Showing papers by "Erich A. Nigg published in 2012"

3D-structured illumination microscopy provides novel insight into architecture of human centrosomes.

Abstract: Centrioles are essential for the formation of cilia and flagella. They also form the core of the centrosome, which organizes microtubule arrays important for cell shape, polarity, motility and division. Here, we have used super-resolution 3D-structured illumination microscopy to analyse the spatial relationship of 18 centriole and pericentriolar matrix (PCM) components of human centrosomes at different cell cycle stages. During mitosis, PCM proteins formed extended networks with interspersed γ-Tubulin. During interphase, most proteins were arranged at specific distances from the walls of centrioles, resulting in ring staining, often with discernible density masses. Through use of site-specific antibodies, we found the C-terminus of Cep152 to be closer to centrioles than the N-terminus, illustrating the power of 3D-SIM to study protein disposition. Appendage proteins showed rings with multiple density masses, and the number of these masses was strongly reduced during mitosis. At the proximal end of centrioles, Sas-6 formed a dot at the site of daughter centriole assembly, consistent with its role in cartwheel formation. Plk4 and STIL co-localized with Sas-6, but Cep135 was associated mostly with mother centrioles. Remarkably, Plk4 formed a dot on the surface of the mother centriole before Sas-6 staining became detectable, indicating that Plk4 constitutes an early marker for the site of nascent centriole formation. Our study provides novel insights into the architecture of human centrosomes and illustrates the power of super-resolution microscopy in revealing the relative localization of centriole and PCM proteins in unprecedented detail.

Cell-cycle-regulated expression of STIL controls centriole number in human cells.

Abstract: Control of centriole number is crucial for genome stability and ciliogenesis. Here, we characterize the role of human STIL, a protein that displays distant sequence similarity to the centriole duplication factors Ana2 in Drosophila and SAS-5 in Caenorhabditis elegans. Using RNA interference, we show that STIL is required for centriole duplication in human cells. Conversely, overexpression of STIL triggers the near-simultaneous formation of multiple daughter centrioles surrounding each mother, which is highly reminiscent of the phenotype produced by overexpression of the polo-like kinase PLK4 or the spindle assembly abnormal protein 6 homolog (SAS-6). We further show, by fluorescence and immunoelectron microscopy, that STIL is recruited to nascent daughter centrioles at the onset of centriole duplication and degraded, in an APC/C(Cdc20-Cdh1)-dependent manner, upon passage through mitosis. We did not detect a stable complex between STIL and SAS-6, but the two proteins resemble each other with regard to both localization and cell cycle control of expression. Thus, STIL cooperates with SAS-6 and PLK4 in the control of centriole number and represents a key centriole duplication factor in human cells.

The autoregulated instability of Polo-like kinase 4 limits centrosome duplication to once per cell cycle

Abstract: Centrioles organize the centrosome, and accurate control of their number is critical for the maintenance of genomic integrity. Centriole duplication occurs once per cell cycle and is controlled by Polo-like kinase 4 (Plk4). We showed previously that Plk4 phosphorylates itself to promote its degradation by the proteasome. Here we demonstrate that this autoregulated instability controls the abundance of endogenous Plk4. Preventing Plk4 autoregulation causes centrosome amplification, stabilization of p53, and loss of cell proliferation; moreover, suppression of p53 allows growth of cells carrying amplified centrosomes. Plk4 autoregulation thus guards against genome instability by limiting centrosome duplication to once per cell cycle.

Aurora B controls kinetochore–microtubule attachments by inhibiting Ska complex–KMN network interaction

Abstract: The KMN network (named according to the acronym for KNL1, Mis12, and Ndc80) and the more recently identified Ska complex (Ska1–3) have been shown to mediate kinetochore (KT)–microtubule (MT) attachments. How these two complexes cooperate to achieve stable end-on attachments remains unknown. In this paper, we show that Aurora B negatively regulates the localization of the Ska complex to KTs and that recruitment of the Ska complex to KTs depends on the KMN network. We identified interactions between members of the KMN and Ska complexes and demonstrated that these interactions are regulated by Aurora B. Aurora B directly phosphorylated Ska1 and Ska3 in vitro, and expression of phosphomimetic mutants of Ska1 and Ska3 impaired Ska KT recruitment and formation of stable KT–MT fibers (K-fibers), disrupting mitotic progression. We propose that Aurora B phosphorylation antagonizes the interaction between the Ska complex and the KMN network, thereby controlling Ska recruitment to KTs and stabilization of KT–MT attachments.

Acentrosomal spindle organization renders cancer cells dependent on the kinesin HSET.

Abstract: Centrosomes represent the major microtubule organizing centers (MTOCs) of animal somatic cells and orchestrate bipolar spindle assembly during mitotic cell division. In meiotic cells, the kinesin HSET compensates for the lack of centrosomes by focusing acentrosomal MTOCs into two spindle poles. By clustering multiple centrosomes into two spindle poles, HSET also mediates bipolar mitosis in cancer cells with supernumerary centrosomes. However, although dispensable in non-transformed human cells, the role of HSET in cancer cells with two centrosomes has remained elusive. In this study, we demonstrate that HSET is required for proper spindle assembly, stable pole-focusing and survival of cancer cells irrespective of normal or supernumerary centrosome number. Strikingly, we detected pronounced acentrosomal MTOC structures in untreated mitotic cancer cells. While in most cancer cells these acentrosomal MTOCs were rapidly incorporated into the assembling bipolar spindle, some cells eventually established bipolar spindles with acentrosomal poles and free centrosomes. These observations demonstrate that acentrosomal MTOCs were functional and that both centrosomal and acentrosomal mechanisms were required for bipolar spindle organization. Our study shows that HSET is critical for clustering acentrosomal and centrosomal MTOCs during spindle formation in human cancer cells with two bona fide centrosomes. Furthermore, we show that in checkpoint-defective cancer cells, acentrosomal spindle formation and HSET-dependence are partially mediated by a constitutive activation of the DNA damage response. In summary, we propose that acentrosomal spindle assembly mechanisms are hyperactive in cancer cells and promote HSET, a key driver of acentrosomal spindle organization, as an attractive target for cancer therapy.

On the regulation, function, and localization of the DNA-dependent ATPase PICH.

Abstract: The putative chromatin remodeling enzyme Plk1-interacting checkpoint helicase (PICH) was discovered as an interaction partner and substrate of the mitotic kinase Plk1. During mitosis PICH associates with centromeres and kinetochores and, most interestingly, constitutes a robust marker for ultrafine DNA bridges (UFBs) that connect separating chromatids in anaphase cells. The precise roles of PICH remain to be clarified. Here, we have used antibody microinjection and siRNA-rescue experiments to study PICH function and localization during M phase progression, with particular emphasis on the role of the predicted ATPase domain and the regulation of PICH localization by Plk1. We show that interference with PICH function results in chromatin bridge formation and micronucleation and that ATPase activity is critical for PICH function. Interestingly, an intact ATPase domain of PICH is required for prevention of chromatin bridge formation but not for UFB resolution, and quantitative analyses of UFB and chromatin bridge frequencies suggest that these structures are of different etiologies. We also show that the ATPase activity of PICH is required for temporal and spatial control of PICH localization to chromatin and that Plk1 likely controls PICH localization through phosphorylation of proteins distinct from PICH itself. This work strengthens the view that PICH is an important, Plk1-regulated enzyme, whose ATPase activity is essential for maintenance of genome integrity. Although not required for the spindle assembly checkpoint, PICH is clearly important for faithful chromosome segregation.

Uncovering the Molecular Machinery of the Human Spindle—An Integration of Wet and Dry Systems Biology

Abstract: The mitotic spindle is an essential molecular machine involved in cell division, whose composition has been studied extensively by detailed cellular biology, high-throughput proteomics, and RNA interference experiments. However, because of its dynamic organization and complex regulation it is difficult to obtain a complete description of its molecular composition. We have implemented an integrated computational approach to characterize novel human spindle components and have analysed in detail the individual candidates predicted to be spindle proteins, as well as the network of predicted relations connecting known and putative spindle proteins. The subsequent experimental validation of a number of predicted novel proteins confirmed not only their association with the spindle apparatus but also their role in mitosis. We found that 75% of our tested proteins are localizing to the spindle apparatus compared to a success rate of 35% when expert knowledge alone was used. We compare our results to the previously published MitoCheck study and see that our approach does validate some findings by this consortium. Further, we predict so-called "hidden spindle hub", proteins whose network of interactions is still poorly characterised by experimental means and which are thought to influence the functionality of the mitotic spindle on a large scale. Our analyses suggest that we are still far from knowing the complete repertoire of functionally important components of the human spindle network. Combining integrated bio-computational approaches and single gene experimental follow-ups could be key to exploring the still hidden regions of the human spindle system.