Centriole engagement: it's not just cohesin any more

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.

Centriole assembly at a glance

The centriole organelle consists of microtubules (MTs) that exhibit a striking 9-fold radial symmetry. Centrioles play fundamental roles across eukaryotes, notably in cell signaling, motility and division. In this Cell Science at a Glance article and accompanying poster, we cover the cellular life cycle of this organelle - from assembly to disappearance - focusing on human centrioles. The journey begins at the end of mitosis when centriole pairs disengage and the newly formed centrioles mature to begin a new duplication cycle. Selection of a single site of procentriole emergence through focusing of polo-like kinase 4 (PLK4) and the resulting assembly of spindle assembly abnormal protein 6 (SAS-6) into a cartwheel element are evoked next. Subsequently, we cover the recruitment of peripheral components that include the pinhead structure, MTs and the MT-connecting A-C linker. The function of centrioles in recruiting pericentriolar material (PCM) and in forming the template of the axoneme are then introduced, followed by a mention of circumstances in which centrioles form de novo or are eliminated.

PCNT is critical for the association and conversion of centrioles to centrosomes during mitosis

A centrosome consists of a pair of centrioles and pericentriolar material (PCM). We manipulated expression of PCNT, a key PCM protein, and investigated roles of PCM in centriole behavior during mitosis. Deletion of PCNT had little effect on the interphase centrosomes. However, centrioles in PCNT-deleted mitotic cells prematurely separated and frequently amplified, revealing that centrioles are limited within the spindle poles by PCNT during mitosis. It is known that specific cleavage of PCNT is necessary for centriole separation during mitotic exit. Delayed centriole separation was observed in G0 phase when a noncleavable PCNT was removed or when PCNT was artificially cleaved by TEV protease. Furthermore, a daughter centriole converts to a mother centriole only after experiencing both mitotic exit and specific PCNT cleavage. Based on the results, we propose that a centriole pair disengages upon entering mitosis but remains associated with the surrounding PCM proteins throughout mitosis. During mitotic exit, specific cleavage of PCNT induces PCM disintegration. As a result, a daughter centriole separates from the mother centriole and converts to a young mother centriole.

Controlling centriole numbers: Geminin family members as master regulators of centriole amplification and multiciliogenesis

To ensure that the genetic material is accurately passed down to daughter cells during mitosis, dividing cells must duplicate their chromosomes and centrosomes once and only once per cell cycle. The same key steps-licensing, duplication, and segregation-control both the chromosome and the centrosome cycle, which must occur in concert to safeguard genome integrity. Aberrations in genome content or centrosome numbers lead to genomic instability and are linked to tumorigenesis. Such aberrations, however, can also be part of the normal life cycle of specific cell types. Multiciliated cells best exemplify the deviation from a normal centrosome cycle. They are post-mitotic cells which massively amplify their centrioles, bypassing the rule for once-per-cell-cycle centriole duplication. Hundreds of centrioles dock to the apical cell surface and generate motile cilia, whose concerted movement ensures fluid flow across epithelia. The early steps that control the generation of multiciliated cells have lately started to be elucidated. Geminin and the vertebrate-specific GemC1 and McIdas are distantly related coiled-coil proteins, initially identified as cell cycle regulators associated with the chromosome cycle. Geminin is required to ensure once-per-cell-cycle genome replication, while McIdas and GemC1 bind to Geminin and are implicated in DNA replication control. Recent findings highlight Geminin family members as early regulators of multiciliogenesis. GemC1 and McIdas specify the multiciliate cell fate by forming complexes with the E2F4/5 transcription factors to switch on a gene expression program leading to centriole amplification and cilia formation. Positive and negative interactions among Geminin family members may link cell cycle control to centriole amplification and multiciliogenesis, acting close to the point of transition from proliferation to differentiation. We review key steps of centrosome duplication and amplification, present the role of Geminin family members in the centrosome and chromosome cycle, and discuss links with disease.

PLK1 regulation of PCNT cleavage ensures fidelity of centriole separation during mitotic exit

Centrioles are duplicated and segregated in close link to the cell cycle. During mitosis, daughter centrioles are disengaged and eventually separated from mother centrioles. New daughter centrioles may be generated only after centriole separation. Therefore, centriole separation is considered a licensing step for centriole duplication. It was previously known that separase specifically cleaves pericentrin (PCNT) during mitotic exit. Here we report that PCNT has to be phosphorylated by PLK1 to be a suitable substrate of separase. Phospho-resistant mutants of PCNT are not cleaved by separase and eventually inhibit centriole separation. Furthermore, phospho-mimetic PCNT mutants rescue centriole separation even in the presence of a PLK1 inhibitor. On the basis on these results, we propose that PLK1 phosphorylation is a priming step for separase-mediated cleavage of PCNT and eventually for centriole separation. PLK1 phosphorylation of PCNT provides an additional layer of regulatory mechanism to ensure the fidelity of centriole separation during mitotic exit.

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.

Developmental alterations in centrosome integrity contribute to the post-mitotic state of mammalian cardiomyocytes

Mammalian cardiomyocytes become post-mitotic shortly after birth. Understanding how this occurs is highly relevant to cardiac regenerative therapy. Yet, how cardiomyocytes achieve and maintain a post-mitotic state is unknown. Here, we show that cardiomyocyte centrosome integrity is lost shortly after birth. This is coupled with relocalization of various centrosome proteins to the nuclear envelope. Consequently, postnatal cardiomyocytes are unable to undergo ciliogenesis and the nuclear envelope adopts the function as cellular microtubule organizing center. Loss of centrosome integrity is associated with, and can promote, cardiomyocyte G0/G1 cell cycle arrest suggesting that centrosome disassembly is developmentally utilized to achieve the post-mitotic state in mammalian cardiomyocytes. Adult cardiomyocytes of zebrafish and newt, which are able to proliferate, maintain centrosome integrity. Collectively, our data provide a novel mechanism underlying the post-mitotic state of mammalian cardiomyocytes as well as a potential explanation for why zebrafish and newts, but not mammals, can regenerate their heart.

DOI:http://dx.doi.org/10.7554/eLife.05563.001

Muscle cells in the heart contract in regular rhythms to pump blood around the body. In humans, rats and other mammals, the vast majority of heart muscle cells lose the ability to divide shortly after birth. Therefore, the heart is unable to replace cells that are lost over the life of the individual, for example, during a heart attack. If too many of these cells are lost, the heart will be unable to pump effectively, which can lead to heart failure. Currently, the only treatment option in humans with heart failure is to perform a heart transplant.

Some animals, such as newts and zebrafish, are able to replace lost heart muscle cells throughout their lifetimes. Thus, these species are able to fully regenerate their hearts even after 20% has been removed. This suggests that it might be possible to manipulate human heart muscle cells to make them divide and regenerate the heart. Recent research has suggested that structures called centrosomes, known to be required to separate copies of the DNA during cell division, are used as a hub to integrate the initial signals that determine whether a cell should divide or not.

Here, Zebrowski et al. studied the centrosomes of heart muscle cells in rats, newts and zebrafish. The experiments show that the centrosomes in rat heart muscle cells are dissembled shortly after birth. Centrosomes are made of several proteins and, in the rat cells, these proteins moved to the membrane that surrounded the nucleus. On the other hand, the centrosomes in the heart muscle cells of the adult newts and zebrafish remained intact.

Further experiments found that that breaking apart the centrosomes of heart muscle cells taken from newborn rats stops these cells from dividing. Zebrowski et al.'s findings suggest that the loss of centrosomes after birth is a possible reason why the hearts of adult humans and other mammals are unable to regenerate after injury. In the future, these findings may aid the development of methods to regenerate human heart muscle and new treatments that may limit division of cancer cells.

DOI:http://dx.doi.org/10.7554/eLife.05563.002

Cep68 can be regulated by Nek2 and SCF complex

Centrosome cohesion maintains centrosomes in close proximity until mitosis, when cell cycle-dependent regulatory signaling events dissolve cohesion and promote centrosome separation in preparation for bipolar spindle assembly at mitosis. Cohesion is regulated by the antagonistic activities of the mitotic NIMA-related kinase 2 (Nek2), protein phosphatase 1, the cohesion fiber components rootletin, centrosomal Nek2-associated protein 1 (C-Nap1) and Cep68. The centrosomal protein Cep68 is essential for centrosome cohesion and dissociates from centrosomes at the onset of mitosis. Here, our cell line studies show the C-terminal 300-400 amino acids of Cep68 are necessary to localize Cep68 to interphase centrosomes while C-terminal 400-500 amino acids might regulate Cep68 dissociation from centrosomes at mitotic onset. In addition, Nek2 was demonstrated to phosphorylate Cep68 in vivo and this phosphorylation appears to promote Cep68 degradation in mitosis. We further show that the SCF complex destroys Cep68 at mitosis through recognition by the beta-Trcp F box component of SCF. Together, the findings provide a new insight into the control of centrosome separation by Cep68 during mitosis.