have isolated and characterised gamma-tubulin cDNA sequences of two further angiosperm species, barley and tobacco. Sequence comparison revealed a phylogenetic tree with distinct clusters corresponding to the systematic position of the species. Furthermore, domains, thought to be exposed in the folded protein and to be candidates for interaction with associated, nucleation-site related proteins, exhibited motifs highly specific of multicellular plants. Strong expression of the gamma-tubulin genes, as determined by Northern blotting, correlated with mitotic activity. Expression dropped distinctly when mitotic activity ceased. Thus, in post-mitotic tissues that showed intricate reshuffling of cortical microtubule arrays related to cell shaping only very low gamma-tubulin steady-state RNA levels were found, contrasting with the situation for alpha-tubulin. The findings indicate that gamma-tubulin expression in plants may be more tightly linked to mitosis, although there is some gamma-tubulin expression at the RNA level even after mitosis. It follows that the post-mitotic changes in microtubular arrays may be less dependent on concurrent gamma-tubulin RNA expression than mitotic cells.

MTOC formation during mitotic exit in fission yeast

Microtubules polymerise from nucleation templates containing γ tubulin. These templates are generally concentrated in discrete structures called microtubule organising centres (MTOCs). In Schizosaccharomyces pombe, an equatorial MTOC (EMTOC) forms mid-way through anaphase B and then disassembles during the final stages of cell separation. We show that the EMTOC was generated by recruiting γ tubulin to the equatorial F-actin ring before it constricted to cleave the cell in two during cytokinesis. The EMTOC was not a continuous ring. It had a variable structure ranging from a horseshoe to a number of short bars. EMTOC integrity depended upon the integrity of the F-actin but not the microtubule cytoskeleton. EMTOC assembly required the activity of both the septation-inducing network (SIN) that regulates the onset of cytokinesis and the anaphase-promoting complex. Activation of the SIN in interphase cells induced F-actin ring formation and contraction and the synthesis of the primary septum but did not promote EMTOC assembly. In contrast, overproduction of the polo-like kinase, Plo1, which also induced multiple rounds of septation in interphase cells, induced EMTOC formation. Thus, the network governing EMTOC formation shared many of the regulatory elements that control cytokinesis but was more complex and revealed an additional function for Plo1 during mitotic exit.

Celogentins A-C, new antimitotic bicyclic peptides from the seeds of Celosia argentea

Three new bicyclic peptides, celogentins A (1), B (2), and C (3), have been isolated together with a known-related peptide, moroidin (4), from the seeds of Celosia argentea, and their structures including absolute stereochemistry were determined by using extensive NMR methods and chemical means. Celogentins A (1), B (2), and C (3) inhibited the polymerization of tubulin, and celogentin C (3) was four times more potent than moroidin (4) in the inhibitory activity. Structure-activity relationship study using moroidin derivatives 5-7 and analogue 8 as well as celogentins A-C (1-3) and moroidin (4) indicates that the bicyclic ring system including unusual non-peptide connections among beta(s)-Leu, Trp, and His residues characteristic of celogentins and moroidin, with ring size and conformations suitable for interaction with tubulin would be important for their biological activity.

A journey into space

The fission yeast, Schizosaccharomyces pombe, has been used as a model eukaryote to study processes such as the cell cycle and cell morphology. In this single-celled organism, growing in a straight line and maintaining the nucleus in the centre of the cell depend on intracellular positional information. Microtubules and microtubular transport are important for generating positional information within the fission yeast cell, and these molecular mechanisms are also probably relevant for generating positional information in other eukaryotic cells.

Re-evaluating centrosome function

Over the past 100 years, the centrosome has risen in status from an enigmatic organelle, located at the focus of microtubules, to a key player in cell-cycle progression and cellular control. A growing body of evidence indicates that centrosomes might not be essential for spindle assembly, whereas recent data indicate that they might be important for initiating S phase and completing cytokinesis. Molecules that regulate centrosome duplication have been identified, and the expanding list of intriguing centrosome-anchored activities, the functions of which have yet to be determined, promises continued discovery.

Structural insight into microtubule function

Microtubules are polymers that are essential for, among other functions, cell transport and cell division in all eukaryotes. The regulation of the microtubule system includes transcription of different tubulin isotypes, folding of alpha/beta-tubulin heterodimers, post-translation modification of tubulin, and nucleotide-based microtubule dynamics, as well as interaction with numerous microtubule-associated proteins that are themselves regulated. The result is the precise temporal and spatial pattern of microtubules that is observed throughout the cell cycle. The recent high-resolution analysis of the structure of tubulin and the microtubule has brought new insight to the study of microtubule function and regulation, as well as the mode of action of antimitotic drugs that disrupt normal microtubule behavior. The combination of structural, genetic, biochemical, and biophysical data should soon give us a fuller understanding of the exquisite details in the regulation of the microtubule cytoskeleton.

Centrosomes enhance the fidelity of cytokinesis in vertebrates and are required for cell cycle progression

When centrosomes are destroyed during prophase by laser microsurgery, vertebrate somatic cells form bipolar acentrosomal mitotic spindles (Khodjakov, A., R.W. Cole, B.R. Oakley, and C.L. Rieder. 2000. Curr. Biol. 10:59-67), but the fate of these cells is unknown. Here, we show that, although these cells lack the radial arrays of astral microtubules normally associated with each spindle pole, they undergo a normal anaphase and usually produce two acentrosomal daughter cells. Relative to controls, however, these cells exhibit a significantly higher (30-50%) failure rate in cytokinesis. This failure correlates with the inability of the spindle to properly reposition itself as the cell changes shape. Also, we destroyed just one centrosome during metaphase and followed the fate of the resultant acentrosomal and centrosomal daughter cells. Within 72 h, 100% of the centrosome-containing cells had either entered DNA synthesis or divided. By contrast, during this period, none of the acentrosomal cells had entered S phase. These data reveal that the primary role of the centrosome in somatic cells is not to form the spindle but instead to ensure cytokinesis and subsequent cell cycle progression.

Structural insights into microtubule function

Microtubules are polymers that are essential for, among other functions, cell transport and cell division in all eukaryotes. The regulation of the microtubule system includes transcription of different tubulin isotypes, folding of /¿-tubulin heterodimers, post-translation modification of tubulin, and nucleotide-based microtubule dynamics, as well as interaction with numerous microtubule-associated proteins that are themselves regulated. The result is the precise temporal and spatial pattern of microtubules that is observed throughout the cell cycle. The recent high-resolution analysis of the structure of tubulin and the microtubule has brought new insight to the study of microtubule function and regulation, as well as the mode of action of antimitotic drugs that disrupt normal microtubule behavior. The combination of structural, genetic, biochemical, and biophysical data should soon give us a fuller understanding of the exquisite details in the regulation of the microtubule cytoskeleton.

Search, capture and signal: games microtubules and centrosomes play

Accurate distribution of the chromosomes in dividing cells requires coupling of cellular polarity cues with both the orientation of the mitotic spindle and cell cycle progression. Work in budding yeast has demonstrated that cytoplasmic dynein and the kinesin Kip3p define redundant pathways that ensure proper spindle orientation. Furthermore, it has been shown that the Kip3p pathway components Kar9p and Bim1p (Yeb1p) form a complex that provides a molecular link between cortical polarity cues and spindle microtubules. Recently, other studies indicated that the cortical localization of Kar9p depends upon actin cables and Myo2p, a type V myosin. In addition, a BUB2-dependent cell cycle checkpoint has been described that inhibits the mitotic exit network and cytokinesis until proper centrosome position is achieved. Combined, these studies provide molecular insight into how cells link cellular polarity, spindle position and cell cycle progression.

Ran induces spindle assembly by reversing the inhibitory effect of importin alpha on TPX2 activity

The small GTPase Ran, bound to GTP, is required for the induction of spindle formation by chromosomes in M phase. High concentrations of Ran.GTP are proposed to surround M phase chromatin. We show that the action of Ran.GTP in spindle formation requires TPX2, a microtubule-associated protein previously known to target a motor protein, Xklp2, to microtubules. TPX2 is normally inactivated by binding to the nuclear import factor, importin alpha, and is displaced from importin alpha by the action of Ran.GTP. TPX2 is required for Ran.GTP and chromatin-induced microtubule assembly in M phase extracts and mediates spontaneous microtubule assembly when present in excess over free importin alpha. Thus, components of the nuclear transport machinery serve to regulate spindle formation in M phase.

The role of Xgrip210 in gamma-tubulin ring complex assembly and centrosome recruitment

The gamma-tubulin ring complex (gammaTuRC), purified from the cytoplasm of vertebrate and invertebrate cells, is a microtubule nucleator in vitro. Structural studies have shown that gammaTuRC is a structure shaped like a lock-washer and topped with a cap. Microtubules are thought to nucleate from the uncapped side of the gammaTuRC. Consequently, the cap structure of the gammaTuRC is distal to the base of the microtubules, giving the end of the microtubule the shape of a pointed cap. Here, we report the cloning and characterization of a new subunit of Xenopus gammaTuRC, Xgrip210. We show that Xgrip210 is a conserved centrosomal protein that is essential for the formation of gammaTuRC. Using immunogold labeling, we found that Xgrip210 is localized to the ends of microtubules nucleated by the gammaTuRC and that its localization is more distal, toward the tip of the gammaTuRC-cap structure, than that of gamma-tubulin. Immunodepletion of Xgrip210 blocks not only the assembly of the gammaTuRC, but also the recruitment of gamma-tubulin and its interacting protein, Xgrip109, to the centrosome. These results suggest that Xgrip210 is a component of the gammaTuRC cap structure that is required for the assembly of the gammaTuRC.

COBRA-1, a rationally-designed epoxy-THF containing compound with potent tubulin depolymerizing activity as a novel anticancer agent

A novel mono-THF containing synthetic anticancer drug, COBRA-1, was designed for targeting a previously unrecognized unique narrow binding cavity on the surface of alpha-tubulin. COBRA-1 inhibited GTP-induced tubulin polymerization in cell-free tubulin turbidity assays. Treatment of human breast cancer and brain tumor (glioblastoma) cells with COBRA-1 caused destruction of microtubule organization and apoptosis. Like other microtubule-interfering agents, COBRA-1 activated the proapoptotic c-Jun N-terminal kinase (JNK) signal transduction pathway, as evidenced by rapid induction of c-jun expression.

Cytoplasmic dynein-mediated assembly of pericentrin and gamma tubulin onto centrosomes

Centrosome assembly is important for mitotic spindle formation and if defective may contribute to genomic instability in cancer. Here we show that in somatic cells centrosome assembly of two proteins involved in microtubule nucleation, pericentrin and gamma tubulin, is inhibited in the absence of microtubules. A more potent inhibitory effect on centrosome assembly of these proteins is observed after specific disruption of the microtubule motor cytoplasmic dynein by microinjection of dynein antibodies or by overexpression of the dynamitin subunit of the dynein binding complex dynactin. Consistent with these observations is the ability of pericentrin to cosediment with taxol-stabilized microtubules in a dynein- and dynactin-dependent manner. Centrosomes in cells with reduced levels of pericentrin and gamma tubulin have a diminished capacity to nucleate microtubules. In living cells expressing a green fluorescent protein-pericentrin fusion protein, green fluorescent protein particles containing endogenous pericentrin and gamma tubulin move along microtubules at speeds of dynein and dock at centrosomes. In Xenopus extracts where gamma tubulin assembly onto centrioles can occur without microtubules, we find that assembly is enhanced in the presence of microtubules and inhibited by dynein antibodies. From these studies we conclude that pericentrin and gamma tubulin are novel dynein cargoes that can be transported to centrosomes on microtubules and whose assembly contributes to microtubule nucleation.

A rationally designed anticancer drug targeting a unique binding cavity of tubulin

A novel mono-THF containing synthetic anticancer drug (WHI-261) was designed for targeting a previously unrecognized unique narrow binding cavity on the surface of tubulin. The anti-cancer activity of WHI-261 was confirmed using MTT assays. The structure-based design, synthesis, and biological activity of WHI-261 are reported.