Controlling centrosome number: licenses and blocks

Hexavalent Chromium Targets Securin to Drive Numerical Chromosome Instability in Human Lung Cells

Hexavalent chromium [Cr(VI)] is a known human lung carcinogen with widespread exposure in environmental and occupational settings. Despite well-known cancer risks, the molecular mechanisms of Cr(VI)-induced carcinogenesis are not well understood, but a major driver of Cr(VI) carcinogenesis is chromosome instability. Previously, we reported Cr(VI) induced numerical chromosome instability, premature centriole disengagement, centrosome amplification, premature centromere division, and spindle assembly checkpoint bypass. A key regulator of these events is securin, which acts by regulating the cleavage ability of separase. Thus, in this study we investigated securin disruption by Cr(VI) exposure. We exposed human lung cells to a particulate Cr(VI) compound, zinc chromate, for acute (24 h) and prolonged (120 h) time points. We found prolonged Cr(VI) exposure caused marked decrease in securin levels and function. After prolonged exposure at the highest concentration, securin protein levels were decreased to 15.3% of control cells, while securin mRNA quantification was 7.9% relative to control cells. Additionally, loss of securin function led to increased separase activity manifested as enhanced cleavage of separase substrates; separase, kendrin, and SCC1. These data show securin is targeted by prolonged Cr(VI) exposure in human lung cells. Thus, a new mechanistic model for Cr(VI)-induced carcinogenesis emerges with centrosome and centromere disruption as key components of numerical chromosome instability, a key driver in Cr(VI) carcinogenesis.

An Emerging Animal Model for Querying the Role of Whole Genome Duplication in Development, Evolution, and Disease

Whole genome duplication (WGD) or polyploidization can occur at the cellular, tissue, and organismal levels. At the cellular level, tetraploidization has been proposed as a driver of aneuploidy and genome instability and correlates strongly with cancer progression, metastasis, and the development of drug resistance. WGD is also a key developmental strategy for regulating cell size, metabolism, and cellular function. In specific tissues, WGD is involved in normal development (e.g., organogenesis), tissue homeostasis, wound healing, and regeneration. At the organismal level, WGD propels evolutionary processes such as adaptation, speciation, and crop domestication. An essential strategy to further our understanding of the mechanisms promoting WGD and its effects is to compare isogenic strains that differ only in their ploidy. Caenorhabditis elegans (C. elegans) is emerging as an animal model for these comparisons, in part because relatively stable and fertile tetraploid strains can be produced rapidly from nearly any diploid strain. Here, we review the use of Caenorhabditis polyploids as tools to understand important developmental processes (e.g., sex determination, dosage compensation, and allometric relationships) and cellular processes (e.g., cell cycle regulation and chromosome dynamics during meiosis). We also discuss how the unique characteristics of the C. elegans WGD model will enable significant advances in our understanding of the mechanisms of polyploidization and its role in development and disease.

Multiple centrosomes enhance migration and immune cell effector functions of mature dendritic cells

Centrosomes play a crucial role during immune cell interactions and initiation of the immune response. In proliferating cells, centrosome numbers are tightly controlled and generally limited to one in G1 and two prior to mitosis. Defects in regulating centrosome numbers have been associated with cell transformation and tumorigenesis. Here, we report the emergence of extra centrosomes in leukocytes during immune activation. Upon antigen encounter, dendritic cells pass through incomplete mitosis and arrest in the subsequent G1 phase leading to tetraploid cells with accumulated centrosomes. In addition, cell stimulation increases expression of polo-like kinase 2, resulting in diploid cells with two centrosomes in G1-arrested cells. During cell migration, centrosomes tightly cluster and act as functional microtubule-organizing centers allowing for increased persistent locomotion along gradients of chemotactic cues. Moreover, dendritic cells with extra centrosomes display enhanced secretion of inflammatory cytokines and optimized T cell responses. Together, these results demonstrate a previously unappreciated role of extra centrosomes for regular cell and tissue homeostasis.

Cartwheel disassembly regulated by CDK1-cyclin B kinase allows human centriole disengagement and licensing

Cartwheel assembly is considered the first step in the initiation of procentriole biogenesis; however, the reason for persistence of the assembled human cartwheel structure from S phase to late mitosis remains unclear. Here, we demonstrate mainly using cell synchronization, RNA interference, immunofluorescence and time-lapse-microscopy, biochemical analysis, and methods that the cartwheel persistently assembles and maintains centriole engagement and centrosome integrity during S phase to late G2 phase. Blockade of the continuous accumulation of centriolar Sas-6, a major cartwheel protein, after procentriole formation induces premature centriole disengagement and disrupts pericentriolar matrix integrity. Additionally, we determined that during mitosis, CDK1-cyclin B phosphorylates Sas-6 at T495 and S510, disrupting its binding to cartwheel component STIL and pericentriolar component Nedd1 and promoting cartwheel disassembly and centriole disengagement. Perturbation of this phosphorylation maintains the accumulation of centriolar Sas-6 and retains centriole engagement during mitotic exit, which results in the inhibition of centriole reduplication. Collectively, these data demonstrate that persistent cartwheel assembly after procentriole formation maintains centriole engagement and that this configuration is relieved through phosphorylation of Sas-6 by CDK1-cyclin B during mitosis in human cells.

Keep Calm and Carry on with Extra Centrosomes

Simple Summary

Precise chromosome segregation during mitosis is a vital event orchestrated by formation of bipolar spindle poles. Supernumerary centrosomes, caused by centrosome amplification, deteriorates mitotic processes, resulting in segregation defects leading to chromosomal instability (CIN). Centrosome amplification is frequently observed in various types of cancer and considered as a significant contributor to destabilization of chromosomes. This review provides a comprehensive overview of causes and consequences of centrosome amplification thoroughly describing molecular mechanisms.

Abstract

Aberrations in the centrosome number and structure can readily be detected at all stages of tumor progression and are considered hallmarks of cancer. Centrosome anomalies are closely linked to chromosome instability and, therefore, are proposed to be one of the driving events of tumor formation and progression. This concept, first posited by Boveri over 100 years ago, has been an area of interest to cancer researchers. We have now begun to understand the processes by which these numerical and structural anomalies may lead to cancer, and vice-versa: how key events that occur during carcinogenesis could lead to amplification of centrosomes. Despite the proliferative advantages that having extra centrosomes may confer, their presence can also lead to loss of essential genetic material as a result of segregational errors and cancer cells must deal with these deadly consequences. Here, we review recent advances in the current literature describing the mechanisms by which cancer cells amplify their centrosomes and the methods they employ to tolerate the presence of these anomalies, focusing particularly on centrosomal clustering.

The Transformation of the Centrosome into the Basal Body: Similarities and Dissimilarities between Somatic and Male Germ Cells and Their Relevance for Male Fertility

The sperm flagellum is essential for the transport of the genetic material toward the oocyte and thus the transmission of the genetic information to the next generation. During the haploid phase of spermatogenesis, i.e., spermiogenesis, a morphological and molecular restructuring of the male germ cell, the round spermatid, takes place that includes the silencing and compaction of the nucleus, the formation of the acrosomal vesicle from the Golgi apparatus, the formation of the sperm tail, and, finally, the shedding of excessive cytoplasm. Sperm tail formation starts in the round spermatid stage when the pair of centrioles moves toward the posterior pole of the nucleus. The sperm tail, eventually, becomes located opposed to the acrosomal vesicle, which develops at the anterior pole of the nucleus. The centriole pair tightly attaches to the nucleus, forming a nuclear membrane indentation. An articular structure is formed around the centriole pair known as the connecting piece, situated in the neck region and linking the sperm head to the tail, also named the head-to-tail coupling apparatus or, in short, HTCA. Finally, the sperm tail grows out from the distal centriole that is now transformed into the basal body of the flagellum. However, a centriole pair is found in nearly all cells of the body. In somatic cells, it accumulates a large mass of proteins, the pericentriolar material (PCM), that together constitute the centrosome, which is the main microtubule-organizing center of the cell, essential not only for the structuring of the cytoskeleton and the overall cellular organization but also for mitotic spindle formation and chromosome segregation. However, in post-mitotic (G1 or G0) cells, the centrosome is transformed into the basal body. In this case, one of the centrioles, which is always the oldest or mother centriole, grows the axoneme of a cilium. Most cells of the body carry a single cilium known as the primary cilium that serves as an antenna sensing the cell's environment. Besides, specialized cells develop multiple motile cilia differing in substructure from the immotile primary cilia that are essential in moving fluids or cargos over the cellular surface. Impairment of cilia formation causes numerous severe syndromes that are collectively subsumed as ciliopathies. This comparative overview serves to illustrate the molecular mechanisms of basal body formation, their similarities, and dissimilarities, in somatic versus male germ cells, by discussing the involved proteins/genes and their expression, localization, and function. The review, thus, aimed to provide a deeper knowledge of the molecular players that is essential for the expansion of clinical diagnostics and treatment of male fertility disorders.

Pericentriolar material

Pimento-Marques and Bettencourt-Dias discuss the composition, assembly and function of pericentriolar material - the proteinaceous material that surrounds the centrioles and forms the centrosome, the main microtubule organizing center found in animal cells.

Centrosome regulation and function in mammalian cortical neurogenesis

As the primary microtubule-organizing center in animal cells, centrosomes regulate microtubule cytoskeleton to support various cellular behaviors. They also serve as the base for nucleating primary cilia, the hub of diverse signaling pathways. Cells typically possess one centrosome that contains two inequal centrioles and undergoes semi-conservative duplication during cell division, resulting in two centrosomes with an inherent asymmetry in age and properties. While the centrosome is ubiquitously present, mutations of centrosome proteins are strongly associated with human microcephaly characterized by a small cerebral cortex, underscoring the importance of an intact centrosome in supporting cortical neurogenesis. Here we review recent advances on centrosome regulation and function in mammalian cortical neural progenitors and discuss the implications for a better understanding of cortical neurogenesis and related disease mechanisms.

ANKRD26 recruits PIDD1 to centriolar distal appendages to activate the PIDDosome following centrosome amplification

Centriole copy number is tightly maintained by the once-per-cycle duplication of these organelles. Centrioles constitute the core of centrosomes, which organize the microtubule cytoskeleton and form the poles of the mitotic spindle. Centrosome amplification is frequently observed in tumors, where it promotes aneuploidy and contributes to invasive phenotypes. In non-transformed cells, centrosome amplification triggers PIDDosome activation as a protective response to inhibit cell proliferation, but how extra centrosomes activate the PIDDosome remains unclear. Using a genome-wide screen, we identify centriole distal appendages as critical for PIDDosome activation in cells with extra centrosomes. The distal appendage protein ANKRD26 is found to interact with and recruit the PIDDosome component PIDD1 to centriole distal appendages, and this interaction is required for PIDDosome activation following centrosome amplification. Furthermore, a recurrent ANKRD26 mutation found in human tumors disrupts PIDD1 localization and PIDDosome activation in cells with extra centrosomes. Our data support a model in which ANKRD26 initiates a centriole-derived signal to limit cell proliferation in response to centrosome amplification.

RACK1 regulates centriole duplication through promoting the activation of polo-like kinase 1 by Aurora A

Breast cancer gene 1 (BRCA1) contributes to the regulation of centrosome number. We previously identified receptor for activated C kinase 1 (RACK1) as a BRCA1-interacting partner. RACK1, a scaffold protein that interacts with multiple proteins through its seven WD40 domains, directly binds to BRCA1 and localizes to centrosomes. RACK1 knockdown suppresses centriole duplication, whereas RACK1 overexpression causes centriole overduplication in a subset of mammary gland-derived cells. In this study, we showed that RACK1 binds directly to polo-like kinase 1 (PLK1) and Aurora A, and promotes the Aurora A-PLK1 interaction. RACK1 knockdown decreased phosphorylated PLK1 (p-PLK1) levels and the centrosomal localization of Aurora A and p-PLK1 in S phase, whereas RACK1 overexpression increased p-PLK1 level and the centrosomal localization of Aurora A and p-PLK1 in interphase, resulting in an increase of cells with abnormal centriole disengagement. Overexpression of cancer-derived RACK1 variants failed to enhance the Aurora A-PLK1 interaction, PLK1 phosphorylation and the centrosomal localization of p-PLK1. These results suggest that RACK1 functions as a scaffold protein that promotes the activation of PLK1 by Aurora A in order to promote centriole duplication.This article has an associated First Person interview with the first author of the paper.

With Age Comes Maturity: Biochemical and Structural Transformation of a Human Centriole in the Making

Centrioles are microtubule-based cellular structures present in most human cells that build centrosomes and cilia. Proliferating cells have only two centrosomes and this number is stringently maintained through the temporally and spatially controlled processes of centriole assembly and segregation. The assembly of new centrioles begins in early S phase and ends in the third G1 phase from their initiation. This lengthy process of centriole assembly from their initiation to their maturation is characterized by numerous structural and still poorly understood biochemical changes, which occur in synchrony with the progression of cells through three consecutive cell cycles. As a result, proliferating cells contain three structurally, biochemically, and functionally distinct types of centrioles: procentrioles, daughter centrioles, and mother centrioles. This age difference is critical for proper centrosome and cilia function. Here we discuss the centriole assembly process as it occurs in somatic cycling human cells with a focus on the structural, biochemical, and functional characteristics of centrioles of different ages.

Centrosomes: The good and the bad for brain development

Centrosomes nucleate and organise the microtubule cytoskeleton in animal cells. These membraneless organelles are key structures for tissue organisation, polarity and growth. Centrosome dysfunction, defined as deviation in centrosome numbers and/or structural integrity, has major impact on brain size and functionality, as compared with other tissues of the organism. In this review, we discuss the contribution of centrosomes to brain growth during development. We discuss in particular the impact of centrosome dysfunction in Drosophila and mammalian neural stem cell division and fitness, which ultimately underlie brain growth defects.

Phosphorylation of keratin 18 serine 52 regulates mother-daughter centriole engagement and microtubule nucleation by cell cycle-dependent accumulation at the centriole

Serine-52 (Ser52) is the major physiologic site of keratin 18 (K18) phosphorylation. Here, we report that serine-52 phosphorylated K18 (phospho-Ser52 K18) accumulated on centrosomes in a cell cycle-dependent manner. Moreover, we found that phospho-Ser52 K18 was located at the proximal end of the mother centriole. Transfection with the K18 Ser52 → Ala (K18 S52A) mutant prevented centriole localization of phospho-Ser52 K18 and resulted in separation of the mother-daughter centrioles. Inhibition of microtubule polymerization led to the disappearance of aggregated phospho-Ser52 K18 on the centrosome; removal of inhibitors resulted in reaccumulation of phospho-Ser52 K18 in microtubule-organizing centers. Transfection with a K18 S52A mutant inhibited microtubule nucleation. These results reveal a cell cycle-dependent change in centrosome localization of phospho-Ser52 k18 and strongly suggest that the phosphorylation status of Ser52 K18 of mother centrioles plays a critical role in maintaining a tight engagement between mother and daughter centrioles and also contributes to microtubule nucleation.

CEP135 isoform dysregulation promotes centrosome amplification in breast cancer cells

The centrosome, composed of two centrioles surrounded by pericentriolar material, is the cell's central microtubule-organizing center. Centrosome duplication is coupled with the cell cycle such that centrosomes duplicate once in S phase. Loss of such coupling produces supernumerary centrosomes, a condition called centrosome amplification (CA). CA promotes cell invasion and chromosome instability, two hallmarks of cancer. We examined the contribution of centriole overduplication to CA and the consequences for genomic stability in breast cancer cells. CEP135, a centriole assembly protein, is dysregulated in some breast cancers. We previously identified a short isoform of CEP135, CEP135mini, that represses centriole duplication. Here, we show that the relative level of full-length CEP135 (CEP135full) to CEP135mini (the CEP135full:mini ratio) is increased in breast cancer cell lines with high CA. Inducing expression of CEP135full in breast cancer cells increases the frequency of CA, multipolar spindles, anaphase-lagging chromosomes, and micronuclei. Conversely, inducing expression of CEP135mini reduces centrosome number. The differential expression of the CEP135 isoforms in vivo is generated by alternative polyadenylation. Directed genetic mutations near the CEP135mini alternative polyadenylation signal reduces the CEP135full:mini ratio and decreases CA. We conclude that dysregulation of CEP135 isoforms promotes centriole overduplication and contributes to chromosome segregation errors in breast cancer cells.

The Cep57-pericentrin module organizes PCM expansion and centriole engagement

Centriole duplication occurs once per cell cycle to ensure robust formation of bipolar spindles and chromosome segregation. Each newly-formed daughter centriole remains connected to its mother centriole until late mitosis. The disengagement of the centriole pair is required for centriole duplication. However, the mechanisms underlying centriole engagement remain poorly understood. Here, we show that Cep57 is required for pericentriolar material (PCM) organization that regulates centriole engagement. Depletion of Cep57 causes PCM disorganization and precocious centriole disengagement during mitosis. The disengaged daughter centrioles acquire ectopic microtubule-organizing-center activity, which results in chromosome mis-segregation. Similar defects are observed in mosaic variegated aneuploidy syndrome patient cells with cep57 mutations. We also find that Cep57 binds to the well-conserved PACT domain of pericentrin. Microcephaly osteodysplastic primordial dwarfism disease pericentrin mutations impair the Cep57-pericentrin interaction and lead to PCM disorganization. Together, our work demonstrates that Cep57 provides a critical interface between the centriole core and PCM.

Centriole disengagement occurs towards mitotic exit and involves cleavage of pericentrin, a component of the pericentriolar material. Here the authors show that depletion of the centrosomal protein Cep57 leads to precocious centriole disengagement, and that Cep57 binds pericentrin.

Development and Evolutionary Constraints in Animals

We review the evolutionary importance of developmental mechanisms in constraining evolutionary changes in animals—in other words, developmental constraints. We focus on hard constraints that can act on macroevolutionary timescales. In particular, we discuss the causes and evolutionary consequences of the ancient metazoan constraint that differentiated cells cannot divide and constraints against changes of phylotypic stages in vertebrates and other higher taxa. We conclude that in all cases these constraints are caused by complex and highly controlled global interactivity of development, the disturbance of which has grave consequences. Mutations that affect such global interactivity almost unavoidably have many deleterious pleiotropic effects, which will be strongly selected against and will lead to long-term evolutionary stasis. The discussed developmental constraints have pervasive consequences for evolution and critically restrict regeneration capacity and body plan evolution.

PPP1R35 ensures centriole homeostasis by promoting centriole-to-centrosome conversion

Centriole-to-centrosome conversion (CCC) safeguards centriole homeostasis by coupling centriole duplication with segregation, and is essential for stabilization of mature vertebrate centrioles naturally devoid of the geometric scaffold or the cartwheel. Here we identified PPP1R35, a putative regulator of the protein phosphatase PP1, as a novel centriolar protein required for CCC. We found that PPP1R35 is enriched at newborn daughter centrioles in S or G2 phase. In the absence of PPP1R35, centriole assembly initiates normally in S phase, but none of the nascent centrioles can form active centrosomes or recruit CEP295, an essential factor for CCC. Instead, all PPP1R35-null centrioles, although stable during their birth in interphase, become disintegrated after mitosis upon cartwheel removal. Surprisingly, we found that neither the centriolar localization nor the function of PPP1R35 in CCC requires the putative PP1-interacting motif. PPP1R35 is thus acting upstream of CEP295 to induce CCC for proper centriole maintenance.

Vesicular trafficking plays a role in centriole disengagement and duplication

Centrosomes are the major microtubule-nucleating and microtubule-organizing centers of cells and play crucial roles in microtubule anchoring, organelle positioning, and ciliogenesis. At the centrosome core lies a tightly associated or “engaged” mother–daughter centriole pair.  During mitotic exit, removal of centrosomal proteins pericentrin and Cep215 promotes “disengagement” by the dissolution of intercentriolar linkers, ensuring a single centriole duplication event per cell cycle.  Herein, we explore a new mechanism involving vesicular trafficking for the removal of centrosomal Cep215. Using small interfering RNA and CRISPR/Cas9 gene-edited cells, we show that the endocytic protein EHD1 regulates Cep215 transport from centrosomes to the spindle midbody, thus facilitating disengagement and duplication. We demonstrate that EHD1 and Cep215 interact and show that Cep215 displays increased localization to vesicles containing EHD1 during mitosis. Moreover, Cep215-containing vesicles are positive for internalized transferrin, demonstrating their endocytic origin. Thus, we describe a novel relationship between endocytic trafficking and the centrosome cycle, whereby vesicles of endocytic origin are used to remove key regulatory proteins from centrosomes to control centriole duplication.

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.

Direct binding of CEP85 to STIL ensures robust PLK4 activation and efficient centriole assembly

Centrosomes are required for faithful chromosome segregation during mitosis. They are composed of a centriole pair that recruits and organizes the microtubule-nucleating pericentriolar material. Centriole duplication is tightly controlled in vivo and aberrations in this process are associated with several human diseases, including cancer and microcephaly. Although factors essential for centriole assembly, such as STIL and PLK4, have been identified, the underlying molecular mechanisms that drive this process are incompletely understood. Combining protein proximity mapping with high-resolution structural methods, we identify CEP85 as a centriole duplication factor that directly interacts with STIL through a highly conserved interaction interface involving a previously uncharacterised domain of STIL. Structure-guided mutational analyses in vivo demonstrate that this interaction is essential for efficient centriolar targeting of STIL, PLK4 activation and faithful daughter centriole assembly. Taken together, our results illuminate a molecular mechanism underpinning the spatiotemporal regulation of the early stages of centriole duplication.

Building the right centriole for each cell type

Loncarek and Bettencourt-Dias review molecular mechanisms of centriole biogenesis amongst different organisms and cell types.

The centriole is a multifunctional structure that organizes centrosomes and cilia and is important for cell signaling, cell cycle progression, polarity, and motility. Defects in centriole number and structure are associated with human diseases including cancer and ciliopathies. Discovery of the centriole dates back to the 19th century. However, recent advances in genetic and biochemical tools, development of high-resolution microscopy, and identification of centriole components have accelerated our understanding of its assembly, function, evolution, and its role in human disease. The centriole is an evolutionarily conserved structure built from highly conserved proteins and is present in all branches of the eukaryotic tree of life. However, centriole number, size, and organization varies among different organisms and even cell types within a single organism, reflecting its cell type–specialized functions. In this review, we provide an overview of our current understanding of centriole biogenesis and how variations around the same theme generate alternatives for centriole formation and function.

Cytoplasmic E2f4 forms organizing centres for initiation of centriole amplification during multiciliogenesis

Abnormal development of multiciliated cells is a hallmark of a variety of human conditions associated with chronic airway diseases, hydrocephalus and infertility. Multiciliogenesis requires both activation of a specialized transcriptional program and assembly of cytoplasmic structures for large-scale centriole amplification that generates basal bodies. It remains unclear, however, what mechanism initiates formation of these multiprotein complexes in epithelial progenitors. Here we show that this is triggered by nucleocytoplasmic translocation of the transcription factor E2f4. After inducing a transcriptional program of centriole biogenesis, E2f4 forms apical cytoplasmic organizing centres for assembly and nucleation of deuterosomes. Using genetically altered mice and E2F4 mutant proteins we demonstrate that centriole amplification is crucially dependent on these organizing centres and that, without cytoplasmic E2f4, deuterosomes are not assembled, halting multiciliogenesis. Thus, E2f4 integrates nuclear and previously unsuspected cytoplasmic events of centriole amplification, providing new perspectives for the understanding of normal ciliogenesis, ciliopathies and cancer.

Multiciliogenesis requires activation of transcriptional and protein assembly programs; however, the mechanisms that initiate the formation of these multiprotein complexes are unclear. Here the authors show that after inducing centriole biogenesis genes, the transcription factor E2f4 is required in the cytoplasm for assembly and nucleation of deuterosomes.

Cdk1 Phosphorylates Drosophila Sas-4 to Recruit Polo to Daughter Centrioles and Convert Them to Centrosomes

Centrosomes and cilia are organized by a centriole pair comprising an older mother and a younger daughter. Centriole numbers are tightly regulated, and daughter centrioles (which assemble in S phase) cannot themselves duplicate or organize centrosomes until they have passed through mitosis. It is unclear how this mitotic “centriole conversion” is regulated, but it requires Plk1/Polo kinase. Here we show that in flies, Cdk1 phosphorylates the conserved centriole protein Sas-4 during mitosis. This creates a Polo-docking site that helps recruit Polo to daughter centrioles and is required for the subsequent recruitment of Asterless (Asl), a protein essential for centriole duplication and mitotic centrosome assembly. Point mutations in Sas-4 that prevent Cdk1 phosphorylation or Polo docking do not block centriole disengagement during mitosis, but block efficient centriole conversion and lead to embryonic lethality. These observations can explain why daughter centrioles have to pass through mitosis before they can duplicate and organize a centrosome.

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Cdk1 phosphorylates Sas-4 to initiate Polo/Plk1 recruitment to daughter centrioles

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Polo recruitment promotes Asterless (Asl) incorporation into daughter centrioles

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Asl incorporation licenses new centrioles to duplicate and organize centrosomes

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These observations help explain why centriole conversion is tied to mitosis

A USP28-53BP1-p53-p21 signaling axis arrests growth after centrosome loss or prolonged mitosis

Precise regulation of centrosome number is critical for accurate chromosome segregation and the maintenance of genomic integrity. In nontransformed cells, centrosome loss triggers a p53-dependent surveillance pathway that protects against genome instability by blocking cell growth. However, the mechanism by which p53 is activated in response to centrosome loss remains unknown. Here, we have used genome-wide CRISPR/Cas9 knockout screens to identify a USP28-53BP1-p53-p21 signaling axis at the core of the centrosome surveillance pathway. We show that USP28 and 53BP1 act to stabilize p53 after centrosome loss and demonstrate this function to be independent of their previously characterized role in the DNA damage response. Surprisingly, the USP28-53BP1-p53-p21 signaling pathway is also required to arrest cell growth after a prolonged prometaphase. We therefore propose that centrosome loss or a prolonged mitosis activate a common signaling pathway that acts to prevent the growth of cells that have an increased propensity for mitotic errors.

The Centrioles, Centrosomes, Basal Bodies, and Cilia of Drosophila melanogaster

Centrioles play a key role in the development of the fly. They are needed for the correct formation of centrosomes, the organelles at the poles of the spindle that can persist as microtubule organizing centers (MTOCs) into interphase. The ability to nucleate cytoplasmic microtubules (MTs) is a property of the surrounding pericentriolar material (PCM). The centriole has a dual life, existing not only as the core of the centrosome but also as the basal body, the structure that templates the formation of cilia and flagellae. Thus the structure and functions of the centriole, the centrosome, and the basal body have an impact upon many aspects of development and physiology that can readily be modeled in Drosophila Centrosomes are essential to give organization to the rapidly increasing numbers of nuclei in the syncytial embryo and for the spatially precise execution of cell division in numerous tissues, particularly during male meiosis. Although mitotic cell cycles can take place in the absence of centrosomes, this is an error-prone process that opens up the fly to developmental defects and the potential of tumor formation. Here, we review the structure and functions of the centriole, the centrosome, and the basal body in different tissues and cultured cells of Drosophila melanogaster, highlighting their contributions to different aspects of development and cell division.

The interplay between centrosomes and the Hippo tumor suppressor pathway

Centrosome amplification is a common feature of both solid and hematological human malignancies. Extra centrosomes are not merely innocent bystanders in cancer cells, but rather promote tumor progression by disrupting normal cellular architecture and generating chromosome instability. Consequently, centrosome amplification correlates with advanced tumor grade and overall poor clinical prognosis. By contrast, extra centrosomes are adversely tolerated in non-transformed cells and hinder cell proliferation. This suggests that in addition to acquiring extra centrosomes, cancer cells must also adapt to overcome the deleterious consequences associated with them. Here, we review evidence that implicates core components of the Hippo tumor suppressor pathway as having key roles in both the direct and indirect regulation of centrosome number. Intriguingly, functional inactivation of the Hippo pathway, which is common across broad spectrum of human cancers, likely represents one key adaptation that enables cancer cells to tolerate extra centrosomes.

14-3-3γ Prevents Centrosome Amplification and Neoplastic Progression

More than 80% of malignant tumors show centrosome amplification and clustering. Centrosome amplification results from aberrations in the centrosome duplication cycle, which is strictly coordinated with DNA-replication-cycle. However, the relationship between cell-cycle regulators and centrosome duplicating factors is not well understood. This report demonstrates that 14-3-3γ localizes to the centrosome and 14-3-3γ loss leads to centrosome amplification. Loss of 14-3-3γ results in the phosphorylation of NPM1 at Thr-199, causing early centriole disjunction and centrosome hyper-duplication. The centrosome amplification led to aneuploidy and increased tumor formation in mice. Importantly, an increase in passage of the 14-3-3γ-knockdown cells led to an increase in the number of cells containing clustered centrosomes leading to the generation of pseudo-bipolar spindles. The increase in pseudo-bipolar spindles was reversed and an increase in the number of multi-polar spindles was observed upon expression of a constitutively active 14-3-3-binding-defective-mutant of cdc25C (S216A) in the 14-3-3γ knockdown cells. The increase in multi-polar spindle formation was associated with decreased cell viability and a decrease in tumor growth. Our findings uncover the molecular basis of regulation of centrosome duplication by 14-3-3γ and inhibition of tumor growth by premature activation of the mitotic program and the disruption of centrosome clustering.

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.

p53 protects against genome instability following centriole duplication failure

Centriole function has been difficult to study because of a lack of specific tools that allow persistent and reversible centriole depletion. Here we combined gene targeting with an auxin-inducible degradation system to achieve rapid, titratable, and reversible control of Polo-like kinase 4 (Plk4), a master regulator of centriole biogenesis. Depletion of Plk4 led to a failure of centriole duplication that produced an irreversible cell cycle arrest within a few divisions. This arrest was not a result of a prolonged mitosis, chromosome segregation errors, or cytokinesis failure. Depleting p53 allowed cells that fail centriole duplication to proliferate indefinitely. Washout of auxin and restoration of endogenous Plk4 levels in cells that lack centrioles led to the penetrant formation of de novo centrioles that gained the ability to organize microtubules and duplicate. In summary, we uncover a p53-dependent surveillance mechanism that protects against genome instability by preventing cell growth after centriole duplication failure.

Integrity of the Pericentriolar Material Is Essential for Maintaining Centriole Association during M Phase

A procentriole is assembled next to the mother centriole during S phase and remains associated until M phase. After functioning as a spindle pole during mitosis, the mother centriole and procentriole are separated at the end of mitosis. A close association of the centriole pair is regarded as an intrinsic block to the centriole reduplication. Therefore, deregulation of this process may cause a problem in the centriole number control, resulting in increased genomic instability. Despite its importance for faithful centriole duplication, the mechanism of centriole separation is not fully understood yet. Here, we report that centriole pairs are prematurely separated in cells whose cell cycle is arrested at M phase by STLC. Dispersal of the pericentriolar material (PCM) was accompanied. This phenomenon was independent of the separase activity but needed the PLK1 activity. Nocodazole effectively inhibited centriole scattering in STLC-treated cells, possibly by reducing the microtubule pulling force around centrosomes. Inhibition of PLK1 also reduced the premature separation of centrioles and the PCM dispersal as well. These results revealed the importance of PCM integrity in centriole association. Therefore, we propose that PCM disassembly is one of the driving forces for centriole separation during mitotic exit.

Centrosome function and assembly in animal cells

It has become clear that the role of centrosomes extends well beyond that of important microtubule organizers. There is increasing evidence that they also function as coordination centres in eukaryotic cells, at which specific cytoplasmic proteins interact at high concentrations and important cell decisions are made. Accordingly, hundreds of proteins are concentrated at centrosomes, including cell cycle regulators, checkpoint proteins and signalling molecules. Nevertheless, several observations have raised the question of whether centrosomes are essential for many cell processes. Recent findings have shed light on the functions of centrosomes in animal cells and on the molecular mechanisms of centrosome assembly, in particular during mitosis. These advances should ultimately allow the in vitro reconstitution of functional centrosomes from their component proteins to unlock the secrets of these enigmatic organelles.

Binding of STIL to Plk4 activates kinase activity to promote centriole assembly

Binding of STIL activates Plk4, and the subsequent phosphorylation of STIL by Plk4 primes the binding of STIL to SAS6 to promote centriole assembly.

Centriole duplication occurs once per cell cycle in order to maintain control of centrosome number and ensure genome integrity. Polo-like kinase 4 (Plk4) is a master regulator of centriole biogenesis, but how its activity is regulated to control centriole assembly is unclear. Here we used gene editing in human cells to create a chemical genetic system in which endogenous Plk4 can be specifically inhibited using a cell-permeable ATP analogue. Using this system, we demonstrate that STIL localization to the centriole requires continued Plk4 activity. Most importantly, we show that direct binding of STIL activates Plk4 by promoting self-phosphorylation of the activation loop of the kinase. Plk4 subsequently phosphorylates STIL to promote centriole assembly in two steps. First, Plk4 activity promotes the recruitment of STIL to the centriole. Second, Plk4 primes the direct binding of STIL to the C terminus of SAS6. Our findings uncover a molecular basis for the timing of Plk4 activation through the cell cycle–regulated accumulation of STIL.

Plk1 relieves centriole block to reduplication by promoting daughter centriole maturation

Centrosome overduplication promotes mitotic abnormalities, invasion and tumorigenesis. Cells regulate the number of centrosomes by limiting centriole duplication to once per cell cycle. The orthogonal orientation between a mother and a daughter centriole, established at the time of centriole duplication, is thought to block further duplication of the mother centriole. Loss of orthogonal orientation (disengagement) between two centrioles during anaphase is considered a licensing event for the next round of centriole duplication. Disengagement requires the activity of Polo-like kinase 1 (Plk1), but how Plk1 drives this process is not clear. Here we employ correlative live/electron microscopy and demonstrate that Plk1 induces maturation and distancing of the daughter centriole, allowing reduplication of the mother centriole even if the original daughter centriole is still orthogonal to it. We find that mother centrioles can undergo reduplication when original daughter centrioles are only ∼80 nm apart, which is the distance centrioles normally reach during prophase.

Evolutionary problems in centrosome and centriole biology

Centrosomes have been an enigma to evolutionary biologists. Either they have been the subject of ill-founded speculation or they have been ignored. Here, we highlight evolutionary paradoxes and problems of centrosome and centriole evolution and seek to understand them in the light of recent advances in centrosome biology. Most evolutionary accounts of centrosome evolution have been based on the hypothesis that centrosomes are replicators, independent of the nucleus and cytoplasm. It is now clear, however, that this hypothesis is not tenable. Instead, centrosomes are formed de novo each cell division, with the presence of an old centrosome regulating, but not essential for, the assembly of a new one. Centrosomes are the microtubule-organizing centres of cells. They can potentially affect sensory and motor characters (as the basal body of cilia), as well as the movements of chromosomes during cell division. This latter role does not seem essential, however, except in male meiosis, and the reasons for this remain unclear. Although the centrosome is absent in some taxa, when it is present, its structure is extraordinarily conserved: in most taxa across eukaryotes, it does not appear to evolve at all. And yet a few insect groups display spectacular hypertrophy of the centrioles. We discuss how this might relate to the unusual reproductive system found in these insects. Finally, we discuss why the fate of centrosomes in sperm and early embryos might differ between different groups of animals.

Centriole maturation requires regulated Plk1 activity during two consecutive cell cycles

Newly formed centrioles in cycling cells undergo a maturation process that is almost two cell cycles long before they become competent to function as microtubule-organizing centers and basal bodies. As a result, each cell contains three generations of centrioles, only one of which is able to form cilia. It is not known how this long and complex process is regulated. We show that controlled Plk1 activity is required for gradual biochemical and structural maturation of the centrioles and timely appendage assembly. Inhibition of Plk1 impeded accumulation of appendage proteins and appendage formation. Unscheduled Plk1 activity, either in cycling or interphase-arrested cells, accelerated centriole maturation and appendage and cilia formation on the nascent centrioles, erasing the age difference between centrioles in one cell. These findings provide a new understanding of how the centriole cycle is regulated and how proper cilia and centrosome numbers are maintained in the cells.

SAS-6 assembly templated by the lumen of cartwheel-less centrioles precedes centriole duplication

Centrioles are 9-fold symmetric structures duplicating once per cell cycle. Duplication involves self-oligomerization of the centriolar protein SAS-6, but how the 9-fold symmetry is invariantly established remains unclear. Here, we found that SAS-6 assembly can be shaped by preexisting (or mother) centrioles. During S phase, SAS-6 molecules are first recruited to the proximal lumen of the mother centriole, adopting a cartwheel-like organization through interactions with the luminal wall, rather than via their self-oligomerization activity. The removal or release of luminal SAS-6 requires Plk4 and the cartwheel protein STIL. Abolishing either the recruitment or the removal of luminal SAS-6 hinders SAS-6 (or centriole) assembly at the outside wall of mother centrioles. After duplication, the lumen of engaged mother centrioles becomes inaccessible to SAS-6, correlating with a block for reduplication. These results lead to a proposed model that centrioles may duplicate via a template-based process to preserve their geometry and copy number.

Procentriole assembly without centriole disengagement - a paradox of male gametogenesis

Disengagement of parent centrioles represents the licensing process to restrict centriole duplication exactly once during the cell cycle. However, we provide compelling evidence that this general rule is overridden in insect gametogenesis, when distinct procentrioles are generated during prophase of the first meiosis while parent centrioles are still engaged. Moreover, the number of procentrioles increases during the following meiotic divisions, and up to four procentrioles were found at the base of each mother centriole. However, procentrioles fail to organize a complete set of A-tubules and are thus unable to function as a template for centriole formation. Such a system, in which procentrioles form but halt growth, represents a unique model to analyze the process of cartwheel assembly and procentriole formation.

Centrosome dysfunction contributes to chromosome instability, chromoanagenesis, and genome reprograming in cancer

The unique ability of centrosomes to nucleate and organize microtubules makes them unrivaled conductors of important interphase processes, such as intracellular payload traffic, cell polarity, cell locomotion, and organization of the immunologic synapse. But it is in mitosis that centrosomes loom large, for they orchestrate, with clockmaker's precision, the assembly and functioning of the mitotic spindle, ensuring the equal partitioning of the replicated genome into daughter cells. Centrosome dysfunction is inextricably linked to aneuploidy and chromosome instability, both hallmarks of cancer cells. Several aspects of centrosome function in normal and cancer cells have been molecularly characterized during the last two decades, greatly enhancing our mechanistic understanding of this tiny organelle. Whether centrosome defects alone can cause cancer, remains unanswered. Until recently, the aggregate of the evidence had suggested that centrosome dysfunction, by deregulating the fidelity of chromosome segregation, promotes and accelerates the characteristic Darwinian evolution of the cancer genome enabled by increased mutational load and/or decreased DNA repair. Very recent experimental work has shown that missegregated chromosomes resulting from centrosome dysfunction may experience extensive DNA damage, suggesting additional dimensions to the role of centrosomes in cancer. Centrosome dysfunction is particularly prevalent in tumors in which the genome has undergone extensive structural rearrangements and chromosome domain reshuffling. Ongoing gene reshuffling reprograms the genome for continuous growth, survival, and evasion of the immune system. Manipulation of molecular networks controlling centrosome function may soon become a viable target for specific therapeutic intervention in cancer, particularly since normal cells, which lack centrosome alterations, may be spared the toxicity of such therapies.

Meiotic HORMA domain proteins prevent untimely centriole disengagement during Caenorhabditis elegans spermatocyte meiosis

In many species where oocytes lack centrosomes, sperm contribute both genetic material and centriole(s) to the zygote. Correct centriole organization during male meiosis is critical to guarantee a normal bipolar mitotic spindle in the zygote. During Caenorhabditis elegans male meiosis, centrioles normally undergo two rounds of duplication, resulting in haploid sperm each containing a single tightly engaged centriole pair. Here we identify an unanticipated role for C. elegans HORMA (Hop1/Rev7/Mad2) domain proteins HTP-1/2 and HIM-3 in regulating centriole disengagement during spermatocyte meiosis. In him-3 and htp-1 htp-2 mutants, centrioles separate inappropriately during meiosis II, resulting in spermatids with disengaged centrioles. Moreover, extra centrosomes are detected in a subset of zygotes. Together, these data implicate HIM-3 and HTP-1/2 in preventing centriole disengagement during meiosis II. We showed previously that HTP-1/2 prevents premature loss of sister chromatid cohesion during the meiotic divisions by inhibiting removal of meiotic cohesin complexes containing the REC-8 subunit. Worms lacking REC-8, or expressing a mutant separase protein with elevated local concentration at centrosomes and in sperm, likewise exhibit inappropriate centriole separation during spermatocyte meiosis. These observations are consistent with HIM-3 and HTP-1/2 preventing centriole disengagement by inhibiting separase-dependent cohesin removal. Our data suggest that the same specialized meiotic mechanisms that function to prevent premature release of sister chromatid cohesion during meiosis I in C. elegans also function to inhibit centriole separation at meiosis II, thereby ensuring that the zygote inherits the appropriate complement of chromosomes and centrioles.

Show me your license, please: deregulation of centriole duplication mechanisms that promote amplification

Centrosomes are organelles involved in generating and organizing the interphase microtubule cytoskeleton, mitotic spindles and cilia. At the centrosome core are a pair of centrioles, structures that act as the duplicating elements of this organelle. Centrioles function to recruit and organize pericentriolar material which nucleates microtubules. While centrioles are relatively simple in construction, the mechanics of centriole biogenesis remain an important yet poorly understood process. More mysterious still are the regulatory mechanisms that oversee centriole assembly. The fidelity of centriole duplication is critical as defects in either the assembly or number of centrioles promote aneuploidy, primary microcephaly, birth defects, ciliopathies and tumorigenesis. In addition, some pathogens employ mechanisms to promote centriole overduplication to the detriment of the host cell. This review summarizes our current understanding of this important topic, highlighting the need for further study if new therapeutics are to be developed to treat diseases arising from defects of centrosome duplication.

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

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.

Transcriptional program of ciliated epithelial cells reveals new cilium and centrosome components and links to human disease

Defects in the centrosome and cilium are associated with a set of human diseases having diverse phenotypes. To further characterize the components that define the function of these organelles we determined the transcriptional profile of multiciliated tracheal epithelial cells. Cultures of mouse tracheal epithelial cells undergoing differentiation in vitro were derived from mice expressing GFP from the ciliated-cell specific FOXJ1 promoter (FOXJ1:GFP). The transcriptional profile of ciliating GFP+ cells from these cultures was defined at an early and a late time point during differentiation and was refined by subtraction of the profile of the non-ciliated GFP- cells. We identified 649 genes upregulated early, when most cells were forming basal bodies, and 73 genes genes upregulated late, when most cells were fully ciliated. Most, but not all, of known centrosome proteins are transcriptionally upregulated early, particularly Plk4, a master regulator of centriole formation. We found that three genes associated with human disease states, Mdm1, Mlf1, and Dyx1c1, are upregulated during ciliogenesis and localize to centrioles and cilia. This transcriptome for mammalian multiciliated epithelial cells identifies new candidate centrosome and cilia proteins, highlights similarities between components of motile and primary cilia, and identifies new links between cilia proteins and human disease.

Oscillation of APC/C activity during cell cycle arrest promotes centrosome amplification

Centrosome duplication is licensed by the disengagement, or 'uncoupling', of centrioles during late mitosis. However, arrest of cells in G2 can trigger premature centriole disengagement. Here, we show that premature disengagement results from untimely activation of the anaphase-promoting complex (APC/C), leading to securin degradation and release of active separase. Although APC/C activation during G2 arrest is dependent on polo-like kinase 1 (Plk1)-mediated degradation of the APC/C inhibitor, early mitotic inhibitor 1 (Emi1), Plk1 also has a second APC/C-independent role in promoting disengagement. Importantly, APC/C and Plk1 activity also stimulates centriole disengagement in response to hydroxyurea or DNA damage-induced cell-cycle arrest and this leads to centrosome amplification. However, the reduplication of disengaged centrioles is dependent on cyclin-dependent kinase 2 (Cdk2) activity and Cdk2 activation coincides with a subsequent inactivation of the APC/C and re-accumulation of cyclin A. Although release from these arrests leads to mitotic entry, the presence of disengaged and/or amplified centrosomes results in the formation of abnormal mitotic spindles that lead to chromosome mis-segregation. Thus, oscillation of APC/C activity during cell cycle arrest promotes both centrosome amplification and genome instability.

Meier-Gorlin syndrome mutations disrupt an Orc1 CDK inhibitory domain and cause centrosome reduplication

Like DNA replication, centrosomes are licensed to duplicate once per cell division cycle to ensure genetic stability. In addition to regulating DNA replication, the Orc1 subunit of the human origin recognition complex controls centriole and centrosome copy number. Here we report that Orc1 harbors a PACT centrosome-targeting domain and a separate domain that differentially inhibits the protein kinase activities of Cyclin E-CDK2 and Cyclin A-CDK2. A cyclin-binding motif (Cy motif) is required for Orc1 to bind Cyclin A and inhibit Cyclin A-CDK2 kinase activity but has no effect on Cyclin E-CDK2 kinase activity. In contrast, Orc1 inhibition of Cyclin E-CDK2 kinase activity occurs by a different mechanism that is affected by Orc1 mutations identified in Meier-Gorlin syndrome patients. The cyclin/CDK2 kinase inhibitory domain of Orc1, when tethered to the PACT domain, localizes to centrosomes and blocks centrosome reduplication. Meier-Gorlin syndrome mutations that disrupt Cyclin E-CDK2 kinase inhibition also allow centrosome reduplication. Thus, Orc1 contains distinct domains that control centrosome copy number and DNA replication. We suggest that the Orc1 mutations present in some Meier-Gorlin syndrome patients contribute to the pronounced microcephaly and dwarfism observed in these individuals by altering centrosome duplication in addition to DNA replication defects.

Shugoshins function as a guardian for chromosomal stability in nuclear division

Accurate chromosome segregation during mitosis and meiosis is regulated and secured by several distinctly different yet intricately connected regulatory mechanisms. As chromosomal instability is a hallmark of a majority of tumors as well as a cause of infertility for germ cells, extensive research in the past has focused on the identification and characterization of molecular components that are crucial for faithful chromosome segregation during cell division. Shugoshins, including Sgo1 and Sgo2, are evolutionarily conserved proteins that function to protect sister chromatid cohesion, thus ensuring chromosomal stability during mitosis and meiosis in eukaryotes. Recent studies reveal that Shugoshins in higher animals play an essential role not only in protecting centromeric cohesion of sister chromatids and assisting bi-orientation attachment at the kinetochores, but also in safeguarding centriole cohesion/engagement during early mitosis. Many molecular components have been identified that play essential roles in modulating/mediating Sgo functions. This review primarily summarizes recent advances on the mechanisms of action of Shugoshins in suppressing chromosomal instability during nuclear division in eukaryotic organisms.

Centrosome asymmetry and inheritance during animal development

The centrosome is a subcellular organelle that is responsible for the majority of microtubule organization. Through this ability, the centrosome is involved in cell division, migration, and polarization. Recent studies have revealed intriguing asymmetries between mother and daughter centrioles as well as between mother and daughter centrosomes, and the involvement of such asymmetries in multiple cellular and developmental processes. This review aims to summarize recent discoveries on such asymmetries in centrioles/centrosomes and the potential implication of their inheritance patterns during cell division and development.

Kendrin is a novel substrate for separase involved in the licensing of centriole duplication

The centrosome, consisting of a pair of centrioles surrounded by pericentriolar material, directs the formation of bipolar spindles during mitosis. Aberrant centrosome number can promote chromosome instability, which is implicated in tumorigenesis. Thus, centrosome duplication needs to be tightly regulated to occur only once per cell cycle. Separase, a cysteine protease that triggers sister chromatid separation, is involved in centriole disengagement, which licenses centrosomes for the next round of duplication. However, at least two questions remain unsolved: what is the substrate relevant to the disengagement, and how does separase, activated at anaphase onset, act on the disengagement that occurs during late mitosis. Here, we show that kendrin, also named pericentrin, is cleaved by activated separase at a consensus site in vivo and in vitro, and this leads to the delayed release of kendrin from the centrosome later in mitosis. Furthermore, we demonstrate that expression of a noncleavable kendrin mutant suppresses centriole disengagement and subsequent centriole duplication. Based on these results, we propose that kendrin is a novel and crucial substrate for separase at the centrosome, protecting the engaged centrioles from premature disengagement and thereby blocking reduplication until the cell passes through mitosis.

Polo-like kinase 4 controls centriole duplication but does not directly regulate cytokinesis

Polo-like kinase 4 (Plk4) plays an essential role in centriole duplication, but recent work led to the conclusion that Plk4 also directly regulates cytokinesis. The consequence of reduced Plk4 levels in human and mouse cells is studied. It is shown that Plk4 controls centriole duplication but does not directly regulate cytokinesis.

Centrioles organize the centrosome, and accurate control of their number is critical for the maintenance of genomic integrity. Centrioles duplicate once per cell cycle, and duplication is coordinated by Polo-like kinase 4 (Plk4). We previously demonstrated that Plk4 accumulation is autoregulated by its own kinase activity. However, loss of heterozygosity of Plk4 in mouse embryonic fibroblasts has been proposed to cause cytokinesis failure as a primary event, leading to centrosome amplification and gross chromosomal abnormalities. Using targeted gene disruption, we show that human epithelial cells with one inactivated Plk4 allele undergo neither cytokinesis failure nor increase in centrosome amplification. Plk4 is shown to localize exclusively at the centrosome, with none in the spindle midbody. Substantial depletion of Plk4 by small interfering RNA leads to loss of centrioles and subsequent spindle defects that lead to a modest increase in the rate of cytokinesis failure. Therefore, Plk4 is a centriole-localized kinase that does not directly regulate cytokinesis.

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

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.

Glypican-1 stimulates Skp2 autoinduction loop and G1/S transition in endothelial cells

The heparan sulfate proteoglycan glypican-1 (GPC1) is involved in tumorigenesis and angiogenesis and is overexpressed frequently in tumor and endothelial cells (ECs) in human gliomas. We demonstrated previously that in brain EC, GPC1 regulates mitotic cyclins and securin as well as mitosis and that GPC1 is required for progression through the cell cycle. To characterize the molecular mechanism underlying cell cycle regulation by GPC1, we systematically investigated its effects on key G(1)/S checkpoint regulators and on major signaling pathways reportedly activated by Dally (Division abnormally delayed) the Drosophila GPC1 homologue. We found that elevated GPC1 affected a wide range of G(1)/S checkpoint regulators, leading to inactivation of the G(1)/S checkpoint and increased S phase entry, apparently by activating the mitogen-independent Skp2 autoinduction loop. Specifically, GPC1 suppressed CDK inhibitors (CKIs), including p21, p27, p16, and p19, and the D cyclins, and induced CDK2 and Skp2. GPC1 may trigger the Skp2 autoinduction loop at least partially by suppressing p21 transcription as knockdown of p21 by RNAi can mimic the effect of GPC1 on the cell cycle regulators related to the loop. Moreover, multiple mitogenic signaling pathways, including ERK MAPK, Wnt and BMP signaling, were significantly stimulated by GPC1 as has been reported for Dally in Drosophila. Notably, the c-Myc oncoprotein, which is frequently up-regulated by both ERK and Wnt signaling and functions as a potent transcription repressor for CKIs as well as D cyclins, was also significantly induced by GPC1. These findings provide mechanistic insights into how GPC1 regulates the cell cycle and proliferation.