Deuterosome-mediated centriole biogenesis

Centriolar defects underlie a primary ciliary dyskinesia phenotype in an adenylate kinase 7 deficient ciliated epithelium

The skin of Xenopus embryos contains numerous multiciliated cells (MCCs), which collectively generate a directed fluid flow across the epithelial surface essential for distributing the overlaying mucous. MCCs develop into highly specialized cells to generate this flow, containing approximately 150 evenly spaced centrioles that give rise to motile cilia. MCC-driven fluid flow can be impaired when ciliary dysfunction occurs, resulting in primary ciliary dyskinesia (PCD) in humans. Mutations in a large number of genes (∼50) have been found to be causative to PCD. Recently, studies have linked low levels of Adenylate Kinase 7 (AK7) gene expression to patients with PCD; however, the mechanism for this link remains unclear. Additionally, AK7 mutations have been linked to multiple PCD patients. Adenylate kinases modulate ATP production and consumption, with AK7 explicitly associated with motile cilia. Here we reproduce an AK7 PCD-like phenotype in Xenopus and describe the cellular consequences that occur with manipulation of AK7 levels. We show that AK7 localizes throughout the cilia in a DPY30 domain-dependent manner, suggesting a ciliary function. Additionally, we find that AK7 overexpression increases centriole number, suggesting a role in regulating centriole biogenesis. We find that in AK7-depleted embryos, cilia number, length, and beat frequency are all reduced, which in turn significantly decreases the tissue-wide mucociliary flow. Additionally, we find a decrease in centriole number and an increase in sub-apical centrioles, implying that AK7 influences both centriole biogenesis and docking, which we propose underlie its defect in ciliogenesis. We find that both the AK domain and the DPY30 domain are required for proper centriole regulation. We propose that AK7 plays a role in PCD by impacting centriole biogenesis and apical docking, ultimately leading to ciliogenesis defects that impair mucociliary clearance.

Spontaneous Calcium Bursts Organize the Apical Actin Cytoskeleton of Multiciliated Cells

Motile cilia perform crucial functions during embryonic development and in adult tissues. They are anchored by an apical actin network that forms microridge-like structures on the surface of multiciliated cells. Using Xenopus as a model system to investigate the mechanisms underlying the formation of these specialized actin structures, we observed stochastic bursts of intracellular calcium concentration in developing multiciliated cells. Through optogenetic manipulation of calcium signaling, we found that individual calcium bursts triggered the fusion and extension of actin structures by activating non-muscle myosin. Repeated cycles of calcium activation promoted assembly and coherence of the maturing apical actin network. Inhibition of the endogenous inositol triphosphate-calcium pathway disrupted the formation of apical actin/microridge-like structures by reducing local centriolar RhoA signaling. This disruption was rescued by transient expression of constitutively active RhoA in multiciliated cells. Our findings identify repetitive calcium bursts as a driving force that promotes the self-organization of the highly specialized actin cytoskeleton of multiciliated cells.

Molecular organization of the distal tip of vertebrate motile cilia

The beating of cilia on multi-ciliated cells (MCCs) is essential for normal development and homeostasis in animals. Unlike basal bodies or axonemes, the distal tips of MCC cilia remain poorly defined. Here, we characterize the molecular organization of the distal tip of vertebrate MCC cilia, revealing two distinct domains occupied by distinct protein constituents. Using frog, mouse, and human MCCs, we find that two largely uncharacterized proteins, Ccdc78 and Ccdc33 occupy a specialized region at the extreme distal tip, and these are required for the normal organization of other tip proteins, including Spef1, Cep104, and Eb3. Ccdc78 and Cccdc33 are also independently required for normal length regulation of MCC cilia. Mechanistically, Ccdc78 and Ccdc33 display robust microtubule-bundling activity both in vivo and in vitro . Thus, we reveal that two previously undefined proteins form a key module for organizing and stabilizing the distal tip of motile cilia in vertebrate MCC. We propose that these proteins represent potential disease loci for motile ciliopathies.

Motile cilia: Key developmental and functional roles in reproductive systems

BACKGROUND

Cilia are specialized microtubule-based organelles that extend from the cell surface and are classified into non-motile and motile types. The assembly and function of cilia are regulated by a complex molecular network that enables motile cilia to generate fluid flow across epithelial surfaces through coordinated beating. These motile cilia are found in the respiratory, nervous, and reproductive systems. In males, motile cilia are found in the efferent ducts and facilitate the transport of sperm from the testis to the epididymis. In females, they are mainly found in the oviducts, where they help to transport, nourish and fertilize eggs, and are also present in the endometrial epithelium.

MATERIAL-METHODS

This review compares the common factors that affect motile cilia in both male and female reproductive tracts, discusses the origin and development of multiciliated cell and cilia within the efferent ducts and oviducts, and enumerates the infertility or related reproductive diseases that may arise due to motile cilia defects.

RESULTS-DISCUSSION

In males, motile cilia in the efferent ducts create turbulence through their beating, which keeps semen suspended and prevents ductal obstruction. In females, motile cilia are distributed on the epithelia of the oviducts and the endometrium. Specifically, motile cilia in the infundibulum of the oviduct aid in capturing oocytes, while cilia in the isthmus region have been found to bind to sperm heads, facilitating the formation of the sperm reservoir. Several common factors, such as miR-34b/c and miR-449, TAp73, Gemc1, and estrogen, etc., have been shown to play crucial regulatory roles in motile cilia within the efferent ducts and oviducts, thereby further influencing fertility outcomes.

CONCLUSIONS

Pathogenic mutations that disrupt ciliary function can impair ciliogenesis or alter the structure of sperm flagella, potentially resulting in infertility. Consequently, motile cilia in both the male and female reproductive tracts are crucial for fertility. There are still numerous unresolved mysteries surrounding these cilia that merit further investigation by researchers, as they hold great significance for the clinical diagnosis and treatment of infertility and related reproductive disorders.

Using Zebrafish to Study Multiciliated Cell Development and Disease States

Multiciliated cells (MCCs) serve many important functions, including fluid propulsion and chemo- and mechanosensing. Diseases ranging from rare conditions to the recent COVID-19 global health pandemic have been linked to MCC defects. In recent years, the zebrafish has emerged as a model to investigate the biology of MCCs. Here, we review the major events in MCC formation including centriole biogenesis and basal body docking. Then, we discuss studies on the role of MCCs in diseases of the brain, respiratory, kidney and reproductive systems, as well as recent findings about the link between MCCs and SARS-CoV-2. Next, we explore why the zebrafish is a useful model to study MCCs and provide a comprehensive overview of previous studies of genetic components essential for MCC development and motility across three major tissues in the zebrafish: the pronephros, brain ependymal cells and nasal placode. Taken together, here we provide a cohesive summary of MCC research using the zebrafish and its future potential for expanding our understanding of MCC-related disease states.

Genetic mechanisms of multiciliated cell development: from fate choice to differentiation in zebrafish and other models

Multiciliated cells (MCCS) form bundles of cilia and their activities are essential for the proper development and physiology of many organ systems. Not surprisingly, defects in MCCs have profound consequences and are associated with numerous disease states. Here, we discuss the current understanding of MCC formation, with a special focus on the genetic and molecular mechanisms of MCC fate choice and differentiation. Furthermore, we cast a spotlight on the use of zebrafish to study MCC ontogeny and several recent advances made in understanding MCCs using this vertebrate model to delineate mechanisms of MCC emergence in the developing kidney.

CCDC78: Unveiling the Function of a Novel Gene Associated with Hereditary Myopathy

CCDC78 was identified as a novel candidate gene for autosomal dominant centronuclear myopathy-4 (CNM4) approximately ten years ago. However, to date, only one family has been described, and the function of CCDC78 remains unclear. Here, we analyze for the first time a family harboring a CCDC78 nonsense mutation to better understand the role of CCDC78 in muscle.

METHODS

We conducted a comprehensive histopathological analysis on muscle biopsies, including immunofluorescent assays to detect multiple sarcoplasmic proteins. We examined CCDC78 transcripts and protein using WB in CCDC78-mutated muscle tissue; these analyses were also performed on muscle, lymphocytes, and fibroblasts from healthy subjects. Subsequently, we conducted RT-qPCR and transcriptome profiling through RNA-seq to evaluate changes in gene expression associated with CCDC78 dysfunction in muscle. Lastly, coimmunoprecipitation (Co-Ip) assays and mass spectrometry (LC-MS/MS) studies were carried out on extracted muscle proteins from both healthy and mutated subjects.

RESULTS

The histopathological features in muscle showed novel histological hallmarks, which included areas of dilated and swollen sarcoplasmic reticulum (SR). We provided evidence of nonsense-mediated mRNA decay (NMD), identified the presence of novel CCDC78 transcripts in muscle and lymphocytes, and identified 1035 muscular differentially expressed genes, including several involved in the SR. Through the Co-Ip assays and LC-MS/MS studies, we demonstrated that CCDC78 interacts with two key SR proteins: SERCA1 and CASQ1. We also observed interactions with MYH1, ACTN2, and ACTA1.

CONCLUSIONS

Our findings provide insight, for the first time, into the interactors and possible role of CCDC78 in skeletal muscle, locating the protein in the SR. Furthermore, our data expand on the phenotype previously associated with CCDC78 mutations, indicating potential histopathological hallmarks of the disease in human muscle. Based on our data, we can consider CCDC78 as the causative gene for CNM4.

An alternative cell cycle coordinates multiciliated cell differentiation

The canonical mitotic cell cycle coordinates DNA replication, centriole duplication and cytokinesis to generate two cells from one1. Some cells, such as mammalian trophoblast giant cells, use cell cycle variants like the endocycle to bypass mitosis2. Differentiating multiciliated cells, found in the mammalian airway, brain ventricles and reproductive tract, are post-mitotic but generate hundreds of centrioles, each of which matures into a basal body and nucleates a motile cilium3,4. Several cell cycle regulators have previously been implicated in specific steps of multiciliated cell differentiation5,6. Here we show that differentiating multiciliated cells integrate cell cycle regulators into a new alternative cell cycle, which we refer to as the multiciliation cycle. The multiciliation cycle redeploys many canonical cell cycle regulators, including cyclin-dependent kinases (CDKs) and their cognate cyclins. For example, cyclin D1, CDK4 and CDK6, which are regulators of mitotic G1-to-S progression, are required to initiate multiciliated cell differentiation. The multiciliation cycle amplifies some aspects of the canonical cell cycle, such as centriole synthesis, and blocks others, such as DNA replication. E2F7, a transcriptional regulator of canonical S-to-G2 progression, is expressed at high levels during the multiciliation cycle. In the multiciliation cycle, E2F7 directly dampens the expression of genes encoding DNA replication machinery and terminates the S phase-like gene expression program. Loss of E2F7 causes aberrant acquisition of DNA synthesis in multiciliated cells and dysregulation of multiciliation cycle progression, which disrupts centriole maturation and ciliogenesis. We conclude that multiciliated cells use an alternative cell cycle that orchestrates differentiation instead of controlling proliferation.

The Influence of a Genetic Variant in CCDC78 on LMNA-Associated Skeletal Muscle Disease

Mutations in the LMNA gene-encoding A-type lamins can cause Limb-Girdle muscular dystrophy Type 1B (LGMD1B). This disease presents with weakness and wasting of the proximal skeletal muscles and has a variable age of onset and disease severity. This variability has been attributed to genetic background differences among individuals; however, such variants have not been well characterized. To identify such variants, we investigated a multigeneration family in which affected individuals are diagnosed with LGMD1B. The primary genetic cause of LGMD1B in this family is a dominant mutation that activates a cryptic splice site, leading to a five-nucleotide deletion in the mature mRNA. This results in a frame shift and a premature stop in translation. Skeletal muscle biopsies from the family members showed dystrophic features of variable severity, with the muscle fibers of some family members possessing cores, regions of sarcomeric disruption, and a paucity of mitochondria, not commonly associated with LGMD1B. Using whole genome sequencing (WGS), we identified 21 DNA sequence variants that segregate with the family members possessing more profound dystrophic features and muscle cores. These include a relatively common variant in coiled-coil domain containing protein 78 (CCDC78). This variant was given priority because another mutation in CCDC78 causes autosomal dominant centronuclear myopathy-4, which causes cores in addition to centrally positioned nuclei. Therefore, we analyzed muscle biopsies from family members and discovered that those with both the LMNA mutation and the CCDC78 variant contain muscle cores that accumulated both CCDC78 and RyR1. Muscle cores containing mislocalized CCDC78 and RyR1 were absent in the less profoundly affected family members possessing only the LMNA mutation. Taken together, our findings suggest that a relatively common variant in CCDC78 can impart profound muscle pathology in combination with a LMNA mutation and accounts for variability in skeletal muscle disease phenotypes.

Formation and function of multiciliated cells

In vertebrates, multiciliated cells (MCCs) are terminally differentiated cells that line the airway tracts, brain ventricles, and reproductive ducts. Each MCC contains dozens to hundreds of motile cilia that beat in a synchronized manner to drive fluid flow across epithelia, the dysfunction of which is associated with a group of human diseases referred to as motile ciliopathies, such as primary cilia dyskinesia. Given the dynamic and complex process of multiciliogenesis, the biological events essential for forming multiple motile cilia are comparatively unelucidated. Thanks to advancements in genetic tools, omics technologies, and structural biology, significant progress has been achieved in the past decade in understanding the molecular mechanism underlying the regulation of multiple motile cilia formation. In this review, we discuss recent studies with ex vivo culture MCC and animal models, summarize current knowledge of multiciliogenesis, and particularly highlight recent advances and their implications.

Deup1 Expression Interferes with Multiciliated Differentiation

A recent study revealed that the loss of Deup1 expression does not affect either centriole amplification or multicilia formation. Therefore, the deuterosome per se is not a platform for amplification of centrioles. In this study, we examine whether gain-of-function of Deup1 affects the development of multiciliated ependymal cells. Our time-lapse study reveals that deuterosomes with an average diameter of 300 nm have two different fates during ependymal differentiation. In the first instance, deuterosomes are scattered and gradually disappear as cells become multiciliated. In the second instance, deuterosomes self-organize into a larger aggregate, called a deuterosome cluster (DC). Unlike scattered deuterosomes, DCs possess centriole components primarily within their large structure. A characteristic of DC-containing cells is that they tend to become primary ciliated rather than multiciliated. Our in utero electroporation study shows that DCs in ependymal tissue are mostly observed at early postnatal stages, but are scarce at late postnatal stages, suggesting the presence of DC antagonists within the differentiating cells. Importantly, from our bead flow assay, ectopic expression of Deup1 significantly impairs cerebrospinal fluid flow. Furthermore, we show that expression of mouse Deup1 in Xenopus embryos has an inhibitory effect on differentiation of multiciliated cells in the epidermis. Taken together, we conclude that the DC formation of Deup1 in multiciliated cells inhibits production of multiple centrioles.

Choroid plexuses carry nodal-like cilia that undergo axoneme regression from early adult stage

Choroid plexuses (ChPs) produce cerebrospinal fluid and sense non-cell-autonomous stimuli to control the homeostasis of the central nervous system. They are mainly composed of epithelial multiciliated cells, whose development and function are still controversial. We have thus characterized the stepwise order of mammalian ChP epithelia cilia formation using a combination of super-resolution-microscopy approaches and mouse genetics. We show that ChP ciliated cells are built embryonically on a treadmill of spatiotemporally regulated events, starting with atypical centriole amplification and ending with the construction of nodal-like 9+0 cilia, characterized by both primary and motile features. ChP cilia undergo axoneme resorption at early postnatal stages through a microtubule destabilization process controlled by the microtubule-severing enzyme spastin and mitigated by polyglutamylation levels. Notably, this phenotype is preserved in humans, suggesting a conserved ciliary resorption mechanism in mammals.

CEP104/FAP256 and associated cap complex maintain stability of the ciliary tip

Cilia are essential organelles that protrude from the cell body. Cilia are made of a microtubule-based structure called the axoneme. In most types of cilia, the ciliary tip is distinct from the rest of the cilium. Here, we used cryo-electron tomography and subtomogram averaging to obtain the structure of the ciliary tip of the ciliate Tetrahymena thermophila. We show that the microtubules at the tip are highly crosslinked with each other and stabilized by luminal proteins, plugs, and cap proteins at the plus ends. In the tip region, the central pair lacks typical projections and twists significantly. By analyzing cells lacking a ciliary tip-enriched protein CEP104/FAP256 by cryo-electron tomography and proteomics, we discovered candidates for the central pair cap complex and explained the potential functions of CEP104/FAP256. These data provide new insights into the function of the ciliary tip and the mechanisms of ciliary assembly and length regulation.

Novel centriolar defects underlie a primary ciliary dyskinesia phenotype in an adenylate kinase 7 deficient ciliated epithelium

The skin of Xenopus embryos contains numerous multiciliated cells (MCCs), which collectively generate a directed fluid flow across the epithelial surface essential for distributing the overlaying mucous. MCCs develop into highly specialized cells to generate this flow, containing approximately 150 evenly spaced centrioles that give rise to motile cilia. MCC-driven fluid flow can be impaired when ciliary dysfunction occurs, resulting in primary ciliary dyskinesia (PCD) in humans. Mutations in a large number of genes (~50) have been found to be causative to PCD. Recently, studies have linked low levels of Adenylate Kinase 7 (AK7) gene expression to patients with PCD; however, the mechanism for this link remains unclear. Additionally, AK7 mutations have been linked to multiple PCD patients. Adenylate kinases modulate ATP production and consumption, with AK7 explicitly associated with motile cilia. Here we reproduce an AK7 PCD-like phenotype in Xenopus and describe the cellular consequences that occur with manipulation of AK7 levels. We show that AK7 localizes throughout the cilia in a DPY30 domain-dependent manner, suggesting a ciliary function. Additionally, we find that AK7 overexpression increases centriole number, suggesting a role in regulating centriole biogenesis. We find that in AK7-depleted embryos, cilia number, length, and beat frequency are all reduced, which in turn, significantly decreases the tissue-wide mucociliary flow. Additionally, we find a decrease in centriole number and an increase in sub-apical centrioles, implying that AK7 influences both centriole biogenesis and docking, which we propose underlie its defect in ciliogenesis. We propose that AK7 plays a role in PCD by impacting centriole biogenesis and apical docking, ultimately leading to ciliogenesis defects that impair mucociliary clearance.

IL-4 drastically decreases deuterosomal and multiciliated cells via alteration in progenitor cell differentiation

BACKGROUND

Allergic inflammation affects the epithelial cell populations resulting in goblet cell hyperplasia and decreased ciliated cells. Recent advances in single-cell RNA sequencing (scRNAseq) have enabled the identification of new cell subtypes and genomic features of single cells. In this study, we aimed to investigate the effect of allergic inflammation in nasal epithelial cell transcriptomes at the single-cell level.

METHODS

We performed scRNAseq in cultured primary human nasal epithelial (HNE) cells and in vivo nasal epithelium. The transcriptomic features and epithelial cell subtypes were determined under IL-4 stimulation, and cell-specific marker genes and proteins were identified.

RESULTS

We confirmed that cultured HNE cells were similar to in vivo epithelial cells through scRNAseq. Cell-specific marker genes were utilized to cluster the cell subtypes, and FOXJ1⁺ -ciliated cells were sub-classified into multiciliated and deuterosomal cells. PLK4 and CDC20B were specific for deuterosomal cells, and SNTN, CPASL, and GSTA2 were specific for multiciliated cells. IL-4 altered the proportions of cell subtypes, resulting in a decrease in multiciliated cells and loss of deuterosomal cells. The trajectory analysis revealed deuterosomal cells as precursor cells of multiciliated cells and deuterosomal cells function as a bridge between club and multiciliated cells. A decrease in deuterosomal cell marker genes was observed in nasal tissue samples with type 2 inflammation.

CONCLUSION

The effects of IL-4 appear to be mediated through the loss of the deuterosomal population, resulting in the reduction in multiciliated cells. This study also newly suggests cell-specific markers that might be pivotal for investigating respiratory inflammatory diseases.

Novel analytical tools reveal that local synchronization of cilia coincides with tissue-scale metachronal waves in zebrafish multiciliated epithelia

Motile cilia are hair-like cell extensions that beat periodically to generate fluid flow along various epithelial tissues within the body. In dense multiciliated carpets, cilia were shown to exhibit a remarkable coordination of their beat in the form of traveling metachronal waves, a phenomenon which supposedly enhances fluid transport. Yet, how cilia coordinate their regular beat in multiciliated epithelia to move fluids remains insufficiently understood, particularly due to lack of rigorous quantification. We combine experiments, novel analysis tools, and theory to address this knowledge gap. To investigate collective dynamics of cilia, we studied zebrafish multiciliated epithelia in the nose and the brain. We focused mainly on the zebrafish nose, due to its conserved properties with other ciliated tissues and its superior accessibility for non-invasive imaging. We revealed that cilia are synchronized only locally and that the size of local synchronization domains increases with the viscosity of the surrounding medium. Even though synchronization is local only, we observed global patterns of traveling metachronal waves across the zebrafish multiciliated epithelium. Intriguingly, these global wave direction patterns are conserved across individual fish, but different for left and right noses, unveiling a chiral asymmetry of metachronal coordination. To understand the implications of synchronization for fluid pumping, we used a computational model of a regular array of cilia. We found that local metachronal synchronization prevents steric collisions, i.e., cilia colliding with each other, and improves fluid pumping in dense cilia carpets, but hardly affects the direction of fluid flow. In conclusion, we show that local synchronization together with tissue-scale cilia alignment coincide and generate metachronal wave patterns in multiciliated epithelia, which enhance their physiological function of fluid pumping.

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

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

PLK4 drives centriole amplification and apical surface area expansion in multiciliated cells

Multiciliated cells (MCCs) are terminally differentiated epithelia that assemble multiple motile cilia used to promote fluid flow. To template these cilia, MCCs dramatically expand their centriole content during a process known as centriole amplification. In cycling cells, the master regulator of centriole assembly Polo-like kinase 4 (PLK4) is essential for centriole duplication; however recent work has questioned the role of PLK4 in centriole assembly in MCCs. To address this discrepancy, we created genetically engineered mouse models and demonstrated that both PLK4 protein and kinase activity are critical for centriole amplification in MCCs. Tracheal epithelial cells that fail centriole amplification accumulate large assemblies of centriole proteins and do not undergo apical surface area expansion. These results show that the initial stages of centriole assembly are conserved between cycling cells and MCCs and suggest that centriole amplification and surface area expansion are coordinated events.

Every day, we inhale thousands of viruses, bacteria and pollution particles. To protect against these threats, cells in our airways produce mucus that traps inhaled particles before they reach the lungs. This mucus then needs to be removed to prevent it from becoming a breeding ground for microbes that may cause a respiratory infection. This is the responsibility of cells covered in tiny hair-like structures called cilia that move together to propel the mucus-trapped particles out of the airways.

These specialized cells can have up to 300 motile cilia on their surface, which grow from structures called centrioles that then anchor the cilia in place. Multiciliated cells are generated from precursor cells that only have two centrioles. Therefore, as these precursors develop, they must produce large numbers of centrioles, considerably more than other cells that only need a couple of extra centrioles during cell division. However, recent studies have questioned whether the precursors of multiciliated cells rely on the same regulatory proteins to produce centrioles as dividing cells.

To help answer this question, LoMastro et al. created genetically engineered mice that lacked or had an inactive form of PLK4, a protein which controls centriole formation in all cell types lacking multiple cilia. This showed that multiciliated cells also need this protein to produce centrioles. LoMastro et al. also found that multiciliated cells became larger while building centrioles, suggesting that this amplification process helps control the cell’s final size.

Defects in motile cilia activity can lead to fluid build-up in the brain, respiratory infections and infertility. Unfortunately, these disorders are difficult to diagnose currently and there is no cure. The findings of LoMastro et al. further our understanding of how motile cilia are built and maintained, and may help future scientists to develop better diagnostic tools and treatments for patients.

Development of a multiciliated cell

Multiciliated cells (MCC) are evolutionary conserved, highly specialized cell types that contain dozens to hundreds of motile cilia that they use to propel fluid directionally. To template these cilia, each MCC produces between 30 and 500 basal bodies via a process termed centriole amplification. Much progress has been made in recent years in understanding the pathways involved in MCC fate determination, differentiation, and ciliogenesis. Recent studies using mammalian cell culture systems, mice, Xenopus, and other model organisms have started to uncover the mechanisms involved in centriole and cilia biogenesis. Yet, how MCC progenitor cells regulate the precise number of centrioles and cilia during their differentiation remains largely unknown. In this review, we will examine recent findings that address this fundamental question.

Experimental and Natural Induction of de novo Centriole Formation

In cycling cells, new centrioles are assembled in the vicinity of pre-existing centrioles. Although this canonical centriole duplication is a tightly regulated process in animal cells, centrioles can also form in the absence of pre-existing centrioles; this process is termed de novo centriole formation. De novo centriole formation is triggered by the removal of all pre-existing centrioles in the cell in various manners. Moreover, overexpression of polo-like kinase 4 (Plk4), a master regulatory kinase for centriole biogenesis, can induce de novo centriole formation in some cell types. Under these conditions, structurally and functionally normal centrioles can be formed de novo. While de novo centriole formation is normally suppressed in cells with intact centrioles, depletion of certain suppressor proteins leads to the ectopic formation of centriole-related protein aggregates in the cytoplasm. It has been shown that de novo centriole formation also occurs naturally in some species. For instance, during the multiciliogenesis of vertebrate epithelial cells, massive de novo centriole amplification occurs to form numerous motile cilia. In this review, we summarize the previous findings on de novo centriole formation, particularly under experimental conditions, and discuss its regulatory mechanisms.

Microridge-like structures anchor motile cilia

Several tissues contain cells with multiple motile cilia that generate a fluid or particle flow to support development and organ functions; defective motility causes human disease. Developmental cues orient motile cilia, but how cilia are locked into their final position to maintain a directional flow is not understood. Here we find that the actin cytoskeleton is highly dynamic during early development of multiciliated cells (MCCs). While apical actin bundles become increasingly more static, subapical actin filaments are nucleated from the distal tip of ciliary rootlets. Anchorage of these subapical actin filaments requires the presence of microridge-like structures formed during MCC development, and the activity of Nonmuscle Myosin II. Optogenetic manipulation of Ezrin, a core component of the microridge actin-anchoring complex, or inhibition of Myosin Light Chain Kinase interfere with rootlet anchorage and orientation. These observations identify microridge-like structures as an essential component of basal body rootlet anchoring in MCCs.

Male infertility-associated Ccdc108 regulates multiciliogenesis via the intraflagellar transport machinery

Motile cilia on the cell surface generate movement and directional fluid flow that is crucial for various biological processes. Dysfunction of these cilia causes human diseases such as sinopulmonary disease and infertility. Here, we show that Ccdc108, a protein linked to male infertility, has an evolutionarily conserved requirement in motile multiciliation. Using Xenopus laevis embryos, Ccdc108 is shown to be required for the migration and docking of basal bodies to the apical membrane in epidermal multiciliated cells (MCCs). We demonstrate that Ccdc108 interacts with the IFT‐B complex, and the ciliation requirement for Ift74 overlaps with Ccdc108 in MCCs. Both Ccdc108 and IFT‐B proteins localize to migrating centrioles, basal bodies, and cilia in MCCs. Importantly, Ccdc108 governs the centriolar recruitment of IFT while IFT licenses the targeting of Ccdc108 to the cilium. Moreover, Ccdc108 is required for the centriolar recruitment of Drg1 and activated RhoA, factors that help establish the apical actin network in MCCs. Together, our studies indicate that Ccdc108 and IFT‐B complex components cooperate in multiciliogenesis.

Emi2 enables centriole amplification during multiciliated cell differentiation

Massive centriole amplification during multiciliated cell (MCC) differentiation is a notable example of organelle biogenesis. This process is thought to be enabled by a derived cell cycle state, but the key cell cycle components required for centriole amplification in MCC progenitors remain poorly defined. Here, we show that emi2 (fbxo43) expression is up-regulated and acts in MCC progenitors after cell cycle exit to transiently inhibit anaphase-promoting complex/cyclosome (APC/C)cdh1 activity. We find that this inhibition is required for the phosphorylation and activation of a key cell cycle kinase, plk1, which acts, in turn, to promote different steps required for centriole amplification and basal body formation, including centriole disengagement, apical migration, and maturation into basal bodies. This emi2-APC/C-plk1 axis is also required to down-regulate gene expression essential for centriole amplification after differentiation is complete. These results identify an emi2-APC/C-plk1 axis that promotes and then terminates centriole assembly and basal body formation during MCC differentiation.

Lrrcc1 and Ccdc61 are conserved effectors of multiciliated cell function

Ciliated epithelia perform essential functions across animal evolution, ranging from locomotion of marine organisms to mucociliary clearance of airways in mammals. These epithelia are composed of multiciliated cells (MCCs) harbouring myriads of motile cilia, which rest on modified centrioles called basal bodies (BBs), and beat coordinately to generate directed fluid flows. Thus, BB biogenesis and organization is central to MCC function. In basal eukaryotes, the coiled-coil domain proteins Lrrcc1 and Ccdc61 were shown to be required for proper BB construction and function. Here, we used the Xenopus embryonic ciliated epidermis to characterize Lrrcc1 and Ccdc61 in vertebrate MCCs. We found that they both encode BB components, localized proximally at the junction with striated rootlets. Knocking down either gene caused defects in BB docking, spacing, and polarization. Moreover, their depletion impaired the apical cytoskeleton, and altered ciliary beating. Consequently, cilia-powered fluid flow was greatly reduced in morphant tadpoles, which displayed enhanced mortality when exposed to pathogenic bacteria. This work illustrates how integration across organizational scales make elementary BB components essential for the emergence of the physiological function of ciliated epithelia.

Essential Roles of Efferent Duct Multicilia in Male Fertility

Cilia are microtubule-based hair-like organelles on the cell surface. Cilia have been implicated in various biological processes ranging from mechanosensation to fluid movement. Ciliary dysfunction leads to a plethora of human diseases, known as ciliopathies. Although non-motile primary cilia are ubiquitous, motile multicilia are found in restricted locations of the body, such as the respiratory tract, the oviduct, the efferent duct, and the brain ventricles. Multicilia beat in a whip-like motion to generate fluid flow over the apical surface of an epithelium. The concerted ciliary motion provides the driving force critical for clearing airway mucus and debris, transporting ova from the ovary to the uterus, maintaining sperm in suspension, and circulating cerebrospinal fluid in the brain. In the male reproductive tract, multiciliated cells (MCCs) were first described in the mid-1800s, but their importance in male fertility remained elusive until recently. MCCs exist in the efferent ducts, which are small, highly convoluted tubules that connect the testis to the epididymis and play an essential role in male fertility. In this review, we will introduce multiciliogenesis, discuss mouse models of male infertility with defective multicilia, and summarize our current knowledge on the biological function of multicilia in the male reproductive tract.

CEP97 phosphorylation by Dyrk1a is critical for centriole separation during multiciliogenesis

Proper cilia formation in multiciliated cells (MCCs) is necessary for appropriate embryonic development and homeostasis. Multicilia share many structural characteristics with monocilia and primary cilia, but there are still significant gaps in our understanding of the regulation of multiciliogenesis. Using the Xenopus embryo, we show that CEP97, which is known as a negative regulator of primary cilia formation, interacts with dual specificity tyrosine phosphorylation regulated kinase 1A (Dyrk1a) to modulate multiciliogenesis. We show that Dyrk1a phosphorylates CEP97, which in turn promotes the recruitment of Polo-like kinase 1 (Plk1), which is a critical regulator of MCC maturation that functions to enhance centriole disengagement in cooperation with the enzyme Separase. Knockdown of either CEP97 or Dyrk1a disrupts cilia formation and centriole disengagement in MCCs, but this defect is rescued by overexpression of Separase. Thus, our study reveals that Dyrk1a and CEP97 coordinate with Plk1 to promote Separase function to properly form multicilia in vertebrate MCCs.

Theoretical Analysis of S, M and N Structural Proteins by the Protein-RNA Recognition Code Leads to Genes/proteins that Are Relevant to the SARS-CoV-2 Life Cycle and Pathogenesis

In this conceptual review, based on the protein-RNA recognition code, some theoretical sequences were detected in the spike (S), membrane (M) and capsid (N) proteins that may post-transcriptionally regulate the host genes/proteins in immune homeostasis, pulmonary epithelial tissue homeostasis, and lipid homeostasis. According to the review of literature, the spectrum of identified genes/proteins shows that the virus promotes IL1α/β-IL1R1 signaling (type 1 immunity) and immunity defense against helminths and venoms (type 2 immunity). In the alteration of homeostasis in the pulmonary epithelial tissue, the virus blocks the function of cilia and the molecular programs that are involved in wound healing (EMT and MET). Additionally, the protein-RNA recognition method described here identifies compatible sequences in the S1A-domain for the post-transcriptional promotion of PIKFYVE, which is one of the critical factors for SARS-CoV-2 entry to the host cell, and for the post-transcriptional repression of xylulokinase XYLB. A decrease in XYLB product (Xu5P) in plasma was proposed as one of the potential metabolomics biomarkers of COVID-19. In summary, the protein-RNA recognition code leads to protein genes relevant to the SARS-CoV-2 life cycle and pathogenesis.

Novel Approach Combining Transcriptional and Evolutionary Signatures to Identify New Multiciliation Genes

Multiciliogenesis is a complex process that allows the generation of hundreds of motile cilia on the surface of specialized cells, to create fluid flow across epithelial surfaces. Dysfunction of human multiciliated cells is associated with diseases of the brain, airway and reproductive tracts. Despite recent efforts to characterize the transcriptional events responsible for the differentiation of multiciliated cells, a lot of actors remain to be identified. In this work, we capitalize on the ever-growing quantity of high-throughput data to search for new candidate genes involved in multiciliation. After performing a large-scale screening using 10 transcriptomics datasets dedicated to multiciliation, we established a specific evolutionary signature involving Otomorpha fish to use as a criterion to select the most likely targets. Combining both approaches highlighted a list of 114 potential multiciliated candidates. We characterized these genes first by generating protein interaction networks, which showed various clusters of ciliated and multiciliated genes, and then by computing phylogenetic profiles. In the end, we selected 11 poorly characterized genes that seem like particularly promising multiciliated candidates. By combining functional and comparative genomics methods, we developed a novel type of approach to study biological processes and identify new promising candidates linked to that process.

Ciliary Dyneins and Dynein Related Ciliopathies

Although ubiquitously present, the relevance of cilia for vertebrate development and health has long been underrated. However, the aberration or dysfunction of ciliary structures or components results in a large heterogeneous group of disorders in mammals, termed ciliopathies. The majority of human ciliopathy cases are caused by malfunction of the ciliary dynein motor activity, powering retrograde intraflagellar transport (enabled by the cytoplasmic dynein-2 complex) or axonemal movement (axonemal dynein complexes). Despite a partially shared evolutionary developmental path and shared ciliary localization, the cytoplasmic dynein-2 and axonemal dynein functions are markedly different: while cytoplasmic dynein-2 complex dysfunction results in an ultra-rare syndromal skeleto-renal phenotype with a high lethality, axonemal dynein dysfunction is associated with a motile cilia dysfunction disorder, primary ciliary dyskinesia (PCD) or Kartagener syndrome, causing recurrent airway infection, degenerative lung disease, laterality defects, and infertility. In this review, we provide an overview of ciliary dynein complex compositions, their functions, clinical disease hallmarks of ciliary dynein disorders, presumed underlying pathomechanisms, and novel developments in the field.

Mechanical stretch scales centriole number to apical area via Piezo1 in multiciliated cells

How cells count and regulate organelle number is a fundamental question in cell biology. For example, most cells restrict centrioles to two in number and assemble one cilium; however, multiciliated cells (MCCs) synthesize hundreds of centrioles to assemble multiple cilia. Aberration in centriole/cilia number impairs MCC function and can lead to pathological outcomes. Yet how MCCs control centriole number remains unknown. Using Xenopus, we demonstrate that centriole number scales with apical area over a remarkable 40-fold change in size. We find that tensile forces that shape the apical area also trigger centriole amplification based on both cell stretching experiments and disruption of embryonic elongation. Unexpectedly, Piezo1, a mechanosensitive ion channel, localizes near each centriole suggesting a potential role in centriole amplification. Indeed, depletion of Piezo1 affects centriole amplification and disrupts its correlation with the apical area in a tension-dependent manner. Thus, mechanical forces calibrate cilia/centriole number to the MCC apical area via Piezo1. Our results provide new perspectives to study organelle number control essential for optimal cell function.

Fine-tuning multiciliated cell differentiation at the post-transcriptional level: contribution of miR-34/449 family members

Cell differentiation is a process that must be precisely regulated for the maintenance of tissue homeostasis. Differentiation towards a multiciliated cell fate is characterized by well-defined stages, where a transcriptional cascade is activated leading to the formation of multiple centrioles and cilia. Centrioles migrate and dock to the apical cell surface and, acting as basal bodies, give rise to multiple motile cilia. The concerted movement of cilia ensures directional fluid flow across epithelia and defects either in their number or structure can lead to disease phenotypes. Micro-RNAs (miRNAs; miRs) are small, non-coding RNA molecules that play an important role in post-transcriptional regulation of gene expression. miR-34b/c and miR-449a/b/c specifically function throughout the differentiation of multiciliated cells, fine-tuning the expression of many different centriole- and cilia-related genes. They strictly regulate the expression levels of genes that are required both for commitment towards the multiciliated cell fate (e.g. Notch) and for the establishment and maintenance of this fate by regulating the expression of transcription factors and structural components of the pathway. Herein we review miR-34 and miR-449 spatiotemporal regulation along with their roles during the different stages of multiciliogenesis.

Biophysical and biochemical properties of Deup1 self-assemblies: a potential driver for deuterosome formation during multiciliogenesis

The deuterosome is a non-membranous organelle involved in large-scale centriole amplification during multiciliogenesis. Deuterosomes are specifically assembled during the process of multiciliogenesis. However, the molecular mechanisms underlying deuterosome formation are poorly understood. In this study, we investigated the molecular properties of deuterosome protein 1 (Deup1), an essential protein involved in deuterosome assembly. We found that Deup1 has the ability to self-assemble into macromolecular condensates both in vitro and in cells. The Deup1-containing structures formed in multiciliogenesis and the Deup1 condensates self-assembled in vitro showed low turnover of Deup1, suggesting that Deup1 forms highly stable structures. Our biochemical analyses revealed that an increase of the concentration of Deup1 and a crowded molecular environment both facilitate Deup1 self-assembly. The self-assembly of Deup1 relies on its N-terminal region, which contains multiple coiled coil domains. Using an optogenetic approach, we demonstrated that self-assembly and the C-terminal half of Deup1 were sufficient to spatially compartmentalize centrosomal protein 152 (Cep152) and polo like kinase 4 (Plk4), master components for centriole biogenesis, in the cytoplasm. Collectively, the present data suggest that Deup1 forms the structural core of the deuterosome through self-assembly into stable macromolecular condensates.This article has an associated First Person interview with the first author of the paper.

Notch signaling induces either apoptosis or cell fate change in multiciliated cells during mucociliary tissue remodeling

Multiciliated cells (MCCs) are extremely highly differentiated, presenting >100 cilia and basal bodies. Therefore, MCC fate is thought to be terminal and irreversible. We analyzed how MCCs are removed from the airway-like mucociliary Xenopus epidermis during developmental tissue remodeling. We found that a subset of MCCs undergoes lateral line-induced apoptosis, but that the majority coordinately trans-differentiate into goblet secretory cells. Both processes are dependent on Notch signaling, while the cellular response to Notch is modulated by Jak/STAT, thyroid hormone, and mTOR signaling. At the cellular level, trans-differentiation is executed through the loss of ciliary gene expression, including foxj1 and pcm1, altered proteostasis, cilia retraction, basal body elimination, as well as the initiation of mucus production and secretion. Our work describes two modes for MCC loss during vertebrate development, the signaling regulation of these processes, and demonstrates that even cells with extreme differentiation features can undergo direct fate conversion.

Xenopus to the rescue: A model to validate and characterize candidate ciliopathy genes

Cilia are present on most vertebrate cells and play a central role in development, growth, and homeostasis. Thus, cilia dysfunction can manifest into an array of diseases, collectively termed ciliopathies, affecting millions of lives worldwide. Yet, our understanding of the gene regulatory networks that control cilia assembly and functions remain incomplete. With the advances in next-generation sequencing technologies, we can now rapidly predict pathogenic variants from hundreds of ciliopathy patients. While the pace of candidate gene discovery is exciting, most of these genes have never been previously implicated in cilia assembly or function. This makes assigning the disease causality difficult. This review discusses how Xenopus, a genetically tractable and high-throughput vertebrate model, has played a central role in identifying, validating, and characterizing candidate ciliopathy genes. The review is focused on multiciliated cells (MCCs) and diseases associated with MCC dysfunction. MCCs harbor multiple motile cilia on their apical surface to generate extracellular fluid flow inside the airway, the brain ventricles, and the oviduct. In Xenopus, these cells are external and present on the embryonic epidermal epithelia, facilitating candidate genes analysis in MCC development in vivo. The ability to introduce patient variants to study their effects on disease progression makes Xenopus a powerful model to improve our understanding of the underlying disease mechanisms and explain the patient phenotype.

Xenopus epidermal and endodermal epithelia as models for mucociliary epithelial evolution, disease, and metaplasia

The Xenopus embryonic epidermis is a powerful model to study mucociliary biology, development, and disease. Particularly, the Xenopus system is being used to elucidate signaling pathways, transcription factor functions, and morphogenetic mechanisms regulating cell fate specification, differentiation and cell function. Thereby, Xenopus research has provided significant insights into potential underlying molecular mechanisms for ciliopathies and chronic airway diseases. Recent studies have also established the embryonic epidermis as a model for mucociliary epithelial remodeling, multiciliated cell trans-differentiation, cilia loss, and mucus secretion. Additionally, the tadpole foregut epithelium is lined by a mucociliary epithelium, which shows remarkable features resembling mammalian airway epithelia, including its endodermal origin and a variable cell type composition along the proximal-distal axis. This review aims to summarize the advantages of the Xenopus epidermis for mucociliary epithelial biology and disease modeling. Furthermore, the potential of the foregut epithelium as novel mucociliary model system is being highlighted. Additional perspectives are presented on how to expand the range of diseases that can be modeled in the frog system, including proton pump inhibitor-associated pneumonia as well as metaplasia in epithelial cells of the airway and the gastroesophageal region.

A role for Cep70 in centriole amplification in multiciliated cells

Centriole amplification in multiciliated cells occurs in a pseudo-cell cycle regulated process that typically utilizes a poorly characterized molecularly dense structure called the deuterosome. We identified the centrosomal protein Cep70 as a novel deuterosome-associated protein that forms a complex with other deuterosome proteins, CCDC78 and Deup1. Cep70 dynamically associates with deuterosomes during centriole amplification in the ciliated epithelia of Xenopus embryos. Cep70 is not found in nascent deuterosomes prior to amplification. However, it becomes localized at deuterosomes at the onset of centriole biogenesis and remains there after the completion of centriole amplification. Deuterosome localization requires a conserved C-terminal "Cep70" motif. Depletion of Cep70 using morpholino oligos or CRISPR/Cas9 editing in F0 embryos leads to a severe decrease in centriole formation in both endogenous MCCs, as well as ectopically induced MCCs. Consistent with a decrease in centrioles, endogenous MCCs have defects in the process of radial intercalation. We propose that Cep70 represents a novel regulator of centriole biogenesis in MCCs.

Potential Regulatory Role of Human-Carboxylesterase-1 Glycosylation in Liver Cancer Cell Growth

We previously reported that human carboxylesterase 1 (CES1), a serine esterase containing a unique N-linked glycosyl group at Asn79 (N79 CES1), is a candidate serological marker of hepatocellular carcinoma (HCC). CES1 is normally present at low-to-undetectable levels in normal human plasma, HCC tumors, and major liver cancer cell lines. To investigate the potential mechanism underlying the suppression of CES1 expression in liver cancer cells, we took advantage of the low detectability of this marker in tumors by overexpressing CES1 in multiple HCC cell lines, including stable Hep3B cells. We found that the population of CES1-overexpressing (OE) cells decreased and that their doubling time was longer compared with mock control liver cancer cells. Using interactive transcriptome, proteome, and subsequent Gene Ontology enrichment analysis of CES1-OE cells, we found substantial decreases in the expression levels of genes involved in cell cycle regulation and proliferation. This antiproliferative function of the N79 glycan of CES1 was further supported by quantitative real-time polymerase chain reaction, flow cytometry, and an apoptosis protein array assay. An analysis of the levels of key signaling target proteins via Western blotting suggested that CES1 overexpression exerted an antiproliferative effect via the PKD1/PKCμ signaling pathway. Similar results were also seen in another HCC cell line (PLC/RFP/5) after transient transfection with CES1 but not in similarly treated non-HCC cell lines (e.g., HeLa and Tera-1 cells), suggesting that CES1 likely exerts a liver cell-type-specific suppressive effect. Given that the N-linked glycosyl group at Asn79 (N79 glycan) of CES1 is known to influence CES1 enzyme activity, we hypothesized that the post-translational modification of CES1 at N79 may be linked to its antiproliferative activity. To investigate the regulatory effect of the N79 glycan on cellular growth, we mutated the single N-glycosylation site in CES1 from Asn to Gln (CES1-N79Q) via site-directed mutagenesis. Fluorescence 2-D difference gel electrophoresis protein expression analysis of cell lysates revealed an increase in cell growth and a decrease in doubling time in cells carrying the N79Q mutation. Thus our results suggest that CES1 exerts an antiproliferative effect in liver cancer cells and that the single N-linked glycosylation at Asn79 plays a potential regulatory role. These functions may underlie the undetectability of CES1 in human HCC tumors and liver cancer cell lines. Mass spectrometry data are available via ProteomeXchange under the identifier PXD021573.

Motile cilia and airway disease

A finely regulated system of airway epithelial development governs the differentiation of motile ciliated cells of the human respiratory tract, conferring the body's mucociliary clearance defence system. Human cilia dysfunction can arise through genetic mutations and this is a cause of debilitating disease morbidities that confer a greatly reduced quality of life. The inherited human motile ciliopathy disorder, primary ciliary dyskinesia (PCD), can arise from mutations in genes affecting various aspects of motile cilia structure and function through deficient production, transport and assembly of cilia motility components or through defective multiciliogenesis. Our understanding about the development of the respiratory epithelium, motile cilia biology and the implications for human pathology has expanded greatly over the past 20 years since isolation of the first PCD gene, rising to now nearly 50 genes. Systems level insights about cilia motility in health and disease have been made possible through intensive molecular and omics (genomics, transcriptomics, proteomics) research, applied in ciliate organisms and in animal and human disease modelling. Here, we review ciliated airway development and the genetic stratification that underlies PCD, for which the underlying genotype can increasingly be connected to biological mechanism and disease prognostics. Progress in this field can facilitate clinical translation of research advances, with potential for great medical impact, e.g. through improvements in ciliopathy disease diagnosis, management, family counselling and by enhancing the potential for future genetically tailored approaches to disease therapeutics.

Building a ciliated epithelium: Transcriptional regulation and radial intercalation of multiciliated cells

The epidermis of the Xenopus embryo has emerged as a powerful tool for studying the development of a ciliated epithelium. Interspersed throughout the epithelium are multiciliated cells (MCCs) with 100+ motile cilia that beat in a coordinated manner to generate fluid flow over the surface of the cell. MCCs are essential for various developmental processes and, furthermore, ciliary dysfunction is associated with numerous pathologies. Therefore, understanding the cellular mechanisms involved in establishing a ciliated epithelium are of particular interest. MCCs originate in the inner epithelial layer of Xenopus skin, where Notch signaling plays a critical role in determining which progenitors will adopt a ciliated cell fate. Then, activation of various transcriptional regulators, such as GemC1 and MCIDAS, initiate the MCC transcriptional program, resulting in centriole amplification and the formation of motile cilia. Following specification and differentiation, MCCs undergo the process of radial intercalation, where cells apically migrate from the inner layer to the outer epithelial layer. This process involves the cooperation of various cytoskeletal networks, activation of various signaling molecules, and changes in cell-ECM and cell-cell adhesion. Coordination of these cellular processes is required for complete incorporation into the outer epithelial layer and generation of a functional ciliated epithelium. Here, we highlight recent advances made in understanding the transcriptional cascades required for MCC specification and differentiation and the coordination of cellular processes that facilitate radial intercalation. Proper regulation of these signaling pathways and processes are the foundation for developing a ciliated epithelium.

Characterisation of centriole biogenesis during multiciliation in planarians

BACKGROUND INFORMATION

Dense multicilia in protozoa and metazoa generate a strong force important for locomotion and extracellular fluid flow. During ciliogenesis, multiciliated cells produce hundreds of centrioles to serve as basal bodies through various pathways including deuterosome-dependent (DD), hyper-activated mother centriole-dependent (MCD) and basal bodydependent (BBD) pathways. The centrosome-free planarian Schmidtea mediterranea is widely used for regeneration studies because its neoblasts are capable of regenerating any body part after injury. However, it is currently unclear how the flatworms generate massive centrioles for multiciliated cells in the pharynx and body epidermis when their cells are initially centriole-free.

RESULTS

In this study, we investigate the progress of centriole amplification during the pharynx regeneration. We observe that the planarian pharyngeal epithelial cells generate their centrioles asynchronously through a de novo pathway. Most of the de novo centrioles are formed individually, whereas the remaining ones are assembled in pairs, possibly by sharing a cartwheel, or in small clusters lacking a nucleation center. Further RNAi experiments show that the known key factors of centriole duplication, including Cep152, Plk4 and Sas6, are crucial for the centriole amplification.

CONCLUSIONS AND SIGNIFICANCE

Our study demonstrates the distinct process of massive centriole biogenesis in S. mediterranea and helps to understand the diversity of centriole biogenesis during evolution.

Development of Ependymal and Postnatal Neural Stem Cells and Their Origin from a Common Embryonic Progenitor

The adult mouse brain contains an extensive neurogenic niche in the lateral walls of the lateral ventricles. This epithelium, which has a unique pinwheel organization, contains multiciliated ependymal (E1) cells and neural stem cells (B1). This postnatal germinal epithelium develops from the embryonic ventricular zone, but the lineage relationship between E1 and B1 cells remains unknown. Distinct subpopulations of radial glia (RG) cells in late embryonic and early postnatal development either expand their apical domain >11-fold to form E1 cells or retain small apical domains that coalesce into the centers of pinwheels to form B1 cells. Using independent methods of lineage tracing, we show that individual RG cells can give rise to clones containing E1 and B1 cells. This study reveals key developmental steps in the formation of the postnatal germinal niche and the shared cellular origin of E1 and B1 cells.

Centriole Number and the Accumulation of Microtubules Modulate the Timing of Apical Insertion during Radial Intercalation

Centrioles are microtubule (MT)-based structures that provide important functions during cell migration, cell division, and cell signaling [1]. Modulating centriole number in 3D cell cultures has been shown to influence protrusive behavior [2-5]. Here, we address in vivo the role of centrioles and the accumulation of MTs on the protrusive behavior required during the initiation of radial intercalation. Radial intercalation is an important developmental process whereby cells undergo polarized movements and interdigitate into a more superficial layer [6, 7]. It is commonly employed during metamorphic events, such as the tissue thinning coupled with expansion or during the introduction of different cell types into an epithelium. During radial intercalation, cells emerge from a basal layer by undergoing a process of apical migration, apical insertion, and expansion [8]. In Xenopus skin, multiciliated cells (MCCs), which contain ∼150 centrioles, and ionocytes (ICs), which contain two centrioles, differentiate during the same developmental window, but MCCs complete intercalation prior to ICs. Here, we utilize this difference in timing to create a quantifiable assay for insertion and find that the timing of insertion is modulated by changes in centriole number and the accumulation of acetylated MTs. Additionally, centrioles align between the nucleus and the leading edge creating an axis of migration with apically oriented (+) ends. Using the MT (-) end protein CAMSAP1 fused to the apically positioned Par6 protein, we have artificially reversed the orientation of MTs and find that the accumulation of MTs in either orientation is sufficient to promote apical insertion.

Propulsive nanomachines: the convergent evolution of archaella, flagella and cilia

Echoing the repeated convergent evolution of flight and vision in large eukaryotes, propulsive swimming motility has evolved independently in microbes in each of the three domains of life. Filamentous appendages – archaella in Archaea, flagella in Bacteria and cilia in Eukaryotes – wave, whip or rotate to propel microbes, overcoming diffusion and enabling colonization of new environments. The implementations of the three propulsive nanomachines are distinct, however: archaella and flagella rotate, while cilia beat or wave; flagella and cilia assemble at their tips, while archaella assemble at their base; archaella and cilia use ATP for motility, while flagella use ion-motive force. These underlying differences reflect the tinkering required to evolve a molecular machine, in which pre-existing machines in the appropriate contexts were iteratively co-opted for new functions and whose origins are reflected in their resultant mechanisms. Contemporary homologies suggest that archaella evolved from a non-rotary pilus, flagella from a non-rotary appendage or secretion system, and cilia from a passive sensory structure. Here, we review the structure, assembly, mechanism and homologies of the three distinct solutions as a foundation to better understand how propulsive nanomachines evolved three times independently and to highlight principles of molecular evolution.

The Repertoire of Serous Ovarian Cancer Non-genetic Heterogeneity Revealed by Single-Cell Sequencing of Normal Fallopian Tube Epithelial Cells

The inter-differentiation between cell states promotes cancer cell survival under stress and fosters non-genetic heterogeneity (NGH). NGH is, therefore, a surrogate of tumor resilience but its quantification is confounded by genetic heterogeneity. Here we show that NGH in serous ovarian cancer (SOC) can be accurately measured when informed by the molecular signatures of the normal fallopian tube epithelium (FTE) cells, the cells of origin of SOC. Surveying the transcriptomes of ∼6,000 FTE cells, predominantly from non-ovarian cancer patients, identified 6 FTE subtypes. We used subtype signatures to deconvolute SOC expression data and found substantial intra-tumor NGH. Importantly, NGH-based stratification of ∼1,700 tumors robustly correlated with survival. Our findings lay the foundation for accurate prognostic and therapeutic stratification of SOC.

Neuronal coordination of motile cilia in locomotion and feeding

Efficient ciliary locomotion and transport require the coordination of motile cilia. Short-range coordination of ciliary beats can occur by biophysical mechanisms. Long-range coordination across large or disjointed ciliated fields often requires nervous system control and innervation of ciliated cells by ciliomotor neurons. The neuronal control of cilia is best understood in invertebrate ciliated microswimmers, but similar mechanisms may operate in the vertebrate body. Here, we review how the study of aquatic invertebrates contributed to our understanding of the neuronal control of cilia. We summarize the anatomy of ciliomotor systems and the physiological mechanisms that can alter ciliary activity. We also discuss the most well-characterized ciliomotor system, that of the larval annelid Platynereis. Here, pacemaker neurons drive the rhythmic activation of cholinergic and serotonergic ciliomotor neurons to induce ciliary arrests and beating. The Platynereis ciliomotor neurons form a distinct part of the larval nervous system. Similar ciliomotor systems likely operate in other ciliated larvae, such as mollusc veligers. We discuss the possible ancestry and conservation of ciliomotor circuits and highlight how comparative experimental approaches could contribute to a better understanding of the evolution and function of ciliary systems.

This article is part of the Theo Murphy meeting issue ‘Unity and diversity of cilia in locomotion and transport’.

Massive centriole production can occur in the absence of deuterosomes in multiciliated cells

Multiciliated cells (MCCs) amplify large numbers of centrioles, which convert into basal bodies that are required for producing multiple motile cilia. Most centrioles amplified by MCCs grow on the surface of organelles called deuterosomes, while a smaller number grow through the centriolar pathway in association with the two parent centrioles. Here we show that MCCs lacking deuterosomes amplify the correct number of centrioles with normal step-wise kinetics. This is achieved through a massive production of centrioles on the surface and in the vicinity of parent centrioles. Therefore, deuterosomes may have evolved to relieve, rather than supplement, the centriolar pathway during multiciliogenesis. Remarkably, MCCs lacking parent centrioles and deuterosomes also amplify the appropriate number of centrioles inside a cloud of pericentriolar and fibrogranular material. These data show that centriole number is set independently of their nucleation platforms and that massive centriole production in MCCs is a robust process that can self-organize.

Dynamics of centriole amplification in centrosome-depleted brain multiciliated progenitors

Reproductive and respiratory organs, along with brain ventricles, are lined by multiciliated epithelial cells (MCC) that generate cilia-powered fluid flows. MCC hijack the centrosome duplication pathway to form hundreds of centrioles and nucleate motile cilia. In these cells, the large majority of procentrioles are formed associated with partially characterized organelles called deuterosomes. We recently challenged the paradigm that deuterosomes and procentrioles are formed de novo by providing data, in brain MCC, suggesting that they are nucleated from the pre-existing centrosomal younger centriole. However, the origin of deuterosomes and procentrioles is still under debate. Here, we further question centrosome importance for deuterosome and centriole amplification. First, we provide additional data confirming that centriole amplification occurs sequentially from the centrosomal region, and that the first procentriole-loaded deuterosomes are associated with the daughter centriole or in the centrosomal centriole vicinity. Then, to further test the requirement of the centrosome in deuterosome and centriole formation, we depleted centrosomal centrioles using a Plk4 inhibitor. We reveal unexpected limited consequences in deuterosome/centriole number in absence of centrosomal centrioles. Notably, in absence of the daughter centriole only, deuterosomes are not seen associated with the mother centriole. In absence of both centrosomal centrioles, procentrioles are still amplified sequentially and with no apparent structural defects. They seem to arise from a focal region, characterized by microtubule convergence and pericentriolar material (PCM) assembly. The relevance of deuterosome association with the daughter centriole as well as the role of the PCM in the focal and sequential genesis of centrioles in absence of centrosomal centrioles are discussed.

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

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