when cilia go bad cilia defects

© 2007 Nature Publishing Group

Cilia (and flagella) are microtubule-based hair-like organelles that extend from the surface of almost all cell types of the human body (FIG. 1, see also the Primary Cilia Resource web page). Although these highly con-served structures are found across a broad range of species, a nearly ubiquitous appearance is observed only in vertebrates. Cilia can be structurally divided into subcompartments that include a basal body, trans-ition zone, axoneme, ciliary membrane and the ciliary tip (BOX 1). Most cell types assemble only one cilium (a monocilium or primary cilium), whereas some cells build cilia bundles that consist of 200–300 individual organelles (multiple cilia).

In contrast to other cell organelles, cilia are only assembled when cells exit the cell cycle from mitosis into a stationary or quiescent and/or differentiated state; and vice versa, entry into the cell cycle is preceded by ciliary resorption1. Cilia and flagella are highly com-plex structures that comprise >650 proteins (see the Ciliomics, Cilia Proteome and Chlamydomonas Flagellar Proteome web pages). The formation of cilia comprises targeting of specific proteins to the basal body area where pre-assembly of axonemal substructures (such as outer dynein arms) occurs2. The transport of proteins and multiprotein precursors across the ciliary compart-ment border and along the length of the axonemes to their functional assembly site is dependent on intraflagellar transport (IFT)3. Proteins are loaded onto the IFT particles at the ciliary base within the cytoplasm and transferred across the ciliary compartment border in a process known as compartmentalized ciliogenesis4. Mutations in genes encoding proteins that participate in IFT cause ciliogenesis defects of both motile and immotile cilia5–8. By contrast, cytosolic ciliogenesis is independent of IFT.

Cilia can either be motile or immotile. The 9+2 axonemes of most motile cilia are assembled by nine peripheral doublet microtubules surrounding two central single microtubules (central pair complex) and contain associated structures including inner and outer dynein arms, radial spokes and nexin links (BOX 1). The 9+0 axonemes that are found in most non-motile cilia lack the two central microtubules and are devoid of dynein arms. The 9+0 axonemes of the motile nodal cilia lack the central microtubules but have dynein arms. Although there are many different classes of cilia with a diversity of variations, all cilia types share the basic structural units composed of the outer microtubule doublets and the ciliary membrane. For example, cilia with a 9+4 axoneme on the notochordal plate of the rabbit embryo have been identified, and it has been proposed that the axonemal structures may vary widely within the vertebrates9.

Four cilia types have been identified in humans and all have been associated with human disease: motile 9+2 cilia (such as respiratory cilia, ependymal cilia); motile 9+0 cilia (nodal cilia); non-motile 9+2 cilia (kinocilium of hair cells10); and non-motile 9+0 cilia (renal monocilia, photoreceptor-connecting cilia). Recent advances have indicated that all cilia (motile or non-motile) throughout the diverse groups of organisms from protists to humans might carry out sensory functions; thus, we prefer to avoid the term ‘sensory cilia’, which is often used interchangeably with ‘non-motile monocilia’ or ‘primary cilia’. In addition, although they have not yet been identified, there might be other motile monocilia types besides nodal cilia.

Although the basic structure of the different types of cilia is obviously similar, they exert various tissue-specific functions during development, tissue morpho-genesis and homeostasis11–13, as discussed in more detail below. Because cilia are located on almost all polarized

*Department of Paediatrics and Adolescent Medicine, University Hospital Freiburg, 79106 Freiburg, Germany. ‡Department of Medicine, University of Cologne, 50937 Köln, Germany. Correspondence to H.O. e-mail: [email protected]:10.1038/nrm2278

Intraflagellar transport(IFT). A ciliaspecific and flagellaspecific transport system that relies on at least 16 different proteins that assemble into transport rafts and move ciliary components across the compartment border and along the peripheral axonemal microtubules to the ciliary tip and back to the cell body. IFT was first described in biflagellate green algae (Chlamydomonas reinhardtii).

CiliogenesisThe processes of cilia assembly and growth that follow and/or accompany cell polarization.

When cilia go bad: cilia defects and ciliopathiesManfred Fliegauf*, Thomas Benzing‡ and Heymut Omran*

Abstract | Defects in the function of cellular organelles such as peroxisomes, lysosomes and mitochondria are well-known causes of human diseases. Recently, another organelle has also been added to this list. Cilia — tiny hair-like organelles attached to the cell surface — are located on almost all polarized cell types of the human body and have been adapted as versatile tools for various cellular functions, explaining why cilia-related disorders can affect many organ systems. Several molecular mechanisms involved in cilia-related disorders have been identified that affect the structure and function of distinct cilia types.

M e c h a n i s M s o f D i s e a s e

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http://www.bowserlab.org/primarycilia/cilialist.html


http://www.sfu.ca/~leroux/ciliome_home.htm

http://www.ciliaproteome.org/

http://labs.umassmed.edu/chlamyfp/index.php


mailto:[email protected]

mailto:[email protected]


Nature Reviews | Molecular Cell Biology

AirwaysRespiratory cilia

BrainEpendymal cilia

Female reproductivesystemFallopian tube cilia

Male reproductivesystemSperm flagella

Inner earKinocilium (red)Stereocilia (green)

EyePhotoreceptorconnecting cilia

Bone/cartilageOsteocyte/chondrocyte cilia

PancreasPancreatic duct cilia

Bile ductCholangiocyte cilia

KidneyRenal cilia

EmbryoNodal cilia

Motile 9+0

Non-motile 9+0

Motile 9+2

Non-motile 9+2

Notochordal plateAn epithelial primordial structure of the notochord (a cylindrical rod of cells). The sheet of notochordal cells is laterally in contact with the roof of the primitive gut and dorsally in contact with the midline cells of the neural plate. The notochordal plate folds off from the roof of the primitive gut to form the notochord.

cell types of the human body, cilia-related disorders — ciliopathies — can affect many organ systems. Ciliopathies can either involve single organs or can occur as multisystemic disorders with phenotypically variable and overlapping disease manifestations14. Here, we focus on the mechanisms by which ciliary dysfunction causes human disorders. notably, most of our knowledge about cilia biology is based on genetic studies of model systems such as Chlamydomonas reinhardtii and mutant mouse models that have also enabled the development of novel therapeutic options for human ciliopathies (BOX 2).

functions of ciliaThe existence of different cilia types indicates that this organelle is likely to have numerous functions.

Motile functions of cilia. The ciliary axoneme com-prises nine peripheral doublets, which have attached dynein arms. Within these large multiprotein com-plexes, axonemal dynein heavy chains exert ciliary movement by ATP-dependent conformational changes and transient binding to neighbouring doublets. The beating of each individual cilium is generated by

Figure 1 | Ciliary dysfunction in human diseases. A monociliated cell is shown in the centre. Motile monocilia (9+0 axoneme, middle left panels) are found at the embryonic node and generate the nodal flow that is essential for determination of left–right body asymmetry. Multiple motile cilia (9+2 axonemal structure, top panels) that transport extracellular fluid along the epithelial surface are located on respiratory epithelial cells, brain ependymal cells and epithelial cells lining the fallopian tubes (panel reproduced with permission from REF. 59 (2005) Elsevier). The sperm flagellum (top right panel; co-stained with antibodies against the dynein heavy chain DNAH5, red) represents a specialized, elongated motile cilium (9+2) that confers motility. Non-motile monocilia (9+0, bottom panels) extend from the surface of most quiescent cells of the body and sense environmental signals such as fluid flow and/or fluid composition. Well-known examples are the monocilia of the tubular epithelia of the kidney, and the epithelia of the bile duct (panel reproduced with permission from REF. 75 (2006) American Physiological Society) and pancreatic ducts (panel reproduced with permission from REF. 129 (2006) Elsevier). The chondrocyte and osteocyte monocilia probably function to sense the amount of strain in bones. The connecting cilia of photoreceptor cells are specialized non-motile cilia (9+0) that connect the inner and outer segments. Non-motile 9+2 cilia (middle right panel) are found in the inner ear (kinocilium, red, arrowhead; stereocilia, green) (panel reproduced with permission from REF. 10 (2005) John Wiley & Sons, Inc.). Besides the four cilia types shown, there might be a high variability of the axonemal structures within vertebrates. In all panels, axonemes were stained (red or green) by indirect immunofluorescence using an antibody against the cilia-specific acetylated a-tubulin isoform. Nuclei were stained using Hoechst or 4′,6-diamidino-2-phenylindole (DAPI).

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coordinated activation and inactivation of the dynein motor proteins within the inner and outer dynein arms along the length of the axoneme15. Ciliary motility is required to move extracellular fluid: the motile 9+0 monocilia at the embryonic node generate an extra-embryonic fluid flow (nodal flow) that is required to determine embryonic left–right asymmetry16. Motility of the multiple 9+2 cilia of respiratory epithelial cells is responsible for mucociliary clearance. Analogously, the multiple 9+2 ependymal cilia mediate ependymal flow17. Furthermore, flagellar motility is required for sperm cells to propel through the female reproductive system.

Non-motile functions. Functions of cilia that are not related to motility are thought to involve sensing of environmental cues. Because cilia protrude from the cell surface, they might act as antennae that receive sig-nals from the periphery. The remote information may be converted into signalling cascades that are initiated within the ciliary compartment and then transduced to the cell body. Consistently, the ciliary membrane (which is continuous with the plasma membrane) contains various cilia-specific receptors, ion channels and signalling molecules. For example, flow-induced passive cilia bending is required for mechanosensation of extracellular fluid flow (for instance, tubular fluid, urine)18. studies in Caenorhabditis elegans have shown that transient receptor potential vanilloid channels in the sensory cilia membranes are transported bidirec-tionally by the IFT system. Therefore, IFT is not only necessary for transport of axonemal components, but is also important for the sensory activity of cilia19. Recent observations indicate that chemosensation, as well as signalling through receptor-dependent pathways such as the sonic hedgehog (sHH), platelet-derived growth fac-tor receptor (PDGFR) pathways or non-canonical Wnt (also known as planar cell polarity (PCP)) pathways, is also mediated through cilia20–22.

cilia in developmentCilia in left–right asymmetry. A link between motile cilia dysfunction and defects in establishing left–right body asymmetry is apparent from the observation that half of individuals with primary ciliary dyskinesia (PCD) exhibit situs inversus totalis (also referred to as Kartagener’s syndrome), which is consistent with randomization of left–right asymmetry11. A simi-lar phenotype was observed in mice with recessive mutations in Lrd, the orthologue of DNAH11, a human axonemal dynein β-chain gene23. During early embryonic development (~7.5 days postcoitum), the rotational movement of nodal cilia at the ventral pole of the murine embryo creates a leftward fluid flow (nodal flow) that is thought to induce breaking of the body symmetry. Indeed, an artificially generated nodal flow independent from ciliary motility was found to be sufficient to determine laterality24. similar lateral-ity breaking mechanisms have also been proposed for zebrafish (involving Kupffer’s vesicle), birds (Hensen’s node), and amphibians (spemann’s organizer)25.

Box 1 | ciliary subcompartments

The ciliary tip harbours the microtubule plus (+) ends (from which axonemes grow) and the switch between the anterograde (kinesin) and retrograde (dynein) intraflagellar transport (IFT) motors. It contains signalling molecules and can undergo morphological changes in response to signalling processes.

The axoneme (see figure part 9) is the structural core of a cilium (without a membrane or soluble material). Its peripheral microtubule doublets, comprising an A‑ and a B‑tubule, are continuous with the microtubules at the transition zone. The doublets are connected by nexin links and are held in place by radial spokes that extend into the axonemal centre. In motile cilia, inner and outer dynein arms are attached to the A‑tubules and mediate ciliary bending by reversibly binding to the neighbouring B‑tubule. The central microtubule pair is surrounded by a sheath. The radial spoke–central pair complex is involved in beat regulation.

The transition zone (parts 5–8) converts the triplet microtubular structure of the basal body into the axonemal doublet structure. Proximal transition fibres (parts 4 and 5) connect each microtubule doublet (without dynein arms) to the membrane and mark the compartment border at which IFT proteins accumulate. The distal part contains stellate fibre arrays (parts 6 and 8) and an amorphous disk structure (part 7) and gives rise to the central microtubules in 9+2 axonemes121,122. The transition zone might contain a gate that controls access to the ciliary compartment.

The basal body (parts 1–4) of each cilium is a specialized centriole (9 × 3 microtubular structure; the tubules of one triplet are depicted as A, B and C in part 3) embedded in pericentriolar material (dark orange) with a proximal amorphous disc (part 1), a cartwheel structure (part 2), a middle piece that lacks appendages (part 3) and transition fibres at the distal end (part 4). In most quiescent cells, the centrioles move to the apical plasma membrane and the mother centriole functions as the microtubule‑organizing centre to nucleate the axonemal microtubules. The daughter centriole remains perpendicular to the mother centriole. In multiciliated cells, centriolar replication is required first.

The ciliary membrane is continuous with the plasma membrane but contains specific signalling molecules that are essential for the function of cilia as antennae. IFT rafts move between the ciliary membrane and the peripheral microtubules and carry membrane anchors. Figure modified with permission from REF. 122 (2003) Blackwell Publishing.

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AnterogradeThe transport direction from the ciliary base to the tip.

RetrogradeThe transport direction from the ciliary tip back to the cell body.

Mucociliary clearanceThe process by which the continuous coordinated beating of respiratory cilia moves the thin mucus layer that covers the airway epithelia towards the pharynx to defend against inhaled pathogens trapped in the mucus.

Ependymal flowThe laminar flow of cerebrospinal fluid through the brain ventricles and the cerebral aqueduct generated by the coordinated beating of ependymal cilia.

Nodal flow hypotheses. Two models have been proposed for how the nodal flow might contribute to left–right asymmetry (FIG. 2). The ‘two cilia’ model26 predicts that nodal flow generated by motile cilia is sensed by non-motile, mechanosensory cilia at the periphery of the node. This model is mainly based on the observation that expression of polycystin-2, a protein thought to be involved in mechanosensation, is restricted to cilia at the periphery of the node, whereas cilia in the centre of the node express lRD but lack polycystin-2. The second model expands on the morphogen gradient model, and predicts that nodal flow results in a leftward gradient of a hypothetical morphogen. Tanaka et al. identified nodal vesicular parcels filled with sonic hedgehog and retinoic acid molecules, which bud off the nodal surface to be transported leftwards by nodal flow where they are smashed to release their contents27. Both models reported an asymmetric Ca2+ release that is probably involved in the subsequent events of left–right determin-ation, which, in turn, is based on asymmetric expression of signalling molecules such as nodal and lefty, and transcription factors such as PITX2 (paired-like homeo-domain-2). nakamura et al. recently suggested that a mechanism known as self-enhancement and lateral-inhibition system (selI) is necessary to generate robust asymmetry28.

Current nodal flow hypotheses cannot sufficiently explain the complex laterality defects that are observed in humans and mice with inborn ciliary motility

defects (FIG. 2). Although most patients with PCD who have DNAH5 or DNAI1 mutations or Dnahc5-mutant mice exhibit situs solitus or situs inversus totalis, a small proportion show partial laterality defects such as situs inversus abdominalis and situs inversus thoracalis29–32. These observations indicate that the reversal of left–right body asymmetry can independently occur along the anterior–posterior axis (corresponding to the upper and lower part of the human body) and is controlled by nodal cilia function. The site of the future diaphragm appears to be the border of this determination. It can be postulated that functional differences of the anterior and posterior part of the node, distinct signalling molecules for determining upper–lower body asymmetry and/or temporal diversity of left–right determination (that is, first the lower and then the upper part) might explain partial laterality defects.

Disorders of development and growthnumerous cilia-related diseases have been described that are associated with developmental defects affecting the central nervous system, the skeleton or other organ sys-tems. several signalling pathways have been implicated in ciliary function.

Hedgehog signalling. loss of activity of the Hedgehog (Hh) signalling pathway33 can cause various birth defects, including holoprosencephaly, polydactyly, craniofacial defects and skeletal malformations34. These abnormalities resem-ble the developmental defects observed in IFT mutant mice (Tg737∆2-3βGal and Tg737orpk mutations; Tg737 is also known as Ift88 (intraflagellar transport-88 homo-logue) or polaris)6,35. An enu (N-ethyl-N-nitrosourea) screen for embryonic patterning defects identified two mouse mutants with phenotypes that are reminiscent of defects of the Hh signalling pathway20. In both strains, mutations in the genes encoding the IFT proteins IFT172 and IFT88 were identified. Further analyses showed that the IFT machinery is essential for Hh sig-nalling downstream of the Hh receptor patched-1 and upstream of direct transcriptional targets of Hh (FIG. 3a). Analyses in the developing limb buds of IFT-mutant mice also suggested altered Hh signalling downstream of patched-1 (REFs 36–37).

In mammals, the main targets of Hh signalling are the glioma (GlI) transcription factors GlI1, GlI2 and GlI3. Haycraft et al. demonstrated that GlI2 and full-length (the activator form) GlI3 functions are disrupted in the Tg737 mutant cells, but that GlI1 and GlI3R (the repressor form) can induce or repress the Hh pathway, respectively, regardless of IFT function. localization of all GlI proteins as well as suppressor of fused (suFu) to the distal tip of cilia in primary limb-bud cell cultures confirmed a prominent role for Hh signalling in the cilium37. Proteolysis of GlI3 into the GlI3R repressor form probably involves suFu function at the ciliary tip (FIG. 3a). Hh signalling therefore appears to require the IFT machinery to shuttle GlI proteins and suFu to the ciliary tip. In the absence of Hh binding to patched-1, smoothened (sMo) is not released from patched-1 and so GlI3 is constantly proteolytically cleaved into the repressor

Box 2 | Therapeutic options

On the basis of the knowledge of the distinct mechanisms involved in many cilia‑related diseases, novel therapeutic options are currently being evaluated to attenuate disease progression. In cystic kidney disease, several molecular mechanisms have been identified that can be targeted by pharmacological therapy.

Dysregulated cell cycle. Pharmacological interventions to slow down the cell‑cycle progression involved in cystic kidney disease have been evaluated123. The cyclin‑dependent kinase (CDK) inhibitor roscovitine (CYC202; a potent inhibitor of CDK2–cyclin E) retarded cystogenesis and improved renal function with a long‑lasting effect in two mouse models of slowly progressing polycystic kidney disease. Roscovitine treatment inhibited the formation of cysts from distinct nephron segments by blocking the cell cycle, by transcriptional regulation and by reduction of apoptosis. This CDK‑selective inhibitor has minimal off‑target kinase activities, and might therefore be a promising candidate for clinical trials of human cystic kidney disease.

Altered downstream signalling. The expression of several mitogen‑activated protein kinases (MAPKs) is dysregulated in the cyst epithelium of pcy (polycystic kidney disease) mice that carry a missense mutation in Nphp3 (encoding nephrocystin‑3), the orthologous gene that is responsible for adolescent nephronophthisis124. This dysregulation is probably a downstream consequence of disturbed renal monocilia function125. Inhibition of extracellular signal‑regulated kinase (ERK)–MAPK signalling attenuated the progression of renal disease in pcy mice, implying that selective targeting of downstream signalling events might be beneficial. Other potential downstream targets for inhibition in polycystic kidney disease include the vasopressin‑2 receptor, epidermal growth‑factor receptor and Src126.

Increased mammalian target of rapamycin (mTOR) activity. Hamartin and tuberin, proteins that are implicated in the formation of renal cysts in tuberous sclerosis, have been found at the ciliary base. They form a heteromeric complex that inhibits mTOR, a kinase that controls cell growth and proliferation. Inhibition of mTOR activity retards cyst formation in rats with polycystic kidney disease127,128. Studies using mTOR inhibitors in patients with cystic kidney disease and tuberous sclerosis are currently underway.

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http://www.ncbi.nlm.nih.gov/sites/entrez?Db=gene&Cmd=ShowDetailView&TermToSearch=1767&ordinalpos=1&itool=EntrezSystem2.PEntrez.Gene.Gene_ResultsPanel.Gene_RVDocSum




http://www.expasy.org/uniprot/P08151



http://www.expasy.org/uniprot/Q9UMX1




Left isomerism (polysplenia)

Right isomerism (asplenia)

Situs solitus Situs inversus totalis

Situs inversus thoracalis Situs inversus abdominalis

Spleen

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GlI3R. GlI3R is then transported by cytoplasmic dynein motors to the cell body and subsequently tar-geted for nuclear entry. Hh binding to patched-1 induces the release of sMo, which can suppress GlI3 process-ing. The active GlI3A form is then transported to the nucleus, where it can activate target genes (FIG. 3a). This concept is compatible with the finding that mutations in Shh, anterograde and retrograde IFT motors (such as Kif3a (kinesin family member-3A) and Dync2h1 (dynein cytoplasmic-2 heavy chain-1), respectively) as well as IFT proteins and GlI3 result in similar developmental defects20,38–41.

Wnt signalling. Mice with defective IFT proteins and mutations in the Bardet–Biedl syndrome (BBs) genes Bbs1, Bbs4 or Bbs6 (also known as Mkks, which encodes McKusick–Kaufman syndrome protein) exhibit pheno-types resembling those observed in mutants of the non-canonical Wnt pathway (also known as the PCP pathway). These include open eyelids, neural tube defects and disrupted cochlear stereociliary bundles42. The evo-lutionarily conserved PCP pathway (FIG. 3b) contributes to the development of polarity in the plane of a cell layer, controlling cellular processes such as cell migration or mitotic spindle orientation43.

Genetic interaction of BBs genes and the PCP gene Ltap (also called Vangl2) in double heterozygous mouse mutants as well as in zebrafish (vangl2), and the obser-vation that the PCP protein vAnGl2 colocalizes with BBs proteins to the basal body and ciliary axoneme, confirmed the hypothesis that cilia are involved in PCP signalling22. Further evidence that ciliary dysfunction contributes to neural tube defects is provided by the dem-onstration that mutations in MKS1 and MKS3, which encode the ciliary proteins MKs1 and meckelin44–45, respectively, are associated with the multisystemic Meckel–Gruber syndrome (MKs type 1–3). This is an autosomal recessive lethal malformation disorder that, among other characteristics, is regularly associated with proximal neural tube defects (encephaloceles). Closure of the neural tube during embryogenesis requires the orientation of polarized epithelial cells in a single plane perpendicular to the apical–basal axis as well as con-vergent extension, which leads to the narrowing and lengthening of tissues during development. Therefore, disruption of the PCP pathway owing to defective cilia-mediated signalling (FIG. 3b) might explain the neural tube defects. Recent studies have also shown that the vertebrate PCP proteins inturned (Int) and fuzzy (Fuz) are essential for ciliogenesis and that, as a consequence, mutant Xenopus laevis embryos also have Hh signalling defects46.

Signalling involving receptors attached to cilia. Besides skeletal patterning defects, Tg737- and polycystin-1 (Pkd1)-mutant mice show stunted growth after birth, which implies defects in appositional and endochon-dral development. An essential role for the primary cilia of osteoblasts and osteocytes in bone development is further supported by the observation that Pkd1-mutant mice show severe skeletal defects, including abnormal

Figure 2 | Human laterality disorders and current models for establishing left–right asymmetry. a | Schematic illustration of normal left–right body asymmetry (situs solitus) and five laterality defects that affect the lungs, heart, liver, stomach and spleen. By their vigorous circular movements, motile monocilia at the embryonic node generate a leftward flow of extra-embryonic fluid (nodal flow). b | The nodal vesicular parcel (NVP) model predicts that vesicles filled with morphogens (such as sonic hedgehog and retinoic acid) are secreted from the right side of the embryonic node and transported to the left side by nodal flow, where they are smashed open by force27. The released contents probably bind to specific transmembrane receptors in the axonemal membrane of cilia on the left side. The consequent initiation of left-sided intracellular Ca2+ release induces downstream signalling events that break bilaterality. In this model, the flow of extra-embryonic fluid is not detected by cilia-based mechanosensation. c | In the two-cilia model, non-sensing motile cilia in the centre of the node create a leftward nodal flow that is mechanically sensed through passive bending of non-motile sensory cilia at the periphery of the node26. Bending of the cilia on the left side leads to a left-sided release of Ca2+ that initiates the establishment of body asymmetry.

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Primary ciliary dyskinesia(PCD). A genetically and phenotypically heterogeneous group of disorders characterized by defective ciliary motility.

NephronophthisisAn autosomal recessive cystic kidney disease characterized by normal or reduced kidney size, cysts at the corticomedullary border and predominant tubulointerstitial fibrosis. Phthisis is a Greek word meaning shrinking or wasting.

formation of the axial skeleton, long bones and spines, disrupted structure and organization of the vertebrae, spina bifida occulta and osteochondrodysplasia47,48. Thus, it has been hypothesized that cilia from chondrocytes, osteoblasts, odontoblasts, fibroblasts and myocytes might sense mechanical strain or chemosensory signals to aid interaction of the cell with its surrounding extracellular matrix. Receptors for extracellular matrix proteins, such as a2, a and β1 integrins, localize to the ciliary membrane of chondrocytes, an observation that supports this concept49.

In nIH3T3 fibroblasts and cultures of mouse embryonic fibroblasts, the primary cilium is important in growth control21. In growth-arrested fibroblasts, the PDGFRa localizes to the primary cilium. ligand-dependent activation of PDGFRa within the ciliary

membrane is followed by activation of AKT and the MeK1/2 (mitogen activated protein kinase (MAPK)–extracellular signal-regulated kinase (eRK) kinase)–eRK1/2 pathways, with MeK1/2 being phosphorylated within the cilium and at the basal body (FIG. 3c). Thus, the MeK1/2–eRK1/2 pathway is an example of a dis-tinct signalling pathway that can be activated within the subcellular ciliary compartment. For other signalling pathways (for example, the RAs pathway), see also the Cilia Proteome database.

Hydrocephalus formation. In the brain ventricles, the synchronized beating of the ependymal 9+2 cilia generates a laminar flow of cerebrospinal fluid above the ependymal cell surface and through the cerebral aqueduct, which is termed ependymal flow17. A link

Figure 3 | models of cilia-generated signalling mechanisms. a | Hedgehog (Hh) signalling in cilia involves the intraflagellar transport machinery, which moves components of the Hh signalling pathway to their functional sites. The transcription factors glioma (GLI) and suppressor of fused (SUFU) are transported to the ciliary tip. GLI is processed to create a transcriptional repressor, which is transported back to the cell body (left panel). On Hh ligand binding to its receptor patched-1 (PTCH1), smoothened (SMO) is released and transported to the ciliary tip, where it turns off GLI processing by interacting with SUFU. The activator form of the GLI transcription factor is transported to the cell body and enters the nucleus where it induces the expression of genes, such as those involved in renal patterning (PAX2, SALL1), cell-cycle regulation (CCND1 (cyclin D1), N-MYC) and GLI family members themselves (GLI1, GLI2) and PTCH1 (self-induction). b | Cilia-mediated signalling acts as a switch between canonical and non-canonical Wnt signalling pathways. In the absence of fluid flow, canonical Wnt signalling predominates. WNT binds to the receptor frizzled, dishevelled (DSH) is recruited to frizzled, and glycogen synthase kinase-3β (GSK3β) is inactivated. β-catenin (β-cat) translocates to the nucleus where it acts as a transcriptional co-activator with members of the lymphoid enhancer binding factor (LEF)–T-cell-specific transcription factor (TCF) family and induces transcription of WNT target genes such as cMYC, AXIN2 or L1CAM (left panel). On mechanosensation of fluid flow, intracellular Ca2+ release causes increased inversin expression. Inversin targets cytoplasmic DSH for anaphase-promoting complex/cyclosome (APC/C)-dependent ubiquitylation and degradation, making it unavailable for canonical Wnt signalling. In non-canonical Wnt signalling (planar cell polarity (PCP) pathway), β-catenin undergoes degradation by a complex of axin, adenomatous polyposis coli (APC) and GSK3β. Inversin does not affect DSH recruitment to the plasma membrane, where DSH is available for non-canonical Wnt signalling. c | The chemosensation-/receptor-based signalling model. Receptors such as platelet-derived growth factor receptor-a (PDGFRa) are located within the axonemal membrane (left panel). Ligands in the tubular flow bind to their receptors, inducing cellular responses through downstream signalling pathways such as the MEK/ERK cascade. IFT, intraflagellar transport; MEK, mitogen activated protein kinase (MAPK)–extracellular signal-regulated kinase (ERK) kinase. Part a modified with permission from REF. 130 (2006) Elsevier.

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Tel Mes Rho Lv III IV

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Situs inversus totalis The complete mirrorimage arrangement of all thoracic and abdominal organs.

Situs solitus The normal position of the viscera (stomach and spleen on the left side, liver on the right side). The threelobed lung is positioned on the right, with the twolobed lung on the left, and the left and right cardiac atria are positioned normally.

Situs inversus abdominalisThe isolated inversion of abdominal organs, but a normal composition of thoracic organs.

Situs inversus thoracalisThe isolated inversion of thoracic organs, but a normal composition of abdominal organs.

HoloprosencephalyA developmental disorder of the brain due to a failure of the embryonic forebrain (the prosencephalon) to form bilateral hemispheres of the cephalon. This causes defects in brain structure and function and also affects the development of the face.

Polydactylysupernumerary fingers or toes. The presence of six fingers or six toes on one or both hands or feet is usually called hexadactyly.

Craniofacial defectsThe developmental abnormalities that affect the head or skull and structures of the face.

between ependymal cilia dysfunction and hydrocephalus (enlargement of the brain ventricles) became apparent by analysis of different animal models. Mice lacking the axonemal dynein heavy chain Dnahc5, the axonemal protein sPAG6 or proteins involved in ciliogenesis, such as IFT88 or FoXJ1 (forkhead box J1; also known as HFH-4), develop hydrocephalus7,17,50,51. Moreover, the spontaneous hydrocephalic WIC-Hyd rat mutant exhibits impaired ependymal ciliary motility52. Hydin (also known as Hy3)-mutant mice develop hydrocepha-lus and have reduced ependymal cilia53. In C. reinhardtii, hydin is required for flagellar motility and localizes to a specific projection on a single microtubule of the cen-tral apparatus54. Mutant flagella are arrested in one of two switch points in the beat cycle: the beginning of the effective stroke or the beginning of the recovery stroke. Therefore, hydrocephalus caused by hydin mutations probably involves defects in the central pair apparatus that result in impaired ciliary motility and ciliary degen-eration and makes the human orthologue an interesting candidate for causing hydrocephalus in humans.

Two cilia-related disease mechanisms have been implicated in hydrocephalus formation. The first was described in Dnahc5-mutant mice, which have dys-motile cilia31. The consequent lack of ependymal flow causes a secondary closure of the aqueduct and subse-quent formation of triventricular hydrocephalus during early postnatal brain development (FIG. 4). In humans, ependymal ciliary dysmotility is not sufficient to cause hydrocephalus but increases the risk of aqueduct closure; hence, there is a higher incidence of this rare disorder in patients with PCD.

The second mechanism for hydrocephalus formation was described for Tg737orpk mice, which exhibit ciliogen-esis defects of both motile and immotile cilia owing to a mutation of the IFT protein IFT88 (REF. 55). However, in these mice, hydrocephalus formation starts at postnatal day 1, and loss of both the coordinated ciliary beat on ventricular ependymal cells and consequent ependy-mal flow does not seem to be the initiating factor for hydrocephalus formation. Rather, Tg737orpk mice have aberrantly formed cilia of the choroid plexus, in which polycystin-1 is mislocalized to the bulb-like structure at the tip, rather than to basal bodies and the distal ciliary axoneme. Therefore, it has been speculated that cilia in Tg737orpk mice have altered mechanosensory func-tions that might lead to altered ion transport activity in the choroid plexus epithelium and, subsequently, to a marked increase in cerebrospinal fluid production.

Dysfunction of the reproductive system sperm immotility (supplementary information s1,s2,s3 (movies)) significantly contributes to male infertility. The composition of sperm tails and respiratory cilia is similar but not identical, which might explain why sperm flagella dyskinesia is often, but not necessarily, associated with PCD and vice versa. The understand-ing of these differences is aided by the demonstration of cell-type-specific spatial localization of axonemal proteins such as outer dynein arm components along the length of the ciliary and flagellar axonemes. In addi-tion, it can be speculated that the assembly processes of human cilia and sperm flagella are not completely identical. For instance, in flies, sperm tail generation

Figure 4 | Hydrocephalus in mice as a result of a lack of ependymal flow. Schematic illustration of the ventricular system during mouse brain development (at embryonic day (E)10, E11.5, E14.5, postnatal day (P)0.5 and in the adult). At E10, the brain has developed from the anterior end of the neural tube into three primary vesicles: telencephalic (Tel; yellow), mesencephalic (Mes; blue) and rhombencephalic (Rho; orange). Later (at E14.5), the two lateral (Lv I and II), third (III) and fourth (IV) ventricles develop. Cerebrospinal fluid, which is predominantly produced in the lateral ventricles (Lv), is transported through the ventricular system and enters the subarachnoid space through foramina at the fourth ventricle (not shown), where it is finally re-absorbed. During late embryonic brain development, the cerebral aqueduct (Aq) connecting the third and fourth ventricle is formed and becomes the narrowest part of the cerebrospinal fluid system (P0.5). In Dnahc5-mutant mice, the lack of ependymal flow owing to immotile ependymal cilia causes closure of the aqueduct and subsequent formation of triventricular hydrocephalus during early postnatal brain development. Figure reproduced with permission from REF. 17 (2004) Oxford University Press.

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http://www.nature.com/nrm/journal/v8/n11/suppinfo/nrm2278.html


Bardet–Biedl syndrome(BBs). A clinically pleiotropic disorder that has a primarily autosomal recessive inheritance pattern (twelve loci, BBS1–BBS12, have been identified so far) and a multitude of symptoms including rod–cone dystrophy, retinitis pigmentosa, obesity, polydactyly, renal abnormalities (such as cystic kidneys), learning disabilities or mental retardation, male hypogonadism and congenital heart defects. BBs proteins localize either to the ciliary base or the axoneme and are involved in subcellular targeting of ciliary proteins. seven of the known BBs proteins assemble into a core complex called the BBsome.

Spina bifidaA developmental abnormality that results from an incomplete closure of the embryonic neural tube and an incompletely formed spinal cord that protrudes through an open gap in the unfused spines of the vertebrae (spina bifida aperta). In the milder form, spina bifida occulta, the spinal cord does not protrude because only a small part of one vertebra is missing and there is no opening to the skin.

Osteochondro-dysplasiaAn abnormal growth of cartilage and bone (individual bones or group of bones). Growth defects of the long bones and/or spine usually cause shortened limbs or a disproportionately shortened body.

Metachronal waveA wavelike movement that is propagated along the epithelial surface, created when cilia on one segment of the epithelium move after another.

CholangiocytesThe epithelial cells of the bile ducts.

involves cytosolic ciliogenesis, in which the axoneme is assembled inside the cytoplasm of the spermatid, independent of IFT4,56. However, observations in Tg737-mutant mice indicate that spermiogenesis in vertebrates requires IFT (G. Pazour, personal communication).

In the female reproductive system, the fallopian tube epithelium undergoes hormonally mediated cyclical morphological changes that affect the ciliated cells and the ciliary beat frequency57,58. The mechanism by which the ovarian duct epithelium responds to various hormonal and neuronal stimuli is incompletely under-stood, although the cilia might function in sensory per-ception and signal transduction from the extracellular environment59–61.

The propulsion of gametes and embryos is accom-plished by complex interaction between muscle contrac-tions, ciliary activity and the flow of tubal secretions62. It is therefore still unclear whether ciliary motility has a prominent role in transport of the fertilized ovum. In female patients with PCD, fertility has been reported to be slightly reduced and, thus, the activity of the oviduct cilia is thought to be partially involved in egg transport63.

airway diseaseRespiratory cilia characteristically beat with a forward effective stroke and a backward recovery stroke in the same plane along the cell surface. Moreover, beating of the cilia bundles located on each single cell, as well as the cilia of the entire cell layer, follow a highly regulated syn-chronized beating mode (supplementary information s4,s5 (movies)). This coordination generates a contin-uous series of metachronal waves that are essential for mucus transport and airway clearance. In patients with PCD, impaired mucociliary clearance causes chronic air-way diseases of the upper (nasopharynx, sinus, middle ear) and lower (bronchi, bronchioles) airways64. Airway cilia from such patients often show characteristic ultrastructural defects of the axoneme, such as absence of some or all inner and/or outer dynein arms, radial spoke defects or microtubule malposition, most of which are associated with characteristic aberrant beat-ing patterns65 (supplementary information s6,s7,s8,s9 (movies)).

Outer dynein arm multiprotein complexes. Identification of genes involved in PCD66 has aided understanding of ciliary beat generation and regulation. Mutations in DNAH5 and DNAI1, which encode axonemal motor components responsible for ciliary movement genera-tion, cause various defects of the distinct outer dynein arm (oDA) multiprotein complexes along the respira-tory ciliary axoneme. Human respiratory cilia contain at least two distinct oDA types: type 1 is located within the proximal ciliary axoneme and contains DnAH5 but not DnAH9, and type 2 contains both DnAH5 and DnAH9 and localizes to the distal ciliary axoneme67. Interestingly, DNAH5 mutations regularly affect oDA assembly of both known human oDA types, whereas DNAI1 mutations mainly affect the assembly of the distally located axonemal oDA complexes. Consistent

with these findings, immotile cilia often arise from DnAH5-mutant axonemes, whereas DnAI1 mutant cilia often retain some residual flickery movement capacity (supplementary information s6,s7 (movies)). similar to mutations of C. reinhardtii orthologues, mutations in both human genes probably result in abnormal pre-assembly of oDA multiprotein complexes. Consequently, dynein heavy chain DnAH5 accumulates at the ciliary base, which is consistent with the current hypothesis that pre-assembly of axonemal multiprotein complex precursors occurs at the basal bodies2,67–69.

Rarely, PCD is caused by mutations in the genes OFD1 (mutated in orofacialdigital syndrome; oFD) and RPGR (retinitis pigmentosa guanosine triphosphatase (GTPase) regulator), which do not primarily affect axonemal motor proteins70,71. Both proteins are localized at the ciliary base. In respiratory cells they are involved in ciliary beat regulation and mutations result in altered beating patterns (supplementary information s9 (movie)). In addition, these proteins are also important in the sensory function of 9+0 cilia (see below).

cystic disorders of the kidney, liver and pancreasEvidence for a common mechanism involving cilia. IFT mutant Tg737orpk mice develop progressive kidney and pancreatic cysts as well as abnormalities involving the hepatic bile ducts5,7,72,73, which suggests that ciliary dysfunction affects the morphogenesis and integrity of kidney, liver and pancreas. All three organs contain simi-lar functional units, tubular systems that transport urine, bile fluid or pancreatic secretions; monocilia extend into the lumen of each. In humans, biliary dysgenesis can occur as an isolated disorder or in combination with polycystic kidney disease.

The frequent association of renal and hepatic cysts in autosomal dominant polycystic kidney disease (ADPKD) and autosomal recessive polycystic kidney disease (ARPKD), or nephronophthisis (nPHP) and liver fibrosis (which occurs in Boichis disease) also sug-gests that a common mechanism might explain each of these conditions. A rodent model of human ARPKD develops cystic kidney and liver disease along with bile-duct defects and shows malformed monocilia of cholangiocytes74,75.

Most proteins associated with various forms of human cystic kidney disorders (sometimes referred to as cystoproteins) indeed localize to distinct ciliary sub-compartments, supporting the hypothesis of cilia-related disease mechanisms76. Proteins involved in BBs, nPHP, oFD type 1, ARPKD and ADPKD localize either to the ciliary base or the ciliary axoneme.

Polycystin, Ca2+ and mechanosensation. The first insights into the functional role of monocilia in tubu-lar cells came from studies in cultured renal collecting duct epithelial cells, which demonstrated a persistent intracellular Ca2+ increase in response to flow-mediated ciliary bending18,77. subsequent analyses showed that this Ca2+ influx required polycystin-1 and polycystin-2, the proteins mutated in ADPKD78. Because depletion of extracellular Ca2+ (as well as inhibition of ryanodine

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receptors) abolished the physiological flow response in wild-type cells, the authors proposed a mechanosen-sory model in which cilia sense fluid movement (FIG. 5). Polycystin-1, located within the cilium, was proposed to function as a flow sensor and to transmit the signal from the extracellular fluid environment to the inter-acting Ca2+-channel polycystin-2. Polycystin-2 then mediates sufficient Ca2+ influx to activate intracellular ryanodine receptors resulting in intracellular Ca2+ release; the increase in intracellular Ca2+ levels probably regulates numerous molecular activities inside the cell that contribute to tissue development and homeostasis in response to tubular flow. several other reports have confirmed that ciliated cells in culture can react to fluid flow79,80. It was also shown that cholangiocyte monocilia can detect and transmit signals from the luminal bile duct flow to the epithelial cells, which is reminiscent of the function of renal monocilia80. However, so far only the studies by Praetorius and spring77 have demonstrated a ciliary mechanosensory capacity by direct mechanical stimulation. Alternatively, considering the mechanisms involved in the nodal vesicular parcel model27, it is also feasible that molecules within the fluid flow might activ-ate chemosensory or receptor-based signalling events (FIG. 3c) to induce the subsequent intracellular Ca2+-release response. Mechanical or chemical fluid-flow sensing might provide morphogenic cues that regulate tubule diameters. Defects due to mutant polycystin pro-teins might generate false signals that indicate a ‘lack of flow’ and might cause a compensatory growth of the tubular cells and subsequent cyst formation.

Mechanosensation and Wnt signalling. Recent findings suggest that signalling from the mechanosensory cilia of tubule cells might involve the PCP pathway (FIG. 3b). similar cystic kidney phenotypes in transgenic mice overexpressing an activated form of β-catenin and in inv/inv mice (a mouse model for infantile nPHP)

implied a role for inversin in Wnt signalling79,81. An inhibitory role for inversin in canonical Wnt signal-ling upstream of the β-catenin degradation complex was demonstrated in renal epithelial cells and X. laevis embryos79. Inversin interacts with the protein dishev-elled and targets it for degradation by the anaphase-promoting complex/cyclosome (APC/C). Consistent with a role in non-canonical Wnt signalling, inversin is required for convergent extension movements in X. laevis gastrulation and elongation of X. laevis animal caps. Furthermore, pronephric cysts in zebrafish caused by inversin knockdown can be rescued by the canonical Wnt signalling inhibitor diversin82. Recently, it was demonstrated that physiological flow conditions lead to upregulated inversin expression and a reduction of β-catenin levels in kidney cells79. Together, these find-ings indicate that inversin might function as a switch between the β-catenin-dependent canonical and the non-canonical PCP pathway. It is therefore tempting to speculate that the start of urine flow during renal development terminates canonical Wnt signalling to facilitate the β-catenin-independent PCP pathway83.

PCP and BBS proteins. Another indication of perturbed PCP signalling in cystic kidney disease came from the analysis of targeted mutations of BBs genes, which are also associated with renal cysts. on the basis of the observation that patients and mice carrying BBs gene mutations display phenotypic defects similar to those of classical defects in PCP, a putative PCP function of BBs genes was explored22. A well-known example of PCP in mammals is the uniform orientation of the hair-cell stereociliary bundles within the cochlea. This orienta-tion is perturbed in Bbs1–/–, Bbs4–/– and Mkks–/– mice, which supports a role for BBs proteins in the establish-ment of PCP. In addition, genetic interaction of both Bbs1 and Mkks in mice and bbs1 and bbs4 in zebrafish with the classical PCP gene Vangl2 was demonstrated22. silencing of Bbs4 results in defective targeting or anchoring of pericentriolar proteins and disorganiza-tion of microtubules. Because BBs4 interacts with peri-centriolar material-1 (PCM1) and p150glued (a subunit of dynactin that links dynactin with dynein), BBs4 is speculated to function as an adaptor that facilitates the loading of cargo onto dynein–dynactin molecular motors, thereby recruiting cargo within the cytosol to the centriolar satellites84. At the basal body, the cargo is then prepared for IFT-dependent transport along the ciliary axoneme. Thus, the correct function of BBs proteins at the basal body might be a prerequisite for the correct localization and function of PCP proteins such as vAnGl2 in the cilium22.

Recently, seven of the known BBs proteins (BBs1, -2, -4, -5, -7, -8 and -9) have been shown to assemble into a ~450 kDa core complex (BBsome) that is proposed to regulate RAB8 (a small GTPase)-dependent vesicular trafficking of membrane proteins from the Golgi into the ciliary membrane85. The BBsome localizes to centriolar satellites in the cytoplasm and to the ciliary membrane. Through the interaction of BBs1 with RABIn8, which localizes at the basal body, the BBsome is recruited to the

Figure 5 | The mechanosensation-based cilia signalling model. The polycystin-1–polycystin-2 complex (PC1–PC2), which is sensitive to shear stress, is localized within the ciliary membrane (left panel). Fluid-induced ciliary bending activates this Ca2+channel. The Ca2+ influx (right panel) causes Ca2+ release from ryanodine-sensitive intracellular stores and subsequent downstream responses such as activating protein-1 (AP1)-dependent gene transcription by the Ca2+-dependent kinase PKCa. Mutations in PC1 or PC2 might disable cilia-mediated mechanosensation, which is normally required for tissue morphogenesis, and thus can cause polycystic kidney disease.

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basal body from the neighbouring centriolar satellites and activates RAB8, which in turn promotes docking and fusion of Golgi vesicles containing newly synthesized ciliary membrane proteins. Disruption of these processes may therefore be the cause of BBs.

PCP, cilia and tubule elongation. The PCP pathway has a key role during tubular elongation, a process in which epithelial cells divide along the longitudinal axis of renal tubules, pancreatic ducts and bile ducts. establishment and maintenance of the tubular geometry requires precise spatial information and correct positioning of the spindle axis, which is controlled by the PCP pathway. The monocilium appears to provide the spatial cues (by sensing tubular fluid flow, for example) to position the centrosome and the mitotic spindle before the next cell division86. Defective cilia function or PCP signalling might cause the tubular epithelial cells to lose spatial orientation and apical–basolateral polarity, to proliferate uncontrollably and to form cysts.

Defects in vision, smell and hearingDisturbed IFT function in retinal degeneration. In retinal rods and cones, 9+0 cilia connect the inner segment of photoreceptors (which contains the nucleus) with the outer segment, which contains the membrane stacks that contain the photo pigment (FIG. 6). All components that are necessary for assembly, maintenance and continuous turnover of the outer segment are synthesized in the cell body and are moved through the connecting cilium by IFT. In mice, targeted mutations that disrupt IFT cause severe retinal degeneration87,88.

A link between human retinal degenerative diseases and dysfunction of the connecting cilium became evident from the observation that mutations in RPGR (which localizes to the connecting cilium) account for 20% of retinitis pigmentosa cases89. Because RPGR interacts with IFT88 as well as with several microtubule motor proteins90, it probably has a role in IFT. In addi-tion, both the RPGR-interacting protein RPGRIP1 and RP1 (retinitis pigmentosa-1), which are involved in the pathogenesis of retinitis pigmentosa, also localize to connecting cilia.

Approximately 20% of all cases of nPHP are asso-ciated with retinal disorders, which might originate from defective IFT within the connecting cilium. Direct interaction of various nPHP proteins with the RPGR–RPGRIP1 complex has been demonstrated91–93, so it can be speculated that the nPHP proteins assemble with RPGR–RPGRIP1 into a large multimeric protein complex within the connecting cilium.

Retinal degeneration (rod–cone dystrophy) is also a main feature of BBs94. All BBs proteins that have been analysed localize to the ciliary axonemes or to the ciliary base95. BBs1, BBs2 and BBs5 contribute either to cilia formation or function, whereas BBs4, BBs7 and BBs8 have been specifically shown to be involved in IFT pro-cesses. Retinitis pigmentosa is also present in Alstrom syndrome (AlMs), which shares many features with BBs. The responsible gene ALMS1 might also be involved in IFT96,97.

Figure 6 | Structure and function of the photoreceptor-connecting cilium. The connecting 9+0 cilia of photoreceptors represent specialized cilia that are the sole transport corridor between the outer and inner photoreceptor segments. These cilia are essential in photoreceptor physiology and, therefore, their dysfunction contributes to retinal degeneration. a | Schematic illustration of a photoreceptor cell and its substructures. b | Immunofluorescence staining of retinal sections using specific markers for ciliary subcompartments. Antibodies against acetylated a-tubulin mark the axoneme (green, left panel). The transition zone was visualized using antibodies against nephrocystin (red, left and right panels), and the basal bodies were stained using antibodies against the pericentriolar marker γ-tubulin (green, right panel). c | Electron microscopy image of a retina showing the localization of the photoreceptor-connecting cilium between the outer and inner segments. d | Schematic illustration of the physiological function of a connecting cilium. Biosynthesis products from the inner segment and turnover products from the outer segment are shuttled through the connecting cilium by the IFT machinery. The localization of several proteins implicated in retinal diseases is indicated: RPGR and nephrocystin are found in the transition zone, usherin is located in the ciliary membrane and BBS proteins are found in basal bodies. BBS, Bardet–Biedl syndrome; BM, Bruch’s membrane; IFT, intraflagellar transport; INL, inner nuclear layer; ONL, outer nuclear layer; OPL, outer plexiform layer; RPE, retinal pigment epithelium; RPGR, retinitis pigmentosa guanosine triphosphatase (GTPase) regulator. Part a modified with permission from REF. 100 (2006) Elsevier.

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BronchiectasisA baglike or cylindrical widening of parts of the bronchial tree, which is usually caused by localized injury of bronchial tissue due to bacterial infections. Affected bronchi are irreversibly damaged.

Chronic sinusitisA permanent or recurrent inflammation of the paranasal sinuses often caused by infections.

HypoosmiaA decreased ability to smell odours.

AnosmiaThe absence of the ability to smell odours.

usher syndrome (usH), is a heterogeneous disease, which, in rare cases, can be associated with bronchiectasis, chronic sinusitis and reduced nasal mucociliary clearance, which is indicative of ciliary dyskinesia98. Dysfunction of the vestibular system can be associated with progressive retinitis pigmentosa. Most usH proteins localize to the photoreceptor-connecting cilia or the periciliary mem-brane99,100, where they are involved in IFT, specifically rhodopsin transport101.

Reduced sense of smell in BBS. Binding of odorant mole-cules to specialized receptors on the membrane of olfac-tory sensory cilia initially induces Ca2+ signals, which are then converted to action potentials via sensory neurons. A link between olfactory cilia dysfunction and hypoosmia and anosmia has become apparent by the observation that patients with BBs are frequently unable to smell102. Mice that lack BBs1 or BBs4 have a severely reduced olfactory ciliated border, defects in the highly specialized ciliated dendritic knobs and trapping of olfactory ciliary proteins in dendrites and cell bodies, which links ciliary function with the sense of smell102,103.

Loss of hearing and balance in Usher syndrome. In verte-brates, both hearing and balance require sensory hair cells of the inner ear that carry multiple actin-based stereocilia and a single tubulin-based kinocilium. Auditory signals are probably transduced via mechanically gated ion channels that convert vibrations into electrical signals by depolari-zation of the hair cell104. Patients with usher syndrome display sensorineural hearing loss and lack vestibular function; the relevant usH proteins possibly function in sterocilia as well as in photoreceptor cilia100,105.

oncogenesisCilia and cell division. Cilia assembly–disassembly seems to be closely linked to cell division. Direct evidence comes from the observation that the IFT protein IFT27 — a Rab-like small G protein —is required for the normal completion of cell division in C. reinhardtii, in addition to its role in flagellar assembly106. Partial knockdown results in cytokinesis defects and elongation of the cell cycle, whereas a more complete knockdown is lethal. In addition, IFT88 localizes to the centrosome throughout the cell cycle and prevents the G1–s transition upon overexpression; it also promotes cell-cycle progression when depleted by RnAi in Hela cells107. Conversely, the centrosomal Aurora A kinase, which promotes mitotic entry in mammalian cells, also induces the rapid disas-sembly and resorption of cilia through activation of histone deacetylase-6 (HDAC6)-dependent tubulin deacetylation108. Most vertebrate cells assemble a primary cilium in the G0–G1 phase of the cell cycle. The centriole from a previously dividing cell can function as the basal body for cilia assembly in a quiescent cell, and centrioles released by cilia disassembly in G1 phase or before or during s phase might function as microtubule-organ-izing centres that are essential for spindle formation109. Therefore, primary cilia are dynamically assembled and resorbed throughout the cell cycle. Dysregulation of these processes might result in oncogenesis, for example,

as a consequence of centrosomal amplification and subsequent genomic instability that is observed in many cancers. The neK (nIMA (never in mitosis gene A)-related kinase) cell-cycle proteins also link the cell cycle to the cilia–centrosome complex110–112. support for the involvement of neK proteins in ciliary function came from the observation that mutations in Nek1 and Nek8 cause cystic kidney disease in mice and zebrafish113,114.

Cystoproteins involved in cell-cycle regulation. Poly-cystin-2 regulates cell proliferation and differentiation by directly interacting with and influencing the nuclear translocation of ID2 (inhibitor of DnA binding-2), a member of the helix–loop–helix protein family115. overexpressing polycystin-2 blocks cell-cycle progres-sion through upregulation of the cyclin-dependent kinase (CDK) inhibitor p21. overexpression of poly-cystin-1, which is required for the ID2–polycystin-2 interaction115, induces p21 expression and directly activates signalling through Janus kinase (JAK)–signal transducer and activator of transcription (sTAT) to regulate the cell cycle116.

In response to fluid flow, the C-terminal tail of the integral plasma membrane protein polycystin-1 is cleaved off by proteolysis, enters the nucleus and directly initiates signalling processes (such as Wnt- and the acti-vating protein-1 (AP1)-mediated pathways) that are modulated by polycystin-2. Because the transcription factor AP1 is involved in various processes, including proliferation, transformation and apoptosis, mechano-sensation in renal tubule epithelia might be directly linked to cell-cycle regulation117.

These polycystin studies indicate that cilia-related proteins could be good tumour-suppressor candidates. Biallelic inactivation of the VHL (von Hippel lindau) tumour-suppressor gene is associated with most spor-adic renal clear cell carcinomas; here, tumorigenic trans-formation is preceded by the formation of renal cysts, which, in turn, are commonly caused by renal mono-cilia dysfunction. The vHl protein localizes to the axonemes of renal monocilia and controls ciliogenesis in kidney cells118–120. In addition, vHl appears to asso-ciate with microtubules and coordinates directional microtubule growth, a prerequisite for ciliogenesis. so, cilia might have a key role in cell-cycle control, and alterations in this process predisposes to cancer. nevertheless, most mutations in cystoproteins do not predispose to renal cancer, indicating that additional effects are required.

conclusions and discussionCilia are highly complex organelles that are involved in numerous functions, from movement of the fertilized ovum to airway clearance. Their dysfunction has been implicated in several disorders; this number is expected to increase. Recent work indicates that cilia assembly and disassembly is closely related to cell-cycle control, and so might attract increased attention in terms of the mechanisms involved in oncogenesis. This knowledge might also aid in the development of novel therapeutic strategies to fight human cancer. The identification of

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2. Fowkes, M. E. & Mitchell, D. R. The role of preassembled cytoplasmic complexes in assembly of flagellar dynein subunits. Mol. Biol. Cell 9, 2337–2347 (1998).

3. Rosenbaum, J. L. & Witman, G. B. Intraflagellar transport. Nature Rev. Mol. Cell Biol. 3, 813–825 (2002). This excellent article provides a detailed and comprehensive overview of IFT and various associated physiological aspects.

4. Avidor-Reiss, T. et al. Decoding cilia function: defining specialized genes required for compartmentalized cilia biogenesis. Cell 117, 527–539 (2004).These authors used phylogenetic screening by comparative genomics to identify genes that are essential for cilia formation and function. They also discuss the difference between IFT-dependent compartmentalized ciliogenesis and cytosolic ciliogenesis, which is independent from IFT.

5. Pazour, G. J. et al. Chlamydomonas IFT88 and its mouse homologue, polycystic kidney disease gene tg737, are required for assembly of cilia and flagella. J. Cell Biol. 151, 709–718 (2000).This article provides evidence that IFT is essential for primary cilia assembly and function in mammals and that defects in cilia assembly can lead to polycystic kidney disease.

6. Murcia, N. S. et al. The Oak Ridge polycystic kidney (orpk) disease gene is required for left–right axis determination. Development 127, 2347–2355 (2000).

7. Taulman, P. D., Haycraft, C. J., Balkovetz, D. F. & Yoder, B. K. Polaris, a protein involved in left–right axis patterning, localizes to basal bodies and cilia. Mol. Biol. Cell 12, 589–599 (2001).

8. Yoder, B. K. et al. Polaris, a protein disrupted in Orpk mutant mice, is required for assembly of renal cilium. Am. J. Physiol. Renal Physiol. 282, 541–552 (2002).

9. Feistel, K. & Blum, M. Three types of cilia including a novel 9+4 axoneme on the notochordal plate of the rabbit embryo. Dev. Dyn. 235, 3348–3358 (2006).

10. Dabdoub, A. & Kelley, M. W. Planar cell polarity and a potential role for a Wnt morphogen gradient in stereociliary bundle orientation in the mammalian inner ear. J. Neurobiol. 64, 446–457 (2005).

11. El Zein, L., Omran, H. & Bouvagnet, P. Lateralization defects and ciliary dyskinesia: lessons from algae. Trends Genet. 19, 162–167 (2003).

12. Ibanez-Tallon, I., Heintz, N. & Omran, H. To beat or not to beat: roles of cilia in development and disease. Hum. Mol. Genet. 12, R27–R35 (2003).

13. Satir, P. & Christensen, S. T. Overview of structure and function of mammalian cilia. Annu. Rev. Physiol. 69, 377–400 (2007).

14. Badano, J. L., Mitsuma, N., Beales, P. L. & Katsanis, N. The ciliopathies: an emerging class of human genetic disorders. Annu. Rev. Genomics Hum. Genet. 7, 125–148 (2006).

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16. Nonaka, S. et al. Randomization of left–right asymmetry due to loss of nodal cilia generating leftward flow of extraembryonic fluid in mice lacking KIF3B motor protein. Cell 95, 829–837 (1998). In this elegant work the authors demonstrate that nodal cilia generate a left-directed flow of extra-embryonic fluid (nodal flow) that is involved in left–right body axis determination.

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new molecular disease mechanisms is also likely to facilitate the development of novel therapeutic strategies in disorders in which, so far, only organ replacement therapy can be offered (for example, cystic kidney disease) (BOX 2).

In this Review, we have discussed how current models and hypotheses cannot completely explain the clinical findings observed in some cilia-related dis-orders (for example, laterality defects), and we therefore anticipate that further research will provide increased insight into the molecular mechanisms involved in these disorders.

Furthermore, additional ultrastructural and sub-cellular localization data are necessary for a better under-standing of cilia-related disorders. Knowledge of the composition and functional role of the ciliary membrane will help to generate novel insights into ciliary function. Finally, many current hypotheses almost exclusively pro-pose a mechanosensory function for primary cilia. only a small number of studies have so far addressed whether cilia are involved in receptor-based signalling (such as chemosensation). We expect that cilia-type specific receptor-based signalling will emerge as an interesting area of research.

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AcknowledgementsWe thank E. Davis and G. Pazour for critical evaluation of the manuscript. H.O. and M.F. are supported by the Deutsche Forschungs-Gemeinschaft. We are grateful for the collabora-tion with the patient support group ‘PCD und Kartagener Syndrom e.V.’. We thank H. Olbrich and N.T. Loges for help with figure preparations.

DaTaBasesEntrez Gene: http://www.ncbi.nlm.nih.gov/sites/entrez?db=geneBbs1| Bbs4 | DNAH5 | DNAI1 | Ift88 | Mkks | OFD1 | Pkd1 | RPGROMIM: http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=OMIMADPKD | Alstrom syndrome | ARPKD | BBS | Kartagener’s syndrome | Meckel–Gruber syndrome | NPHP | orofacialdigital syndrome | PCD | Usher syndromeUniProtKB: http://ca.expasy.org/sprotGLI1 | GLI2 | GLI3 | patched-1 | polycystin-2 | SMO | SUFU

fURTheR infoRMaTionHeymut Omran’s homepage: http://www.uniklinik-freiburg.de/kinderklinik/live/forschung/omran.htmlChlamydomonas Flagellar Proteome: http://labs.umassmed.edu/chlamyfp/index.phpCilia Proteome database: http://www.ciliaproteome.orgCiliomics: http://www.sfu.ca/~leroux/ciliome_home.htmPrimary Cilia Resource: www.bowserlab.org/primarycilia/cilialist.html

sUPPLeMenTaRY infoRMaTionSee online article: S1 (movie) | S2 (movie) | S3 (movie) | S4 (movie) | S5 (movie) | S6 (movie) | S7 (movie) | S8 (movie) | S9 (movie)

all linkS are aCTive in THe online pdf

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http://www.ncbi.nlm.nih.gov/sites/entrez?db=gene

http://www.ncbi.nlm.nih.gov/sites/entrez?db=gene










http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=OMIM

http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=OMIM












http://ca.expasy.org/sprot







http://www.expasy.org/uniprot/Q9UMX1

http://www.uniklinik-freiburg.de/kinderklinik/live/forschung/omran.html

http://www.uniklinik-freiburg.de/kinderklinik/live/forschung/omran.html


http://www.ciliaproteome.org

http://www.sfu.ca/~leroux/ciliome_home.htm













CORRIGENDUM

When cilia go bad: cilia defects and ciliopathiesManfred Fliegauf, Thomas Benzing & Heymut Omran Nature Reviews Molecular Cell Biology 8, 880–893 (2007); doi:10.1038/nrm2278

The authors would like to add the following text (in italics) and references (see references *,‡ and § below) to the article, and include a change to Figure 2a.

On page 883 of the article, in the first paragraph following the main heading ‘Disorders of development and growth’, the text and references should read as follows:‘Numerous cilia-related diseases have been described that are associated with developmental defects affecting the central nervous system, the skeleton or other organ systems. Several signalling pathways have been implicated in ciliary function. In Chlamydomonas, it has been shown that the IFT machinery is directly involved in cilium-generated signalling*‡.’

On page 890 of the article, in the last paragraph of the section entitled ‘Oncogenesis’ under the subheading ‘Cystoproteins involved in cell-cycle regulation’, an additional reference should have been included as follows:‘These polycystin studies indicate that cilia-related proteins could be good tumour-suppressor candidates. Biallelic inactivation of the VHL (von Hippel Lindau) tumour-suppressor gene is associated with most sporadic renal clear cell carcinomas; here, tumorigenic transformation is preceded by the formation of renal cysts, which, in turn, are commonly caused by renal monocilia dysfunction. The VHL protein localizes to the axonemes of renal monocilia and controls ciliogenesis in kidney cells118–120,§….’

* Wang, Q., Pan, J. & Snell, W. J. Intraflagellar transport particles participate directly in cilium-generated signaling in Chlamydomonas. Cell 125, 549–562 (2006).‡ Pan, J. & Snell, W. J. Kinesin-II is required for flagellar sensory transduction during fertilization in Chlamydomonas. Mol. Biol. Cell 13, 1417–1426 (2002).§ Thoma, C.R. et al. pVHL and GSK3β are components of a primary cilium-maintenance signalling network. Nature Cell Biol. 9, 588–595 (2007).

In Figure 2a, the hearts and lungs depicted for ‘left isomerism (polysplenia)’ and ‘right isomerism (asplenia)’ have been changed (see below).


Left isomerism (polysplenia)

Right isomerism (asplenia)

Situs solitus Situs inversus totalis

Situs inversus thoracalis Situs inversus abdominalis

Spleen

Stomach

Liver

Heart

Lung

lu lu

a Laterality defects

Lung

when cilia go bad cilia defects

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