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Timely and accurate assembly of the mitotic spindle is critical for the faithful segregation of chromosomes, and centrosome separation is a key step in this process. The timing of centrosome separation varies dramatically between cell types; however, the mechanisms responsible for these differences and its significance are unclear. Here, we show that activation of epidermal growth factor receptor (EGFR) signaling determines the timing of centrosome separation. Premature separation of centrosomes decreases the requirement for the major mitotic kinesin Eg5 for spindle assembly, accelerates mitosis, and decreases the rate of chromosome missegregation. Importantly, EGF stimulation impacts upon centrosome separation and mitotic progression to different degrees in different cell lines. Cells with high EGFR levels fail to arrest in mitosis upon Eg5 inhibition. This has important implications for cancer therapy because cells with high centrosomal response to EGF are more susceptible to combinatorial inhibition of EGFR and Eg5., (Copyright © 2013 Elsevier Inc. All rights reserved.)

Stil (Sil, SCL/TAL1 interrupting locus) is a cytosolic and centrosomal protein expressed in proliferating cells that is required for mouse and zebrafish neural development and is mutated in familial microcephaly. Recently the Drosophila melanogaster ortholog of Stil was found to be important for centriole duplication. Consistent with this finding, we report here that mouse embryonic fibroblasts lacking Stil are characterized by slow growth, low mitotic index and absence of clear centrosomes. We hypothesized that Stil regulates mitosis through the tumor suppressor Chfr, an E3 ligase that blocks mitotic entry in response to mitotic stress. Mouse fibroblasts lacking Stil by genomic or RNA interference approaches, as well as E9.5 Stil(-/-) embryos, express high levels of the Chfr protein and reduced levels of the Chfr substrate Plk1. Exogenous expression of Stil, knockdown of Chfr or overexpression of Plk1 reverse the abnormal mitotic phenotypes of fibroblasts lacking Stil. We further demonstrate that Stil increases Chfr auto-ubiquitination and reduces its protein stability. Thus, Stil is required for centrosome organization, entry into mitosis and cell proliferation, and these functions are at least partially mediated by Chfr and its targets. This is the first identification of a negative regulator of the Chfr mitotic checkpoint.

Primary microcephaly, Seckel syndrome, and microcephalic osteodysplastic primordial dwarfism type II (MOPD II) are disorders exhibiting marked microcephaly, with small brain sizes reflecting reduced neuron production during fetal life. Although primary microcephaly can be caused by mutations in microcephalin (MCPH1), cells from patients with Seckel syndrome and MOPD II harbor mutations in ataxia telangiectasia and Rad3 related (ATR) or pericentrin (PCNT), leading to disturbed ATR signaling. In this study, we show that a lack of MCPH1 or PCNT results in a loss of Chk1 from centrosomes with subsequently deregulated activation of centrosomal cyclin B-Cdk1.

Centrosomes consist of a pair of barrel-shaped microtubule assemblies called centrioles, surrounded by a pericentriolar matrix. The only well-characterized functions of centrosomes is to recognize both interphase microtubule arrays responsible for cell polarity and the mitotic spindle, which mediates the strictly bipolar separations of chromosomes. In addition to these established functions it has been speculated that centrosomes might be involved in several different cell cycle regulatory events like entry into mitosis, cytokinesis, G(1)/S transition and monitoring of DNA damage. These assumptions are mainly based on a rapidly growing list of centrosome-associated regulatory proteins such as p53, Brca1, Chk1, Chk2, TopBP1, Aurora-A, Plk1, cyclin B1, and Cdk1. However, only very few direct links between their localization to the centrosome and specific cellular functions have been unraveled until recently. This review will focus on recent advances in the understanding of the role of centrosomes as integrators of positive and negative pathways for mitotic entry.

Summary Chromosomal instability is a hallmark of cancer and correlates with the presence of extra centrosomes, which originate from centriole overduplication. Overduplicated centrioles lead to the formation of centriole rosettes, which mature into supernumerary centrosomes in the subsequent cell cycle. While extra centrosomes promote chromosome missegregation by clustering into pseudo-bipolar spindles, the contribution of centriole rosettes to chromosome missegregation is unknown. We used multi-modal imaging of cells with conditional centriole overduplication to show that mitotic rosettes in bipolar spindles frequently harbor unequal centriole numbers, leading to biased chromosome capture that favors binding to the prominent pole. This results in chromosome missegregation and aneuploidy. Rosette mitoses lead to viable offspring and significantly contribute to progeny production. We further show that centrosome abnormalities in primary human malignancies frequently consist of centriole rosettes. As asymmetric centriole rosettes generate mitotic errors that can be propagated, rosette mitoses are sufficient to cause chromosome missegregation in cancer. [ABSTRACT FROM AUTHOR]

Abstract: With regard to cancer development the centrosome has been the center of attraction of scientists for already more than a 100years. After the initial assumption that amplified centrosomes and abnormal mitotic arrangements might be a cause of cancer at the beginning of the last century, enormous efforts have been undertaken to clarify the relevance of centrosome amplification in tumorigenesis. In the meantime, centrosome amplification has been observed in most, both solid and hematological, cancer entities and by now is viewed as a “hallmark” of cancer cells. In this review we summarize basics in centrosome biology and what is known about the emergence of amplified centrosomes. In addition, we discuss how centrosome amplification might cause aneuploidy thereby leading to malignant transformation of cells. Furthermore, we present recent insights into the role of centrosome amplification in tumor formation based on work in model systems. [Copyright &y& Elsevier]

Abstract: Centrosome abnormalities occur commonly in cancer, and contribute to chromosomal instability and tumorigenesis. New evidence on a phylogenetically conserved mechanism termed ‘centrosomal clustering’ provides exciting insights into how cells with supernumerary centrosomes adapt to avoid lethal multipolar divisions. Here, we highlight the emerging molecular basis of centrosome clustering, and its impact on asymmetric divisions of stem cells, chromosomal (in)stability and malignant transformation. Finally, pharmacological inhibition of centrosome clustering promises to selectively target tumor cells. [Copyright &y& Elsevier]

Entry into mitosis occurs after activation of Cdk1, resulting in chromosome condensation in the nucleus and centrosome separation, as well as increased microtubule nucleation activity in the cytoplasm. The active cyclin-B1-Cdk1 complex first appears at the centrosome, suggesting that the centrosome may facilitate the activation of mitotic regulators required for the commitment of cells to mitosis. However, the signalling pathways involved in controlling the initial activation of Cdk1 at the centrosome remain largely unknown. Here, we show that human Chk1 kinase localizes to interphase, but not mitotic, centrosomes. Chemical inhibition of Chk1 resulted in premature centrosome separation and activation of centrosome-associated Cdk1. Forced immobilization of kinase-inactive Chk1 to centrosomes also resulted in premature Cdk1 activation. Conversely, under such conditions wild-type Chk1 impaired activation of centrosome-associated Cdk1, thereby resulting in DNA endoreplication and centrosome amplification. Activation of centrosomal Cdk1 in late prophase seemed to be mediated by cytoplasmic Cdc25B, whose activity is controlled by centrosome-associated Chk1. These results suggest that centrosome-associated Chk1 shields centrosomal Cdk1 from unscheduled activation by cytoplasmic Cdc25B, thereby contributing to proper timing of the initial steps of cell division, including mitotic spindle formation. [ABSTRACT FROM AUTHOR]

Centrosomes, which mediate accurate chromosome segregation during mitosis, undergo duplication precisely once per cell division at the G1/S boundary. Recently, we described centrosome aberrations as a possible cause of aneuploidy in acute myeloid leukemia (AML) and found a correlation of the percentage of cells carrying abnormal centrosomes to their cytogenetic risk profile. To elucidate the molecular events responsible for the development of centrosome aberrations in AML, tumor RNA of 29 AML samples was hybridized to cDNA microarrays. The microarrays comprised some 2800 different genes with relevance to hematopoiesis, tumorigenesis and mitosis and included a set of 359 centrosome-associated genes. We identified two gene expression signatures, which allowed an accurate classification according to the extent of centrosome aberrations and the ploidy status in 28 of 29 patients each. Specifically, 18 genes were present in both signatures, including genes that code for cell cycle regulatory proteins (cyclin A2, cyclin D3, cyclin H, CDK6, p18INK4c, p21Cip1, PAK1) and centrosome-associated proteins (pericentrin, a2-tubulin, NUMA1, TUBGCP2, PRKAR2A). In conclusion, the high expression of centrosome-associated genes matches the description of centrosome aberrations in several tumor types. Moreover, in AML the identification of G1/S-phase stimulatory genes suggests that one mechanism of aneuploidy induction might be the deregulation of centrosome replication at the G1/S boundary.Oncogene (2004) 23, 2379-2384. doi:10.1038/sj.onc.1207401 Published online 9 February 2004 [ABSTRACT FROM AUTHOR]

A letter to the editor is presented in response to the article "Cep63 Recruits Cdk1 to the Centrosome: Implications for Regulation of Mitotic Entry, Centrosome Amplification, and Genome Maintenance," by H. Löffler et al. in the 2011 issue.