Human Cep192 and Cep152 cooperate in Plk4 recruitment and centriole duplication

Emerging insights into symmetry breaking in centriole duplication: updated view on centriole duplication theory

Centriole duplication occurs once per cell cycle. Since only a single daughter centriole is assembled adjacent to each mother centriole, symmetry around the mother centriole must be broken in the process of centriole duplication. Recent studies have established that Plk4, a master kinase for centriole duplication, can self-assemble into condensates, and have suggested that this Plk4 self-assembly is the key to symmetry breaking. Here, we present the current hypotheses for how Plk4 could break symmetry around the mother centriole via autonomous regulation. After this initial symmetry-breaking process, the ring-to-dot conversion of Plk4 around the mother centriole completes the selection of the site for procentriole formation. We also discuss how this dynamic transition contributes to the strict regulation of centriole duplication.

A molecular mechanism for the procentriole recruitment of Ana2

McLamarrah et al. characterize an early step in centriole duplication. They show that Plk4 hyperphosphorylates Ana2, which increases the affinity of Ana2 for the G-box domain of Sas4, promoting Ana2’s accumulation at the procentriole and, consequently, daughter centriole formation.

During centriole duplication, a preprocentriole forms at a single site on the mother centriole through a process that includes the hierarchical recruitment of a conserved set of proteins, including the Polo-like kinase 4 (Plk4), Ana2/STIL, and the cartwheel protein Sas6. Ana2/STIL is critical for procentriole assembly, and its recruitment is controlled by the kinase activity of Plk4, but how this works remains poorly understood. A structural motif called the G-box in the centriole outer wall protein Sas4 interacts with a short region in the N terminus of Ana2/STIL. Here, we show that binding of Ana2 to the Sas4 G-box enables hyperphosphorylation of the Ana2 N terminus by Plk4. Hyperphosphorylation increases the affinity of the Ana2–G-box interaction, and, consequently, promotes the accumulation of Ana2 at the procentriole to induce daughter centriole formation.

TRIM37 controls cancer-specific vulnerability to PLK4 inhibition

Centrosomes catalyze microtubule formation for mitotic spindle assembly¹. Centrosomes duplicate once per cell cycle in a process controlled the kinase PLK4^2,3. Following chemical PLK4 inhibition, cell division in the absence of centrosome duplication generates centrosome-less cells that exhibit delayed, acentrosomal spindle assembly⁴. Whether PLK4 inhibitors can be leveraged for cancer treatment is not yet clear. Here, we show that acentrosomal spindle assembly following PLK4 inhibition depends on levels of the centrosomal ubiquitin ligase TRIM37. Low TRIM37 accelerates acentrosomal spindle assembly and improves proliferation following PLK4 inhibition, whereas high TRIM37 inhibits acentrosomal spindle assembly, leading to mitotic failure and cessation of proliferation. The Chr17q region containing the TRIM37 gene is frequently amplified in neuroblastoma and in breast cancer^5–8, which renders these cancer types highly sensitive to PLK4 inhibition. TRIM37 inactivation improves acentrosomal mitosis because TRIM37 prevents PLK4 self-assembly into centrosome-independent condensates that serve as ectopic microtubule-organizing centers. By contrast, elevated TRIM37 expression inhibits acentrosomal spindle assembly via a distinct mechanism that involves degradation of the centrosomal component CEP192. Thus, TRIM37 is a critical determinant of mitotic vulnerability to PLK4 inhibition. Linkage of TRIM37 to prevalent cancer-associated genomic changes, including 17q gain in neuroblastoma and 17q23 amplification in breast cancer, may offer an opportunity to use PLK4 inhibition to trigger selective mitotic failure and provide new avenues to treatments for these cancers.

FAM46C/TENT5C functions as a tumor suppressor through inhibition of Plk4 activity

Polo like kinase 4 (Plk4) is a tightly regulated serine threonine kinase that governs centriole duplication. Increased Plk4 expression, which is a feature of many common human cancers, causes centriole overduplication, mitotic irregularities, and chromosomal instability. Plk4 can also promote cancer invasion and metastasis through regulation of the actin cytoskeleton. Herein we demonstrate physical interaction of Plk4 with FAM46C/TENT5C, a conserved protein of unknown function until recently. FAM46C localizes to centrioles, inhibits Plk4 kinase activity, and suppresses Plk4-induced centriole duplication. Interference with Plk4 function by FAM46C was independent of the latter's nucleotidyl transferase activity. In addition, FAM46C restrained cancer cell invasion and suppressed MDA MB-435 cancer growth in a xenograft model, opposing the effect of Plk4. We demonstrate loss of FAM46C in patient-derived colorectal cancer tumor tissue that becomes more profound with advanced clinical stage. These results implicate FAM46C as a tumor suppressor that acts by inhibiting Plk4 activity.

A self-assembled cylindrical platform for Plk4-induced centriole biogenesis

The centrosome, a unique membraneless multiprotein organelle, plays a pivotal role in various cellular processes that are critical for promoting cell proliferation. Faulty assembly or organization of the centrosome results in abnormal cell division, which leads to various human disorders including cancer, microcephaly and ciliopathy. Recent studies have provided new insights into the stepwise self-assembly of two pericentriolar scaffold proteins, Cep63 and Cep152, into a near-micrometre-scale higher-order structure whose architectural properties could be crucial for proper execution of its biological function. The construction of the scaffold architecture appears to be centrally required for tight control of a Ser/Thr kinase called Plk4, a key regulator of centriole duplication, which occurs precisely once per cell cycle. In this review, we will discuss a new paradigm for understanding how pericentrosomal scaffolds are self-organized into a new functional entity and how, on the resulting structural platform, Plk4 undergoes physico-chemical conversion to trigger centriole biogenesis.

Primary cilia biogenesis and associated retinal ciliopathies

The primary cilium is a ubiquitous microtubule-based organelle that senses external environment and modulates diverse signaling pathways in different cell types and tissues. The cilium originates from the mother centriole through a complex set of cellular events requiring hundreds of distinct components. Aberrant ciliogenesis or ciliary transport leads to a broad spectrum of clinical entities with overlapping yet highly variable phenotypes, collectively called ciliopathies, which include sensory defects and syndromic disorders with multi-organ pathologies. For efficient light detection, photoreceptors in the retina elaborate a modified cilium known as the outer segment, which is packed with membranous discs enriched for components of the phototransduction machinery. Retinopathy phenotype involves dysfunction and/or degeneration of the light sensing photoreceptors and is highly penetrant in ciliopathies. This review will discuss primary cilia biogenesis and ciliopathies, with a focus on the retina, and the role of CP110-CEP290-CC2D2A network. We will also explore how recent technologies can advance our understanding of cilia biology and discuss new paradigms for developing potential therapies of retinal ciliopathies.

Cep57 and Cep57l1 function redundantly to recruit the Cep63-Cep152 complex for centriole biogenesis

The Cep63-Cep152 complex located at the mother centriole recruits Plk4 to initiate centriole biogenesis. How the complex is targeted to mother centrioles, however, is unclear. In this study, we show that Cep57 and its paralog, Cep57l1, colocalize with Cep63 and Cep152 at the proximal end of mother centrioles in both cycling cells and multiciliated cells undergoing centriole amplification. Both Cep57 and Cep57l1 bind to the centrosomal targeting region of Cep63. The depletion of both proteins, but not either one, blocks loading of the Cep63-Cep152 complex to mother centrioles and consequently prevents centriole duplication. We propose that Cep57 and Cep57l1 function redundantly to ensure recruitment of the Cep63-Cep152 complex to the mother centrioles for procentriole formation.

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.

A Proximity Mapping Journey into the Biology of the Mammalian Centrosome/Cilium Complex

The mammalian centrosome/cilium complex is composed of the centrosome, the primary cilium and the centriolar satellites, which together regulate cell polarity, signaling, proliferation and motility in cells and thereby development and homeostasis in organisms. Accordingly, deregulation of its structure and functions is implicated in various human diseases including cancer, developmental disorders and neurodegenerative diseases. To better understand these disease connections, the molecular underpinnings of the assembly, maintenance and dynamic adaptations of the centrosome/cilium complex need to be uncovered with exquisite detail. Application of proximity-based labeling methods to the centrosome/cilium complex generated spatial and temporal interaction maps for its components and provided key insights into these questions. In this review, we first describe the structure and cell cycle-linked regulation of the centrosome/cilium complex. Next, we explain the inherent biochemical and temporal limitations in probing the structure and function of the centrosome/cilium complex and describe how proximity-based labeling approaches have addressed them. Finally, we explore current insights into the knowledge we gained from the proximity mapping studies as it pertains to centrosome and cilium biogenesis and systematic characterization of the centrosome, cilium and centriolar satellite interactomes.

Direct interaction between CEP85 and STIL mediates PLK4-driven directed cell migration

PLK4 has emerged as a prime target for cancer therapeutics, and its overexpression is frequently observed in various types of human cancer. Recent studies have further revealed an unexpected oncogenic activity of PLK4 in regulating cancer cell migration and invasion. However, the molecular basis behind the role of PLK4 in these processes still remains only partly understood. Our previous work has demonstrated that an intact CEP85-STIL binding interface is necessary for robust PLK4 activation and centriole duplication. Here, we show that CEP85 and STIL are also required for directional cancer cell migration. Mutational and functional analyses reveal that the interactions between CEP85, STIL and PLK4 are essential for effective directional cell motility. Mechanistically, we show that PLK4 can drive the recruitment of CEP85 and STIL to the leading edge of cells to promote protrusive activity, and that downregulation of CEP85 and STIL leads to a reduction in ARP2 (also known as ACTR2) phosphorylation and reorganization of the actin cytoskeleton, which in turn impairs cell migration. Collectively, our studies provide molecular insight into the important role of the CEP85-STIL complex in modulating PLK4-driven cancer cell migration.This article has an associated First Person interview with the first author of the paper.

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.

Mitotic kinase anchoring proteins: the navigators of cell division

The coordinated activities of many protein kinases, acting on multiple protein substrates, ensures the error-free progression through mitosis of eukaryotic cells. Enormous research effort has thus been devoted to studying the roles and regulation of these mitotic kinases, and to the identification of their physiological substrates. Central for the timely deployment of specific protein kinases to their appropriate substrates during the cell division cycle are the many anchoring proteins, which serve critical regulatory roles. Through direct association, anchoring proteins are capable of modulating the catalytic activity and/or sub-cellular distribution of the mitotic kinases they associate with. The key roles of some anchoring proteins in cell division are well-established, whilst others are still being unearthed. Here, we review the current knowledge on anchoring proteins for some mitotic kinases, and highlight how targeting anchoring proteins for inhibition, instead of the mitotic kinases themselves, could be advantageous for disrupting the cell division cycle.

The Centrosome Linker and Its Role in Cancer and Genetic Disorders

Centrosome cohesion, the joining of the two centrosomes of a cell, is increasingly appreciated as a major regulator of cell functions such as Golgi organization and cilia positioning. One major element of centrosome cohesion is the centrosome linker that consists of a growing number of proteins. The timely disassembly of the centrosome linker enables centrosomes to separate and assemble a functional bipolar mitotic spindle that is crucial for maintaining genomic integrity. Exciting new findings link centrosome linker defects to cell transformation and genetic disorders. We review recent data on the molecular mechanisms of the assembly and disassembly of the centrosome linker, and discuss how defects in the proper execution of these processes cause DNA damage and genomic instability leading to disease.

Regulated protein stabilization underpins the functional interplay among basal body components in Trypanosoma brucei

The basal body in the human parasite Trypanosoma brucei is structurally equivalent to the centriole in animals and functions in the nucleation of axonemal microtubules in the flagellum. T. brucei lacks many evolutionarily conserved centriolar protein homologs and constructs the basal body through unknown mechanisms. Two evolutionarily conserved centriole/basal body cartwheel proteins, TbSAS-6 and TbBLD10, and a trypanosome-specific protein, BBP65, play essential roles in basal body biogenesis in T. brucei, but how they cooperate in the regulation of basal body assembly remains elusive. Here using RNAi, endogenous epitope tagging, immunofluorescence microscopy, and 3D-structured illumination super-resolution microscopy, we identified a new trypanosome-specific protein named BBP164 and found that it has an essential role in basal body biogenesis in T. brucei Further investigation of the functional interplay among BBP164 and the other three regulators of basal body assembly revealed that BBP164 and BBP65 are interdependent for maintaining their stability and depend on TbSAS-6 and TbBLD10 for their stabilization in the basal body. Additionally, TbSAS-6 and TbBLD10 are independent from each other and from BBP164 and BBP65 for maintaining their stability in the basal body. These findings demonstrate that basal body cartwheel proteins are required for stabilizing other basal body components and uncover that regulation of protein stability is an unusual control mechanism for assembly of the basal body in T. brucei.

Centriolar satellite biogenesis and function in vertebrate cells

Centriolar satellites are non-membranous cytoplasmic granules that concentrate in the vicinity of the centrosome, the major microtubule-organizing centre (MTOC) in animal cells. Originally assigned as conduits for the transport of proteins towards the centrosome and primary cilium, the complexity of satellites is starting to become apparent. Recent studies defined the satellite proteome and interactomes, placing hundreds of proteins from diverse pathways in association with satellites. In addition, studies on cells lacking satellites have revealed that the centrosome can assemble in their absence, whereas studies on acentriolar cells have demonstrated that satellite assembly is independent from an intact MTOC. A role for satellites in ciliogenesis is well established; however, their contribution to other cellular functions is poorly understood. In this Review, we discuss the developments in our understanding of centriolar satellite assembly and function, and why satellites are rapidly becoming established as governors of multiple cellular processes. We highlight the composition and biogenesis of satellites and what is known about the regulation of these aspects. Furthermore, we discuss the evolution from thinking of satellites as mere facilitators of protein trafficking to the centrosome to thinking of them being key regulators of protein localization and cellular proteostasis for a diverse set of pathways, making them of broader interest to fields beyond those focused on centrosomes and ciliogenesis.

Studying Centriole Duplication and Elongation in Human Cells

Centrioles assemble centrosomes and cilia/flagella, which are microtubule-based structures with key roles in cell division, polarity, motility, and signaling. Centriole biogenesis is a tightly regulated process, and deregulation of centriole numbers and structure can have dramatic consequences for cellular function and integrity. However, their small size poses a challenge to study them. Here, we describe protocols that allow the identification and assessment of true centrioles and that provide straightforward strategies to study the role of new candidate proteins in centriole duplication and elongation.

NudC-like protein 2 restrains centriole amplification by stabilizing HERC2

Centriole duplication is tightly controlled to occur once per cell cycle, and disruption of this synchrony causes centriole amplification, which is frequently observed in many cancers. Our previous work showed that nuclear distribution gene C (NudC)-like protein 2 (NudCL2) localizes to centrosomes; however, little is known about the role of NudCL2 in the regulation of centrosome function. Here, we find that NudCL2 is required for accurate centriole duplication by stabilizing the E3 ligase HECT domain and RCC1-like domain-containing protein 2 (HERC2). Knockout (KO) of NudCL2 using CRISPR/Cas9-based genome editing or depletion of NudCL2 using small interfering RNA causes significant centriole amplification. Overexpression of NudCL2 significantly suppresses hydroxyurea-induced centriole overduplication. Quantitative proteomic analysis reveals that HERC2 is downregulated in NudCL2 KO cells. NudCL2 is shown to interact with and stabilize HERC2. Depletion of HERC2 leads to the similar defects to that in NudCL2-downregulated cells, and ectopic expression of HERC2 effectively rescues the centriole amplification caused by the loss of NudCL2, whereas the defects induced by HERC2 depletion cannot be reversed by exogenous expression of NudCL2. Either loss of NudCL2 or depletion of HERC2 leads to the accumulation of ubiquitin-specific peptidase 33 (USP33), a centrosomal protein that positively regulates centriole duplication. Moreover, knockdown of USP33 reverses centriole amplification in both NudCL2 KO and HERC2-depleted cells. Taken together, our data suggest that NudCL2 plays an important role in maintaining the fidelity of centriole duplication by stabilizing HERC2 to control USP33 protein levels, providing a previously undescribed mechanism restraining centriole amplification.

The Centrosome and the Primary Cilium: The Yin and Yang of a Hybrid Organelle

Centrosomes and primary cilia are usually considered as distinct organelles, although both are assembled with the same evolutionary conserved, microtubule-based templates, the centrioles. Centrosomes serve as major microtubule- and actin cytoskeleton-organizing centers and are involved in a variety of intracellular processes, whereas primary cilia receive and transduce environmental signals to elicit cellular and organismal responses. Understanding the functional relationship between centrosomes and primary cilia is important because defects in both structures have been implicated in various diseases, including cancer. Here, we discuss evidence that the animal centrosome evolved, with the transition to complex multicellularity, as a hybrid organelle comprised of the two distinct, but intertwined, structural-functional modules: the centriole/primary cilium module and the pericentriolar material/centrosome module. The evolution of the former module may have been caused by the expanding cellular diversification and intercommunication, whereas that of the latter module may have been driven by the increasing complexity of mitosis and the requirement for maintaining cell polarity, individuation, and adhesion. Through its unique ability to serve both as a plasma membrane-associated primary cilium organizer and a juxtanuclear microtubule-organizing center, the animal centrosome has become an ideal integrator of extracellular and intracellular signals with the cytoskeleton and a switch between the non-cell autonomous and the cell-autonomous signaling modes. In light of this hypothesis, we discuss centrosome dynamics during cell proliferation, migration, and differentiation and propose a model of centrosome-driven microtubule assembly in mitotic and interphase cells. In addition, we outline the evolutionary benefits of the animal centrosome and highlight the hierarchy and modularity of the centrosome biogenesis networks.

Pericentrin-mediated SAS-6 recruitment promotes centriole assembly

The centrosome is composed of two centrioles surrounded by a microtubule-nucleating pericentriolar material (PCM). Although centrioles are known to regulate PCM assembly, it is less known whether and how the PCM contributes to centriole assembly. Here we investigate the interaction between centriole components and the PCM by taking advantage of fission yeast, which has a centriole-free, PCM-containing centrosome, the SPB. Surprisingly, we observed that several ectopically-expressed animal centriole components such as SAS-6 are recruited to the SPB. We revealed that a conserved PCM component, Pcp1/pericentrin, interacts with and recruits SAS-6. This interaction is conserved and important for centriole assembly, particularly its elongation. We further explored how yeasts kept this interaction even after centriole loss and showed that the conserved calmodulin-binding region of Pcp1/pericentrin is critical for SAS-6 interaction. Our work suggests that the PCM not only recruits and concentrates microtubule-nucleators, but also the centriole assembly machinery, promoting biogenesis close by.

Atypical function of a centrosomal module in WNT signalling drives contextual cancer cell motility

Centrosomes control cell motility, polarity and migration that is thought to be mediated by their microtubule-organizing capacity. Here we demonstrate that WNT signalling drives a distinct form of non-directional cell motility that requires a key centrosome module, but not microtubules or centrosomes. Upon exosome mobilization of PCP-proteins, we show that DVL2 orchestrates recruitment of a CEP192-PLK4/AURKB complex to the cell cortex where PLK4/AURKB act redundantly to drive protrusive activity and cell motility. This is mediated by coordination of formin-dependent actin remodelling through displacement of cortically localized DAAM1 for DAAM2. Furthermore, abnormal expression of PLK4, AURKB and DAAM1 is associated with poor outcomes in breast and bladder cancers. Thus, a centrosomal module plays an atypical function in WNT signalling and actin nucleation that is critical for cancer cell motility and is associated with more aggressive cancers. These studies have broad implications in how contextual signalling controls distinct modes of cell migration.

PLK4 promotes centriole duplication by phosphorylating STIL to link the procentriole cartwheel to the microtubule wall

Centrioles play critical roles in organizing the assembly of the mitotic spindle and templating the formation of primary cilia. Centriole duplication occurs once per cell cycle and is regulated by Polo-like kinase 4 (PLK4). Although significant progress has been made in understanding centriole composition, we have limited knowledge of how PLK4 activity controls specific steps in centriole formation. Here, we show that PLK4 phosphorylates its centriole substrate STIL on a conserved site, S428, to promote STIL binding to CPAP. This phospho-dependent binding interaction is conserved in Drosophila and facilitates the stable incorporation of both STIL and CPAP into the centriole. We propose that procentriole assembly requires PLK4 to phosphorylate STIL in two different regions: phosphorylation of residues in the STAN motif allow STIL to bind SAS6 and initiate cartwheel assembly, while phosphorylation of S428 promotes the binding of STIL to CPAP, linking the cartwheel to microtubules of the centriole wall.

A cell’s DNA is the chemical instruction manual for everything it does. Each cell in our bodies contains over two meters of DNA, which is divided into 46 packages of information called chromosomes. When the body needs to make more cells, for example during growth or repair, existing cells divide in two in order to replicate themselves. This means that they also need to copy all of their DNA and then deliver identical sets of chromosomes to each new cell.

Animal cells use structures called centrioles to help them divide their sets of chromosomes accurately. When cells are about to divide, they make a new set of centrioles by assembling a variety of proteins. This assembly process must be carefully controlled; if too many or too few centrioles are built, cell division errors can occur that lead to the generation of new cells with abnormal numbers of chromosomes.

The enzyme PLK4 helps to assemble centrioles, but its exact role in the construction process has remained largely unknown. For example, how it might modify different components of the centriole, and why this matters, is poorly understood.

By performing cell biological and biochemical experiments using human cells, Moyer and Holland show that PLK4 interacts with a protein called STIL that is found in the central part of the centriole. The modification of STIL at a specific location by PLK4 was needed to link it to another protein in the outer wall of the centriole, and was also necessary for the cells to build new centrioles. Cells in which PLK4 was unable to modify STIL had too few centrioles when they were beginning to divide.

Testing the activity of PLK4 in fruit flies revealed that it plays a similar role as in human cells. This suggests that the modification of STIL by PLK4 is important for normal cell division across different species.

The results presented by Moyer and Holland help us to understand how dividing cells build the complex machinery that enables them to pass on their genetic material accurately. Future work that builds on these findings could provide insight into human diseases, such as brain development disorders and cancer, where centrioles are either defective or present in the wrong number.

Precision Revisited: Targeting Microcephaly Kinases in Brain Tumors

Glioblastoma multiforme and medulloblastoma are the most frequent high-grade brain tumors in adults and children, respectively. Standard therapies for these cancers are mainly based on surgical resection, radiotherapy, and chemotherapy. However, intrinsic or acquired resistance to treatment occurs almost invariably in the first case, and side effects are unacceptable in the second. Therefore, the development of new, effective drugs is a very important unmet medical need. A critical requirement for developing such agents is to identify druggable targets required for the proliferation or survival of tumor cells, but not of other cell types. Under this perspective, genes mutated in congenital microcephaly represent interesting candidates. Congenital microcephaly comprises a heterogeneous group of disorders in which brain volume is reduced, in the absence or presence of variable syndromic features. Genetic studies have clarified that most microcephaly genes encode ubiquitous proteins involved in mitosis and in maintenance of genomic stability, but the effects of their inactivation are particularly strong in neural progenitors. It is therefore conceivable that the inhibition of the function of these genes may specifically affect the proliferation and survival of brain tumor cells. Microcephaly genes encode for a few kinases, including CITK, PLK4, AKT3, DYRK1A, and TRIO. In this review, we summarize the evidence indicating that the inhibition of these molecules could exert beneficial effects on different aspects of brain cancer treatment.

Polo-like kinase 4 maintains centriolar satellite integrity by phosphorylation of centrosomal protein 131 (CEP131)

The centrosome, consisting of two centrioles surrounded by a dense network of proteins, is the microtubule-organizing center of animal cells. Polo-like kinase 4 (PLK4) is a Ser/Thr protein kinase and the master regulator of centriole duplication, but it may play additional roles in centrosome function. To identify additional proteins regulated by PLK4, we generated an RPE-1 human cell line with a genetically engineered "analog-sensitive" PLK4AS, which genetically encodes chemical sensitivity to competitive inhibition via a bulky ATP analog. We used this transgenic line in an unbiased multiplex phosphoproteomic screen. Several hits were identified and validated as direct PLK4 substrates by in vitro kinase assays. Among them, we confirmed Ser-78 in centrosomal protein 131 (CEP131, also known as AZI1) as a direct substrate of PLK4. Using immunofluorescence microscopy, we observed that although PLK4-mediated phosphorylation of Ser-78 is dispensable for CEP131 localization, ciliogenesis, and centriole duplication, it is essential for maintaining the integrity of centriolar satellites. We also found that PLK4 inhibition or use of a nonphosphorylatable CEP131 variant results in dispersed centriolar satellites. Moreover, replacement of endogenous WT CEP131 with an S78D phosphomimetic variant promoted aggregation of centriolar satellites. We conclude that PLK4 phosphorylates CEP131 at Ser-78 to maintain centriolar satellite integrity.

Molecular architecture of a cylindrical self-assembly at human centrosomes

The cell is constructed by higher-order structures and organelles through complex interactions among distinct structural constituents. The centrosome is a membraneless organelle composed of two microtubule-derived structures called centrioles and an amorphous mass of pericentriolar material. Super-resolution microscopic analyses in various organisms revealed that diverse pericentriolar material proteins are concentrically localized around a centriole in a highly organized manner. However, the molecular nature underlying these organizations remains unknown. Here we show that two human pericentriolar material scaffolds, Cep63 and Cep152, cooperatively generate a heterotetrameric α-helical bundle that functions in conjunction with its neighboring hydrophobic motifs to self-assemble into a higher-order cylindrical architecture capable of recruiting downstream components, including Plk4, a key regulator for centriole duplication. Mutations disrupting the self-assembly abrogate Plk4-mediated centriole duplication. Because pericentriolar material organization is evolutionarily conserved, this work may offer a paradigm for investigating the assembly and function of centrosomal scaffolds in various organisms.

The centrosome is a membraneless organelle composed of two centrioles and an amorphous pericentriolar material but the overall centrosome organizations remains unknown. Here authors show that two scaffold proteins, Cep63 and Cep152, self-assemble into a higher-order cylindrical architecture capable of recruiting downstream components, including Plk4.

Mechanism and Regulation of Centriole and Cilium Biogenesis

The centriole is an ancient microtubule-based organelle with a conserved nine-fold symmetry. Centrioles form the core of centrosomes, which organize the interphase microtubule cytoskeleton of most animal cells and form the poles of the mitotic spindle. Centrioles can also be modified to form basal bodies, which template the formation of cilia and play central roles in cellular signaling, fluid movement, and locomotion. In this review, we discuss developments in our understanding of the biogenesis of centrioles and cilia and the regulatory controls that govern their structure and number. We also discuss how defects in these processes contribute to a spectrum of human diseases and how new technologies have expanded our understanding of centriole and cilium biology, revealing exciting avenues for future exploration.

Bridging centrioles and PCM in proper space and time

Throughout biology, specifying cellular events at the correct location and time is necessary for ensuring proper function. The formation of robust microtubule organizing centers (MTOCs) in mitosis is one such event that must be restricted in space to centrosomes to prevent ectopic MTOC formation elsewhere in the cell, a situation that can result in multipolar spindle formation and aneuploidy. The process of reaching maximum centrosome MTOC activity in late G2, known as centrosome maturation, ensures accurate timing of nuclear envelope breakdown and proper chromosome attachment. Although centrosome maturation has been recognized for over a century, the spatial and temporal regulatory mechanisms that direct MTOC activation are poorly understood. Here, we review Sas-4/CPAP, Asterless/Cep152, Spd-2/Cep192, and PLP/Pericentrin, a group of proteins we refer to as 'bridge' proteins that reside at the surface of centrioles, perfectly positioned to serve as the gatekeepers of proper centrosome maturation at the perfect place and time.

Albatross/FBF1 contributes to both centriole duplication and centrosome separation

The centrosome is a small but important organelle that participates in centriole duplication, spindle formation, and ciliogenesis. Each event is regulated by key enzymatic reactions, but how these processes are integrated remains unknown. Recent studies have reported that ciliogenesis is controlled by distal appendage proteins such as FBF1, also known as Albatross. However, the precise role of Albatross in the centrosome cycle, including centriole duplication and centrosome separation, remains to be determined. Here, we report a novel function for Albatross at the proximal ends of centrioles. Using Albatross monospecific antibodies, full-length constructs, and siRNAs for rescue experiments, we found that Albatross mediates centriole duplication by recruiting HsSAS-6, a cartwheel protein of centrioles. Moreover, Albatross participates in centrosome separation during mitosis by recruiting Plk1 to residue S348 of Albatross after its phosphorylation. Taken together, our results show that Albatross is a novel protein that spatiotemporally integrates different aspects of centrosome function, namely ciliogenesis, centriole duplication, and centrosome separation.

Regulating a key mitotic regulator, polo-like kinase 1 (PLK1)

During cell division, duplicated genetic material is separated into two distinct daughter cells. This process is essential for initial tissue formation during development and to maintain tissue integrity throughout an organism's lifetime. To ensure the efficacy and efficiency of this process, the cell employs a variety of regulatory and signaling proteins that function as mitotic regulators and checkpoint proteins. One vital mitotic regulator is polo-like kinase 1 (PLK1), a highly conserved member of the polo-like kinase family. Unique from its paralogues, it functions specifically during mitosis as a regulator of cell division. PLK1 is spatially and temporally enriched at three distinct subcellular locales; the mitotic centrosomes, kinetochores, and the cytokinetic midbody. These localization patterns allow PLK1 to phosphorylate specific downstream targets to regulate mitosis. In this review, we will explore how polo-like kinases were originally discovered and diverged into the five paralogues (PLK1-5) in mammals. We will then focus specifically on the most conserved, PLK1, where we will discuss what is known about how its activity is modulated, its role during the cell cycle, and new, innovative tools that have been developed to examine its function and interactions in cells.

Autoamplification and Competition Drive Symmetry Breaking: Initiation of Centriole Duplication by the PLK4-STIL Network

Centrioles, the cores of centrosomes and cilia, duplicate every cell cycle to ensure their faithful inheritance. How only a single procentriole is produced on each mother centriole remains enigmatic. We propose the first mechanistic biophysical model for procentriole initiation which posits that interactions between kinase PLK4 and its activator-substrate STIL are central for procentriole initiation. The model recapitulates the transition from a uniform "ring" of PLK4 surrounding the mother centriole to a single PLK4 "spot" that initiates procentriole assembly. This symmetry breaking requires autocatalytic activation of PLK4 and enhanced centriolar anchoring of PLK4 by phosphorylated STIL. We find that in situ degradation of active PLK4 cannot break symmetry. The model predicts that competition between transient PLK4 activity maxima for PLK4-STIL complexes destabilizes the PLK4 ring and produces instead a single PLK4 spot. Weakening of competition by overexpression of PLK4 and STIL causes progressive addition of supernumerary procentrioles, as observed experimentally.

Emerging Picture of Deuterosome-Dependent Centriole Amplification in MCCs

Multiciliated cells (MCCs) have several hair-like structures called cilia, which are required to propel substances on their surface. A cilium is organized from a basal body which resembles a hollow microtubule structure called a centriole. In terminally differentiated MCCs, hundreds of new basal bodies/centrioles are formed via two parallel pathways: the centriole- and deuterosome-dependent pathways. The deuterosome-dependent pathway is also referred to as "de novo" because unlike the centriole-dependent pathway which requires pre-existing centrioles, in the de novo pathway multiple new centrioles are organized around non-microtubule structures called deuterosomes. In the last five years, some deuterosome-specific markers have been identified and concurrent advancements in the super-resolution techniques have significantly contributed to gaining insights about the major stages of centriole amplification during ciliogenesis. Altogether, a new picture is emerging which also challenges the previous notion that deuterosome pathway is de novo. This review is primarily focused on studies that have contributed towards the better understanding of deuterosome-dependent centriole amplification and presents a developing model about the major stages identified during this process.

PPP1R35 is a novel centrosomal protein that regulates centriole length in concert with the microcephaly protein RTTN

Centrosome structure, function, and number are finely regulated at the cellular level to ensure normal mammalian development. Here, we characterize PPP1R35 as a novel bona fide centrosomal protein and demonstrate that it is critical for centriole elongation. Using quantitative super-resolution microscopy mapping and live-cell imaging we show that PPP1R35 is a resident centrosomal protein located in the proximal lumen above the cartwheel, a region of the centriole that has eluded detailed characterization. Loss of PPP1R35 function results in decreased centrosome number and shortened centrioles that lack centriolar distal and microtubule wall associated proteins required for centriole elongation. We further demonstrate that PPP1R35 acts downstream of, and forms a complex with, RTTN, a microcephaly protein required for distal centriole elongation. Altogether, our study identifies a novel step in the centriole elongation pathway centered on PPP1R35 and elucidates downstream partners of the microcephaly protein RTTN.

Aurora-PLK1 cascades as key signaling modules in the regulation of mitosis

Mitosis is controlled by reversible protein phosphorylation involving specific kinases and phosphatases. A handful of major mitotic protein kinases, such as the cyclin B-CDK1 complex, the Aurora kinases, and Polo-like kinase 1 (PLK1), cooperatively regulate distinct mitotic processes. Research has identified proteins and mechanisms that integrate these kinases into signaling cascades that guide essential mitotic events. These findings have important implications for our understanding of the mechanisms of mitotic regulation and may advance the development of novel antimitotic drugs. We review collected evidence that in vertebrates, the Aurora kinases serve as catalytic subunits of distinct complexes formed with the four scaffold proteins Bora, CEP192, INCENP, and TPX2, which we deem "core" Aurora cofactors. These complexes and the Aurora-PLK1 cascades organized by Bora, CEP192, and INCENP control crucial aspects of mitosis and all pathways of spindle assembly. We compare the mechanisms of Aurora activation in relation to the different spindle assembly pathways and draw a functional analogy between the CEP192 complex and the chromosomal passenger complex that may reflect the coevolution of centrosomes, kinetochores, and the actomyosin cleavage apparatus. We also analyze the roles and mechanisms of Aurora-PLK1 signaling in the cell and centrosome cycles and in the DNA damage response.

Revisiting Centrioles in Nematodes-Historic Findings and Current Topics

Theodor Boveri is considered as the “father” of centrosome biology. Boveri’s fundamental findings have laid the groundwork for decades of research on centrosomes. Here, we briefly review his early work on centrosomes and his first description of the centriole. Mainly focusing on centriole structure, duplication, and centriole assembly factors in C. elegans, we will highlight the role of this model in studying germ line centrosomes in nematodes. Last but not least, we will point to future directions of the C. elegans centrosome field.

Abnormally high expression of POLD1, MCM2, and PLK4 promotes relapse of acute lymphoblastic leukemia

This study aimed to explore the underlying mechanism of relapsed acute lymphoblastic leukemia (ALL).Datasets of GSE28460 and GSE18497 were downloaded from Gene Expression Omnibus (GEO). Differentially expressed genes (DEGs) between diagnostic and relapsed ALL samples were identified using Limma package in R, and a Venn diagram was drawn. Next, functional enrichment analyses of co-regulated DEGs were performed. Based on the String database, protein-protein interaction network and module analyses were also conducted. Moreover, transcription factors and miRNAs targeting co-regulated DEGs were predicted using the WebGestalt online tool.A total of 71 co-regulated DEGs were identified, including 56 co-upregulated genes and 15 co-downregulated genes. Functional enrichment analyses showed that upregulated DEGs were significantly enriched in the cell cycle, and DNA replication, and repair related pathways. POLD1, MCM2, and PLK4 were hub proteins in both protein-protein interaction network and module, and might be potential targets of E2F. Additionally, POLD1 and MCM2 were found to be regulated by miR-520H via E2F1.High expression of POLD1, MCM2, and PLK4 might play positive roles in the recurrence of ALL, and could serve as potential therapeutic targets for the treatment of relapsed ALL.

Haploid genetic screens identify genetic vulnerabilities to microtubule-targeting agents

The absence of biomarkers to accurately predict anticancer therapy response remains a major obstacle in clinical oncology. We applied a genome‐wide loss‐of‐function screening approach in human haploid cells to characterize genetic vulnerabilities to classical microtubule‐targeting agents. Using docetaxel and vinorelbine, two well‐established chemotherapeutic agents, we sought to identify genetic alterations sensitizing human HAP1 cells to these drugs. Despite the fact that both drugs act on microtubules, a set of distinct genes were identified whose disruption affects drug sensitivity. For docetaxel, this included a number of genes with a function in mitosis, while for vinorelbine we identified inactivation of FBXW7,RB1, and NF2, three frequently mutated tumor suppressor genes, as sensitizing factors. We validated these genes using independent knockout clones and confirmed FBXW7 as an important regulator of the mitotic spindle assembly. Upon FBXW7 depletion, vinorelbine treatment led to decreased survival of cells due to defective mitotic progression and subsequent mitotic catastrophe. We show that haploid insertional mutagenesis screens are a useful tool to study genetic vulnerabilities to classical chemotherapeutic drugs by identifying thus far unknown sensitivity factors. These results provide a rationale for investigating patient response to vinca alkaloid‐based anticancer treatment in relation to the mutational status of these three tumor suppressor genes, and could in the future lead to the establishment of novel predictive biomarkers or suggest new drug combinations based on molecular mechanisms of drug sensitivity.

Once and only once: mechanisms of centriole duplication and their deregulation in disease

Centrioles are conserved microtubule-based organelles that form the core of the centrosome and act as templates for the formation of cilia and flagella. Centrioles have important roles in most microtubule-related processes, including motility, cell division and cell signalling. To coordinate these diverse cellular processes, centriole number must be tightly controlled. In cycling cells, one new centriole is formed next to each pre-existing centriole in every cell cycle. Advances in imaging, proteomics, structural biology and genome editing have revealed new insights into centriole biogenesis, how centriole numbers are controlled and how alterations in these processes contribute to diseases such as cancer and neurodevelopmental disorders. Moreover, recent work has uncovered the existence of surveillance pathways that limit the proliferation of cells with numerical centriole aberrations. Owing to this progress, we now have a better understanding of the molecular mechanisms governing centriole biogenesis, opening up new possibilities for targeting these pathways in the context of human disease.

Uncoordinated centrosome cycle underlies the instability of non-diploid somatic cells in mammals

Mammalian somatic cells are more stable as diploids, but the mechanisms underlying this stability are unclear. Yaguchi et al. show that changes in centriole licensing compromise the control of centrosome number in haploid or tetraploid human cells, suggesting that the ploidy-dependent control of the centrosome cycle explains the instability of non-diploid karyotypes.

In animals, somatic cells are usually diploid and are unstable when haploid for unknown reasons. In this study, by comparing isogenic human cell lines with different ploidies, we found frequent centrosome loss specifically in the haploid state, which profoundly contributed to haploid instability through subsequent mitotic defects. We also found that the efficiency of centriole licensing and duplication changes proportionally to ploidy level, whereas that of DNA replication stays constant. This caused gradual loss or frequent overduplication of centrioles in haploid and tetraploid cells, respectively. Centriole licensing efficiency seemed to be modulated by astral microtubules, whose development scaled with ploidy level, and artificial enhancement of aster formation in haploid cells restored centriole licensing efficiency to diploid levels. The ploidy–centrosome link was observed in different mammalian cell types. We propose that incompatibility between the centrosome duplication and DNA replication cycles arising from different scaling properties of these bioprocesses upon ploidy changes underlies the instability of non-diploid somatic cells in mammals.

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.

Noncanonical Biogenesis of Centrioles and Basal Bodies

Centrioles and basal bodies (CBBs) organize centrosomes and cilia within eukaryotic cells. These organelles are composed of microtubules and hundreds of proteins performing multiple functions such as signaling, cytoskeleton remodeling, and cell motility. The CBB is present in all branches of the eukaryotic tree of life and, despite its ultrastructural and protein conservation, there is diversity in its function, occurrence (i.e., presence/absence), and modes of biogenesis across species. In this review, we provide an overview of the multiple pathways through which CBBs are formed in nature, with a special focus on the less studied, noncanonical ways. Despite the differences among each mechanism herein presented, we highlighted some of their common principles. These principles, governing different steps of biogenesis, ensure that CBBs may perform a multitude of functions in a huge diversity of organisms but yet retained their robustness in structure throughout evolution.

The SKP1-Cullin-F-box E3 ligase βTrCP and CDK2 cooperate to control STIL abundance and centriole number

Deregulation of centriole duplication has been implicated in cancer and primary microcephaly. Accordingly, it is important to understand how key centriole duplication factors are regulated. E3 ubiquitin ligases have been implicated in controlling the levels of several duplication factors, including PLK4, STIL and SAS-6, but the precise mechanisms ensuring centriole homeostasis remain to be fully understood. Here, we have combined proteomics approaches with the use of MLN4924, a generic inhibitor of SCF E3 ubiquitin ligases, to monitor changes in the cellular abundance of centriole duplication factors. We identified human STIL as a novel substrate of SCF-βTrCP. The binding of βTrCP depends on a DSG motif within STIL, and serine 395 within this motif is phosphorylated in vivo SCF-βTrCP-mediated degradation of STIL occurs throughout interphase and mutations in the DSG motif causes massive centrosome amplification, attesting to the physiological importance of the pathway. We also uncover a connection between this new pathway and CDK2, whose role in centriole biogenesis remains poorly understood. We show that CDK2 activity protects STIL against SCF-βTrCP-mediated degradation, indicating that CDK2 and SCF-βTrCP cooperate via STIL to control centriole biogenesis.

FBXL13 directs the proteolysis of CEP192 to regulate centrosome homeostasis and cell migration

Aberrant centrosome organisation with ensuing alterations of microtubule nucleation capacity enables tumour cells to proliferate and invade despite increased genomic instability. CEP192 is a key factor in the initiation process of centrosome duplication and in the control of centrosome microtubule nucleation. However, regulatory means of CEP192 have remained unknown. Here, we report that FBXL13, a binding determinant of SCF (SKP1-CUL1-F-box)-family E3 ubiquitin ligases, is enriched at centrosomes and interacts with the centrosomal proteins Centrin-2, Centrin-3, CEP152 and CEP192. Among these, CEP192 is specifically targeted for proteasomal degradation by FBXL13. Accordingly, induced FBXL13 expression downregulates centrosomal γ-tubulin and disrupts centrosomal microtubule arrays. In addition, depletion of FBXL13 induces high levels of CEP192 and γ-tubulin at the centrosomes with the consequence of defects in cell motility. Together, we characterise FBXL13 as a novel regulator of microtubule nucleation activity and highlight a role in promoting cell motility with potential tumour-promoting implications.

Distinct roles of Cep192 and Cep152 in acentriolar MTOCs and spindle formation during mouse oocyte maturation

Mammalian oocytes lack a centriole that acts as a microtubule organization center (MTOC) in most somatic cells. During oocyte maturation, MTOCs undergo remodeling processes, including decondensation, fragmentation, and self-organization. However, the underlying mechanisms of MTOC remodeling in mouse oocytes are not well understood. We showed that two pericentriolar proteins, Cep192 and Cep152, play crucial roles during MTOC remodeling in mouse oocytes. Cep192 is present in MTOCs at all stages of oocyte maturation, and its depletion induces ablation of MTOCs, delay in spindle formation, and abnormal chromosomal alignment in spindles. In the case of Cep152, its localization on MTOCs is limited at the germinal vesicle stage and then disappears from the MTOCs after the germinal vesicle breakdown stage. Cep152 exclusion from MTOCs is involved in the fragmentation of MTOCs, and it is regulated by cyclin-dependent kinase 1 activity. Our results demonstrate the different roles of Cep192 and Cep152 in MTOC remodeling and a novel regulatory mechanism during meiotic spindle formation in mouse oocytes.-Lee, I.-W., Jo, Y.-J., Jung, S.-M., Wang, H.-Y., Kim, N.-H., Namgoong, S. Distinct roles of Cep192 and Cep152 in acentriolar MTOCs and spindle formation during mouse oocyte maturation.

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.

Two-step phosphorylation of Ana2 by Plk4 is required for the sequential loading of Ana2 and Sas6 to initiate procentriole formation

The conserved process of centriole duplication requires Plk4 kinase to recruit and promote interactions between Sas6 and Sas5/Ana2/STIL (respective nomenclature of worms/flies/humans). Plk4-mediated phosphorylation of Ana2/STIL in its conserved STAN motif has been shown to promote its interaction with Sas6. However, STAN motif phosphorylation is not required for recruitment of Ana2 to the centriole. Here we show that in Drosophila, Ana2 loads onto the site of procentriole formation ahead of Sas6 in a process that also requires Plk4. However, whereas Plk4 is first recruited to multiple sites around the ring of zone II at the periphery of the centriole, Ana2 is recruited to a single site in telophase before Plk4 becomes finally restricted to this same single site. When we over-ride the auto-destruction of Plk4, it remains localized to multiple sites in the outer ring of the centriole and, if catalytically active, recruits Ana2 to these sites. Thus, it is the active form of Plk4 that promotes Ana2's recruitment to the centriole. We now show that Plk4 phosphorylates Ana2 at a site other than the STAN motif, which lies in a conserved region we term the ANST (ANa2-STil) motif. Mutation of this site, S38, to a non-phosphorylatable residue prevents the procentriole loading of Ana2 and blocks centriole duplication. Thus the initiation of procentriole formation requires Plk4 to first phosphorylate a single serine residue in the ANST motif to promote Ana2's recruitment and, secondly, to phosphorylate four residues in the STAN motif enabling Ana2 to recruit Sas6. We discuss these findings in light of the multiple Plk4 phosphorylation sites on Ana2.

PLK4: a link between centriole biogenesis and cancer

INTRODUCTION

Polo like kinase (PLK) is known to play a pivotal role in various cell cycle processes to perpetuate proper division and growth of the cells. Polo like kinase-4 (PLK4) is one such kinase that appears in low abundance and plays a well-characterized role in centriole duplication. PLK4 deregulation (i.e. both overexpression and depletion of PLK4), leads to altered mitotic fidelity and thereby triggers tumorigenesis. Hence, over the last few years PLK4 has emerged as a potential therapeutic target for the treatment of various advanced cancers. Areas covered: In this review, we discuss the basic structure, expression, localization and functions of PLK4 along with its regulation by various proteins. We also discuss the role of altered PLK4 activity in the onset of cancer and the current pre-clinical and clinical inhibitors to regulate PLK4. Expert opinion: PLK4 mediated centriole duplication has a crucial role in maintaining mitotic correctness in normal cells, while its deregulation has a greater impact on genesis of cancer. Henceforth, a deep knowledge of the PLK4 levels, its role and interactions with various proteins in cancer is required to design effective inhibitors for clinical use.

Recent advances in pericentriolar material organization: ordered layers and scaffolding gels

The centrosome is an unusual organelle that lacks a surrounding membrane, raising the question of what limits its size and shape. Moreover, while electron microscopy (EM) has provided a detailed view of centriole architecture, there has been limited understanding of how the second major component of centrosomes, the pericentriolar material (PCM), is organized. Here, we summarize exciting recent findings from super-resolution fluorescence imaging, structural biology, and biochemical reconstitution that together reveal the presence of ordered layers and complex gel-like scaffolds in the PCM. Moreover, we discuss how this is leading to a better understanding of the process of microtubule nucleation, how alterations in PCM size are regulated in cycling and differentiated cells, and why mutations in PCM components lead to specific human pathologies.

Centriole Biogenesis: From Identifying the Characters to Understanding the Plot

The centriole is a beautiful microtubule-based organelle that is critical for the proper execution of many fundamental cellular processes, including polarity, motility, and division. Centriole biogenesis, the making of this miniature architectural wonder, has emerged as an exemplary model to dissect the mechanisms governing the assembly of a eukaryotic organelle. Centriole biogenesis relies on a set of core proteins whose contributions to the assembly process have begun to be elucidated. Here, we review current knowledge regarding the mechanisms by which these core characters function in an orderly fashion to assemble the centriole. In particular, we discuss how having the correct proteins at the right place and at the right time is critical to first scaffold, then initiate, and finally execute the centriole assembly process, thus underscoring fundamental principles governing organelle biogenesis.

A novel genetic syndrome with STARD9 mutation and abnormal spindle morphology

Intellectual disability (ID) is one of neurodevelopmental disorders characterized by serious defects in both intelligence and adaptive behavior. Although it has been suggested that genetic aberrations associated with the process of cell division underlie ID, the cytological evidence for mitotic defects in actual patient's cells is rarely reported. Here, we report a novel mutation in the STARD9 (also known as KIF16A) gene found in a patient with severe ID, characteristic features, epilepsy, acquired microcephaly, and blindness. Using whole-exome sequence analysis, we sequenced potential candidate genes in the patient. We identified a homozygous single-nucleotide deletion creating a premature stop codon in the STARD9 gene. STARD9 encodes a 4,700 amino acid protein belonging to the kinesin superfamily. Depletion of STARD9 or overexpression of C-terminally truncated STARD9 mutants were known to induce spindle assembly defects in human culture cells. To determine cytological features in the patient cells, we isolated lymphoblast cells from the patient, and performed immunofluorescence analysis. Remarkably, mitotic defects, including multipolar spindle formation, fragmentation of pericentriolar materials and centrosome amplification, were observed in the cells. Taken together, our findings raise the possibility that controlled expression of full-length STARD9 is necessary for proper spindle assembly in cell division during human development. We propose that mutations in STARD9 result in abnormal spindle morphology and cause a novel genetic syndrome with ID.

The Zebrafish curly fry Is Required for Proper Centrosome and Mitotic Spindle Assembly

The zebrafish curly fry (cfy) mutation leads to a dramatic increase in mitotic index and cell death starting during neural tube formation. The mutant phenotype is cell autonomous and does not result from defects in apical/basal polarity within the neuroepithelium. The increase in mitotic index could be due to increased proliferation or cell cycle arrest in mitosis. cfy embryos were analyzed to examine these two possibilities. By labeling embryos with a pulse of BrdU and anti-phospho-histone 3 and examining the DNA content by fluorescence activated cell sorting, we show that cfy mutants exhibit no increase in proliferation, but a significant increase in the number of cells arrested in mitosis. Furthermore, time-lapse microscopy in vivo confirmed that a great majority of dividing cells arrest during mitosis and that these mitotically arrested cells die in cfy embryos. Finally, immunostaining and confocal microscopy in cfy mutant embryos revealed that mitotic cells in mutants contain aberrant centrosomes and often exhibit monopolar spindles, thereby leading to mitotic cell cycle arrest. Our results suggest that the cfy gene is required for proper centrosome assembly and mitotic spindle formation, therefore critical for normal cell division.