Neurl4, a novel daughter centriole protein, prevents formation of ectopic microtubule organizing centres

Mesoscale regulation of microtubule-organizing centers by the E3 ligase TRIM37

Centrosomes ensure accurate chromosome segregation during cell division. Although the regulation of centrosome number is well established, less is known about the suppression of noncentrosomal microtubule-organizing centers (ncMTOCs). The E3 ligase TRIM37, implicated in Mulibrey nanism and 17q23-amplified cancers, has emerged as a key regulator of both centrosomes and ncMTOCs. Yet, the mechanism by which TRIM37 achieves enzymatic activation to target these mesoscale structures had thus far remained unknown. Here we elucidate the activation process of TRIM37, unveiling a process that initiates with TRAF domain-directed substrate recognition followed by B-box domain-mediated oligomerization and culminates in RING domain dimerization. Using optogenetics, we demonstrate that the E3 activity of TRIM37 is directly coupled to the assembly state of its substrates, being activated only when centrosomal proteins cluster into higher-order assemblies resembling MTOCs. This regulatory framework provides a mechanistic basis for understanding TRIM37-driven pathologies and echoes the restriction of the human immunodeficiency virus capsid by TRIM5, thus unveiling a conserved activation blueprint among TRIM proteins to control turnover of complexes assembled at the mesoscale level.

Mesoscale regulation of MTOCs by the E3 ligase TRIM37

Centrosomes ensure accurate chromosome segregation during cell division. Although the regulation of centrosome number is well-established, less is known about the suppression of non-centrosomal MTOCs (ncMTOCs). The E3 ligase TRIM37, implicated in Mulibrey nanism and 17q23-amplified cancers, has emerged as a key regulator of both centrosomes and ncMTOCs. Yet, the mechanism by which TRIM37 achieves enzymatic activation to target these mesoscale structures had remained unknown. Here, we elucidate TRIM37's activation process, beginning with TRAF domain-directed substrate recognition, progressing through B-box domain-mediated oligomerization, and culminating in RING domain dimerization. Using optogenetics, we demonstrate that TRIM37's E3 activity is directly coupled to the assembly state of its substrates, activating only when centrosomal proteins cluster into higher-order assemblies resembling MTOCs. This regulatory framework provides a mechanistic basis for understanding TRIM37-driven pathologies and, by echoing TRIM5's restriction of the HIV capsid, unveils a conserved activation blueprint among TRIM proteins for controlling mesoscale assembly turnover.

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.

Centriole elimination during Caenorhabditis elegans oogenesis initiates with loss of the central tube protein SAS-1

In most metazoans, centrioles are lost during oogenesis, ensuring that the zygote is endowed with the correct number of two centrioles, which are paternally contributed. How centriole architecture is dismantled during oogenesis is not understood. Here, we analyze with unprecedent detail the ultrastructural and molecular changes during oogenesis centriole elimination in Caenorhabditis elegans. Centriole elimination begins with loss of the so-called central tube and organelle widening, followed by microtubule disassembly. The resulting cluster of centriolar proteins then disappears gradually, usually moving in a microtubule- and dynein-dependent manner to the plasma membrane. Our analysis indicates that neither Polo-like kinases nor the PCM, which modulate oogenesis centriole elimination in Drosophila, do so in C. elegans. Furthermore, we demonstrate that the central tube protein SAS-1 normally departs initially from the organelle, which loses integrity earlier in sas-1 mutants. Overall, our work provides novel mechanistic insights regarding the fundamental process of oogenesis centriole elimination.

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.

ODF2 Negatively Regulates CP110 Levels at the Centrioles/Basal Bodies to Control the Biogenesis of Primary Cilia

Primary cilia are essential sensory organelles that develop when an inhibitory cap consisting of CP110 and other proteins is eliminated. The degradation of CP110 by the ubiquitin-dependent proteasome pathway mediated by NEURL4 and HYLS1 removes the inhibitory cap. Here, we investigated the suitability of rapamycin-mediated dimerization for centriolar recruitment and asked whether the induced recruitment of NEURL4 or HYLS1 to the centriole promotes primary cilia development and CP110 degradation. We used rapamycin-mediated dimerization with ODF2 to induce their targeted recruitment to the centriole. We found decreased CP110 levels in the transfected cells, but independent of rapamycin-mediated dimerization. By knocking down ODF2, we showed that ODF2 controls CP110 levels. The overexpression of ODF2 is not sufficient to promote the formation of primary cilia, but the overexpression of NEURL4 or HYLS1 is. The co-expression of ODF2 and HYLS1 resulted in the formation of tube-like structures, indicating an interaction. Thus, ODF2 controls primary cilia formation by negatively regulating the concentration of CP110 levels. Our data suggest that ODF2 most likely acts as a scaffold for the binding of proteins such as NEURL4 or HYLS1 to mediate CP110 degradation.

LiveG4ID-Seq for Profiling the Dynamic Landscape of Chromatin G-Quadruplexes During Cell Cycle in Living Cells

G-quadruplex (G4) structures exist in the single-stranded DNA of chromatin and regulate genome function. However, the native chromatin G4 landscape in living cells has yet to be fully characterized. Herein, a genetic-encoded live-cell G4 identifier probe (LiveG4ID) is constructed and its cellular localization, biocompatibility, and G4-binding specificity is evaluated. By coupling LiveG4ID with cleavage under targets and tagmentation (CUT&Tag), LiveG4ID-seq, a method for mapping native chromatin G4 landscape in living cells with high accuracy is established. Compared to the conventional G4 CUT&Tag method, LiveG4ID-seq can identify more chromatin G4 signals and have a higher ratio of true positive signals. Using LiveG4ID-seq, the dynamic landscape of chromatin G4 structures during the cell cycle is profiled. It is discovered that chromatin G4 structures are prevalent in the promoter regions of cell cycle-specific genes, even in the early M phase when the chromatin is condensed. These data demonstrate the capacity of LiveG4ID-seq to profile a more accurate G4 landscape in living cells and promote future studies on chromatin G4 structures.

Common genetic risk factors in ASD and ADHD co-occurring families

Autism spectrum disorder (ASD) and attention-deficit/hyperactivity disorder (ADHD) are two major neurodevelopmental disorders that frequently co-occur. However, the genetic mechanism of the co-occurrence remains unclear. The New Jersey Language and Autism Genetics Study (NJLAGS) collected more than 100 families with at least one member affected by ASD. NJLAGS families show a high prevalence of ADHD and provide a good opportunity to study shared genetic risk factors for ASD and ADHD. The linkage study of the NJLAGS families revealed regions on chromosomes 12 and 17 that are significantly associated with ADHD. Using whole-genome sequencing data on 272 samples from 73 NJLAGS families, we identified potential risk genes for ASD and ADHD. Within the linkage regions, we identified 36 genes that are associated with ADHD using a pedigree-based gene prioritization approach. KDM6B (Lysine Demethylase 6B) is the highest-ranking gene, which is a known risk gene for neurodevelopmental disorders, including ASD and ADHD. At the whole-genome level, we identified 207 candidate genes from the analysis of both small variants and structure variants, including both known and novel genes. Using enrichment and protein-protein interaction network analyses, we identified gene ontology terms and pathways enriched for ASD and ADHD candidate genes, such as cilia function and cation channel activity. Candidate genes and pathways identified in our study improve the understanding of the genetic etiology of ASD and ADHD and will lead to new diagnostic or therapeutic interventions for ASD and ADHD in the future.

Appearing and disappearing acts of cilia

The past few decades have seen a rise in research on vertebrate cilia and ciliopathy, with interesting collaborations between basic and clinical scientists. This work includes studies on ciliary architecture, composition, evolution, and organelle generation and its biological role. The human body has cells that harbour any of the following four types of cilia: 9+0 motile, 9+0 immotile, 9+2 motile, and 9+2 immotile. Depending on the type, cilia play an important role in cell/fluid movement, mating, sensory perception, and development. Defects in cilia are associated with a wide range of human diseases afflicting the brain, heart, kidneys, respiratory tract, and reproductive system. These are commonly known as ciliopathies and affect millions of people worldwide. Due to their complex genetic etiology, diagnosis and therapy have remained elusive. Although model organisms like Chlamydomonas reinhardtii have been a useful source for ciliary research, reports of a fascinating and rewarding translation of this research into mammalian systems, especially humans, are seen. The current review peeks into one of the complex features of this organelle, namely its birth, the common denominators across the formation of both 9+0 and 9+2 ciliary types, the molecules involved in ciliogenesis, and the steps that go towards regulating their assembly and disassembly.

Proteolytic control in ciliogenesis: Temporal restriction or early initiation?

Cellular processes are highly dependent on a dynamic proteome that undergoes structural and functional rearrangements to allow swift conversion between different cellular states. By inducing proteasomal degradation of inhibitory or stimulating factors, ubiquitylation is particularly well suited to trigger such transitions. One prominent example is the remodelling of the centrosome upon cell cycle exit, which is required for the formation of primary cilia - antenna-like structures on the surface of most cells that act as integrative hubs for various extracellular signals. Over the last decade, many reports on ubiquitin-related events involved in the regulation of ciliogenesis have emerged. Very often, these processes are considered to be initiated ad hoc, that is, directly before its effect on cilia biogenesis becomes evident. While such a temporal restriction may hold true for the majority of events, there is evidence that some of them are initiated earlier during the cell cycle. Here, we provide an overview of ubiquitin-dependent processes in ciliogenesis and discuss available data that indicate such an early onset of proteolytic regulation within preceding cell cycle stages.

Experimental and Natural Induction of de novo Centriole Formation

In cycling cells, new centrioles are assembled in the vicinity of pre-existing centrioles. Although this canonical centriole duplication is a tightly regulated process in animal cells, centrioles can also form in the absence of pre-existing centrioles; this process is termed de novo centriole formation. De novo centriole formation is triggered by the removal of all pre-existing centrioles in the cell in various manners. Moreover, overexpression of polo-like kinase 4 (Plk4), a master regulatory kinase for centriole biogenesis, can induce de novo centriole formation in some cell types. Under these conditions, structurally and functionally normal centrioles can be formed de novo. While de novo centriole formation is normally suppressed in cells with intact centrioles, depletion of certain suppressor proteins leads to the ectopic formation of centriole-related protein aggregates in the cytoplasm. It has been shown that de novo centriole formation also occurs naturally in some species. For instance, during the multiciliogenesis of vertebrate epithelial cells, massive de novo centriole amplification occurs to form numerous motile cilia. In this review, we summarize the previous findings on de novo centriole formation, particularly under experimental conditions, and discuss its regulatory mechanisms.

Neuralized-like protein 4 (NEURL4) mediates ADP-ribosylation of mitochondrial proteins

ADP-ribosylation is a reversible post-translational modification where an ADP-ribose moiety is covalently attached to target proteins by ADP-ribosyltransferases (ARTs). Although best known for its nuclear roles, ADP-ribosylation is increasingly recognized as a key regulatory strategy across cellular compartments. ADP-ribosylation of mitochondrial proteins has been widely reported, but the exact nature of mitochondrial ART enzymes is debated. We have identified neuralized-like protein 4 (NEURL4) as a mitochondrial ART enzyme and show that most ART activity associated with mitochondria is lost in the absence of NEURL4. The NEURL4-dependent ADP-ribosylome in mitochondrial extracts from HeLa cells includes numerous mitochondrial proteins previously shown to be ADP-ribosylated. In particular, we show that NEURL4 is required for the regulation of mtDNA integrity via poly-ADP-ribosylation of mtLIG3, the rate-limiting enzyme for base excision repair (BER). Collectively, our studies reveal that NEURL4 acts as the main mitochondrial ART enzyme under physiological conditions and provide novel insights in the regulation of mitochondria homeostasis through ADP-ribosylation.

Centrosome maturation - in tune with the cell cycle

Centrosomes are the main microtubule-organizing centres, playing essential roles in the organization of the cytoskeleton during interphase, and in the mitotic spindle, which controls chromosome segregation, during cell division. Centrosomes also act as the basal body of cilia, regulating cilium length and affecting extracellular signal reception as well as the integration of intracellular signalling pathways. Centrosomes are self-replicative and duplicate once every cell cycle to generate two centrosomes. The core support structure of the centrosome consists of two molecularly distinct centrioles. The mother (mature) centriole exhibits accessory appendages and is surrounded by both pericentriolar material and centriolar satellites, structures that the daughter (immature) centriole lacks. In this Review, we discuss what is currently known about centrosome duplication, its dialogue with the cell cycle and the sequential acquisition of specific components during centriole maturation. We also describe our current understanding of the mature centriolar structures that are required to build a cilium. Altogether, the built-in centrosome asymmetries that stem from the two centrosomes inheriting molecularly different centrioles sets the foundation for cell division being an intrinsically asymmetric process.

LUBAC regulates ciliogenesis by promoting CP110 removal from the mother centriole

Primary cilia transduce diverse signals in embryonic development and adult tissues. Defective ciliogenesis results in a series of human disorders collectively known as ciliopathies. The CP110–CEP97 complex removal from the mother centriole is an early critical step for ciliogenesis, but the underlying mechanism for this step remains largely obscure. Here, we reveal that the linear ubiquitin chain assembly complex (LUBAC) plays an essential role in ciliogenesis by targeting the CP110–CEP97 complex. LUBAC specifically generates linear ubiquitin chains on CP110, which is required for CP110 removal from the mother centriole in ciliogenesis. We further identify that a pre-mRNA splicing factor, PRPF8, at the distal end of the mother centriole acts as the receptor of the linear ubiquitin chains to facilitate CP110 removal at the initial stage of ciliogenesis. Thus, our study reveals a direct mechanism of regulating CP110 removal in ciliogenesis and implicates the E3 ligase LUBAC as a potential therapy target of cilia-associated diseases, including ciliopathies and cancers.

TRIM37 prevents formation of centriolar protein assemblies by regulating Centrobin

TRIM37 is an E3 ubiquitin ligase mutated in Mulibrey nanism, a disease with impaired organ growth and increased tumor formation. TRIM37 depletion from tissue culture cells results in supernumerary foci bearing the centriolar protein Centrin. Here, we characterize these centriolar protein assemblies (Cenpas) to uncover the mechanism of action of TRIM37. We find that an atypical de novo assembly pathway can generate Cenpas that act as microtubule-organizing centers (MTOCs), including in Mulibrey patient cells. Correlative light electron microscopy reveals that Cenpas are centriole-related or electron-dense structures with stripes. TRIM37 regulates the stability and solubility of Centrobin, which accumulates in elongated entities resembling the striped electron dense structures upon TRIM37 depletion. Furthermore, Cenpas formation upon TRIM37 depletion requires PLK4, as well as two parallel pathways relying respectively on Centrobin and PLK1. Overall, our work uncovers how TRIM37 prevents Cenpas formation, which would otherwise threaten genome integrity.

Dynamic centriolar localization of Polo and Centrobin in early mitosis primes centrosome asymmetry

Centrosomes, the main microtubule organizing centers (MTOCs) of metazoan cells, contain an older "mother" and a younger "daughter" centriole. Stem cells either inherit the mother or daughter-centriole-containing centrosome, providing a possible mechanism for biased delivery of cell fate determinants. However, the mechanisms regulating centrosome asymmetry and biased centrosome segregation are unclear. Using 3D-structured illumination microscopy (3D-SIM) and live-cell imaging, we show in fly neural stem cells (neuroblasts) that the mitotic kinase Polo and its centriolar protein substrate Centrobin (Cnb) accumulate on the daughter centriole during mitosis, thereby generating molecularly distinct mother and daughter centrioles before interphase. Cnb's asymmetric localization, potentially involving a direct relocalization mechanism, is regulated by Polo-mediated phosphorylation, whereas Polo's daughter centriole enrichment requires both Wdr62 and Cnb. Based on optogenetic protein mislocalization experiments, we propose that the establishment of centriole asymmetry in mitosis primes biased interphase MTOC activity, necessary for correct spindle orientation.

The Singularity of the Drosophila Male Germ Cell Centriole: The Asymmetric Distribution of Sas4 and Sas6

Drosophila spermatocytes have giant centrioles that display unique properties. Both the parent centrioles maintain a distinct cartwheel and nucleate a cilium-like region that persists during the meiotic divisions and organizes a structured sperm axoneme. Moreover, the parent centrioles are morphologically undistinguishable, unlike vertebrate cells in which mother and daughter centrioles have distinct structural features. However, our immunofluorescence analysis of the parent centrioles in mature primary spermatocytes revealed an asymmetric accumulation of the typical Sas4 and Sas6 proteins. Notably, the fluorescence intensity of Sas4 and Sas6 at the daughter centrioles is greater than the intensity found at the mother ones. In contrast, the centrioles of wing imaginal disc cells display an opposite condition in which the loading of Sas4 and Sas6 at the mother centrioles is greater. These data underlie a subtle asymmetry among the parent centrioles and point to a cell type diversity of the localization of the Sas4 and Sas6 proteins.

SFI1 promotes centriole duplication by recruiting USP9X to stabilize the microcephaly protein STIL

In mammals, centrioles participate in brain development, and human mutations affecting centriole duplication cause microcephaly. Here, we identify a role for the mammalian homologue of yeast SFI1, involved in the duplication of the yeast spindle pole body, as a critical regulator of centriole duplication in mammalian cells. Mammalian SFI1 interacts with USP9X, a deubiquitylase associated with human syndromic mental retardation. SFI1 localizes USP9X to the centrosome during S phase to deubiquitylate STIL, a critical regulator of centriole duplication. USP9X-mediated deubiquitylation protects STIL from degradation. Consistent with a role for USP9X in stabilizing STIL, cells from patients with USP9X loss-of-function mutations have reduced STIL levels. Together, these results demonstrate that SFI1 is a centrosomal protein that localizes USP9X to the centrosome to stabilize STIL and promote centriole duplication. We propose that the USP9X protection of STIL to facilitate centriole duplication underlies roles of both proteins in human neurodevelopment.

Primary Cilia as Signaling Hubs in Health and Disease

Primary cilia detect extracellular cues and transduce these signals into cells to regulate proliferation, migration, and differentiation. Here, the function of primary cilia as signaling hubs of growth factors and morphogens is in focus. First, the molecular mechanisms regulating the assembly and disassembly of primary cilia are described. Then, the role of primary cilia in mediating growth factor and morphogen signaling to maintain human health and the potential mechanisms by which defects in these pathways contribute to human diseases, such as ciliopathy, obesity, and cancer are described. Furthermore, a novel signaling pathway by which certain growth factors stimulate cell proliferation through suppression of ciliogenesis is also described, suggesting novel therapeutic targets in cancer.

Coordination of Embryogenesis by the Centrosome in Drosophila melanogaster

The first 3 h of Drosophila melanogaster embryo development are exemplified by rapid nuclear divisions within a large syncytium, transforming the zygote to the cellular blastoderm after 13 successive cleavage divisions. As the syncytial embryo develops, it relies on centrosomes and cytoskeletal dynamics to transport nuclei, maintain uniform nuclear distribution throughout cleavage cycles, ensure generation of germ cells, and coordinate cellularization. For the sake of this review, we classify six early embryo stages that rely on processes coordinated by the centrosome and its regulation of the cytoskeleton. The first stage features migration of one of the female pronuclei toward the male pronucleus following maturation of the first embryonic centrosomes. Two subsequent stages distribute the nuclei first axially and then radially in the embryo. The remaining three stages involve centrosome-actin dynamics that control cortical plasma membrane morphogenesis. In this review, we highlight the dynamics of the centrosome and its role in controlling the six stages that culminate in the cellularization of the blastoderm embryo.

Multi-omic tumor data reveal diversity of molecular mechanisms that correlate with survival

Outcomes for cancer patients vary greatly even within the same tumor type, and characterization of molecular subtypes of cancer holds important promise for improving prognosis and personalized treatment. This promise has motivated recent efforts to produce large amounts of multidimensional genomic (multi-omic) data, but current algorithms still face challenges in the integrated analysis of such data. Here we present Cancer Integration via Multikernel Learning (CIMLR), a new cancer subtyping method that integrates multi-omic data to reveal molecular subtypes of cancer. We apply CIMLR to multi-omic data from 36 cancer types and show significant improvements in both computational efficiency and ability to extract biologically meaningful cancer subtypes. The discovered subtypes exhibit significant differences in patient survival for 27 of 36 cancer types. Our analysis reveals integrated patterns of gene expression, methylation, point mutations, and copy number changes in multiple cancers and highlights patterns specifically associated with poor patient outcomes.

A distal centriolar protein network controls organelle maturation and asymmetry

A long-standing mystery in the centrosome field pertains to the origin of asymmetry within the organelle. The removal of daughter centriole-specific/enriched proteins (DCPs) and acquisition of distal appendages on the future mother centriole are two important steps in the generation of asymmetry. We find that DCPs are recruited sequentially, and their removal is abolished in cells lacking Talpid3 or C2CD3. We show that removal of certain DCPs constitutes another level of control for distal appendage (DA) assembly. Remarkably, we also find that Talpid3 forms a distal centriolar multi-functional hub that coordinates the removal of specific DCPs, DA assembly, and recruitment of ciliary vesicles through distinct regions mutated in ciliopathies. Finally, we show that Talpid3, C2CD3, and OFD1 differentially regulate the assembly of sub-distal appendages, the CEP350/FOP/CEP19 module, centriolar satellites, and actin networks. Our work extends the spatial and functional understanding of proteins that control organelle maturation and asymmetry, ciliogenesis, and human disease.

The regulation of cilium assembly and disassembly in development and disease

The primary cilium is an antenna-like organelle assembled on most types of quiescent and differentiated mammalian cells. This immotile structure is essential for interpreting extracellular signals that regulate growth, development and homeostasis. As such, ciliary defects produce a spectrum of human diseases, termed ciliopathies, and deregulation of this important organelle also plays key roles during tumor formation and progression. Recent studies have begun to clarify the key mechanisms that regulate ciliary assembly and disassembly in both normal and tumor cells, highlighting new possibilities for therapeutic intervention. Here, we review these exciting new findings, discussing the molecular factors involved in cilium formation and removal, the intrinsic and extrinsic control of cilium assembly and disassembly, and the relevance of these processes to mammalian cell growth and disease.

The role of ubiquitination in the regulation of primary cilia assembly and disassembly

The primary cilium is a cellular antenna found on the surface of many eukaryotic cells, whose main role is to sense and transduce signals that regulate growth, development, and differentiation. Although once believed to be a vestigial organelle without important function, it has become clear that defects in primary cilium are responsible for a wide variety of genetic diseases affecting many organs and tissues, including the brain, eyes, heart, kidneys, liver, and pancreas. The primary cilium is mainly present in quiescent and differentiated cells, and controls must exist to ensure that this organelle is assembled or disassembled at the right time. Although many protein components required for building the cilium have been identified, mechanistic details of how these proteins are spatially and temporally regulated and how these regulations are connected to external cues are beginning to emerge. This review article highlights the role of ubiquitination and in particular, E3 ubiquitin ligases and deubiquitinases, in the control of primary cilia assembly and disassembly.

A novel Cep120-dependent mechanism inhibits centriole maturation in quiescent cells

The two centrioles of the centrosome in quiescent cells are inherently asymmetric structures that differ in age, morphology and function. How these asymmetric properties are established and maintained during quiescence remains unknown. Here, we show that a daughter centriole-associated ciliopathy protein, Cep120, plays a critical inhibitory role at daughter centrioles. Depletion of Cep120 in quiescent mouse and human cells causes accumulation of pericentriolar material (PCM) components including pericentrin, Cdk5Rap2, ninein and Cep170. The elevated PCM levels result in increased microtubule-nucleation activity at the centrosome. Consequently, loss of Cep120 leads to aberrant dynein-dependent trafficking of centrosomal proteins, dispersal of centriolar satellites, and defective ciliary assembly and signaling. Our results indicate that Cep120 helps to maintain centrosome homeostasis by inhibiting untimely maturation of the daughter centriole, and defines a potentially new molecular defect underlying the pathogenesis of ciliopathies such as Jeune Asphyxiating Thoracic Dystrophy and Joubert syndrome.

Among the countless components of an animal cell, microtubules perform many important roles. These hollow filaments support the cell’s shape and help to transport different materials around within it. They also form a hair-like projection on the cell surface called the primary cilium, which helps the cell sense its environment. Most microtubules in an animal cell are organized by a structure called the centrosome, which has two smaller cylindrical structures called centrioles at its core. In cells that are not dividing, these two centrioles are different in age. The older of the two centrioles was assembled at least two cell divisions ago and is commonly called the “mother” centriole. The younger one, which was assembled the previous time the cell divided, is called the “daughter” centriole.

Most activities at the centrosome are controlled by the mother centriole. For example, the mother centriole contains protein complexes called appendages that allow it to dock at the cell surface and build the cilium. The mother centriole also contains a complex of proteins called the pericentriolar material, which helps it assemble microtubules and anchor them in place. In contrast, the daughter centriole lacks appendages, does not form a cilium, has less pericentriolar material and so assembles fewer microtubules. Why the daughter centriole cannot recruit these protein complexes remains a mystery.

One possibly important difference between mother and daughter centrioles is that daughter centrioles in non-dividing cells have much higher levels of a protein called Cep120. Now, Betleja et al. have studied the role of this protein in more detail. Experiments with mouse and human cells show that Cep120 plays an important inhibitory role at the daughter centriole. When the production of Cep120 was blocked, more pericentriolar material associated with the daughter centriole, and more microtubules were assembled by the centrosome. This interfered with the movement of other proteins to the centrosome, which ultimately disrupted both the centrosome’s ability to assemble cilia and the cell’s ability to sense its environment.

The findings of Betleja et al. show that a Cep120-dependent mechanism actively regulates the centrosome’s function in non-dividing cells. These experiments uncover a potentially new type of molecular defect that may be responsible for diseases caused by faulty cilia, such as Joubert Syndrome and Jeune Asphyxiating Thoracic Dystrophy. The next challenge will be to understand how Cep120 inhibits the levels of pericentriolar material only at the daughter centriole but not the mother centriole.

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.

Network Analysis of UBE3A/E6AP-Associated Proteins Provides Connections to Several Distinct Cellular Processes

Perturbations in activity and dosage of the UBE3A ubiquitin-ligase have been linked to Angelman syndrome and autism spectrum disorders. UBE3A was initially identified as the cellular protein hijacked by the human papillomavirus E6 protein to mediate the ubiquitylation of p53, a function critical to the oncogenic potential of these viruses. Although a number of substrates have been identified, the normal cellular functions and pathways affected by UBE3A are largely unknown. Previously, we showed that UBE3A associates with HERC2, NEURL4, and MAPK6/ERK3 in a high-molecular-weight complex of unknown function that we refer to as the HUN complex (HERC2, UBE3A, and NEURL4). In this study, the combination of two complementary proteomic approaches with a rigorous network analysis revealed cellular functions and pathways in which UBE3A and the HUN complex are involved. In addition to finding new UBE3A-associated proteins, such as MCM6, SUGT1, EIF3C, and ASPP2, network analysis revealed that UBE3A-associated proteins are connected to several fundamental cellular processes including translation, DNA replication, intracellular trafficking, and centrosome regulation. Our analysis suggests that UBE3A could be involved in the control and/or integration of these cellular processes, in some cases as a component of the HUN complex, and also provides evidence for crosstalk between the HUN complex and CAMKII interaction networks. This study contributes to a deeper understanding of the cellular functions of UBE3A and its potential role in pathways that may be affected in Angelman syndrome, UBE3A-associated autism spectrum disorders, and human papillomavirus-associated cancers.

Engaging Anaphase Catastrophe Mechanisms to Eradicate Aneuploid Cancers

Cancer cells often have supernumerary centrosomes that promote genomic instability, a pathognomonic feature of cancer. During mitosis, cancer cells with supernumerary centrosomes undergo bipolar cell division by clustering centrosomes into two poles. When supernumerary centrosome clustering is antagonized, cancer cells are forced to undergo multipolar division leading to death of daughter cells. This proapoptotic pathway, called anaphase catastrophe, preferentially eliminates aneuploid cancer cells and malignant tumors in engineered mouse models. Anaphase catastrophe occurs through the loss or inhibition of the centrosomal protein CP110, a direct cyclin-dependent kinase 1 (CDK1) and CDK2 target. Intriguingly, CP110 is repressed by the KRAS oncoprotein. This sensitizes KRAS-driven lung cancers (an unmet medical need) to respond to CDK2 inhibitors. Anaphase catastrophe-inducing agents like CDK1 and CDK2 antagonists are lethal to cancer cells with supernumerary centrosomes, but can relatively spare normal cells with two centrosomes. This mechanism is proposed to provide a therapeutic window in the cancer clinic following treatment with a CDK1 or CDK2 inhibitor. Taken together, anaphase catastrophe is a clinically tractable mechanism that promotes death of neoplastic tumors with aneuploidy, a hallmark of cancer. Mol Cancer Ther; 17(4); 724-31. ©2018 AACR.

RNF8/UBC13 ubiquitin signaling suppresses synapse formation in the mammalian brain

Although ubiquitin ligases have been implicated in autism, their roles and mechanisms in brain development remain incompletely understood. Here, we report that in vivo knockdown or conditional knockout of the autism-linked ubiquitin ligase RNF8 or associated ubiquitin-conjugating enzyme UBC13 in rodent cerebellar granule neurons robustly increases the number of parallel fiber presynaptic boutons and functional parallel fiber/Purkinje cell synapses. In contrast to the role of nuclear RNF8 in proliferating cells, RNF8 operates in the cytoplasm in neurons to suppress synapse differentiation in vivo. Proteomics analyses reveal that neuronal RNF8 interacts with the HECT domain protein HERC2 and scaffold protein NEURL4, and knockdown of HERC2 or NEURL4 phenocopies the inhibition of RNF8/UBC13 signaling on synapse differentiation. In behavior analyses, granule neuron-specific knockout of RNF8 or UBC13 impairs cerebellar-dependent learning. Our study defines RNF8 and UBC13 as components of a novel cytoplasmic ubiquitin-signaling network that suppresses synapse formation in the brain.

NEURL4 regulates the transcriptional activity of tumor suppressor protein p53 by modulating its oligomerization

p53 is a transcription factor that regulates important cellular processes related to tumor suppression, including induction of senescence, apoptosis, and DNA repair as well as the inhibition of angiogenesis and cell migration. Therefore, it is critical to understand the molecular mechanism that regulates it. p53 tetramerization is a key step in its activation process and the regulation of this oligomerization, an important control point. The E3 ubiquitin ligase HERC2 controls the p53 transcriptional activity by regulation of its oligomerization state. HERC2-interacting proteins such as the adaptor-like protein with six neuralized domains NEURL4 are also candidates to regulate p53 activity. Here, we demonstrate the existence of an interaction network between NEURL4, HERC2 and p53 proteins. We report a functional interaction between NEURL4 and p53, involving the C-terminal region of p53 and the neuralized domains 3 and 4 of NEURL4. Through this interaction, NEURL4 regulates the transcriptional activity of p53. Thus, NEURL4 depletion reduced the transcriptional activity whereas NEURL4 overexpression increased it. In both cases, p53 stability was not affected. Although NEURL4 may interact with p53 independently of the E3 ubiquitin ligase HERC2, we observed that both proteins are needed to regulate the transcriptional activity of p53. Clonogenic assays confirmed the functional relevance of this interaction observing a decrease in cell growth by NEURL4 overexpression correlated to the increase of cellular cycle inhibitor p21 by p53 activation. Under these conditions, NEURL4 activated p53 oligomerization. All these findings identify NEURL4 as a novel regulator of the p53’s signaling.

The Centrioles, Centrosomes, Basal Bodies, and Cilia of Drosophila melanogaster

Centrioles play a key role in the development of the fly. They are needed for the correct formation of centrosomes, the organelles at the poles of the spindle that can persist as microtubule organizing centers (MTOCs) into interphase. The ability to nucleate cytoplasmic microtubules (MTs) is a property of the surrounding pericentriolar material (PCM). The centriole has a dual life, existing not only as the core of the centrosome but also as the basal body, the structure that templates the formation of cilia and flagellae. Thus the structure and functions of the centriole, the centrosome, and the basal body have an impact upon many aspects of development and physiology that can readily be modeled in Drosophila Centrosomes are essential to give organization to the rapidly increasing numbers of nuclei in the syncytial embryo and for the spatially precise execution of cell division in numerous tissues, particularly during male meiosis. Although mitotic cell cycles can take place in the absence of centrosomes, this is an error-prone process that opens up the fly to developmental defects and the potential of tumor formation. Here, we review the structure and functions of the centriole, the centrosome, and the basal body in different tissues and cultured cells of Drosophila melanogaster, highlighting their contributions to different aspects of development and cell division.

The daughter centriole controls ciliogenesis by regulating Neurl-4 localization at the centrosome

During ciliogenesis, the daughter centriole is found next to the mother, but its role in this process is not known. Loukil et al. show that a daughter centriole is necessary for primary cilia formation by promoting Neurl-4–dependent removal of the negative ciliogenesis regulator CP110 from the mother.

The two centrioles of the centrosome differ in age and function. Although the mother centriole mediates most centrosome-dependent processes, the role of the daughter remains poorly understood. A recent study has implicated the daughter centriole in centriole amplification in multiciliated cells, but its contribution to primary ciliogenesis is unclear. We found that manipulations that prevent daughter centriole formation or induce its separation from the mother abolish ciliogenesis. This defect was caused by stabilization of the negative ciliogenesis regulator CP110 and was corrected by CP110 depletion. CP110 dysregulation may be caused by effects on Neurl-4, a daughter centriole–associated ubiquitin ligase cofactor, which was required for ciliogenesis. Centrosome-targeted Neurl-4 was sufficient to restore ciliogenesis in cells with manipulated daughter centrioles. Interestingly, early during ciliogenesis, Neurl-4 transiently associated with the mother centriole in a process that required mother–daughter centriole proximity. Our data support a model in which the daughter centriole promotes ciliogenesis through Neurl-4–dependent regulation of CP110 levels at the mother centriole.

Cep78 controls centrosome homeostasis by inhibiting EDD-DYRK2-DDB1VprBP

The centrosome plays a critical role in various cellular processes including cell division and cilia formation, and deregulation of centrosome homeostasis is a hallmark feature of many human diseases. Here, we show that centrosomal protein of 78 kDa (Cep78) localizes to mature centrioles and directly interacts with viral protein R binding protein (VprBP). Although VprBP is a component of two distinct E3 ubiquitin ligases, EDD-DYRK2-DDB1VprBP and CRL4VprBP, Cep78 binds specifically to EDD-DYRK2-DDB1VprBP and inhibits its activity. A pool of EDD-DYRK2-DDB1VprBP is active at the centrosome and mediates ubiquitination of CP110, a novel centrosomal substrate. Deregulation of Cep78 or EDD-DYRK2-DDB1VprBP perturbs CP110 ubiquitination and protein stability, thereby affecting centriole length and cilia assembly. Mechanistically, ubiquitination of CP110 entails its phosphorylation by DYRK2 and binding to VprBP Cep78 specifically impedes the transfer of ubiquitin from EDD to CP110 without affecting CP110 phosphorylation and binding to VprBP Thus, we identify Cep78 as a new player that regulates centrosome homeostasis by inhibiting the final step of the enzymatic reaction catalyzed by EDD-DYRK2-DDB1VprBP.

Centrosomes and cancer: revisiting a long-standing relationship

Over a century ago, centrosome aberrations were postulated to cause cancer by promoting genome instability. The mechanisms governing centrosome assembly and function are increasingly well understood, allowing for a timely reappraisal of this postulate. This Review discusses recent advances that shed new light on the relationship between centrosomes and cancer, and raise the possibility that centrosome aberrations contribute to this disease in different ways than initially envisaged.

The Parkinson's Disease-Associated Protein Kinase LRRK2 Modulates Notch Signaling through the Endosomal Pathway

Leucine-rich repeat kinase 2 (LRRK2) is a key molecule in the pathogenesis of familial and idiopathic Parkinson’s disease (PD). We have identified two novel LRRK2-associated proteins, a HECT-type ubiquitin ligase, HERC2, and an adaptor-like protein with six repeated Neuralized domains, NEURL4. LRRK2 binds to NEURL4 and HERC2 via the LRRK2 Ras of complex proteins (ROC) domain and NEURL4, respectively. HERC2 and NEURL4 link LRRK2 to the cellular vesicle transport pathway and Notch signaling, through which the LRRK2 complex promotes the recycling of the Notch ligand Delta-like 1 (Dll1)/Delta (Dl) through the modulation of endosomal trafficking. This process negatively regulates Notch signaling through cis-inhibition by stabilizing Dll1/Dl, which accelerates neural stem cell differentiation and modulates the function and survival of differentiated dopaminergic neurons. These effects are strengthened by the R1441G ROC domain-mutant of LRRK2. These findings suggest that the alteration of Notch signaling in mature neurons is a component of PD etiology linked to LRRK2.

LRRK2 is linked to autosomal dominant late-onset Parkinson’s disease, suggesting that LRRK2 gain-of-function mutations lead to age-dependent degeneration of the midbrain dopaminergic neurons. In this study, we describe two novel LRRK2-associated proteins HERC2 and NEURL4, which are a ubiquitin ligase and an adaptor-like protein, respectively. HERC2 and NEURL4 direct LRRK2 to Notch signaling pathway, in which the LRRK2-NEURL4-HERC2 complex promotes the recycling of the Notch ligand Delta-like 1 (Dll1)/Delta (Dl) through the modulation of endosomal trafficking. As a result, the amounts of Dll1/D1 on the plasma membrane are increased, which affects negatively Notch signaling through cis-inhibition. The effect is enhanced by a Parkinson’s-disease associated mutation of LRRK2. Inhibition of Notch signaling in adult dopaminergic neurons impairs its functions and survival. These findings indicate a possible link between Notch pathway and Parkinson’s disease.

MDM1 is a microtubule-binding protein that negatively regulates centriole duplication

MDM1 is a microtubule-binding protein that localizes to centrioles. 3D-SIM microscopy shows MDM1 to be closely associated with the centriole barrel, likely residing in the centriole lumen. MDM1 overexpression and depletion experiments suggest that MDM1 is a negative regulator of centriole duplication.

Mouse double-minute 1 (Mdm1) was originally identified as a gene amplified in transformed mouse cells and more recently as being highly up-regulated during differentiation of multiciliated epithelial cells, a specialized cell type having hundreds of centrioles and motile cilia. Here we show that the MDM1 protein localizes to centrioles of dividing cells and differentiating multiciliated cells. 3D-SIM microscopy showed that MDM1 is closely associated with the centriole barrel, likely residing in the centriole lumen. Overexpression of MDM1 suppressed centriole duplication, whereas depletion of MDM1 resulted in an increase in granular material that likely represents early intermediates in centriole formation. We show that MDM1 binds microtubules in vivo and in vitro. We identified a repeat motif in MDM1 that is required for efficient microtubule binding and found that these repeats are also present in CCSAP, another microtubule-binding protein. We propose that MDM1 is a negative regulator of centriole duplication and that its function is mediated through microtubule binding.

Specific CP110 Phosphorylation Sites Mediate Anaphase Catastrophe after CDK2 Inhibition: Evidence for Cooperation with USP33 Knockdown

Chromosomal instability (CIN) is a hallmark of solid tumor biology and is implicated in carcinogenesis. Preferentially eliminating malignant cells by targeting CIN and aneuploidy is an attractive antineoplastic strategy. We previously reported that CDK2 antagonism causes lung cancer cells to undergo anaphase catastrophe and apoptosis through inhibition of phosphorylation of the centrosomal protein CP110. Cells with activating KRAS mutations were particularly sensitive to CDK2 inhibition due to downregulation of CP110 protein levels. This study investigated mechanisms of CDK2 antagonism that mediate anaphase catastrophe via changes in CP110 protein expression and how activated KRAS affects CP110 levels in lung cancers. Site-directed mutagenesis revealed candidate CDK phosphorylation sites of CP110 (residues Ser 170 and Thr 194) critical for conferring anaphase catastrophe by altering centrosome clustering in mitosis. Intriguingly, KRAS mutation can promote CP110 protein degradation by upregulating the ubiquitin ligase SCF(cyclinF), which targets CP110 protein for destabilization. Finally, CDK2 inhibitor response was enhanced when combined with knockdown of the deubiquitinase USP33 that in turn accelerates CP110 protein degradation. Thus, this study provides molecular pharmacologic insights into how CP110 expression regulates response to CDK2 inhibition. An improved understanding of in vitro antineoplastic mechanisms of combining CDK2 antagonism with induced CP110 repression provides a rationale for exploring clinical consequences of this strategy. Taken together, preclinical findings obtained from combining CDK2 inhibition with USP33 repression have implications for treating patients with non-small cell lung cancers.

Neurl4 contributes to germ cell formation and integrity in Drosophila

Primordial germ cells (PGCs) form at the posterior pole of the Drosophila embryo, and then migrate to their final destination in the gonad where they will produce eggs or sperm. Studies of the different stages in this process, including assembly of germ plasm in the oocyte during oogenesis, specification of a subset of syncytial embryonic nuclei as PGCs, and migration, have been informed by genetic analyses. Mutants have defined steps in the process, and the identities of the affected genes have suggested biochemical mechanisms. Here we describe a novel PGC phenotype. When Neurl4 activity is reduced, newly formed PGCs frequently adopt irregular shapes and appear to bud off vesicles. PGC number is also reduced, an effect exacerbated by a separate role for Neurl4 in germ plasm formation during oogenesis. Like its mammalian homolog, Drosophila Neurl4 protein is concentrated in centrosomes and downregulates centrosomal protein CP110. Reducing CP110 activity suppresses the abnormal PGC morphology of Neurl4 mutants. These results extend prior analyses of Neurl4 in cultured cells, revealing a heightened requirement for Neurl4 in germ-line cells in Drosophila.

Proteomic analysis and identification of cellular interactors of the giant ubiquitin ligase HERC2

HERC2 is a large E3 ubiquitin ligase with multiple structural domains that has been implicated in an array of cellular processes. Mutations in HERC2 are linked to developmental delays and impairment caused by nervous system dysfunction, such as Angelman Syndrome and autism-spectrum disorders. However, HERC2 cellular activity and regulation remain poorly understood. We used a broad proteomic approach to survey the landscape of cellular proteins that interact with HERC2. We identified nearly 300 potential interactors, a subset of which we validated binding to HERC2. The potential HERC2 interactors included the eukaryotic translation initiation factor 3 complex, the intracellular transport COPI coatomer complex, the glycogen regulator phosphorylase kinase, beta-catenin, PI3 kinase, and proteins involved in fatty acid transport and iron homeostasis. Through a complex bioinformatic analysis of potential interactors, we linked HERC2 to cellular processes including intracellular protein trafficking and transport, metabolism of cellular energy, and protein translation. Given its size, multidomain structure, and association with various cellular activities, HERC2 may function as a scaffold to integrate protein complexes and bridge critical cellular pathways. This work provides a significant resource with which to interrogate HERC2 function more deeply and evaluate its contributions to mechanisms governing cellular homeostasis and disease.

RBM14 prevents assembly of centriolar protein complexes and maintains mitotic spindle integrity

Formation of a new centriole adjacent to a pre-existing centriole occurs only once per cell cycle. Despite being crucial for genome integrity, the mechanisms controlling centriole biogenesis remain elusive. Here, we identify RBM14 as a novel suppressor of assembly of centriolar protein complexes. Depletion of RBM14 in human cells induces ectopic formation of centriolar protein complexes through function of the STIL/CPAP complex. Intriguingly, the formation of such structures seems not to require the cartwheel structure that normally acts as a scaffold for centriole formation, whereas they can retain pericentriolar material and microtubule nucleation activity. Moreover, we find that, upon RBM14 depletion, a part of the ectopic centriolar protein complexes in turn assemble into structures more akin to centrioles, presumably by incorporating HsSAS-6, a cartwheel component, and cause multipolar spindle formation. We further demonstrate that such structures assemble in the cytoplasm even in the presence of pre-existing centrioles. This study sheds light on the possibility that ectopic formation of aberrant structures related to centrioles may contribute to genome instability and tumorigenesis.

The CP110-interacting proteins Talpid3 and Cep290 play overlapping and distinct roles in cilia assembly

Talpid3 and Cep290 promote proper ciliary vesicle formation by regulating centriolar satellite accretion and Rab8a localization.

We have identified Talpid3/KIAA0586 as a component of a CP110-containing protein complex important for centrosome and cilia function. Talpid3 assembles a ring-like structure at the extreme distal end of centrioles. Ablation of Talpid3 resulted in an aberrant distribution of centriolar satellites involved in protein trafficking to centrosomes as well as cilia assembly defects, reminiscent of loss of Cep290, another CP110-associated protein. Talpid3 depletion also led to mislocalization of Rab8a, a small GTPase thought to be essential for ciliary vesicle formation. Expression of activated Rab8a suppressed cilia assembly defects provoked by Talpid3 depletion, suggesting that Talpid3 affects cilia formation through Rab8a recruitment and/or activation. Remarkably, ultrastructural analyses showed that Talpid3 is required for centriolar satellite dispersal, which precedes the formation of mature ciliary vesicles, a process requiring Cep290. These studies suggest that Talpid3 and Cep290 play overlapping and distinct roles in ciliary vesicle formation through regulation of centriolar satellite accretion and Rab8a.

CP110 and its network of partners coordinately regulate cilia assembly

Cilia are hair-like protrusions found at the surface of most eukaryotic cells. They can be divided into two types, motile and non-motile. Motile cilia are found in a restricted number of cell types, are generally present in large numbers, and beat in a coordinated fashion to generate fluid flow or locomotion. Non-motile or primary cilia, on the other hand, are detected in many different cell types, appear once per cell, and primarily function to transmit signals from the extracellular milieu to the cell nucleus. Defects in cilia formation, function, or maintenance are known to cause a bewildering set of human diseases, or ciliopathies, typified by retinal degeneration, renal failure and cystic kidneys, obesity, liver dysfunction, and neurological disorders. A common denominator between motile and primary cilia is their structural similarity, as both types of cilia are composed of an axoneme, the ciliary backbone that is made up of microtubules emanating from a mother centriole/basal body anchored to the cell membrane, surrounded by a ciliary membrane continuous with the plasma membrane. This structural similarity is indicative of a universal mechanism of cilia assembly involving a common set of molecular players and a sophisticated, highly regulated series of molecular events. In this review, we will mainly focus on recent advances in our understanding of the regulatory mechanisms underlying cilia assembly, with special attention paid to the centriolar protein, CP110, its interacting partner Cep290, and the various downstream molecular players and events leading to intraflagellar transport (IFT), a process that mediates the bidirectional movement of protein cargos along the axoneme and that is essential for cilia formation and maintenance.

Discovering regulators of centriole biogenesis through siRNA-based functional genomics in human cells

Centrioles are essential for forming cilia, flagella, and centrosomes and are thus critical for a range of fundamental cellular processes. Despite their importance, the mechanisms governing centriole biogenesis remain incompletely understood. We performed a high-content genome-wide small-interfering-RNA-based screen to identify genes regulating centriole formation in human cells. We designed an algorithm to automatically detect GFP-Centrin foci that, combined with subsequent manual analysis, allowed us to identify 44 genes required for centriole formation and 32 genes needed for restricting centriole number. Detailed follow-up characterization uncovered that the C2 domain protein C2CD3 is required for distal centriole formation and suggests that it functions in the basal body to template primary cilia. Moreover, we found that the E3 ubiquitin ligase TRIM37 prevents centriole reduplication events. We developed a dynamic web interface containing all images and numerical features as a powerful resource to investigate facets of centrosome biology.

USP33 regulates centrosome biogenesis via deubiquitination of the centriolar protein CP110

Centrosome duplication is critical for cell division, and genome instability can result if duplication is not restricted to a single round per cell cycle. Centrosome duplication is controlled in part by CP110, a centriolar protein that positively regulates centriole duplication while restricting centriole elongation and ciliogenesis. Maintenance of normal CP110 levels is essential, as excessive CP110 drives centrosome over-duplication and suppresses ciliogenesis, whereas its depletion inhibits centriole amplification and leads to highly elongated centrioles and aberrant assembly of cilia in growing cells. CP110 levels are tightly controlled, partly through ubiquitination by the ubiquitin ligase complex SCF(cyclin F) during G2 and M phases of the cell cycle. Here, using human cells, we report a new mechanism for the regulation of centrosome duplication that requires USP33, a deubiquitinating enzyme that is able to regulate CP110 levels. USP33 interacts with CP110 and localizes to centrioles primarily in S and G2/M phases, the periods during which centrioles duplicate and elongate. USP33 potently and specifically deubiquitinates CP110, but not other cyclin-F substrates. USP33 activity antagonizes SCF(cyclin F)-mediated ubiquitination and promotes the generation of supernumerary centriolar foci, whereas ablation of USP33 destabilizes CP110 and thereby inhibits centrosome amplification and mitotic defects. To our knowledge, we have identified the first centriolar deubiquitinating enzyme whose expression regulates centrosome homeostasis by countering cyclin-F-mediated destruction of a key substrate. Our results point towards potential therapeutic strategies for inhibiting tumorigenesis associated with centrosome amplification.

A cyclin without cyclin-dependent kinases: cyclin F controls genome stability through ubiquitin-mediated proteolysis

Cell cycle transitions are driven by the periodic oscillations of cyclins, which bind and activate cyclin-dependent kinases (CDKs) to phosphorylate target substrates. Cyclin F uses a substrate recruitment strategy similar to that of the other cyclins, but its associated catalytic activity is substantially different. Indeed, cyclin F is the founding member of the F-box family of proteins, which are the substrate recognition subunits of Skp1-Cul1-F-box protein (SCF) ubiquitin ligase complexes. Here, we discuss cyclin F function and recently identified substrates of SCF(cyclin)(F) involved in deoxyribonucleotide triphosphate (dNTP) production, centrosome duplication, and spindle formation. We highlight the relevance of cyclin F in controlling genome stability through ubiquitin-mediated proteolysis and the implications for cancer development.

Functional analysis of the NHR2 domain indicates that oligomerization of Neuralized regulates ubiquitination and endocytosis of Delta during Notch signaling

The Notch pathway plays an integral role in development by regulating cell fate in a wide variety of multicellular organisms. A critical step in the activation of Notch signaling is the endocytosis of the Notch ligands Delta and Serrate. Ligand endocytosis is regulated by one of two E3 ubiquitin ligases, Neuralized (Neur) or Mind bomb. Neur is comprised of a C-terminal RING domain, which is required for Delta ubiquitination, and two Neur homology repeat (NHR) domains. We have previously shown that the NHR1 domain is required for Delta trafficking. Here we show that the NHR1 domain also affects the binding and internalization of Serrate. Furthermore, we show that the NHR2 domain is required for Neur function and that a point mutation in the NHR2 domain (Gly430) abolishes Neur ubiquitination activity and affects ligand internalization. Finally, we provide evidence that Neur can form oligomers in both cultured cells and fly tissues, which regulate Neur activity and, by extension, ligand internalization.

Identification and proteomic analysis of distinct UBE3A/E6AP protein complexes

The E6AP ubiquitin ligase catalyzes the high-risk human papillomaviruses' E6-mediated ubiquitylation of p53, contributing to the neoplastic progression of cells infected by these viruses. Defects in the activity and the dosage of E6AP are linked to Angelman syndrome and to autism spectrum disorders, respectively, highlighting the need for precise control of the enzyme. With the exception of HERC2, which modulates the ubiquitin ligase activity of E6AP, little is known about the regulation or function of E6AP normally. Using a proteomic approach, we have identified and validated several new E6AP-interacting proteins, including HIF1AN, NEURL4, and mitogen-activated protein kinase 6 (MAPK6). E6AP exists as part of several different protein complexes, including the proteasome and an independent high-molecular-weight complex containing HERC2, NEURL4, and MAPK6. In examining the functional consequence of its interaction with the proteasome, we found that UBE3C (another proteasome-associated ubiquitin ligase), but not E6AP, contributes to proteasomal processivity in mammalian cells. We also found that E6 associates with the HERC2-containing high-molecular-weight complex through its binding to E6AP. These proteomic studies reveal a level of complexity for E6AP that has not been previously appreciated and identify a number of new cellular proteins through which E6AP may be regulated or functioning.