A distal centriolar protein network controls organelle maturation and asymmetry

Centriole structural integrity defects are a crucial feature of hydrolethalus syndrome

Hydrolethalus syndrome (HLS) is a lethal, autosomal recessive ciliopathy caused by the mutation of the conserved centriole protein HYLS1. How HYLS1 controls centriole function is poorly understood. Here, we show that mice harboring the HYLS1 disease mutation die shortly after birth and exhibit developmental defects that recapitulate several manifestations of HLS. These phenotypes arise from a loss of centriole integrity that causes tissue-specific defects in cilia assembly and function. We show that HYLS1 is recruited to the centriole by CEP120 and stabilizes the localization of centriole inner scaffold proteins that ensure the integrity of the centriolar microtubule wall. The HLS disease mutation reduced the centriole localization of HYLS1 and caused degeneration of the centriole distal end. We propose that tissue-specific defects in centriole integrity caused by the HYLS1 mutation prevent ciliogenesis and contribute to HLS phenotypes.

Centrobin serves as a safeguard to guide timely centriole maturation during the cell cycle

Centrioles assemble and segregate in link to the cell cycle. Daughter centrioles assemble at S phase, and become young mother centrioles after M phase. Since distal appendages (DAs) are installed to young mother centrioles at the second G2/M transition phase, it takes one and a half cell cycle for a daughter centriole to fully mature into an old mother centriole. Here, we investigated specific roles of centrobin on centriole maturation by tracing its centriole localization throughout the cell cycle. Centrobin instantly places at the nascent daughter centrioles during the S phase, maintains its localization through subsequent cell cycle as these daughter centrioles mature into young mother centrioles, and detaches from the young mother centriole during the G2 phase, prior to DA installation. Centrobin KO cells exhibit two DA-installed centrioles, due to premature DA installation in daughter centrioles, and can produce doublet cilia from two DA-installed basal bodies. We also present evidence that direct phosphorylation of Plk1 is crucial for centrobin attachment to centrioles during G2 and M phases. Finally, premature DA installation was also observed in centrobin KO mice. Our results collectively demonstrate that centrobin serves as a safeguard to guide timely centriole maturation during the cell cycle.

The tubulin poly-glutamylase complex, TPGC, is required for phosphatidyl inositol homeostasis and cilium assembly and maintenance

The tubulin poly-glutamylase complex (TPGC) is comprised of TTLL1 and at least five associated proteins that promote the addition of glutamate residues to tubulin tails of microtubules. Despite its discovery two decades ago, the enzyme has been refractory to characterization owing to its complex multimeric nature and the inability to detect poly-glutamylase activity after assembling the six-subunit complex. We now show that TPGC is the key enzyme driving centriolar and ciliary poly-glutamylation. We identified two novel TPGC subunits, TBC1D19 and KIAA1841, and showed that both components play an essential role in the assembly of the eight-subunit holo-enzyme. Remarkably, we were able to reconstitute the activity of TPGC with all eight subunits. TBC1D19 and KIAA1841 were essential for assembly and activity, and loss of TBC1D19 strongly compromised multiple tubulin modifications, including axonemal poly-glutamylation. TBC1D19 loss abolished transport of Arl13b and other ciliary membrane proteins, abrogating primary cilium assembly. Structural modeling revealed an essential role for TBC1D19 and KIAA1841 in complex assembly, microtubule binding, and preferential poly-glutamylation of α-tubulin. We found that TBC1D19 loss abrogated the ciliary localization of phosphatidyl inositol phosphatase, INPP5E, triggering cilium instability. Ciliogenesis in TBC1D19 null cells could be restored through inhibition of a specific phosphatidyl inositol phosphate (PIP) kinase, PIP5K1c, suggesting that TBC1D19 is required to instigate and maintain PIP homeostasis during ciliogenesis. Collectively, our data show that TPGC is a multi-functional enzyme essential for cilium assembly and maintenance.

The primary cilia: Orchestrating cranial neural crest cell development

Primary cilia (hereafter "cilia") are microtubule-based antenna-like organelles projecting from the surface of vertebrate cells. Cilia can serve as cellular antennae controlling cell growth and differentiation. Absent or dysfunctional cilia frequently lead to craniofacial anomalies known as craniofacial ciliopathies. However, the detailed pathological mechanisms of craniofacial ciliopathies remain unclear. This perspective discusses our current understanding of the role of cilia in cranial neural crest cells. We also describe potential mechanisms of ciliogenesis in cranial neural crest cells, which may contribute to unraveling the complex pathogenesis of craniofacial ciliopathies.

Pathogenic KIAA0586/TALPID3 variants are associated with defects in primary and motile cilia

Pathogenic variants in KIAA0586/TALPID3 are associated with the ciliopathy Joubert syndrome (JS). We report individuals with KIAA0586/TALPID3 variants affected by primary and motile cilia defects leading to JS and chronic destructive airway disease. DNA variants were detected in three families by sequencing. In two unrelated families, a deep-intronic variant (KIAA0586/TALPID3:c.3990 + 3186G>A) activated a cryptic exon. We performed histological and functional analyses in native and air-liquid interface (ALI) cultured respiratory cells. Primary cilia lengths were measured in patient-derived fibroblasts. Our data associate KIAA0586/TALPID3 variants with a syndrome combining JS and chronic destructive airway disease, reduced number of motile cilia, disorganized basal body location, and ciliary clearance malfunction. Additionally, patient-derived cell lines showed primary cilia defects. Disease causing KIAA0586/TALPID3 variants, including a deep-intronic sequence variant, were associated with primary and motile cilia defects in JS patients. The combination of JS and respiratory symptoms should be considered indicative for KIAA0586/TALPID3 sequence alterations.

Use of patient-derived cell models for characterization of compound heterozygous hypomorphic C2CD3 variants in a patient with isolated nephronophthisis

BACKGROUND

Primary ciliopathies are a heterogeneous group of rare disorders predominantly caused by autosomal-recessive genetic variants that disrupt non-motile ciliary function. They often manifest as a syndromic phenotype, frequently involving the kidney. Biallelic pathogenic variants in C2CD3 disrupt ciliogenesis and Sonic Hedgehog (SHH) signaling, resulting in a severe ciliopathy (Orofaciodigital syndrome XIV, OMIM 615948). We present compound heterozygous missense variants in C2CD3 that partially disrupt ciliary function in a patient with isolated renal disease.

METHODS

Exome sequencing identified biallelic C2CD3 missense variants (p.Pro168Leu; p.Thr2079Met). Patient-derived fibroblasts and urinary renal epithelial cells (URECs), and human RPE-1 C2CD3 knockout (KO) cell-lines were used for in vitro studies.

RESULTS

Cilia length was significantly shorter in patient-derived fibroblasts compared to an unaffected sibling (2.309 vs. 2.850 μm, P < 0.0001), while URECs showed significantly shortened cilia (2.068 vs. 2.807 μm, P < 0.0001) and a 40.8% reduction in ciliation (P < 0.001). The latter was not observed in fibroblasts, suggesting a kidney-specific effect. SHH signaling was dysregulated in patient cells as expression of GLI3 activator protein and GLI1 mRNA was significantly reduced. C2CD3 localization to the basal body was significantly reduced in patient URECs. Finally, rescue experiments in C2CD3 KO RPE-1 cells corroborated these findings by demonstrating a reduced capacity to restore ciliogenesis for each variant.

CONCLUSION

Biallelic hypomorphic missense variants in C2CD3 may contribute to an isolated nephronophthisis phenotype with impaired ciliogenesis and SHH signaling. Our findings underscore the importance of functional testing to characterize candidate gene-disease relationships in patients with nephropathy of unknown etiology.

Targeted Deletion in the Basal Body Protein Talpid3 Leads to Loss of Primary Cilia in Embryonic Stem Cells and Defective Lineage-Specific Differentiation

Talpid3 is a basal body protein required for the formation of primary cilia, an organelle involved in signal transduction. Here, we asked if Talpid3 has a role in the regulation of differentiation and/or self-renewal of ES cells and whether cells lacking cilia due to a deletion in Talpid3 can be reprogrammed to induced pluripotent stem (iPS) cells. We show that mouse embryonic limb fibroblasts which lack primary cilia with a targeted deletion in the Talpid3 (Ta3) gene can be efficiently reprogrammed to iPS cells. Furthermore, vector-free Ta3-/- iPS cells retain ES cell features and are able to self-renew. However, both Ta3-/- iPS and ES cells are unable to form visceral endoderm and differentiate poorly into neurons. The observed defects are not a consequence of reprogramming since Ta3-/- ES cells also exhibit this phenotype. Thus, Talpid3 and primary cilia are required for some differentiation events but appear to be dispensable for stem cell self-renewal and reprogramming.

Centriole structural integrity defects are a crucial feature of Hydrolethalus Syndrome

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

High-throughput electron tomography identifies centriole over-elongation as an early event in plasma cell disorders

Plasma cell disorders are clonal outgrowths of pre-malignant or malignant plasma cells, characterized by extensive chromosomal aberrations. Centrosome abnormalities are a major driver of chromosomal instability in cancer but their origin, incidence, and composition in primary tumor cells is poorly understood. Using cutting-edge, semi-automated high-throughput electron tomography, we characterized at nanoscale 1386 centrioles in CD138pos plasma cells from eight healthy donors and 21 patients with plasma cell disorders, and 722 centrioles from different control populations. In plasma cells from healthy individuals, over-elongated centrioles accumulated with age. In plasma cell disorders, centriole over-elongation was notably frequent in early, pre-malignant disease stages, became less pronounced in overt multiple myeloma, and almost entirely disappeared in aggressive plasma cell leukemia. Centrioles in other types of patient-derived B cell neoplasms showed no over-elongation. In contrast to current belief, centriole length appears to be highly variable in long-lived, healthy plasma cells, and over-elongation and structural aberrations are common in this cell type. Our data suggest that structural centrosome aberrations accumulate with age in healthy CD138pos plasma cells and may thus play an important role in early aneuploidization as an oncogenic driver in plasma cell disorders.

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

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

Control of centrosome distal appendages assembly and disassembly

Centrosomes are microtubule organizing centers involved in chromosome segregation, spindle orientation, cell motility and cilia formation. In recent years, they have also emerged as key modulators of asymmetric cell division. Centrosomes are composed of two centrioles that initiate duplication in S phase. The conservative nature of centriole duplication means that the two centrioles of a G1 cell are of different ages. They are also structurally different as only the older centriole carry appendages, an assembly of a subset of proteins primarily required for cilia formation. In a growing tissue, the non-motile, primary cilium acts as a mechano- and sensory organelle that influences cell behavior via modulation of signaling pathways. Here, we discuss the most recent findings about distal appendage composition and function, as well as cell cycle-specific regulation and their implications in various diseases.

Centriolar satellites expedite mother centriole remodeling to promote ciliogenesis

Centrosomes are orbited by centriolar satellites, dynamic multiprotein assemblies nucleated by Pericentriolar material 1 (PCM1). To study the requirement for centriolar satellites, we generated mice lacking PCM1, a crucial component of satellites. Pcm1-/- mice display partially penetrant perinatal lethality with survivors exhibiting hydrocephalus, oligospermia, and cerebellar hypoplasia, and variably expressive phenotypes such as hydronephrosis. As many of these phenotypes have been observed in human ciliopathies and satellites are implicated in cilia biology, we investigated whether cilia were affected. PCM1 was dispensable for ciliogenesis in many cell types, whereas Pcm1-/- multiciliated ependymal cells and human PCM1-/- retinal pigmented epithelial 1 (RPE1) cells showed reduced ciliogenesis. PCM1-/- RPE1 cells displayed reduced docking of the mother centriole to the ciliary vesicle and removal of CP110 and CEP97 from the distal mother centriole, indicating compromised early ciliogenesis. Similarly, Pcm1-/- ependymal cells exhibited reduced removal of CP110 from basal bodies in vivo. We propose that PCM1 and centriolar satellites facilitate efficient trafficking of proteins to and from centrioles, including the departure of CP110 and CEP97 to initiate ciliogenesis, and that the threshold to trigger ciliogenesis differs between cell types.

Structure and function of distal and subdistal appendages of the mother centriole

Centrosomes are composed of centrioles surrounded by pericentriolar material. The two centrioles in G1 phase are distinguished by the localization of their appendages in the distal and subdistal regions; the centriole possessing both types of appendage is older and referred to as the mother centriole, whereas the other centriole lacking appendages is the daughter centriole. Both distal and subdistal appendages in vertebrate cells consist of multiple proteins assembled in a hierarchical manner. Distal appendages function mainly in the initial process of ciliogenesis, and subdistal appendages are involved in microtubule anchoring, mitotic spindle regulation and maintenance of ciliary signaling. Mutations in genes encoding components of both appendage types are implicated in ciliopathies and developmental defects. In this Review, we discuss recent advances in knowledge regarding the composition and assembly of centriolar appendages, as well as their roles in development and disease.

The central scaffold protein CEP350 coordinates centriole length, stability, and maturation

The centriole is the microtubule-based backbone that ensures integrity, function, and cell cycle-dependent duplication of centrosomes. Mostly unclear mechanisms control structural integrity of centrioles. Here, we show that the centrosome protein CEP350 functions as scaffold that coordinates distal-end properties of centrioles such as length, stability, and formation of distal and subdistal appendages. CEP350 fulfills these diverse functions by ensuring centriolar localization of WDR90, recruiting the proteins CEP78 and OFD1 to the distal end of centrioles and promoting the assembly of subdistal appendages that have a role in removing the daughter-specific protein Centrobin. The CEP350-FOP complex in association with CEP78 or OFD1 controls centriole microtubule length. Centrobin safeguards centriole distal end stability, especially in the compromised CEP350-/- cells, while the CEP350-FOP-WDR90 axis secures centriole integrity. This study identifies CEP350 as a guardian of the distal-end region of centrioles without having an impact on the proximal PCM part.

The evolutionary conserved proteins CEP90, FOPNL, and OFD1 recruit centriolar distal appendage proteins to initiate their assembly

In metazoa, cilia assembly is a cellular process that starts with centriole to basal body maturation, migration to the cell surface, and docking to the plasma membrane. Basal body docking involves the interaction of both the distal end of the basal body and the transition fibers/distal appendages, with the plasma membrane. Mutations in numerous genes involved in basal body docking and transition zone assembly are associated with the most severe ciliopathies, highlighting the importance of these events in cilium biogenesis. In this context, the ciliate Paramecium has been widely used as a model system to study basal body and cilia assembly. However, despite the evolutionary conservation of cilia assembly events across phyla, whether the same molecular players are functionally conserved, is not fully known. Here, we demonstrated that CEP90, FOPNL, and OFD1 are evolutionary conserved proteins crucial for ciliogenesis. Using ultrastructure expansion microscopy, we unveiled that these proteins localize at the distal end of both centrioles/basal bodies in Paramecium and mammalian cells. Moreover, we found that these proteins are recruited early during centriole duplication on the external surface of the procentriole. Functional analysis performed both in Paramecium and mammalian cells demonstrate the requirement of these proteins for distal appendage assembly and basal body docking. Finally, we show that mammalian centrioles require another component, Moonraker (MNR), to recruit OFD1, FOPNL, and CEP90, which will then recruit the distal appendage proteins CEP83, CEP89, and CEP164. Altogether, we propose that this OFD1, FOPNL, and CEP90 functional module is required to determine in mammalian cells the future position of distal appendage proteins.

CEP90, FOPNL and OFD1 form an evolutionary conserved module which promotes the assembly of centriolar distal appendages. This study uses ultrastructure expansion microscopy to reveal the recruitment of this module on early-born procentrioles to in turn recruit centriolar distal appendage proteins, proposing that this dictates the future location of distal appendages.

Roles of the actin cytoskeleton in ciliogenesis

Primary cilia play a key role in the ability of cells to respond to extracellular stimuli, such as signaling molecules and environmental cues. These sensory organelles are crucial to the development of many organ systems, and defects in primary ciliogenesis lead to multisystemic genetic disorders, known as ciliopathies. Here, we review recent advances in the understanding of several key aspects of the regulation of ciliogenesis. Primary ciliogenesis is thought to take different pathways depending on cell type, and some recent studies shed new light on the cell-type-specific mechanisms regulating ciliogenesis at the apical surface in polarized epithelial cells, which are particularly relevant for many ciliopathies. Furthermore, recent findings have demonstrated the importance of actin cytoskeleton dynamics in positively and negatively regulating multiple stages of ciliogenesis, including the vesicular trafficking of ciliary components and the positioning and docking of the basal body. Finally, studies on the formation of motile cilia in multiciliated epithelial cells have revealed requirements for actin remodeling in this process too, as well as showing evidence of an additional alternative ciliogenesis pathway.

PLK1 controls centriole distal appendage formation and centrobin removal via independent pathways

Centrioles are central structural elements of centrosomes and cilia. In human cells daughter centrioles are assembled adjacent to existing centrioles in S-phase and reach their full functionality with the formation of distal and subdistal appendages one-and-a-half cell cycle later, as they exit their second mitosis. Current models postulate that the centriolar protein centrobin acts as placeholder for distal appendage proteins that must be removed to complete distal appendage formation. Here, we investigated in non-transformed human epithelial RPE1 cells the mechanisms controlling centrobin removal and its effect on distal appendage formation. Our data are consistent with a speculative model in which centrobin is removed from older centrioles due to a higher affinity for the newly born daughter centrioles, under the control of the centrosomal kinase Plk1. This removal also depends on the presence of subdistal appendage proteins on the oldest centriole. Removing centrobin, however, is not required for the recruitment of distal appendage proteins, even though this process is equally dependent on Plk1. We conclude that Plk1 kinase regulates centrobin removal and distal appendage formation during centriole maturation via separate pathways.

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.

Regulators of tubulin polyglutamylation control nuclear shape and cilium disassembly by balancing microtubule and actin assembly

Cytoskeletal networks play an important role in regulating nuclear morphology and ciliogenesis. However, the role of microtubule (MT) post-translational modifications in nuclear shape regulation and cilium disassembly has not been explored. Here we identified a novel regulator of the tubulin polyglutamylase complex (TPGC), C11ORF49/CSTPP1, that regulates cytoskeletal organization, nuclear shape, and cilium disassembly. Mechanistically, loss of C11ORF49/CSTPP1 impacts the assembly and stability of the TPGC, which modulates long-chain polyglutamylation levels on microtubules (MTs) and thereby balances the binding of MT-associated proteins and actin nucleators. As a result, loss of TPGC leads to aberrant, enhanced assembly of MTs that penetrate the nucleus, which in turn leads to defects in nuclear shape, and disorganization of cytoplasmic actin that disrupts the YAP/TAZ pathway and cilium disassembly. Further, we showed that C11ORF49/CSTPP1-TPGC plays mechanistically distinct roles in the regulation of nuclear shape and cilium disassembly. Remarkably, disruption of C11ORF49/CSTPP1-TPGC also leads to developmental defects in vivo. Our findings point to an unanticipated nexus that links tubulin polyglutamylation with nuclear shape and ciliogenesis.

Centrosome maturation - in tune with the cell cycle

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

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.

Myosin VI regulates ciliogenesis by promoting the turnover of the centrosomal/satellite protein OFD1

The actin motor protein myosin VI is a multivalent protein with diverse functions. Here, we identified and characterised a myosin VI ubiquitous interactor, the oral‐facial‐digital syndrome 1 (OFD1) protein, whose mutations cause malformations of the face, oral cavity, digits and polycystic kidney disease. We found that myosin VI regulates the localisation of OFD1 at the centrioles and, as a consequence, the recruitment of the distal appendage protein Cep164. Myosin VI depletion in non‐tumoural cell lines causes an aberrant localisation of OFD1 along the centriolar walls, which is due to a reduction in the OFD1 mobile fraction. Finally, loss of myosin VI triggers a severe defect in ciliogenesis that could be, at least partially, ascribed to an impairment in the autophagic removal of OFD1 from satellites. Altogether, our results highlight an unprecedent layer of regulation of OFD1 and a pivotal role of myosin VI in coordinating the formation of the distal appendages and primary cilium with important implications for the genetic disorders known as ciliopathies.

CEP120-mediated KIAA0753 recruitment onto centrioles is required for timely neuronal differentiation and germinal zone exit in the developing cerebellum

Here, Chang et al. report that CEP120, a JS-associated protein involved in centriole biogenesis and cilia assembly, regulates timely neuronal differentiation and the departure of granule neuron progenitors (GNPs) from their germinal zone during cerebellar development. Their findings reveal a close interplay between CEP120 and KIAA0753 for the germinal zone exit and timely neuronal differentiation of GNPs during cerebellar development, and mutations in CEP120 and KIAA0753 may participate in the heterotopia and cerebellar hypoplasia observed in JS patients.

Joubert syndrome (JS) is a recessive ciliopathy in which all affected individuals have congenital cerebellar vermis hypoplasia. Here, we report that CEP120, a JS-associated protein involved in centriole biogenesis and cilia assembly, regulates timely neuronal differentiation and the departure of granule neuron progenitors (GNPs) from their germinal zone during cerebellar development. Our results show that depletion of Cep120 perturbs GNP cell cycle progression, resulting in a delay of cell cycle exit in vivo. To dissect the potential mechanism, we investigated the association between CEP120 interactome and the JS database and identified KIAA0753 (a JS-associated protein) as a CEP120-interacting protein. Surprisingly, we found that CEP120 recruits KIAA0753 to centrioles, and that loss of this interaction induces accumulation of GNPs in the germinal zone and impairs neuronal differentiation. Importantly, the replenishment of wild-type CEP120 rescues the above defects, whereas expression of JS-associated CEP120 mutants, which hinder KIAA0753 recruitment, does not. Together, our data reveal a close interplay between CEP120 and KIAA0753 for the germinal zone exit and timely neuronal differentiation of GNPs during cerebellar development, and mutations in CEP120 and KIAA0753 may participate in the heterotopia and cerebellar hypoplasia observed in JS patients.

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.

Stem cells' centrosomes: How can organelles identified 130 years ago contribute to the future of regenerative medicine?

At the core of regenerative medicine lies the expectation of repair or replacement of damaged tissues or whole organs. Donor scarcity and transplant rejection are major obstacles, and exactly the obstacles that stem cell-based therapy promises to overcome. These therapies demand a comprehensive understanding of the asymmetric division of stem cells, i.e. their ability to produce cells with identical potency or differentiated cells. It is believed that with better understanding, researchers will be able to direct stem cell differentiation. Here, we describe extraordinary advances in manipulating stem cell fate that show that we need to focus on the centrosome and the centrosome-derived primary cilium. This belief comes from the fact that this organelle is the vehicle that coordinates the asymmetric division of stem cells. This is supported by studies that report the significant role of the centrosome/cilium in orchestrating signaling pathways that dictate stem cell fate. We anticipate that there is sufficient evidence to place this organelle at the center of efforts that will shape the future of regenerative medicine.

A ciliopathy complex builds distal appendages to initiate ciliogenesis

Cells inherit two centrioles, the older of which is uniquely capable of generating a cilium. Using proteomics and superresolved imaging, we identify a module that we term DISCO (distal centriole complex). The DISCO components CEP90, MNR, and OFD1 underlie human ciliopathies. This complex localizes to both distal centrioles and centriolar satellites, proteinaceous granules surrounding centrioles. Cells and mice lacking CEP90 or MNR do not generate cilia, fail to assemble distal appendages, and do not transduce Hedgehog signals. Disrupting the satellite pools does not affect distal appendage assembly, indicating that it is the centriolar populations of MNR and CEP90 that are critical for ciliogenesis. CEP90 recruits the most proximal known distal appendage component, CEP83, to root distal appendage formation, an early step in ciliogenesis. In addition, MNR, but not CEP90, restricts centriolar length by recruiting OFD1. We conclude that DISCO acts at the distal centriole to support ciliogenesis by restraining centriole length and assembling distal appendages, defects in which cause human ciliopathies.

Biophysical and Quantitative Principles of Centrosome Biogenesis and Structure

The centrosome is a main orchestrator of the animal cellular microtubule cytoskeleton. Dissecting its structure and assembly mechanisms has been a goal of cell biologists for over a century. In the last two decades, a good understanding of the molecular constituents of centrosomes has been achieved. Moreover, recent breakthroughs in electron and light microscopy techniques have enabled the inspection of the centrosome and the mapping of its components with unprecedented detail. However, we now need a profound and dynamic understanding of how these constituents interact in space and time. Here, we review the latest findings on the structural and molecular architecture of the centrosome and how its biogenesis is regulated, highlighting how biophysical techniques and principles as well as quantitative modeling are changing our understanding of this enigmatic cellular organelle. Expected final online publication date for the Annual Review of Cell and Developmental Biology, Volume 37 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

CEP78 functions downstream of CEP350 to control biogenesis of primary cilia by negatively regulating CP110 levels

CEP78 is a centrosomal protein implicated in ciliogenesis and ciliary length control, and mutations in the CEP78 gene cause retinal cone-rod dystrophy associated with hearing loss. However, the mechanism by which CEP78 affects cilia formation is unknown. Based on a recently discovered disease-causing CEP78 p.L150S mutation, we identified the disease-relevant interactome of CEP78. We confirmed that CEP78 interacts with the EDD1-DYRK2-DDB1^VPRBP E3 ubiquitin ligase complex, which is involved in CP110 ubiquitination and degradation, and identified a novel interaction between CEP78 and CEP350 that is weakened by the CEP78^L150S mutation. We show that CEP350 promotes centrosomal recruitment and stability of CEP78, which in turn leads to centrosomal recruitment of EDD1. Consistently, cells lacking CEP78 display significantly increased cellular and centrosomal levels of CP110, and depletion of CP110 in CEP78-deficient cells restored ciliation frequency to normal. We propose that CEP78 functions downstream of CEP350 to promote ciliogenesis by negatively regulating CP110 levels via an EDD1-dependent mechanism.

Centrosomes in asymmetric cell division

Asymmetric cell division (ACD) is a strategy for achieving cell diversity. Research carried out over the last two decades has shown that in some cell types that divide asymmetrically, mother and daughter centrosomes are noticeably different from one another in structure, behaviour, and fate, and that robust ACD depends upon centrosome function. Here, I review the latest advances in this field with special emphasis on the complex structure-function relationship of centrosomes with regards to ACD and on mechanistic insight derived from cell types that divide symmetrically but is likely to be relevant in ACD. I also include a comment arguing for the need to investigate the centrosome cycle in other cell types that divide asymmetrically.

The entangled relationship between cilia and actin

Primary cilia are microtubule-based sensory cell organelles that are vital for tissue and organ development. They act as an antenna, receiving and transducing signals, enabling communication between cells. Defects in ciliogenesis result in severe genetic disorders collectively termed ciliopathies. In recent years, the importance of the direct and indirect involvement of actin regulators in ciliogenesis came into focus as it was shown that F-actin polymerisation impacts ciliation. The ciliary basal body was further identified as both a microtubule and actin organising centre. In the current review, we summarize recent studies on F-actin in and around primary cilia, focusing on different actin regulators and their effect on ciliogenesis, from the initial steps of basal body positioning and regulation of ciliary assembly and disassembly. Since primary cilia are also involved in several intracellular signalling pathways such as planar cell polarity (PCP), subsequently affecting actin rearrangements, the multiple effectors of this pathway are highlighted in more detail with a focus on the feedback loops connecting actin networks and cilia proteins. Finally, we elucidate the role of actin regulators in the development of ciliopathy symptoms and cancer.

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.

TALPID3 and ANKRD26 selectively orchestrate FBF1 localization and cilia gating

Transition fibers (TFs) regulate cilia gating and make the primary cilium a distinct functional entity. However, molecular insights into the biogenesis of a functional cilia gate remain elusive. In a forward genetic screen in Caenorhabditis elegans, we uncover that TALP-3, a homolog of the Joubert syndrome protein TALPID3, is a TF-associated component. Genetic analysis reveals that TALP-3 coordinates with ANKR-26, the homolog of ANKRD26, to orchestrate proper cilia gating. Mechanistically, TALP-3 and ANKR-26 form a complex with key gating component DYF-19, the homolog of FBF1. Co-depletion of TALP-3 and ANKR-26 specifically impairs the recruitment of DYF-19 to TFs. Interestingly, in mammalian cells, TALPID3 and ANKRD26 also play a conserved role in coordinating the recruitment of FBF1 to TFs. We thus report a conserved protein module that specifically regulates the functional component of the ciliary gate and suggest a correlation between defective gating and ciliopathy pathogenesis.

Most cells possess sensory cilia, which need to be gated properly. Here the authors show that the C. elegans proteins TALP-3 and ANKR-26 coordinate cilia gating in the context of transition fibers and that this mechanism is conserved in mammalian cells and likely implicated in certain ciliopathies.

Molecular diagnosis of kidney transplant failure based on urine

In light of the organ shortage, there is a great responsibility to assess postmortal organs for which procurement has been consented and to increase the life span of transplanted organs. The former responsibility has moved many centers to accept extended criteria organs. The latter responsibility requires an exact diagnosis and, if possible, omission of the harmful influence on the transplant. We report the course of a kidney transplant that showed a steady decline of function over a decade, displaying numerous cysts of different sizes. Clinical workup excluded the most frequent causes of chronic transplant failure. The filed allocation documents mentioned the donor's disease of oral-facial-digital syndrome, a rare ciliopathy, which can also affect the kidney. Molecular diagnosis was performed by culturing donor tubular cells from the recipient´s urine more than 10 years after transplantation. Next-generation panel sequencing with DNA from tubular urinary cells revealed a novel truncating mutation in OFD1, which sufficiently explains the features of the kidney transplants, also found in the second kidney allograft. Despite this severe donor disease, lifesaving transplantation with good long-term outcome was enabled for 5 recipients.

Nek2 kinase displaces distal appendages from the mother centriole prior to mitosis

Distal appendages (DAs) of the mother centriole are essential for the initial steps of ciliogenesis in G1/G0 phase of the cell cycle. DAs are released from centrosomes in mitosis by an undefined mechanism. Here, we show that specific DAs lose their centrosomal localization at the G2/M transition in a manner that relies upon Nek2 kinase activity to ensure low DA levels at mitotic centrosomes. Overexpression of active Nek2A, but not kinase-dead Nek2A, prematurely displaced DAs from the interphase centrosomes of immortalized retina pigment epithelial (RPE1) cells. This dramatic impact was also observed in mammary epithelial cells with constitutively high levels of Nek2. Conversely, Nek2 knockout led to incomplete dissociation of DAs and cilia in mitosis. As a consequence, we observed the presence of a cilia remnant that promoted the asymmetric inheritance of ciliary signaling components and supported cilium reassembly after cell division. Together, our data establish Nek2 as an important kinase that regulates DAs before mitosis.

Actin-based regulation of ciliogenesis - The long and the short of it

The primary cilia is found on the mammalian cell surface where it serves as an antenna for the reception and transmission of a variety of cellular signaling pathways. At its core the cilium is a microtubule-based organelle, but it is clear that its assembly and function are dependent upon the coordinated regulation of both actin and microtubule dynamics. In particular, the discovery that the centrosome is able to act as both a microtubule and actin organizing centre implies that both cytoskeletal networks are acting directly on the process of cilia assembly. In this review, we set our recent results with the formin FHDC1 in the context of current reports that show each stage of ciliogenesis is impacted by changes in actin dynamics. These include direct effects of actin filament assembly on basal body positioning, vesicle trafficking to and entry into the cilium, cilia length, cilia membrane organization and cilia-dependent signaling.

CEP120 interacts with C2CD3 and Talpid3 and is required for centriole appendage assembly and ciliogenesis

Centrosomal protein 120 (CEP120) was originally identified as a daughter centriole-enriched protein that participates in centriole elongation. Recent studies showed that CEP120 gene mutations cause complex ciliopathy phenotypes in humans, including Joubert syndrome and Jeune asphyxiating thoracic dystrophy, suggesting that CEP120 plays an additional role in ciliogenesis. To investigate the potential roles of CEP120 in centriole elongation and cilia formation, we knocked out the CEP120 gene in p53-deficient RPE1 cells using the CRISPR/Cas9 editing system, and performed various analyses. We herein report that loss of CEP120 produces short centrioles with no apparent distal and subdistal appendages. CEP120 knockout was also associated with defective centriole elongation, impaired recruitment of C2CD3 and Talpid3 to the distal ends of centrioles, and consequent defects in centriole appendage assembly and cilia formation. Interestingly, wild-type CEP120 interacts with C2CD3 and Talpid3, whereas a disease-associated CEP120 mutant (I975S) has a low affinity for C2CD3 binding and perturbs cilia assembly. Together, our findings reveal a novel role of CEP120 in ciliogenesis by showing that it interacts with C2CD3 and Talpid3 to assemble centriole appendages and by illuminating the molecular mechanism through which the CEP120 (I975S) mutation causes complex ciliopathies.