LRRC45 contributes to early steps of axoneme extension

Renal ciliopathies

Primary cilia are essential cellular organelles with pivotal roles in many signalling pathways. Here we provide an overview of the role of primary cilia within the kidney, starting with primary ciliary structure and key protein complexes. We then highlight the specialised functions of primary cilia, emphasising their role in a group of diseases known as renal ciliopathies. These conditions include forms of polycystic kidney disease, nephronophthisis, and other syndromic ciliopathies, such as Joubert syndrome and Bardet-Biedl syndrome. We explore models of renal ciliopathies, both in vitro and in vivo, shedding light on the molecular mechanisms underlying these diseases including Wnt and Hedgehog signalling pathways, inflammation, and cellular metabolism. Finally, we discuss therapeutic approaches, from current treatments to cutting-edge preclinical research and clinical trials.

Biallelic Variants in LRRC45 Impair Ciliogenesis and Cause a Severe Neurological Disorder

Leucine - rich repeat containing 45 protein (LRRC45) protein localizes at the proximal end of centrioles and forms a component of the proteinaceous linker between them, with an important role in centrosome cohesion. In addition, a pool of it localizes at the distal appendages of the modified parent centriole that forms the primary cilium and it has essential functions in the establishment of the transition zone and axonemal extension during early ciliogenesis. Here, we describe three individuals from two unrelated families with severe central nervous system anomalies. Exome sequencing identified biallelic variants in LRRC45 in the affected individuals: P1: c.1402-2A>G; P2 and P3: c.1262G>C (p.Arg421Thr). Investigation of the variant c.1402-2A>G in patient-derived skin fibroblasts revealed that it triggers aberrant splicing, leading to an abnormal LRRC45 transcript that lacks exon 14. Consistent with this the mRNA and protein levels of LRRC45 were drastically reduced in P1-derived fibroblast cells compared to the controls. P1 fibroblasts showed a significant reduction of primary cilia frequency and length. In silico modeling of the missense variant in P2/P3 suggested a destabilizing effect on LRRC45. Given these findings, we propose that the pathogenic loss-of-function variants in LRRC45 are associated with a novel spectrum of neurological ciliopathy phenotypes.

Myristoylated Neuronal Calcium Sensor-1 captures the preciliary vesicle at distal appendages

The primary cilium is a microtubule-based organelle that cycles through assembly and disassembly. In many cell types, formation of the cilium is initiated by recruitment of preciliary vesicles to the distal appendage of the mother centriole. However, the distal appendage mechanism that directly captures preciliary vesicles is yet to be identified. In an accompanying paper, we show that the distal appendage protein, CEP89, is important for the preciliary vesicle recruitment, but not for other steps of cilium formation (Kanie et al., 2025). The lack of a membrane-binding motif in CEP89 suggests that it may indirectly recruit preciliary vesicles via another binding partner. Here, we identify Neuronal Calcium Sensor-1 (NCS1) as a stoichiometric interactor of CEP89. NCS1 localizes to the position between CEP89 and the centriole-associated vesicle marker, RAB34, at the distal appendage. This localization was completely abolished in CEP89 knockouts, suggesting that CEP89 recruits NCS1 to the distal appendage. Similar to CEP89 knockouts, preciliary vesicle recruitment as well as subsequent cilium formation was perturbed in NCS1 knockout cells. The ability of NCS1 to recruit the preciliary vesicle is dependent on its myristoylation motif and NCS1 knockout cells expressing a myristoylation defective mutant failed to rescue the vesicle recruitment defect despite localizing properly to the centriole. In sum, our analysis reveals the first known mechanism for how the distal appendage recruits the preciliary vesicles.

A hierarchical pathway for assembly of the distal appendages that organize primary cilia

Distal appendages are ninefold symmetric blade-like structures attached to the distal end of the mother centriole. These structures are critical for the formation of the primary cilium, by regulating at least four critical steps: preciliary vesicle recruitment, recruitment and initiation of intraflagellar transport (IFT), and removal of CP110. While specific proteins that localize to the distal appendages have been identified, how exactly each protein functions to achieve the multiple roles of the distal appendages is poorly understood. Here, we comprehensively analyze known and newly discovered distal appendage proteins (CEP83, SCLT1, CEP164, TTBK2, FBF1, CEP89, KIZ, ANKRD26, PIDD1, LRRC45, NCS1, CEP15) for their precise localization, order of recruitment, and their roles in each step of cilia formation. Using CRISPR-Cas9 knockouts, we show that the order of the recruitment of the distal appendage proteins is highly interconnected and a more complex hierarchy. Our analysis highlights two protein modules, CEP83-SCLT1 and CEP164-TTBK2, as critical for structural assembly of distal appendages. Functional assays revealed that CEP89 selectively functions in the RAB34+ vesicle recruitment, while deletion of the integral components, CEP83-SCLT1-CEP164-TTBK2, severely compromised all four steps of cilium formation. Collectively, our analyses provide a more comprehensive view of the organization and the function of the distal appendage, paving the way for molecular understanding of ciliary assembly.

LRRC45 promotes lung cancer proliferation and progression by enhancing c-MYC, slug, MMP2, and MMP9 expression

BACKGROUND

The leucine-rich repeat-containing (LRRC) superfamily members are known for their significant roles in tumorigenesis and cellular proliferation. However, the specific regulatory role of LRRC45 in lung cancer remains unexplored. This study investigated the impact and underlying mechanisms of LRRC45 on the proliferative, migratory, and invasive capacities of lung adenocarcinoma (LUAD) cells, potentially identifying new targets for therapeutic intervention.

MATERIAL AND METHODS

The importance of LRRC45 in lung cancer was analyzed using the online databases of UCSC Xena, TCGA, TISIDB, and UALCAN, whereas to detect target gene expression, we used the qRT-PCR, Western blot, and immunofluorescence confocal. The cell growth was monitored by colony formation assay and migration was examined by cell migration assay. Finally, a xenograft mouse tumor model using A549 ​cells was used to explore the in vivo effect of LRRC45 in lung cancer.

RESULTS

Inhibition of LRRC45 expression led to a notable decrease in proliferation, migration, and invasion of A549 and H1299 ​cells. LRRC45 silencing significantly reduced the tumor volume and improved the mice's survival. Additionally, inhibition of LRRC45 expression dramatically suppressed c-MYC, Slug, MMP2, and MMP9 expression. Overexpression of c-MYC and/or Slug in the LRRC45-deficient cells can partially or totally restore the LRRC45 deficiency-suppressed growth. Moreover, the overexpression of MMP2 and/or MMP9 could partially or totally restore LRRC45 deficiency-reduced cell metastasis.

CONCLUSIONS

LRRC45 could promote the proliferative, migrative, and invasive capacities of lung cancer cells by increasing c-MYC, Slug, MMP2, and MMP9 expression, indicating the therapeutic implications and potential significance of these pathways in lung cancer.

TonEBP inhibits ciliogenesis by controlling aurora kinase A and regulating centriolar satellite integrity

BACKGROUND

Primary cilia on the surface of eukaryotic cells serve as sensory antennas for the reception and transmission in various cell signaling pathways. They are dynamic organelles that rapidly form during differentiation and cell cycle exit. Defects in these organelles cause a group of wide-ranging disorders called ciliopathies. Tonicity-responsive enhancer-binding protein (TonEBP) is a pleiotropic stress protein that mediates various physiological and pathological cellular responses. TonEBP is well-known for its role in adaptation to a hypertonic environment, to which primary cilia have been reported to contribute. Furthermore, TonEBP is involved in a wide variety of other signaling pathways, such as Sonic Hedgehog and WNT signaling, that promote primary ciliogenesis, suggesting a possible regulatory role. However, the functional relationship between TonEBP and primary ciliary formation remains unclear.

METHODS

TonEBP siRNAs and TonEBP-mCherry plasmids were used to examine their effects on cell ciliation rates, assembly and disassembly processes, and regulators. Serum starvation was used as a condition to induce ciliogenesis.

RESULTS

We identified a novel pericentriolar localization for TonEBP. The results showed that TonEBP depletion facilitates the formation of primary cilia, whereas its overexpression results in fewer ciliated cells. Moreover, TonEBP controlled the expression and activity of aurora kinase A, a major negative regulator of ciliogenesis. Additionally, TonEBP overexpression inhibited the loss of CP110 from the mother centrioles during the early stages of primary cilia assembly. Finally, TonEBP regulated the localization of PCM1 and AZI1, which are necessary for primary cilia formation.

CONCLUSIONS

This study proposes a novel role for TonEBP as a pericentriolar protein that regulates the integrity of centriolar satellite components. This regulation has shown to have a negative effect on ciliogenesis. Investigations into cilium assembly and disassembly processes suggest that TonEBP acts upstream of the aurora kinase A - histone deacetylase 6 signaling pathway and affects basal body formation to control ciliogenesis. Taken together, our data proposes previously uncharacterized regulation of primary cilia assembly by TonEBP.

The ARPKD Protein DZIP1L Regulates Ciliary Protein Entry by Modulating the Architecture and Function of Ciliary Transition Fibers

Serving as the cell's sensory antennae, primary cilia are linked to numerous human genetic diseases when they malfunction. DZIP1L, identified as one of the genetic causes of human autosomal recessive polycystic kidney disease (ARPKD), is an evolutionarily conserved ciliary basal body protein. Although it has been reported that DZIP1L is involved in the ciliary entry of PKD proteins, the underlying mechanism remains elusive. Here, an uncharacterized role of DZIP1L is reported in modulating the architecture and function of transition fibers (TFs), striking ciliary base structures essential for selective cilia gating. Using C. elegans as a model, C01G5.7 (hereafter termed DZIP-1) is identified as the sole homolog of DZIP1L, which specifically localizes to TFs. While DZIP-1 or ANKR-26 (the ortholog of ANKRD26) deficiency shows subtle impact on TFs, co-depletion of DZIP-1 and ANKR-26 disrupts TF assembly and cilia gating for soluble and membrane proteins, including the ortholog of ADPKD protein polycystin-2. Notably, the synergistic role for DZIP1L and ANKRD26 in the formation and function of TFs is highly conserved in mammalian cilia. Hence, the findings illuminate an evolutionarily conserved role of DZIP1L in TFs architecture and function, highlighting TFs as a vital part of the ciliary gate implicated in ciliopathies ARPKD.

Identification of 30 transition fibre proteins in Trypanosoma brucei reveals a complex and dynamic structure

Transition fibres and distal appendages surround the distal end of mature basal bodies and are essential for ciliogenesis, but only a few of the proteins involved have been identified and functionally characterised. Here, through genome-wide analysis, we have identified 30 transition fibre proteins (TFPs) and mapped their arrangement in the flagellated eukaryote Trypanosoma brucei. We discovered that TFPs are recruited to the mature basal body before and after basal body duplication, with differential expression of five TFPs observed at the assembling new flagellum compared to the existing fixed-length old flagellum. RNAi-mediated depletion of 17 TFPs revealed six TFPs that are necessary for ciliogenesis and a further three TFPs that are necessary for normal flagellum length. We identified nine TFPs that had a detectable orthologue in at least one basal body-forming eukaryotic organism outside of the kinetoplastid parasites. Our work has tripled the number of known transition fibre components, demonstrating that transition fibres are complex and dynamic in their composition throughout the cell cycle, which relates to their essential roles in ciliogenesis and flagellum length regulation.

Non-canonical role for the BAF complex subunit DPF3 in mitosis and ciliogenesis

DPF3, along with other subunits, is a well-known component of the BAF chromatin remodeling complex, which plays a key role in regulating chromatin remodeling activity and gene expression. Here, we elucidated a non-canonical localization and role for DPF3. We showed that DPF3 dynamically localizes to the centriolar satellites in interphase and to the centrosome, spindle midzone and bridging fiber area, and midbodies during mitosis. Loss of DPF3 causes kinetochore fiber instability, unstable kinetochore-microtubule attachment and defects in chromosome alignment, resulting in altered mitotic progression, cell death and genomic instability. In addition, we also demonstrated that DPF3 localizes to centriolar satellites at the base of primary cilia and is required for ciliogenesis by regulating axoneme extension. Taken together, these findings uncover a moonlighting dual function for DPF3 during mitosis and ciliogenesis.

The intercentriolar fibers function as docking sites of centriolar satellites for cilia assembly

Two mother centrioles in an animal cell are linked by intercentriolar fibers that have CROCC/rootletin as their main building block. Here, we investigated the regulatory role of intercentriolar/rootlet fibers in cilia assembly. The cilia formation rates were significantly reduced in the CEP250/C-NAP1 and CROCC/rootletin knockout (KO) cells, irrespective of the departure of the young mother centrioles from the basal bodies. In addition, centriolar satellites were dispersed throughout the cytoplasm in the CEP250 and CROCC KO cells. We observed that PCM1 directly binds to CROCC. Their interaction is critical not only for the accumulation of centriolar satellites near the centrosomes/basal bodies but also for cilia formation. Finally, we observed that the centriolar satellite proteins are localized at the intercentriolar/rootlet fibers in the kidney epithelial cells. Based on these findings, we propose that the intercentriolar/rootlet fibers function as docking sites for centriolar satellites near the centrosomes/basal bodies and facilitate the cilia assembly process.

A stress-induced cilium-to-PML-NB route drives senescence initiation

Cellular senescence contributes to tissue homeostasis and age-related pathologies. However, how senescence is initiated in stressed cells remains vague. Here, we discover that exposure to irradiation, oxidative or inflammatory stressors induces transient biogenesis of primary cilia, which are then used by stressed cells to communicate with the promyelocytic leukemia nuclear bodies (PML-NBs) to initiate senescence responses in human cells. Mechanistically, a ciliary ARL13B-ARL3 GTPase cascade negatively regulates the association of transition fiber protein FBF1 and SUMO-conjugating enzyme UBC9. Irreparable stresses downregulate the ciliary ARLs and release UBC9 to SUMOylate FBF1 at the ciliary base. SUMOylated FBF1 then translocates to PML-NBs to promote PML-NB biogenesis and PML-NB-dependent senescence initiation. Remarkably, Fbf1 ablation effectively subdues global senescence burden and prevents associated health decline in irradiation-treated mice. Collectively, our findings assign the primary cilium a key role in senescence induction in mammalian cells and, also, a promising target in future senotherapy strategies.

Morphologically intact airways in lung fibrosis have an abnormal proteome

Honeycombing is a histological pattern consistent with Usual Interstitial Pneumonia (UIP). Honeycombing refers to cystic airways located at sites of dense fibrosis with marked mucus accumulation. Utilizing laser capture microdissection coupled mass spectrometry (LCM-MS), we interrogated the fibrotic honeycomb airway cells and fibrotic uninvolved airway cells (distant from honeycomb airways and morphologically intact) in specimens from 10 patients with UIP. Non-fibrotic airway cell specimens from 6 patients served as controls. Furthermore, we performed LCM-MS on the mucus plugs found in 6 patients with UIP and 6 patients with mucinous adenocarcinoma. The mass spectrometry data were subject to both qualitative and quantitative analysis and validated by immunohistochemistry. Surprisingly, fibrotic uninvolved airway cells share a similar protein profile to honeycomb airway cells, showing deregulation of the slit and roundabout receptor (Slit and Robo) pathway as the strongest category. We find that (BPI) fold-containing family B member 1 (BPIFB1) is the most significantly increased secretome-associated protein in UIP, whereas Mucin-5AC (MUC5AC) is the most significantly increased in mucinous adenocarcinoma. We conclude that fibrotic uninvolved airway cells share pathological features with fibrotic honeycomb airway cells. In addition, fibrotic honeycomb airway cells are enriched in mucin biogenesis proteins with a marked derangement in proteins essential for ciliogenesis. This unbiased spatial proteomic approach generates novel and testable hypotheses to decipher fibrosis progression.

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.

Myristoylated Neuronal Calcium Sensor-1 captures the ciliary vesicle at distal appendages

The primary cilium is a microtubule-based organelle that cycles through assembly and disassembly. In many cell types, formation of the cilium is initiated by recruitment of ciliary vesicles to the distal appendage of the mother centriole. However, the distal appendage mechanism that directly captures ciliary vesicles is yet to be identified. In an accompanying paper, we show that the distal appendage protein, CEP89, is important for thef ciliary vesicle recruitment, but not for other steps of cilium formation (Tomoharu Kanie, Love, Fisher, Gustavsson, & Jackson, 2023). The lack of a membrane binding motif in CEP89 suggests that it may indirectly recruit ciliary vesicles via another binding partner. Here, we identify Neuronal Calcium Sensor-1 (NCS1) as a stoichiometric interactor of CEP89. NCS1 localizes to the position between CEP89 and a ciliary vesicle marker, RAB34, at the distal appendage. This localization was completely abolished in CEP89 knockouts, suggesting that CEP89 recruits NCS1 to the distal appendage. Similarly to CEP89 knockouts, ciliary vesicle recruitment as well as subsequent cilium formation was perturbed in NCS1 knockout cells. The ability of NCS1 to recruit the ciliary vesicle is dependent on its myristoylation motif and NCS1 knockout cells expressing myristoylation defective mutant failed to rescue the vesicle recruitment defect despite localizing proper localization to the centriole. In sum, our analysis reveals the first known mechanism for how the distal appendage recruits the ciliary vesicles.

A hierarchical pathway for assembly of the distal appendages that organize primary cilia

Distal appendages are nine-fold symmetric blade-like structures attached to the distal end of the mother centriole. These structures are critical for formation of the primary cilium, by regulating at least four critical steps: ciliary vesicle recruitment, recruitment and initiation of intraflagellar transport (IFT), and removal of CP110. While specific proteins that localize to the distal appendages have been identified, how exactly each protein functions to achieve the multiple roles of the distal appendages is poorly understood. Here we comprehensively analyze known and newly discovered distal appendage proteins (CEP83, SCLT1, CEP164, TTBK2, FBF1, CEP89, KIZ, ANKRD26, PIDD1, LRRC45, NCS1, C3ORF14) for their precise localization, order of recruitment, and their roles in each step of cilia formation. Using CRISPR-Cas9 knockouts, we show that the order of the recruitment of the distal appendage proteins is highly interconnected and a more complex hierarchy. Our analysis highlights two protein modules, CEP83-SCLT1 and CEP164-TTBK2, as critical for structural assembly of distal appendages. Functional assay revealed that CEP89 selectively functions in RAB34+ ciliary vesicle recruitment, while deletion of the integral components, CEP83-SCLT1-CEP164-TTBK2, severely compromised all four steps of cilium formation. Collectively, our analyses provide a more comprehensive view of the organization and the function of the distal appendage, paving the way for molecular understanding of ciliary assembly.

Tissue-specific requirement of sodium channel and clathrin linker 1 (Sclt1) for ciliogenesis during limb development

Primary cilia have essential roles as signaling centers during development and adult homeostasis. Disruption of ciliary structure or function causes congenital human disorders called ciliopathies. Centriolar distal appendage (DAP) proteins are important for anchoring cilia to the membrane. However, the exact functions of DAP during in vivo ciliogenesis and animal development remain poorly understood. Here, we showed that the DAP component sodium channel and clathrin linker 1 (Sclt1) mutant mice had abnormal craniofacial and limb development with postnatal lethality. In mutant embryos, most of the affected tissues had defects in DAP recruitment to the basal body and docking to the membrane that resulted in reduced ciliogenesis and disrupted hedgehog (Hh) signaling in limb bud mesenchymal cells. However, limb digit formation and ciliogenesis in Sclt1 mutant mice were differentially affected between the fore- and hindlimb buds. The forelimbs developed normally in Sclt1 mutants, but the hindlimbs had preaxial polydactyly. Heterozygous loss of Cep83, another core DAP component, in Sclt1 mutant mice, caused forelimb and hindlimb polydactyly. These findings revealed the tissue-specific differential requirement of DAPs. Taken together, these results indicated that during limb development the ciliary base components, DAPs, play an essential role in ciliogenesis and Hh signaling in vivo in a position-dependent manner.

The centrosomal protein 83 (CEP83) regulates human pluripotent stem cell differentiation toward the kidney lineage

During embryonic development, the mesoderm undergoes patterning into diverse lineages including axial, paraxial, and lateral plate mesoderm (LPM). Within the LPM, the so-called intermediate mesoderm (IM) forms kidney and urogenital tract progenitor cells, while the remaining LPM forms cardiovascular, hematopoietic, mesothelial, and additional progenitor cells. The signals that regulate these early lineage decisions are incompletely understood. Here, we found that the centrosomal protein 83 (CEP83), a centriolar component necessary for primary cilia formation and mutated in pediatric kidney disease, influences the differentiation of human-induced pluripotent stem cells (hiPSCs) toward IM. We induced inactivating deletions of CEP83 in hiPSCs and applied a 7-day in vitro protocol of IM kidney progenitor differentiation, based on timed application of WNT and FGF agonists. We characterized induced mesodermal cell populations using single-cell and bulk transcriptomics and tested their ability to form kidney structures in subsequent organoid culture. While hiPSCs with homozygous CEP83 inactivation were normal regarding morphology and transcriptome, their induced differentiation into IM progenitor cells was perturbed. Mesodermal cells induced after 7 days of monolayer culture of CEP83-deficient hiPCS exhibited absent or elongated primary cilia, displayed decreased expression of critical IM genes (PAX8, EYA1, HOXB7), and an aberrant induction of LPM markers (e.g. FOXF1, FOXF2, FENDRR, HAND1, HAND2). Upon subsequent organoid culture, wildtype cells differentiated to form kidney tubules and glomerular-like structures, whereas CEP83-deficient cells failed to generate kidney cell types, instead upregulating cardiomyocyte, vascular, and more general LPM progenitor markers. Our data suggest that CEP83 regulates the balance of IM and LPM formation from human pluripotent stem cells, identifying a potential link between centriolar or ciliary function and mesodermal lineage induction.

Emerging roles of centrosome cohesion

The centrosome, consisting of centrioles and the associated pericentriolar material, is the main microtubule-organizing centre (MTOC) in animal cells. During most of interphase, the two centrosomes of a cell are joined together by centrosome cohesion into one MTOC. The most dominant element of centrosome cohesion is the centrosome linker, an interdigitating, fibrous network formed by the protein C-Nap1 anchoring a number of coiled-coil proteins including rootletin to the proximal end of centrioles. Alternatively, centrosomes can be kept together by the action of the minus end directed kinesin motor protein KIFC3 that works on interdigitating microtubules organized by both centrosomes and probably by the actin network. Although cells connect the two interphase centrosomes by several mechanisms into one MTOC, the general importance of centrosome cohesion, particularly for an organism, is still largely unclear. In this article, we review the functions of the centrosome linker and discuss how centrosome cohesion defects can lead to diseases.

Molecular diagnoses in the congenital malformations caused by ciliopathies cohort of the 100,000 Genomes Project

BACKGROUND

Primary ciliopathies represent a group of inherited disorders due to defects in the primary cilium, the 'cell's antenna'. The 100,000 Genomes Project was launched in 2012 by Genomics England (GEL), recruiting National Health Service (NHS) patients with eligible rare diseases and cancer. Sequence data were linked to Human Phenotype Ontology (HPO) terms entered by recruiting clinicians.

METHODS

Eighty-three prescreened probands were recruited to the 100,000 Genomes Project suspected to have congenital malformations caused by ciliopathies in the following disease categories: Bardet-Biedl syndrome (n=45), Joubert syndrome (n=14) and 'Rare Multisystem Ciliopathy Disorders' (n=24). We implemented a bespoke variant filtering and analysis strategy to improve molecular diagnostic rates for these participants.

RESULTS

We determined a research molecular diagnosis for n=43/83 (51.8%) probands. This is 19.3% higher than previously reported by GEL (n=27/83 (32.5%)). A high proportion of diagnoses are due to variants in non-ciliopathy disease genes (n=19/43, 44.2%) which may reflect difficulties in clinical recognition of ciliopathies. n=11/83 probands (13.3%) had at least one causative variant outside the tiers 1 and 2 variant prioritisation categories (GEL's automated triaging procedure), which would not be reviewed in standard 100,000 Genomes Project diagnostic strategies. These include four structural variants and three predicted to cause non-canonical splicing defects. Two unrelated participants have biallelic likely pathogenic variants in LRRC45, a putative novel ciliopathy disease gene.

CONCLUSION

These data illustrate the power of linking large-scale genome sequence to phenotype information. They demonstrate the value of research collaborations in order to maximise interpretation of genomic data.

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.

The Role of Centrosome Distal Appendage Proteins (DAPs) in Nephronophthisis and Ciliogenesis

The primary cilium is found in most mammalian cells and plays a functional role in tissue homeostasis and organ development by modulating key signaling pathways. Ciliopathies are a group of genetically heterogeneous disorders resulting from defects in cilia development and function. Patients with ciliopathic disorders exhibit a range of phenotypes that include nephronophthisis (NPHP), a progressive tubulointerstitial kidney disease that commonly results in end-stage renal disease (ESRD). In recent years, distal appendages (DAPs), which radially project from the distal end of the mother centriole, have been shown to play a vital role in primary ciliary vesicle docking and the initiation of ciliogenesis. Mutations in the genes encoding these proteins can result in either a complete loss of the primary cilium, abnormal ciliary formation, or defective ciliary signaling. DAPs deficiency in humans or mice commonly results in NPHP. In this review, we outline recent advances in our understanding of the molecular functions of DAPs and how they participate in nephronophthisis development.

Cellular and molecular actors of myeloid cell fusion: podosomes and tunneling nanotubes call the tune

Different types of multinucleated giant cells (MGCs) of myeloid origin have been described; osteoclasts are the most extensively studied because of their importance in bone homeostasis. MGCs are formed by cell-to-cell fusion, and most types have been observed in pathological conditions, especially in infectious and non-infectious chronic inflammatory contexts. The precise role of the different MGCs and the mechanisms that govern their formation remain poorly understood, likely due to their heterogeneity. First, we will introduce the main populations of MGCs derived from the monocyte/macrophage lineage. We will then discuss the known molecular actors mediating the early stages of fusion, focusing on cell-surface receptors involved in the cell-to-cell adhesion steps that ultimately lead to multinucleation. Given that cell-to-cell fusion is a complex and well-coordinated process, we will also describe what is currently known about the evolution of F-actin-based structures involved in macrophage fusion, i.e., podosomes, zipper-like structures, and tunneling nanotubes (TNT). Finally, the localization and potential role of the key fusion mediators related to the formation of these F-actin structures will be discussed. This review intends to present the current status of knowledge of the molecular and cellular mechanisms supporting multinucleation of myeloid cells, highlighting the gaps still existing, and contributing to the proposition of potential disease-specific MGC markers and/or therapeutic targets.

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.

Teasing out function from morphology: Similarities between primary cilia and immune synapses

Immune synapses are formed between immune cells to facilitate communication and coordinate the immune response. The reorganization of receptors involved in recognition and signaling creates a transient area of plasma membrane specialized in signaling and polarized secretion. Studies on the formation of the immune synapse between cytotoxic T lymphocytes (CTLs) and their targets uncovered a critical role for centrosome polarization in CTL function and suggested a striking parallel between the synapse and primary cilium. Since these initial observations, a plethora of further morphological, functional, and molecular similarities have been identified between these two fascinating structures. In this review, we describe how advances in imaging and molecular techniques have revealed additional parallels as well as functionally significant differences and discuss how comparative studies continue to shed light on the molecular mechanisms underlying the functions of both the immune synapse and primary cilium.

Role of DZIP1-CBY-FAM92 transition zone complex in the basal body to membrane attachment and ciliary budding

Cilia play important signaling or motile functions in various organisms. In Human, cilia dysfunctions are responsible for a wide range of diseases, called ciliopathies. Cilia assembly is a tightly controlled process, which starts with the conversion of the centriole into a basal body, leading to the formation of the ciliary bud that protrudes inside a ciliary vesicle and/or ultimately at the cell surface. Ciliary bud formation is associated with the assembly of the transition zone (TZ), a complex architecture of proteins of the ciliary base which plays critical functions in gating proteins in and out of the ciliary compartment. Many proteins are involved in the assembly of the TZ, which shows structural and functional variations in different cell types or organisms. In this review, we discuss how a particular complex, composed of members of the DZIP1, CBY and FAM92 families of proteins, is required for the initial stages of cilia assembly leading to ciliary bud formation and how their functional hierarchy contributes to TZ assembly. Moreover, we summarize how evidences in Drosophila reveal functional differences of the DZIP1-CBY-FAM92 complex in the different ciliated tissues of this organism. Whereas it is essential for proper TZ assembly in the two types of ciliated tissues, it is involved in stable anchoring of basal bodies to the plasma membrane in male germ cells. Overall, the DZIP1-CBY-FAM92 complex reveals a molecular assembly pathway required for the initial stages of ciliary bud formation and that is conserved from Drosophila to Human.

Regulation of polycystin expression, maturation and trafficking

The major autosomal dominant polycystic kidney disease (ADPKD) genes, PKD1 and PKD2, are wildly expressed at the organ and tissue level. PKD1 encodes polycystin 1 (PC1), a large membrane associated receptor-like protein that can complex with the PKD2 product, PC2. Various cellular locations have been described for both PC1, including the plasma membrane and extracellular vesicles, and PC2, especially the endoplasmic reticulum (ER), but compelling evidence indicates that the primary cilium, a sensory organelle, is the key site for the polycystin complex to prevent PKD. As with other membrane proteins, the ER biogenesis pathway is key to appropriately folding, performing quality control, and exporting fully folded PC1 to the Golgi apparatus. There is a requirement for binding with PC2 and cleavage of PC1 at the GPS for this folding and export to occur. Six different monogenic defects in this pathway lead to cystic disease development, with PC1 apparently particularly sensitive to defects in this general protein processing pathway. Trafficking of membrane proteins, and the polycystins in particular, through the Golgi to the primary cilium have been analyzed in detail, but at this time, there is no clear consensus on a ciliary targeting sequence required to export proteins to the cilium. After transitioning though the trans-Golgi network, polycystin-bearing vesicles are likely sorted to early or recycling endosomes and then transported to the ciliary base, possibly via docking to transition fibers (TF). The membrane-bound polycystin complex then undergoes facilitated trafficking through the transition zone, the diffusion barrier at the base of the cilium, before entering the cilium. Intraflagellar transport (IFT) may be involved in moving the polycystins along the cilia, but data also indicates other mechanisms. The ciliary polycystin complex can be ubiquitinated and removed from cilia by internalization at the ciliary base and may be sent back to the plasma membrane for recycling or to lysosomes for degradation. Monogenic defects in processes regulating the protein composition of cilia are associated with syndromic disorders involving many organ systems, reflecting the pleotropic role of cilia during development and for tissue maintenance. Many of these ciliopathies have renal involvement, likely because of faulty polycystin signaling from cilia. Understanding the expression, maturation and trafficking of the polycystins helps understand PKD pathogenesis and suggests opportunities for therapeutic intervention.

Phosphorylation of multiple proteins involved in ciliogenesis by Tau Tubulin kinase 2

Primary cilia are organelles necessary for proper implementation of developmental and homeostasis processes. To initiate their assembly, coordinated actions of multiple proteins are needed. Tau tubulin kinase 2 (TTBK2) is a key player in the cilium assembly pathway, controlling the final step of cilia initiation. The function of TTBK2 in ciliogenesis is critically dependent on its kinase activity; however, the precise mechanism of TTBK2 action has so far not been fully understood due to the very limited information about its relevant substrates. In this study, we demonstrate that CEP83, CEP89, CCDC92, Rabin8, and DVL3 are substrates of TTBK2 kinase activity. Further, we characterize a set of phosphosites of those substrates and CEP164 induced by TTBK2 in vitro and in vivo. Intriguingly, we further show that identified TTBK2 phosphosites and consensus sequence delineated from those are distinct from motifs previously assigned to TTBK2. Finally, we show that TTBK2 is also required for efficient phosphorylation of many S/T sites in CEP164 and provide evidence that TTBK2-induced phosphorylations of CEP164 modulate its function, which in turn seems relevant for the process of cilia formation. In summary, our work provides important insight into the substrates-TTBK2 kinase relationship and suggests that phosphorylation of substrates on multiple sites by TTBK2 is probably involved in the control of ciliogenesis in human cells.

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.

The Centrosome Linker and Its Role in Cancer and Genetic Disorders

Centrosome cohesion, the joining of the two centrosomes of a cell, is increasingly appreciated as a major regulator of cell functions such as Golgi organization and cilia positioning. One major element of centrosome cohesion is the centrosome linker that consists of a growing number of proteins. The timely disassembly of the centrosome linker enables centrosomes to separate and assemble a functional bipolar mitotic spindle that is crucial for maintaining genomic integrity. Exciting new findings link centrosome linker defects to cell transformation and genetic disorders. We review recent data on the molecular mechanisms of the assembly and disassembly of the centrosome linker, and discuss how defects in the proper execution of these processes cause DNA damage and genomic instability leading to disease.

Phosphorylation of CEP83 by TTBK2 is necessary for cilia initiation

Primary cilia are microtubule-based organelles that play important roles in development and tissue homeostasis. Tau-tubulin kinase-2 (TTBK2) is genetically linked to spinocerebellar ataxia type 11, and its kinase activity is crucial for ciliogenesis. Although it has been shown that TTBK2 is recruited to the centriole by distal appendage protein CEP164, little is known about TTBK2 substrates associated with its role in ciliogenesis. Here, we perform superresolution microscopy and discover that serum starvation results in TTBK2 redistribution from the periphery toward the root of distal appendages. Our biochemical analyses uncover CEP83 as a bona fide TTBK2 substrate with four phosphorylation sites characterized. We also demonstrate that CEP164-dependent TTBK2 recruitment to distal appendages is required for subsequent CEP83 phosphorylation. Specifically, TTBK2-dependent CEP83 phosphorylation is important for early ciliogenesis steps, including ciliary vesicle docking and CP110 removal. In summary, our results reveal a molecular mechanism of kinase regulation in ciliogenesis and identify CEP83 as a key substrate of TTBK2 during cilia initiation.

Cep128 associates with Odf2 to form the subdistal appendage of the centriole

The mother centriole in a cell has two appendages, the distal appendage (DA) and subdistal appendage (SDA), which have roles in generating cilia and organizing the cellular microtubular network, respectively. In the knockout (KO) cells of Odf2, the component of the DA and SDA, both appendages simultaneously disappear. However, the molecular mechanisms by which the DA and SDA form independently but close to each other downstream of Odf2 are unknown. Here, using super-resolution structured illumination microscopy (SR-SIM), we found that the signal for GFP-tagged Odf2 overlapped considerably with that of immunofluorescently labeled Cep128. We further found that Cep128 knockdown (KD) caused the dissociation of other SDA components from the centriole, including centriolin, Ndel1, ninein and Cep170, whereas Odf2 was still associated with the centriole. In contrast, the DA components remained associated with the centriole in Cep128 KD cells. Consistent with this observation, we identified Cep128 as an Odf2-interacting protein by immunoprecipitation. Taken with the finding that Cep128 deletion decreased the stability of centriolar microtubules, our results indicate that Cep128 associates with Odf2 in the hierarchical assembly of SDA components to elicit the microtubule-organizing function.