Elsa Logarinho

Different spindle checkpoint proteins monitor microtubule attachment and tension at kinetochores in Drosophila cells.

The spindle assembly checkpoint detects errors in kinetochore attachment to the spindle including insufficient microtubule occupancy and absence of tension across bi-oriented kinetochore pairs. Here, we analyse how the kinetochore localization of the Drosophila spindle checkpoint proteins Bub1, Mad2, Bub3 and BubR1, behave in response to alterations in microtubule binding or tension. To analyse the behaviour in the absence of tension, we treated S2 cells with low doses of taxol to disrupt microtubule dynamics and tension, but not kinetochore-microtubule occupancy. Under these conditions, we found that Mad2 and Bub1 do not accumulate at metaphase kinetochores whereas BubR1 does. Consistently, in mono-oriented chromosomes, both kinetochores accumulate BubR1 whereas Bub1 and Mad2 only localize at the unattached kinetochore. To study the effect of tension we analysed the kinetochore localization of spindle checkpoint proteins in relation to tension-sensitive kinetochore phosphorylation recognised by the 3F3/2 antibody. Using detergent-extracted S2 cells as a system in which kinetochore phosphorylation can be easily manipulated, we observed that BubR1 and Bub3 accumulation at kinetochores is dependent on the presence of phosphorylated 3F3/2 epitopes. However, Bub1 and Mad2 localize at kinetochores regardless of the 3F3/2 phosphorylation state. Altogether, our results suggest that spindle checkpoint proteins sense distinct aspects of kinetochore interaction with the spindle, with Mad2 and Bub1 monitoring microtubule occupancy while BubR1 and Bub3 monitor tension across attached kinetochores.

Localization of the Drosophila checkpoint control protein Bub3 to the kinetochore requires Bub1 but not Zw10 or Rod

Abstract. We report here the isolation and molecular characterization of the Drosophila homolog of the mitotic checkpoint control protein Bub3. The Drosophila Bub3 protein is associated with the centromere/kinetochore of chromosomes in larval neuroblasts whose spindle assembly checkpoints have been activated by incubation with the microtubule-depolymerizing agent colchicine. Drosophila Bub3 is also found at the kinetochore regions in mitotic larval neuroblasts and in meiotic primary and secondary spermatocytes, with the strong signal seen during prophase and prometaphase becoming increasingly weaker after the chromosomes have aligned at the metaphase plate. We further show that the localization of Bub3 to the kinetochore is disrupted by mutations in the gene encoding the Drosophila homolog of the spindle assembly checkpoint protein Bub1. Combined with recent findings showing that the kinetochore localization of Bub1 conversely depends upon Bub3, these results support the hypothesis that the spindle assembly checkpoint proteins exist as a multiprotein complex recruited as a unit to the kinetochore. In contrast, we demonstrate that the kinetochore constituents Zw10 and Rod are not needed for the binding of Bub3 to the kinetochore. This suggests that the kinetochore is assembled in at least two relatively independent pathways.

The human spindle assembly checkpoint protein Bub3 is required for the establishment of efficient kinetochore-microtubule attachments.

The spindle assembly checkpoint monitors the status of kinetochore-microtubule (K-MT) attachments and delays anaphase onset until full metaphase alignment is achieved. Recently, the role of spindle assembly checkpoint proteins was expanded with the discovery that BubR1 and Bub1 are implicated in the regulation of K-MT attachments. One unsolved question is whether Bub3, known to form cell cycle constitutive complexes with both BubR1 and Bub1, is also required for proper chromosome-to-spindle attachments. Using RNA interference and high-resolution microscopy, we analyzed K-MT interactions in Bub3-depleted cells and compared them to those in Bub1- or BubR1-depleted cells. We found that Bub3 is essential for the establishment of correct K-MT attachments. In contrast to BubR1 depletion, which severely compromises chromosome attachment and alignment, we found Bub3 and Bub1 depletions to produce defective K-MT attachments that, however, still account for significant chromosome congression. After Aurora B inhibition, alignment defects become severer in Bub3- and Bub1-depleted cells, while partially rescued in BubR1-depleted cells, suggesting that Bub3 and Bub1 depletions perturb K-MT attachments distinctly from BubR1. Interestingly, misaligned chromosomes in Bub3- and Bub1-depleted cells were found to be predominantly bound in a side-on configuration. We propose that Bub3 promotes the formation of stable end-on bipolar attachments.

Kinetochore-microtubule interactions “in check” by Bub1, Bub3 and BubR1: The dual task of attaching and signalling

The spindle assembly checkpoint (SAC) prevents anaphase onset until all chromosomes accomplish proper bipolar attachments to the mitotic spindle and come under tension, thereby ensuring the fidelity of chromosome segregation. Despite significant advances in our understanding of SAC signalling, a clear link between checkpoint signalling and the molecular mechanisms underlying chromosome attachment to microtubules has not been established so far. However, independent studies from many groups have interestingly found that the bone-a-fide Bub1, BubR1 and Bub3 SAC proteins are themselves required for proper kinetochore-microtubule (K-MT) interactions. Here, we review these findings and discuss the specific contribution of each of these proteins in the regulation of K-MT attachment, taking into consideration their interdependencies for kinetochore localization as well as their relationship with other proteins with a known role in chromosome attachment and congression.

Mitotic spindle multipolarity without centrosome amplification

Mitotic spindle bipolarity is essential for faithful segregation of chromosomes during cell division. Multipolar spindles are often seen in human cancers and are usually associated with supernumerary centrosomes that result from centrosome overduplication or cytokinesis failure. A less-understood path to multipolar spindle formation may arise due to loss of spindle pole integrity in response to spindle and/or chromosomal forces. Here we discuss the different routes leading to multipolar spindle formation, focusing on spindle multipolarity without centrosome amplification. We also present the distinct and common features between these pathways and discuss their therapeutic implications.

CLASPs prevent irreversible multipolarity by ensuring spindle-pole resistance to traction forces during chromosome alignment

Loss of spindle-pole integrity during mitosis leads to multipolarity independent of centrosome amplification. Multipolar-spindle conformation favours incorrect kinetochore–microtubule attachments, compromising faithful chromosome segregation and daughter-cell viability. Spindle-pole organization influences and is influenced by kinetochore activity, but the molecular nature behind this critical force balance is unknown. CLASPs are microtubule-, kinetochore- and centrosome-associated proteins whose functional perturbation leads to three main spindle abnormalities: monopolarity, short spindles and multipolarity. The first two reflect a role at the kinetochore–microtubule interface through interaction with specific kinetochore partners, but how CLASPs prevent spindle multipolarity remains unclear. Here we found that human CLASPs ensure spindle-pole integrity after bipolarization in response to CENP-E- and Kid-mediated forces from misaligned chromosomes. This function is independent of end-on kinetochore–microtubule attachments and involves the recruitment of ninein to residual pericentriolar satellites. Distinctively, multipolarity arising through this mechanism often persists through anaphase. We propose that CLASPs and ninein confer spindle-pole resistance to traction forces exerted during chromosome congression, thereby preventing irreversible spindle multipolarity and aneuploidy.

Cdc42p controls yeast-cell shape and virulence of Paracoccidioides brasiliensis

Paracoccidioides brasiliensis is characterized by a multiple budding phenotype and a polymorphic cell growth, leading to the formation of cells with extreme variations in shape and size. Since Cdc42 is a pivotal molecule in establishing and maintaining polarized growth for diverse cell types, as well as during pathogenesis of certain fungi, we evaluated its role during cell growth and virulence of the yeast-form of P. brasiliensis. We used antisense technology to knock-down PbCDC42s expression in P. brasiliensis yeast cells, promoting a decrease in cell size and more homogenous cell growth, altering the typical polymorphism of wild-type cells. Reduced expression levels also lead to increased phagocytosis and decreased virulence in a mouse model of infection. We provide genetic evidences underlying Pbcdc42p as an important protein during host-pathogen interaction and the relevance of the polymorphic nature and cell size in the pathogenesis of P. brasiliensis.

Chromosome mis-segregation and cytokinesis failure in trisomic human cells

Cancer cells display aneuploid karyotypes and typically mis-segregate chromosomes at high rates, a phenotype referred to as chromosomal instability (CIN). To test the effects of aneuploidy on chromosome segregation and other mitotic phenotypes we used the colorectal cancer cell line DLD1 (2n = 46) and two variants with trisomy 7 or 13 (DLD1+7 and DLD1+13), as well as euploid and trisomy 13 amniocytes (AF and AF+13). We found that trisomic cells displayed higher rates of chromosome mis-segregation compared to their euploid counterparts. Furthermore, cells with trisomy 13 displayed a distinctive cytokinesis failure phenotype. We showed that up-regulation of SPG20 expression, brought about by trisomy 13 in DLD1+13 and AF+13 cells, is sufficient for the cytokinesis failure phenotype. Overall, our study shows that aneuploidy can induce chromosome mis-segregation. Moreover, we identified a trisomy 13-specific mitotic phenotype that is driven by up-regulation of a gene encoded on the aneuploid chromosome. DOI: http://dx.doi.org/10.7554/eLife.05068.001

IFN-γ–Dependent Activation of Macrophages during Experimental Infections by Mycobacterium ulcerans Is Impaired by the Toxin Mycolactone

Buruli ulcer, caused by Mycobacterium ulcerans infections, is a necrotizing skin disease whose pathogenesis is associated with the exotoxin mycolactone. Despite the relevance of this emergent disease, little is known on the immune response against the pathogen. Following the recent demonstration of an intramacrophage growth phase for M. ulcerans, we investigated the biological relevance of IFN-γ and the antimycobacterial mechanisms activated by this cytokine in M. ulcerans-infected macrophages. Three M. ulcerans strains were tested: 5114 (mutant mycolactone-negative, avirulent strain); 94–1327 (intermediate virulence); and 98–912 (high virulence). We show in this study that IFN-γ is expressed in mouse-infected tissues and that IFN-γ–deficient mice display increased susceptibility to infection with strains 5114 and, to a lesser extent, 94–1327, but not with the highly virulent strain. Accordingly, IFN-γ–activated cultured macrophages controlled the proliferation of the avirulent and the intermediate virulent strains. Addition of mycolactone purified from strain 98–912 to cultures of IFN-γ–activated macrophages infected with the mycolactone-negative strain led to a dose-dependent inhibition of the IFN-γ–induced protective mechanisms, involving phagosome maturation/acidification and increased NO production, therefore resulting in increased bacterial burdens. Our findings suggest that the protection mediated by IFN-γ in M. ulcerans-infected macrophages is impaired by the local buildup of mycolactone.

Absence of ataxin-3 leads to cytoskeletal disorganization and increased cell death.

Ataxin-3 (ATXN3) is a widely expressed protein that binds to ubiquitylated proteins, has deubiquitylating activity in vitro and is thought to modulate substrate degradation through the ubiquitin-proteasome pathway. Expansion of a polyglutamine tract in ATXN3 causes Machado-Joseph disease, a late-onset neurodegenerative disorder characterized by ubiquitin-positive aggregate formation and specific neuronal death. Although ATXN3 has been involved in transcriptional repression and in the ubiquitin-proteasome pathway, its biological function is still unknown. In this work, we show that depletion of ATXN3 using small-interference RNA (siRNA) causes a prominent phenotype in both human and mouse cell lines. A mild increase in ubiquitylation occurs and cells exhibit ubiquitin-positive foci, which is consistent with ATXN3 putative function as a deubiquitylating enzyme. In addition, siATXN3-silenced cells exhibit marked morphological changes such as rounder shape and loss of adhesion protrusions. At a structural level, the microtubule, microfilament and intermediate filament networks are severely compromised and disorganized. This cytoskeletal phenotype is reversible and dependent on ATXN3 levels. Cell-extracellular matrix connection is also affected in ATXN3-depleted cells as talin expression is reduced in the focal adhesions and lower levels of alpha-1 integrin subunit are expressed at their surface. Although the cytoskeletal and adhesion problems do not originate any major change in the cell cycle of siATXN3-depleted cells, cell death is increased in siATXN3 cultures compared to controls. In summary, in this work we show that the absence of ATXN3 leads to an overt cytoskeletal/adhesion defect raising the possibility that this protein may play a role in the cytoskeleton.