Unraveling the mysteries of centriolar satellites: time to rewrite the textbooks about the centrosome/cilium complex

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

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

Glutamylation of centrosomes ensures their function by recruiting microtubule nucleation factors

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

Navigating centriolar satellites: the role of PCM1 in cellular and organismal processes

Centriolar satellites are ubiquitous membrane-less organelles that play critical roles in numerous cellular and organismal processes. They were initially discovered through electron microscopy as cytoplasmic granules surrounding centrosomes in vertebrate cells. These structures remained enigmatic until the identification of pericentriolar material 1 protein (PCM1) as their molecular marker, which has enabled their in-depth characterization. Recently, centriolar satellites have come into the spotlight due to their links to developmental and neurodegenerative disorders. This review presents a comprehensive summary of the major advances in centriolar satellite biology, with a focus on studies that investigated their biology associated with the essential scaffolding protein PCM1. We begin by exploring the molecular, cellular, and biochemical properties of centriolar satellites, laying the groundwork for a deeper understanding of their functions and mechanisms at both cellular and organismal levels. We then examine the implications of their dysregulation in various diseases, particularly highlighting their emerging roles in neurodegenerative and developmental disorders, as revealed by organismal models of PCM1. We conclude by discussing the current state of knowledge and posing questions about the adaptable nature of these organelles, thereby setting the stage for future research.

Conservation of OFD1 Protein Motifs: Implications for Discovery of Novel Interactors and the OFD1 Function

OFD1 is a protein involved in many cellular processes, including cilia biogenesis, mitotic spindle assembly, translation, autophagy and the repair of double-strand DNA breaks. Despite many potential interactors identified in high-throughput studies, only a few have been directly confirmed with their binding sites identified. We performed an analysis of the evolutionary conservation of the OFD1 sequence in three clades: 80 Tetrapoda, 144 Vertebrata or 26 Animalia species, and identified 59 protein-binding motifs localized in the OFD1 regions conserved in various clades. Our results indicate that OFD1 contains 14 potential post-translational modification (PTM) sites targeted by at least eight protein kinases, seven motifs bound by proteins recognizing phosphorylated aa residues and a binding site for phosphatase 2A. Moreover, OFD1 harbors both a motif that enables its phosphorylation by mitogen-activated protein kinases (MAPKs) and a specific docking site for these proteins. Generally, our results suggest that OFD1 forms a scaffold for interaction with many proteins and is tightly regulated by PTMs and ligands. Future research on OFD1 should focus on the regulation of OFD1 function and localization.

Convergence of autism proteins at the cilium

Hundreds of high-confidence autism genes have been identified, yet the relevant etiological mechanisms remain unclear. Gene ontology analyses have repeatedly identified enrichment of proteins with annotated functions in gene expression regulation and neuronal communication. However, proteins are often pleiotropic and these annotations are inherently incomplete. Our recent autism functional genetics work has suggested that these genes may share a common mechanism at the cilium, a membrane-bound organelle critical for neurogenesis, brain patterning, and neuronal activity-all processes strongly implicated in autism. Moreover, autism commonly co-occurs with conditions that are known to involve ciliary-related pathologies, including congenital heart disease, hydrocephalus, and blindness. However, the role of autism genes at the cilium has not been systematically investigated. Here we demonstrate that autism proteins spanning disparate functional annotations converge in expression, localization, and function at cilia, and that patients with pathogenic variants in these genes have cilia-related co-occurring conditions and biomarkers of disrupted ciliary function. This degree of convergence among genes spanning diverse functional annotations strongly suggests that cilia are relevant to autism, as well as to commonly co-occurring conditions, and that this organelle should be explored further for therapeutic potential.

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

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

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.

Coiled-coil domain containing 159 is required for spermatid head and tail assembly in mice†

The centrosome is critical for maintaining the sperm head-tail connection and the formation of flagellar microtubules. In this study, we found that in mouse testes, CCDC159 (coiled-coil domain-containing protein 159) is specifically localized to the head-tail coupling apparatus (HTCA) of spermatids, a structure that ensures sperm head-tail tight conjunction. CCDC159 contains a C-terminal coiled-coil domain that functions as the centrosomal localization signal. Gene knockout (KO) of Ccdc159 in mice resulted in acephalic spermatozoa, abnormal flagella, and male infertility. To explore the mechanism behind CCDC159 regulating spermatogenesis, we identified CCDC159-binding proteins using a yeast two-hybrid screen and speculated that CCDC159 participates in HTCA assembly by regulating protein phosphatase PP1 activity. Further RNA-sequencing analyses of Ccdc159 KO testes revealed numerous genes involved in male gamete generation that were downregulated. Together, our results show that CCDC159 in spermatids is a novel centrosomal protein anchoring the sperm head to the tail. Considering the limitation of KO mouse model in clarifying the biological function of CCDC159 in spermatogenesis, a gene-rescue experiment will be performed in the future.

Necrosulfonamide causes oxidation of PCM1 and impairs ciliogenesis and autophagy

Centriolar satellites are high-order assemblies, scaffolded by the protein PCM1, that gravitate as particles around the centrosome and play pivotal roles in fundamental cellular processes notably ciliogenesis and autophagy. Despite stringent control mechanisms involving phosphorylation and ubiquitination, the landscape of post-translational modifications shaping these structures remains elusive. Here, we report that necrosulfonamide (NSA), a small molecule known for binding and inactivating the pivotal effector of cell death by necroptosis MLKL, intersects with centriolar satellites, ciliogenesis, and autophagy independently of MLKL. NSA functions as a potent redox cycler and triggers the oxidation and aggregation of PCM1 alongside select partners, while minimally impacting the overall distribution of centriolar satellites. Additionally, NSA-mediated ROS production disrupts ciliogenesis and leads to the accumulation of autophagy markers, partially alleviated by PCM1 deletion. Together, these results identify PCM1 as a redox sensor protein and provide new insights into the interplay between centriolar satellites and autophagy.

Lack of CCDC146, a ubiquitous centriole and microtubule-associated protein, leads to non-syndromic male infertility in human and mouse

From a cohort of 167 infertile patients suffering from multiple morphological abnormalities of the flagellum (MMAF), pathogenic bi-allelic mutations were identified in the CCDC146 gene. In somatic cells, CCDC146 is located at the centrosome and at multiple microtubule-related organelles during mitotic division, suggesting that it is a microtubule-associated protein (MAP). To decipher the molecular pathogenesis of infertility associated with CCDC146 mutations, a Ccdc146 knock-out (KO) mouse line was created. KO male mice were infertile, and sperm exhibited a phenotype identical to CCDC146 mutated patients. CCDC146 expression starts during late spermiogenesis. In the spermatozoon, the protein is conserved but is not localized to centrioles, unlike in somatic cells, rather it is present in the axoneme at the level of microtubule doublets. Expansion microscopy associated with the use of the detergent sarkosyl to solubilize microtubule doublets suggests that the protein may be a microtubule inner protein (MIP). At the subcellular level, the absence of CCDC146 impacted all microtubule-based organelles such as the manchette, the head-tail coupling apparatus (HTCA), and the axoneme. Through this study, a new genetic cause of infertility and a new factor in the formation and/or structure of the sperm axoneme were characterized.

Emerging insights into CP110 removal during early steps of ciliogenesis

The primary cilium is an antenna-like projection from the plasma membrane that serves as a sensor of the extracellular environment and a crucial signaling hub. Primary cilia are generated in most mammalian cells, and their physiological significance is highlighted by the large number of severe developmental disorders or ciliopathies that occur when primary ciliogenesis is impaired. Primary ciliogenesis is a tightly regulated process, and a central early regulatory step is the removal of a key mother centriole capping protein, CP110 (also known as CCP110). This uncapping allows vesicles docked on the distal appendages of the mother centriole to fuse to form a ciliary vesicle, which is bent into a ciliary sheath as the microtubule-based axoneme grows and extends from the mother centriole. When the mother centriole migrates toward the plasma membrane, the ciliary sheath fuses with the plasma membrane to form the primary cilium. In this Review, we outline key early steps of primary ciliogenesis, focusing on several novel mechanisms for removal of CP110. We also highlight examples of ciliopathies caused by genetic variants that encode key proteins involved in the early steps of ciliogenesis.

A Chemically Inducible Organelle Rerouting Assay to Probe Primary Cilium Assembly, Maintenance, and Disassembly in Cultured Cells

The primary cilium is a conserved, microtubule-based organelle that protrudes from the surface of most vertebrate cells as well as sensory cells of many organisms. It transduces extracellular chemical and mechanical cues to regulate diverse cellular processes during development and physiology. Loss-of-function studies via RNA interference and CRISPR/Cas9-mediated gene knockouts have been the main tool for elucidating the functions of proteins, protein complexes, and organelles implicated in cilium biology. However, these methods are limited in studying acute spatiotemporal functions of proteins as well as the connection between their cellular positioning and functions. A powerful approach based on inducible recruitment of plus or minus end-directed molecular motors to the protein of interest enables fast and precise control of protein activity in time and in space. In this chapter, we present a chemically inducible heterodimerization method for functional perturbation of centriolar satellites, an emerging membrane-less organelle involved in cilium biogenesis and function. The method we present is based on rerouting of centriolar satellites to the cell center or the periphery in mammalian epithelial cells. We also describe how this method can be applied to study the temporal functions of centriolar satellites during primary cilium assembly, maintenance, and disassembly.

The centrosomal protein 131 participates in the regulation of mitochondrial apoptosis

Centriolar satellites are multiprotein aggregates that orbit the centrosome and govern centrosome homeostasis and primary cilia formation. In contrast to the scaffold PCM1, which nucleates centriolar satellites and has been linked to microtubule dynamics, autophagy, and intracellular trafficking, the functions of its interactant CEP131 beyond ciliogenesis remain unclear. Using a knockout strategy in a non-ciliary T-cell line, we report that, although dispensable for centriolar satellite assembly, CEP131 participates in optimal tubulin glycylation and polyglutamylation, and microtubule regrowth. Our unsupervised label-free proteomic analysis by quantitative mass spectrometry further uncovered mitochondrial and apoptotic signatures. CEP131-deficient cells showed an elongated mitochondrial network. Upon cell death inducers targeting mitochondria, knockout cells displayed delayed cytochrome c release from mitochondria, subsequent caspase activation, and apoptosis. This mitochondrial permeabilization defect was intrinsic, and replicable in vitro with isolated organelles. These findings extend CEP131 functions to life-and-death decisions and propose ways to interfere with mitochondrial apoptosis.

CCDC15 localizes to the centriole inner scaffold and controls centriole length and integrity

Centrioles are microtubule-based organelles responsible for forming centrosomes and cilia, which serve as microtubule-organizing, signaling, and motility centers. Biogenesis and maintenance of centrioles with proper number, size, and architecture are vital for their functions during development and physiology. While centriole number control has been well-studied, less is understood about their maintenance as stable structures with conserved size and architecture during cell division and ciliary motility. Here, we identified CCDC15 as a centriole protein that colocalizes with and interacts with the inner scaffold, a crucial centriolar subcompartment for centriole size control and integrity. Using ultrastructure expansion microscopy, we found that CCDC15 depletion affects centriole length and integrity, leading to defective cilium formation, maintenance, and response to Hedgehog signaling. Moreover, loss-of-function experiments showed CCDC15's role in recruiting both the inner scaffold protein POC1B and the distal SFI1/Centrin-2 complex to centrioles. Our findings reveal players and mechanisms of centriole architectural integrity and insights into diseases linked to centriolar defects.

Organization, functions, and mechanisms of the BBSome in development, ciliopathies, and beyond

The BBSome is an octameric protein complex that regulates ciliary transport and signaling. Mutations in BBSome subunits are closely associated with ciliary defects and lead to ciliopathies, notably Bardet-Biedl syndrome. Over the past few years, there has been significant progress in elucidating the molecular organization and functions of the BBSome complex. An improved understanding of BBSome-mediated biological events and molecular mechanisms is expected to help advance the development of diagnostic and therapeutic approaches for BBSome-related diseases. Here, we review the current literature on the structural assembly, transport regulation, and molecular functions of the BBSome, emphasizing its roles in cilium-related processes. We also provide perspectives on the pathological role of the BBSome in ciliopathies as well as how these can be exploited for therapeutic benefit.

Pleiotropy of autism-associated chromatin regulators

Gene ontology analyses of high-confidence autism spectrum disorder (ASD) risk genes highlight chromatin regulation and synaptic function as major contributors to pathobiology. Our recent functional work in vivo has additionally implicated tubulin biology and cellular proliferation. As many chromatin regulators, including the ASD risk genes ADNP and CHD3, are known to directly regulate both tubulins and histones, we studied the five chromatin regulators most strongly associated with ASD (ADNP, CHD8, CHD2, POGZ and KMT5B) specifically with respect to tubulin biology. We observe that all five localize to microtubules of the mitotic spindle in vitro in human cells and in vivo in Xenopus. Investigation of CHD2 provides evidence that mutations present in individuals with ASD cause a range of microtubule-related phenotypes, including disrupted localization of the protein at mitotic spindles, cell cycle stalling, DNA damage and cell death. Lastly, we observe that ASD genetic risk is significantly enriched among tubulin-associated proteins, suggesting broader relevance. Together, these results provide additional evidence that the role of tubulin biology and cellular proliferation in ASD warrants further investigation and highlight the pitfalls of relying solely on annotated gene functions in the search for pathological mechanisms.

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.

CSPP1 stabilizes growing microtubule ends and damaged lattices from the luminal side

Microtubules are dynamic cytoskeletal polymers, and their organization and stability are tightly regulated by numerous cellular factors. While regulatory proteins controlling the formation of interphase microtubule arrays and mitotic spindles have been extensively studied, the biochemical mechanisms responsible for generating stable microtubule cores of centrioles and cilia are poorly understood. Here, we used in vitro reconstitution assays to investigate microtubule-stabilizing properties of CSPP1, a centrosome and cilia-associated protein mutated in the neurodevelopmental ciliopathy Joubert syndrome. We found that CSPP1 preferentially binds to polymerizing microtubule ends that grow slowly or undergo growth perturbations and, in this way, resembles microtubule-stabilizing compounds such as taxanes. Fluorescence microscopy and cryo-electron tomography showed that CSPP1 is deposited in the microtubule lumen and inhibits microtubule growth and shortening through two separate domains. CSPP1 also specifically recognizes and stabilizes damaged microtubule lattices. These data help to explain how CSPP1 regulates the elongation and stability of ciliary axonemes and other microtubule-based structures.

CCDC66 regulates primary cilium length and signaling via interactions with transition zone and axonemal proteins

The primary cilium is a microtubule-based organelle that serves as a hub for many signaling pathways. It functions as part of the centrosome or cilium complex, which also contains the basal body and the centriolar satellites. Little is known about the mechanisms by which the microtubule-based ciliary axoneme is assembled with a proper length and structure, particularly in terms of the activity of microtubule-associated proteins (MAPs) and the crosstalk between the different compartments of the centrosome or cilium complex. Here, we analyzed CCDC66, a MAP implicated in cilium biogenesis and ciliopathies. Live-cell imaging revealed that CCDC66 compartmentalizes between centrosomes, centriolar satellites, and the ciliary axoneme and tip during cilium biogenesis. CCDC66 depletion in human cells causes defects in cilium assembly, length and morphology. Notably, CCDC66 interacts with the ciliopathy-linked MAPs CEP104 and CSPP1, and regulates axonemal length and Hedgehog pathway activation. Moreover, CCDC66 is required for the basal body recruitment of transition zone proteins and intraflagellar transport B (IFT-B) machinery. Overall, our results establish CCDC66 as a multifaceted regulator of the primary cilium and provide insight into how ciliary MAPs and subcompartments cooperate to ensure assembly of functional cilia.

ATM- and ATR-induced primary ciliogenesis promotes cisplatin resistance in pancreatic ductal adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) is one of the most lethal cancers because of its late diagnosis and chemoresistance. Primary cilia, the cellular antennae, are observed in most human cells to maintain development and differentiation. Primary cilia are gradually lost during the progression of pancreatic cancer and are eventually absent in PDAC. Here, we showed that cisplatin-resistant PDAC regrew primary cilia. Additionally, genetic or pharmacological disruption of primary cilia sensitized PDAC to cisplatin treatment. Mechanistically, ataxia telangiectasia mutated (ATM) and ATM and RAD3-related (ATR), tumor suppressors that initiate DNA damage responses, promoted the excessive formation of centriolar satellites (EFoCS) and autophagy activation. Disruption of EFoCS and autophagy inhibited primary ciliogenesis, sensitizing PDAC cells to cisplatin treatment. Collectively, our findings revealed an unexpected interplay among the DNA damage response, primary cilia, and chemoresistance in PDAC and deciphered the molecular mechanism by which ATM/ATR-mediated EFoCS and autophagy cooperatively regulate primary ciliogenesis.

Cilia proteins getting to work - how do they commute from the cytoplasm to the base of cilia?

Cilia are multifunctional organelles that originated with the last eukaryotic common ancestor and play central roles in the life cycles of diverse organisms. The motile flagella that move single cells like sperm or unicellular organisms, the motile cilia on animal multiciliated cells that generate fluid flow in organs, and the immotile primary cilia that decorate nearly all cells in animals share many protein components in common, yet each also requires specialized proteins to perform their specialized functions. Despite a now-advanced understanding of how such proteins are transported within cilia, we still know very little about how they are transported from their sites of synthesis through the cytoplasm to the ciliary base. Here, we review the literature concerning this underappreciated topic in ciliary cell biology. We discuss both general mechanisms, as well as specific examples of motor-driven active transport and passive transport via diffusion-and-capture. We then provide deeper discussion of specific, illustrative examples, such as the diverse array of protein subunits that together comprise the intraflagellar transport (IFT) system and the multi-protein axonemal dynein motors that drive beating of motile cilia. We hope this Review will spur further work, shedding light not only on ciliogenesis and ciliary signaling, but also on intracellular transport in general.

Moonlighting at the Poles: Non-Canonical Functions of Centrosomes

Centrosomes are best known as the microtubule organizing centers (MTOCs) of eukaryotic cells. In addition to their classic role in chromosome segregation, centrosomes play diverse roles unrelated to their MTOC activity during cell proliferation and quiescence. Metazoan centrosomes and their functional doppelgängers from lower eukaryotes, the spindle pole bodies (SPBs), act as important structural platforms that orchestrate signaling events essential for cell cycle progression, cellular responses to DNA damage, sensory reception and cell homeostasis. Here, we provide a critical overview of the unconventional and often overlooked roles of centrosomes/SPBs in the life cycle of eukaryotic cells.

LUZP1: A new player in the actin-microtubule cross-talk

LUZP1 (leucine zipper protein 1) was first described as being important for embryonic development. Luzp1 null mice present defective neural tube closure and cardiovascular problems, which cause perinatal death. Since then, LUZP1 has also been implicated in the etiology of diseases like the 1p36 and the Townes-Brocks syndromes, and the molecular mechanisms involving this protein started being uncovered. Proteomics studies placed LUZP1 in the interactomes of the centrosome-cilium interface, centriolar satellites, and midbody. Concordantly, LUZP1 is an actin and microtubule-associated protein, which localizes to the centrosome, the basal body of primary cilia, the midbody, actin filaments and cellular junctions. LUZP1, like its interactor EPLIN, is an actin-stabilizing protein and a negative regulator of primary cilia formation. Moreover, through the regulation of actin, LUZP1 has been implicated in the regulation of cell cycle progression, cell migration and epithelial cell apical constriction. This review discusses the latest findings concerning LUZP1 molecular functions and implications in disease development.

Estrogens—Origin of Centrosome Defects in Human Cancer?

Estrogens are associated with a variety of diseases and play important roles in tumor development and progression. Centrosome defects are hallmarks of human cancers and contribute to ongoing chromosome missegragation and aneuploidy that manifest in genomic instability and tumor progression. Although several mechanisms underlie the etiology of centrosome aberrations in human cancer, upstream regulators are hardly known. Accumulating experimental and clinical evidence points to an important role of estrogens in deregulating centrosome homeostasis and promoting karyotype instability. Here, we will summarize existing literature of how natural and synthetic estrogens might contribute to structural and numerical centrosome defects, genomic instability and human carcinogenesis.

Function of Centriolar Satellites and Regulation by Post-Translational Modifications

Centriolar satellites are small membrane-less granules that gravitate around the centrosome. Recent advances in defining the satellite proteome and interactome have unveiled hundreds of new satellite components thus illustrating the complex nature of these particles. Although initially linked to the homeostasis of centrosome and the formation of primary cilia, these composite and highly dynamic structures appear to participate in additional cellular processes, such as proteostasis, autophagy, and cellular stress. In this review, we first outline the main features and many roles of centriolar satellites. We then discuss how post-translational modifications, such as phosphorylation and ubiquitination, shape their composition and functions. This is of particular interest as interfering with these processes may provide ways to manipulate these structures.

Aurora Kinase A proximity map reveals centriolar satellites as regulators of its ciliary function

Aurora kinase A (AURKA) is a conserved kinase that plays crucial roles in numerous cellular processes. Although AURKA overexpression is frequent in human cancers, its pleiotropic functions and multifaceted regulation present challenges in its therapeutic targeting. Key to overcoming these challenges is to identify and characterize the full range of AURKA interactors, which are often weak and transient. Previous proteomic studies were limited in monitoring dynamic and non-mitotic AURKA interactions. Here, we generate the proximity interactome of AURKA in asynchronous cells, which consists of 440 proteins involving multiple biological processes and cellular compartments. Importantly, AURKA has extensive proximate and physical interactions to centriolar satellites, key regulators of the primary cilium. Loss-of-function experiments identify satellites as negative regulators of AURKA activity, abundance, and localization in quiescent cells. Notably, loss of satellites activates AURKA at the basal body, decreases centrosomal IFT88 levels, and causes ciliogenesis defects. Collectively, our results provide a resource for dissecting spatiotemporal regulation of AURKA and uncover its proteostatic regulation by satellites as a new mechanism for its ciliary functions.

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

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

Centrosome organization and functions

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

The Functionally Unannotated Proteome of Human Male Tissues: A Shared Resource to Uncover New Protein Functions Associated with Reproductive Biology

In the context of the Human Proteome Project, we built an inventory of 412 functionally unannotated human proteins for which experimental evidence at the protein level exists (uPE1) and which are highly expressed in tissues involved in human male reproduction. We implemented a strategy combining literature mining, bioinformatics tools to collate annotation and experimental information from specific molecular public resources, and efficient visualization tools to put these unknown proteins into their biological context (protein complexes, tissue and subcellular location, expression pattern). The gathered knowledge allowed pinpointing five uPE1 for which a function has recently been proposed and which should be updated in protein knowledge bases. Furthermore, this bioinformatics strategy allowed to build new functional hypotheses for five other uPE1s in link with phenotypic traits that are specific to male reproductive function such as ciliogenesis/flagellum formation in germ cells (CCDC112 and TEX9), chromatin remodeling (C3orf62) and spermatozoon maturation (CCDC183). We also discussed the enigmatic case of MAGEB proteins, a poorly documented cancer/testis antigen subtype. Tools used and computational outputs produced during this study are freely accessible via ProteoRE (http://www.proteore.org), a Galaxy-based instance, for reuse purposes. We propose these five uPE1s should be investigated in priority by expert laboratories and hope that this inventory and shared resources will stimulate the interest of the community of reproductive biology.

WDR90 is a centriolar microtubule wall protein important for centriole architecture integrity

Centrioles are characterized by a nine-fold arrangement of microtubule triplets held together by an inner protein scaffold. These structurally robust organelles experience strenuous cellular processes such as cell division or ciliary beating while performing their function. However, the molecular mechanisms underlying the stability of microtubule triplets, as well as centriole architectural integrity remain poorly understood. Here, using ultrastructure expansion microscopy for nanoscale protein mapping, we reveal that POC16 and its human homolog WDR90 are components of the microtubule wall along the central core region of the centriole. We further found that WDR90 is an evolutionary microtubule associated protein. Finally, we demonstrate that WDR90 depletion impairs the localization of inner scaffold components, leading to centriole structural abnormalities in human cells. Altogether, this work highlights that WDR90 is an evolutionary conserved molecular player participating in centriole architecture integrity.

Cells are made up of compartments called organelles that perform specific roles. A cylindrical organelle called the centriole is important for a number of cellular processes, ranging from cell division to movement and signaling. Each centriole contains nine blades made up of protein filaments called microtubules, which link together to form a cylinder. This well-known structure can be found in a variety of different species. Yet, it is unclear how centrioles are able to maintain this stable architecture whilst carrying out their various different cell roles.

In early 2020, a group of researchers discovered a scaffold protein at the center of centrioles that helps keep the microtubule blades stable. Further investigation suggested that another protein called WDR90 may also help centrioles sustain their cylindrical shape. However, the exact role of this protein was poorly understood.

To determine the role of WDR90, Steib et al. – including many of the researchers involved in the 2020 study – used a method called Ultrastructure Expansion Microscopy to precisely locate the WDR90 protein in centrioles. This revealed that WDR90 is located on the microtubule wall of centrioles in green algae and human cells grown in the lab. Further experiments showed that the protein binds directly to microtubules and that removing WDR90 from human cells causes centrioles to lose their scaffold proteins and develop structural defects.

This investigation provides fundamental insights into the structure and stability of centrioles. It shows that single proteins are key components in supporting the structural integrity of organelles and shaping their overall architecture. Furthermore, these findings demonstrate how ultrastructure expansion microscopy can be used to determine the role of individual proteins within a complex structure.

Microtubule-associated proteins and emerging links to primary cilium structure, assembly, maintenance, and disassembly

The primary cilium is a microtubule-based structure that protrudes from the cell surface in diverse eukaryotic organisms. It functions as a key signaling center that decodes a variety of mechanical and chemical stimuli and plays fundamental roles in development and homeostasis. Accordingly, structural and functional defects of the primary cilium have profound effects on the physiology of multiple organ systems including kidney, retina, and central nervous system. At the core of the primary cilium is the microtubule-based axoneme, which supports the cilium shape and acts as the scaffold for bidirectional transport of cargoes into and out of cilium. Advances in imaging, proteomics, and structural biology have revealed new insights into the ultrastructural organization and composition of the primary cilium, the mechanisms that underlie its biogenesis and functions, and the pathologies that result from their deregulation termed ciliopathies. In this viewpoint, we first discuss the recent studies that identified the three-dimensional native architecture of the ciliary axoneme and revealed that it is considerably different from the well-known '9 + 0' paradigm. Moving forward, we explore emerging themes in the assembly and maintenance of the axoneme, with a focus on how microtubule-associated proteins regulate its structure, length, and stability. This far more complex picture of the primary cilium structure and composition, as well as the recent technological advances, open up new avenues for future research.

Spatial Characterization of the Human Centrosome Proteome Opens Up New Horizons for a Small but Versatile Organelle

After a century of research, the human centrosome continues to fascinate. Based on immunofluorescence and confocal microscopy, an extensive inventory of the protein components of the human centrosome, and the centriolar satellites, with the important contribution of over 300 novel proteins localizing to these compartments is presented. A network of candidate centrosome proteins involved in ubiquitination, including six interaction partners of the Kelch-like protein 21, and an additional network of protein phosphatases, together supporting the suggested role of the centrosome as an interactive hub for cell signaling, is identified. Analysis of multi-localization across cellular organelles analyzed within the Human Protein Atlas (HPA) project shows how multi-localizing proteins are particularly overrepresented in centriolar satellites, supporting the dynamic nature and wide range of functions for this compartment. In summary, the spatial dissection of the human centrosome and centriolar satellites described here provides a comprehensive knowledgebase for further exploration of their proteomes.

Acute inhibition of centriolar satellite function and positioning reveals their functions at the primary cilium

Centriolar satellites are dynamic, membraneless granules composed of over 200 proteins. They store, modify, and traffic centrosome and primary cilium proteins, and help to regulate both the biogenesis and some functions of centrosomes and cilium. In most cell types, satellites cluster around the perinuclear centrosome, but their integrity and cellular distribution are dynamically remodeled in response to different stimuli, such as cell cycle cues. Dissecting the specific and temporal functions and mechanisms of satellites and how these are influenced by their cellular positioning and dynamics has been challenging using genetic approaches, particularly in ciliated and proliferating cells. To address this, we developed a chemical-based trafficking assay to rapidly and efficiently redistribute satellites to either the cell periphery or center, and fuse them into stable clusters in a temporally controlled way. Induced satellite clustering at either the periphery or center resulted in antagonistic changes in the pericentrosomal levels of a subset of proteins, revealing a direct and selective role for their positioning in protein targeting and sequestration. Systematic analysis of the interactome of peripheral satellite clusters revealed enrichment of proteins implicated in cilium biogenesis and mitosis. Importantly, induction of peripheral satellite targeting in ciliated cells revealed a function for satellites not just for efficient cilium assembly but also in the maintenance of steady-state cilia and in cilia disassembly by regulating the structural integrity of the ciliary axoneme. Finally, perturbing satellite distribution and dynamics inhibited their mitotic dissolution, and mitotic progression was perturbed only in cells with centrosomal satellite clustering. Collectively, our results for the first time showed a direct link between satellite functions and their pericentrosomal clustering, suggested new mechanisms underlying satellite functions during cilium assembly, and provided a new tool for probing temporal satellite functions in different contexts

What happens when centriolar satellites are not in the right place at the right time? By redistributing satellites to the periphery or center of the cell and assessing the consequences of their mispositioning, this study reveals novel functions for satellites during mitosis, cilium maintenance, and cilium disassembly and suggests new mechanisms.

A Proximity Mapping Journey into the Biology of the Mammalian Centrosome/Cilium Complex

The mammalian centrosome/cilium complex is composed of the centrosome, the primary cilium and the centriolar satellites, which together regulate cell polarity, signaling, proliferation and motility in cells and thereby development and homeostasis in organisms. Accordingly, deregulation of its structure and functions is implicated in various human diseases including cancer, developmental disorders and neurodegenerative diseases. To better understand these disease connections, the molecular underpinnings of the assembly, maintenance and dynamic adaptations of the centrosome/cilium complex need to be uncovered with exquisite detail. Application of proximity-based labeling methods to the centrosome/cilium complex generated spatial and temporal interaction maps for its components and provided key insights into these questions. In this review, we first describe the structure and cell cycle-linked regulation of the centrosome/cilium complex. Next, we explain the inherent biochemical and temporal limitations in probing the structure and function of the centrosome/cilium complex and describe how proximity-based labeling approaches have addressed them. Finally, we explore current insights into the knowledge we gained from the proximity mapping studies as it pertains to centrosome and cilium biogenesis and systematic characterization of the centrosome, cilium and centriolar satellite interactomes.