Towards a molecular architecture of centriole assembly

The kinase ZYG-1 phosphorylates the cartwheel protein SAS-5 to drive centriole assembly in C. elegans

Centrioles organize centrosomes, the cell's primary microtubule-organizing centers (MTOCs). Centrioles double in number each cell cycle, and mis-regulation of this process is linked to diseases such as cancer and microcephaly. In C. elegans, centriole assembly is controlled by the Plk4 related-kinase ZYG-1, which recruits the SAS-5-SAS-6 complex. While the kinase activity of ZYG-1 is required for centriole assembly, how it functions has not been established. Here we report that ZYG-1 physically interacts with and phosphorylates SAS-5 on 17 conserved serine and threonine residues in vitro. Mutational scanning reveals that serine 10 and serines 331/338/340 are indispensable for proper centriole assembly. Embryos expressing SAS-5S10A exhibit centriole assembly failure, while those expressing SAS-5S331/338/340A possess extra centrioles. We show that in the absence of serine 10 phosphorylation, the SAS-5-SAS-6 complex is recruited to centrioles, but is not stably incorporated, possibly due to a failure to coordinately recruit the microtubule-binding protein SAS-4. Our work defines the critical role of phosphorylation during centriole assembly and reveals that ZYG-1 might play a role in preventing the formation of excess centrioles.

Centriole structural integrity defects are a crucial feature of Hydrolethalus Syndrome

Hydrolethalus Syndrome (HLS) is a lethal, autosomal recessive ciliopathy caused by the mutation of the conserved centriole protein HYLS1. However, how HYLS1 facilitates the centriole-based templating of cilia is poorly understood. Here, we show that mice harboring the HYLS1 disease mutation die shortly after birth and exhibit developmental defects that recapitulate several manifestations of the human disease. These phenotypes arise from tissue-specific defects in cilia assembly and function caused by a loss of centriole integrity. We show that HYLS1 is recruited to the centriole by CEP120 and functions to recruit centriole inner scaffold proteins that stabilize the centriolar microtubule wall. The HLS mutation disrupts the interaction of HYLS1 with CEP120 leading to HYLS1 displacement and degeneration of the centriole distal end. We propose that tissue-specific defects in centriole integrity caused by the HYLS1 mutation prevent ciliogenesis and drive HLS phenotypes.

Prolonged overexpression of PLK4 leads to formation of centriole rosette clusters that are connected via canonical centrosome linker proteins

Centrosome amplification is a hallmark of cancer and PLK4 is one of the responsible factors for cancer associated centrosome amplification. Increased PLK4 levels was also shown to contribute to generation of cells with centriole amplification in mammalian tissues as olfactory neuron progenitor cells. PLK4 overexpression generates centriole rosette (CR) structures which harbor more than two centrioles each. Long term PLK4 overexpression results with centrosome amplification, but the maturation of amplified centrioles in CRs and linking of PLK4 induced amplified centrosomes has not yet been investigated in detail. Here, we show evidence for generation of large clustered centrosomes which have more than 2 centriole rosettes and define these structures as centriole rosette clusters (CRCs) in cells that have high PLK4 levels for 2 consecutive cell cycles. In addition, we show that PLK4 induced CRs follow normal centrosomal maturation processes and generate CRC structures that are inter-connected with canonical centrosomal linker proteins as C-Nap1, Rootletin and Cep68 in the second cell cycle after PLK4 induction. Increased PLK4 levels in cells with C-Nap1 and Rootletin knock-out resulted with distanced CRs and CRCs in interphase, while Nek2 knock-out inhibited separation of CRCs in prometaphase, providing functional evidence for the binding of CRC structures with centrosomal linker proteins. Taken together, these results suggest a cell cycle dependent model for PLK4 induced centrosome amplification which occurs in 2 consecutive cell cycles: (i) CR state in the first cell cycle, and (ii) CRC state in the second cell cycle.

Therapeutic potential of targeting polo-like kinase 4

Polo-like kinase 4 (PLK4), a highly conserved serine/threonine kinase, masterfully regulates centriole duplication in a spatiotemporal manner to ensure the fidelity of centrosome duplication and proper mitosis. Abnormal expression of PLK4 contributes to genomic instability and associates with a poor prognosis in cancer. Inhibition of PLK4 is demonstrated to exhibit significant efficacy against various types of human cancers, further highlighting its potential as a promising therapeutic target for cancer treatment. As such, numerous small-molecule inhibitors with distinct chemical scaffolds targeting PLK4 have been extensively investigated for the treatment of different human cancers, with several undergoing clinical evaluation (e.g., CFI-400945). Here, we review the structure, distribution, and biological functions of PLK4, encapsulate its intricate regulatory mechanisms of expression, and highlighting its multifaceted roles in cancer development and metastasis. Moreover, the recent advancements of PLK4 inhibitors in patent or literature are summarized, and their therapeutic potential as monotherapies or combination therapies with other anticancer agents are also discussed.

PLK4 self-phosphorylation drives the selection of a single site for procentriole assembly

Polo-like kinase 4 (PLK4) is a key regulator of centriole biogenesis, but how PLK4 selects a single site for procentriole assembly remains unclear. Using ultrastructure expansion microscopy, we show that PLK4 localizes to discrete sites along the wall of parent centrioles. While there is variation in the number of sites PLK4 occupies on the parent centriole, most PLK4 localize at a dominant site that directs procentriole assembly. Inhibition of PLK4 activity leads to stable binding of PLK4 to the centriole and increases occupancy to a maximum of nine sites. We show that self-phosphorylation of an unstructured linker promotes the release of active PLK4 from the centriole to drive the selection of a single site for procentriole assembly. Preventing linker phosphorylation blocks PLK4 turnover, leading to supernumerary sites of PLK4 localization and centriole amplification. Therefore, self-phosphorylation is a major driver of the spatial patterning of PLK4 at the centriole and plays a critical role in selecting a single centriole duplication site.

Towards understanding centriole elimination

Centrioles are microtubule-based structures crucial for forming flagella, cilia and centrosomes. Through these roles, centrioles are critical notably for proper cell motility, signalling and division. Recent years have advanced significantly our understanding of the mechanisms governing centriole assembly and architecture. Although centrioles are typically very stable organelles, persisting over many cell cycles, they can also be eliminated in some cases. Here, we review instances of centriole elimination in a range of species and cell types. Moreover, we discuss potential mechanisms that enable the switch from a stable organelle to a vanishing one. Further work is expected to provide novel insights into centriole elimination mechanisms in health and disease, thereby also enabling scientists to readily manipulate organelle fate.

Centrosome Movements Are TUBG1-Dependent

The centrosome of mammalian cells is in constant movement and its motion plays a part in cell differentiation and cell division. The purpose of this study was to establish the involvement of the TUBG meshwork in centrosomal motility. In live cells, we used a monomeric red-fluorescence-protein-tagged centrin 2 gene and a green-fluorescence-protein-tagged TUBG1 gene for labeling the centrosome and the TUBG1 meshwork, respectively. We found that centrosome movements occurred in cellular sites rich in GTPase TUBG1 and single-guide RNA mediated a reduction in the expression of TUBG1, altering the motility pattern of centrosomes. We propose that the TUBG1 meshwork enables the centrosomes to move by providing them with an interacting platform that mediates positional changes. These findings uncover a novel regulatory mechanism that controls the behavior of centrosomes.

FOXG1 targets BMP repressors and cell cycle inhibitors in human neural progenitor cells

FOXG1 is a critical transcription factor in human brain where loss-of-function mutations cause a severe neurodevelopmental disorder, while increased FOXG1 expression is frequently observed in glioblastoma. FOXG1 is an inhibitor of cell patterning and an activator of cell proliferation in chordate model organisms but different mechanisms have been proposed as to how this occurs. To identify genomic targets of FOXG1 in human neural progenitor cells (NPCs), we engineered a cleavable reporter construct in endogenous FOXG1 and performed chromatin immunoprecipitation (ChIP) sequencing. We also performed deep RNA sequencing of NPCs from two females with loss-of-function mutations in FOXG1 and their healthy biological mothers. Integrative analyses of RNA and ChIP sequencing data showed that cell cycle regulation and Bone Morphogenic Protein (BMP) repression gene ontology categories were over-represented as FOXG1 targets. Using engineered brain cell lines, we show that FOXG1 specifically activates SMAD7 and represses CDKN1B. Activation of SMAD7 which inhibits BMP signaling may be one way that FOXG1 patterns the forebrain, while repression of cell cycle regulators such as CDKN1B may be one way that FOXG1 expands the NPC pool to ensure proper brain size. Our data reveal novel mechanisms on how FOXG1 may control forebrain patterning and cell proliferation in human brain development.

Scanning electron microscopy of human islet cilia

Human islet primary cilia are vital glucose-regulating organelles whose structure remains uncharacterized. Scanning electron microscopy (SEM) is a useful technique for studying the surface morphology of membrane projections like cilia, but conventional sample preparation does not reveal the submembrane axonemal structure, which holds key implications for ciliary function. To overcome this challenge, we combined SEM with membrane-extraction techniques to examine primary cilia in native human islets. Our data show well-preserved cilia subdomains which demonstrate both expected and unexpected ultrastructural motifs. Morphometric features were quantified when possible, including axonemal length and diameter, microtubule conformations, and chirality. We further describe a ciliary ring, a structure that may be a specialization in human islets. Key findings are correlated with fluorescence microscopy and interpreted in the context of cilia function as a cellular sensor and communications locus in pancreatic islets.

The cilium-centrosome axis in coupling cell cycle exit and cell fate

The centrosome is an evolutionarily conserved, ancient organelle whose role in cell division was first described over a century ago. The structure and function of the centrosome as a microtubule-organizing center, and of its extracellular extension - the primary cilium - as a sensory antenna, have since been extensively studied, but the role of the cilium-centrosome axis in cell fate is still emerging. In this Opinion piece, we view cellular quiescence and tissue homeostasis from the vantage point of the cilium-centrosome axis. We focus on a less explored role in the choice between distinct forms of mitotic arrest - reversible quiescence and terminal differentiation, which play distinct roles in tissue homeostasis. We outline evidence implicating the centrosome-basal body switch in stem cell function, including how the cilium-centrosome complex regulates reversible versus irreversible arrest in adult skeletal muscle progenitors. We then highlight exciting new findings in other quiescent cell types that suggest signal-dependent coupling of nuclear and cytoplasmic events to the centrosome-basal body switch. Finally, we propose a framework for involvement of this axis in mitotically inactive cells and identify future avenues for understanding how the cilium-centrosome axis impacts central decisions in tissue homeostasis.

Scanning electron microscopy of human islet cilia

Human islet primary cilia are vital glucose-regulating organelles whose structure remains uncharacterized. Scanning electron microscopy (SEM) is a useful technique for studying the surface morphology of membrane projections like primary cilia, but conventional sample preparation does not reveal the sub-membrane axonemal structure which holds key implications for cilia function. To overcome this challenge, we combined SEM with membrane-extraction techniques to examine cilia in native human islets. Our data show well-preserved cilia subdomains which demonstrate both expected and unexpected ultrastructural motifs. Morphometric features were quantified when possible, including axonemal length and diameter, microtubule conformations and chirality. We further describe a novel ciliary ring, a structure that may be a specialization in human islets. Key findings are correlated with fluorescence microscopy and interpreted in the context of cilia function as a cellular sensor and communications locus in pancreatic islets.

Bld10p/Cep135 determines the number of triplets in the centriole independently of the cartwheel

The conserved nine-fold structural symmetry of the centriole is thought to be generated by cooperation between two mechanisms, one dependent on and the other independent of the cartwheel, a sub-centriolar structure consisting of a hub and nine spokes. However, the molecular entity of the cartwheel-independent mechanism has not been elucidated. Here, using Chlamydomonas reinhardtii mutants, we show that Bld10p/Cep135, a conserved centriolar protein that connects cartwheel spokes and triplet microtubules, plays a central role in this mechanism. Using immunoelectron microscopy, we localized hemagglutinin epitopes attached to distinct regions of Bld10p along two lines that connect adjacent triplets. Consistently, conventional and cryo-electron microscopy identified crosslinking structures at the same positions. In centrioles formed in the absence of the cartwheel, truncated Bld10p was found to significantly reduce the inter-triplet distance and frequently form eight-microtubule centrioles. These results suggest that the newly identified crosslinks are comprised of part of Bld10p/Cep135. We propose that Bld10p determines the inter-triplet distance in the centriole and thereby regulates the number of triplets in a cartwheel-independent manner.

Ciliate cortical organization and dynamics for cell motility: Comparing ciliates and vertebrates

The generation of efficient fluid flow is crucial for organismal development and homeostasis, sexual reproduction, and motility. Multi-ciliated cells possess fields of motile cilia that beat in synchrony to propel fluid. Ciliary arrays are remarkably conserved in their organization and function. Ciliates have polarized multi-ciliary arrays (MCAs) to promote fluid flow for cell motility. The ciliate cortex is decorated with hundreds of basal bodies (BB) forming linear rows along the cell's anterior-posterior axis. BBs scaffold and position cilia to form the organized ciliary array. Nascent BBs assemble at the base of BBs. As nascent BBs mature, they integrate into the cortical BB and cytoskeletal network and nucleate their own cilium. The organization of MCAs is balanced between cortical stability and cortical dynamism. The cortical cytoskeletal network both establishes and maintains a stable organization of the MCA in the face of mechanical forces exerted by ciliary beating. At the same time, MCA organization is plastic, such that it remodels for optimal ciliary mobility during development and in response to environmental conditions. Such plasticity promotes effective feeding and ecological behavior required for these organisms. Together, these properties allow an organism to effectively sense, adapt to, and move through its environment.

How COVID-19 Hijacks the Cytoskeleton: Therapeutic Implications

The SARS-CoV-2 virus invades and replicates within host cells by “hijacking” biomolecular machinery, gaining control of the microtubule cytoskeleton. After attaching to membrane receptors and entering cells, the SARS-CoV-2 virus co-opts the dynamic intra-cellular cytoskeletal network of microtubules, actin, and the microtubule-organizing center, enabling three factors that lead to clinical pathology: (1) viral load due to intra-cellular trafficking, (2) cell-to-cell spread by filopodia, and (3) immune dysfunction, ranging from hyper-inflammatory cytokine storm to ineffective or absent response. These factors all depend directly on microtubules and the microtubule-organizing center, as do cell functions such as mitosis and immune cell movement. Here we consider how the SARS-CoV-2 virus may “hijack” cytoskeletal functions by docking inside the microtubule-organizing center’s centriole “barrels”, enabling certain interactions between the virus’s positively charged spike (“S”) proteins and negatively charged C-termini of the microtubules that the centriole comprises, somewhat like fingers on a keyboard. This points to the potential benefit of therapies aimed not directly at the virus but at the microtubules and microtubule-organizing center of the host cell on which the virus depends. These therapies could range from anti-microtubule drugs to low-intensity ultrasound (megahertz mechanical vibrations) externally applied to the vagus nerve at the neck and/or to the spleen (since both are involved in mediating inflammatory response). Given that ultrasound imaging machines suitable for vagal/splenic ultrasound are available for clinical trials in every hospital, we recommend an alternative therapeutic approach for COVID-19 based on addressing and normalizing the host cell microtubules and microtubule-organizing centers co-opted by the SARS-CoV-2 virus.

Atypical and distinct microtubule radial symmetries in the centriole and the axoneme of Lecudina tuzetae

The centriole is a minute cylindrical organelle present in a wide range of eukaryotic species. Most centrioles have a signature 9-fold radial symmetry of microtubules that is imparted onto the axoneme of the cilia and flagella they template, with 9 centriolar microtubule doublets growing into 9 axonemal microtubule doublets. There are exceptions to the 9-fold symmetrical arrangement of axonemal microtubules in some species, with lower or higher fold symmetries. In the few cases where this has been examined, such alterations in axonemal symmetries are grounded in likewise alterations in centriolar symmetries. Here, we examine the question of microtubule number continuity between centriole and axoneme in flagellated gametes of the gregarine Lecudina tuzetae, which have been reported to exhibit a 6-fold radial symmetry of axonemal microtubules. We used time-lapse differential interference microscopy to identify the stage at which flagellated gametes are present. Thereafter, using electron microscopy and ultrastructure-expansion microscopy coupled to STimulated Emission Depletion (STED) super-resolution imaging, we uncover that a 6- or 5-fold radial symmetry in the axoneme is accompanied by an 8-fold radial symmetry in the centriole. We conclude that the transition between centriolar and axonemal microtubules can be characterized by unexpected plasticity. [Media: see text] [Media: see text] [Media: see text].

A novel HIF1α-STIL-FOXM1 axis regulates tumor metastasis

BACKGROUND

Metastasis is the major cause of morbidity and mortality in cancer that involves in multiple steps including epithelial-mesenchymal transition (EMT) process. Centrosome is an organelle that functions as the major microtubule organizing center (MTOC), and centrosome abnormalities are commonly correlated with tumor aggressiveness. However, the conclusive mechanisms indicating specific centrosomal proteins participated in tumor progression and metastasis remain largely unknown.

METHODS

The expression levels of centriolar/centrosomal genes in various types of cancers were first examined by in silico analysis of the data derived from The Cancer Genome Atlas (TCGA), Gene Expression Omnibus (GEO), and European Bioinformatics Institute (EBI) datasets. The expression of STIL (SCL/TAL1-interrupting locus) protein in clinical specimens was further assessed by Immunohistochemistry (IHC) analysis and the oncogenic roles of STIL in tumorigenesis were analyzed using in vitro and in vivo assays, including cell migration, invasion, xenograft tumor formation, and metastasis assays. The transcriptome differences between low- and high-STIL expression cells were analyzed by RNA-seq to uncover candidate genes involved in oncogenic pathways. The quantitative polymerase chain reaction (qPCR) and reporter assays were performed to confirm the results. The chromatin immunoprecipitation (ChIP)-qPCR assay was applied to demonstrate the binding of transcriptional factors to the promoter.

RESULTS

The expression of STIL shows the most significant increase in lung and various other types of cancers, and is highly associated with patients' survival rate. Depletion of STIL inhibits tumor growth and metastasis. Interestingly, excess STIL activates the EMT pathway, and subsequently enhances cancer cell migration and invasion. Importantly, we reveal an unexpected role of STIL in tumor metastasis. A subset of STIL translocate into nucleus and associate with FOXM1 (Forkhead box protein M1) to promote tumor metastasis and stemness via FOXM1-mediated downstream target genes. Furthermore, we demonstrate that hypoxia-inducible factor 1α (HIF1α) directly binds to the STIL promoter and upregulates STIL expression under hypoxic condition.

CONCLUSIONS

Our findings indicate that STIL promotes tumor metastasis through the HIF1α-STIL-FOXM1 axis, and highlight the importance of STIL as a promising therapeutic target for lung cancer treatment.

WDR62 regulates mouse oocyte meiotic maturation related to p-JNK and H3K9 trimethylation

WDR62 (WD40-repeat protein 62) participates in diverse biological process, especially mitotic spindle organization via regulating centriole biogenesis and the function of centriole-associated protein. However, the role of WDR62 exerts in spindle assembly and meiotic progression control in oocytes lacking typical centrosomes remains obscure. In a previous study, we reported that WDR62 is involved in spindle migration and asymmetric cytokinesis in mouse oocyte meiosis. In the current study, another novel function of WDR62 regulating cell cycle progression through meiotic spindle formation during oocyte meiotic maturation was found. Knockdown of WDR62 through siRNA microinjection disrupted the meiotic cell cycle and induced metaphase-I (MI) arrest coupled with severe spindle abnormality, chromosome misalignment, and aneuploid generation. Moreover, WDR62 depletion induced defective kinetochore-microtubule attachments (K-MT) and activated spindle assembly checkpoint (SAC), which could trigger the arrest of meiotic progression. Further study demonstrated that depletion of WDR62 was associated with an aberrant location of p-JNK and reduced its expression level; concomitantly, status of H3K9 trimethylation was also altered. In addition, phenotypes similar to WDR62 depletion were observed during the function-loss analysis of p-JNK using a specific inhibitor (SP600125), which signifies that WDR62 is important for spindle organization and meiotic progression, and this function might be via its regulation of p-JNK. In conclusion, this study revealed that WDR62 functions in multiple ways during oocyte meiotic maturation, which could be related to p-JNK and H3K9 trimethylation.

The APC/C targets the Cep152-Cep63 complex at the centrosome to regulate mitotic spindle assembly

The control of protein abundance is a fundamental regulatory mechanism during mitosis. The anaphase-promoting complex/cyclosome (APC/C) is the main protein ubiquitin ligase responsible for the temporal regulation of mitotic progression. It has been proposed that the APC/C might fulfil other functions, including assembly of the mitotic spindle. Here, we show that the APC/C localizes to centrosomes, the organizers of the eukaryotic microtubule cytoskeleton, specifically during mitosis. Recruitment of the APC/C to spindle poles requires the centrosomal protein Cep152, and we identified Cep152 as both an APC/C interaction partner and an APC/C substrate. Previous studies have shown that Cep152 forms a complex with Cep57 and Cep63. The APC/C-mediated ubiquitylation of Cep152 at the centrosome releases Cep57 from this inhibitory complex and enables its interaction with pericentrin, a critical step in promoting microtubule nucleation. Thus, our study extends the function of the APC/C from being a regulator of mitosis to also acting as a positive governor of spindle assembly. The APC/C thereby integrates control of these two important processes in a temporal manner.

Microenvironment-mediated cancer dormancy: Insights from metastability theory

Dormancy is an evolutionarily conserved protective mechanism widely observed in nature. A pathological example is found during cancer metastasis, where cancer cells disseminate from the primary tumor, home to secondary organs, and enter a growth-arrested state, which could last for decades. Recent studies have pointed toward the microenvironment being heavily involved in inducing, preserving, or ceasing this dormant state, with a strong focus on identifying specific molecular mechanisms and signaling pathways. Increasing evidence now suggests the existence of an interplay between intracellular as well as extracellular biochemical and mechanical cues in guiding such processes. Despite the inherent complexities associated with dormancy, proliferation, and growth of cancer cells and tumor tissues, viewing these phenomena from a physical perspective allows for a more global description, independent from many details of the systems. Building on the analogies between tissues and fluids and thermodynamic phase separation concepts, we classify a number of proposed mechanisms in terms of a thermodynamic metastability of the tumor with respect to growth. This can be governed by interaction with the microenvironment in the form of adherence (wetting) to a substrate or by mechanical confinement of the surrounding extracellular matrix. By drawing parallels with clinical and experimental data, we advance the notion that the local energy minima, or metastable states, emerging in the tissue droplet growth kinetics can be associated with a dormant state. Despite its simplicity, the provided framework captures several aspects associated with cancer dormancy and tumor growth.

Endoderm development requires centrioles to restrain p53-mediated apoptosis in the absence of ERK activity

Centrioles comprise the heart of centrosomes, microtubule-organizing centers. To study the function of centrioles in lung and gut development, we genetically disrupted centrioles throughout the mouse endoderm. Surprisingly, removing centrioles from the endoderm did not disrupt intestinal growth or development but blocked lung branching. In the lung, acentriolar SOX2-expressing airway epithelial cells apoptosed. Loss of centrioles activated p53, and removing p53 restored survival of SOX2-expressing cells, lung branching, and mouse viability. To investigate how endodermal p53 activation specifically killed acentriolar SOX2-expressing cells, we assessed ERK, a prosurvival cue. ERK was active throughout the intestine and in the distal lung buds, correlating with tolerance to centriole loss. Pharmacologically inhibiting ERK activated apoptosis in acentriolar cells, revealing that ERK activity protects acentriolar cells from apoptosis. Therefore, centrioles are largely dispensable for endodermal growth and the spatial distribution of ERK activity in the endoderm shapes the developmental consequences of centriolar defects and p53 activation.

The centriolar tubulin code

Centrioles are microtubule-based cell organelles present in most eukaryotes. They participate in the control of cell division as part of the centrosome, the major microtubule-organizing center of the cell, and are also essential for the formation of primary and motile cilia. During centriole assembly as well as across its lifetime, centriolar tubulin display marks defined by post-translational modifications (PTMs), such as glutamylation or acetylation. To date, the functions of these PTMs at centrioles are not well understood, although pioneering experiments suggest a role in the stability of this organelle. Here, we review the current knowledge regarding PTMs at centrioles with a particular focus on a possible link between these modifications and centriole's architecture, and propose possible hypothesis regarding centriolar tubulin PTMs's function.

Centrosome instability: when good centrosomes go bad

The centrosome is a tiny cytoplasmic organelle that organizes and constructs massive molecular machines to coordinate diverse cellular processes. Due to its many roles during both interphase and mitosis, maintaining centrosome homeostasis is essential to normal health and development. Centrosome instability, divergence from normal centrosome number and structure, is a common pathognomonic cellular state tightly associated with cancers and other genetic diseases. As novel connections are investigated linking the centrosome to disease, it is critical to understand the breadth of centrosome functions to inspire discovery. In this review, we provide an introduction to normal centrosome function and highlight recent discoveries that link centrosome instability to specific disease states.

p53 Activation in Genetic Disorders: Different Routes to the Same Destination

The tumor suppressor p53 is critical for preventing neoplastic transformation and tumor progression. Inappropriate activation of p53, however, has been observed in a number of human inherited disorders that most often affect development of the brain, craniofacial region, limb skeleton, and hematopoietic system. Genes related to these developmental disorders are essentially involved in transcriptional regulation/chromatin remodeling, rRNA metabolism, DNA damage-repair pathways, telomere maintenance, and centrosome biogenesis. Perturbation of these activities or cellular processes may result in p53 accumulation in cell cultures, animal models, and perhaps humans as well. Mouse models of several p53 activation-associated disorders essentially recapitulate human traits, and inactivation of p53 in these models can alleviate disorder-related phenotypes. In the present review, we focus on how dysfunction of the aforementioned biological processes causes developmental defects via excessive p53 activation. Notably, several disease-related genes exert a pleiotropic effect on those cellular processes, which may modulate the magnitude of p53 activation and establish or disrupt regulatory loops. Finally, we discuss potential therapeutic strategies for genetic disorders associated with p53 misactivation.

Deficient adaptation to centrosome duplication defects in neural progenitors causes microcephaly and subcortical heterotopias

Congenital microcephaly (MCPH) is a neurodevelopmental disease associated with mutations in genes encoding proteins involved in centrosomal and chromosomal dynamics during mitosis. Detailed MCPH pathogenesis at the cellular level is still elusive, given the diversity of MCPH genes and lack of comparative in vivo studies. By generating a series of CRISPR/Cas9-mediated genetic KOs, we report here that — whereas defects in spindle pole proteins (ASPM, MCPH5) result in mild MCPH during development — lack of centrosome (CDK5RAP2, MCPH3) or centriole (CEP135, MCPH8) regulators induces delayed chromosome segregation and chromosomal instability in neural progenitors (NPs). Our mouse model of MCPH8 suggests that loss of CEP135 results in centriole duplication defects, TP53 activation, and cell death of NPs. Trp53 ablation in a Cep135-deficient background prevents cell death but not MCPH, and it leads to subcortical heterotopias, a malformation seen in MCPH8 patients. These results suggest that MCPH in some MCPH patients can arise from the lack of adaptation to centriole defects in NPs and may lead to architectural defects if chromosomally unstable cells are not eliminated during brain development.

Ubiquitin signaling in the control of centriole duplication

The centrosome plays an essential role in maintaining genetic stability, ciliogenesis and cell polarisation. The core of the centrosome is made up of two centrioles that duplicate precisely once during every cell cycle to generate two centrosomes that are required for bipolar spindle assembly and chromosome segregation. Abundance of centriole proteins at optimal levels and their recruitment to the centrosome are tightly regulated in time and space in order to restrict aberrant duplication of centrioles, a phenomenon that is observed in many cancers. Recent advances have conclusively shown that dedicated ubiquitin ligase-dependent protein degradation machineries are involved in governing centriole duplication. These studies revealed intricate mechanistic insights into how the ubiquitin ligases target different centriole proteins. In certain cases, a specific ubiquitin ligase targets a number of substrate proteins that co-regulate centriole assembly, prompting the possibility that substrate-targeting occurs during formation of the sub-centriolar structures. There are also instances where a specific centriole duplication protein is targeted by several ubiquitin ligases at different stages of the cell cycle, suggesting synchronised actions. Recent evidence also indicated a direct association of E3 ubiquitin ligase with the centrioles, supporting the notion that substrate-targeting occurs in the organelle itself. In this review, we highlight these advances by underlining the mechanisms of how different ubiquitin ligase machineries control centriole duplication and discuss our views on their coordination.

CCDC57 Cooperates with Microtubules and Microcephaly Protein CEP63 and Regulates Centriole Duplication and Mitotic Progression

Centrosomes function in key cellular processes ranging from cell division to cellular signaling. Their dysfunction is linked to cancer and developmental disorders. Here, we identify CCDC57 as a pleiotropic regulator of centriole duplication, mitosis, and ciliogenesis. Combining proximity mapping with superresolution imaging, we show that CCDC57 localizes to the proximal end of centrioles and interacts with the microcephaly protein CEP63, centriolar satellite proteins, and microtubules. Loss of CCDC57 causes defects in centriole duplication and results in a failure to localize CEP63 and CEP152 to the centrosome. Additionally, CCDC57 depletion perturbs mitotic progression both in wild-type and centriole-less cells. Importantly, its centrosome-targeting region is required for its interaction with CEP63 and functions during centriole duplication and cilium assembly, whereas the microtubule-targeting region is required for its mitotic functions. Together, our results identify CCDC57 as a critical interface between centrosome and microtubule-mediated cellular processes that are deregulated in microcephaly.

From Proteomic Mapping to Invasion-Metastasis-Cascade Systemic Biomarkering and Targeted Drugging of Mutant BRAF-Dependent Human Cutaneous Melanomagenesis

Simple Summary

Despite the recent advances in human malignancy therapy, metastasis and chemoresistance remain the principal causes of cancer-derived deaths. Given the fatal forms of cutaneous metastatic melanoma, we herein employed primary (WM115) and metastatic (WM266-4) melanoma cells, both obtained from the same patient, to identify novel biomarkers and therapeutic agents. Through state-of-the-art technologies including deep proteome landscaping, immunofluorescence phenotyping, and drug toxicity screening, we were able to describe new molecular programs, oncogenic drivers, and drug regimens, controlling the invasion-metastasis cascade during BRAF^V600D-dependent melanomagenesis. It proved that proteomic navigation could foster the development of systemic biomarkering and targeted drugging for successful treatment of advanced disease.

Abstract

Melanoma is classified among the most notoriously aggressive human cancers. Despite the recent progress, due to its propensity for metastasis and resistance to therapy, novel biomarkers and oncogenic molecular drivers need to be promptly identified for metastatic melanoma. Hence, by employing nano liquid chromatography-tandem mass spectrometry deep proteomics technology, advanced bioinformatics algorithms, immunofluorescence, western blotting, wound healing protocols, molecular modeling programs, and MTT assays, we comparatively examined the respective proteomic contents of WM115 primary (n = 3955 proteins) and WM266-4 metastatic (n = 6681 proteins) melanoma cells. It proved that WM115 and WM266-4 cells have engaged hybrid epithelial-to-mesenchymal transition/mesenchymal-to-epithelial transition states, with TGF-β controlling their motility in vitro. They are characterized by different signatures of SOX-dependent neural crest-like stemness and distinct architectures of the cytoskeleton network. Multiple signaling pathways have already been activated from the primary melanoma stage, whereas HIF1α, the major hypoxia-inducible factor, can be exclusively observed in metastatic melanoma cells. Invasion-metastasis cascade-specific sub-routines of activated Caspase-3-triggered apoptosis and LC3B-II-dependent constitutive autophagy were also unveiled. Importantly, WM115 and WM266-4 cells exhibited diverse drug response profiles, with epirubicin holding considerable promise as a beneficial drug for metastatic melanoma clinical management. It is the proteome navigation that enables systemic biomarkering and targeted drugging to open new therapeutic windows for advanced disease.

Optimization strategies for expression and purification of soluble N-terminal domain of human centriolar protein SAS-6 in Escherichia coli

Spindle assembly abnormal protein 6 (SAS-6), a highly conserved centriolar protein, constitutes the center of the cartwheel assembly that scaffolds centrioles early in their biogenesis. Abnormalities in cartwheel assembly lead to chromosomal dysfunctions. The molecular structure of human SAS-6 (HsSAS-6) and cartwheel hub and how they direct centriole symmetry is unknown. No crystal structure of wildtype HsSAS-6 has been reported to date, since soluble recombinant partial/full-length HsSAS-6 expression and purification posed grand challenges. In the present study we have explored optimization of ten different N terminal SAS-6 fusion proteins expression in a variety of E. coli hosts. During optimization we have included some of the most commonly used purification tags: Histidine tag, maltose-binding protein (MBP), small ubiquitin-related modifier (SUMO) tag and modified MBP tag with surface entropy reduction mutations. We demonstrate several levels of tag assisted solubility and stable expression strategies. We find that the MBP tag accompanied by Surface Entropy Reduction mutations (MBP/SER) in a fixed arm approach rescues the folded SAS-6N protein with significantly improved solubility. This expression of HsSAS-6N in E. coli Rosetta DE3 pLysS expression strain gave rise to high protein expression yielding around 6.0-11.5 mg of soluble protein per liter of growth culture.

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

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

Centrosome organization and functions

The centrosome, discovered near 1875, was named by Boveri when proposing the chromosomal theory of heredity. After a long eclipse, a considerable amount of molecular data has been accumulated on the centrosome and its biogenesis in the last 30 years, summarized regularly in excellent reviews. Major questions are still at stake in 2021 however, as we lack a comprehensive view of the centrosome functions. I will first try to see how progress towards a unified view of the role of centrosomes during evolution is possible, and then review recent data on only some of the many important questions raised by this organelle.

Centrosomes in mitotic spindle assembly and orientation

The centrosome is present in most animal cells and functions as the major microtubule-organizing center to ensure faithful chromosome segregation during cell division. As cells transition from interphase to mitosis, the duplicated centrosomes separate and move to opposite sides of the cell where the spindle assembles. Centrosomes not only nucleate but also organize microtubules of the mitotic spindle. The mitotic spindle is anchored to the cell cortex by the astral microtubules emanating from the centrosomes. Proper orientation of the mitotic spindle is essential for correct cell division. Centrosome-localized polo-like kinase Plk1 has been linked to regulation of proper spindle orientation. A number of proteins including MISP and NuMA have been implicated in the Plk1-mediated spindle orientation pathway.

Extracellular vesicles and post-translational protein deimination signatures in haemolymph of the American lobster (Homarus americanus)

The American lobster (Homarus americanus) is a commercially important crustacean with an unusual long life span up to 100 years and a comparative animal model of longevity. Therefore, research into its immune system and physiology is of considerable importance both for industry and comparative immunology studies. Peptidylarginine deiminases (PADs) are a phylogenetically conserved enzyme family that catalyses post-translational protein deimination via the conversion of arginine to citrulline. This can lead to structural and functional protein changes, sometimes contributing to protein moonlighting, in health and disease. PADs also regulate the cellular release of extracellular vesicles (EVs), which is an important part of cellular communication, both in normal physiology and in immune responses. Hitherto, studies on EVs in Crustacea are limited and neither PADs nor associated protein deimination have been studied in a Crustacean species. The current study assessed EV and deimination signatures in haemolymph of the American lobster. Lobster EVs were found to be a poly-dispersed population in the 10-500 nm size range, with the majority of smaller EVs, which fell within 22-115 nm. In lobster haemolymph, 9 key immune and metabolic proteins were identified to be post-translationally deiminated, while further 41 deiminated protein hits were identified when searching against a Crustacean database. KEGG (Kyoto encyclopedia of genes and genomes) and GO (gene ontology) enrichment analysis of these deiminated proteins revealed KEGG and GO pathways relating to a number of immune, including anti-pathogenic (viral, bacterial, fungal) and host-pathogen interactions, as well as metabolic pathways, regulation of vesicle and exosome release, mitochondrial function, ATP generation, gene regulation, telomerase homeostasis and developmental processes. The characterisation of EVs, and post-translational deimination signatures, reported in lobster in the current study, and the first time in Crustacea, provides insights into protein moonlighting functions of both species-specific and phylogenetically conserved proteins and EV-mediated communication in this long-lived crustacean. The current study furthermore lays foundation for novel biomarker discovery for lobster aquaculture.

Structural bioinformatics predicts that the Retinitis Pigmentosa-28 protein of unknown function FAM161A is a homologue of the microtubule nucleation factor Tpx2

Background: FAM161A is a microtubule-associated protein conserved widely across eukaryotes, which is mutated in the inherited blinding disease Retinitis Pigmentosa-28. FAM161A is also a centrosomal protein, being a core component of a complex that forms an internal skeleton of centrioles. Despite these observations about the importance of FAM161A, current techniques used to examine its sequence reveal no homologies to other proteins.

Methods: Sequence profiles derived from multiple sequence alignments of FAM161A homologues were constructed by PSI-BLAST and HHblits, and then used by the profile-profile search tool HHsearch, implemented online as HHpred, to identify homologues. These in turn were used to create profiles for reverse searches and pair-wise searches. Multiple sequence alignments were also used to identify amino acid usage in functional elements.

Results: FAM161A has a single homologue: the targeting protein for Xenopus kinesin-like protein-2 (Tpx2), which is a strong hit across more than 200 residues. Tpx2 is also a microtubule-associated protein, and it has been shown previously by a cryo-EM molecular structure to nucleate microtubules through two small elements: an extended loop and a short helix. The homology between FAM161A and Tpx2 includes these elements, as FAM161A has three copies of the loop, and one helix that has many, but not all, properties of the one in Tpx2.

Conclusions: FAM161A and ­its homologues are predicted to be a previously unknown variant of Tpx2, and hence bind microtubules in the same way. This prediction allows precise, testable molecular models to be made of FAM161A-microtubule complexes.

Non-Canonical Functions of the Gamma-Tubulin Meshwork in the Regulation of the Nuclear Architecture

Simple Summary

The appearance of a cell is connected to its function. For example, the fusiform of smooth muscle cells is adapted to facilitate muscle contraction, the lobed nucleus in white blood cells assists with the migratory behavior of these immune cells, and the condensed nucleus in sperm aids in their swimming efficiency. Thus, changes in appearance have been used for decades by doctors as a diagnostic method for human cancers. Here, we summarize our knowledge of how a cell maintains the shape of the nuclear compartment. Specifically, we discuss the role of a novel protein meshwork, the gamma-tubulin meshwork, in the regulation of nuclear morphology and as a therapeutic target against cancer.

Abstract

The nuclear architecture describes the organization of the various compartments in the nucleus of eukaryotic cells, where a plethora of processes such as nucleocytoplasmic transport, gene expression, and assembly of ribosomal subunits occur in a dynamic manner. During the different phases of the cell cycle, in post-mitotic cells and after oncogenic transformation, rearrangements of the nuclear architecture take place, and, among other things, these alterations result in reorganization of the chromatin and changes in gene expression. A member of the tubulin family, γtubulin, was first identified as part of a multiprotein complex that allows nucleation of microtubules. However, more than a decade ago, γtubulin was also characterized as a nuclear protein that modulates several crucial processes that affect the architecture of the nucleus. This review presents the latest knowledge regarding changes that arise in the nuclear architecture of healthy cells and under pathological conditions and, more specifically, considers the particular involvement of γtubulin in the modulation of the biology of the nuclear compartment.

Differential turnover of Nup188 controls its levels at centrosomes and role in centriole duplication

NUP188 encodes a scaffold component of the nuclear pore complex (NPC) and has been implicated as a congenital heart disease gene through an ill-defined function at centrioles. Here, we explore the mechanisms that physically and functionally segregate Nup188 between the pericentriolar material (PCM) and NPCs. Pulse-chase fluorescent labeling indicates that Nup188 populates centrosomes with newly synthesized protein that does not exchange with NPCs even after mitotic NPC breakdown. In addition, the steady-state levels of Nup188 are controlled by the sensitivity of the PCM pool, but not the NPC pool, to proteasomal degradation. Proximity-labeling and super-resolution microscopy show that Nup188 is vicinal to the inner core of the interphase centrosome. Consistent with this, we demonstrate direct binding between Nup188 and Cep152. We further show that Nup188 functions in centriole duplication at or upstream of Sas6 loading. Together, our data establish Nup188 as a component of PCM needed to duplicate the centriole with implications for congenital heart disease mechanisms.

Architecture of the centriole cartwheel-containing region revealed by cryo-electron tomography

Centrioles are evolutionarily conserved barrels of microtubule triplets that form the core of the centrosome and the base of the cilium. While the crucial role of the proximal region in centriole biogenesis has been well documented, its native architecture and evolutionary conservation remain relatively unexplored. Here, using cryo-electron tomography of centrioles from four evolutionarily distant species, we report on the architectural diversity of the centriole's proximal cartwheel-bearing region. Our work reveals that the cartwheel central hub is constructed from a stack of paired rings with cartwheel inner densities inside. In both Paramecium and Chlamydomonas, the repeating structural unit of the cartwheel has a periodicity of 25 nm and consists of three ring pairs, with 6 radial spokes emanating and merging into a single bundle that connects to the microtubule triplet via the D2-rod and the pinhead. Finally, we identified that the cartwheel is indirectly connected to the A-C linker through the triplet base structure extending from the pinhead. Together, our work provides unprecedented evolutionary insights into the architecture of the centriole proximal region, which underlies centriole biogenesis.

WBP11 is required for splicing the TUBGCP6 pre-mRNA to promote centriole duplication

Centriole duplication occurs once in each cell cycle to maintain centrosome number. A previous genome-wide screen revealed that depletion of 14 RNA splicing factors leads to a specific defect in centriole duplication, but the cause of this deficit remains unknown. Here, we identified an additional pre-mRNA splicing factor, WBP11, as a novel protein required for centriole duplication. Loss of WBP11 results in the retention of ∼200 introns, including multiple introns in TUBGCP6, a central component of the γ-TuRC. WBP11 depletion causes centriole duplication defects, in part by causing a rapid decline in the level of TUBGCP6. Several additional splicing factors that are required for centriole duplication interact with WBP11 and are required for TUBGCP6 expression. These findings provide insight into how the loss of a subset of splicing factors leads to a failure of centriole duplication. This may have clinical implications because mutations in some spliceosome proteins cause microcephaly and/or growth retardation, phenotypes that are strongly linked to centriole defects.

Tetrahymena Poc5 is a transient basal body component that is important for basal body maturation

Basal bodies (BBs) are microtubule-based organelles that act as a template for and stabilize cilia at the cell surface. Centrins ubiquitously associate with BBs and function in BB assembly, maturation and stability. Human POC5 (hPOC5) is a highly conserved centrin-binding protein that binds centrins through Sfi1p-like repeats and is required for building full-length, mature centrioles. Here, we use the BB-rich cytoskeleton of Tetrahymena thermophila to characterize Poc5 BB functions. Tetrahymena Poc5 (TtPoc5) uniquely incorporates into assembling BBs and is then removed from mature BBs prior to ciliogenesis. Complete genomic knockout of TtPOC5 leads to a significantly increased production of BBs, yet a markedly reduced ciliary density, both of which are rescued by reintroduction of TtPoc5. A second Tetrahymena POC5-like gene, SFR1, is similarly implicated in modulating BB production. When TtPOC5 and SFR1 are co-deleted, cell viability is compromised and BB overproduction is exacerbated. Overproduced BBs display defective transition zone formation and a diminished capacity for ciliogenesis. This study uncovers a requirement for Poc5 in building mature BBs, providing a possible functional link between hPOC5 mutations and impaired cilia.This article has an associated First Person interview with the first author of the paper.

FAM110B Inhibits Non-Small Cell Lung Cancer Cell Proliferation and Invasion Through Inactivating Wnt/β-Catenin Signaling

PURPOSE

FAM110B is a member of the FAM110 family (family with sequence similarity 110), which is a component of the centrosome associated proteins. Previous studies have shown that FAM110B may be involved in the process of cell cycle and may play a role in carcinogenesis and tumor progression. Using an online database, we found that FAM110B may predict favorable prognosis in non-small cell lung cancer (NSCLC). Therefore, the role of FAM110B playing in NSCLC needs to be further investigated.

PATIENTS AND METHODS

Online databases and immunohistochemistry were used to predict the expression and prognostic value of FAM110B in NSCLC samples. Immunofluorescence staining was used to investigate the subcellular distribution of FAM110B. Western blot, MTT, colony formation, and matrigel invasion assay were used to detect the expression and the effect of FAM110B on mediating proliferation and invasion in NSCLC cell lines.

RESULTS

In this study, immunohistochemistry results showed that FAM110B expression significantly correlated with early TNM staging (P=0.020) and negative regional lymph node metastasis (P=0.006). Kaplan-Meier survival analysis found that the median survival time of patients with positive FAM110B expression (56.181±2.348 months) was significantly longer than those with negative FAM110B expression (47.701±2.997 months, P=0.024). Moreover, overexpression of FAM110B inhibited the proliferation and invasion of A549, H1299, and LK2 cell lines. Conversely, FAM110B RNAi exerted opposite effects in the above cell lines. Furthermore, FAM110B overexpression downregulated the active β-catenin, phosphorylation of GSK-3β (p-GSK-3β), cyclin B1, cyclin D1, MMP2, and MMP7, and upregulated the phosphorylation of β-catenin (p-β-catenin) in A549 and H1299 cells. Besides, the FAM110B-induced depressions of p-GSK-3β and active β-catenin were reversed after being treated with Wnt/β-catenin inhibitor, XAV-939.

CONCLUSION

In summary, our results demonstrated that the overexpression of FAM110B restricts the proliferation and invasion of NSCLC cells by inhibiting Wnt/β-catenin signaling. Our study reveals the antitumor function of FAM110B in NSCLC and indicates that FAM110B is a potential therapeutic target.

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.

The Emerging Role of ncRNAs and RNA-Binding Proteins in Mitotic Apparatus Formation

Mounting experimental evidence shows that non-coding RNAs (ncRNAs) serve a wide variety of biological functions. Recent studies suggest that a part of ncRNAs are critically important for supporting the structure of subcellular architectures. Here, we summarize the current literature demonstrating the role of ncRNAs and RNA-binding proteins in regulating the assembly of mitotic apparatus, especially focusing on centrosomes, kinetochores, and mitotic spindles.

Centrioles and Ciliary Structures during Male Gametogenesis in Hexapoda: Discovery of New Models

Centrioles are-widely conserved barrel-shaped organelles present in most organisms. They are indirectly involved in the organization of the cytoplasmic microtubules both in interphase and during the cell division by recruiting the molecules needed for microtubule nucleation. Moreover, the centrioles are required to assemble cilia and flagella by the direct elongation of their microtubule wall. Due to the importance of the cytoplasmic microtubules in several aspects of the cell life, any defect in centriole structure can lead to cell abnormalities that in humans may result in significant diseases. Many aspects of the centriole dynamics and function have been clarified in the last years, but little attention has been paid to the exceptions in centriole structure that occasionally appeared within the animal kingdom. Here, we focused our attention on non-canonical aspects of centriole architecture within the Hexapoda. The Hexapoda is one of the major animal groups and represents a good laboratory in which to examine the evolution and the organization of the centrioles. Although these findings represent obvious exceptions to the established rules of centriole organization, they may contribute to advance our understanding of the formation and the function of these organelles.

Targeting the centrosome and polo-like kinase 4 in osteosarcoma

It has been historically uncertain if extra centrosomes are a cause or consequence of tumorigenesis. Experiments have recently established that overexpression of polo-like kinase 4 (PLK4) promotes centrosome amplification with consequential promotion of cellular aneuploidy. Furthermore, centrosome amplification drives spontaneous tumorigenesis in mice. Tissues lacking normal functional p53 tolerate extra centrosomes, whereas p53 proficient tissues initiate proliferative arrest in this circumstance. Extra centrosomes trigger activation of the multi-protein PIDDosome complex, with Caspase-2 effecting cleavage of the p53-negative regulator mouse double minute 2, consequent stabilization of p53 and p21-dependent arrest of the cell cycle. The co-occurrence of cellular aneuploidy, complex chromosomal rearrangements and p53 dysfunction is a striking feature of some osteosarcomas. It is postulated that small-molecule PLK4 inhibitors such as CFI-400945, which are in development, may have utility in osteosarcoma given these findings.

The ubiquitin ligase FBXW7 targets the centriolar assembly protein HsSAS-6 for degradation and thereby regulates centriole duplication

Formation of a single new centriole from a pre-existing centriole is strictly controlled to maintain correct centrosome number and spindle polarity in cells. However, the mechanisms that govern this process are incompletely understood. Here, using several human cell lines, immunofluorescence and structured illumination microscopy methods, and ubiquitination assays, we show that the E3 ubiquitin ligase F-box and WD repeat domain-containing 7 (FBXW7), a subunit of the SCF ubiquitin ligase, down-regulates spindle assembly 6 homolog (HsSAS-6), a key protein required for procentriole cartwheel assembly, and thereby regulates centriole duplication. We found that FBXW7 abrogation stabilizes HsSAS-6 and increases its recruitment to the mother centriole at multiple sites, leading to supernumerary centrioles. Ultrastructural analyses revealed that FBXW7 is broadly localized on the mother centriole and that its presence is reduced at the site where the HsSAS-6-containing procentriole is formed. This observation suggested that FBXW7 restricts procentriole assembly to a specific site to generate a single new centriole. In contrast, during HsSAS-6 overexpression, FBXW7 strongly associated with HsSAS-6 at the centriole. We also found that SCF^FBXW7 interacts with HsSAS-6 and targets it for ubiquitin-mediated degradation. Further, we identified putative phosphodegron sites in HsSAS-6, whose substitutions rendered it insensitive to FBXW7-mediated degradation and control of centriole number. In summary, SCF^FBXW7 targets HsSAS-6 for degradation and thereby controls centriole biogenesis by restraining HsSAS-6 recruitment to the mother centriole, a molecular mechanism that controls supernumerary centrioles/centrosomes and the maintenance of bipolar spindles.

CEP44 ensures the formation of bona fide centriole wall, a requirement for the centriole-to-centrosome conversion

Centrosomes are essential organelles with functions in microtubule organization that duplicate once per cell cycle. The first step of centrosome duplication is the daughter centriole formation followed by the pericentriolar material recruitment to this centriole. This maturation step was termed centriole-to-centrosome conversion. It was proposed that CEP295-dependent recruitment of pericentriolar proteins drives centriole conversion. Here we show, based on the analysis of proteins that promote centriole biogenesis, that the developing centriole structure helps drive centriole conversion. Depletion of the luminal centriole protein CEP44 that binds to the A-microtubules and interacts with POC1B affecting centriole structure and centriole conversion, despite CEP295 binding to centrioles. Impairment of POC1B, TUBE1 or TUBD1, which disturbs integrity of centriole microtubules, also prevents centriole-to-centrosome conversion. We propose that the CEP295, CEP44, POC1B, TUBE1 and TUBD1 centriole biogenesis pathway that functions in the centriole lumen and on the cytoplasmic side is essential for the centriole-to-centrosome conversion.

During cell division, centrosomes duplicate and newly formed centrioles must undergo centriole-to-centrosome conversion, but the molecular details are unclear. Here, the authors report that the centriole microtubule-triplet 9-fold structure scaffolds pericentriolar proteins and permits the conversion of centrioles to fully functional centrosomes.

Choreography of the centrosome

More than a century ago, the centrosome was discovered and described as “the true division organ of the cell”. Electron microscopy revealed that a centrosome is an amorphous structure or pericentriolar protein matrix that surrounds a pair of well-organized centrioles. Today, the importance of the centrosome as a microtubule-organizing center and coordinator of the mitotic spindle is questioned, because centrioles are absent in up to half of all known eukaryotic species, and various mechanisms for acentrosomal microtubule nucleation have been described. This review recapitulates the known functions of centrosome movements in cellular homeostasis and discusses knowledge gaps in this field.

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.