Structure and function of vertebrate cilia, towards a new taxonomy

Multicilia dynamically transduce Sonic Hedgehog signaling to regulate choroid plexus functions

The choroid plexus is a major site for cerebrospinal fluid (CSF) production, characterized by a multiciliated epithelial monolayer that regulates CSF production. We demonstrate that defective choroid plexus ciliogenesis or intraflagellar transport yields neonatal hydrocephalus, at least in part due to increased water channel Aqp1 and ion transporter Atp1a2 expression. We demonstrate choroid plexus multicilia as sensory cilia, transducing both canonical and non-canonical Sonic Hedgehog (Shh) signaling. Interestingly, it is the non-canonical Shh signaling that represses Aqp1 and Atp1a2 expression by the Smoothened (Smo)/Gαi/cyclic AMP (cAMP) pathway. Choroid plexus multicilia exhibit unique ciliary ultrastructure, carrying features of both primary and motile cilia. Unlike most cilia that elongate during maturation, choroid plexus ciliary length decreases during development, causing a decline of Shh signaling intensity in the developing choroid plexus, a derepression of Aqp1 and Atp1a2, and, ultimately, increased CSF production. Hence, the developmental dynamics of choroid plexus multicilia dampens the Shh signaling intensity to promote CSF production.

Multicilia dynamically transduce Shh signaling to regulate choroid plexus functions

Choroid plexus is a major site for cerebrospinal fluid (CSF) production, characterized by a multiciliated epithelial monolayer that regulates CSF production. We demonstrate that defective choroid plexus ciliogenesis or Intraflagellar transport yields neonatal hydrocephalus, at least in part, due to increased water channel Aqp1 and ion transporter Atp1a2 expression. We demonstrate choroid plexus multicilia as sensory cilia, transducing both canonical and non-canonical Shh signaling. Interestingly, it is the non-canonical Shh signaling that represses Aqp1 and Atp1a2 expression by Smo/Gαi/cAMP pathway. Choroid plexus multicilia exhibit unique ciliary ultrastructure, carrying features of both primary and motile cilia. Unlike most cilia that elongate during maturation, choroid plexus ciliary length decreases during development, causing a decline of Shh signaling intensity in developing choroid plexus, a derepression of Aqp1 and Atp1a2, and ultimately, an increased CSF production. Hence, developmental dynamics of choroid plexus multicilia dampens the Shh signaling intensity to promote CSF production.

Macro- and microscopic anatomy of the digestive tract in the red-eared slider (Emydidae: Trachemys scripta elegans)

The red-eared sliders (Emydidae: Trachemys scripta) is characterised by a high adaptability to a variety of environment and threatens the habitat of Japanese native species. The ability to digest a variety of diets may attribute to the high adaptive capacity of this species to various environments, however, the digestive morphology remains scarcely described in red-eared sliders. In this study, we investigated the macro- and microscopic anatomy of the esophagus, stomach, small intestine, and large intestine in red-eared sliders. All segments of the digestive tract had longitudinal mucosal folds, the height and width of which varied in each segment of the digestive tract. The stomach had the highest and widest mucosal folds. The mucosal folds in the proximal-to-middle small intestine exhibited a zigzag shape, whereas those in the distal small intestine were linear. The wall of the digestive tract regularly consisted of mucosa, submucosa, tunica muscularis, and tunica adventitia or serosa. In each segment of the digestive tract, the epithelial structure was different. The esophagus and small intestine were lined by the pseudostratified columnar epithelium. In both segments, the basal part of the pseudostratified epithelium included proliferating cell nuclear antigen (PCNA)-positive proliferating cells. The stomach and large intestine were lined by the simple columnar epithelium. In the stomach and large intestine, PCNA-positive proliferating cells were present in the neck of the proper gastric gland and crypt-like structures, respectively. The proper gastric gland was composed of oxynticopeptic and mucous cells. This study revealed the detailed macro- and microscopic anatomy of the digestive tract in red-eared sliders. Overall, our findings may provide an anatomical basis for understanding the relationship between morphology and function in the digestive tract of turtles.

Cilia and Cancer: From Molecular Genetics to Therapeutic Strategies

Cilia are microtubule-based organelles that project from the cell surface with motility or sensory functions. Primary cilia work as antennae to sense and transduce extracellular signals. Cilia critically control proliferation by mediating cell-extrinsic signals and by regulating cell cycle entry. Recent studies have shown that primary cilia and their associated proteins also function in autophagy and genome stability, which are important players in oncogenesis. Abnormal functions of primary cilia may contribute to oncogenesis. Indeed, defective cilia can either promote or suppress cancers, depending on the cancer-initiating mutation, and the presence or absence of primary cilia is associated with specific cancer types. Together, these findings suggest that primary cilia play important, but distinct roles in different cancer types, opening up a completely new avenue of research to understand the biology and treatment of cancers. In this review, we discuss the roles of primary cilia in promoting or inhibiting oncogenesis based on the known or predicted functions of cilia and cilia-associated proteins in several key processes and related clinical implications.

Evidence for intraflagellar transport in butterfly spermatocyte cilia

In the model organism insect Drosophila melanogaster short cilia assemble on spermatocytes that elaborate into 1.8 mm long flagella during spermatid differentiation. A unique feature of these cilia/flagella is their lack of dependence on intraflagellar transport (IFT) for their assembly. Here, we show that in the common butterfly Pieris brassicae, the spermatocyte cilia are exceptionally long: about 40 μm compared to less than 1 μm in Drosophila. By transmission electron microscopy, we show that P. brassicae spermatocytes display several features not found in melanogaster, including compelling evidence of IFT structures and features of motile cilia.

Identification of TEKTIN1-expressing multiciliated cells during spontaneous differentiation of non-human primate embryonic stem cells

Tektins are a group of microtubule-stabilizing proteins necessary for cilia and flagella assembly. TEKTIN1 (TEKT1) is used as a sperm marker for monitoring germ cell differentiation in embryonic stem (ES) and induced pluripotent stem (iPS) cells. Although upregulation of TEKT1 has been reported during spontaneous differentiation of ES and iPS cells, it is unclear which cells express TEKT1. To identify TEKT1-expressing cells, we established an ES cell line derived from cynomolgus monkeys (Macaca fascicularis), which expresses Venus controlled by the TEKT1 promoter. Venus expression was detected at 5 weeks of differentiation on the surface of the embryoid body (EB), and it gradually increased with the concomitant formation of a leash-like structure at the EB periphery. Motile cilia were observed on the surface of the Venus-positive leash-like structure after 8 weeks of differentiation. The expression of cilia markers as well as TEKT1-5 and 9 + 2 microtubule structures, which are characteristic of motile cilia, were detected in Venus-positive cells. These results demonstrated that TEKT1-expressing cells are multiciliated epithelial-like cells that form a leash-like structure during the spontaneous differentiation of ES and iPS cells. These findings will provide a new research strategy for studying cilia biology, including ciliogenesis and ciliopathies.

Schmidtea mediterranea as a Model Organism to Study the Molecular Background of Human Motile Ciliopathies

Cilia and flagella are evolutionarily conserved organelles that form protrusions on the surface of many growth-arrested or differentiated eukaryotic cells. Due to the structural and functional differences, cilia can be roughly classified as motile and non-motile (primary). Genetically determined dysfunction of motile cilia is the basis of primary ciliary dyskinesia (PCD), a heterogeneous ciliopathy affecting respiratory airways, fertility, and laterality. In the face of the still incomplete knowledge of PCD genetics and phenotype-genotype relations in PCD and the spectrum of PCD-like diseases, a continuous search for new causative genes is required. The use of model organisms has been a great part of the advances in understanding molecular mechanisms and the genetic basis of human diseases; the PCD spectrum is not different in this respect. The planarian model (Schmidtea mediterranea) has been intensely used to study regeneration processes, and-in the context of cilia-their evolution, assembly, and role in cell signaling. However, relatively little attention has been paid to the use of this simple and accessible model for studying the genetics of PCD and related diseases. The recent rapid development of the available planarian databases with detailed genomic and functional annotations prompted us to review the potential of the S. mediterranea model for studying human motile ciliopathies.

The choroid plexus: a missing link in our understanding of brain development and function

Studies of the choroid plexus lag behind those of the more widely known blood-brain barrier, despite a much longer history. This review has two overall aims. The first is to outline long-standing areas of research where there are unanswered questions, such as control of cerebrospinal fluid (CSF) secretion and blood flow. The second aim is to review research over the past 10 years where the focus has shifted to the idea that there are choroid plexuses located in each of the brain's ventricles that make specific contributions to brain development and function through molecules they generate for delivery via the CSF. These factors appear to be particularly important for aspects of normal brain growth. Most research carried out during the twentieth century dealt with the choroid plexus, a brain barrier interface making critical contributions to the composition and stability of the brain's internal environment throughout life. More recent research in the twenty-first century has shown the importance of choroid plexus-generated CSF in neurogenesis, influence of sex and other hormones on choroid plexus function, and choroid plexus involvement in circadian rhythms and sleep. The advancement of technologies to facilitate delivery of brain-specific therapies via the CSF to treat neurological disorders is a rapidly growing area of research. Conversely, understanding the basic mechanisms and implications of how maternal drug exposure during pregnancy impacts the developing brain represents another key area of research.

Three-dimensional flagella structures from animals' closest unicellular relatives, the Choanoflagellates

In most eukaryotic organisms, cilia and flagella perform a variety of life-sustaining roles related to environmental sensing and motility. Cryo-electron microscopy has provided considerable insight into the morphology and function of flagellar structures, but studies have been limited to less than a dozen of the millions of known eukaryotic species. Ultrastructural information is particularly lacking for unicellular organisms in the Opisthokonta clade, leaving a sizeable gap in our understanding of flagella evolution between unicellular species and multicellular metazoans (animals). Choanoflagellates are important aquatic heterotrophs, uniquely positioned within the opisthokonts as the metazoans' closest living unicellular relatives. We performed cryo-focused ion beam milling and cryo-electron tomography on flagella from the choanoflagellate species Salpingoeca rosetta. We show that the axonemal dyneins, radial spokes, and central pair complex in S. rosetta more closely resemble metazoan structures than those of unicellular organisms from other suprakingdoms. In addition, we describe unique features of S. rosetta flagella, including microtubule holes, microtubule inner proteins, and the flagellar vane: a fine, net-like extension that has been notoriously difficult to visualize using other methods. Furthermore, we report barb-like structures of unknown function on the extracellular surface of the flagellar membrane. Together, our findings provide new insights into choanoflagellate biology and flagella evolution between unicellular and multicellular opisthokonts.

Fluorescence imaging of beta cell primary cilia

Primary cilia are slender cell-surface organelles that project into the intercellular space. In pancreatic beta cells, primary cilia coordinate a variety of cell responses including GPCR signaling, calcium influx, and insulin secretion, along with likely many underappreciated roles in islet development and differentiation. To study cilia function in islet biology, direct visualization of primary cilia by microscopic methods is often a necessary first step. Ciliary abundance, distribution, and morphology are heterogeneous among islet cells and are best visualized by fluorescence microscopy, the tools for which are readily accessible to most researchers. Here we present a collection of fluorescence imaging methods that we have adopted and optimized for the observation of primary cilia in mouse and human islets. These include conventional confocal microscopy using fixed islets and pancreas sections, live-cell imaging with cilia-targeted biosensors and probes, cilia motion recordings, and quantitative analysis of primary cilia waveform in the ex vivo environment. We discuss practical considerations and limitations of our approaches as well as new tools on the horizon to facilitate the observation of primary cilia in pancreatic islets.

Depletion of Ift88 in thymic epithelial cells affects thymic synapse and T-cell differentiation in aged mice

Primary cilia are ubiquitous hair-like organelles, usually projecting from the cell surface. They are essential for the organogenesis and homeostasis of various physiological functions, and their dysfunction leads to a plethora of human diseases. However, there are few reports on the role of primary cilia in the immune system; therefore, we focused on their role in the thymus that nurtures immature lymphocytes to full-fledged T cells. We detected primary cilia on the thymic epithelial cell (TEC) expressing transforming growth factor β (TGF-β) receptor in the basal body, and established a line of an intraflagellar transport protein 88 (Ift88) knockout mice lacking primary cilia in TECs (Ift88-TEC null mutant) to clarify their precise role in thymic organogenesis and T-cell differentiation. The Ift88-TEC null mutant mice showed stunted cilia or lack of cilia in TECs. The intercellular contact between T cells and the "thymic synapse" of medullary TECs was slightly disorganized in Ift88-TEC null mutants. Notably, the CD4- and CD8-single positive thymocyte subsets increased significantly. The absence or disorganization of thymic cilia downregulated the TGF-β signaling cascade, increasing the number of single positive thymocytes. To our knowledge, this is the first study reporting the physiological role of primary cilia and Ift88 in regulating the differentiation of the thymus and T cells.

INPP5E and Coordination of Signaling Networks in Cilia

Primary cilia are ubiquitous mechanosensory organelles that specifically coordinate a series of cellular signal transduction pathways to control cellular physiological processes during development and in tissue homeostasis. Defects in the function or structure of primary cilia have been shown to be associated with a large range of diseases called ciliopathies. Inositol polyphosphate-5-phosphatase E (INPP5E) is an inositol polyphosphate 5-phosphatase that is localized on the ciliary membrane by anchorage via its C-terminal prenyl moiety and hydrolyzes both phosphatidylinositol-4, 5-bisphosphate (PtdIns(4,5)P2) and PtdIns(3,4,5)P3, leading to changes in the phosphoinositide metabolism, thereby resulting in a specific phosphoinositide distribution and ensuring proper localization and trafficking of proteins in primary cilia. In addition, INPP5E also works synergistically with cilia membrane-related proteins by playing key roles in the development and maintenance homeostasis of cilia. The mutation of INPP5E will cause deficiency of primary cilia signaling transduction, ciliary instability and ciliopathies. Here, we present an overview of the role of INPP5E and its coordination of signaling networks in primary cilia.

The highly conserved FOXJ1 target CFAP161 is dispensable for motile ciliary function in mouse and Xenopus

Cilia are protrusions of the cell surface and composed of hundreds of proteins many of which are evolutionary and functionally well conserved. In cells assembling motile cilia the expression of numerous ciliary components is under the control of the transcription factor FOXJ1. Here, we analyse the evolutionary conserved FOXJ1 target CFAP161 in Xenopus and mouse. In both species Cfap161 expression correlates with the presence of motile cilia and depends on FOXJ1. Tagged CFAP161 localises to the basal bodies of multiciliated cells of the Xenopus larval epidermis, and in mice CFAP161 protein localises to the axoneme. Surprisingly, disruption of the Cfap161 gene in both species did not lead to motile cilia-related phenotypes, which contrasts with the conserved expression in cells carrying motile cilia and high sequence conservation. In mice mutation of Cfap161 stabilised the mutant mRNA making genetic compensation triggered by mRNA decay unlikely. However, genes related to microtubules and cilia, microtubule motor activity and inner dyneins were dysregulated, which might buffer the Cfap161 mutation.

Investigating the Role of the microRNA-34/449 Family in Male Infertility: A Critical Analysis and Review of the Literature

There is a great body of evidence suggesting that in both humans and animal models the microRNA-34/449 (miR-34/449) family plays a crucial role for normal testicular functionality as well as for successful spermatogenesis, regulating spermatozoa maturation and functionality. This review and critical analysis aims to summarize the potential mechanisms via which miR-34/449 dysregulation could lead to male infertility. Existing data indicate that miR-34/449 family members regulate ciliogenesis in the efferent ductules epithelium. Upon miR-34/449 dysregulation, ciliogenesis in the efferent ductules is significantly impaired, leading to sperm aggregation and agglutination as well as to defective reabsorption of the seminiferous tubular fluids. These events in turn cause obstruction of the efferent ductules and thus accumulation of the tubular fluids resulting to high hydrostatic pressure into the testis. High hydrostatic pressure progressively leads to testicular dysfunction as well as to spermatogenic failure and finally to male infertility, which could range from severe oligoasthenozoospermia to azoospermia. In addition, miR-34/449 family members act as significant regulators of spermatogenesis with an essential role in controlling expression patterns of several spermatogenesis-related proteins. It is demonstrated that these microRNAs are meiotic specific microRNAs as their expression is relatively higher at the initiation of meiotic divisions during spermatogenesis. Moreover, data indicate that these molecules are essential for proper formation as well as for proper function of spermatozoa per se. MicroRNA-34/449 family seems to exert significant anti-oxidant and anti-apoptotic properties and thus contribute to testicular homeostatic regulation. Considering the clinical significance of these microRNAs, data indicate that the altered expression of the miR-34/449 family members is strongly associated with several aspects of male infertility. Most importantly, miR-34/449 levels in spermatozoa, in testicular tissues as well as in seminal plasma seem to be directly associated with severity of male infertility, indicating that these microRNAs could serve as potential sensitive biomarkers for an accurate individualized differential diagnosis, as well as for the assessment of the severity of male factor infertility. In conclusion, dysregulation of miR-34/449 family detrimentally affects male reproductive potential, impairing both testicular functionality as well as spermatogenesis. Future studies are needed to verify these conclusions.

Maturation of the Olfactory Sensory Neuron and Its Cilia

Olfactory sensory neurons (OSNs) are bipolar neurons, unusual because they turn over continuously and have a multiciliated dendrite. The extensive changes in gene expression accompanying OSN differentiation in mice are largely known, especially the transcriptional regulators responsible for altering gene expression, revealing much about how differentiation proceeds. Basal progenitor cells of the olfactory epithelium transition into nascent OSNs marked by Cxcr4 expression and the initial extension of basal and apical neurites. Nascent OSNs become immature OSNs within 24-48 h. Immature OSN differentiation requires about a week and at least 2 stages. Early-stage immature OSNs initiate expression of genes encoding key transcriptional regulators and structural proteins necessary for further neuritogenesis. Late-stage immature OSNs begin expressing genes encoding proteins important for energy production and neuronal homeostasis that carry over into mature OSNs. The transition to maturity depends on massive expression of one allele of one odorant receptor gene, and this results in expression of the last 8% of genes expressed by mature OSNs. Many of these genes encode proteins necessary for mature function of axons and synapses or for completing the elaboration of non-motile cilia, which began extending from the newly formed dendritic knobs of immature OSNs. The cilia from adjoining OSNs form a meshwork in the olfactory mucus and are the site of olfactory transduction. Immature OSNs also have a primary cilium, but its role is unknown, unlike the critical role in proliferation and differentiation played by the primary cilium of the olfactory epithelium's horizontal basal cell.

The single flagellum of Leishmania has a fixed polarisation of its asymmetric beat

Eukaryotic flagella undertake different beat types as necessary for different functions; for example, the Leishmania parasite flagellum undergoes a symmetric tip-to-base beat for forward swimming and an asymmetric base-to-tip beat to rotate the cell. In multi-ciliated tissues or organisms, the asymmetric beats are coordinated, leading to movement of the cell, organism or surrounding fluid. This coordination involves a polarisation of power stroke direction. Here, we asked whether the asymmetric beat of the single Leishmania flagellum also has a fixed polarisation. We developed high frame rate dual-colour fluorescence microscopy to visualise flagellar-associated structures in live swimming cells. This showed that the asymmetric Leishmania beat is polarised, with power strokes only occurring in one direction relative to the asymmetric flagellar machinery. Polarisation of bending was retained in deletion mutants whose flagella cannot beat but have a static bend. Furthermore, deletion mutants for proteins required for asymmetric extra-axonemal and rootlet-like flagellum-associated structures also retained normal polarisation. Leishmania beat polarisation therefore likely arises from either the nine-fold rotational symmetry of the axoneme structure or is due to differences between the outer doublet decorations.

Highlighted Article: By using high speed, high-resolution fluorescence microscopy of swimming Leishmania cells, we showed that the asymmetric flagellar beat always wafts in the same direction and investigate which structures are involved.

Physiological and Pathophysiological Aspects of Primary Cilia-A Literature Review with View on Functional and Structural Relationships in Cartilage

Cilia are cellular organelles that project from the cell. They occur in nearly all non-hematopoietic tissues and have different functions in different tissues. In mesenchymal tissues primary cilia play a crucial role in the adequate morphogenesis during embryological development. In mature articular cartilage, primary cilia fulfil chemo- and mechanosensitive functions to adapt the cellular mechanisms on extracellular changes and thus, maintain tissue homeostasis and morphometry. Ciliary abnormalities in osteoarthritic cartilage could represent pathophysiological relationships between ciliary dysfunction and tissue deformation. Nevertheless, the molecular and pathophysiological relationships of ‘Primary Cilia’ (PC) in the context of osteoarthritis is not yet fully understood. The present review focuses on the current knowledge about PC and provide a short but not exhaustive overview of their role in cartilage.

New perspectives on GPCRs: GPCR heterodimer formation (melanocortin receptor) and GPCR on primary cilia (melanin concentrating hormone receptor)

GPCRs are the largest family of receptors accounting for about 30% of the current drug targets. However, it is difficult to fully elucidate the mechanisms regulating intracellular GPCR signal regulation. It is thus important to consider and investigate GPCRs with respect to endogenous situations. Our group has been investigating GPCRs involved in body color (teleost and amphibian) and eating (vertebrate). Here, I review two independent GPCR systems (heterodimer formation and primary ciliated GPCR) that can be breakthroughs in GPCR research. In teleosts, MCRs form heterodimers, which significantly reduce their affinity for acetylated ligands. In mammals, MCHR1 is localized in the ciliary membrane and shortens the length of the primary cilia through a unique signal from the ciliary membrane. Considering these two new GPCR concepts is expected to advance the overall view of the GPCR system.

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.

Olfactory Loss and Dysfunction in Ciliopathies: Molecular Mechanisms and Potential Therapies

BACKGROUND

Ciliopathies are a class of inherited pleiotropic genetic disorders in which alterations in cilia assembly, maintenance, and/or function exhibit penetrance in the multiple organ systems. Olfactory dysfunction is one such clinical manifestation that has been shown in both patients and model organisms. Existing therapies for ciliopathies are limited to the treatment or management of symptoms. The last decade has seen an increase in potential curative therapeutic options including small molecules and biologics. Recent work in multiciliated olfactory sensory neurons has demonstrated the capacity of targeted gene therapy to restore ciliation in terminally differentiated cells and rescue olfactory function. This review will discuss the current understanding of the penetrance of ciliopathies in the olfactory system. Importantly, it will highlight both pharmacological and biological approaches, and their potential therapeutic value in the olfactory system and other ciliated tissues.

METHODS

We undertook a structured and comprehensive search of peer-reviewed research literature encompassing in vitro, in vivo, model organism, and clinical studies. From these publications, we describe the olfactory system, and discuss the penetrance of ciliopathies and impact of cilia loss on olfactory function. In addition, we outlined the developing therapies for ciliopathies across different organ and cell culture systems, and discussed their potential therapeutic application to the mammalian olfactory system.

RESULTS

One-hundred sixty-one manuscripts were included in the review, centering on the understanding of olfactory penetrance of ciliopathies, and discussing the potential therapeutic options for ciliopathies in the context of the mammalian olfactory system. Forty-four manuscripts were used to generate a table listing the known congenital causes of olfactory dysfunction, with the first ten listed are linked to ciliopathies. Twenty-three manuscripts were used to outline the potential of small molecules for the olfactory system. Emphasis was placed on HDAC6 inhibitors and lithium, both of which were shown to stabilize microtubule structures, contributing to ciliogenesis and cilia lengthening. Seventy-five manuscripts were used to describe gene therapy and gene therapeutic strategies. Included were the implementation of adenoviral, adeno-associated virus (AAV), and lentiviral vectors to treat ciliopathies across different organ systems and application toward the olfactory system. Thus far, adenoviral and AAVmeditated ciliary restoration demonstrated successful proof-of-principle preclinical studies. In addition, gene editing, ex vivo gene therapy, and transplantation could serve as alternative therapeutic and long-term approaches. But for all approaches, additional assessment of vector immunogenicity, specificity, and efficacy need further investigation. Currently, ciliopathy treatments are limited to symptomatic management with no curative options. However, the accessibility and amenability of the olfactory system to treatment would facilitate development and advancement of a viable therapy.

CONCLUSION

The findings of this review highlight the contribution of ciliopathies to a growing list of congenial olfactory dysfunctions. Promising results from other organ systems imply the feasibility of biologics, with results from gene therapies proving to be a viable therapeutic option for ciliopathies and olfactory dysfunction.

The Roles of Primary Cilia in Cardiovascular Diseases

Primary cilia are microtubule-based organelles found in most mammalian cell types. Cilia act as sensory organelles that transmit extracellular clues into intracellular signals for molecular and cellular responses. Biochemical and molecular defects in primary cilia are associated with a wide range of diseases, termed ciliopathies, with phenotypes ranging from polycystic kidney disease, liver disorders, mental retardation, and obesity to cardiovascular diseases. Primary cilia in vascular endothelia protrude into the lumen of blood vessels and function as molecular switches for calcium (Ca²⁺) and nitric oxide (NO) signaling. As mechanosensory organelles, endothelial cilia are involved in blood flow sensing. Dysfunction in endothelial cilia contributes to aberrant fluid-sensing and thus results in vascular disorders, including hypertension, aneurysm, and atherosclerosis. This review focuses on the most recent findings on the roles of endothelial primary cilia within vascular biology and alludes to the possibility of primary cilium as a therapeutic target for cardiovascular disorders.

The evolutionary conserved FOXJ1 target gene Fam183b is essential for motile cilia in Xenopus but dispensable for ciliary function in mice

The transcription factor FOXJ1 is essential for the formation of motile cilia throughout the animal kingdom. Target genes therefore likely constitute an important part of the motile cilia program. Here, we report on the analysis of one of these targets, Fam183b, in Xenopus and mice. Fam183b encodes a protein with unknown function which is conserved from the green algae Chlamydomonas to humans. Fam183b is expressed in tissues harbouring motile cilia in both mouse and frog embryos. FAM183b protein localises to basal bodies of cilia in mIMCD3 cells and of multiciliated cells of the frog larval epidermis. In addition, FAM183b interacts with NUP93, which also localises to basal bodies. During frog embryogenesis, Fam183b was dispensable for laterality specification and brain development, but required for ciliogenesis and motility of epidermal multiciliated cells and nephrostomes, i.e. the embryonic kidney. Surprisingly, mice homozygous for a null allele did not display any defects indicative of disrupted motile ciliary function. The lack of a cilia phenotype in mouse and the limited requirements in frog contrast with high sequence conservation and the correlation of gene expression with the presence of motile cilia. This finding may be explained through compensatory mechanisms at sites where no defects were observed in our FAM183b-loss-of-function studies.

N-terminal dual lipidation-coupled molecular targeting into the primary cilium

The primary cilium functions as an "antenna" for cell signaling, studded with characteristic transmembrane receptors and soluble protein factors, raised above the cell surface. In contrast to the transmembrane proteins, targeting mechanisms of nontransmembrane ciliary proteins are poorly understood. We focused on a pathogenic mutation that abolishes ciliary localization of retinitis pigmentosa 2 protein and revealed a dual acylation-dependent ciliary targeting pathway. Short N-terminal sequences which contain myristoylation and palmitoylation sites are sufficient to target a marker protein into the cilium in a palmitoylation-dependent manner. A Golgi-localized palmitoyltransferase DHHC-21 was identified as the key enzyme controlling this targeting pathway. Rapid turnover of the targeted protein was ensured by cholesterol-dependent membrane fluidity, which balances highly and less-mobile populations of the molecules within the cilium. This targeting signal was found in a set of signal transduction molecules, suggesting a general role of this pathway in proper ciliary organization, and dysfunction in ciliary disorders.

Sensing the cilium, digital capture of ciliary data for comparative genomics investigations

Background

Cilia are specialized, hair-like structures that project from the cell bodies of eukaryotic cells. With increased understanding of the distribution and functions of various types of cilia, interest in these organelles is accelerating. To effectively use this great expansion in knowledge, this information must be made digitally accessible and available for large-scale analytical and computational investigation. Capture and integration of knowledge about cilia into existing knowledge bases, thus providing the ability to improve comparative genomic data analysis, is the objective of this work.

Methods

We focused on the capture of information about cilia as studied in the laboratory mouse, a primary model of human biology. The workflow developed establishes a standard for capture of comparative functional data relevant to human biology. We established the 310 closest mouse orthologs of the 302 human genes defined in the SYSCILIA Gold Standard set of ciliary genes. For the mouse genes, we identified biomedical literature for curation and used Gene Ontology (GO) curation paradigms to provide functional annotations from these publications.

Results

Employing a methodology for comprehensive capture of experimental data about cilia genes in structured, digital form, we established a workflow for curation of experimental literature detailing molecular function and roles of cilia proteins starting with the mouse orthologs of the human SYSCILIA gene set. We worked closely with the GO Consortium ontology development editors and the SYSCILIA Consortium to improve the representation of ciliary biology within the GO. During the time frame of the ontology improvement project, we have fully curated 134 of these 310 mouse genes, resulting in an increase in the number of ciliary and other experimental annotations.

Conclusions

We have improved the GO annotations available for mouse genes orthologous to the human genes in the SYSCILIA Consortium’s Gold Standard set. In addition, ciliary terminology in the GO itself was improved in collaboration with GO ontology developers and the SYSCILIA Consortium. These improvements to the GO terms for the functions and roles of ciliary proteins, along with the increase in annotations of the corresponding genes, enhance the representation of ciliary processes and localizations and improve access to these data during large-scale bioinformatic analyses.

Electronic supplementary material

The online version of this article (10.1186/s13630-018-0057-0) contains supplementary material, which is available to authorized users.

Motile cilia of human airway epithelia contain hedgehog signaling components that mediate noncanonical hedgehog signaling

Previous studies localized the hedgehog (HH) signaling system to primary cilia. We discovered that motile cilia on airway epithelia also contain HH signaling proteins, indicating that like primary cilia, these motile cilia have an important sensory function. However, in contrast to the function of HH signaling in most primary cilia, sonic hedgehog (SHH) elicits noncanonical signaling, reducing cellular levels of cAMP. These findings suggest that airway SHH may quiet airway defenses. Involvement of SHH in lung disease and positioning of motile cilia where they sample SHH and other ligands in the airway lumen suggest that noncanonical HH signaling might modulate airway responses to the environment in health and disease.

Differentiated airway epithelia produce sonic hedgehog (SHH), which is found in the thin layer of liquid covering the airway surface. Although previous studies showed that vertebrate HH signaling requires primary cilia, as airway epithelia mature, the cells lose primary cilia and produce hundreds of motile cilia. Thus, whether airway epithelia have apical receptors for SHH has remained unknown. We discovered that motile cilia on airway epithelial cells have HH signaling proteins, including patched and smoothened. These cilia also have proteins affecting cAMP-dependent signaling, including Gα_i and adenylyl cyclase 5/6. Apical SHH decreases intracellular levels of cAMP, which reduces ciliary beat frequency and pH in airway surface liquid. These results suggest that apical SHH may mediate noncanonical HH signaling through motile cilia to dampen respiratory defenses at the contact point between the environment and the lung, perhaps counterbalancing processes that stimulate airway defenses.

The Gene Ontology of eukaryotic cilia and flagella

BACKGROUND

Recent research into ciliary structure and function provides important insights into inherited diseases termed ciliopathies and other cilia-related disorders. This wealth of knowledge needs to be translated into a computational representation to be fully exploitable by the research community. To this end, members of the Gene Ontology (GO) and SYSCILIA Consortia have worked together to improve representation of ciliary substructures and processes in GO.

METHODS

Members of the SYSCILIA and Gene Ontology Consortia suggested additions and changes to GO, to reflect new knowledge in the field. The project initially aimed to improve coverage of ciliary parts, and was then broadened to cilia-related biological processes. Discussions were documented in a public tracker. We engaged the broader cilia community via direct consultation and by referring to the literature. Ontology updates were implemented via ontology editing tools.

RESULTS

So far, we have created or modified 127 GO terms representing parts and processes related to eukaryotic cilia/flagella or prokaryotic flagella. A growing number of biological pathways are known to involve cilia, and we continue to incorporate this knowledge in GO. The resulting expansion in GO allows more precise representation of experimentally derived knowledge, and SYSCILIA and GO biocurators have created 199 annotations to 50 human ciliary proteins. The revised ontology was also used to curate mouse proteins in a collaborative project. The revised GO and annotations, used in comparative 'before and after' analyses of representative ciliary datasets, improve enrichment results significantly.

CONCLUSIONS

Our work has resulted in a broader and deeper coverage of ciliary composition and function. These improvements in ontology and protein annotation will benefit all users of GO enrichment analysis tools, as well as the ciliary research community, in areas ranging from microscopy image annotation to interpretation of high-throughput studies. We welcome feedback to further enhance the representation of cilia biology in GO.

Identification of FOXJ1 effectors during ciliogenesis in the foetal respiratory epithelium and embryonic left-right organiser of the mouse

Formation of motile cilia in vertebrate embryos is essential for proper development and tissue function. Key regulators of motile ciliogenesis are the transcription factors FOXJ1 and NOTO, which are conserved throughout vertebrates. Downstream target genes of FOXJ1 have been identified in a variety of species, organs and cultured cell lines; in murine embryonic and foetal tissues, however, FOXJ1 and NOTO effectors have not been comprehensively analysed and our knowledge of the downstream genetic programme driving motile ciliogenesis in the mammalian lung and ventral node is fragmentary. We compared genome-wide expression profiles of undifferentiated E14.5 vs. abundantly ciliated E18.5 micro-dissected airway epithelia as well as Foxj1⁺ vs. Foxj1-deficient foetal (E16.5) lungs of the mouse using microarray hybridisation. 326 genes deregulated in both screens are candidates for FOXJ1-dependent, ciliogenesis-associated factors at the endogenous onset of motile ciliogenesis in the lung, including 123 genes that have not been linked to ciliogenesis before; 46% of these novel factors lack known homologues outside mammals. Microarray screening of Noto⁺ vs. Noto null early headfold embryos (E7.75) identified 59 of the lung candidates as NOTO/FOXJ1-dependent factors in the embryonic left-right organiser that carries a different subtype of motile cilia. For several uncharacterised factors from this small overlap - including 1700012B09Rik, 1700026L06Rik and Fam183b - we provide extended experimental evidence for a ciliary function.

MicroRNAs as key regulators of GTPase-mediated apical actin reorganization in multiciliated epithelia

Multiciliated cells (MCCs), which are present in specialized vertebrate tissues such as mucociliary epithelia, project hundreds of motile cilia from their apical membrane. Coordinated ciliary beating in MCCs contributes to fluid propulsion in several biological processes. In a previous work, we demonstrated that microRNAs of the miR-34/449 family act as new conserved regulators of MCC differentiation by specifically repressing cell cycle genes and the Notch pathway. Recently, we have shown that miR-34/449 also modulate small GTPase pathways to promote, in a later stage of differentiation, the assembly of the apical actin network, a prerequisite for proper anchoring of centrioles-derived neo-synthesized basal bodies. We characterized several miR-34/449 targets related to small GTPase pathways including R-Ras, which represents a key and conserved regulator during MCC differentiation. Direct RRAS repression by miR-34/449 is necessary for apical actin meshwork assembly, notably by allowing the apical relocalization of the actin binding protein Filamin-A near basal bodies. Our studies establish miR-34/449 as central players that orchestrate several steps of MCC differentiation program by regulating distinct signaling pathways.

9 + 0 and 9 + 2 cilia are randomly dispersed in the mouse node

The initial determination of left-right asymmetry is an essential process in embryonic development. In mouse embryo, cilia in the node play an important role generating the nodal flow that subsequently triggers left-right determination in the embryo. Although nodal cilia have historically been thought to have a 9 + 0 axonemal configuration, the existence of 9 + 2 cilia has been reported so far. Because the distribution of those two types of cilia within the node has not yet been reported, we assessed the arrangement of 9 + 0 and 9 + 2 cilia in the node. In this study, we concluded that most of the nodal cilia were 9 + 0 in structure and there were much fewer 9 + 2 cilia than 9 + 0 cilia. Furthermore, the two types of cilia were randomly distributed in the node with no regularity. In addition, we studied the embryonic origin of the crown cells surrounding the node to better understand their identity.

Observation of the Ciliary Movement of Choroid Plexus Epithelial Cells Ex Vivo

The choroid plexus is located in the ventricular wall of the brain, the main function of which is believed to be production of cerebrospinal fluid. Choroid plexus epithelial cells (CPECs) covering the surface of choroid plexus tissue harbor multiple unique cilia, but most of the functions of these cilia remain to be investigated. To uncover the function of CPEC cilia with particular reference to their motility, an ex vivo observation system was developed to monitor ciliary motility during embryonic, perinatal and postnatal periods. The choroid plexus was dissected out of the brain ventricle and observed under a video-enhanced contrast microscope equipped with differential interference contrast optics. Under this condition, a simple and quantitative method was developed to analyze the motile profiles of CPEC cilia for several hours ex vivo. Next, the morphological changes of cilia during development were observed by scanning electron microscopy to elucidate the relationship between the morphological maturity of cilia and motility. Interestingly, this method could delineate changes in the number and length of cilia, which peaked at postnatal day (P) 2, while the beating frequency reached a maximum at P10, followed by abrupt cessation at P14. These techniques will enable elucidation of the functions of cilia in various tissues. While related techniques have been published in a previous report(1), the current study focuses on detailed techniques to observe the motility and morphology of CPEC cilia ex vivo.

Stem cells and fluid flow drive cyst formation in an invertebrate excretory organ

Cystic kidney diseases (CKDs) affect millions of people worldwide. The defining pathological features are fluid-filled cysts developing from nephric tubules due to defective flow sensing, cell proliferation and differentiation. The underlying molecular mechanisms, however, remain poorly understood, and the derived excretory systems of established invertebrate models (Caenorhabditis elegans and Drosophila melanogaster) are unsuitable to model CKDs. Systematic structure/function comparisons revealed that the combination of ultrafiltration and flow-associated filtrate modification that is central to CKD etiology is remarkably conserved between the planarian excretory system and the vertebrate nephron. Consistently, both RNA-mediated genetic interference (RNAi) of planarian orthologues of human CKD genes and inhibition of tubule flow led to tubular cystogenesis that share many features with vertebrate CKDs, suggesting deep mechanistic conservation. Our results demonstrate a common evolutionary origin of animal excretory systems and establish planarians as a novel and experimentally accessible invertebrate model for the study of human kidney pathologies.

Unique among ciliopathies: primary ciliary dyskinesia, a motile cilia disorder

Primary ciliary dyskinesia (PCD) is a ciliopathy, but represents the sole entity from this class of disorders that results from the dysfunction of motile cilia. Characterized by respiratory problems appearing in childhood, infertility, and situs defects in ~50% of individuals, PCD has an estimated prevalence of approximately 1 in 10,000 live births. The diagnosis of PCD can be prolonged due to a lack of disease awareness, coupled with the fact that symptoms can be confused with other more common genetic disorders, such as cystic fibrosis, or environmental insults that result in frequent respiratory infections. A primarily autosomal recessive disorder, PCD is genetically heterogeneous with >30 causal genes identified, posing significant challenges to genetic diagnosis. Here, we provide an overview of PCD as a disorder underscored by impaired ciliary motility; we discuss the recent advances towards uncovering the genetic basis of PCD; we discuss the molecular knowledge gained from PCD gene discovery, which has improved our understanding of motile ciliary assembly; and we speculate on how accelerated diagnosis, together with detailed phenotypic data, will shape the genetic and functional architecture of this disorder.

Interactome analysis reveals that FAM161A, deficient in recessive retinitis pigmentosa, is a component of the Golgi-centrosomal network

Defects in FAM161A, a protein of unknown function localized at the cilium of retinal photoreceptor cells, cause retinitis pigmentosa, a form of hereditary blindness. By using different fragments of this protein as baits to screen cDNA libraries of human and bovine retinas, we defined a yeast two-hybrid-based FAM161A interactome, identifying 53 bona fide partners. In addition to statistically significant enrichment in ciliary proteins, as expected, this interactome revealed a substantial bias towards proteins from the Golgi apparatus, the centrosome and the microtubule network. Validation of interaction with key partners by co-immunoprecipitation and proximity ligation assay confirmed that FAM161A is a member of the recently recognized Golgi-centrosomal interactome, a network of proteins interconnecting Golgi maintenance, intracellular transport and centrosome organization. Notable FAM161A interactors included AKAP9, FIP3, GOLGA3, KIFC3, KLC2, PDE4DIP, NIN and TRIP11. Furthermore, analysis of FAM161A localization during the cell cycle revealed that this protein followed the centrosome during all stages of mitosis, likely reflecting a specific compartmentalization related to its role at the ciliary basal body during the G0 phase. Altogether, these findings suggest that FAM161A's activities are probably not limited to ciliary tasks but also extend to more general cellular functions, highlighting possible novel mechanisms for the molecular pathology of retinal disease.

Cilia in the choroid plexus: their roles in hydrocephalus and beyond

Cilia are whip-like projections that are widely conserved in eukaryotes and function as a motile propeller and/or sensory platform to detect various extracellular stimuli. In vertebrates, cilia are ubiquitously found in most cells, showing structural and functional diversities depending on the cell type. In this review, we focus on the structure and function of cilia in choroid plexus epithelial cells (CPECs). CPECs form one or two dozen non-motile 9+0 cilia, which display transient acquisition of motility during development. Genetic malfunction of cilia can lead to failure of multiple organs including the brain. Especially, several groups have demonstrated that the defects in CPEC cilia cause the communicating form of hydrocephalus. In order to elucidate the molecular mechanisms underlying the hydrocephalus, we have previously demonstrated that the cilia possess an NPFF receptor for autocrine signaling to regulate transepithelial fluid transport. In this perspective, we also discuss the potential involvement of cilia in the other aspects of choroid plexus functions, such as the regulation of brain development and neuroinflammation.

Human FAM154A (SAXO1) is a microtubule-stabilizing protein specific to cilia and related structures

Cilia and flagella are microtubule-based organelles present at the surface of most cells, ranging from protozoa to vertebrates, in which these structures are implicated in processes from morphogenesis to cell motility. In vertebrate neurons, microtubule-associated MAP6 proteins stabilize cold-resistant microtubules through their Mn and Mc modules, and play a role in synaptic plasticity. Although centrioles, cilia and flagella have cold-stable microtubules, MAP6 proteins have not been identified in these organelles, suggesting that additional proteins support this role in these structures. Here, we characterize human FAM154A (hereafter referred to as hSAXO1) as the first human member of a widely conserved family of MAP6-related proteins specific to centrioles and cilium microtubules. Our data demonstrate that hSAXO1 binds specifically to centriole and cilium microtubules. We identify, in vivo and in vitro, hSAXO1 Mn modules as responsible for microtubule binding and stabilization as well as being necessary for ciliary localization. Finally, overexpression and knockdown studies show that hSAXO1 modulates axoneme length. Taken together, our findings suggest a fine regulation of hSAXO1 localization and important roles in cilium biogenesis and function.

Molecular characterization of the apical organ of the anthozoan Nematostella vectensis

Apical organs are sensory structures present in many marine invertebrate larvae where they are considered to be involved in their settlement, metamorphosis and locomotion. In bilaterians they are characterised by a tuft of long cilia and receptor cells and they are associated with groups of neurons, but their relatively low morphological complexity and dispersed phylogenetic distribution have left their evolutionary relationship unresolved. Moreover, since apical organs are not present in the standard model organisms, their development and function are not well understood. To provide a foundation for a better understanding of this structure we have characterised the molecular composition of the apical organ of the sea anemone Nematostella vectensis. In a microarray-based comparison of the gene expression profiles of planulae with either a wildtype or an experimentally expanded apical organ, we identified 78 evolutionarily conserved genes, which are predominantly or specifically expressed in the apical organ of Nematostella. This gene set comprises signalling molecules, transcription factors, structural and metabolic genes. The majority of these genes, including several conserved, but previously uncharacterized ones, are potentially involved in different aspects of the development or function of the long cilia of the apical organ. To demonstrate the utility of this gene set for comparative analyses, we further analysed the expression of a subset of previously uncharacterized putative orthologs in sea urchin larvae and detected expression for twelve out of eighteen of them in the apical domain. Our study provides a molecular characterization of the apical organ of Nematostella and represents an informative tool for future studies addressing the development, function and evolutionary history of apical organ cells.

Taste receptors in innate immunity

Taste receptors were first identified on the tongue, where they initiate a signaling pathway that communicates information to the brain about the nutrient content or potential toxicity of ingested foods. However, recent research has shown that taste receptors are also expressed in a myriad of other tissues, from the airway and gastrointestinal epithelia to the pancreas and brain. The functions of many of these extraoral taste receptors remain unknown, but emerging evidence suggests that bitter and sweet taste receptors in the airway are important sentinels of innate immunity. This review discusses taste receptor signaling, focusing on the G-protein-coupled receptors that detect bitter, sweet, and savory tastes, followed by an overview of extraoral taste receptors and in-depth discussion of studies demonstrating the roles of taste receptors in airway innate immunity. Future research on extraoral taste receptors has significant potential for identification of novel immune mechanisms and insights into host-pathogen interactions.

Females remate more frequently when mated with sperm-deficient males

Polyandry is a source of sexual conflict and males often try to limit female promiscuity. Consequently, male manipulation of receptivity via antiaphrodisiacs and copulatory plugs that prevent female remating can be a source of sexual conflict. This sexual conflict may be intensified when females must remate for fertility insurance. Male red-sided garter snakes produce a large, gelatinous copulatory plug that has been proposed to 1) physically prevent remating and 2) contain an antiaphrodisiac that reduces female receptivity. These males may become sperm depleted because of their dissociated reproductive pattern. If a female mates with a sperm deficient male and is also rendered unreceptive to further mating, then this represents a serious conflict. We tested whether female remating frequency is affected when females are mated with a male that produces a sperm-less copulatory plug. We show that females are significantly more likely to remate after mating with vasectomized males than intact males, even though vasectomized males still produce a copulatory plug. These results suggest that the ejaculate material of the plug does not contain an antiaphrodisiac. Instead, females may use sperm as a cue for post-copulatory mate assessment and seek to remate for the direct benefit of fertility insurance if they have mated with sperm-depleted males.

Autophagy and regulation of cilia function and assembly

Motile and primary cilia (PC) are microtubule-based structures located at the cell surface of many cell types. Cilia govern cellular functions ranging from motility to integration of mechanical and chemical signaling from the environment. Recent studies highlight the interplay between cilia and autophagy, a conserved cellular process responsible for intracellular degradation. Signaling from the PC recruits the autophagic machinery to trigger autophagosome formation. Conversely, autophagy regulates ciliogenesis by controlling the levels of ciliary proteins. The cross talk between autophagy and ciliated structures is a novel aspect of cell biology with major implications in development, physiology and human pathologies related to defects in cilium function.

Primary cilium-associated genes mediate bone marrow stromal cell response to hypoxia

Currently there is intense interest in using mesenchymal stem cells (MSC) for therapeutic interventions in many diseases and conditions. To accelerate the therapeutic use of stem cells we must understand how they sense their environment. Primary cilia are an extracellular sensory organelle present on most growth arrested cells that transduce information about the cellular environment into cells, triggering signaling cascades that have profound effects on development, cell cycle, proliferation, differentiation and migration. Migrating cells likely encounter differing oxygen tensions, therefore we investigated the effect of oxygen tension on cilia. Using bone marrow stromal cells (BMSCs, also known as bone marrow-derived mesenchymal stem cells) we found that oxygen tension significantly affected the length of cilia in primary BMSCs. Chronic exposure to hypoxia specifically down-regulated genes involved in hedgehog signaling and re-localized the Smo and Gli2 proteins to cilia. Investigating the effects of chemotactic migration on cilia, we observed significantly longer cilia in migrating cells which was again, strongly influenced by oxygen tension. Finally, using computational modeling we identified links between migration and ciliation signaling pathways, characterizing the novel role of HSP90 and PI3K signaling in regulating BMSC cilia length. These findings enhance our current understanding of BMSC adaptions to hypoxia and advance our knowledge of BMSC biology and cilia regulation.

Primary cilium: an elaborate structure that blocks cell division?

A primary cilium is a microtubule-based membranous protrusion found in almost all cell types. A primary cilium has a "9+0" axoneme that distinguishes this ancient organelle from the canonical motile "9+2" cilium. A primary cilium is the sensory center of the cell that regulates cell proliferation and embryonic development. The primary ciliary pocket is a specialized endocytic membrane domain in the basal region. The basal body of a primary cilium exists as a form of the centriole during interphase of the cell cycle. Although conventional thinking suggests that the cell cycle regulates centrosomal changes, recent studies suggest the opposite, that is, centrosomal changes regulate the cell cycle. In this regard, centrosomal kinase Aurora kinase A (AurA), Polo-like kinase 1 (Plk1), and NIMA related Kinase (Nek or Nrk) propel cell cycle progression by promoting primary cilia disassembly which indicates a non-mitotic function. However, the persistence of primary cilia during spermatocyte division challenges the dominate idea of the incompatibility of primary cilia and cell division. In this review, we demonstrate the detailed structure of primary cilia and discuss the relationship between primary cilia disassembly and cell cycle progression on the background of various mitotic kinases.

Switching on cilia: transcriptional networks regulating ciliogenesis

Cilia play many essential roles in fluid transport and cellular locomotion, and as sensory hubs for a variety of signal transduction pathways. Despite having a conserved basic morphology, cilia vary extensively in their shapes and sizes, ultrastructural details, numbers per cell, motility patterns and sensory capabilities. Emerging evidence indicates that this diversity, which is intimately linked to the different functions that cilia perform, is in large part programmed at the transcriptional level. Here, we review our understanding of the transcriptional control of ciliary biogenesis, highlighting the activities of FOXJ1 and the RFX family of transcriptional regulators. In addition, we examine how a number of signaling pathways, and lineage and cell fate determinants can induce and modulate ciliogenic programs to bring about the differentiation of distinct cilia types.

Motile cilia harbor serum response factor as a mechanism of environment sensing and injury response in the airway

Nonmotile primary cilia are recognized as important sensory organelles during development and normal biological functioning. For example, recent work demonstrates that transcriptional regulators of the sonic hedgehog signaling pathway localize to primary cilia and participate in sensing and transducing signals regarding the cellular environment. In contrast, motile cilia are traditionally viewed as mechanical machinery, vital for the movement of solutes and clearance of bacteria and debris, but not participants in cellular sensing and signaling mechanisms. Recently, motile cilia were found to harbor receptors responsible for sensing and responding to environmental stimuli. However, no transcription factors are known to be regulated by cilia localization as a sensing mechanism in vertebrates. Using a mouse model of organic dust-induced airway inflammation, we found that the transcription factor serum response factor (SRF) localizes to motile cilia of airway epithelial cells and alters its localization in response to inflammatory stimuli. Furthermore, inhibition of SRF signaling using the small molecule CCG-1423 reduces organic dust-induced IL-8 release from bronchial epithelial cells and stimulates cilia beat frequency in ciliated mouse tracheal epithelial cells. Immunohistochemical analyses reveal that SRF localizes to the cilia of mouse brain ependymal and ovarian epithelial cells as well. These data reveal a novel mechanism by which a transcription factor localizes to motile cilia and modulates cell activities including cilia motility and inflammation response. These data challenge current dogma regarding motile cilia functioning and may lead to significant contributions in understanding motile ciliary signaling dynamics, as well as mechanisms involving SRF-mediated responses to inflammation and injury.

Fused (Stk36) is a ciliary protein required for central pair assembly and motile cilia orientation in the mammalian oviduct

BACKGROUND

Motile cilia on the inner lining of the oviductal epithelium play a central role in ovum transport toward the uterus and subsequent fertilization by sperm. While the basic ultrastructure of 9+2 motile cilia (nine peripheral microtubule doublets surrounding a central pair) has been characterized, many important steps of ciliogenesis remain poorly understood.

RESULTS

Our previous studies on mammalian Fused (Fu) (Stk36), a putative serine-threonine kinase, reveal a critical function of Fu in central pair construction and cilia orientation of motile cilia that line the tracheal and ependymal epithelia. These findings identify a novel regulatory component for these processes. In this study, we show that Fu is expressed in the multi-ciliated oviductal epithelium in several vertebrates, suggesting a conserved function of Fu in the oviduct. In support of this, analysis of Fu-deficient mouse oviducts uncovers a similar role of Fu in central pair construction and cilia orientation. We also demonstrate that Fu localizes to motile cilia and physically associates with kinesin Kif27 located at the cilium base and known central pair components Spag16 and Pcdp1.

CONCLUSIONS

Our results delineate a novel pathway for central pair apparatus assembly and add important insight to the biogenesis and function of oviductal motile cilia.

Developmental changes in ciliary motility on choroid plexus epithelial cells during the perinatal period

Cilia have crucial roles in various developmental and physiological events. Previously, we reported that choroid plexus epithelial cells (CPECs) have multiple, nonmotile 9+0 cilia, but the cilia exhibit transient motility with variable axonemal arrangements in the neonatal period. These features make these cilia unique, as they do not fit in to the traditional categories of primary or motile cilia, and their physiological roles remain elusive. To address this issue, we studied ciliary motility on CPECs through development, with particular interest in the embryonic period. In the fetal choroid plexus of the lateral ventricles, the proportion of cells with motile cilia and their beat frequency increased over time. The ciliary motility profiles peaked near the day of birth, and gradually declined in the two weeks thereafter. The dynamic changes in ciliary motility correlated with changes in Dnahc11 expression. We demonstrated previously that the ciliary motility at P2 was insufficient to produce detectable fluid flow; thus it appears that CPEC cilia do not produce fluid flow at any point during development. Together, our results suggest that a temporally regulated, unique function of CPEC cilia may exist during the perinatal period.

CMF22 is a broadly conserved axonemal protein and is required for propulsive motility in Trypanosoma brucei

The eukaryotic flagellum (or cilium) is a broadly conserved organelle that provides motility for many pathogenic protozoa and is critical for normal development and physiology in humans. Therefore, defining core components of motile axonemes enhances understanding of eukaryotic biology and provides insight into mechanisms of inherited and infectious diseases in humans. In this study, we show that component of motile flagella 22 (CMF22) is tightly associated with the flagellar axoneme and is likely to have been present in the last eukaryotic common ancestor. The CMF22 amino acid sequence contains predicted IQ and ATPase associated with a variety of cellular activities (AAA) motifs that are conserved among CMF22 orthologues in diverse organisms, hinting at the importance of these domains in CMF22 function. Knockdown by RNA interference (RNAi) and rescue with an RNAi-immune mRNA demonstrated that CMF22 is required for propulsive cell motility in Trypanosoma brucei. Loss of propulsive motility in CMF22-knockdown cells was due to altered flagellar beating patterns, rather than flagellar paralysis, indicating that CMF22 is essential for motility regulation and likely functions as a fundamental regulatory component of motile axonemes. CMF22 association with the axoneme is weakened in mutants that disrupt the nexin-dynein regulatory complex, suggesting potential interaction with this complex. Our results provide insight into the core machinery required for motility of eukaryotic flagella.