Human Cep192 and Cep152 cooperate in Plk4 recruitment and centriole duplication

Cell cycle progression in response to oxygen levels

Hypoxia' or decreases in oxygen availability' results in the activation of a number of different responses at both the whole organism and the cellular level. These responses include drastic changes in gene expression, which allow the organism (or cell) to cope efficiently with the stresses associated with the hypoxic insult. A major breakthrough in the understanding of the cellular response to hypoxia was the discovery of a hypoxia sensitive family of transcription factors known as the hypoxia inducible factors (HIFs). The hypoxia response mounted by the HIFs promotes cell survival and energy conservation. As such, this response has to deal with important cellular process such as cell division. In this review, the integration of oxygen sensing with the cell cycle will be discussed. HIFs, as well as other components of the hypoxia pathway, can influence cell cycle progression. The role of HIF and the cell molecular oxygen sensors in the control of the cell cycle will be reviewed.

Proximity interactions among centrosome components identify regulators of centriole duplication

The centrosome consists of a pair of centrioles and surrounding pericentriolar material (PCM). Many vertebrate cells also have an array of granules, termed centriolar satellites, that localize around the centrosome and are associated with centrosome and cilium function. Centriole duplication occurs once per cell cycle and is effected by a set of proteins including PLK4, CEP192, CEP152, CEP63, and CPAP. Information on the relationships between these components is limited due to the difficulty in assaying interactions in the context of the centrosome. Here, we used proximity-dependent biotin identification (BioID) to identify proximity interactions among centriole duplication proteins. PLK4, CEP192, and CEP152 BioID identified known physically interacting proteins and a new interaction between CEP152 and CDK5RAP2 consistent with a function of CEP152 in PCM recruitment. BioID for CEP63 and its paralog CCDC67 revealed extensive proximity interactions with centriolar satellite proteins. Focusing on these satellite proteins identified two new regulators of centriole duplication, CCDC14 and KIAA0753. Both proteins colocalize with CEP63 to satellites, bind to CEP63, and identify other satellite proteins by BioID. KIAA0753 positively regulates centriole duplication and CEP63 centrosome localization, whereas CCDC14 negatively regulates both processes. These results suggest that centriolar satellites have a previously unappreciated function in regulating centriole duplication.

Centriole biogenesis in multiciliated cells

Centrioles duplicate only once per cell cycle in proliferating cells, whereas in multiciliated cells, hundreds of centrioles form almost simultaneously. The molecular control mechanisms that govern centriole amplification in multiciliated cells are largely unknown. Two studies highlight Deup1 and CCDC78 as key players in this process.

PIPKIγ targets to the centrosome and restrains centriole duplication

Centriole biogenesis depends on the polo-like kinase (PLK4) and a small group of structural proteins. The spatiotemporal regulation of these proteins at pre-existing centrioles is essential to ensure that centriole duplication occurs once per cell cycle. Here, we report that phosphatidylinositol 4-phosphate 5-kinase type-1 gamma (PIP5K1C, hereafter referred to as PIPKIγ) plays an important role in centriole fidelity. PIPKIγ localized in a ring-like pattern in the intermediate pericentriolar materials around the proximal end of the centriole in G1, S and G2 phases, but not in M phase. This localization was dependent upon an association with centrosomal protein of 152 KDa (CEP152). Without detaining cells in S or M phase, the depletion of PIPKIγ led to centriole amplification in a manner that was dependent upon PLK4 and spindle assembly abnormal protein 6 homolog (SAS6). The expression of exogenous PIPKIγ reduced centriole amplification that occurred as a result of endogenous PIPKIγ depletion, hydroxyurea treatment or PLK4 overexpression, suggesting that PIPKIγ is likely to function at the PLK4 level to restrain centriole duplication. Importantly, we found that PIPKIγ bound to the cryptic polo-box domain of PLK4 and that this binding reduced the kinase activity of PLK4. Together, our findings suggest that PIPKIγ is a novel negative regulator of centriole duplication, which acts by modulating the homeostasis of PLK4 activity.

Nek2 and Plk4: prognostic markers, drivers of breast tumorigenesis and drug resistance

The Nek2 and Plk4 kinases serve as crucial regulators of mitotic processes such as the centrosome duplication cycle and spindle assembly. Deregulation of these processes can trigger chromosome instability and aneuploidy, which are hallmarks of many solid tumors, including breast cancer. Emerging data from the literature illustrated various functions of Nek2 in breast cancer models, with compelling evidence of its prognostic value in breast tumors. The two kinases control distinct steps in the centrosome-centriole cycle and their dysregulation lead to centrosome amplification, marked by the presence of more than two centrosomes within the cell. We found single or composite overexpression of these kinases in breast tumor samples, regardless of subtype, which strongly associated with poor prognosis. Interestingly, in a panel of established cell lines, both kinases are highly expressed in Her2-positive breast cancer cells exhibiting centrosome amplification and trastuzumab resistance. In summary, it appears that Nek2 and Plk4 might synergize to promote breast tumorigenesis and may also be involved in tamoxifen and trastuzumab resistance.

The role of mitotic kinases in coupling the centrosome cycle with the assembly of the mitotic spindle

The centrosome acts as the major microtubule-organizing center (MTOC) for cytoskeleton maintenance in interphase and mitotic spindle assembly in vertebrate cells. It duplicates only once per cell cycle in a highly spatiotemporally regulated manner. When the cell undergoes mitosis, the duplicated centrosomes separate to define spindle poles and monitor the assembly of the bipolar mitotic spindle for accurate chromosome separation and the maintenance of genomic stability. However, centrosome abnormalities occur frequently and often lead to monopolar or multipolar spindle formation, which results in chromosome instability and possibly tumorigenesis. A number of studies have begun to dissect the role of mitotic kinases, including NIMA-related kinases (Neks), cyclin-dependent kinases (CDKs), Polo-like kinases (Plks) and Aurora kinases, in regulating centrosome duplication, separation and maturation and subsequent mitotic spindle assembly during cell cycle progression. In this Commentary, we review the recent research progress on how these mitotic kinases are coordinated to couple the centrosome cycle with the cell cycle, thus ensuring bipolar mitotic spindle fidelity. Understanding this process will help to delineate the relationship between centrosomal abnormalities and spindle defects.

Hierarchical recruitment of Plk4 and regulation of centriole biogenesis by two centrosomal scaffolds, Cep192 and Cep152

Centrosomes play an important role in various cellular processes, including spindle formation and chromosome segregation. They are composed of two orthogonally arranged centrioles, whose duplication occurs only once per cell cycle. Accurate control of centriole numbers is essential for the maintenance of genomic integrity. Although it is well appreciated that polo-like kinase 4 (Plk4) plays a central role in centriole biogenesis, how it is recruited to centrosomes and whether this step is necessary for centriole biogenesis remain largely elusive. Here we showed that Plk4 localizes to distinct subcentrosomal regions in a temporally and spatially regulated manner, and that Cep192 and Cep152 serve as two distinct scaffolds that recruit Plk4 to centrosomes in a hierarchical order. Interestingly, Cep192 and Cep152 competitively interacted with the cryptic polo box of Plk4 through their homologous N-terminal sequences containing acidic-α-helix and N/Q-rich motifs. Consistent with these observations, the expression of either one of these N-terminal fragments was sufficient to delocalize Plk4 from centrosomes. Furthermore, loss of the Cep192- or Cep152-dependent interaction with Plk4 resulted in impaired centriole duplication that led to delayed cell proliferation. Thus, the spatiotemporal regulation of Plk4 localization by two hierarchical scaffolds, Cep192 and Cep152, is critical for centriole biogenesis.

Amorphous no more: subdiffraction view of the pericentriolar material architecture

The centrosome influences the shape, orientation and activity of the microtubule cytoskeleton. The pericentriolar material (PCM), determines this functionality by providing a dynamic platform for nucleating microtubules and acts as a nexus for molecular signaling. Although great strides have been made in understanding PCM activity, its diffraction-limited size and amorphous appearance on electron microscopy (EM) have limited analysis of its high-order organization. Here, we outline current knowledge of PCM architecture and assembly, emphasizing recent super-resolution imaging studies that revealed the PCM has a layered structure made of fibers and matrices conserved from flies to humans. Notably, these studies debunk the long-standing view of an amorphous PCM and provide a paradigm to dissect the supramolecular organization of organelles in cells.

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.

Deuterosome-mediated centriole biogenesis

The ability of cells to faithfully duplicate their two centrioles once per cell cycle is critical for proper mitotic progression and chromosome segregation. Multiciliated cells represent an interesting variation of centriole duplication in that these cells generate greater than 100 centrioles, which form the basal bodies of their motile cilia. This centriole amplification is proposed to require a structure termed the deuterosome, thought to be capable of promoting de novo centriole biogenesis. Here, we begin to molecularly characterize the deuterosome and identify it as a site for the localization of Cep152, Plk4, and SAS6. Additionally we identify CCDC78 as a centriole-associated and deuterosome protein that is essential for centriole amplification. Overexpression of Cep152, but not Plk4, SAS6, or CCDC78, drives overamplification of centrioles. However, in CCDC78 morphants, Cep152 fails to localize to the deuterosome and centriole biogenesis is impaired, indicating that CCDC78-mediated recruitment of Cep152 is required for deuterosome-mediated centriole biogenesis.