Centriole Assembly Requires Both Centriolar and Pericentriolar Material Proteins

A conserved role for centriolar satellites in translation of centrosomal and ciliary proteins

Centriolar satellites are cytoplasmic particles found in the vicinity of centrosomes and cilia whose specific functional contribution has long been unclear. Here, we identify Combover as the Drosophila ortholog of the main scaffolding component of satellites, PCM1. Like PCM1, Combover localizes to cytoplasmic foci containing centrosomal proteins and its depletion or mutation results in centrosomal and ciliary phenotypes. Strikingly, however, the concentration of satellites near centrosomes and cilia is not a conserved feature, nor do Combover foci display directed movement. Proximity interaction analysis revealed not only centrosomal and ciliary proteins, but also RNA-binding proteins and proteins involved in quality control. Further work in Drosophila and vertebrate cells found satellites to be associated with centrosomal and ciliary mRNAs, as well as evidence for protein synthesis occurring directly at satellites. Given that PCM1 depletion does not affect overall protein levels, we propose that satellites instead promote the coordinate synthesis of centrosomal and ciliary proteins, thereby facilitating the formation of protein complexes.

Glutamylation of centrosomes ensures their function by recruiting microtubule nucleation factors

Centrosomes are tubulin-based organelles that undergo glutamylation, a post-translational modification that conjugates glutamic acid residues to tubulins. Although centrosomal glutamylation has been known for several decades, how this modification regulates centrosome structure and function remains unclear. To address this long-standing issue, we developed a method to spatiotemporally reduce centrosomal glutamylation by recruiting an engineered deglutamylase to centrosomes. We found that centrosome structure remains largely unaffected by centrosomal hypoglutamylation. Intriguingly, glutamylation physically recruits, via electrostatic forces, the NEDD1/CEP192/γ-tubulin complex to centrosomes, ensuring microtubule nucleation and proper trafficking of centriolar satellites. The consequent defect in centriolar satellite trafficking leads to reduced levels of the ciliogenesis factor Talpid3, suppressing ciliogenesis. Centrosome glutamylation also promotes proper mitotic spindle formation and mitosis. In summary, our study provides a new approach to spatiotemporally manipulate glutamylation at centrosomes, and offers novel insights into how centrosomes are organized and regulated by glutamylation.

PLK4: Master Regulator of Centriole Duplication and Its Therapeutic Potential

Centrosomes catalyze the assembly of a microtubule-based bipolar spindle, essential for the precise chromosome segregation during cell division. At the center of this process lies Polo-Like Kinase 4 (PLK4), the master regulator that controls the duplication of the centriolar core to ensure the correct balance of two centrosomes per dividing cell. Disruptions in centrosome number or function can lead to genetic disorders such as primary microcephaly or drive tumorigenesis via centrosome amplification. In this context, several chemical inhibitors of PLK4 have emerged as promising therapeutic candidates. The inhibition of PLK4 results in the emergence of acentrosomal cells, which undergo prolonged and error-prone mitosis. This aberrant mitotic duration triggers a "mitotic stopwatch" mechanism that activates the tumor suppressor p53, halting cellular proliferation. However, in a multitude of cancers, the efficacy of this mitotic surveillance mechanism is compromised by mutations that incapacitate p53. Recent investigations have unveiled p53-independent vulnerabilities in cancers characterized by chromosomal gain or amplification of 17q23, which encodes for the ubiquitin ligase TRIM37, in response to PLK4 inhibition, particularly in neuroblastoma and breast cancer. This review encapsulates the latest advancements in our understanding of centriole duplication and acentrosomal cell division in the context of TRIM37 amplification, positioning PLK4 as a compelling target for innovative cancer therapeutics.

Molecular architectures of centrosomes in C. elegans embryos visualized by cryo-electron tomography

Centrosomes organize microtubules that are essential for mitotic divisions in animal cells. They consist of centrioles surrounded by pericentriolar material (PCM). Questions related to mechanisms of centriole assembly, PCM organization, and spindle microtubule formation remain unanswered, partly due to limited availability of molecular-resolution structural data inside cells. Here, we use cryo-electron tomography to visualize centrosomes across the cell cycle in cells isolated from C. elegans embryos. We describe a pseudo-timeline of centriole assembly and identify distinct structural features in both mother and daughter centrioles. We find that centrioles and PCM microtubules differ in protofilament number (13 versus 11), which could be explained by atypical γ-tubulin ring complexes with 11-fold symmetry identified at the minus ends of short PCM microtubule segments. We further characterize a porous and disordered network that forms the interconnected PCM. Thus, our work builds a three-dimensional structural atlas that helps explain how centrosomes assemble, grow, and achieve function.

Centrioles generate two scaffolds with distinct biophysical properties to build mitotic centrosomes

Mitotic centrosomes assemble when centrioles recruit large amounts of pericentriolar material (PCM) around themselves. The PCM comprises hundreds of proteins, and there is much debate about its physical nature. Here, we show that Drosophila Spd-2 (human CEP192) fluxes out from centrioles, recruiting Polo and Aurora A kinases to catalyze the assembly of two distinct mitotic-PCM scaffolds: a Polo-dependent Cnn scaffold, and an Aurora A-dependent TACC scaffold, which exhibit solid- and liquid-like behaviors, respectively. Both scaffolds can independently recruit PCM proteins, but both are required for proper centrosome assembly, with the Cnn scaffold providing mechanical strength, and the TACC scaffold concentrating centriole and centrosome proteins. Recruiting Spd-2 to synthetic beads injected into early embryos reconstitutes key aspects of mitotic centrosome assembly on the bead surface, and this depends on Spd-2's ability to recruit Polo and Aurora A. Thus, Spd-2 orchestrates the assembly of two scaffolds, with distinct biophysical properties, that cooperate to build mitotic centrosomes in flies.

MLL/WDR5 complex recruits centriolar satellite protein Cep72 to regulate microtubule nucleation and spindle formation

Dysfunction of the centrosome, the major microtubule-organizing center of the cell, is implicated in microcephaly. Haploinsufficiency of mixed-lineage leukemia (MLL/KMT2A) protein causes Wiedemann-Steiner syndrome (WSS), a neurodevelopmental disorder associated with microcephaly. However, whether MLL has a function at the centrosome is not clear. Here, we show that loss of the MLL/WDR5 complex affects microtubule nucleation and regrowth. MLL/WDR5 localize to the pericentriolar material and interact with centriolar satellite protein Cep72 and γ-tubulin ring complex proteins (γ-TuRCs). MLL/WDR5 promote the localization of γ-TuRCs and structural proteins like AKAP9 to the centrosome during interphase and mitosis, a phenotype also observed in cells derived from patients with WSS. During mitosis, loss of MLL, WDR5, and Cep72 affects spindle formation and leads to misaligned chromosomes. Last, we show that MLL and WDR5 recruit Cep72 to the centrosome. Our studies provide insight into an undiscovered role of MLL at the centrosome and elucidate how centriolar satellite proteins like Cep72 can be recruited to the centrosome.

Structural insights into SSNA1 self-assembly and its microtubule binding for centriole maintenance

SSNA-1 is a fibrillar protein localized at the area where dynamic microtubule remodeling occurs including centrosomes. Despite the important activities of SSNA1 to microtubules such as nucleation, co-polymerization, and lattice sharing microtubule branching, the underlying molecular mechanism have remained unclear due to a lack of structural information. Here, we determined the cryo-EM structure of C. elegans SSNA-1 at 4.55 Å resolution and evaluated its role during embryonic development in C. elegans. We found that SSNA1 forms an anti-parallel coiled-coil, and its self-assembly is facilitated by the overhangs of 16 residues at its C-terminus, which dock on the adjacent coiled-coil to form a triple-stranded helical junction. Notably, the microtubule-binding region is within the triple-stranded junction, highlighting that self-assembly of SSNA-1 facilitates effective microtubule interaction by creating hubs along a fibril. Furthermore, our genetical analysis elucidated that deletion of SSNA-1 resulted in a significant reduction in embryonic viability and the formation of multipolar spindles during cell division. Interestingly, when the ability of SSNA-1 self-assembly was impaired, embryonic viability stayed low, comparable to that of the knockout strain. Our study provides molecular insights into the self-assembly mechanisms of SSNA-1, shedding light on its role in controlling microtubule binding and cell division through the regulation of centriole stability.

The C. elegans homolog of Sjögren's Syndrome Nuclear Antigen 1 is required for the structural integrity of the centriole and bipolar mitotic spindle assembly

Centrioles play central roles in ciliogenesis and mitotic spindle assembly. Once assembled, centrioles exhibit long-term stability, a property essential for maintaining numerical control. How centriole stability is achieved and how it is lost in certain biological contexts are still not completely understood. In this study we show that SSNA-1, the Caenorhabditis elegans ortholog of Sjogren's Syndrome Nuclear Antigen 1, is a centriole constituent that localizes close to the microtubule outer wall, while also exhibiting a developmentally regulated association with centriole satellite-like structures. A complete deletion of the ssna-1 gene results in an embryonic lethal phenotype marked by the appearance of extra centrioles and spindle poles. We show that SSNA-1 genetically interacts with the centriole stability factor SAS-1 and is required post assembly for centriole structural integrity. In SSNA-1's absence, centrioles assemble but fracture leading to extra spindle poles. However, if the efficiency of cartwheel assembly is reduced, the absence of SSNA-1 results in daughter centriole loss and monopolar spindle formation, indicating that the cartwheel and SSNA-1 cooperate to stabilize the centriole during assembly. Our work thus shows that SSNA-1 contributes to centriole stability during and after assembly, thereby ensuring proper centriole number.

Maternal inheritance of functional centrioles in two parthenogenetic nematodes

Centrioles are the core constituent of centrosomes, microtubule-organizing centers involved in directing mitotic spindle assembly and chromosome segregation in animal cells. In sexually reproducing species, centrioles degenerate during oogenesis and female meiosis is usually acentrosomal. Centrioles are retained during male meiosis and, in most species, are reintroduced with the sperm during fertilization, restoring centriole numbers in embryos. In contrast, the presence, origin, and function of centrioles in parthenogenetic species is unknown. We found that centrioles are maternally inherited in two species of asexual parthenogenetic nematodes and identified two different strategies for maternal inheritance evolved in the two species. In Rhabditophanes diutinus, centrioles organize the poles of the meiotic spindle and are inherited by both the polar body and embryo. In Disploscapter pachys, the two pairs of centrioles remain close together and are inherited by the embryo only. Our results suggest that maternally-inherited centrioles organize the embryonic spindle poles and act as a symmetry-breaking cue to induce embryo polarization. Thus, in these parthenogenetic nematodes, centrioles are maternally-inherited and functionally replace their sperm-inherited counterparts in sexually reproducing species.

Novel biallelic SASS6 variants associated with primary microcephaly and fetal growth restriction

Primary microcephaly is characterized by a head circumference prenatally or at birth that falls below three standard deviations from age-, ethnic-, and sex-specific norms. Genetic defects are one of the underlying causes of primary microcephaly. Since 2014, five variants of the SASS6 gene have been identified as the cause of MCPH 14 in three reported families. In this study, we present the genetic findings of members of a nonconsanguineous Chinese couple with a history of microcephaly and fetal growth restriction (FGR) during their first pregnancy. Utilizing trio whole-exome sequencing, we identified compound heterozygous variants involving a frameshift NM_194292.3:c.450_453del p.(Lys150AsnfsTer7) variant and a splice region NM_194292.3:c.1674+3A>G variant within the SASS6 gene in the affected fetus. Moreover, reverse transcriptase-polymerase chain reaction from RNA of the mother's peripheral blood leukocytes revealed that the c.1674+3A>G variant led to the skipping of exon 14 and an inframe deletion. To the best of our knowledge, the association between FGR and SASS6-related microcephaly has not been reported, and our findings confirm the pivotal role of SASS6 in microcephaly pathogenesis and reveal an expanded view of the phenotype and mutation spectrum associated with this gene.

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.

Time-series reconstruction of the molecular architecture of human centriole assembly

Centriole biogenesis, as in most organelle assemblies, involves the sequential recruitment of sub-structural elements that will support its function. To uncover this process, we correlated the spatial location of 24 centriolar proteins with structural features using expansion microscopy. A time-series reconstruction of protein distributions throughout human procentriole assembly unveiled the molecular architecture of the centriole biogenesis steps. We found that the process initiates with the formation of a naked cartwheel devoid of microtubules. Next, the bloom phase progresses with microtubule blade assembly, concomitantly with radial separation and rapid cartwheel growth. In the subsequent elongation phase, the tubulin backbone grows linearly with the recruitment of the A-C linker, followed by proteins of the inner scaffold (IS). By following six structural modules, we modeled 4D assembly of the human centriole. Collectively, this work provides a framework to investigate the spatial and temporal assembly of large macromolecules.

Native molecular architectures of centrosomes in C. elegans embryos

Centrosomes organize microtubules that are essential for mitotic divisions in animal cells. They consist of centrioles surrounded by Pericentriolar Material (PCM). Questions related to mechanisms of centriole assembly, PCM organization, and microtubule formation remain unanswered, in part due to limited availability of molecular-resolution structural analyses in situ. Here, we use cryo-electron tomography to visualize centrosomes across the cell cycle in cells isolated from C. elegans embryos. We describe a pseudo-timeline of centriole assembly and identify distinct structural features including a cartwheel in daughter centrioles, and incomplete microtubule doublets surrounded by a star-shaped density in mother centrioles. We find that centriole and PCM microtubules differ in protofilament number (13 versus 11) indicating distinct nucleation mechanisms. This difference could be explained by atypical γ-tubulin ring complexes with 11-fold symmetry identified at the minus ends of short PCM microtubules. We further characterize a porous and disordered network that forms the interconnected PCM. Thus, our work builds a three-dimensional structural atlas that helps explain how centrosomes assemble, grow, and achieve function.

Centrosome amplification and aneuploidy driven by the HIV-1-induced Vpr•VprBP•Plk4 complex in CD4+ T cells

HIV-1 infection elevates the risk of developing various cancers, including T-cell lymphoma. Whether HIV-1-encoded proteins directly contribute to oncogenesis remains unknown. We observe that approximately 1-5% of CD4+ T cells from the blood of people living with HIV-1 exhibit over-duplicated centrioles, suggesting that centrosome amplification underlies the development of HIV-1-associated cancers by driving aneuploidy. Through affinity purification, biochemical, and cellular analyses, we discover that Vpr, an accessory protein of HIV-1, hijacks the centriole duplication machinery and induces centrosome amplification and aneuploidy. Mechanistically, Vpr forms a cooperative ternary complex with an E3 ligase subunit, VprBP, and polo-like kinase 4 (Plk4). Unexpectedly, however, the complex enhances Plk4's functionality by promoting its relocalization to the procentriole assembly and induces centrosome amplification. Loss of either Vpr's C-terminal 17 residues or VprBP acidic region, the two elements required for binding to Plk4 cryptic polo-box, abrogates Vpr's capacity to induce these events. Furthermore, HIV-1 WT, but not its Vpr mutant, induces multiple centrosomes and aneuploidy in human primary CD4+ T cells. We propose that the Vpr•VprBP•Plk4 complex serves as a molecular link that connects HIV-1 infection to oncogenesis and that inhibiting the Vpr C-terminal motif may reduce the occurrence of HIV-1-associated cancers.

A phylogenetic profiling approach identifies novel ciliogenesis genes in Drosophila and C. elegans

Cilia are cellular projections that perform sensory and motile functions in eukaryotic cells. A defining feature of cilia is that they are evolutionarily ancient, yet not universally conserved. In this study, we have used the resulting presence and absence pattern in the genomes of diverse eukaryotes to identify a set of 386 human genes associated with cilium assembly or motility. Comprehensive tissue-specific RNAi in Drosophila and mutant analysis in C. elegans revealed signature ciliary defects for 70-80% of novel genes, a percentage similar to that for known genes within the cluster. Further characterization identified different phenotypic classes, including a set of genes related to the cartwheel component Bld10/CEP135 and two highly conserved regulators of cilium biogenesis. We propose this dataset defines the core set of genes required for cilium assembly and motility across eukaryotes and presents a valuable resource for future studies of cilium biology and associated disorders.

A primary microcephaly-associated sas-6 mutation perturbs centrosome duplication, dendrite morphogenesis, and ciliogenesis in Caenorhabditis elegans

The human SASS6(I62T) missense mutation has been linked with the incidence of primary microcephaly in a Pakistani family, although the mechanisms by which this mutation causes disease remain unclear. The SASS6(I62T) mutation corresponds to SAS-6(L69T) in Caenorhabditis elegans. Given that SAS-6 is highly conserved, we modeled this mutation in C. elegans and examined the sas-6(L69T) effect on centrosome duplication, ciliogenesis, and dendrite morphogenesis. Our studies revealed that all the above processes are perturbed by the sas-6(L69T) mutation. Specifically, C. elegans carrying the sas-6(L69T) mutation exhibit an increased failure of centrosome duplication in a sensitized genetic background. Further, worms carrying this mutation also display shortened phasmid cilia, an abnormal phasmid cilia morphology, shorter phasmid dendrites, and chemotaxis defects. Our data show that the centrosome duplication defects caused by this mutation are only uncovered in a sensitized genetic background, indicating that these defects are mild. However, the ciliogenesis and dendritic defects caused by this mutation are evident in an otherwise wild-type background, indicating that they are stronger defects. Thus, our studies shed light on the novel mechanisms by which the sas-6(L69T) mutation could contribute to the incidence of primary microcephaly in humans.

Extensive programmed centriole elimination unveiled in C. elegans embryos

Centrioles are critical for fundamental cellular processes, including signaling, motility, and division. The extent to which centrioles are present after cell cycle exit in a developing organism is not known. The stereotypical lineage of Caenorhabditis elegans makes it uniquely well-suited to investigate this question. Using notably lattice light-sheet microscopy, correlative light electron microscopy, and lineage assignment, we found that ~88% of cells lose centrioles during embryogenesis. Our analysis reveals that centriole elimination is stereotyped, occurring invariably at a given time in a given cell type. Moreover, we established that experimentally altering cell fate results in corresponding changes in centriole fate. Overall, we uncovered the existence of an extensive centriole elimination program, which we anticipate to be paradigmatic for a broad understanding of centriole fate regulation.

PCM1 labeling reveals myonuclear and nuclear dynamics in skeletal muscle across species

Myonuclei transcriptionally regulate muscle fibers during homeostasis and adaptation to exercise. Their subcellular location and quantity are important when characterizing phenotypes of myopathies, the effect of treatments, and understanding the roles of satellite cells in muscle adaptation and muscle "memory." Difficulties arise in identifying myonuclei due to their proximity to the sarcolemma and closely residing interstitial cell neighbors. We aimed to determine to what extent (pericentriolar material-1) PCM1 is a specific marker of myonuclei in vitro and in vivo. Single isolated myofibers and cross sections from mice and humans were studied from several models including wild-type and Lamin A/C mutant mice after functional overload and damage and recovery in humans following forced eccentric contractions. Fibers were immunolabeled for PCM1, Pax7, and DNA. C2C12 myoblasts were also studied to investigate changes in PCM1 localization during myogenesis. PCM1 was detected at not only the nuclear envelope of myonuclei in mature myofibers and in newly formed myotubes but also centrosomes in proliferating myogenic precursors, which may or may not fuse to join the myofiber syncytium. PCM1 was also detected in nonmyogenic nuclei near the sarcolemma, especially in regenerating areas of the Lmna^+/ΔK32 mouse and damaged human muscle. Although PCM1 is not completely specific to myonuclei, the impact that PCM1+ macrophages and interstitial cells have on myonuclei counts would be small in healthy muscle. PCM1 may prove useful as a marker of satellite cell dynamics due to the distinct change in localization during differentiation, revealing satellite cells in their quiescent (PCM1-), proliferating (PCM1+ centrosome), and prefusion states (PCM1+ nuclear envelope).

A combined in silico and in vivo approach to the structure-function annotation of SPD-2 provides mechanistic insight into its functional diversity

Centrosomes are organelles that function as hubs of microtubule nucleation and organization, with key roles in organelle positioning, asymmetric cell division, ciliogenesis, and signaling. Aberrant centrosome number, structure or function is linked to neurodegenerative diseases, developmental abnormalities, ciliopathies, and tumor development. A major regulator of centrosome biogenesis and function in C. elegans is the conserved Spindle-defective protein 2 (SPD-2), a homolog of the human CEP-192 protein. CeSPD-2 is required for centrosome maturation, centriole duplication, spindle assembly and possibly cell polarity establishment. Despite its importance, the specific molecular mechanism of CeSPD-2 regulation and function is poorly understood. Here, we combined computational analysis with cell biology approaches to uncover possible structure-function relationships of CeSPD-2 that may shed mechanistic light on its function. Domain prediction analysis corroborated and refined previously identified coiled-coils and ASH (Aspm-SPD-2 Hydin) domains and identified new domains: a GEF domain, an Ig-like domain, and a PDZ-like domain. In addition to these predicted structural features, CeSPD-2 is also predicted to be intrinsically disordered. Surface electrostatic maps identified a large basic region unique to the ASH domain of CeSPD-2. This basic region overlaps with most of the residues predicted to be involved in protein-protein interactions. In vivo, ASH::GFP localized to centrosomes and centrosome-associated microtubules. Our analysis groups ASH domains, PapD, Usher chaperone domains, and Major Sperm Protein (MSP) domains into a single superfold within the larger Immunoglobulin superfamily. This study lays the groundwork for designing rational hypothesis-based experiments to uncover the mechanisms of CeSPD-2 function in vivo.Abbreviations: AIR, Aurora kinase; ASH, Aspm-SPD-2 Hydin; ASP, Abnormal Spindle Protein; ASPM, Abnormal Spindle-like Microcephaly-associated Protein; CC, coiled-coil; CDK, Cyclin-dependent Kinase; Ce, Caenorhabditis elegans; CEP, Centrosomal Protein; CPAP, centrosomal P4.1-associated protein; D, Drosophila; GAP, GTPase activating protein; GEF, GTPase guanine nucleotide exchange factor; Hs, Homo sapiens/Human; Ig, Immunoglobulin; MAP, Microtubule associated Protein; MSP, Major Sperm Protein; MDP, Major Sperm Domain-Containing Protein; OCRL-1, Golgi endocytic trafficking protein Inositol polyphosphate 5-phosphatase; PAR, abnormal embryonic PARtitioning of the cytosol; PCM, Pericentriolar material; PCMD, pericentriolar matrix deficient; PDZ, PSD95/Dlg-1/zo-1; PLK, Polo like kinase; RMSD, Root Mean Square Deviation; SAS, Spindle assembly abnormal proteins; SPD, Spindle-defective protein; TRAPP, TRAnsport Protein Particle; Xe, Xenopus; ZYG, zygote defective protein.

Duplication and Segregation of Centrosomes during Cell Division

During its division the cell must ensure the equal distribution of its genetic material in the two newly created cells, but it must also distribute organelles such as the Golgi apparatus, the mitochondria and the centrosome. DNA, the carrier of heredity, located in the nucleus of the cell, has made it possible to define the main principles that regulate the progression of the cell cycle. The cell cycle, which includes interphase and mitosis, is essentially a nuclear cycle, or a DNA cycle, since the interphase stages names (G1, S, G2) phases are based on processes that occur exclusively with DNA. However, centrosome duplication and segregation are two equally important events for the two new cells that must inherit a single centrosome. The centrosome, long considered the center of the cell, is made up of two small cylinders, the centrioles, made up of microtubules modified to acquire a very high stability. It is the main nucleation center of microtubules in the cell. Apart from a few exceptions, each cell in G1 phase has only one centrosome, consisting in of two centrioles and pericentriolar materials (PCM), which must be duplicated before the cell divides so that the two new cells formed inherit a single centrosome. The centriole is also the origin of the primary cilia, motile cilia and flagella of some cells.

Global cellular response to chemical perturbation of PLK4 activity and abnormal centrosome number

Centrosomes act as the main microtubule organizing center (MTOC) in metazoans. Centrosome number is tightly regulated by limiting centriole duplication to a single round per cell cycle. This control is achieved by multiple mechanisms, including the regulation of the protein kinase PLK4, the most upstream facilitator of centriole duplication. Altered centrosome numbers in mouse and human cells cause p53-dependent growth arrest through poorly defined mechanisms. Recent work has shown that the E3 ligase TRIM37 is required for cell cycle arrest in acentrosomal cells. To gain additional insights into this process, we undertook a series of genome-wide CRISPR/Cas9 screens to identify factors important for growth arrest triggered by treatment with centrinone B, a selective PLK4 inhibitor. We found that TRIM37 is a key mediator of growth arrest after partial or full PLK4 inhibition. Interestingly, PLK4 cellular mobility decreased in a dose-dependent manner after centrinone B treatment. In contrast to recent work, we found that growth arrest after PLK4 inhibition correlated better with PLK4 activity than with mitotic length or centrosome number. These data provide insights into the global response to changes in centrosome number and PLK4 activity and extend the role for TRIM37 in regulating the abundance, localization, and function of centrosome proteins.

Centrioles generate a local pulse of Polo/PLK1 activity to initiate mitotic centrosome assembly

Mitotic centrosomes are formed when centrioles start to recruit large amounts of pericentriolar material (PCM) around themselves in preparation for mitosis. This centrosome "maturation" requires the centrioles and also Polo/PLK1 protein kinase. The PCM comprises several hundred proteins and, in Drosophila, Polo cooperates with the conserved centrosome proteins Spd-2/CEP192 and Cnn/CDK5RAP2 to assemble a PCM scaffold around the mother centriole that then recruits other PCM client proteins. We show here that in Drosophila syncytial blastoderm embryos, centrosomal Polo levels rise and fall during the assembly process-peaking, and then starting to decline, even as levels of the PCM scaffold continue to rise and plateau. Experiments and mathematical modelling indicate that a centriolar pulse of Polo activity, potentially generated by the interaction between Polo and its centriole receptor Ana1 (CEP295 in humans), could explain these unexpected scaffold assembly dynamics. We propose that centrioles generate a local pulse of Polo activity prior to mitotic entry to initiate centrosome maturation, explaining why centrioles and Polo/PLK1 are normally essential for this process.

A modified TurboID approach identifies tissue-specific centriolar components in C. elegans

Proximity-dependent labeling approaches such as BioID have been a great boon to studies of protein-protein interactions in the context of cytoskeletal structures such as centrosomes which are poorly amenable to traditional biochemical approaches like immunoprecipitation and tandem affinity purification. Yet, these methods have so far not been applied extensively to invertebrate experimental models such as C. elegans given the long labeling times required for the original promiscuous biotin ligase variant BirA*. Here, we show that the recently developed variant TurboID successfully probes the interactomes of both stably associated (SPD-5) and dynamically localized (PLK-1) centrosomal components. We further develop an indirect proximity labeling method employing a GFP nanobody-TurboID fusion, which allows the identification of protein interactors in a tissue-specific manner in the context of the whole animal. Critically, this approach utilizes available endogenous GFP fusions, avoiding the need to generate multiple additional strains for each target protein and the potential complications associated with overexpressing the protein from transgenes. Using this method, we identify homologs of two highly conserved centriolar components, Cep97 and BLD10/Cep135, which are present in various somatic tissues of the worm. Surprisingly, neither protein is expressed in early embryos, likely explaining why these proteins have escaped attention until now. Our work expands the experimental repertoire for C. elegans and opens the door for further studies of tissue-specific variation in centrosome architecture.

Structures of SAS-6 coiled coil hold implications for the polarity of the centriolar cartwheel

Centrioles are eukaryotic organelles that template the formation of cilia and flagella, as well as organize the microtubule network and the mitotic spindle in animal cells. Centrioles have proximal-distal polarity and a 9-fold radial symmetry imparted by a likewise symmetrical central scaffold, the cartwheel. The spindle assembly abnormal protein 6 (SAS-6) self-assembles into 9-fold radially symmetric ring-shaped oligomers that stack via an unknown mechanism to form the cartwheel. Here, we uncover a homo-oligomerization interaction mediated by the coiled-coil domain of SAS-6. Crystallographic structures of Chlamydomonas reinhardtii SAS-6 coiled-coil complexes suggest this interaction is asymmetric, thereby imparting polarity to the cartwheel. Using a cryoelectron microscopy (cryo-EM) reconstitution assay, we demonstrate that amino acid substitutions disrupting this asymmetric association also impair SAS-6 ring stacking. Our work raises the possibility that the asymmetric interaction inherent to SAS-6 coiled-coil provides a polar element for cartwheel assembly, which may assist the establishment of the centriolar proximal-distal axis.

Cell cycle arrest induced by trichoplein depletion is independent of cilia assembly

Cilia assembly and centriole duplication are closely coordinated with cell cycle progression, and inhibition of cilia disassembly impedes cell cycle progression. The centrosomal protein trichoplein (TCHP) has been shown to promote cell cycle progression in the G₁ -S phase by disassembling cilia. In this study, we showed that deletion of TCHP not only prevented the progression to the S phase but also resulted in cell cycle exit and entrance into G₀ phase. Surprisingly, we found that loss of TCHP-induced G₀ arrest could not be reversed by blocking the assembly of cilia. In cells without IFT20 or CEP164, two genes encoding key factors for ciliogenesis, depletion of TCHP still led to G₀ arrest. Mechanistically, we also found that TCHP depletion-induced cell cycle arrest was not mediated through a centrosome surveillance mechanism, but inhibition of Rb or concomitant inhibition of both Rb and p53 signaling pathways was required to reverse the cell cycle phenotype. In conclusion, our study provides new insights into the function of TCHP in cell cycle progression.

Daughter centrioles assemble preferentially towards the nuclear envelope in Drosophila syncytial embryos

Centrosomes are important organizers of microtubules within animal cells. They comprise a pair of centrioles surrounded by the pericentriolar material, which nucleates and organizes the microtubules. To maintain centrosome numbers, centrioles must duplicate once and only once per cell cycle. During S-phase, a single new ‘daughter’ centriole is built orthogonally on one side of each radially symmetric ‘mother’ centriole. Mis-regulation of duplication can result in the simultaneous formation of multiple daughter centrioles around a single mother centriole, leading to centrosome amplification, a hallmark of cancer. It remains unclear how a single duplication site is established. It also remains unknown whether this site is pre-defined or randomly positioned around the mother centriole. Here, we show that within Drosophila syncytial embryos daughter centrioles preferentially assemble on the side of the mother facing the nuclear envelope, to which the centrosomes are closely attached. This positional preference is established early during duplication and remains stable throughout daughter centriole assembly, but is lost in centrosomes forced to lose their connection to the nuclear envelope. This shows that non-centrosomal cues influence centriole duplication and raises the possibility that these external cues could help establish a single duplication site.

Nine-fold symmetry of centriole: The joint efforts of its core proteins

The centriole is a widely conserved organelle required for the assembly of centrosomes, cilia, and flagella. Its striking feature - the nine-fold symmetrical structure, was discovered over 70 years ago by transmission electron microscopy, and since elaborated mostly by cryo-electron microscopy and super-resolution microscopy. Here, we review the discoveries that led to the current understanding of how the nine-fold symmetrical structure is built. We focus on the recent findings of the centriole structure in high resolution, its assembly pathways, and its nine-fold distributed components. We propose a model that the assembly of the nine-fold symmetrical centriole depends on the concerted efforts of its core proteins.

Structural and Functional Insights into the Microtubule Organizing Centers of Toxoplasma gondii and Plasmodium spp

Microtubule organizing centers (MTOCs) perform critical cellular tasks by nucleating, stabilizing, and anchoring microtubule's minus ends. These capacities impact tremendously a wide array of cellular functions ranging from ascribing cell shape to orchestrating cell division and generating motile structures, among others. The phylum Apicomplexa comprises over 6000 single-celled obligate intracellular parasitic species. Many of the apicomplexan are well known pathogens such as Toxoplasma gondii and the Plasmodium species, causative agents of toxoplasmosis and malaria, respectively. Microtubule organization in these parasites is critical for organizing the cortical cytoskeleton, enabling host cell penetration and the positioning of large organelles, driving cell division and directing the formation of flagella in sexual life stages. Apicomplexans are a prime example of MTOC diversity displaying multiple functional and structural MTOCs combinations within a single species. This diversity can only be fully understood in light of each organism's specific MT nucleation requirements and their evolutionary history. Insight into apicomplexan MTOCs had traditionally been limited to classical ultrastructural work by transmission electron microscopy. However, in the past few years, a large body of molecular insight has emerged. In this work we describe the latest insights into nuclear MTOC biology in two major human and animal disease causing Apicomplexans: Toxoplasma gondii and Plasmodium spp.

Sub-centrosomal mapping identifies augmin-γTuRC as part of a centriole-stabilizing scaffold

Centriole biogenesis and maintenance are crucial for cells to generate cilia and assemble centrosomes that function as microtubule organizing centers (MTOCs). Centriole biogenesis and MTOC function both require the microtubule nucleator γ-tubulin ring complex (γTuRC). It is widely accepted that γTuRC nucleates microtubules from the pericentriolar material that is associated with the proximal part of centrioles. However, γTuRC also localizes more distally and in the centriole lumen, but the significance of these findings is unclear. Here we identify spatially and functionally distinct subpopulations of centrosomal γTuRC. Luminal localization is mediated by augmin, which is linked to the centriole inner scaffold through POC5. Disruption of luminal localization impairs centriole integrity and interferes with cilium assembly. Defective ciliogenesis is also observed in γTuRC mutant fibroblasts from a patient suffering from microcephaly with chorioretinopathy. These results identify a non-canonical role of augmin-γTuRC in the centriole lumen that is linked to human disease.

PCMD-1 bridges the centrioles and the pericentriolar material scaffold in C. elegans

Correct cell division relies on the formation of a bipolar spindle. In animal cells, microtubule nucleation at the spindle poles is facilitated by the pericentriolar material (PCM), which assembles around a pair of centrioles. Although centrioles are essential for PCM assembly, the proteins that anchor the PCM to the centrioles are less known. Here, we investigate the molecular function of PCMD-1 in bridging the PCM and the centrioles in Caenorhabditis elegans. We demonstrate that the centrosomal recruitment of PCMD-1 is dependent on the outer centriolar protein SAS-7. The most C-terminal part of PCMD-1 is sufficient to target it to the centrosome, and the coiled-coil domain promotes its accumulation by facilitating self-interaction. We reveal that PCMD-1 interacts with the PCM scaffold protein SPD-5, the mitotic kinase PLK-1 and the centriolar protein SAS-4. Using an ectopic translocation assay, we show that PCMD-1 can selectively recruit downstream PCM scaffold components to an ectopic location in the cell, indicating that PCMD-1 is able to anchor the PCM scaffold proteins at the centrioles. Our work suggests that PCMD-1 is an essential functional bridge between the centrioles and the PCM.

Plk4 triggers autonomous de novo centriole biogenesis and maturation

Centrioles form centrosomes and cilia. In most proliferating cells, centrioles assemble through canonical duplication, which is spatially, temporally, and numerically regulated by the cell cycle and the presence of mature centrioles. However, in certain cell types, centrioles assemble de novo, yet by poorly understood mechanisms. Herein, we established a controlled system to investigate de novo centriole biogenesis, using Drosophila melanogaster egg explants overexpressing Polo-like kinase 4 (Plk4), a trigger for centriole biogenesis. We show that at a high Plk4 concentration, centrioles form de novo, mature, and duplicate, independently of cell cycle progression and of the presence of other centrioles. Plk4 concentration determines the temporal onset of centriole assembly. Moreover, our results suggest that distinct biochemical kinetics regulate de novo and canonical biogenesis. Finally, we investigated which other factors modulate de novo centriole assembly and found that proteins of the pericentriolar material (PCM), and in particular γ-tubulin, promote biogenesis, likely by locally concentrating critical components.

From tip to toe - dressing centrioles in γTuRC

Centrioles are microtubule-based cylindrical structures that assemble the centrosome and template the formation of cilia. The proximal part of centrioles is associated with the pericentriolar material, a protein scaffold from which microtubules are nucleated. This activity is mediated by the γ-tubulin ring complex (γTuRC) whose central role in centrosomal microtubule organization has been recognized for decades. However, accumulating evidence suggests that γTuRC activity at this organelle is neither restricted to the pericentriolar material nor limited to microtubule nucleation. Instead, γTuRC is found along the entire centriole cylinder, at subdistal appendages, and inside the centriole lumen, where its canonical function as a microtubule nucleator might be supplemented or replaced by a function in microtubule anchoring and centriole stabilization, respectively. In this Opinion, we discuss recent insights into the expanded repertoire of γTuRC activities at centrioles and how distinct subpopulations of γTuRC might act in concert to ensure centrosome and cilia biogenesis and function, ultimately supporting cell proliferation, differentiation and homeostasis. We propose that the classical view of centrosomal γTuRC as a pericentriolar material-associated microtubule nucleator needs to be revised.

Tuning SAS-6 architecture with monobodies impairs distinct steps of centriole assembly

Centrioles are evolutionarily conserved multi-protein organelles essential for forming cilia and centrosomes. Centriole biogenesis begins with self-assembly of SAS-6 proteins into 9-fold symmetrical ring polymers, which then stack into a cartwheel that scaffolds organelle formation. The importance of this architecture has been difficult to decipher notably because of the lack of precise tools to modulate the underlying assembly reaction. Here, we developed monobodies against Chlamydomonas reinhardtii SAS-6, characterizing three in detail with X-ray crystallography, atomic force microscopy and cryo-electron microscopy. This revealed distinct monobody-target interaction modes, as well as specific consequences on ring assembly and stacking. Of particular interest, monobody MB_CRS6-15 induces a conformational change in CrSAS-6, resulting in the formation of a helix instead of a ring. Furthermore, we show that this alteration impairs centriole biogenesis in human cells. Overall, our findings identify monobodies as powerful molecular levers to alter the architecture of multi-protein complexes and tune centriole assembly.

An acentriolar centrosome at the C. elegans ciliary base

In animal cells, the functions of the microtubule cytoskeleton are coordinated by centriole-based centrosomes via γ-tubulin complexes embedded in the pericentriolar material or PCM.¹ PCM assembly has been best studied in the context of mitosis, where centriolar SPD-2 recruits PLK-1, which in turn phosphorylates key scaffolding components like SPD-5 and CNN to promote expansion of the PCM polymer.^2-4 To what extent these mechanisms apply to centrosomes in interphase or in differentiated cells remains unclear.⁵ Here, we examine a novel type of centrosome found at the ciliary base of C. elegans sensory neurons, which we show plays important roles in neuronal morphogenesis, cellular trafficking, and ciliogenesis. These centrosomes display similar dynamic behavior to canonical, mitotic centrosomes, with a stable PCM scaffold and dynamically localized client proteins. Unusually, however, they are not organized by centrioles, which degenerate early in terminal differentiation.⁶ Yet, PCM not only persists but continues to grow with key scaffolding proteins including SPD-5 expressed under control of the RFX transcription factor DAF-19. This assembly occurs in the absence of the mitotic regulators SPD-2, AIR-1 and PLK-1, but requires tethering by PCMD-1, a protein which also plays a role in the initial, interphase recruitment of PCM in early embryos.⁷ These results argue for distinct mechanisms for mitotic and non-mitotic PCM assembly, with only the former requiring PLK-1 phosphorylation to drive rapid expansion of the scaffold polymer.

Centrosomes: An acentriolar MTOC at the ciliary base

Centrioles are microtubule-based organelles that are embedded within pericentriolar material (PCM). Together, they comprise the centrosome, a microtubule-organizing center. PCM can sometimes exist in the absence of centrioles, but a new example of acentriolar PCM in neurons offers deeper insight into the relationship between these two entities.

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.

PLK4-phosphorylated NEDD1 facilitates cartwheel assembly and centriole biogenesis initiations

Centrosome duplication occurs under strict spatiotemporal regulation once per cell cycle, and it begins with cartwheel assembly and daughter centriole biogenesis at the lateral sites of the mother centrioles. However, although much of this process is understood, how centrosome duplication is initiated remains unclear. Here, we show that cartwheel assembly followed by daughter centriole biogenesis is initiated on the NEDD1-containing layer of the pericentriolar material (PCM) by the recruitment of SAS-6 to the mother centriole under the regulation of PLK4. We found that PLK4-mediated phosphorylation of NEDD1 at its S325 amino acid residue directly promotes both NEDD1 binding to SAS-6 and recruiting SAS-6 to the centrosome. Overexpression of phosphomimicking NEDD1 mutant S325E promoted cartwheel assembly and daughter centriole biogenesis initiations, whereas overexpression of nonphosphorylatable NEDD1 mutant S325A abolished the initiations. Collectively, our results demonstrate that PLK4-regulated NEDD1 facilitates initiation of the cartwheel assembly and of daughter centriole biogenesis in mammals.

Dynein-dynactin segregate meiotic chromosomes in C. elegans spermatocytes

The microtubule motor cytoplasmic dynein 1 (dynein) and its essential activator dynactin have conserved roles in spindle assembly and positioning during female meiosis and mitosis, but their contribution to male meiosis remains poorly understood. Here, we characterize the G33S mutation in the C. elegans dynactin subunit DNC-1, which corresponds to G59S in human p150^Glued that causes motor neuron disease. In spermatocytes, dnc-1(G33S) delays spindle assembly and penetrantly inhibits anaphase spindle elongation in meiosis I, which prevents the segregation of homologous chromosomes. By contrast, chromosomes segregate without errors in the early dnc-1(G33S) embryo. Deletion of the DNC-1 N-terminus shows that defective meiosis in dnc-1(G33S) spermatocytes is not due to the inability of DNC-1 to interact with microtubules. Instead, our results suggest that the DNC-1(G33S) protein, which is aggregation prone in vitro, is less stable in spermatocytes than the early embryo, resulting in different phenotypic severity in the two dividing tissues. Thus, the dnc-1(G33S) mutant reveals that dynein-dynactin drive meiotic chromosome segregation in spermatocytes and illustrates that the extent to which protein misfolding leads to loss of function can vary significantly between cell types.

Polo-like kinase 1 independently controls microtubule-nucleating capacity and size of the centrosome

Centrosomes are composed of a centriolar core surrounded by a pericentriolar material (PCM) matrix that docks microtubule-nucleating γ-tubulin complexes. During mitotic entry, the PCM matrix increases in size and nucleating capacity in a process called centrosome maturation. Polo-like kinase 1 (PLK1) is recruited to centrosomes and phosphorylates PCM matrix proteins to drive their self-assembly, which leads to PCM expansion. Here, we show that in addition to controlling PCM expansion, PLK1 independently controls the generation of binding sites for γ-tubulin complexes on the PCM matrix. Selectively preventing the generation of PLK1-dependent γ-tubulin docking sites led to spindle defects and impaired chromosome segregation without affecting PCM expansion, highlighting the importance of phospho-regulated centrosomal γ-tubulin docking sites in spindle assembly. Inhibiting both γ-tubulin docking and PCM expansion by mutating substrate target sites recapitulated the effects of loss of centrosomal PLK1 on the ability of centrosomes to catalyze spindle assembly.

Identification of compounds that bind the centriolar protein SAS-6 and inhibit its oligomerization

Centrioles are key eukaryotic organelles that are responsible for the formation of cilia and flagella, and for organizing the microtubule network and the mitotic spindle in animals. Centriole assembly requires oligomerization of the essential protein spindle assembly abnormal 6 (SAS-6), which forms a structural scaffold templating the organization of further organelle components. A dimerization interaction between SAS-6 N-terminal "head" domains was previously shown to be essential for protein oligomerization in vitro and for function in centriole assembly. Here, we developed a pharmacophore model allowing us to assemble a library of low-molecular-weight ligands predicted to bind the SAS-6 head domain and inhibit protein oligomerization. We demonstrate using NMR spectroscopy that a ligand from this family binds at the head domain dimerization site of algae, nematode, and human SAS-6 variants, but also that another ligand specifically recognizes human SAS-6. Atomistic molecular dynamics simulations starting from SAS-6 head domain crystallographic structures, including that of the human head domain which we now resolve, suggest that ligand specificity derives from favorable Van der Waals interactions with a hydrophobic cavity at the dimerization site.

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.

Principal Postulates of Centrosomal Biology. Version 2020

The centrosome, which consists of two centrioles surrounded by pericentriolar material, is a unique structure that has retained its main features in organisms of various taxonomic groups from unicellular algae to mammals over one billion years of evolution. In addition to the most noticeable function of organizing the microtubule system in mitosis and interphase, the centrosome performs many other cell functions. In particular, centrioles are the basis for the formation of sensitive primary cilia and motile cilia and flagella. Another principal function of centrosomes is the concentration in one place of regulatory proteins responsible for the cell's progression along the cell cycle. Despite the existing exceptions, the functioning of the centrosome is subject to general principles, which are discussed in this review.

Novel features of centriole polarity and cartwheel stacking revealed by cryo-tomography

Centrioles are polarized microtubule‐based organelles that seed the formation of cilia, and which assemble from a cartwheel containing stacked ring oligomers of SAS‐6 proteins. A cryo‐tomography map of centrioles from the termite flagellate Trichonympha spp. was obtained previously, but higher resolution analysis is likely to reveal novel features. Using sub‐tomogram averaging (STA) in T. spp. and Trichonympha agilis, we delineate the architecture of centriolar microtubules, pinhead, and A‐C linker. Moreover, we report ~25 Å resolution maps of the central cartwheel, revealing notably polarized cartwheel inner densities (CID). Furthermore, STA of centrioles from the distant flagellate Teranympha mirabilis uncovers similar cartwheel architecture and a distinct filamentous CID. Fitting the CrSAS‐6 crystal structure into the flagellate maps and analyzing cartwheels generated in vitro indicate that SAS‐6 rings can directly stack onto one another in two alternating configurations: with a slight rotational offset and in register. Overall, improved STA maps in three flagellates enabled us to unravel novel architectural features, including of centriole polarity and cartwheel stacking, thus setting the stage for an accelerated elucidation of underlying assembly mechanisms.

The centriolar cartwheel structure: symmetric, stacked, and polarized

An accurate centriolar structure is crucial for organelle function, necessitating the existence of molecular mechanisms for the tight control of centriole assembly. Formation of an initial scaffold, the cartwheel, assists the correct placement of centriolar proteins during assembly and templates key structural parameters of the organelle. Past work illustrated how cartwheel and centriolar symmetry are linked, and grounded organelle symmetry and diameter to the properties of the centriolar protein SAS-6. However, questions remained over how centriole polarity and length are controlled. Recent advances in resolving cartwheel structure and cell biology showed that these assemblies are polarized and that their length is under the control of a homeostatic mechanism. These cartwheel properties may, in turn, influence the centriolar polarity and length.

Tissue specific requirement of Drosophila Rcd4 for centriole duplication and ciliogenesis

Rcd4 is a poorly characterized Drosophila centriole component whose mammalian counterpart, PPP1R35, is suggested to function in centriole elongation and conversion to centrosomes. Here, we show that rcd4 mutants exhibit fewer centrioles, aberrant mitoses, and reduced basal bodies in sensory organs. Rcd4 interacts with the C-terminal part of Ana3, which loads onto the procentriole during interphase, ahead of Rcd4 and before mitosis. Accordingly, depletion of Ana3 prevents Rcd4 recruitment but not vice versa. We find that neither Ana3 nor Rcd4 participates directly in the mitotic conversion of centrioles to centrosomes, but both are required to load Ana1, which is essential for such conversion. Whereas ana3 mutants are male sterile, reflecting a requirement for Ana3 for centriole development in the male germ line, rcd4 mutants are fertile and have male germ line centrioles of normal length. Thus, Rcd4 is essential in somatic cells but is not absolutely required in spermatogenesis, indicating tissue-specific roles in centriole and basal body formation.

Loss of CPAP in developing mouse brain and its functional implication for human primary microcephaly

Primary microcephaly (MCPH) is a neurodevelopmental disorder characterized by small brain size with mental retardation. CPAP (also known as CENPJ), a known microcephaly-associated gene, plays a key role in centriole biogenesis. Here, we generated a previously unreported conditional knockout allele in the mouse Cpap gene. Our results showed that conditional Cpap deletion in the central nervous system preferentially induces formation of monopolar spindles in radial glia progenitors (RGPs) at around embryonic day 14.5 and causes robust apoptosis that severely disrupts embryonic brains. Interestingly, microcephalic brains with reduced apoptosis are detected in conditional Cpap gene-deleted mice that lose only one allele of p53 (also known as Trp53), while simultaneous removal of p53 and Cpap rescues RGP death. Furthermore, Cpap deletion leads to cilia loss, RGP mislocalization, junctional integrity disruption, massive heterotopia and severe cerebellar hypoplasia. Together, these findings indicate that complete CPAP loss leads to severe and complex phenotypes in developing mouse brain, and provide new insights into the causes of MCPH.

Drosophila Sas-6, Ana2 and Sas-4 self-organise into macromolecular structures that can be used to probe centriole and centrosome assembly

Centriole assembly requires a small number of conserved proteins. The precise pathway of centriole assembly has been difficult to study, as the lack of any one of the core assembly proteins [Plk4, Ana2 (the homologue of mammalian STIL), Sas-6, Sas-4 (mammalian CPAP) or Asl (mammalian Cep152)] leads to the absence of centrioles. Here, we use Sas-6 and Ana2 particles (SAPs) as a new model to probe the pathway of centriole and centrosome assembly. SAPs form in Drosophila eggs or embryos when Sas-6 and Ana2 are overexpressed. SAP assembly requires Sas-4, but not Plk4, whereas Asl helps to initiate SAP assembly but is not required for SAP growth. Although not centrioles, SAPs recruit and organise many centriole and centrosome components, nucleate microtubules, organise actin structures and compete with endogenous centrosomes to form mitotic spindle poles. SAPs require Asl to efficiently recruit pericentriolar material (PCM), but Spd-2 (the homologue of mammalian Cep192) can promote some PCM assembly independently of Asl. These observations provide new insights into the pathways of centriole and centrosome assembly.