The primary cilium coordinates signaling pathways in cell cycle control and migration during development and tissue repair

Primary cilia are decreased in breast cancer: analysis of a collection of human breast cancer cell lines and tissues

Primary cilia (PC) are solitary, sensory organelles that are critical for several signaling pathways. PC were detected by immunofluorescence of cultured cells and breast tissues. After growth for 7 days in vitro, PC were detected in ∼70% of breast fibroblasts and in 7-19% of epithelial cells derived from benign breast (184A1 and MCF10A). In 11 breast cancer cell lines, PC were present at a low frequency in four (from 0.3% to 4% of cells), but were absent in the remainder. The cancer cell lines with PC were all of the basal B subtype, which is analogous to the clinical triple-negative breast cancer subtype. Furthermore, the frequency of PC decreased with increasing degree of transformation/progression in the MCF10 and MDA-MB-435/LCC6 isogenic models of cancer progression. In histologically normal breast tissues, PC were frequent in fibroblasts and myoepithelial cells and less common in luminal epithelial cells. Of 26 breast cancers examined, rare PC were identified in cancer epithelial cells of only one cancer, which was of the triple-negative subtype. These data indicate a decrease or loss of PC in breast cancer and an association of PC with the basal B subtype. This manuscript contains online supplemental material at http://www.jhc.org. Please visit this article online to view these materials.

Functional genomic screen for modulators of ciliogenesis and cilium length

Primary cilia are evolutionarily conserved cellular organelles that organize diverse signaling pathways1,2. Defects in the formation or function of primary cilia are associated with a spectrum of human diseases and developmental abnormalities3. Genetic screens in model organisms have discovered core machineries of cilium assembly and maintenance4. However, regulatory molecules that coordinate the biogenesis of primary cilia with other cellular processes, including cytoskeletal organization, vesicle trafficking and cell-cell adhesion, remain to be identified. Here we report the results of a functional genomic screen using RNA interference (RNAi) to identify human genes involved in ciliogenesis control. The screen identified 36 positive and 13 negative ciliogenesis modulators, which include molecules involved in actin dynamics and vesicle trafficking. Further investigation demonstrated that blocking actin assembly facilitates ciliogenesis by stabilizing the pericentrosomal preciliary compartment (PPC), a previously uncharacterized compact vesiculotubular structure storing transmembrane proteins destined for cilia during the early phase of ciliogenesis. PPC was labeled by recycling endosome markers. Moreover, knockdown of modulators that are involved in the endocytic recycling pathway affected the formation of PPC as well as ciliogenesis. Our results uncover a critical regulatory step that couples actin dynamics and endocytic recycling with ciliogenesis, and also provide potential target molecules for future study.

Temporal relationship between primary and motile ciliogenesis in airway epithelial cells

Cilia are traditionally classified as motile or primary. Motile cilia are restricted to specific populations of well-differentiated epithelial cells, including those in the airway, brain ventricles, and oviducts. Primary cilia are nonmotile, solitary structures that are present in many cell types, and often have sensory functions such as in the retina and renal tubules. Primary cilia were also implicated in the regulation of fundamental processes in development. Rare depictions of primary cilia in embryonic airways led us to hypothesize that primary cilia in airway cells are temporally related to motile ciliogenesis. We identified primary cilia in undifferentiated, cultured airway epithelial cells from mice and humans and in developing lungs. The solitary cilia in the airways express proteins considered unique to primary cilia, including polycystin-1 and polycystin-2. A temporal analysis of airway epithelial cell differentiation showed that cells with primary cilia acquire markers of motile ciliogenesis, suggesting that motile ciliated cells originate from primary ciliated cells. Whereas motile ciliogenesis requires Foxj1, primary ciliogenesis does not, and the expression of Foxj1 was associated with a loss of primary cilia, just before the appearance of motile cilia. Primary cilia were not found in well-differentiated airway epithelial cells. However, after injury, they appear in the luminal layer of epithelium and in basal cells. The transient nature of primary cilia, together with the temporal and spatial patterns of expression in the development and repair of airway epithelium, suggests a critical role of primary cilia in determining outcomes during airway epithelial cell differentiation.

Directional cell migration and chemotaxis in wound healing response to PDGF-AA are coordinated by the primary cilium in fibroblasts

Cell motility and migration play pivotal roles in numerous physiological and pathophysiological processes including development and tissue repair. Cell migration is regulated through external stimuli such as platelet-derived growth factor-AA (PDGF-AA), a key regulator in directional cell migration during embryonic development and a chemoattractant during postnatal migratory responses including wound healing. We previously showed that PDGFRalpha signaling is coordinated by the primary cilium in quiescent cells. However, little is known about the function of the primary cilium in cell migration. Here we used micropipette analysis to show that a normal chemosensory response to PDGF-AA in fibroblasts requires the primary cilium. In vitro and in vivo wound healing assays revealed that in ORPK mouse (IFT88(Tg737Rpw)) fibroblasts, where ciliary assembly is defective, chemotaxis towards PDGF-AA is absent, leading to unregulated high speed and uncontrolled directional cell displacement during wound closure, with subsequent defects in wound healing. These data suggest that in coordination with cytoskeletal reorganization, the fibroblast primary cilium functions via ciliary PDGFRalpha signaling to monitor directional movement during wound healing.

Polycystic kidney disease, cilia, and planar polarity

Cystic kidney diseases are characterized by dilated or cystic kidney tubular segments. Changes in planar cell polarity, flow sensing, and/or proliferation have been proposed to explain these disorders. Over the last few years, several groups have suggested that ciliary dysfunction is a central component of cyst formation. We review evidence for and against each of these models, stressing some of the inconsistencies that should be resolved if an accurate understanding of cyst formation is to be achieved. We also comment on data supporting a model in which ciliary function could play different roles at different developmental stages and on the relevance of dissecting potential differences between pathways required for tubule formation and/or maintenance.

Primary cilia and the cell cycle

Cilia are microtubule-based structures that protrude from the cell surface and function as sensors for mechanical and chemical environmental cues that regulate cellular differentiation or division. In metazoans, ciliary signaling is important during organismal development and in the homeostasis controls of adult tissues, with receptors for the Hedgehog, platelet derived growth factor (PDGF), Wnt, and other signaling cascades arrayed and active along the ciliary membrane. In normal cells, cilia are dynamically regulated during cell cycle progression: present in G0 and G1 cells, and usually in S/G2 cells, but almost invariably resorbed before mitotic entry, to reappear post-cytokinesis. This periodic resorption and reassembly of cilia, specified by the intrinsic cell cycle the intrinsic cell cycle machinery, influences the susceptibility of cells to the influence of extrinsic signals with cilia-associated receptors. Pathogenic conditions of mammals associated with loss of or defects in ciliary integrity include a number of developmental disorders, cystic syndromes in adults, and some cancers. With the continuing expansion of the list of human diseases associated with ciliary abnormalities, the identification of the cellular mechanisms regulating ciliary growth and disassembly has become a topic of intense research interest. Although these mechanisms are far from being understood, a number of recent studies have begun to identify key regulatory factors that may begin to offer insight into disease pathogenesis and treatment. In this chapter we will discuss the current state of knowledge regarding cell cycle control of ciliary dynamics, and provide general methods that can be applied to investigate cell cycle-dependent ciliary growth and disassembly.

Analyzing primary cilia by multiphoton microscopy

In this chapter, a technique is outlined for the use of immunohistochemistry (IHC) followed by multiphoton microscopy (MPM) for the analysis of incidence, length, and 3D orientation of the axoneme of the primary cilium. Although the application presented specifically emphasizes localizations in tenocytes and chondrocytes, the technique is applicable to cells in a wide range of connective tissues. The primary advantages of utilizing MPM as opposed to TEM for these kinds of ciliary analyses are the rapidity of the technique for preparation of the samples and the ability to collect data from multiple cells simultaneously. Using MPM, the axoneme, basal body, and associated centriole can be visualized by specific IHC with localizing antibodies. However, the resolution achieved through TEM analyses allows the complex morphology of the primary cilium to be visualized, and this remains the primary advantage of TEM versus MPM. SHG, which occurs only with MPM, allows visualization of collagen fibrils and is particularly advantageous for localizing primary cilia associated with cells in connective tissues. This, and the deep penetration with less photobleaching, are the primary advantages of MPM compared to confocal microscopy. As with any microscopical technique, the protocol needs to be optimized for any given tissue. In particular, additional antigen retrieval techniques to enhance the unmasking of specific epitopes for antibody binding may be required for adaptation of this approach to other dense connective tissues with complex spatial organizations such as intervertebral disc or meniscus.

From central to rudimentary to primary: the history of an underappreciated organelle whose time has come. The primary cilium

For the first time, the history of the central flagellum/primary cilium has been explored systematically and in depth. It is a long and informative story about the course of scientific discovery, memory loss and rediscovery. The progress of our story is saltatory, pushed onward by innovations in technology and retarded by socio-scientific issues of linguistic and temporal chauvinism. Over one hundred and fifty years passed between the discovery of this organelle and full appreciation of its important functions. The main character in our story is an organelle that was relegated to a very minor role in the cellular opera for a very long time, until its rather sudden promotion to a central role in orchestrating many of the sensory and signaling events of the cell. Although early investigators speculated on just such a role for the primary cilium as early as 1898, it was over one hundred years before proof for this hypothesis was forthcoming.

Cildb: a knowledgebase for centrosomes and cilia

Ciliopathies, pleiotropic diseases provoked by defects in the structure or function of cilia or flagella, reflect the multiple roles of cilia during development, in stem cells, in somatic organs and germ cells. High throughput studies have revealed several hundred proteins that are involved in the composition, function or biogenesis of cilia. The corresponding genes are potential candidates for orphan ciliopathies. To study ciliary genes, model organisms are used in which particular questions on motility, sensory or developmental functions can be approached by genetics. In the course of high throughput studies of cilia in Paramecium tetraurelia, we were confronted with the problem of comparing our results with those obtained in other model organisms. We therefore developed a novel knowledgebase, Cildb, that integrates ciliary data from heterogeneous sources. Cildb links orthology relationships among 18 species to high throughput ciliary studies, and to OMIM data on human hereditary diseases. The web interface of Cildb comprises three tools, BioMart for complex queries, BLAST for sequence homology searches and GBrowse for browsing the human genome in relation to OMIM information for human diseases. Cildb can be used for interspecies comparisons, building candidate ciliary proteomes in any species, or identifying candidate ciliopathy genes.

Database URL:http://cildb.cgm.cnrs-gif.fr

Arrestin development: emerging roles for beta-arrestins in developmental signaling pathways

Arrestins were identified as mediators of G protein-coupled receptor (GPCR) desensitization and endocytosis. However, it is now clear that they scaffold many intracellular signaling networks to modulate the strength and duration of signaling by diverse types of receptors--including those relevant to the Hedgehog, Wnt, Notch, and TGFbeta pathways--and downstream kinases such as the MAPK and Akt/PI3K cascades. The involvement of arrestins in many discrete developmental signaling events suggests an indispensable role for these multifaceted molecular scaffolds.

Zebrafish ift57, ift88, and ift172 intraflagellar transport mutants disrupt cilia but do not affect hedgehog signaling

Cilia formation requires intraflagellar transport (IFT) proteins. Recent studies indicate that mammalian Hedgehog (Hh) signaling requires cilia. It is unclear, however, if the requirement for cilia and IFT proteins in Hh signaling represents a general rule for all vertebrates. Here we examine zebrafish ift57, ift88, and ift172 mutants and morphants for defects in Hh signaling. Although ift57 and ift88 mutants and morphants contained residual maternal protein, the cilia were disrupted. In contrast to previous genetic studies in mouse, mutations in zebrafish IFT genes did not affect the expression of Hh target genes in the neural tube and forebrain and had no quantitative effect on Hh target gene expression. Zebrafish IFT mutants also exhibited no dramatic changes in the craniofacial skeleton, somite formation, or motor neuron patterning. Thus, our data indicate the requirement for cilia in the Hh signal transduction pathway may not represent a universal mechanism in vertebrates.

Joubert syndrome: insights into brain development, cilium biology, and complex disease

Joubert syndrome (JS) is a primarily autosomal recessive condition characterized by hypotonia, ataxia, abnormal eye movements, and intellectual disability with a distinctive mid-hindbrain malformation (the "molar tooth sign"). Variable features include retinal dystrophy, cystic kidney disease, liver fibrosis and polydactyly. Recently, substantial progress has been made in our understanding of the genetic basis of JS, including identification of seven causal genes (NPHP1, AHI1, CEP290, RPGRIP1L, TMEM67/MKS3, ARL13B and CC2D2A). Despite this progress, the known genes account for <50% of cases and few strong genotype-phenotype correlations exist in JS; however, genetic testing can be prioritized based on clinical features. While all seven JS genes have been implicated in the function of the primary cilium/basal body organelle (PC/BB), little is known about how the PC/BB is required for brain, kidney, retina and liver development/function, nor how disruption of PC/BB function leads to diseases of these organs. Recent work on the function of the PC/BB indicates that the organelle is required for multiple signaling pathways including sonic hedgehog, WNT and platelet derived growth factor. Due to shared clinical features and underlying molecular pathophysiology, JS is included in the rapidly expanding group of disorders called ciliopathies. The ciliopathies are emerging as models for more complex diseases, where sequence variants in multiple genes contribute to the phenotype expressed in any given patient.

The primary cilium coordinates early cardiogenesis and hedgehog signaling in cardiomyocyte differentiation

Defects in the assembly or function of primary cilia, which are sensory organelles, are tightly coupled to developmental defects and diseases in mammals. Here, we investigated the function of the primary cilium in regulating hedgehog signaling and early cardiogenesis. We report that the pluripotent P19.CL6 mouse stem cell line, which can differentiate into beating cardiomyocytes, forms primary cilia that contain essential components of the hedgehog pathway, including Smoothened, Patched-1 and Gli2. Knockdown of the primary cilium by Ift88 and Ift20 siRNA or treatment with cyclopamine, an inhibitor of Smoothened, blocks hedgehog signaling in P19.CL6 cells, as well as differentiation of the cells into beating cardiomyocytes. E11.5 embryos of the Ift88(tm1Rpw) (Ift88-null) mice, which form no cilia, have ventricular dilation, decreased myocardial trabeculation and abnormal outflow tract development. These data support the conclusion that cardiac primary cilia are crucial in early heart development, where they partly coordinate hedgehog signaling.

Primary cilia and signaling pathways in mammalian development, health and disease

Although first described as early as 1898 and long considered a vestigial organelle of little functional importance, the primary cilium has become one of the hottest research topics in modern cell biology and physiology. Primary cilia are nonmotile sensory organelles present in a single copy on the surface of most growth-arrested or differentiated mammalian cells, and defects in their assembly or function are tightly coupled to many developmental defects, diseases and disorders. In normal tissues, the primary cilium coordinates a series of signal transduction pathways, including Hedgehog, Wnt, PDGFRalpha and integrin signaling. In the kidney, the primary cilium may function as a mechano-, chemo- and osmosensing unit that probes the extracellular environment and transmits signals to the cell via, e.g., polycystins, which depend on ciliary localization for appropriate function. Indeed, hypomorphic mutations in the mouse ift88 (previously called Tg737) gene, which encodes a ciliogenic intraflagellar transport protein, result in malformation of primary cilia, and in the collecting ducts of kidney tubules this is accompanied by development of autosomal recessive polycystic kidney disease (PKD). While PKD was one of the first diseases to be linked to dysfunctional primary cilia, defects in this organelle have subsequently been associated with many other phenotypes, including cancer, obesity, diabetes as well as a number of developmental defects. Collectively, these disorders of the cilium are now referred to as the ciliopathies. In this review, we provide a brief overview of the structure and function of primary cilia and some of their roles in coordinating signal transduction pathways in mammalian development, health and disease.