Search, capture and signal: games microtubules and centrosomes play

EB1 and APC bind to mDia to stabilize microtubules downstream of Rho and promote cell migration

Lysophosphatidic acid (LPA) stimulates Rho GTPase and its effector, the formin mDia, to capture and stabilize microtubules in fibroblasts. We investigated whether mammalian EB1 and adenomatous polyposis coli (APC) function downstream of Rho-mDia in microtubule stabilization. A carboxy-terminal APC-binding fragment of EB1 (EB1-C) functioned as a dominant-negative inhibitor of microtubule stabilization induced by LPA or active mDia. Knockdown of EB1 with small interfering RNAs also prevented microtubule stabilization. Expression of either full-length EB1 or APC, but not an APC-binding mutant of EB1, was sufficient to stabilize microtubules. Binding and localization studies showed that EB1, APC and mDia may form a complex at stable microtubule ends. Furthermore, EB1-C, but not an APC-binding mutant, inhibited fibroblast migration in an in vitro wounding assay. These results show an evolutionarily conserved pathway for microtubule capture, and suggest that mDia functions as a scaffold protein for EB1 and APC to stabilize microtubules and promote cell migration.

Nuclear migration and positioning in filamentous fungi

Genetic analyses of nuclear distribution mutants have indicated that functions of the microtubule motor, cytoplasmic dynein, and its regulators are important for nuclear positioning in filamentous fungi. Here we review these studies and also present the need to further dissect how dynein and its associated microtubule cytoskeleton are involved mechanistically in nuclear positioning in the multinucleated hyphae.

Functional characterization of Dma1 and Dma2, the budding yeast homologues of Schizosaccharomyces pombe Dma1 and human Chfr

Proper transmission of genetic information requires correct assembly and positioning of the mitotic spindle, responsible for driving each set of sister chromatids to the two daughter cells, followed by cytokinesis. In case of altered spindle orientation, the spindle position checkpoint inhibits Tem1-dependent activation of the mitotic exit network (MEN), thus delaying mitotic exit and cytokinesis until errors are corrected. We report a functional analysis of two previously uncharacterized budding yeast proteins, Dma1 and Dma2, 58% identical to each other and homologous to human Chfr and Schizosaccharomyces pombe Dma1, both of which have been previously implicated in mitotic checkpoints. We show that Dma1 and Dma2 are involved in proper spindle positioning, likely regulating septin ring deposition at the bud neck. DMA2 overexpression causes defects in septin ring disassembly at the end of mitosis and in cytokinesis. The latter defects can be rescued by either eliminating the spindle position checkpoint protein Bub2 or overproducing its target, Tem1, both leading to MEN hyperactivation. In addition, dma1Delta dma2Delta cells fail to activate the spindle position checkpoint in response to the lack of dynein, whereas ectopic expression of DMA2 prevents unscheduled mitotic exit of spindle checkpoint mutants treated with microtubule-depolymerizing drugs. Although their primary functions remain to be defined, our data suggest that Dma1 and Dma2 might be required to ensure timely MEN activation in telophase.

A Role for the FEAR Pathway in Nuclear Positioning during Anaphase

In budding yeast, cells lacking separase function exit mitosis with an undivided nucleus localized to the daughter cell. Here we show that the inability to separate sister chromatids per se is not sufficient to cause the daughter preference. Rather, separase affects nuclear positioning as part of the Cdc14 early anaphase release (FEAR) pathway. The role of the FEAR pathway in nuclear positioning is exerted during anaphase and is not shared by the mitotic exit network. We find that the nuclear segregation defect in FEAR mutants does not stem from nonfunctional spindle poles or the absence of cytoplasmic microtubules. Instead, the concomitant inactivation of sister chromatid separation and the FEAR pathway uncovered a mother-directed force in anaphase that was previously masked by the elongating spindle. We propose that at anaphase onset, the FEAR pathway activates cytoplasmic microtubule-associated forces that facilitate chromosome segregation to the mother cell.

Cell cycle-dependent nuclear localization of yeast RNase III is required for efficient cell division

Members of the double-stranded RNA-specific ribonuclease III (RNase III) family were shown to affect cell division and chromosome segregation, presumably through an RNA interference-dependent mechanism. Here, we show that in Saccharomyces cerevisiae, where the RNA interference machinery is not conserved, an orthologue of RNase III (Rnt1p) is required for progression of the cell cycle and nuclear division. The deletion of Rnt1p delayed cells in both G1 and G2/M phases of the cell cycle. Nuclear division and positioning at the bud neck were also impaired in Deltarnt1 cells. The cell cycle defects were restored by the expression of catalytically inactive Rnt1p, indicating that RNA cleavage is not essential for cell cycle progression. Rnt1p was found to exit from the nucleolus to the nucleoplasm in the G2/M phase, and perturbation of its localization pattern delayed the progression of cell division. A single mutation in the Rnt1p N-terminal domain prevented its accumulation in the nucleoplasm and slowed exit from mitosis without any detectable effects on RNA processing. Together, the data reveal a new role for a class II RNase III in the cell cycle and suggest that at least some members of the RNase III family possess catalysis-independent functions.

The spindle assembly and spindle position checkpoints

The mitotic spindle segregates chromosomes to opposite ends of the cell in preparation for cell division. Chromosome attachment to the spindle is monitored by the spindle assembly checkpoint, and at least in yeast cells, penetration of one spindle pole into the bud is monitored by the spindle position checkpoint. We review the historical origins of these checkpoints and recent progress in understanding their surveillance pathways. We also highlight fascinating but as yet unresolved questions, and examine crosstalk between the checkpoints.

Maternally expressed and partially redundant β-tubulins inCaenorhabditis elegansare autoregulated

The mitotic spindle, which partitions replicated chromosomes to daughter cells during cell division, is composed of microtubule assemblies of α/β-tubulin heterodimers. Positioning of the mitotic spindle influences the size and location of daughter cells, and can be important for the proper partitioning of developmental determinants. We describe two semi-dominant mis-sense mutations in tbb-2, one of two C. elegans β-tubulin genes that are maternally expressed and together are required for microtubule-dependent processes in the early embryo. These mutations result in a posteriorly displaced and misoriented mitotic spindle during the first cell division. In contrast, a probable tbb-2 null allele is recessive, and when homozygous results in less severe spindle positioning defects and only partially penetrant embryonic lethality. Two of the tbb-2 mutations result in reduced levels of TBB-2 protein, and increased levels of a second maternally expressed β-tubulin, TBB-1. However, levels of TBB-1 are not increased in a tbb-2 mutant with an allele that does not result in reduced levels of TBB-2 protein. We conclude that feedback regulation influences maternal β-tubulin expression in C. elegans, but cannot fully restore normal microtubule function in the absence of one β-tubulin isoform.

The Drosophila kinesin-like protein KLP67A is essential for mitotic and male meiotic spindle assembly

We have performed a mutational analysis together with RNA interference to determine the role of the kinesin-like protein KLP67A in Drosophila cell division. During both mitosis and male meiosis, Klp67A mutations cause an increase in MT length and disrupt discrete aspects of spindle assembly, as well as cytokinesis. Mutant cells exhibit greatly enlarged metaphase spindle as a result of excessive MT polymerization. The analysis of both living and fixed cells also shows perturbations in centrosome separation, chromosome segregation, and central spindle assembly. These data demonstrate that the MT plus end-directed motor KLP67A is essential for spindle assembly during mitosis and male meiosis and suggest that the regulation of MT plus-end polymerization is a key determinant of spindle architecture throughout cell division.

Mutations in a beta-tubulin disrupt spindle orientation and microtubule dynamics in the early Caenorhabditis elegans embryo

The early Caenorhabditis elegans embryo contains abundant transcripts for two alpha- and two beta-tubulins, raising the question of whether each isoform performs specialized functions or simply contributes to total tubulin levels. Our identification of two recessive, complementing alleles of a beta-tubulin that disrupt nuclear-centrosome centration and rotation in the early embryo originally suggested that this tubulin, tbb-2, has specialized functions. However, embryos from tbb-2 deletion worms do not have defects in nuclear-centrosome centration and rotation suggesting that the complementing alleles are not null mutations. Both complementing alleles have distinct effects on microtubule dynamics and show allele-specific interactions with the two embryonically expressed alpha-tubulins: One of the alleles causes microtubules to be cold stable and resistant to the microtubule-depolymerizing drug benomyl, whereas the other causes cell cycle-specific defects in microtubule polymerization. Gene-specific RNA interference targeting all four embryonically expressed tubulin genes singly and in all double combinations showed that the tubulin isoforms in the early embryo are largely functionally redundant with the exception of tbb-2. tbb-2 is required for centrosome stabilization during anaphase of the first cell division, suggesting that tbb-2 may be specialized for interactions with the cell cortex.

Orientation of asymmetric stem cell division by the APC tumor suppressor and centrosome

Stem cell self-renewal can be specified by local signals from the surrounding microenvironment, or niche. However, the relation between the niche and the mechanisms that ensure the correct balance between stem cell self-renewal and differentiation is poorly understood. Here, we show that dividing Drosophila male germline stem cells use intracellular mechanisms involving centrosome function and cortically localized Adenomatous Polyposis Coli tumor suppressor protein to orient mitotic spindles perpendicular to the niche, ensuring a reliably asymmetric outcome in which one daughter cell remains in the niche and self-renews stem cell identity, whereas the other, displaced away, initiates differentiation.

CaSPA2 is important for polarity establishment and maintenance in Candida albicans

Saccharomyces cerevisiae Spa2p is a component of polarisome that controls cell polarity. Here, we have characterized the role of its homologue, CaSpa2p, in the polarized growth in Candida albicans. During yeast growth, GFP-tagged CaSpa2p localized to distinct growth sites in a cell cycle-dependent manner, while during hyphal growth it persistently localized to hyphal tips throughout the cell cycle. Persistent tip localization of the protein was also observed in Catup1Delta and Canrg1Delta, mutants constitutive for filamentous growth. Caspa2Delta exhibited defects in polarity establishment and maintenance, such as random budding and failure to confine growth to a small surface area leading to round cells with wide, elongated bud necks and markedly thicker hyphae. It was also defective in nuclear positioning, presumably a result of defective interactions between cytoplasmic microtubules with certain polarity determinants. The highly conserved SHD-I and SHD-V domains were found to be important and responsible for different aspects of CaSpa2p function. Caspa2Delta exhibited no virulence in the mouse systemic candidiasis model. Because of the existence of distinct growth forms and the easy control of the switch between them in vitro, C. albicans may serve as a useful model in cell polarity research.

Novel role of cytoplasmic dynein motor in maintenance of the nuclear number in conidia through organized conidiation in Aspergillus oryzae

Cytoplasmic dynein is a minus-end-directed, microtubule-dependent motor protein complex. DhcA, cytoplasmic dynein heavy chain in Aspergillus oryzae, contained four P-loops involved in ATP binding which were conserved as in cytoplasmic dynein heavy chains of other organisms. The amino acid sequence of A. oryzae DhcA was similar to cytoplasmic dynein heavy chains from other organisms except for the N-terminus of Saccharomyces cerevisiae Dyn1. Disruption of dhcA gene in the region encoding four P-loop motifs resulted in a defective growth and perturbed distribution of nuclei and vacuoles. The dhcA disruptant exhibited an abnormal morphology of conidial heads and conidia with an increased nuclear number. The present study implicates a novel role of cytoplasmic dynein in maintenance of the nuclear number in conidia through an organized conidiation.

The distribution of active force generators controls mitotic spindle position

During unequal cell divisions a mitotic spindle is eccentrically positioned before cell cleavage. To determine the basis of the net force imbalance that causes spindle displacement in one-cell Caenorhabditis elegans embryos, we fragmented centrosomes with an ultraviolet laser. Analysis of the mean and variance of fragment speeds suggests that the force imbalance is due to a larger number of force generators pulling on astral microtubules of the posterior aster relative to the anterior aster. Moreover, activation of heterotrimeric guanine nucleotide- binding protein (Gprotein) alpha subunits is required to generate these astral forces.

EB1-microtubule interactions in Xenopus egg extracts: role of EB1 in microtubule stabilization and mechanisms of targeting to microtubules

EB1 targets to polymerizing microtubule ends, where it is favorably positioned to regulate microtubule polymerization and confer molecular recognition of the microtubule end. In this study, we focus on two aspects of the EB1-microtubule interaction: regulation of microtubule dynamics by EB1 and the mechanism of EB1 association with microtubules. Immunodepletion of EB1 from cytostatic factor-arrested M-phase Xenopus egg extracts dramatically reduced microtubule length; this was complemented by readdition of EB1. By time-lapse microscopy, EB1 increased the frequency of microtubule rescues and decreased catastrophes, resulting in increased polymerization and decreased depolymerization and pausing. Imaging of EB1 fluorescence revealed a novel structure: filamentous extensions on microtubule plus ends that appeared during microtubule pauses; loss of these extensions correlated with the abrupt onset of polymerization. Fluorescent EB1 localized to comets at the polymerizing plus ends of microtubules in cytostatic factor extracts and uniformly along the lengths of microtubules in interphase extracts. The temporal decay of EB1 fluorescence from polymerizing microtubule plus ends predicted a dissociation half-life of seconds. Fluorescence recovery after photobleaching also revealed dissociation and rebinding of EB1 to the microtubule wall with a similar half-life. EB1 targeting to microtubules is thus described by a combination of higher affinity binding to polymerizing ends and lower affinity binding along the wall, with continuous dissociation. The latter is likely to be attenuated in interphase. The highly conserved effect of EB1 on microtubule dynamics suggests it belongs to a core set of regulatory factors conserved in higher organisms, and the complex pattern of EB1 targeting to microtubules could be exploited by the cell for coordinating microtubule behaviors.

The microtubule plus-end proteins EB1 and dynactin have differential effects on microtubule polymerization

Several microtubule-binding proteins including EB1, dynactin, APC, and CLIP-170 localize to the plus-ends of growing microtubules. Although these proteins can bind to microtubules independently, evidence for interactions among them has led to the hypothesis of a plus-end complex. Here we clarify the interaction between EB1 and dynactin and show that EB1 binds directly to the N-terminus of the p150(Glued) subunit. One function of a plus-end complex may be to regulate microtubule dynamics. Overexpression of either EB1 or p150(Glued) in cultured cells bundles microtubules, suggesting that each may enhance microtubule stability. The morphology of these bundles, however, differs dramatically, indicating that EB1 and dynactin may act in different ways. Disruption of the dynactin complex augments the bundling effect of EB1, suggesting that dynactin may regulate the effect of EB1 on microtubules. In vitro assays were performed to elucidate the effects of EB1 and p150(Glued) on microtubule polymerization, and they show that p150(Glued) has a potent microtubule nucleation effect, whereas EB1 has a potent elongation effect. Overall microtubule dynamics may result from a balance between the individual effects of plus-end proteins. Differences in the expression and regulation of plus-end proteins in different cell types may underlie previously noted differences in microtubule dynamics.

PAR-dependent and geometry-dependent mechanisms of spindle positioning

During intrinsically asymmetric division, the spindle is oriented onto a polarized axis specified by a group of conserved PAR proteins. Extrinsic geometric asymmetry generated by cell shape also affects spindle orientation in some systems, but how intrinsic and extrinsic mechanisms coexist without interfering with each other is unknown. In some asymmetrically dividing cells of the wild-type Caenorhabditis elegans embryo, nuclear rotation directed toward the anterior cortex orients the forming spindle. We find that in such cells, a PAR-dependent mechanism dominates and causes rotation onto the polarized axis, regardless of cell shape. However, when geometric asymmetry is removed, free nuclear rotation in the center of the cell is observed, indicating that the anterior-directed nature of rotation in unaltered embryos is an effect of cell shape. This free rotation is inconsistent with the prevailing model for nuclear rotation, the specialized cortical site model. In contrast, in par-3 mutant embryos, a geometry-dependent mechanism becomes active and causes directed nuclear rotation. These results lead to the model that in wild-type embryos both PAR-3 and PAR-2 are essential for nuclear rotation in asymmetrically dividing cells, but that PAR-3 inhibits geometry-dependent rotation in nonpolarized cells, thus preventing cell shape from interfering with spindle orientation.

Determinants of S. cerevisiae dynein localization and activation: implications for the mechanism of spindle positioning

BACKGROUND

During anaphase in budding yeast, dynein inserts the mitotic spindle across the neck between mother and daughter cells. The mechanism of dynein-dependent spindle positioning is thought to involve recruitment of dynein to the cell cortex followed by capture of astral microtubules (aMTs).

RESULTS

We report the native-level localization of the dynein heavy chain and characterize the effects of mutations in dynein regulators on its intracellular distribution. Budding yeast dynein displays discontinuous localization along aMTs, with enrichment at the spindle pole body and aMT plus ends. Loss of Bik1p (CLIP-170), the cargo binding domain of Bik1p, or Pac1p (LIS1) resulted in diminished targeting of dynein to aMTs. By contrast, loss of dynactin or a mutation in the second P loop domain of dynein resulted in an accumulation of dynein on the plus ends of aMTs. Unexpectedly, loss of Num1p, a proposed dynein cortical anchor, also resulted in selective accumulation of dynein on the plus ends of anaphase aMTs.

CONCLUSIONS

We propose that, rather than first being recruited to the cell cortex, dynein is delivered to the cortex on the plus ends of polymerizing aMTs. Dynein may then undergo Num1p-dependent activation and transfer to the region of cortical contact. Based on the similar effects of loss of Num1p and loss of dynactin on dynein localization, we suggest that Num1p might also enhance dynein motor activity or processivity, perhaps by clustering dynein motors.

Kar9 asymmetrical loading on spindle poles mediates proper spindle alignment in budding yeast

In the February 21 issue of Cell, demonstrate that asymmetrical loading of Kar9 onto astral microtubules (MTs) emanating from the bud-ward-directed spindle pole ensures delivery of this spindle pole to the bud. Kar9 mediates alignment of the spindle with the cell polarity axis through a Myo2-dependent mechanism that reorients astral MTs toward the bud.

Control of Lte1 localization by cell polarity determinants and Cdc14

BACKGROUND

The putative guanine nucleotide exchange factor Lte1 plays an essential role in promoting exit from mitosis at low temperatures. Lte1 is thought to activate a Ras-like signaling cascade, the mitotic exit network (MEN). MEN promotes the release of the protein phosphatase Cdc14 from the nucleolus during anaphase, and this release is a prerequisite for exit from mitosis. Lte1 is present throughout the cell during G1 but is sequestered in the bud during S phase and mitosis by an unknown mechanism.

RESULTS

We show that anchorage of Lte1 in the bud requires septins, the cell polarity determinants Cdc42 and Cla4, and Kel1. Lte1 physically associates with Kel1 and requires Kel1 for its localization in the bud, suggesting a role for Kel1 in anchoring Lte1 at the bud cortex. Our data further implicate the PAK-like protein kinase Cla4 in controlling Lte1 phosphorylation and localization. CLA4 is required for Lte1 phosphorylation and bud localization. Furthermore, when overexpressed, CLA4 induces Lte1 phosphorylation and localization to regions of polarized growth. Finally, we show that Cdc14, directly or indirectly, controls Lte1 dephosphorylation and delocalization from the bud during exit from mitosis.

CONCLUSION

Restriction of Lte1 to the bud cortex depends on the cortical proteins Cdc42 and Kel1 and the septin ring. Cla4 and Cdc14 promote and demote Lte1 localization at and from the bud cortex, respectively, suggesting not only that the phosphorylation status of Lte1 controls its localization but also indicating that Cla4 and Cdc14 are key regulators of the spatial asymmetry of Lte1.

Hypertonicity-induced projections reflect cell polarity in mouse metaphase II oocytes: involvement of microtubules, microfilaments, and chromosomes

A previous study showed that with hypertonic sucrose treatment, a projection is formed in mouse metaphase II (MII) oocytes in proximity to the spindle and chromosomes, where a polarized cortical domain is located. However, little is known about the mechanisms involved in this process. Here, we designed a series of experiments to test the hypothesis that hypertonicity is the induction factor for the formation of projections in mouse MII oocytes. Our hypothesis was supported by the following evidence: 1) different concentrations of sucrose affected the formation and shape of projections, whereas serum or basic media had little effect; 2) other hypertonic sugar solutions could also induce projection formation; and 3) projections could also be induced by hypertonic NaCl solution. We then tested the hypothesis that the cytoskeleton was involved in the formation of hypertonicity-induced projections. This was investigated by culturing MII- and germinal vesicle-stage mouse oocytes in the presence or absence of cytoskeletal inhibitors, including cytochalasin B (disruption of actin filaments), nocodazole (disruption of microtubules), and taxol (polymerization of tubulin molecules). We found that none of the cytoskeletal inhibitors alone could prevent hypertonicity-induced projection formation, whereas the combination of cytochalasin B with nocodazole or with taxol blocked the formation of these projections in most matured oocytes. When immature oocytes were incubated in cytochalasin B, but not in nocodazole or taxol, the formation of an actin-rich domain and the peripheral positioning of the spindle were blocked during maturation; hence, no projections were formed, even after hypertonic sucrose treatment. Based on these observations, we propose that three components are necessary for projection formation: 1) a polarized cortical patch (e.g., an actin-rich domain), 2) rigid submembrane structures (e.g., a spindle and/or chromosomes), and 3) solid connections between the above. Any disturbance of one of these factors will affect the hypertonicity-induced projection formation. Hypertonicity-induced projection in mouse oocytes thus provides an experimental model for studies regarding cell polarity and the interaction between membrane and submembrane components.

Kar9p-independent microtubule capture at Bud6p cortical sites primes spindle polarity before bud emergence in Saccharomyces cerevisiae

Spindle orientation is critical for accurate chromosomal segregation in eukaryotic cells. In the yeast Saccharomyces cerevisiae, orientation of the mitotic spindle is achieved by a program of microtubule-cortex interactions coupled to spindle morphogenesis. We previously implicated Bud6p in directing microtubule capture throughout this program. Herein, we have analyzed cells coexpressing GFP:Bud6 and GFP:Tub1 fusions, providing a kinetic view of Bud6p-microtubule interactions in live cells. Surprisingly, even during the G1 phase, microtubule capture at the recent division site and the incipient bud is dictated by Bud6p. These contacts are eliminated in bud6 delta cells but are proficient in kar9 delta cells. Thus, Bud6p cues microtubule capture, as soon as a new cell polarity axis is established independent of Kar9p. Bud6p increases the duration of interactions and promotes distinct modes of cortical association within the bud and neck regions. In particular, microtubule shrinkage and growth at the cortex rarely occur away from Bud6p sites. These are the interactions selectively impaired at the bud cortex in bud6 delta cells. Finally, interactions away from Bud6p sites within the bud differ from those occurring at the mother cell cortex, pointing to the existence of an independent factor controlling cortical contacts in mother cells after bud emergence.

Microtubule capture: IQGAP and CLIP-170 expand the repertoire

Rho GTPases regulate microtubule capture near the cell cortex to polarize cells. What is surprising is the repertoire of interactions between proteins at the ends of microtubules and their cortical targets. The microtubule tip protein CLIP-170 has now been found to interact with the Cdc42/Rac effector IQGAP and mediate transient capture of microtubules.

Evidence that an interaction between EB1 and p150(Glued) is required for the formation and maintenance of a radial microtubule array anchored at the centrosome

EB1 is a microtubule tip-associated protein that interacts with the APC tumor suppressor protein and components of the dynein/dynactin complex. We have found that the C-terminal 50 and 84 amino acids (aa) of EB1 were sufficient to mediate the interactions with APC and dynactin, respectively. EB1 formed mutually exclusive complexes with APC and dynactin, and a direct interaction between EB1 and p150(Glued) was identified. EB1-GFP deletion mutants demonstrated a role for the N-terminus in mediating the EB1-microtubule interaction, whereas C-terminal regions contributed to both its microtubule tip localization and a centrosomal localization. Cells expressing the last 84 aa of EB1 fused to GFP (EB1-C84-GFP) displayed profound defects in microtubule organization and centrosomal anchoring. EB1-C84-GFP expression severely inhibited microtubule regrowth, focusing, and anchoring in transfected cells during recovery from nocodazole treatment. The recruitment of gamma-tubulin and p150(Glued) to centrosomes was also inhibited. None of these effects were seen in cells expressing the last 50 aa of EB1 fused to GFP. Furthermore, EB1-C84-GFP expression did not induce Golgi apparatus fragmentation. We propose that a functional interaction between EB1 and p150(Glued) is required for microtubule minus end anchoring at centrosomes during the assembly and maintenance of a radial microtubule array.

The association of 14-3-3gamma and actin plays a role in cell division and apoptosis in astrocytes

The 14-3-3 protein family plays critical regulatory roles in signaling pathways in cell division and apoptosis. 14-3-3gamma is mainly expressed in brain. Using primary cultures of cerebral cortical astrocytes, we investigated the relationships between 14-3-3gamma proteins and actin in astrocytes in cell division and under ischemia. Our results showed that endogenous 14-3-3gamma proteins in immature astrocytes appeared filamentous and co-localized with filamentous actin (F-actin). During certain stages of mitosis, 14-3-3gamma proteins first aggregated and then formed a ring-like structure that surrounded the daughter nuclei and enclosed the F-actin. In 4-week-old cultures of astrocytes, 14-3-3gamma proteins appeared as punctate aggregates in the cytoplasm. Under ischemia, 14-3-3gamma proteins formed filamentous structures and were closely associated with F-actin in surviving astrocytes. However, in apoptotic astrocytes, the intensity of immunostaining of 14-3-3gamma proteins in the cytoplasm decreased. The proteins aggregated around the nucleus and dissociated from the actin filaments. Reciprocal co-immunoprecipitations demonstrated that endogenous 14-3-3gamma proteins bound to detergent-soluble actin and the level of binding increased after 4h of ischemia. As actin is a critical structural protein heavily involved in cell division and apoptotic death, our findings suggest that 14-3-3gamma proteins play a role in cytoskeletal function during the process of cell division and apoptosis in astrocytes in association with actin.

A role for regulated binding of p150(Glued) to microtubule plus ends in organelle transport

A subset of microtubule-associated proteins, including cytoplasmic linker protein (CLIP)-170, dynactin, EB1, adenomatous polyposis coli, cytoplasmic dynein, CLASPs, and LIS-1, has been shown recently to target to the plus ends of microtubules. The mechanisms and functions of this binding specificity are not understood, although a role in encouraging microtubule elongation has been proposed. To extend previous work on the role of dynactin in organelle transport, we analyzed p150(Glued) by live-cell imaging. Time-lapse analysis of p150(Glued) revealed targeting to the plus ends of growing microtubules, requiring the NH2-terminal cytoskeleton-associated protein-glycine rich domain, but not EB1 or CLIP-170. Effectors of protein kinase A modulated microtubule binding and suggested p150(Glued) phosphorylation as a factor in plus-end binding specificity. Using a phosphosensitive monoclonal antibody, we mapped the site of p150(Glued) phosphorylation to Ser-19. In vivo and in vitro analysis of phosphorylation site mutants revealed that p150(Glued) phosphorylation mediates dynamic binding to microtubules. To address the function of dynamic binding, we imaged GFP-p150(Glued) during the dynein-dependent transport of Golgi membranes. Live-cell analysis revealed a transient interaction between Golgi membranes and GFP-p150(Glued)-labeled microtubules just prior to transport, implicating microtubules and dynactin in a search-capture mechanism for minus-end-directed organelles.

SRC-1 and Wnt signaling act together to specify endoderm and to control cleavage orientation in early C. elegans embryos

In early C. elegans embryos, signaling between a posterior blastomere, P2, and a ventral blastomere, EMS, specifies endoderm and orients the division axis of the EMS cell. Although Wnt signaling contributes to this polarizing interaction, no mutants identified to date abolish P2/EMS signaling. Here, we show that two tyrosine kinase-related genes, src-1 and mes-1, are required for the accumulation of phosphotyrosine between P2 and EMS. Moreover, src-1 and mes-1 mutants strongly enhance endoderm and EMS spindle rotation defects associated with Wnt pathway mutants. SRC-1 and MES-1 signal bidirectionally to control cell fate and division orientation in both EMS and P2. Our findings suggest that Wnt and Src signaling function in parallel to control developmental outcomes within a single responding cell.

Mechanisms of spindle positioning: focus on flies and worms

Accurate spindle positioning is crucial for spatial control of cell division. During metazoan development, coordination between polarity cues and spindle position also ensures correct segregation of cell fate determinants. Converging evidence indicates that spindle positioning is achieved through interactions between cortical anchors and the plus ends of microtubules, generating pulling forces acting on spindle poles. This article discusses recent findings that indicate how this mechanism might be used for spindle positioning during Drosophila and Caenorhabditis elegans development.

Dynactin is necessary for synapse stabilization

We present evidence that synapse retraction occurs during normal synaptic growth at the Drosophila neuromuscular junction (NMJ). An RNAi-based screen to identify the molecular mechanisms that regulate synapse retraction identified Arp-1/centractin, a subunit of the dynactin complex. Arp-1 dsRNA enhances synapse retraction, and this effect is phenocopied by a mutation in P150/Glued, also a dynactin component. The Glued protein is enriched within the presynaptic nerve terminal, and presynaptic expression of a dominant-negative Glued transgene enhances retraction. Retraction is associated with a local disruption of the synaptic microtubule cytoskeleton. Electrophysiological, ultrastructural, and immunohistochemical data support a model in which presynaptic retraction precedes disassembly of the postsynaptic apparatus. Our data suggests that dynactin functions locally within the presynaptic arbor to promote synapse stability.

LIS1, CLIP-170's key to the dynein/dynactin pathway

CLIP-170 is a plus-end tracking protein which may act as an anticatastrophe factor. It has been proposed to mediate the association of dynein/dynactin to microtubule (MT) plus ends, and it also binds to kinetochores in a dynein/dynactin-dependent fashion, both via its C-terminal domain. This domain contains two zinc finger motifs (proximal and distal), which are hypothesized to mediate protein-protein interactions. LIS1, a protein implicated in brain development, acts in several processes mediated by the dynein/dynactin pathway by interacting with dynein and other proteins. Here we demonstrate colocalization and direct interaction between CLIP-170 and LIS1. In mammalian cells, LIS1 recruitment to kinetochores is dynein/dynactin dependent, and recruitment there of CLIP-170 is dependent on its site of binding to LIS1, located in the distal zinc finger motif. Overexpression of CLIP-170 results in a zinc finger-dependent localization of a phospho-LIS1 isoform and dynactin to MT bundles, raising the possibility that CLIP-170 and LIS1 regulate dynein/dynactin binding to MTs. This work suggests that LIS1 is a regulated adapter between CLIP-170 and cytoplasmic dynein at sites involved in cargo-MT loading, and/or in the control of MT dynamics.

Post-karyokinesis centrosome movement leaves a trail of unanswered questions

The centrosome is a complex structure composed of a large number of proteins (pericentriolar material, PCM) usually organized around a pair of centrioles (or a centriole duplex). This structure is capable of nucleating and organizing microtubules, duplication, and motility. In general, episodes of dramatic centrosome movement correlate with periods of cellular reorganization and nowhere is cellular reorganization more apparent, or more important, than in the periods before and after cell division. It is now clear that centrosome movement occurs not only prior to cell division but also at its completion, in concert with cytokinesis. The focus of this review is the newly emerging picture of centrosome activity during the post-karyokinesis period and the role that this activity might play in the transition of cells from mitosis to interphase.

Dynein at the cortex

Cytoplasmic dynein is a minus end directed microtubule motor protein with numerous functions during interphase and mitosis. Recent evidence has identified several roles mediated by a fraction of cytoplasmic dynein associated with the cell cortex. So far, these include nuclear migration, mitotic spindle orientation, and cytoskeletal reorientation during wound healing, but others are likely. The possibility that a cortically bound form of dynein might represent its most ancient evolutionary state is discussed.

Polyploids require Bik1 for kinetochore-microtubule attachment

The attachment of kinetochores to spindle microtubules (MTs) is essential for maintaining constant ploidy in eukaryotic cells. Here, biochemical and imaging data is presented demonstrating that the budding yeast CLIP-170 orthologue Bik1is a component of the kinetochore-MT binding interface. Strikingly, Bik1 is not required for viability in haploid cells, but becomes essential in polyploids. The ploidy-specific requirement for BIK1 enabled us to characterize BIK1 without eliminating nonhomologous genes, providing a new approach to circumventing the overlapping function that is a common feature of the cytoskeleton. In polyploid cells, Bik1 is required before anaphase to maintain kinetochore separation and therefore contributes to the force that opposes the elastic recoil of attached sister chromatids. The role of Bik1 in kinetochore separation appears to be independent of the role of Bik1 in regulating MT dynamics. The finding that a protein involved in kinetochore-MT attachment is required for the viability of polyploids has potential implications for cancer therapeutics.

EB 1 immunofluorescence reveals an increase in growing astral microtubule length and number during anaphase in NRK-52E cells

Spindle positioning in animal cells is thought to rely upon the interaction of astral microtubules with the cell cortex. Information on the dynamics of astral microtubules during this process is scarce, in part because of the difficulty in visualising these microtubules by light microscopy. EB1 is a protein which specifically localises to growing microtubule distal tips. Immunostaining for EB1 therefore represents a powerful method for visualising the distribution of growing microtubule tips within cells. In this study we used EB1 immunostaining in mitotic NRK-52E cells to quantitatively analyse the length and number of growing astral microtubules during metaphase and anaphase. We observed a dramatic increase in growing astral microtubule length and number during anaphase. Furthermore, drug treatments which specifically destroyed astral microtubules resulted in an increase in misaligned anaphase but not metaphase spindles. We suggest that an anaphase-specific increase in growing astral microtubule length and number facilitates the maintenance of a correctly aligned spindle in mitotic NRK-52E cells.

A mammalian Partner of inscuteable binds NuMA and regulates mitotic spindle organization

Asymmetric cell division requires the orientation of mitotic spindles along the cell-polarity axis. In Drosophila neuroblasts, this involves the interaction of the proteins Inscuteable (Insc) and Partner of inscuteable (Pins). We report here that a human Pins-related protein, called LGN, is instead essential for the assembly and organization of the mitotic spindle. LGN is cytoplasmic in interphase cells, but associates with the spindle poles during mitosis. Ectopic expression of LGN disrupts spindle-pole organization and chromosome segregation. Silencing of LGN expression by RNA interference also disrupts spindle-pole organization and prevents normal chromosome segregation. We found that LGN binds the nuclear mitotic apparatus protein NuMA, which tethers spindles at the poles, and that this interaction is required for the LGN phenotype. Anti-LGN antibodies and the LGN-binding domain of NuMA both trigger microtubule aster formation in mitotic Xenopus egg extracts, and the NuMA-binding domain of LGN blocks aster assembly in egg extracts treated with taxol. Thus, we have identified a mammalian Pins homologue as a key regulator of spindle organization during mitosis.

Asymmetric cell division in fucoid algae: a role for cortical adhesions in alignment of the mitotic apparatus

The first cell division in zygotes of the fucoid brown alga Pelvetia compressa is asymmetric and we are interested in the mechanism controlling the alignment of this division. Since the division plane bisects the mitotic apparatus, we investigated the timing and mechanism of spindle alignments. Centrosomes, which give rise to spindle poles, aligned with the growth axis in two phases--a premetaphase rotation of the nucleus and centrosomes followed by a postmetaphase alignment that coincided with the separation of the mitotic spindle poles during anaphase and telophase. The roles of the cytoskeleton and cell cortex in the two phases of alignment were analyzed by treatment with pharmacological agents. Treatments that disrupted cytoskeleton or perturbed cortical adhesions inhibited pre-metaphase alignment and we propose that this rotational alignment is effected by microtubules anchored at cortical adhesion sites. Postmetaphase alignment was not affected by any of the treatments tested, and may be dependent on asymmetric cell morphology.

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.

Control of spindle polarity and orientation in Saccharomyces cerevisiae

Control of mitotic spindle orientation represents a major strategy for the generation of cell diversity during development of metazoans. Studies in the budding yeast Saccharomyces cerevisiae have contributed towards our present understanding of the general principles underlying the regulation of spindle positioning in an asymmetrically dividing cell. In S. cerevisiae, the mitotic spindle must orient along the cell polarity axis, defined by the site of bud emergence, to ensure correct nuclear division between the mother and daughter cells. Establishment of spindle polarity dictates this process and relies on the concerted control of spindle pole function and a precise program of cues originating from the cell cortex that directs cytoplasmic microtubule attachments during spindle morphogenesis. These cues cross talk with the machinery responsible for bud-site selection, indicating that orientation of the spindle in yeast cells is mechanistically coupled to the definition of a polarity axis and the division plane. Here, we propose a model integrating the inherently asymmetric properties of the spindle pathway with the program of positional information contributing towards orienting the spindle in budding yeast. Because the basic machinery orienting the spindle in higher-eukaryotic cells appears to be conserved, it might be expected that similar principles govern centrosome asymmetry in the course of metazoan development.