Identification of novel ciliogenesis factors using a new in vivo model for mucociliary epithelial development

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.

Xenopus as a model system for studying pigmentation and pigmentary disorders

Human pigmentary disorders encompass a broad spectrum of phenotypic changes arising from disruptions in various stages of melanocyte formation, the melanogenesis process, or the transfer of pigment from melanocytes to keratinocytes. A large number of pigmentation genes associated with pigmentary disorders have been identified, many of them awaiting in vivo confirmation. A more comprehensive understanding of the molecular basis of pigmentary disorders requires a vertebrate animal model where changes in pigmentation are easily observable in vivo and can be combined to genomic modifications and gain/loss-of-function tools. Here we present the amphibian Xenopus with its unique features that fulfill these requirements. Changes in pigmentation are particularly easy to score in Xenopus embryos, allowing whole-organism based phenotypic screening. The development and behavior of Xenopus melanocytes closely mimic those observed in mammals. Interestingly, both Xenopus and mammalian skins exhibit comparable reactions to ultraviolet radiation. This review highlights how Xenopus constitutes an alternative and complementary model to the more commonly used mouse and zebrafish, contributing to the advancement of knowledge in melanocyte cell biology and related diseases.

C. elegans PPEF-type phosphatase (Retinal degeneration C ortholog) functions in diverse classes of cilia to regulate nematode behaviors

Primary (non-motile) cilia represent structurally and functionally diverse organelles whose roles as specialized cellular antenna are central to animal cell signaling pathways, sensory physiology and development. An ever-growing number of ciliary proteins, including those found in vertebrate photoreceptors, have been uncovered and linked to human disorders termed ciliopathies. Here, we demonstrate that an evolutionarily-conserved PPEF-family serine-threonine phosphatase, not functionally linked to cilia in any organism but associated with rhabdomeric (non-ciliary) photoreceptor degeneration in the Drosophila rdgC (retinal degeneration C) mutant, is a bona fide ciliary protein in C. elegans. The nematode protein, PEF-1, depends on transition zone proteins, which make up a 'ciliary gate' in the proximal-most region of the cilium, for its compartmentalization within cilia. Animals lacking PEF-1 protein function display structural defects to several types of cilia, including potential degeneration of microtubules. They also exhibit anomalies to cilium-dependent behaviors, including impaired responses to chemical, temperature, light, and noxious CO2 stimuli. Lastly, we demonstrate that PEF-1 function depends on conserved myristoylation and palmitoylation signals. Collectively, our findings broaden the role of PPEF proteins to include cilia, and suggest that the poorly-characterized mammalian PPEF1 and PPEF2 orthologs may also have ciliary functions and thus represent ciliopathy candidates.

Foxi1 regulates multiple steps of mucociliary development and ionocyte specification through transcriptional and epigenetic mechanisms

Foxi1 is a master regulator of ionocytes (ISCs / INCs) across species and organs. Two subtypes of ISCs exist, and both α- and β-ISCs regulate pH- and ion-homeostasis in epithelia. Gain and loss of FOXI1 function are associated with human diseases, including Pendred syndrome, male infertility, renal acidosis and cancers. Foxi1 functions were predominantly studied in the context of ISC specification, however, reports indicate additional functions in early and ectodermal development. Here, we re-investigated the functions of Foxi1 in Xenopus laevis embryonic mucociliary epidermis development and found a novel function for Foxi1 in the generation of Notch-ligand expressing mucociliary multipotent progenitors (MPPs). We demonstrate that Foxi1 has multiple concentration-dependent functions: At low levels, Foxi1 confers ectodermal competence through transcriptional and epigenetic mechanisms, while at high levels, Foxi1 induces a multi-step process of ISC specification and differentiation. We further describe how foxi1 expression is affected through auto- and Notch-regulation, how Ubp1 and Dmrt2 regulate ISC subtype differentiation, and how this developmental program affects Notch signaling as well as mucociliary patterning. Together, we reveal novel functions for Foxi1 in Xenopus mucociliary epidermis formation, relevant to our understanding of vertebrate development and human disease.

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.

Actin Depolymerizing Factor Destrin Regulates Cilia Development and Function during Vertebrate Embryogenesis

The actin cytoskeleton plays fundamental roles in ciliogenesis and the actin depolymerizing factor destrin regulates actin dynamics by treadmilling actin filaments and increasing globular actin pools. However, the specific developmental roles of destrin in ciliogenesis have not been fully elucidated. Here, we investigated the function of destrin in ciliogenesis using Xenopus laevis and human retinal pigmented epithelial (hRPE1) cells. We discovered the loss of destrin increased the number of multiciliated cells in the Xenopus epithelium and impeded cilia motility. Additionally, destrin depletion remarkably reduced the length of primary cilia in the Xenopus neural tube and hRPE1 cells by affecting actin dynamics. Immunofluorescence using markers of ciliary components indicated that destrin controls the directionality and polarity of basal bodies and axonemal elongation by modulating actin dynamics, independent of basal body docking. In conclusion, destrin plays a significant role during vertebrate ciliogenesis regulating both primary and multicilia development. Our data suggest new insights for understanding the roles of actin dynamics in cilia development.

Localisation and function of key axonemal microtubule inner proteins and dynein docking complex members reveal extensive diversity among vertebrate motile cilia

Vertebrate motile cilia are classified as (9+2) or (9+0), based on the presence or absence of the central pair apparatus, respectively. Cryogenic electron microscopy analyses of (9+2) cilia have uncovered an elaborate axonemal protein composition. The extent to which these features are conserved in (9+0) cilia remains unclear. CFAP53, a key axonemal filamentous microtubule inner protein (fMIP) and a centriolar satellites component, is essential for motility of (9+0), but not (9+2) cilia. Here, we show that in (9+2) cilia, CFAP53 functions redundantly with a paralogous fMIP, MNS1. MNS1 localises to ciliary axonemes, and combined loss of both proteins in zebrafish and mice caused severe outer dynein arm loss from (9+2) cilia, significantly affecting their motility. Using immunoprecipitation, we demonstrate that, whereas MNS1 can associate with itself and CFAP53, CFAP53 is unable to self-associate. We also show that additional axonemal dynein-interacting proteins, two outer dynein arm docking (ODAD) complex members, show differential localisation between types of motile cilia. Together, our findings clarify how paralogous fMIPs, CFAP53 and MNS1, function in regulating (9+2) versus (9+0) cilia motility, and further emphasise extensive structural diversity among these organelles.

Label-free proteomic comparison reveals ciliary and nonciliary phenotypes of IFT-A mutants

DIFFRAC is a powerful method for systematically comparing proteome content and organization between samples in a high-throughput manner. By subjecting control and experimental protein extracts to native chromatography and quantifying the contents of each fraction using mass spectrometry, it enables the quantitative detection of alterations to protein complexes and abundances. Here, we applied DIFFRAC to investigate the consequences of genetic loss of Ift122, a subunit of the intraflagellar transport-A (IFT-A) protein complex that plays a vital role in the formation and function of cilia and flagella, on the proteome of Tetrahymena thermophila. A single DIFFRAC experiment was sufficient to detect changes in protein behavior that mirrored known effects of IFT-A loss and revealed new biology. We uncovered several novel IFT-A-regulated proteins, which we validated through live imaging in Xenopus multiciliated cells, shedding new light on both the ciliary and non-ciliary functions of IFT-A. Our findings underscore the robustness of DIFFRAC for revealing proteomic changes in response to genetic or biochemical perturbation.

JNK regulates ciliogenesis through the interflagellar transport complex and actin networks

The c-Jun N-terminal kinase (JNK) regulates various important physiological processes. Although the JNK pathway has been under intense investigation for over 20 yr, its complexity is still perplexing, with multiple protein partners underlying the diversity of its activity. We show that JNK is associated with the basal bodies in both primary and motile cilia. Loss of JNK disrupts basal body migration and docking and leads to severe ciliogenesis defects. JNK's involvement in ciliogenesis stems from a dual role in the regulation of the actin networks of multiciliated cells (MCCs) and the establishment of the intraflagellar transport-B core complex. JNK signaling is also critical for the maintenance of the actin networks and ciliary function in mature MCCs. JNK is implicated in the development of diabetes, neurodegeneration, and liver disease, all of which have been linked to ciliary dysfunction. Our work uncovers a novel role of JNK in ciliogenesis and ciliary function that could have important implications for JNK's role in the disease.

The adaptive microbiome hypothesis and immune interactions in amphibian mucus

The microbiome is known to provide benefits to hosts, including extension of immune function. Amphibians are a powerful immunological model for examining mucosal defenses because of an accessible epithelial mucosome throughout their developmental trajectory, their responsiveness to experimental treatments, and direct interactions with emerging infectious pathogens. We review amphibian skin mucus components and describe the adaptive microbiome as a novel process of disease resilience where competitive microbial interactions couple with host immune responses to select for functions beneficial to the host. We demonstrate microbiome diversity, specificity of function, and mechanisms for memory characteristic of an adaptive immune response. At a time when industrialization has been linked to losses in microbiota important for host health, applications of microbial therapies such as probiotics may contribute to immunotherapeutics and to conservation efforts for species currently threatened by emerging diseases.

A single-cell, time-resolved profiling of Xenopus mucociliary epithelium reveals nonhierarchical model of development

The specialized cell types of the mucociliary epithelium (MCE) lining the respiratory tract enable continuous airway clearing, with its defects leading to chronic respiratory diseases. The molecular mechanisms driving cell fate acquisition and temporal specialization during mucociliary epithelial development remain largely unknown. Here, we profile the developing Xenopus MCE from pluripotent to mature stages by single-cell transcriptomics, identifying multipotent early epithelial progenitors that execute multilineage cues before specializing into late-stage ionocytes and goblet and basal cells. Combining in silico lineage inference, in situ hybridization, and single-cell multiplexed RNA imaging, we capture the initial bifurcation into early epithelial and multiciliated progenitors and chart cell type emergence and fate progression into specialized cell types. Comparative analysis of nine airway atlases reveals an evolutionary conserved transcriptional module in ciliated cells, whereas secretory and basal types execute distinct function-specific programs across vertebrates. We uncover a continuous nonhierarchical model of MCE development alongside a data resource for understanding respiratory biology.

Label-free proteomic comparison reveals ciliary and non-ciliary phenotypes of IFT-A mutants

DIFFRAC is a powerful method for systematically comparing proteome content and organization between samples in a high-throughput manner. By subjecting control and experimental protein extracts to native chromatography and quantifying the contents of each fraction using mass spectrometry, it enables the quantitative detection of alterations to protein complexes and abundances. Here, we applied DIFFRAC to investigate the consequences of genetic loss of Ift122, a subunit of the intraflagellar transport-A (IFT-A) protein complex that plays a vital role in the formation and function of cilia and flagella, on the proteome of Tetrahymena thermophila . A single DIFFRAC experiment was sufficient to detect changes in protein behavior that mirrored known effects of IFT-A loss and revealed new biology. We uncovered several novel IFT-A-regulated proteins, which we validated through live imaging in Xenopus multiciliated cells, shedding new light on both the ciliary and non-ciliary functions of IFT-A. Our findings underscore the robustness of DIFFRAC for revealing proteomic changes in response to genetic or biochemical perturbation.

Temporal Notch signaling regulates mucociliary cell fates through Hes-mediated competitive de-repression

Tissue functions are determined by the types and ratios of cells present, but little is known about self-organizing principles establishing correct cell type compositions. Mucociliary airway clearance relies on the correct balance between secretory and ciliated cells, which is regulated by Notch signaling across mucociliary systems. Using the airway-like Xenopus epidermis, we investigate how cell fates depend on signaling, how signaling levels are controlled, and how Hes transcription factors regulate cell fates. We show that four mucociliary cell types each require different Notch levels and that their specification is initiated sequentially by a temporal Notch gradient. We describe a novel role for Foxi1 in the generation of Delta-expressing multipotent progenitors through Hes7.1. Hes7.1 is a weak repressor of mucociliary genes and overcomes maternal repression by the strong repressor Hes2 to initiate mucociliary development. Increasing Notch signaling then inhibits Hes7.1 and activates first Hes4, then Hes5.10, which selectively repress cell fates. We have uncovered a self-organizing mechanism of mucociliary cell type composition by competitive de-repression of cell fates by a set of differentially acting repressors. Furthermore, we present an in silico model of this process with predictive abilities.

INTS13 variants causing a recessive developmental ciliopathy disrupt assembly of the Integrator complex

Oral-facial-digital (OFD) syndromes are a heterogeneous group of congenital disorders characterized by malformations of the face and oral cavity, and digit anomalies. Mutations within 12 cilia-related genes have been identified that cause several types of OFD, suggesting that OFDs constitute a subgroup of developmental ciliopathies. Through homozygosity mapping and exome sequencing of two families with variable OFD type 2, we identified distinct germline variants in INTS13, a subunit of the Integrator complex. This multiprotein complex associates with RNA Polymerase II and cleaves nascent RNA to modulate gene expression. We determined that INTS13 utilizes its C-terminus to bind the Integrator cleavage module, which is disrupted by the identified germline variants p.S652L and p.K668Nfs*9. Depletion of INTS13 disrupts ciliogenesis in human cultured cells and causes dysregulation of a broad collection of ciliary genes. Accordingly, its knockdown in Xenopus embryos leads to motile cilia anomalies. Altogether, we show that mutations in INTS13 cause an autosomal recessive ciliopathy, which reveals key interactions between components of the Integrator complex.

STIL Acts as an Oncogenetic Driver in a Primary Cilia-Dependent Manner in Human Cancer

SCL/TAL1 Interrupting locus (STIL) is a ciliary-related gene involved in regulating the cell cycle and duplication of centrioles in dividing cells. STIL has been found disordered in multiple cancers and driven carcinogenesis. However, the molecular mechanisms and biological functions of STIL in cancers remain ambiguous. Here, we systematically analyzed the genetic alterations, molecular mechanisms, and clinical relevance of STIL across >10,000 samples representing 33 cancer types in The Cancer Genome Atlas (TCGA) dataset. We found that STIL expression is up-regulated in most cancer types compared with their adjacent normal tissues. The expression dysregulation of STIL was affected by copy number variation, mutation, and DNA methylation. High STIL expression was associated with worse outcomes and promoted the progression of cancers. Gene Ontology (GO) enrichment analysis and Gene Set Variation Analysis (GSVA) further revealed that STIL is involved in cell cycle progression, Mitotic spindle, G2M checkpoint, and E2F targets pathways across cancer types. STIL expression was negatively correlated with multiple genes taking part in ciliogenesis and was positively correlated with several genes which participated with centrosomal duplication or cilia degradation. Moreover, STIL silencing could promote primary cilia formation and inhibit cell cycle protein expression in prostate and kidney cancer cell lines. The phenotype and protein expression alteration due to STIL silencing could be reversed by IFT88 silencing in cancer cells. These results revealed that STIL could regulate the cell cycle through primary cilia in tumor cells. In summary, our results revealed the importance of STIL in cancers. Targeting STIL might be a novel therapeutic approach for cancers.

The Scf/Kit pathway implements self-organized epithelial patterning

How global patterns emerge from individual cell behaviors is poorly understood. In the Xenopus embryonic epidermis, multiciliated cells (MCCs) are born in a random pattern within an inner mesenchymal layer and subsequently intercalate at regular intervals into an outer epithelial layer. Using video microscopy and mathematical modeling, we found that regular pattern emergence involves mutual repulsion among motile immature MCCs and affinity toward outer-layer intercellular junctions. Consistently, Arp2/3-mediated actin remodeling is required for MCC patterning. Mechanistically, we show that the Kit tyrosine kinase receptor, expressed in MCCs, and its ligand Scf, expressed in outer-layer cells, are both required for regular MCC distribution. Membrane-associated Scf behaves as a potent adhesive cue for MCCs, while its soluble form promotes their mutual repulsion. Finally, Kit expression is sufficient to confer order to a disordered heterologous cell population. This work reveals how a single signaling system can implement self-organized large-scale patterning.

A 'tad' of hope in the fight against airway disease

Xenopus tadpoles have emerged as a powerful in vivo model system to study mucociliary epithelia such as those found in the human airways. The tadpole skin has mucin-secreting cells, motile multi-ciliated cells, ionocytes (control local ionic homeostasis) and basal stem cells. This cellular architecture is very similar to the large airways of the human lungs and represents an easily accessible and experimentally tractable model system to explore the molecular details of mucociliary epithelia. Each of the cell types in the tadpole skin has a human equivalent and a conserved network of genes and signalling pathways for their differentiation has been discovered. Great insight into the function of each of the cell types has been achieved using the Xenopus model and this has enhanced our understanding of airway disease. This simple model has already had a profound impact on the field but, as molecular technologies (e.g. gene editing and live imaging) continue to develop apace, its use for understanding individual cell types and their interactions will likely increase. For example, its small size and genetic tractability make it an ideal model for live imaging of a mucociliary surface especially during environmental challenges such as infection. Further potential exists for the mimicking of human genetic mutations that directly cause airway disease and for the pre-screening of drugs against novel therapeutic targets.

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.

Functional partitioning of a liquid-like organelle during assembly of axonemal dyneins

Ciliary motility is driven by axonemal dyneins that are assembled in the cytoplasm before deployment to cilia. Motile ciliopathy can result from defects in the dyneins themselves or from defects in factors required for their cytoplasmic pre-assembly. Recent work demonstrates that axonemal dyneins, their specific assembly factors, and broadly-acting chaperones are concentrated in liquid-like organelles in the cytoplasm called DynAPs (Dynein Axonemal Particles). Here, we use in vivo imaging in Xenopus to show that inner dynein arm (IDA) and outer dynein arm (ODA) subunits are partitioned into non-overlapping sub-regions within DynAPs. Using affinity- purification mass-spectrometry of in vivo interaction partners, we also identify novel partners for inner and outer dynein arms. Among these, we identify C16orf71/Daap1 as a novel axonemal dynein regulator. Daap1 interacts with ODA subunits, localizes specifically to the cytoplasm, is enriched in DynAPs, and is required for the deployment of ODAs to axonemes. Our work reveals a new complexity in the structure and function of a cell-type specific liquid-like organelle that is directly relevant to human genetic disease.

The FOXJ1 target Cfap206 is required for sperm motility, mucociliary clearance of the airways and brain development

Cilia are complex cellular protrusions consisting of hundreds of proteins. Defects in ciliary structure and function, many of which have not been characterised molecularly, cause ciliopathies: a heterogeneous group of human syndromes. Here, we report on the FOXJ1 target gene Cfap206, orthologues of which so far have only been studied in Chlamydomonas and Tetrahymena In mouse and Xenopus, Cfap206 was co-expressed with and dependent on Foxj1 CFAP206 protein localised to the basal body and to the axoneme of motile cilia. In Xenopus crispant larvae, the ciliary beat frequency of skin multiciliated cells was enhanced and bead transport across the epidermal mucociliary epithelium was reduced. Likewise, Cfap206 knockout mice revealed ciliary phenotypes. Electron tomography of immotile knockout mouse sperm flagella indicated a role in radial spoke formation reminiscent of FAP206 function in Tetrahymena Male infertility, hydrocephalus and impaired mucociliary clearance of the airways in the absence of laterality defects in Cfap206 mutant mice suggests that Cfap206 may represent a candidate for the subgroup of human primary ciliary dyskinesias caused by radial spoke defects.

Alpha-tocopherol exerts protective function against the mucotoxicity of particulate matter in amphibian and human goblet cells

Exposure to particulate matter (PM) in ambient air is known to increase the risk of cardiovascular disorders and mortality. The cytotoxicity of PM is mainly due to the abnormal increase of reactive oxygen species (ROS), which damage cellular components such as DNA, RNA, and proteins. The correlation between PM exposure and human disorders, including mortality, is based on long-term exposure. In this study we have investigated acute responses of mucus-secreting goblet cells upon exposure to PM derived from a heavy diesel engine. To this end, we employed the mucociliary epithelium of amphibian embryos and human Calu-3 cells to examine PM mucotoxicity. Our data suggest that acute exposure to PM significantly impairs mucus secretion and results in the accumulation of mucus vesicles in the cytoplasm of goblet cells. RNA-seq analysis revealed that acute responses to PM exposure significantly altered gene expression patterns; however, known regulators of mucus production and the secretory pathway were not significantly altered. Interestingly, pretreatment with α-tocopherol nearly recovered the hyposecretion of mucus from both amphibian and human goblet cells. We believe this study demonstrates the mucotoxicity of PM and the protective function of α-tocopherol on mucotoxicity caused by acute PM exposure from heavy diesel engines.

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.

Rare Human Diseases: Model Organisms in Deciphering the Molecular Basis of Primary Ciliary Dyskinesia

Primary ciliary dyskinesia (PCD) is a recessive heterogeneous disorder of motile cilia, affecting one per 15,000-30,000 individuals; however, the frequency of this disorder is likely underestimated. Even though more than 40 genes are currently associated with PCD, in the case of approximately 30% of patients, the genetic cause of the manifested PCD symptoms remains unknown. Because motile cilia are highly evolutionarily conserved organelles at both the proteomic and ultrastructural levels, analyses in the unicellular and multicellular model organisms can help not only to identify new proteins essential for cilia motility (and thus identify new putative PCD-causative genes), but also to elucidate the function of the proteins encoded by known PCD-causative genes. Consequently, studies involving model organisms can help us to understand the molecular mechanism(s) behind the phenotypic changes observed in the motile cilia of PCD affected patients. Here, we summarize the current state of the art in the genetics and biology of PCD and emphasize the impact of the studies conducted using model organisms on existing knowledge.

ΔN-Tp63 Mediates Wnt/β-Catenin-Induced Inhibition of Differentiation in Basal Stem Cells of Mucociliary Epithelia

Mucociliary epithelia provide a first line of defense against pathogens. Impaired regeneration and remodeling of mucociliary epithelia are associated with dysregulated Wnt/β-catenin signaling in chronic airway diseases, but underlying mechanisms remain elusive, and studies yield seemingly contradicting results. Employing the Xenopus mucociliary epidermis, the mouse airway, and human airway Basal cells, we characterize the evolutionarily conserved roles of Wnt/β-catenin signaling in vertebrates. In multiciliated cells, Wnt is required for cilia formation during differentiation. In Basal cells, Wnt prevents specification of epithelial cell types by activating ΔN-TP63, a master transcription factor, which is necessary and sufficient to mediate the Wnt-induced inhibition of specification and is required to retain Basal cells during development. Chronic Wnt activation leads to remodeling and Basal cell hyperplasia, which are reversible in vivo and in vitro, suggesting Wnt inhibition as a treatment option in chronic lung diseases. Our work provides important insights into mucociliary signaling, development, and disease.

Na+/H+ Exchangers Are Required for the Development and Function of Vertebrate Mucociliary Epithelia

Na+/H+ exchangers (NHEs) represent a highly conserved family of ion transporters that regulate pH homeostasis. NHEs as well as other proton transporters were previously linked to the regulation of the Wnt signaling pathway, cell polarity signaling, and mucociliary function. Furthermore, mutations in the gene SLC9A3 (encoding NHE3) were detected as additional risk factors for airway infections in cystic fibrosis patients. Here, we used the Xenopus embryonic mucociliary epidermis as well as human airway epithelial cells (HAECs) as models to investigate the functional roles of NHEs in mucociliary development and regeneration. In Xenopus embryos, NHEs 1-3 were expressed during epidermal development, and loss of NHE function impaired mucociliary clearance in tadpoles. Clearance defects were caused by reduced cilia formation, disrupted alignment of basal bodies in multiciliated cells (MCCs), and dysregulated mucociliary gene expression. These data also suggested that NHEs may contribute to the activation of Wnt signaling in mucociliary epithelia. In HAECs, pharmacological inhibition of NHE function also caused defective ciliation and regeneration in airway MCCs. Collectively, our data revealed a requirement for NHEs in vertebrate mucociliary epithelia and linked NHE activity to cilia formation and function in differentiating MCCs. Our results provide an entry point for the understanding of the contribution of NHEs to signaling, development, and pathogenesis in the human respiratory tract.

Notch1 is asymmetrically distributed from the beginning of embryogenesis and controls the ventral center

Based on functional evidence, we have previously demonstrated that early ventral Notch1 activity restricts dorsoanterior development in Xenopus We found that Notch1 has ventralizing properties and abolishes the dorsalizing activity of β-catenin by reducing its steady state levels, in a process that does not require β-catenin phosphorylation by glycogen synthase kinase 3β. In the present work, we demonstrate that Notch1 mRNA and protein are enriched in the ventral region from the beginning of embryogenesis in Xenopus This is the earliest sign of ventral development, preceding the localized expression of wnt8a, bmp4 and Ventx genes in the ventral center and the dorsal accumulation of nuclear β-catenin. Knockdown experiments indicate that Notch1 is necessary for the normal expression of genes essential for ventral-posterior development. These results indicate that during early embryogenesis ventrally located Notch1 promotes the development of the ventral center. Together with our previous evidence, these results suggest that ventral enrichment of Notch1 underlies the process by which Notch1 participates in restricting nuclear accumulation of β-catenin to the dorsal side.

Xenopus: An alternative model system for identifying muco-active agents

The airway epithelium in human plays a central role as the first line of defense against environmental contaminants. Most respiratory diseases such as chronic obstructive pulmonary disease (COPD), asthma, and respiratory infections, disturb normal muco-ciliary functions by stimulating the hypersecretion of mucus. Several muco-active agents have been used to treat hypersecretion symptoms in patients. Current muco-active reagents control mucus secretion by modulating either airway inflammation, cholinergic parasympathetic nerve activities or by reducing the viscosity by cleaving crosslinking in mucin and digesting DNAs in mucus. However, none of the current medication regulates mucus secretion by directly targeting airway goblet cells. The major hurdle for screening potential muco-active agents that directly affect the goblet cells, is the unavailability of in vivo model systems suitable for high-throughput screening. In this study, we developed a high-throughput in vivo model system for identifying muco-active reagents using Xenopus laevis embryos. We tested mucus secretion under various conditions and developed a screening strategy to identify potential muco-regulators. Using this novel screening technique, we identified narasin as a potential muco-regulator. Narasin treatment of developing Xenopus embryos significantly reduced mucus secretion. Furthermore, the human lung epithelial cell line, Calu-3, responded similarly to narasin treatment, validating our technique for discovering muco-active reagents.

Protein localization screening in vivo reveals novel regulators of multiciliated cell development and function

Multiciliated cells (MCCs) drive fluid flow in diverse tubular organs and are essential for the development and homeostasis of the vertebrate central nervous system, airway and reproductive tracts. These cells are characterized by dozens or hundreds of motile cilia that beat in a coordinated and polarized manner. In recent years, genomic studies have not only elucidated the transcriptional hierarchy for MCC specification but also identified myriad new proteins that govern MCC ciliogenesis, cilia beating and cilia polarization. Interestingly, this burst of genomic data has also highlighted that proteins with no obvious role in cilia do, in fact, have important ciliary functions. Understanding the function of proteins with little prior history of study presents a special challenge, especially when faced with large numbers of such proteins. Here, we define the subcellular localization in MCCs of ∼200 proteins not previously implicated in cilia biology. Functional analyses arising from the screen provide novel links between actin cytoskeleton and MCC ciliogenesis.

Functional characterization of the mucus barrier on the Xenopus tropicalis skin surface

The production of mucus helps to trap pathogens, preventing their entry into the body, while it also acts as an interface for many important physiological events (e.g., gas and nutrient exchange). In mammalian models, a detailed study of mucus and its component parts is hindered by the difficulty in accessing these internally located tissues. The Xenopus tropicalis tadpole skin offers a complementary nonmammalian model system to study mucosal epithelia. Using this, we identify a mucin, similar to human mucins, that protects against infection. This system offers an experimentally tractable approach to study mucins and the mucus barrier and their role in conferring protection at mucosal surfaces.

Mucosal surfaces represent critical routes for entry and exit of pathogens. As such, animals have evolved strategies to combat infection at these sites, in particular the production of mucus to prevent attachment and to promote subsequent movement of the mucus/microbe away from the underlying epithelial surface. Using biochemical, biophysical, and infection studies, we have investigated the host protective properties of the skin mucus barrier of the Xenopus tropicalis tadpole. Specifically, we have characterized the major structural component of the barrier and shown that it is a mucin glycoprotein (Otogelin-like or Otogl) with similar sequence, domain organization, and structural properties to human gel-forming mucins. This mucin forms the structural basis of a surface barrier (∼6 μm thick), which is depleted through knockdown of Otogl. Crucially, Otogl knockdown leads to susceptibility to infection by the opportunistic pathogen Aeromonas hydrophila. To more accurately reflect its structure, tissue localization, and function, we have renamed Otogl as Xenopus Skin Mucin, or MucXS. Our findings characterize an accessible and tractable model system to define mucus barrier function and host–microbe interactions.

Manipulating and Analyzing Cell Type Composition of the Xenopus Mucociliary Epidermis

The Xenopus embryonic epidermis serves as a model to investigate the development, cell biology, and regeneration of vertebrate mucociliary epithelia. Its fast development as well as the ease of manipulation and analysis in this system facilitate novel approaches and sophisticated experiments addressing the principle mechanisms of mucociliary signaling, transcriptional regulation, and morphogenesis. This protocol describes how cell type composition can be manipulated and analyzed, and how mucociliary organoids can be generated and used for "omics"-type of experiments.

The role of nitric oxide during embryonic epidermis development of Xenopus laevis

Nitric oxide (NO) is a potent radical molecule that participates in various biological processes such as vasodilation, cell proliferation, immune response and neurotransmission. NO mainly activates soluble guanylate cyclase, leading to cGMP production and activation of protein kinase G and its downstream targets. Here we report the essential role of NO during embryonic epidermis development. Xenopus embryonic epidermis has become a useful model reflecting human epithelial tissue composition. The developing epidermis of Xenopus laevis is formed from specialized ionocytes, multi-ciliated, goblet and small secretory cells. We found that NO is mainly produced in multi-ciliated cells and ionocytes. Production of NO during early developmental stages is required for formation of multi-ciliated cells, ionocytes and small secretory cells by regulation of epidermal-specific gene expression. The data from this research indicate a novel role of NO during development, which supports recent findings of NO production in human mucociliary and epithelium development.

Summary: Embryonic epidermis development is influenced by nitric oxide, where it has been linked to the development of ionocytes, multi-ciliated cells and small secretory cells.

Genetic kidney diseases: Caenorhabditis elegans as model system

Despite its apparent simplicity, the nematode Caenorhabditis elegans has a high rating as a model in molecular and developmental biology and biomedical research. C. elegans has no excretory system comparable with the mammalian kidney but many of the genes and molecular pathways involved in human kidney diseases are conserved in C. elegans. The plethora of genetic, molecular and imaging tools available in C. elegans has enabled major discoveries in renal research and advanced our understanding of the pathogenesis of genetic kidney diseases. In particular, studies in C. elegans have pioneered the fundamental role of cilia for cystic kidney diseases. In addition, proteins of the glomerular filtration barrier and podocytes are critical for cell recognition, assembly of functional neuronal circuits, mechanosensation and signal transduction in C. elegans. C. elegans has also proved tremendously valuable for aging research and the Von Hippel-Lindau tumor suppressor gene has been shown to modulate lifespan in the nematode. Further, studies of the excretory canal, membrane transport and ion channel function in C. elegans have provided insights into mechanisms of tubulogenesis and cellular homeostasis. This review recounts the way that C. elegans can be used to investigate various aspects of genetic and molecular nephrology. This model system opens up an exciting and new area of study of renal development and diseases.

La-related protein 6 controls ciliated cell differentiation

Background

La-related protein 6 (LARP6) is an evolutionally conserved RNA-binding protein. Vertebrate LARP6 binds the 5′ stem-loop found in mRNAs encoding type I collagen to regulate their translation, but other target mRNAs and additional functions for LARP6 are unknown. The aim of this study was to elucidate an additional function of LARP6 and to evaluate the importance of its function during development.

Methods

To uncover the role of LARP6 in development, we utilized Morpholino Oligos to deplete LARP6 protein in Xenopus embryos. Then, embryonic phenotypes and ciliary structures of LAPR6 morphants were examined. To identify the molecular mechanism underlying ciliogenesis regulated by LARP6, we tested the expression level of cilia-related genes, which play important roles in ciliogenesis, by RT-PCR or whole mount in situ hybridization (WISH).

Results

We knocked down LARP6 in Xenopus embryos and found neural tube closure defects. LARP6 mutant, which compromises the collagen synthesis, could rescue these defects. Neural tube closure defects are coincident with lack of cilia, antenna-like cellular organelles with motility- or sensory-related functions, in the neural tube. The absence of cilia at the epidermis was also observed in LARP6 morphants, and this defect was due to the absence of basal bodies which are formed from centrioles and required for ciliary assembly. In the process of multi-ciliated cell (MCC) differentiation, mcidas, which activates the transcription of genes required for centriole formation during ciliogenesis, could partially restore MCCs in LARP6 morphants. In addition, LARP6 likely controls the expression of mcidas in a Notch-independent manner.

Conclusions

La-related protein 6 is involved in ciliated cell differentiation during development by controlling the expression of cilia-related genes including mcidas. This LARP6 function involves a mechanism that is distinct from its established role in binding to collagen mRNAs and regulating their translation.

Electronic supplementary material

The online version of this article (doi:10.1186/s13630-017-0047-7) contains supplementary material, which is available to authorized users.

What we can learn from a tadpole about ciliopathies and airway diseases: Using systems biology in Xenopus to study cilia and mucociliary epithelia

Over the past years, the Xenopus embryo has emerged as an incredibly useful model organism for studying the formation and function of cilia and ciliated epithelia in vivo. This has led to a variety of findings elucidating the molecular mechanisms of ciliated cell specification, basal body biogenesis, cilia assembly, and ciliary motility. These findings also revealed the deep functional conservation of signaling, transcriptional, post-transcriptional, and protein networks employed in the formation and function of vertebrate ciliated cells. Therefore, Xenopus research can contribute crucial insights not only into developmental and cell biology, but also into the molecular mechanisms underlying cilia related diseases (ciliopathies) as well as diseases affecting the ciliated epithelium of the respiratory tract in humans (e.g., chronic lung diseases). Additionally, systems biology approaches including transcriptomics, genomics, and proteomics have been rapidly adapted for use in Xenopus, and broaden the applications for current and future translational biomedical research. This review aims to present the advantages of using Xenopus for cilia research, highlight some of the evolutionarily conserved key concepts and mechanisms of ciliated cell biology that were elucidated using the Xenopus model, and describe the potential for Xenopus research to address unresolved questions regarding the molecular mechanisms of ciliopathies and airway diseases.

TTC25 Deficiency Results in Defects of the Outer Dynein Arm Docking Machinery and Primary Ciliary Dyskinesia with Left-Right Body Asymmetry Randomization

Multiprotein complexes referred to as outer dynein arms (ODAs) develop the main mechanical force to generate the ciliary and flagellar beat. ODA defects are the most common cause of primary ciliary dyskinesia (PCD), a congenital disorder of ciliary beating, characterized by recurrent infections of the upper and lower airways, as well as by progressive lung failure and randomization of left-right body asymmetry. Using a whole-exome sequencing approach, we identified recessive loss-of-function mutations within TTC25 in three individuals from two unrelated families affected by PCD. Mice generated by CRISPR/Cas9 technology and carrying a deletion of exons 2 and 3 in Ttc25 presented with laterality defects. Consistently, we observed immotile nodal cilia and missing leftward flow via particle image velocimetry. Furthermore, transmission electron microscopy (TEM) analysis in TTC25-deficient mice revealed an absence of ODAs. Consistent with our findings in mice, we were able to show loss of the ciliary ODAs in humans via TEM and immunofluorescence (IF) analyses. Additionally, IF analyses revealed an absence of the ODA docking complex (ODA-DC), along with its known components CCDC114, CCDC151, and ARMC4. Co-immunoprecipitation revealed interaction between the ODA-DC component CCDC114 and TTC25. Thus, here we report TTC25 as a new member of the ODA-DC machinery in humans and mice.

Mitochondrial Transplantation Attenuates Airway Hyperresponsiveness by Inhibition of Cholinergic Hyperactivity

Increased cholinergic activity has been highlighted in the pathogenesis of airway hyperresponsiveness, and alternations of mitochondrial structure and function appear to be involved in many lung diseases including airway hyperresponsiveness. It is crucial to clarify the cause-effect association between mitochondrial dysfunction and cholinergic hyperactivity in the pathogenesis of airway hyperresponsiveness. Male SD rats and cultured airway epithelial cells were exposed to cigarette smoke plus lipopolysaccharide administration; mitochondria isolated from airway epithelium were delivered into epithelial cells in vitro and in vivo. Both the cigarette smoke plus lipopolysaccharide-induced cholinergic hyperactivity in vitro and the airway hyperresponsiveness to acetylcholine in vivo were reversed by the transplantation of exogenous mitochondria. The rescue effects of exogenous mitochondria were imitated by the elimination of excessive reactive oxygen species or blockage of muscarinic M₃ receptor, but inhibited by M receptor enhancer. Mitochondrial transplantation effectively attenuates cigarette smoke plus lipopolysaccharide-stimulated airway hyperresponsiveness through the inhibition of ROS-enhanced epithelial cholinergic hyperactivity.

Developmental aspects of the direct-developing frog Adelophryne maranguapensis

Direct development in amphibians is characterized by the loss of aquatic breeding. The anuran Adelophryne maranguapensis is one example of a species with direct development, and it is endemic to the state of Ceará, Brazil. Detailed morphological features of A. maranguapensis embryos and the stages of sequential development have not been described before. Here, we analyzed all available genetic sequence tags in A. maranguapensis (tyr exon 1, pomc and rag1) and compared them with sequences from other species of Adelophryne frogs. We describe the A. maranguapensis reproductive tract and embryonic body development, with a focus on the limbs, tail, ciliated cells of the skin, and the egg tooth, which were analyzed using scanning electron microscopy. Histological analyses revealed ovaries containing oocytes surrounded by follicular cells, displaying large nuclei with nucleoli inside. Early in development, the body is unpigmented, and the neural tube forms dorsally to the yolk vesicle, typical of a direct-developing frog embryo. The hindlimbs develop earlier than the forelimbs. Ciliated cells are abundant during the early stages of skin development and are less common during later stages. The egg tooth appears in the later stages and develops as a keratinized microridge structure. The developmental profile of A. maranguapensis presented here will contribute to our understanding of the direct-development model and may help preserve this endangered native Brazilian frog. genesis 54:257-271, 2016. © 2016 Wiley Periodicals, Inc.

Ccdc11 is a novel centriolar satellite protein essential for ciliogenesis and establishment of left-right asymmetry

Mutations in CCDC11 cause aberrant placement of internal organs and congenital heart disease in humans. Ccdc11 is a novel component of centriolar satellites and plays a critical role in motile and sensory ciliogenesis. The results implicate centriolar satellites in the pathology of left–right patterning and heart disease.

The establishment of left–right (L-R) asymmetry in vertebrates is dependent on the sensory and motile functions of cilia during embryogenesis. Mutations in CCDC11 disrupt L-R asymmetry and cause congenital heart disease in humans, yet the molecular and cellular functions of the protein remain unknown. Here we demonstrate that Ccdc11 is a novel component of centriolar satellites—cytoplasmic granules that serve as recruitment sites for proteins destined for the centrosome and cilium. Ccdc11 interacts with core components of satellites, and its loss disrupts the subcellular organization of satellite proteins and perturbs primary cilium assembly. Ccdc11 colocalizes with satellite proteins in human multiciliated tracheal epithelia, and its loss inhibits motile ciliogenesis. Similarly, depletion of CCDC11 in Xenopus embryos causes defective assembly and motility of cilia in multiciliated epidermal cells. To determine the role of CCDC11 during vertebrate development, we generated mutant alleles in zebrafish. Loss of CCDC11 leads to defective ciliogenesis in the pronephros and within the Kupffer’s vesicle and results in aberrant L-R axis determination. Our results highlight a critical role for Ccdc11 in the assembly and function of motile cilia and implicate centriolar satellite–associated proteins as a new class of proteins in the pathology of L-R patterning and congenital heart disease.

Ptbp1 and Exosc9 knockdowns trigger skin stability defects through different pathways

In humans, genetic diseases affecting skin integrity (genodermatoses) are generally caused by mutations in a small number of genes that encode structural components of the dermal-epidermal junctions. In this article, we first show that inactivation of both exosc9, which encodes a component of the RNA exosome, and ptbp1, which encodes an RNA-binding protein abundant in Xenopus embryonic skin, impairs embryonic Xenopus skin development, with the appearance of dorsal blisters along the anterior part of the fin. However, histological and electron microscopy analyses revealed that the two phenotypes are distinct. Exosc9 morphants are characterized by an increase in the apical surface of the goblet cells, loss of adhesion between the sensorial and peridermal layers, and a decrease in the number of ciliated cells within the blisters. Ptbp1 morphants are characterized by an altered goblet cell morphology. Gene expression profiling by deep RNA sequencing showed that the expression of epidermal and genodermatosis-related genes is also differentially affected in the two morphants, indicating that alterations in post-transcriptional regulations can lead to skin developmental defects through different routes. Therefore, the developing larval epidermis of Xenopus will prove to be a useful model for dissecting the post-transcriptional regulatory network involved in skin development and stability with significant implications for human diseases.

miR-34/449 control apical actin network formation during multiciliogenesis through small GTPase pathways

Vertebrate multiciliated cells (MCCs) contribute to fluid propulsion in several biological processes. We previously showed that microRNAs of the miR-34/449 family trigger MCC differentiation by repressing cell cycle genes and the Notch pathway. Here, using human and Xenopus MCCs, we show that beyond this initial step, miR-34/449 later promote the assembly of an apical actin network, required for proper basal bodies anchoring. Identification of miR-34/449 targets related to small GTPase pathways led us to characterize R-Ras as a key regulator of this process. Protection of RRAS messenger RNA against miR-34/449 binding impairs actin cap formation and multiciliogenesis, despite a still active RhoA. We propose that miR-34/449 also promote relocalization of the actin binding protein Filamin-A, a known RRAS interactor, near basal bodies in MCCs. Our study illustrates the intricate role played by miR-34/449 in coordinating several steps of a complex differentiation programme by regulating distinct signalling pathways.

MicroRNAs of the miR-34/449 family initiate formation of multiciliated cells through the suppression of cell cycle genes and Notch. Here the authors show that miR-34/449 also regulate the assembly of an apical actin network necessary for basal body anchoring by regulating the expression of R-Ras.

BMP signalling controls the construction of vertebrate mucociliary epithelia

Despite the importance of mucociliary epithelia in animal physiology, the mechanisms controlling their establishment are poorly understood. Using the developing Xenopus epidermis and regenerating human upper airways, we reveal the importance of BMP signalling for the construction of vertebrate mucociliary epithelia. In Xenopus, attenuation of BMP activity is necessary for the specification of multiciliated cells (MCCs), ionocytes and small secretory cells (SSCs). Conversely, BMP activity is required for the proper differentiation of goblet cells. Our data suggest that the BMP and Notch pathways interact to control fate choices in the developing epidermis. Unexpectedly, BMP activity is also necessary for the insertion of MCCs, ionocytes and SSCs into the surface epithelium. In human, BMP inhibition also strongly stimulates the formation of MCCs in normal and pathological (cystic fibrosis) airway samples, whereas BMP overactivation has the opposite effect. This work identifies the BMP pathway as a key regulator of vertebrate mucociliary epithelium differentiation and morphogenesis.

TGF-β Signaling Regulates the Differentiation of Motile Cilia

The cilium is a small cellular organelle with motility- and/or sensory-related functions that plays a crucial role during developmental and homeostatic processes. Although many molecules or signal transduction pathways that control cilia assembly have been reported, the mechanisms of ciliary length control have remained enigmatic. Here, we report that Smad2-dependent transforming growth factor β (TGF-β) signaling impacts the length of motile cilia at the Xenopus left-right (LR) organizer, the gastrocoel roof plate (GRP), as well as at the neural tube and the epidermis. Blocking TGF-β signaling resulted in the absence of the transition zone protein B9D1/MSKR-1 from cilia in multi-ciliated cells (MCCs) of the epidermis. Interestingly, this TGF-β activity is not mediated by Mcidas, Foxj1, and RFX2, the known major regulators of ciliogenesis. These data indicate that TGF-β signaling is crucial for the function of the transition zone, which in turn may affect the regulation of cilia length.

Characterization of tetratricopeptide repeat-containing proteins critical for cilia formation and function

Cilia formation and function require a special set of trafficking machinery termed intraflagellar transport (IFT), consisting mainly of protein complexes IFT-A, IFT-B, BBSome, and microtubule-dependent molecular motors. Tetratricopeptide repeat-containing (TTC) proteins are widely involved in protein complex formation. Nine of them are known to serve as components of the IFT or BBSome complexes. How many TTC proteins are cilia-related and how they function, however, remain unclear. Here we show that twenty TTC genes were upregulated by at least 2-fold during the differentiation of cultured mouse tracheal epithelial cells (MTECs) into multiciliated cells. Our systematic screen in zebrafish identified four novel TTC genes, ttc4, -9c, -36, and -39c, that are critical for cilia formation and motility. Accordingly, their zebrafish morphants displayed typical ciliopathy-related phenotypes, including curved body, abnormal otolith, hydrocephalus, and defective left-right patterning. The morphants of ttc4 and ttc25, a known cilia-related gene, additionally showed pronephric cyst formation. Immunoprecipitation indicated associations of TTC4, -9c, -25, -36, and -39c with components or entire complexes of IFT-A, IFT-B, or BBSome, implying their participations in IFT or IFT-related activities. Our results provide a global view for the relationship between TTC proteins and cilia.

ATP4a is required for development and function of the Xenopus mucociliary epidermis - a potential model to study proton pump inhibitor-associated pneumonia

Proton pump inhibitors (PPIs), which target gastric H(+)/K(+)ATPase (ATP4), are among the most commonly prescribed drugs. PPIs are used to treat ulcers and as a preventative measure against gastroesophageal reflux disease in hospitalized patients. PPI treatment correlates with an increased risk for airway infections, i.e. community- and hospital-acquired pneumonia. The cause for this correlation, however, remains elusive. The Xenopus embryonic epidermis is increasingly being used as a model to study airway-like mucociliary epithelia. Here we use this model to address how ATP4 inhibition may affect epithelial function in human airways. We demonstrate that atp4a knockdown interfered with the generation of cilia-driven extracellular fluid flow. ATP4a and canonical Wnt signaling were required in the epidermis for expression of foxj1, a transcriptional regulator of motile ciliogenesis. The ATP4/Wnt module activated foxj1 downstream of ciliated cell fate specification. In multiciliated cells (MCCs) of the epidermis, ATP4a was also necessary for normal myb expression, apical actin formation, basal body docking and alignment of basal bodies. Furthermore, ATP4-dependent Wnt/β-catenin signaling in the epidermis was a prerequisite for foxa1-mediated specification of small secretory cells (SSCs). SSCs release serotonin and other substances into the medium, and thereby regulate ciliary beating in MCCs and protect the epithelium against infection. Pharmacological inhibition of ATP4 in the mature mucociliary epithelium also caused a loss of MCCs and led to impaired mucociliary clearance. These data strongly suggest that PPI-associated pneumonia in human patients might, at least in part, be linked to dysfunction of mucociliary epithelia of the airways.

ERK7 regulates ciliogenesis by phosphorylating the actin regulator CapZIP in cooperation with Dishevelled

Cilia are essential for embryogenesis and maintenance of homeostasis, but little is known about the signalling pathways that regulate ciliogenesis. Here, we identify ERK7, an atypical mitogen-activated protein kinase, as a key regulator of ciliogenesis. ERK7 is strongly expressed in ciliated tissues of Xenopus embryos. ERK7 knockdown markedly diminishes both the number and the length of cilia in multiciliated cells, and it inhibits the apical migration of basal bodies. Moreover, ERK7 knockdown results in a loss of the apical actin meshwork, which is required for the proper migration of basal bodies. We find that the actin regulator CapZIP, which has been shown to regulate ciliogenesis in a phosphorylation-dependent manner, is an ERK7 substrate, and that Dishevelled, which has also been shown to regulate ciliogenesis, facilitates ERK7 phosphorylation of CapZIP through binding to both ERK7 and CapZIP. Collectively, these results identify an ERK7/Dishevelled/CapZIP axis that regulates ciliogenesis.