Stabilization of cartwheel-less centrioles for duplication requires CEP295-mediated centriole-to-centrosome conversion

Drosophila Alms1 proteins regulate centriolar cartwheel assembly by enabling Plk4-Ana2 amplification loop

Centrioles play a central role in cell division by recruiting pericentriolar material (PCM) to form the centrosome. Alterations in centriole number or function lead to various diseases including cancer or microcephaly. Centriole duplication is a highly conserved mechanism in eukaryotes. Here, we show that the two Drosophila orthologs of the Alström syndrome protein 1 (Alms1a and Alms1b) are unexpected novel players of centriole duplication in fly. Using Ultrastructure Expansion Microscopy, we reveal that Alms1a is a PCM protein that is loaded proximally on centrioles at the onset of procentriole formation, whereas Alms1b caps the base of mature centrioles. We demonstrate that chronic loss of Alms1 proteins (with RNA null alleles) affects PCM maturation, whereas their acute loss (in RNAi KD) completely disrupts procentriole formation before Sas-6 cartwheel assembly. We establish that Alms1 proteins are required for the amplification of the Plk4-Ana2 pool at the duplication site and the subsequent Sas-6 recruitment. Thus, Alms1 proteins are novel critical but highly buffered regulators of PCM and cartwheel assembly in flies.

The conserved Spd-2/CEP192 domain adopts a unique protein fold to promote centrosome scaffold assembly

Centrosomes form when centrioles assemble pericentriolar material (PCM) around themselves. Spd-2/CEP192 proteins, defined by a conserved "Spd-2 domain" (SP2D) comprising two closely spaced AspM-Spd-2-Hydin (ASH) domains, play a critical role in centrosome assembly. Here, we show that the SP2D does not target Drosophila Spd-2 to centrosomes but rather promotes PCM scaffold assembly. Crystal structures of the human and honeybee SP2D reveal an unusual "extended cradle" structure mediated by a conserved interaction interface between the two ASH domains. Mutations predicted to perturb this interface, including a human mutation associated with male infertility and Mosaic Variegated Aneuploidy, disrupt PCM scaffold assembly in flies. The SP2D is monomeric in solution, but the Drosophila SP2D can form higher-order oligomers upon phosphorylation by PLK1 (Polo-like kinase 1). Crystal-packing interactions and AlphaFold predictions suggest how SP2Ds might self-assemble, and mutations associated with one such potential dimerization interface markedly perturb SP2D oligomerization in vitro and PCM scaffold assembly in vivo.

A delta-tubulin/epsilon-tubulin/Ted protein complex is required for centriole architecture

Centrioles have a unique, conserved architecture formed by three linked, 'triplet', microtubules arranged in ninefold symmetry. The mechanisms by which these triplet microtubules are formed remain unclear but likely involve the noncanonical tubulins delta-tubulin and epsilon-tubulin. Previously, we found that human cells lacking delta-tubulin or epsilon-tubulin form abnormal centrioles, characterized by an absence of triplet microtubules, lack of central core protein POC5, and a futile cycle of centriole formation and disintegration (Wang et al., 2017). Here, we show that human cells lacking either TEDC1 or TEDC2 have similar abnormalities. Using ultrastructure expansion microscopy, we observed that mutant centrioles elongate to the same length as control centrioles in G2 phase and fail to recruit central core scaffold proteins. Remarkably, mutant centrioles also have an expanded proximal region. During mitosis, these mutant centrioles further elongate before fragmenting and disintegrating. All four proteins physically interact and TEDC1 and TEDC2 can form a subcomplex in the absence of the tubulins, supporting an AlphaFold Multimer model of the tetramer. TEDC1 and TEDC2 localize to centrosomes and are mutually dependent on each other and on delta-tubulin and epsilon-tubulin for localization. Our results demonstrate that delta-tubulin, epsilon-tubulin, TEDC1, and TEDC2 function together to promote robust centriole architecture, laying the foundation for future studies on the mechanisms underlying the assembly of triplet microtubules and their interactions with centriole structure.

Multimodal mechanisms of human centriole engagement and disengagement

Centrioles are unique cellular structures that replicate to produce identical copies, ensuring accurate chromosome segregation during mitosis. A new centriole, the "daughter", is assembled adjacent to an existing "mother" centriole. Only after the daughter centriole is fully developed as a complete replica, does it disengage and become the core of a new functional centrosome. The mechanisms preventing precocious disengagement of the immature daughter centriole have remained unclear. Here, we identify three key mechanisms maintaining mother-daughter centriole engagement: the cartwheel, the torus, and the pericentriolar material (PCM). Among these, the torus critically establishes the characteristic orthogonal engagement. We also demonstrate that engagement mediated by the cartwheel and torus is progressively released during centriole maturation. This release involves structural changes in the daughter, known as centriole blooming and distancing, respectively. Disrupting these structural transitions blocks subsequent steps, preventing centriole disengagement and centrosome conversion in the G1 phase. This study provides a comprehensive understanding of how the maturing daughter centriole progressively disengages from its mother through multiple steps, ensuring its complete structure and conversion into an independent centrosome.

Centrosome biogenesis and maintenance in homeostasis and disease

Recent technological advances in proteomics and microscopy techniques, such as cryo-electron microscopy (cryoEM) and expansion microscopy (ExM), have enhanced our understanding of centrosome structure, biogenesis, and regulation. Here we discuss new insights into centrosome structure, highlight new regulatory mechanisms in centrosome biogenesis, and explore emerging concepts in centrosome maintenance and plasticity across different contexts. Furthermore, we review how centrosome biogenesis and homeostasis are dysregulated in various pathological conditions. We finalise by outlining outstanding questions in the field, how the mechanisms discussed are regulated across multiple contexts, the balance between centriole stability and plasticity, and the therapeutic potential of targeting centrosome dysfunction in disease.

A novel homozygous missense variant in POC1B causes cone dystrophy in a consanguineous Pakistani family

BACKGROUND

Cone dystrophy is a heterogeneous hereditary retinal disorder with disease symptoms appearing in the late first or early second decades of life.

METHODS

A consanguineous Pakistani family with three affected individuals underwent detailed clinical and genetic investigation.

RESULTS

The proband, a 63-years old male, showed severely reduced day vision, a visual acuity of counting fingers (CF), color vision deficiency, high myopia and photophobia. Fundus images showed bilateral peripapillary atrophy, bilateral dull foveal reflex, tilted disc, tessellated fundus, and hyperfluorescence at the peripheral superior temporal arcade and leak at an early stage. OCT macula and angiography findings suggested traction near the disc in the right eye and sub-retinal fluid at the fovea in the left eye. Retinal layers were normal toward the periphery but disorganized near the disc. Full visual field tests showed bilateral central scotoma, while single visual field tests indicated bilateral generalized depression of the visual field. The proband showed normal bilateral intraocular pressure, normal choroidal vessels, and unremarkable anterior segment. Exome sequencing identified a novel homozygous missense variant (POC1B:NM_172240.3:c.1391T>C;p.L464P) in the proband. Existing evidence supported pathogenicity of the identified variant in the family.

CONCLUSION

In conclusion, we document a first-ever Pakistani family with POC1B-related cone dystrophy.

Interactions of N- and C-terminal parts of Ana1 permitting centriole duplication but not elongation

The conserved process of centriole duplication requires the establishment of a Sas6-centred cartwheel initiated by Plk4's phosphorylation of Ana1/STIL. Subsequently, the centriole undergoes conversion to a centrosome requiring its radial expansion and elongation, mediated by a network requiring interactions between Cep135, Ana1/Cep295 and Asterless/Cep152. Here, we show that mutant alleles encoding overlapping N- and C-terminal parts of Ana1 are capable of intragenic complementation to rescue radial expansion. This permits the recruitment of Asl and thereby centriole duplication and mechanosensory cilia formation to restore the coordination defects of these mutants. This genetic combination also rescues centriole duplication in the male germ line but does not rescue the elongation of the triplet microtubule-containing centrioles of primary spermatocytes. Consequently, these males are coordinated but sterile. Such centriole elongation is rescued by the continuous, full-length Ana1 sequence. We define a region that when deleted within otherwise intact Ana1 does not permit primary spermatocyte centrioles to elongate but still allows recruitment of Asl. Our findings point to differing demands upon the physical organization of Ana1 for the distinct processes of radial expansion and elongation of centrioles.

Permanent cilia loss during cerebellar granule cell neurogenesis involves withdrawal of cilia maintenance and centriole capping

Brain neurons utilize the primary cilium as a privileged compartment to detect and respond to extracellular ligands such as Sonic hedgehog (SHH). However, cilia in cerebellar granule cell (GC) neurons disassemble during differentiation through ultrastructurally unique intermediates, a process we refer to as cilia deconstruction. In addition, mature neurons do not reciliate despite having docked centrioles. Here, we identify molecular changes that accompany cilia deconstruction and centriole docking in GC neurons. We used single cell transcriptomic and immunocytological analyses to compare the transcript levels and subcellular localization of proteins between progenitor, differentiating, and mature GCs. Differentiating GCs lacked transcripts for key activators of premitotic cilia resorption, indicating that cilia disassembly in differentiating cells is distinct from premitotic cilia resorption. Instead, during differentiation, transcripts of many genes required for cilia maintenance-specifically those encoding components of intraflagellar transport, pericentrosomal material, and centriolar satellites-decreased. The abundance of several corresponding proteins in and around cilia and centrosomes also decreased. These changes coincided with downregulation of SHH signaling prior to differentiation, even in a mutant with excessive SHH activation. Finally, mother centrioles in maturing granule neurons recruited the cap complex protein, CEP97. These data suggest that a global, developmentally programmed decrease in cilium maintenance in differentiating GCs mediates cilia deconstruction, while capping of docked mother centrioles prevents cilia regrowth and dysregulated SHH signaling. Our study provides mechanistic insights expanding our understanding of permanent cilia loss in multiple tissue-specific contexts.

CDK5RAP2 activates microtubule nucleator γTuRC by facilitating template formation and actin release

To organize microtubules, cells tightly control the activity of the microtubule nucleator γ-tubulin ring complex (γTuRC). The open ring-shaped γTuRC was proposed to nucleate microtubules by a template mechanism. However, its splayed structure does not match microtubule symmetry, leaving it unclear how γTuRC becomes an efficient nucleator. Here, we identify the mechanism of γTuRC activation by CDK5RAP2 centrosomin motif 1 (CM1). Using cryoelectron microscopy (cryo-EM), we find that activation involves binding of multiple CM1 dimers to five distinct sites around the outside of the γTuRC cone, which crucially depends on regulatory modules formed by MZT2 and the N-terminal extensions of GCP2 subunits. CM1 binding promotes lateral interactions between GCP subunits to facilitate microtubule-like conformations and release of luminal actin that is integral to non-activated γTuRC. We propose a model where generation of γTuRC with an expanded conformational range, rather than perfect symmetry, is sufficient to boost nucleation activity.

An interaction network of inner centriole proteins organised by POC1A-POC1B heterodimer crosslinks ensures centriolar integrity

Centriole integrity, vital for cilia formation and chromosome segregation, is crucial for human health. The inner scaffold within the centriole lumen composed of the proteins POC1B, POC5 and FAM161A is key to this integrity. Here, we provide an understanding of the function of inner scaffold proteins. We demonstrate the importance of an interaction network organised by POC1A-POC1B heterodimers within the centriole lumen, where the WD40 domain of POC1B localises close to the centriole wall, while the POC5-interacting WD40 of POC1A resides in the centriole lumen. The POC1A-POC5 interaction and POC5 tetramerization are essential for inner scaffold formation and centriole stability. The microtubule binding proteins FAM161A and MDM1 by binding to POC1A-POC1B, likely positioning the POC5 tetramer near the centriole wall. Disruption of POC1A or POC1B leads to centriole microtubule defects and deletion of both genes causes centriole disintegration. These findings provide insights into organisation and function of the inner scaffold.

Interactions of N- and C-terminal parts of Ana1 permitting centriole duplication but not elongation

The conserved process of centriole duplication requires establishment of a Sas6-centred cartwheel initiated by Plk4's phosphorylation of Ana1/STIL. Subsequently the centriole undergoes conversion to a centrosome requiring its radial expansion and elongation, mediated by a network requiring interactions between Cep135, Ana1/Cep295, and Asterless/Cep152. Here we show that mutant alleles encoding overlapping N- and C-terminal parts of Ana1 are capable of intragenic complementation to rescue radial expansion. This permits recruitment of Asl and thereby centriole duplication and mechanosensory cilia formation to restore the coordination defects of these mutants. This genetic combination also rescues centriole duplication in the male germ line but does not rescue the elongation of the triplet microtubule-containing centrioles of primary spermatocytes and consequently these males are coordinated but sterile. Such centriole elongation is rescued by the continuous, full-length Ana1 sequence. We define a region that when deleted within otherwise intact Ana1 does not permit primary spermatocyte centrioles to elongate but still allows recruitment of Asl. Our findings point to differing demands upon the physical organization of Ana1 for the distinct processes of radial expansion and elongation of centrioles.

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.

Primary cilia formation requires the Leigh syndrome-associated mitochondrial protein NDUFAF2

Mitochondria-related neurodegenerative diseases have been implicated in the disruption of primary cilia function. Mutation in an intrinsic mitochondrial complex I component NDUFAF2 has been identified in Leigh syndrome, a severe inherited mitochondriopathy. Mutations in ARMC9, which encodes a basal body protein, cause Joubert syndrome, a ciliopathy with defects in the brain, kidney, and eye. Here, we report a mechanistic link between mitochondria metabolism and primary cilia signaling. We discovered that loss of NDUFAF2 caused both mitochondrial and ciliary defects in vitro and in vivo and identified NDUFAF2 as a binding partner for ARMC9. We also found that NDUFAF2 was both necessary and sufficient for cilia formation and that exogenous expression of NDUFAF2 rescued the ciliary and mitochondrial defects observed in cells from patients with known ARMC9 deficiency. NAD+ supplementation restored mitochondrial and ciliary dysfunction in ARMC9-deficient cells and zebrafish and ameliorated the ocular motility and motor deficits of a patient with ARMC9 deficiency. The present results provide a compelling mechanistic link, supported by evidence from human studies, between primary cilia and mitochondrial signaling. Importantly, our findings have significant implications for the development of therapeutic approaches targeting ciliopathies.

Antiparallel microtubule bundling supports KIF15-driven mitotic spindle assembly

The spindle is a bipolar microtubule-based machine that is crucial for accurate chromosome segregation. Spindle bipolarity is generated by Eg5 (a kinesin-5), a conserved motor that drives spindle assembly by localizing to and sliding apart antiparallel microtubules. In the presence of Eg5 inhibitors (K5Is), KIF15 (a kinesin-12) can promote spindle assembly, resulting in K5I-resistant cells (KIRCs). However, KIF15 is a less potent motor than Eg5, suggesting that other factors may contribute to spindle formation in KIRCs. Protein Regulator of Cytokinesis 1 (PRC1) preferentially bundles antiparallel microtubules, and we previously showed that PRC1 promotes KIF15-microtubule binding, leading us to hypothesize that PRC1 may enhance KIF15 activity in KIRCs. Here, we demonstrate that: 1) loss of PRC1 in KIRCs decreases spindle bipolarity, 2) overexpression of PRC1 increases spindle formation efficiency in KIRCs, 3) overexpression of PRC1 protects K5I naïve cells against the K5I S-trityl-L-cysteine (STLC), and 4) PRC1 overexpression promotes the establishment of K5I resistance. These effects are not fully reproduced by a TPX2, a microtubule bundler with no known preference for microtubule orientation. These results suggest a model wherein PRC1-mediated bundling of microtubules creates a more favorable microtubule architecture for KIF15-driven mitotic spindle assembly in the context of Eg5 inhibition.

9-fold symmetry is not essential for centriole elongation and formation of new centriole-like structures

As daughter centrioles assemble during G2, they recruit conserved Ana3/RTTN followed by its partner Rcd4/PPP1R35. Together, this contributes to the subsequent recruitment of Ana1/CEP295, required for the centriole's conversion to a centrosome. Here, we show that Rcd4/PPP1R35 is also required to maintain 9-fold centriole symmetry in the Drosophila male germline; its absence causes microtubule triplets to disperse into a reduced number of doublet or singlet microtubules. rcd4-null mutant spermatocytes display skinny centrioles that elongate normally and localize centriolar components correctly. Mutant spermatocytes also have centrioles of normal girth that splay at their proximal ends when induced to elongate by Ana1 overexpression. Skinny and splayed spermatid centrioles can still recruit a proximal centriole-like (PCL) structure marking a capability to initiate features of centriole duplication in developing sperm. Thus, stable 9-fold symmetry of microtubule triplets is not essential for centriole growth, correct longitudinal association of centriole components, and aspects of centriole duplication.

Ana1/CEP295 is an essential player in the centrosome maintenance program regulated by Polo kinase and the PCM

Centrioles are part of centrosomes and cilia, which are microtubule organising centres (MTOC) with diverse functions. Despite their stability, centrioles can disappear during differentiation, such as in oocytes, but little is known about the regulation of their structural integrity. Our previous research revealed that the pericentriolar material (PCM) that surrounds centrioles and its recruiter, Polo kinase, are downregulated in oogenesis and sufficient for maintaining both centrosome structural integrity and MTOC activity. We now show that the expression of specific components of the centriole cartwheel and wall, including ANA1/CEP295, is essential for maintaining centrosome integrity. We find that Polo kinase requires ANA1 to promote centriole stability in cultured cells and eggs. In addition, ANA1 expression prevents the loss of centrioles observed upon PCM-downregulation. However, the centrioles maintained by overexpressing and tethering ANA1 are inactive, unlike the MTOCs observed upon tethering Polo kinase. These findings demonstrate that several centriole components are needed to maintain centrosome structure. Our study also highlights that centrioles are more dynamic than previously believed, with their structural stability relying on the continuous expression of multiple components.

Bi-allelic variants in CEP295 cause Seckel-like syndrome presenting with primary microcephaly, developmental delay, intellectual disability, short stature, craniofacial and digital abnormalities

BACKGROUND

Pathogenic variants in the centrosome protein (CEP) family have been implicated in primary microcephaly, Seckel syndrome, and classical ciliopathies. However, most CEP genes remain unlinked to specific Mendelian genetic diseases in humans. We sought to explore the roles of CEP295 in human pathology.

METHODS

Whole-exome sequencing was performed to screen for pathogenic variants in patients with severe microcephaly. Patient-derived fibroblasts and CEP295-depleted U2OS and RPE1 cells were used to clarify the underlying pathomechanisms, including centriole/centrosome development, cell cycle and proliferation changes, and ciliogenesis. Complementary experiments using CEP295 mRNA were performed to determine the pathogenicity of the identified missense variant.

FINDINGS

Here, we report bi-allelic variants of CEP295 in four children from two unrelated families, characterized by severe primary microcephaly, short stature, developmental delay, intellectual disability, facial deformities, and abnormalities of fingers and toes, suggesting a Seckel-like syndrome. Mechanistically, depletion of CEP295 resulted in a decrease in the numbers of centrioles and centrosomes and triggered p53-dependent G1 cell cycle arrest. Moreover, loss of CEP295 causes extensive primary ciliary defects in both patient-derived fibroblasts and RPE1 cells. The results from complementary experiments revealed that the wild-type CEP295, but not the mutant protein, can correct the developmental defects of the centrosome/centriole and cilia in the patient-derived skin fibroblasts.

INTERPRETATION

This study reports CEP295 as a causative gene of the syndromic microcephaly phenotype in humans. Our study also demonstrates that defects in CEP295 result in primary ciliary defects.

FUNDING

A full list of funding bodies that contributed to this study can be found under "Acknowledgments."

Centrosome linker diversity and its function in centrosome clustering and mitotic spindle formation

The centrosome linker joins the two interphase centrosomes of a cell into one microtubule organizing center. Despite increasing knowledge on linker components, linker diversity in different cell types and their role in cells with supernumerary centrosomes remained unexplored. Here, we identified Ninein as a C-Nap1-anchored centrosome linker component that provides linker function in RPE1 cells while in HCT116 and U2OS cells, Ninein and Rootletin link centrosomes together. In interphase, overamplified centrosomes use the linker for centrosome clustering, where Rootletin gains centrosome linker function in RPE1 cells. Surprisingly, in cells with centrosome overamplification, C-Nap1 loss prolongs metaphase through persistent activation of the spindle assembly checkpoint indicated by BUB1 and MAD1 accumulation at kinetochores. In cells lacking C-Nap1, the reduction of microtubule nucleation at centrosomes and the delay in nuclear envelop rupture in prophase probably cause mitotic defects like multipolar spindle formation and chromosome mis-segregation. These defects are enhanced when the kinesin HSET, which normally clusters multiple centrosomes in mitosis, is partially inhibited indicating a functional interplay between C-Nap1 and centrosome clustering in mitosis.

Postmitotic centriole disengagement and maturation leads to centrosome amplification in polyploid trophoblast giant cells

DNA replication is normally coupled with centriole duplication in the cell cycle. Trophoblast giant cells (TGCs) of the placenta undergo endocycles resulting in polyploidy but their centriole state is not known. We used a cell culture model for TGC differentiation to examine centriole and centrosome number and properties. Before differentiation, trophoblast stem cells (TSCs) have either two centrioles before duplication or four centrioles after. We find that the average nuclear area increases approximately eight-fold over differentiation, but most TGCs do not have more than four centrioles. However, these centrioles become disengaged, acquire centrosome proteins, and can nucleate microtubules. In addition, some TGCs undergo further duplication and disengagement of centrioles, resulting in substantially higher numbers. Live imaging revealed that disengagement and separation are centriole autonomous and can occur asynchronously. Centriole amplification, when present, occurs by the standard mechanism of one centriole generating one procentriole. PLK4 inhibition blocks centriole formation in differentiating TGCs but does not affect endocycle progression. In summary, centrioles in TGC endocycles undergo disengagement and conversion to centrosomes. This increases centrosome number but to a limited extent compared with DNA reduplication.

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.

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.

Long-range migration of centrioles to the apical surface of the olfactory epithelium

Olfactory sensory neurons (OSNs) in vertebrates detect odorants using multiple cilia, which protrude from the end of the dendrite and require centrioles for their formation. In mouse olfactory epithelium, the centrioles originate in progenitor cells near the basal lamina, often 50-100 μm from the apical surface. It is unknown how centrioles traverse this distance or mature to form cilia. Using high-resolution expansion microscopy, we found that centrioles migrate together, with multiple centrioles per group and multiple groups per OSN, during dendrite outgrowth. Centrioles were found by live imaging to migrate slowly, with a maximum rate of 0.18 µm/minute. Centrioles in migrating groups were associated with microtubule nucleation factors, but acquired rootletin and appendages only in mature OSNs. The parental centriole had preexisting appendages, formed a single cilium before other centrioles, and retained its unique appendage configuration in the mature OSN. We developed an air-liquid interface explant culture system for OSNs and used it to show that centriole migration can be perturbed ex vivo by stabilizing microtubules. We consider these results in the context of a comprehensive model for centriole formation, migration, and maturation in this important sensory cell type.

Evolutionary conservation of centriole rotational asymmetry in the human centrosome

Centrioles are formed by microtubule triplets in a nine-fold symmetric arrangement. In flagellated protists and in animal multiciliated cells, accessory structures tethered to specific triplets render the centrioles rotationally asymmetric, a property that is key to cytoskeletal and cellular organization in these contexts. In contrast, centrioles within the centrosome of animal cells display no conspicuous rotational asymmetry. Here, we uncover rotationally asymmetric molecular features in human centrioles. Using ultrastructure expansion microscopy, we show that LRRCC1, the ortholog of a protein originally characterized in flagellate green algae, associates preferentially to two consecutive triplets in the distal lumen of human centrioles. LRRCC1 partially co-localizes and affects the recruitment of another distal component, C2CD3, which also has an asymmetric localization pattern in the centriole lumen. Together, LRRCC1 and C2CD3 delineate a structure reminiscent of a filamentous density observed by electron microscopy in flagellates, termed the 'acorn'. Functionally, the depletion of LRRCC1 in human cells induced defects in centriole structure, ciliary assembly and ciliary signaling, supporting that LRRCC1 cooperates with C2CD3 to organizing the distal region of centrioles. Since a mutation in the LRRCC1 gene has been identified in Joubert syndrome patients, this finding is relevant in the context of human ciliopathies. Taken together, our results demonstrate that rotational asymmetry is an ancient property of centrioles that is broadly conserved in human cells. Our work also reveals that asymmetrically localized proteins are key for primary ciliogenesis and ciliary signaling in human cells.

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.

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.

Coding variants in the PCNT and CEP295 genes contribute to breast cancer risk in Chinese women

BACKGROUND

Centrioles play pivotal roles in the assembly of centrosomes, their dysfunction is associated with multiple inherited diseases or cancers. To date, few studies have focused on the associations between coding single nucleotide polymorphisms (SNPs) in the centriole duplication cycle genes and the risk of breast cancer in Chinese women.

METHODS

Twenty-one SNPs were selected from the coding regions of 10 critical centriole genes. The associations between the selected SNPs and breast cancer susceptibility were assessed in a case-control study of Chinese women, which included 1032 cases and 1063 controls. Potential biological functions in the influence of protein stability and the profile of expression quantitative trait loci (eQTL) of the identified SNPs were further evaluated using in silico databases.

RESULTS

Multivariate logistic regression analyses revealed that a missense SNP rs7279204 in PCNT was significantly associated with an increased risk of breast cancer (additive model: adjusted OR=1.19, 95% CI: 1.02-1.38), while a missense SNP rs77922978 in CEP295 was significantly associated with a decreased risk of breast cancer (additive model: adjusted OR=0.74, 95% CI: 0.56-0.97). Stratification analyses suggested that rs7279204 and rs77922978 exhibited different effects among later first live birth, ER-negative and PR-negative women (P<0.05). Moreover, rs77922978 showed significant differences for ER and PR status strata (heterogeneity test P=0.028, P=0.046). In addition, bioinformatic analyses indicated that the two variants may possess potential functions of reducing the protein stability of their host genes. Further eQTL analysis showed that the rs7279204 was not only correlated with the expression of its host gene PCNT, but also correlated with the expression of its nearby genes, implying its potential roles in regulation of some cancer susceptibility genes.

CONCLUSIONS

The SNPs rs7279204 and rs77922978 within the coding region of the PCNT and CEP295 genes may contribute to the susceptibility of breast cancer in Han Chinese population.

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.

Ana1 helps recruit Polo to centrioles to promote mitotic PCM assembly and centriole elongation

Polo kinase (PLK1 in mammals) is a master cell cycle regulator that is recruited to various subcellular structures, often by its polo-box domain (PBD), which binds to phosphorylated S-pS/pT motifs. Polo/PLK1 kinases have multiple functions at centrioles and centrosomes, and we have previously shown that in Drosophila phosphorylated Sas-4 initiates Polo recruitment to newly formed centrioles, while phosphorylated Spd-2 recruits Polo to the pericentriolar material (PCM) that assembles around mother centrioles in mitosis. Here, we show that Ana1 (Cep295 in humans) also helps to recruit Polo to mother centrioles in Drosophila. If Ana1-dependent Polo recruitment is impaired, mother centrioles can still duplicate, disengage from their daughters and form functional cilia, but they can no longer efficiently assemble mitotic PCM or elongate during G2. We conclude that Ana1 helps recruit Polo to mother centrioles to specifically promote mitotic centrosome assembly and centriole elongation in G2, but not centriole duplication, centriole disengagement or cilia assembly. This article has an associated First Person interview with the first author of the paper.

Triple deletion of TP53, PCNT, and CEP215 promotes centriole amplification in the M phase

Supernumerary centrioles are frequently observed in diverse types of cancer cells. In this study, we investigated the mechanism underlying the generation of supernumerary centrioles during the M phase. We generated the TP53;PCNT;CEP215 triple knockout (KO) cells and determined the configurations of the centriole during the cell cycle. The triple KO cells exhibited a precocious separation of centrioles and unscheduled centriole assembly in the M phase. Supernumerary centrioles in the triple KO cells were present throughout the cell cycle; however, among all the centrioles, only two maintained an intact composition, including CEP135, CEP192, CEP295 and CEP152. Intact centrioles were formed during the S phase and the rest of the centrioles may be generated during the M phase. M-phase-assembled centrioles lacked the ability to organize microtubules in the interphase; however, a fraction of them may acquire pericentriolar material to organize microtubules during the M phase. Taken together, our work reveals the heterogeneity of the supernumerary centrioles in the triple KO cells.  .

Human centrosome organization and function in interphase and mitosis

Centrosomes were first described by Edouard Van Beneden and named and linked to chromosome segregation by Theodor Boveri around 1870. In the 1960-1980s, electron microscopy studies have revealed the remarkable ultrastructure of a centriole -- a nine-fold symmetrical microtubular assembly that resides within a centrosome and organizes it. Less than two decades ago, proteomics and genomic screens conducted in multiple species identified hundreds of centriole and centrosome core proteins and revealed the evolutionarily conserved nature of the centriole assembly pathway. And now, super resolution microscopy approaches and improvements in cryo-tomography are bringing an unparalleled nanoscale-detailed picture of the centriole and centrosome architecture. In this chapter, we summarize the current knowledge about the architecture of human centrioles. We discuss the structured organization of centrosome components in interphase, focusing on localization/function relationship. We discuss the process of centrosome maturation and mitotic spindle pole assembly in centriolar and acentriolar cells, emphasizing recent literature.

Genotoxic stress-activated DNA-PK-p53 cascade and autophagy cooperatively induce ciliogenesis to maintain the DNA damage response

The DNA-PK maintains cell survival when DNA damage occurs. In addition, aberrant activation of the DNA-PK induces centrosome amplification, suggesting additional roles for this kinase. Here, we showed that the DNA-PK-p53 cascade induced primary cilia formation (ciliogenesis), thus maintaining the DNA damage response under genotoxic stress. Treatment with genotoxic drugs (etoposide, neocarzinostatin, hydroxyurea, or cisplatin) led to ciliogenesis in human retina (RPE1), trophoblast (HTR8), lung (A459), and mouse Leydig progenitor (TM3) cell lines. Upon genotoxic stress, several DNA damage signaling were activated, but only the DNA-PK-p53 cascade contributed to ciliogenesis, as pharmacological inhibition or genetic depletion of this pathway decreased genotoxic stress-induced ciliogenesis. Interestingly, in addition to localizing to the nucleus, activated DNA-PK localized to the base of the primary cilium (mother centriole) and daughter centriole. Genotoxic stress also induced autophagy. Inhibition of autophagy initiation or lysosomal degradation or depletion of ATG7 decreased genotoxic stress-induced ciliogenesis. Besides, inhibition of ciliogenesis by depletion of IFT88 or CEP164 attenuated the genotoxic stress-induced DNA damage response. Thus, our study uncovered the interplay among genotoxic stress, the primary cilium, and the DNA damage response.

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.

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.

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

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

With Age Comes Maturity: Biochemical and Structural Transformation of a Human Centriole in the Making

Centrioles are microtubule-based cellular structures present in most human cells that build centrosomes and cilia. Proliferating cells have only two centrosomes and this number is stringently maintained through the temporally and spatially controlled processes of centriole assembly and segregation. The assembly of new centrioles begins in early S phase and ends in the third G1 phase from their initiation. This lengthy process of centriole assembly from their initiation to their maturation is characterized by numerous structural and still poorly understood biochemical changes, which occur in synchrony with the progression of cells through three consecutive cell cycles. As a result, proliferating cells contain three structurally, biochemically, and functionally distinct types of centrioles: procentrioles, daughter centrioles, and mother centrioles. This age difference is critical for proper centrosome and cilia function. Here we discuss the centriole assembly process as it occurs in somatic cycling human cells with a focus on the structural, biochemical, and functional characteristics of centrioles of different ages.

Asymmetric clustering of centrosomes defines the early evolution of tetraploid cells

Tetraploidy has long been of interest to both cell and cancer biologists, partly because of its documented role in tumorigenesis. A common model proposes that the extra centrosomes that are typically acquired during tetraploidization are responsible for driving tumorigenesis. However, tetraploid cells evolved in culture have been shown to lack extra centrosomes. This observation raises questions about how tetraploid cells evolve and more specifically about the mechanisms(s) underlying centrosome loss. Here, using a combination of fixed cell analysis, live cell imaging, and mathematical modeling, we show that populations of newly formed tetraploid cells rapidly evolve in vitro to retain a near-tetraploid chromosome number while losing the extra centrosomes gained at the time of tetraploidization. This appears to happen through a process of natural selection in which tetraploid cells that inherit a single centrosome during a bipolar division with asymmetric centrosome clustering are favored for long-term survival.

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

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

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

Super-resolution microscopy: successful applications in centrosome study and beyond

Centrosome is the main microtubule-organizing center in most animal cells. Its core structure, centriole, also assembles cilia and flagella that have important sensing and motility functions. Centrosome has long been recognized as a highly conserved organelle in eukaryotic species. Through electron microscopy, its ultrastructure was revealed to contain a beautiful nine-symmetrical core 60 years ago, yet its molecular basis has only been unraveled in the past two decades. The emergence of super-resolution microscopy allows us to explore the insides of a centrosome, which is smaller than the diffraction limit of light. Super-resolution microscopy also enables the compartmentation of centrosome proteins into different zones and the identification of their molecular interactions and functions. This paper compiles the centrosome architecture knowledge that has been revealed in recent years and highlights the power of several super-resolution techniques.

Poc1B and Sas-6 Function Together during the Atypical Centriole Formation in Drosophila melanogaster

Insects and mammals have atypical centrioles in their sperm. However, it is unclear how these atypical centrioles form. Drosophila melanogaster sperm has one typical centriole called the giant centriole (GC) and one atypical centriole called the proximal centriole-like structure (PCL). During early sperm development, centriole duplication factors such as Ana2 and Sas-6 are recruited to the GC base to initiate PCL formation. The centriolar protein, Poc1B, is also recruited at this initiation stage, but its precise role during PCL formation is unclear. Here, we show that Poc1B recruitment was dependent on Sas-6, that Poc1B had effects on cellular and PCL Sas-6, and that Poc1B and Sas-6 were colocalized in the PCL/centriole core. These findings suggest that Sas-6 and Poc1B interact during PCL formation. Co-overexpression of Ana2 and Sas-6 induced the formation of ectopic particles that contained endogenous Poc1 proteins and were composed of PCL-like structures. These structures were disrupted in Poc1 mutant flies, suggesting that Poc1 proteins stabilize the PCL-like structures. Lastly, Poc1B and Sas-6 co-overexpression also induced the formation of PCL-like structures, suggesting that they can function together during the formation of the PCL. Overall, our findings suggest that Poc1B and Sas-6 function together during PCL formation.

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

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

Pericentrin-mediated SAS-6 recruitment promotes centriole assembly

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

It takes two (centrioles) to tango

Cells that divide during embryo development require precisely two centrioles during interphase and four centrioles during mitosis. This precise number is maintained by allowing each centriole to nucleate only one centriole per cell cycle (i.e. centriole duplication). Yet, how the first cell of the embryo, the zygote, obtains two centrioles has remained a mystery in most mammals and insects. The mystery arose because the female gamete (oocyte) is thought to have no functional centrioles and the male gamete (spermatozoon) is thought to have only one functional centriole, resulting in a zygote with a single centriole. However, recent studies in fruit flies, beetles and mammals, including humans, suggest an alternative explanation: spermatozoa have a typical centriole and an atypical centriole. The sperm typical centriole has a normal structure but distinct protein composition, whereas the sperm atypical centriole is distinct in both. During fertilization, the atypical centriole is released into the zygote, nucleates a new centriole and participates in spindle pole formation. Thus, the spermatozoa's atypical centriole acts as a second centriole in the zygote. Here, we review centriole biology in general and especially in reproduction, we describe the discovery of the spermatozoon atypical centriole, and we provide an updated model for centriole inherence during sexual reproduction. While we focus on humans and other non-rodent mammals, we also provide a broader evolutionary perspective.

Mechanism and Regulation of Centriole and Cilium Biogenesis

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

Centrosome Amplification and Tumorigenesis: Cause or Effect?

Centrosome amplification is a feature of multiple tumour types and has been postulated to contribute to both tumour initiation and tumour progression. This chapter focuses on the mechanisms by which an increase in centrosome number might lead to an increase or decrease in tumour progression and the role of proteins that regulate centrosome number in driving tumorigenesis.

PCNT is critical for the association and conversion of centrioles to centrosomes during mitosis

A centrosome consists of a pair of centrioles and pericentriolar material (PCM). We manipulated expression of PCNT, a key PCM protein, and investigated roles of PCM in centriole behavior during mitosis. Deletion of PCNT had little effect on the interphase centrosomes. However, centrioles in PCNT-deleted mitotic cells prematurely separated and frequently amplified, revealing that centrioles are limited within the spindle poles by PCNT during mitosis. It is known that specific cleavage of PCNT is necessary for centriole separation during mitotic exit. Delayed centriole separation was observed in G0 phase when a noncleavable PCNT was removed or when PCNT was artificially cleaved by TEV protease. Furthermore, a daughter centriole converts to a mother centriole only after experiencing both mitotic exit and specific PCNT cleavage. Based on the results, we propose that a centriole pair disengages upon entering mitosis but remains associated with the surrounding PCM proteins throughout mitosis. During mitotic exit, specific cleavage of PCNT induces PCM disintegration. As a result, a daughter centriole separates from the mother centriole and converts to a young mother centriole.

Dual inhibition of Kif15 by oxindole and quinazolinedione chemical probes

The mitotic spindle is a microtubule-based machine that segregates a replicated set of chromosomes during cell division. Many cancer drugs alter or disrupt the microtubules that form the mitotic spindle. Microtubule-dependent molecular motors that function during mitosis are logical alternative mitotic targets for drug development. Eg5 (Kinesin-5) and Kif15 (Kinesin-12), in particular, are an attractive pair of motor proteins, as they work in concert to drive centrosome separation and promote spindle bipolarity. Furthermore, we hypothesize that the clinical failure of Eg5 inhibitors may be (in part) due to compensation by Kif15. In order to test this idea, we screened a small library of kinase inhibitors and identified GW108X, an oxindole that inhibits Kif15 in vitro. We show that GW108X has a distinct mechanism of action compared with a commercially available Kif15 inhibitor, Kif15-IN-1 and may serve as a lead with which to further develop Kif15 inhibitors as clinically relevant agents.

ALMS1 and Alström syndrome: a recessive form of metabolic, neurosensory and cardiac deficits

Alström syndrome (AS) is characterised by metabolic deficits, retinal dystrophy, sensorineural hearing loss, dilated cardiomyopathy and multi-organ fibrosis. Elucidating the function of the mutated gene, ALMS1, is critical for the development of specific treatments and may uncover pathways relevant to a range of other disorders including common forms of obesity and type 2 diabetes. Interest in ALMS1 is heightened by the recent discovery of its involvement in neonatal cardiomyocyte cell cycle arrest, a process with potential relevance to regenerative medicine. ALMS1 encodes a ~ 0.5 megadalton protein that localises to the base of centrioles. Some studies have suggested a role for this protein in maintaining centriole-nucleated sensory organelles termed primary cilia, and AS is now considered to belong to the growing class of human genetic disorders linked to ciliary dysfunction (ciliopathies). However, mechanistic details are lacking, and recent studies have implicated ALMS1 in several processes including endosomal trafficking, actin organisation, maintenance of centrosome cohesion and transcription. In line with a more complex picture, multiple isoforms of the protein likely exist and non-centrosomal sites of localisation have been reported. This review outlines the evidence for both ciliary and extra-ciliary functions of ALMS1.

The ciliopathy gene product Cep290 is required for primary cilium formation and microtubule network organization

The mammalian centrosome/cilium complex is composed of the centrosome, the primary cilium, and the centriolar satellites, which together function in key cellular processes including signaling. Defective assembly, maintenance, and function of the centrosome/ cilium complex cause the human genetic diseases known as ciliopathies, which are characterized by a multitude of developmental syndromes including retinal degeneration and kidney cysts. The molecular mechanisms underlying pathogenesis in ciliopathies remain poorly understood, which requires structural and functional characterization of the mutated ciliopathy proteins at the cellular level. To this end, we elucidated the function and regulation of Cep290, which is the most frequently mutated gene in ciliopathies and importantly its functions remain poorly understood. First, we generated Cep290-null cells using the CRISPR/Cas9 genome editing approach. Using functional assays, we showed that Cep290-null cells do not ciliate and that they have defects in centriolar satellites dynamics and interphase microtubule organization. The centriolar satellites were tightly clustered around the centrosome in Cep290-null cells, and the interphase microtubule network lost its radial organization. Our results provide phenotypic insight into the disease mechanisms of Cep290 ciliopathy mutations and also the tools for studying genotype/phenotype relationships in ciliopathies.