Centrosomes back in the limelight

Epigenetic Signatures of Centrosomes Are Novel Targets in Cancer Diagnosis: Insights from an Analysis of the Cancer Genome Atlas

The centrosome plays a central role for cellular signaling and is critical for several fundamental cellular processes in human cells. Centrosome abnormalities have been linked to multiple solid tumors and hematological malignancies. We sought to explore the potential role of the DNA methylation, a critical epigenetic modification, of centrosome-related genes in different cancers. The 450K array DNA methylation data and RNA-seq data were downloaded for ~4000 tumor samples and ~500 normal controls from The Cancer Genome Atlas (TCGA) project, covering 11 major cancer types. Cancers with more than 30 normal controls were retained for analysis. Differentially modified CpGs of centrosome genes were identified, and cancer-specific epigenetic models were developed using a machine-learning algorithm for each cancer type. The association between the methylation level of differential CpGs and the corresponding gene expression, as well as the co-localization of the differential CpGs and cis-regulatory elements were evaluated. In total, 2761 CpGs located on 160 centrosome genes for 6 cancers were included in the analysis. Cancer-specific models demonstrated a high accuracy in terms of the area under the receiver operating characteristic (ROC) curve (AUC > 0.9) in five cancers and showed tissue specificity. This study enhanced our understanding of the epigenetic mechanisms underlying the DNA methylation of centrosome-related genes in cancers, and showed the potential of these epigenetic modifications as novel cancer biomarkers.

Tools used to assay genomic instability in cancers and cancer meiomitosis

Genomic instability is a defining characteristic of cancer and the analysis of DNA damage at the chromosome level is a crucial part of the study of carcinogenesis and genotoxicity. Chromosomal instability (CIN), the most common level of genomic instability in cancers, is defined as the rate of loss or gain of chromosomes through successive divisions. As such, DNA in cancer cells is highly unstable. However, the underlying mechanisms remain elusive. There is a debate as to whether instability succeeds transformation, or if it is a by-product of cancer, and therefore, studying potential molecular and cellular contributors of genomic instability is of high importance. Recent work has suggested an important role for ectopic expression of meiosis genes in driving genomic instability via a process called meiomitosis. Improving understanding of these mechanisms can contribute to the development of targeted therapies that exploit DNA damage and repair mechanisms. Here, we discuss a workflow of novel and established techniques used to assess chromosomal instability as well as the nature of genomic instability such as double strand breaks, micronuclei, and chromatin bridges. For each technique, we discuss their advantages and limitations in a lab setting. Lastly, we provide detailed protocols for the discussed techniques.

Fussing About Fission: Defining Variety Among Mainstream and Exotic Apicomplexan Cell Division Modes

Cellular reproduction defines life, yet our textbook-level understanding of cell division is limited to a small number of model organisms centered around humans. The horizon on cell division variants is expanded here by advancing insights on the fascinating cell division modes found in the Apicomplexa, a key group of protozoan parasites. The Apicomplexa display remarkable variation in offspring number, whether karyokinesis follows each S/M-phase or not, and whether daughter cells bud in the cytoplasm or bud from the cortex. We find that the terminology used to describe the various manifestations of asexual apicomplexan cell division emphasizes either the number of offspring or site of budding, which are not directly comparable features and has led to confusion in the literature. Division modes have been primarily studied in two human pathogenic Apicomplexa, malaria-causing Plasmodium spp. and Toxoplasma gondii, a major cause of opportunistic infections. Plasmodium spp. divide asexually by schizogony, producing multiple daughters per division round through a cortical budding process, though at several life-cycle nuclear amplifications stages, are not followed by karyokinesis. T. gondii divides by endodyogeny producing two internally budding daughters per division round. Here we add to this diversity in replication mechanisms by considering the cattle parasite Babesia bigemina and the pig parasite Cystoisospora suis. B. bigemina produces two daughters per division round by a “binary fission” mechanism whereas C. suis produces daughters through both endodyogeny and multiple internal budding known as endopolygeny. In addition, we provide new data from the causative agent of equine protozoal myeloencephalitis (EPM), Sarcocystis neurona, which also undergoes endopolygeny but differs from C. suis by maintaining a single multiploid nucleus. Overall, we operationally define two principally different division modes: internal budding found in cyst-forming Coccidia (comprising endodyogeny and two forms of endopolygeny) and external budding found in the other parasites studied (comprising the two forms of schizogony, binary fission and multiple fission). Progressive insights into the principles defining the molecular and cellular requirements for internal vs. external budding, as well as variations encountered in sexual stages are discussed. The evolutionary pressures and mechanisms underlying apicomplexan cell division diversification carries relevance across Eukaryota.

The Hydrophobic Patch Directs Cyclin B to Centrosomes to Promote Global CDK Phosphorylation at Mitosis

The cyclin-dependent kinases (CDKs) are the major cell-cycle regulators that phosphorylate hundreds of substrates, controlling the onset of S phase and M phase [1, 2, 3]. However, the patterns of substrate phosphorylation increase are not uniform, as different substrates become phosphorylated at different times as cells proceed through the cell cycle [4, 5]. In fission yeast, the correct ordering of CDK substrate phosphorylation can be established by the activity of a single mitotic cyclin-CDK complex [6, 7]. Here, we investigate the substrate-docking region, the hydrophobic patch, on the fission yeast mitotic cyclin Cdc13 as a potential mechanism to correctly order CDK substrate phosphorylation. We show that the hydrophobic patch targets Cdc13 to the yeast centrosome equivalent, the spindle pole body (SPB), and disruption of this motif prevents both centrosomal localization of Cdc13 and the onset of mitosis but does not prevent S phase. CDK phosphorylation in mitosis is compromised for approximately half of all mitotic CDK substrates, with substrates affected generally being those that require the highest levels of CDK activity to become phosphorylated and those that are located at the SPB. Our experiments suggest that the hydrophobic patch of mitotic cyclins contributes to CDK substrate selection by directing the localization of Cdc13-CDK to centrosomes and that this localization of CDK contributes to the CDK substrate phosphorylation necessary to ensure proper entry into mitosis. Finally, we show that mutation of the hydrophobic patch prevents cyclin B1 localization to centrosomes in human cells, suggesting that this mechanism of cyclin-CDK spatial regulation may be conserved across eukaryotes.

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The hydrophobic patch of human and yeast cyclin B directs it to the centrosome

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Loss of the yeast cyclin B hydrophobic patch allows S phase but prevents mitosis

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Compartmentalized mitotic CDK phosphorylation relies on the hydrophobic patch

How the Other Half Lives: What p53 Does When It Is Not Being a Transcription Factor

It has been four decades since the discovery of p53, the designated ‘Guardian of the Genome’. P53 is primarily known as a master transcription factor and critical tumor suppressor, with countless studies detailing the mechanisms by which it regulates a host of gene targets and their consequent signaling pathways. However, transcription-independent functions of p53 also strongly define its tumor-suppressive capabilities and recent findings shed light on the molecular mechanisms hinted at by earlier efforts. This review highlights the transcription-independent mechanisms by which p53 influences the cellular response to genomic instability (in the form of replication stress, centrosome homeostasis, and transposition) and cell death. We also pinpoint areas for further investigation in order to better understand the context dependency of p53 transcription-independent functions and how these are perturbed when TP53 is mutated in human cancer.

TgCep250 is dynamically processed through the division cycle and is essential for structural integrity of the Toxoplasma centrosome

The apicomplexan centrosome has a unique bipartite structure comprising an inner and outer core responsible for the nuclear cycle (mitosis) and budding cycles (cytokinesis), respectively. Although these two cores are always associated, they function independently to facilitate polyploid intermediates in the production of many progeny per replication round. Here, we describe the function of a large coiled-coil protein in Toxoplasma gondii, TgCep250, in connecting the two centrosomal cores and promoting their structural integrity. Throughout the cell cycle, TgCep250 localizes to the inner core but, associated with proteolytic processing, is also present on the outer core during the onset of cell division. In the absence of TgCep250, stray centrosome inner and outer core foci were observed. The detachment between centrosomal inner and outer cores was found in only one of the centrosomes during cell division, indicating distinct states of mother and daughter centrosomes. In mammals, Cep250 processing is required for centrosomal splitting and is mediated by Nek phopsphorylation. However, we show that neither the nonoverlapping spatiotemporal localization of TgNek1 and TgCep250 nor the distinct phenotypes upon their respective depletion support conservation of this mechanism in Toxoplasma. In conclusion, TgCep250 has a tethering function tailored to the unique bipartite centrosome in the Apicomplexa.

Plasmodium centrin PbCEN-4 localizes to the putative MTOC and is dispensable for malaria parasite proliferation

Centrins are calmodulin-like phosphoproteins present in the centrosome and play an active role in the duplication, separation and organization of centrosomal structures such as the microtubule-organizing centre (MTOC) during mitosis. They are also major components of the basal body of flagella and cilia. In Plasmodium spp., the parasite that causes malaria, mitosis is closed during asexual replication and the MTOC is embedded within the intact nuclear membrane. The MTOC has been named the centriolar plaque and is similar to the spindle pole body in yeast. In all phases of asexual replication, repeated rounds of nuclear division precede cell division. However, our knowledge of the location and function of centrins during this process is limited. Previous studies have identified four putative centrins in the human parasite Plasmodiumfalciparum. We report here the cellular localization of an alveolate-specific centrin (PbCEN-4) during the atypical cell division of asexual replicative stages, using live cell imaging with the rodent malaria parasite P. berghei as a model system. We show that this centrin forms a multi-protein complex with other centrins, but is dispensable for parasite proliferation.

Summary: This study examines the localization of malaria parasite centrin PbCEN4 at the parasite MTOC during closed endomitosis and shows it to be dispensable for proliferation.

Inactivation of PLK4-STIL Module Prevents Self-Renewal and Triggers p53-Dependent Differentiation in Human Pluripotent Stem Cells

Centrioles account for centrosomes and cilia formation. Recently, a link between centrosomal components and human developmental disorders has been established. However, the exact mechanisms how centrosome abnormalities influence embryogenesis and cell fate are not understood. PLK4-STIL module represents a key element of centrosome duplication cycle. We analyzed consequences of inactivation of the module for early events of embryogenesis in human embryonic stem cells (hESCs) and human induced pluripotent stem cells (hiPSCs). We demonstrate that blocking of PLK4 or STIL functions leads to centrosome loss followed by both p53-dependent and -independent defects, including prolonged cell divisions, upregulation of p53, chromosome instability, and, importantly, reduction of pluripotency markers and induction of differentiation. We show that the observed loss of key stem cells properties is connected to alterations in mitotic timing and protein turnover. In sum, our data define a link between centrosome, its regulators, and the control of pluripotency and differentiation in PSCs.

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Blocking of PLK4-STIL module in hESCs/hiPSCs leads to:

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Centrosome loss, prolonged and error-prone mitosis;

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p53-dependent differentiation;

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Reduction of pluripotency linked to altered protein turnover

Cell polarity: having and making sense of direction-on the evolutionary significance of the primary cilium/centrosome organ in Metazoa

Cell-autonomous polarity in Metazoans is evolutionarily conserved. I assume that permanent polarity in unicellular eukaryotes is required for cell motion and sensory reception, integration of these two activities being an evolutionarily constrained function. Metazoans are unique in making cohesive multicellular organisms through complete cell divisions. They evolved a primary cilium/centrosome (PC/C) organ, ensuring similar functions to the basal body/flagellum of unicellular eukaryotes, but in different cells, or in the same cell at different moments. The possibility that this innovation contributed to the evolution of individuality, in being instrumental in the early specification of the germ line during development, is further discussed. Then, using the example of highly regenerative organisms like planarians, which have lost PC/C organ in dividing cells, I discuss the possibility that part of the remodelling necessary to reach a new higher-level unit of selection in multi-cellular organisms has been triggered by conflicts among individual cell polarities to reach an organismic polarity. Finally, I briefly consider organisms with a sensorimotor organ like the brain that requires exceedingly elongated polarized cells for its activity. I conclude that beyond critical consequences for embryo development, the conservation of cell-autonomous polarity in Metazoans had far-reaching implications for the evolution of individuality.

The connections of Wnt pathway components with cell cycle and centrosome: side effects or a hidden logic?

Wnt signaling cascade has developed together with multicellularity to orchestrate the development and homeostasis of complex structures. Wnt pathway components - such as β-catenin, Dishevelled (DVL), Lrp6, and Axin-- are often dedicated proteins that emerged in evolution together with the Wnt signaling cascade and are believed to function primarily in the Wnt cascade. It is interesting to see that in recent literature many of these proteins are connected with cellular functions that are more ancient and not limited to multicellular organisms - such as cell cycle regulation, centrosome biology, or cell division. In this review, we summarize the recent literature describing this crosstalk. Specifically, we attempt to find the answers to the following questions: Is the response to Wnt ligands regulated by the cell cycle? Is the centrosome and/or cilium required to activate the Wnt pathway? How do Wnt pathway components regulate the centrosomal cycle and cilia formation and function? We critically review the evidence that describes how these connections are regulated and how they help to integrate cell-to-cell communication with the cell and the centrosomal cycle in order to achieve a fine-tuned, physiological response.

Towards a molecular architecture of the centrosome in Toxoplasma gondii

Toxoplasma gondii is the causative agent of toxoplasmosis. The pathogenicity of this unicellular parasite is tightly linked to its ability to efficiently proliferate within its host. Tachyzoites, the fast dividing form of the parasite, divide by endodyogeny. This process involves a single round of DNA replication, closed nuclear mitosis, and assembly of two daughter cells within a mother. The successful completion of endodyogeny relies on the temporal and spatial coordination of a plethora of simultaneous events. It has been shown that the Toxoplasma centrosome serves as signaling hub which nucleates spindle microtubules during mitosis and organizes the scaffolding of daughter cells components during cytokinesis. In addition, the centrosome is essential for inheriting both the apicoplast (a chloroplast-like organelle) and the Golgi apparatus. A growing body of evidence supports the notion that the T. gondii centrosome diverges in protein composition, structure and organization from its counterparts in higher eukaryotes making it an attractive source of potentially druggable targets. Here, we summarize the current knowledge on T. gondii centrosomal proteins and extend the putative centrosomal protein repertoire by in silico identification of mammalian centrosomal protein orthologs. We propose a working model for the organization and architecture of the centrosome in Toxoplasma parasites. Experimental validation of our proposed model will uncover how each predicted protein translates into the biology of centrosome, cytokinesis, karyokinesis, and organelle inheritance in Toxoplasma parasites.

Klp10A modulates the localization of centriole-associated proteins during Drosophila male gametogenesis

Mutations in Klp10A, a microtubule-depolymerising Kinesin-13, lead to overly long centrioles in Drosophila male germ cells. We demonstrated that the loss of Klp10A modifies the distribution of typical proteins involved in centriole assembly and function. In the absence of Klp10A the distribution of Drosophila pericentrin-like protein (Dplp), Sas-4 and Sak/Plk4 that are restricted in control testes to the proximal end of the centriole increase along the centriole length. Remarkably, the cartwheel is lacking or it appears abnormal in mutant centrioles, suggesting that this structure may spatially delimit protein localization. Moreover, the parent centrioles that in control cells have the same dimensions grow at different rates in mutant testes with the mother centrioles longer than the daughters. Daughter centrioles have often an ectopic position with respect to the proximal end of the mothers and failed to recruit Dplp.

Mother centrioles are kicked out so that starfish zygote can grow

Most oocytes eliminate their centrioles during meiotic divisions through unclear mechanisms. In this issue, Borrego-Pinto et al. (2016. J Cell. Biol. http://dx.doi.org/10.1083/jcb.201510083) show that mother centrioles need to be eliminated from starfish oocytes by extrusion into the polar bodies for successful embryo development.

Rapid centriole assembly in Naegleria reveals conserved roles for both de novo and mentored assembly

Centrioles are eukaryotic organelles whose number and position are critical for cilia formation and mitosis. Many cell types assemble new centrioles next to existing ones ("templated" or mentored assembly). Under certain conditions, centrioles also form without pre-existing centrioles (de novo). The synchronous differentiation of Naegleria amoebae to flagellates represents a unique opportunity to study centriole assembly, as nearly 100% of the population transitions from having no centrioles to having two within minutes. Here, we find that Naegleria forms its first centriole de novo, immediately followed by mentored assembly of the second. We also find both de novo and mentored assembly distributed among all major eukaryote lineages. We therefore propose that both modes are ancestral and have been conserved because they serve complementary roles, with de novo assembly as the default when no pre-existing centriole is available, and mentored assembly allowing precise regulation of number, timing, and location of centriole assembly.

A comparative overview of the sperm centriolar complex in mammals and birds: Variations on a theme

This paper presents an overview of the structure, function and anomalies of the sperm centriolar complex (CC) on a comparative basis between mammals and birds. The information is based on selected references from the literature supplemented by original observations on spermiogenesis and sperm structure in disparate mammalian (cheetah and cane rat) and avian (ostrich, rhea and emu) species. Whereas the basic structure of the CC (a diplosome surrounded by pericentriolar material) is similar in Aves and Mammalia, certain differences are apparent. Centriole reduction does not generally occur in birds, but when present as in oscines, involves the loss of the proximal centriole. In ratites, the distal centriole forms the core of the entire midpiece and incorporates the outer dense fibres in addition to initiating axoneme formation. The elements of the connecting piece are not segmented in birds and less complex in basic design than in mammals. The functions of the various components of the CC appear to be similar in birds and mammals. Despite obvious differences in sperm head shape, the centrosomal anomalies afflicting both vertebrate groups demonstrate structural uniformity across species and display a similar range of defects. Most abnormalities result from defective migration and alignment of the CC relative to the nucleus. The most severe manifestation is that of acephalic sperm, while angled tail attachment, abaxial and multiflagellate sperm reflect additional defective forms. The stump-tail defect is not observed in birds. A comparison of defective sperm formation and centrosomal dysfunction at the molecular level is currently difficult owing to the paucity of relevant information on avian sperm.

LIN-41 inactivation leads to delayed centrosome elimination and abnormal chromosome behavior during female meiosis in Caenorhabditis elegans

During oogenesis, two successive meiotic cell divisions occur without functional centrosomes because of the inactivation and subsequent elimination of maternal centrosomes during the diplotene stage of meiosis I. Despite being a conserved phenomenon in most metazoans, the means by which this centrosome behavior is controlled during female meiosis remain elusive. Here, we conducted a targeted RNAi screening in the Caenorhabditis elegans gonad to identify novel regulators of centrosome behavior during oogenesis. We screened 513 genes known to be essential for embryo production and directly visualized GFP-γ-tubulin to monitor centrosome behavior at all stages of oogenesis. In the screening, we found that RNAi-mediated inactivation of 33 genes delayed the elimination of GFP-γ-tubulin at centrosomes during oogenesis, whereas inactivation of nine genes accelerated the process. Depletion of the TRIM-NHL protein LIN-41 led to a significant delay in centrosome elimination and to the separation and reactivation of centrosomes during oogenesis. Upon LIN-41 depletion, meiotic chromosomes were abnormally condensed and pulled toward one of the two spindle poles around late pachytene even though the spindle microtubules emanated from both centrosomes. Overall, our work provides new insights into the regulation of centrosome behavior to ensure critical meiotic events and the generation of intact oocytes.

Conserved molecular interactions in centriole-to-centrosome conversion

Centrioles are required to assemble centrosomes for cell division and cilia for motility and signalling. New centrioles assemble perpendicularly to pre-existing ones in G1-S and elongate throughout S and G2. Fully elongated daughter centrioles are converted into centrosomes during mitosis to be able to duplicate and organize pericentriolar material in the next cell cycle. Here we show that centriole-to-centrosome conversion requires sequential loading of Cep135, Ana1 (Cep295) and Asterless (Cep152) onto daughter centrioles during mitotic progression in both Drosophila melanogaster and human. This generates a molecular network spanning from the inner- to outermost parts of the centriole. Ana1 forms a molecular strut within the network, and its essential role can be substituted by an engineered fragment providing an alternative linkage between Asterless and Cep135. This conserved architectural framework is essential for loading Asterless or Cep152, the partner of the master regulator of centriole duplication, Plk4. Our study thus uncovers the molecular basis for centriole-to-centrosome conversion that renders daughter centrioles competent for motherhood.

Molecular Pathways Underlying Projection Neuron Production and Migration during Cerebral Cortical Development

Glutamatergic neurons of the mammalian cerebral cortex originate from radial glia (RG) progenitors in the ventricular zone (VZ). During corticogenesis, neuroblasts migrate toward the pial surface using two different migration modes. One is multipolar (MP) migration with random directional movement, and the other is locomotion, which is a unidirectional movement guided by the RG fiber. After reaching their final destination, the neurons finalize their migration by terminal translocation, which is followed by maturation via dendrite extension to initiate synaptogenesis and thereby complete neural circuit formation. This switching of migration modes during cortical development is unique in mammals, which suggests that the RG-guided locomotion mode may contribute to the evolution of the mammalian neocortical 6-layer structure. Many factors have been reported to be involved in the regulation of this radial neuronal migration process. In general, the radial migration can be largely divided into four steps; (1) maintenance and departure from the VZ of neural progenitor cells, (2) MP migration and transition to bipolar cells, (3) RG-guided locomotion, and (4) terminal translocation and dendrite maturation. Among these, many different gene mutations or knockdown effects have resulted in failure of the MP to bipolar transition (step 2), suggesting that it is a critical step, particularly in radial migration. Moreover, this transition occurs at the subplate layer. In this review, we summarize recent advances in our understanding of the molecular mechanisms underlying each of these steps. Finally, we discuss the evolutionary aspects of neuronal migration in corticogenesis.

Centrosomes and cancer: revisiting a long-standing relationship

Over a century ago, centrosome aberrations were postulated to cause cancer by promoting genome instability. The mechanisms governing centrosome assembly and function are increasingly well understood, allowing for a timely reappraisal of this postulate. This Review discusses recent advances that shed new light on the relationship between centrosomes and cancer, and raise the possibility that centrosome aberrations contribute to this disease in different ways than initially envisaged.

Subdiffraction resolution microscopy methods for analyzing centrosomes organization

In this chapter, we describe the current methods of examining the structure of centrosomes by fluorescence subdiffraction microscopy. By using recently developed microscopy techniques, centrosomal proteins can now be examined in cells with a resolution of only a few nanometers, a level of molecular detail beyond the reach of traditional cell biology methods as confocal and widefield microscopy. We emphasize imaging by three-dimensional structured illumination microscopy, stochastic optical reconstruction microscopy, and quantitative approaches to image data analysis.

The Drosophila centriole: conversion of doublets to triplets within the stem cell niche

We report here that two distinct centriole lineages exist in Drosophila: somatic centrioles usually composed by microtubule doublets and germ line centrioles characterized by triplets. Remarkably, the transition from doublets to triplets in the testis occurs within the stem cell niche with the formation of the C-tubule. We demonstrated that the old mother centriole that stays in the apical cytoplasm of the male germline stem cells (GSCs) is invariably composed by triplets, whereas its daughter is always built by mixed doublets and triplets. This difference represents the first documentation of a structural asymmetry between mother and daughter centrioles in Drosophila GSCs and may reflect a correlation between the architecture of parent centrioles and their ability to recruit centrosomal proteins. We also found that the old mother centriole is linked to the cell membrane by distinct projections that may play an important role in keeping its apical position during centrosome separation.