Ciliogenesis defects in embryos lacking inturned or fuzzy function are associated with failure of planar cell polarity and Hedgehog signaling

Murine dishevelled 3 functions in redundant pathways with dishevelled 1 and 2 in normal cardiac outflow tract, cochlea, and neural tube development

Dishevelled (Dvl) proteins are important signaling components of both the canonical β-catenin/Wnt pathway, which controls cell proliferation and patterning, and the planar cell polarity (PCP) pathway, which coordinates cell polarity within a sheet of cells and also directs convergent extension cell (CE) movements that produce narrowing and elongation of the tissue. Three mammalian Dvl genes have been identified and the developmental roles of Dvl1 and Dvl2 were previously determined. Here, we identify the functions of Dvl3 in development and provide evidence of functional redundancy among the three murine Dvls. Dvl3^−/− mice died perinatally with cardiac outflow tract abnormalities, including double outlet right ventricle and persistent truncus arteriosis. These mutants also displayed a misorientated stereocilia in the organ of Corti, a phenotype that was enhanced with the additional loss of a single allele of the PCP component Vangl2/Ltap (LtapLp/+). Although neurulation appeared normal in both Dvl3^−/− and LtapLp/+ mutants, Dvl3^+/−;LtapLp/+ combined mutants displayed incomplete neural tube closure. Importantly, we show that many of the roles of Dvl3 are also shared by Dvl1 and Dvl2. More severe phenotypes were observed in Dvl3 mutants with the deficiency of another Dvl, and increasing Dvl dosage genetically with Dvl transgenes demonstrated the ability of Dvls to compensate for each other to enable normal development. Interestingly, global canonical Wnt signaling appeared largely unaffected in the double Dvl mutants, suggesting that low Dvl levels are sufficient for functional canonical Wnt signals. In summary, we demonstrate that Dvl3 is required for cardiac outflow tract development and describe its importance in the PCP pathway during neurulation and cochlea development. Finally, we establish several developmental processes in which the three Dvls are functionally redundant.

Multi-gene families, comprising a set of very similar genes with shared nucleotide sequences, are common in mammals. Individual family members may be expressed in different places and perform separate functions. Alternatively, the genes may have redundant functions, but distinct dosage requirements. Mammals share three Dishevelled (Dvl) family members and while the roles of Dvl1 and Dvl2 have been described previously, the functions of Dvl3 have remained elusive. Here, we show that the lack of Dvl3 in mice affects the formation of the heart, neural tube, and inner ear. We further show that the defects in these tissues are much more severe when the mice are deficient in more than one Dvl family member, indicating redundant functions for these genes. Congenital heart disease affects approximately 75 in every 1,000 live human births, and approximately 30% of these diseases are due to disruptions in the outflow tract, the region affected in mice lacking Dvl genes. Neural tube defects, similar to those observed in the Dvl mutants, are also common in humans. The animal models described here provide useful tools to elucidate the genetic mechanisms that underlie these abnormalities and may provide novel ways of treating these disorders in the future.

Developmental changes in frequency of the ciliary somatostatin receptor 3 protein

Primary cilia extend from the surface of most vertebrate cells and display several signaling molecules, including the somatostatin receptor 3 (SSTR3), enabling cilia to play essential roles as chemical, osmotic and mechanical sensors. The SSTR3 is widely distributed in the adult rat brain, and also influences cell proliferation and apoptosis. To establish whether the SSTR3 is positioned to influence these developmental processes, we examined, using immunohistochemistry, the embryonic and postnatal development of SSTR3 expression in the rat hippocampal formation, and its association with newly born and mature neurons in adult rats. Elongated SSTR3-immunoreactive (-ir) cilia first appeared in the hippocampal formation CA3 region of postnatal day (P) 0 animals, and their density increased to high levels by P2, remained at high levels through to P30, but were at low levels in 5-month old rats. A similar developmental pattern was observed in the CA1 region, where SSTR3-ir ciliated structures were first detected on P2. In contrast, density levels in the granular cell layer of the dentate gyrus were very high by P30, and remained elevated in adult rats. SSTR3-ir cilia did not colocalize with neuroblasts in the hippocampal formation or olfactory bulb, but appeared to be localized to more mature cells in these regions. A few SSTR3-ir neurons were also observed in the hippocampal formation. These findings support the hypothesis that the ciliary SSTR3 is well positioned to influence the cell cycle and apoptotic processes during postnatal development, and in neurogenic regions of the adult rat brain.

Convergent extension movements and ciliary function are mediated by ofd1, a zebrafish orthologue of the human oral-facial-digital type 1 syndrome gene

In humans, OFD1 is mutated in oral-facial-digital type I syndrome leading to prenatal death in hemizygous males and dysmorphic faces and brain malformations, with polycystic kidneys presenting later in life in heterozygous females. To elucidate the function of Ofd1, we have studied its function during zebrafish embryonic development. In wild-type embryos, ofd1 mRNA is widely expressed and Ofd1-green fluorescent protein (GFP) fusion localizes to the centrosome/basal body. Disrupting Ofd1 using antisense morpholinos (MOs) led to bent body axes, hydrocephalus and oedema. Laterality was randomized in the brain, heart and viscera, likely a consequence of shorter cilia with disrupted axonemes and perturbed intravesicular fluid flow in Kupffer's vesicle. Embryos injected with ofd1 MOs also displayed convergent extension (CE) defects, which were enhanced by loss of Slb/Wnt11 or Tri/Vangl2, two proteins functioning in a non-canonical Wnt/Planar Cell Polarity (PCP) pathway. Pronephric glomerular midline fusion was compromised in vangl2 and ofd1 loss of function embryos and we suggest this anomaly may be a novel CE defect. Thus, Ofd1 is required for ciliary motility and function in zebrafish, supporting data showing that Ofd1 is essential for primary cilia function in mice. In addition, our data show that Ofd1 is important for CE during gastrulation, consistent with data linking primary cilia and non-canonical Wnt/PCP signalling.

Organelle positioning and cell polarity

In spite of conspicuous differences in their polarized architecture, swimming unicellular eukaryotes and migrating cells from metazoa display a conserved hierarchical interlocking of the main cellular compartments, in which the microtubule network has a dominant role. A microtubule array can organize the distribution of endomembranes owing to a cell-wide and polarized extension around a unique nucleus-associated structure. The nucleus-associated structure in animal cells contains a highly conserved organelle, the centriole or basal body. This organelle has a defined polarity that can be transmitted to the cell. Its conservative mode of duplication seems to be a core mechanism for the transmission of polarities through cell division.

Primary cilia in osteoarthritic chondrocytes: from chondrons to clusters

Osteoarthritis (OA) is a common joint disease characterized by articular cartilage degeneration. The etiology of OA is unknown. Because several previous studies have shown that primary cilia play critical roles in joint development, this study examined the incidence and morphology of primary cilia in chondrocytes during joint degeneration in a naturally occurring bovine model of OA. Primary cilia were detected using antibodies to acetylated alpha-tubulin in normal cartilage as well as in mild and severe OA tissue. In normal cartilage, cilia number and length were lowest in the superficial zone and increased with distance from the articular surface. In OA tissue, the incidence and length of cilia increased at the eroding articulating surface, resulting in an overall increased proportion of cilia. This is the first study to show that primary cilia are present on chondrocytes throughout OA progression and that the overall percentage of ciliated cells within the degenerating cartilage increases with OA severity.

Hedgehog: functions and mechanisms

The Hedgehog (Hh) family of proteins control cell growth, survival, and fate, and pattern almost every aspect of the vertebrate body plan. The use of a single morphogen for such a wide variety of functions is possible because cellular responses to Hh depend on the type of responding cell, the dose of Hh received, and the time cells are exposed to Hh. The Hh gradient is shaped by several proteins that are specifically required for Hh processing, secretion, and transport through tissues. The mechanism of cellular response, in turn, incorporates multiple feedback loops that fine-tune the level of signal sensed by the responding cells. Germline mutations that subtly affect Hh pathway activity are associated with developmental disorders, whereas somatic mutations activating the pathway have been linked to multiple forms of human cancer. This review focuses broadly on our current understanding of Hh signaling, from mechanisms of action to cellular and developmental functions. In addition, we review the role of Hh in the pathogenesis of human disease and the possibilities for therapeutic intervention.

Dishevelled links basal body docking and orientation in ciliated epithelial cells

Some epithelia contain cells with multiple motile cilia that beat in a concerted manner. New tools and experimental systems have facilitated molecular studies of cilium biogenesis and the coordinated planar polarization of cilia that leads to their concerted motility. A recent elegant study using embryonic frog epidermis demonstrates that Dishevelled, a key regulator of both the Wnt-beta-catenin and planar cell polarity pathways, controls both the docking and planar polarization of ciliary basal bodies.

Role of intraflagellar transport and primary cilia in skeletal development

Primary cilia are nonmotile microtubule-based appendages extending from the surface of almost all vertebrate cells. The process of intraflagellar transport (IFT) is responsible for building and maintaining the structure and function of primary cilia. Disruption of Kif3a, a component of the Kinesin-II motor complex, disables anterograde IFT and leads to failure in the formation and maintenance of cilia. Likewise, the absence of IFT88/Tg737/Polaris, a core component of the IFT particle, results in the loss of cilia. Although primary cilia were described on chondrocytes almost 40 years ago, only recently has the functional significance of IFT and cilia in skeletal development been uncovered through the use of mouse models containing mutations or deletions in genes required to make and maintain cilia. Together, the results indicate that primary cilia/IFT are involved in coordinating multiple signaling pathways within the skeleton.

Planar polarity genes in the Drosophila wing regulate the localisation of the FH3-domain protein Multiple Wing Hairs to control the site of hair production

The core planar polarity proteins play important roles in coordinating cell polarity, in part by adopting asymmetric subcellular localisations that are likely to serve as cues for cell polarisation by as yet uncharacterised pathways. Here we describe the role of Multiple Wing Hairs (Mwh), a novel formin homology 3 (FH3)-domain protein, which acts downstream of the core polarity proteins to restrict the production of actin-rich prehairs to distal cell edges in the Drosophila pupal wing. Mwh appears to function as a repressor of actin filament formation and, in its absence, ectopic actin bundles are seen across the entire apical surface of cells. We show that the proximally localised core polarity protein Strabismus acts via the downstream effector proteins Inturned, Fuzzy and Fritz to stabilise Mwh in apico-proximal cellular regions. In addition, the distally localised core polarity protein Frizzled positively promotes prehair initiation, suggesting that both proximal and distal cellular cues act together to ensure accurate prehair placement.

Building it up and taking it down: the regulation of vertebrate ciliogenesis

Primary cilia project from the surface of most vertebrate cells, and function in sensation and signaling during both development and adult tissue homeostasis. Mounting evidence links ciliary defects with a wide variety of diseases, underscoring the importance of understanding how these dynamic organelles are assembled and maintained. However, despite their physiological and clinical relevance, the logic and machinery that regulate ciliogenesis remain largely enigmatic. Here, we summarize emerging data that connect the assembly and disassembly of the primary cilium to cell cycle progression and we examine how determinants of cell architecture, including the planar cell polarity pathway, may regulate ciliogenesis. Additionally, identification of the genes underlying diverse ciliopathies in human patients is shedding light on the regulation of the formation of this complex organelle.

Loss of Fat4 disrupts PCP signaling and oriented cell division and leads to cystic kidney disease

Tissue organization in Drosophila is regulated by the core planar cell polarity (PCP) proteins Frizzled, Dishevelled, Prickle, Van Gogh and Flamingo. Core PCP proteins are conserved in mammals and function in mammalian tissue organization. Recent studies have identified another group of Drosophila PCP proteins, consisting of the protocadherins Fat and Dachsous (Ds) and the transmembrane protein Four-jointed (Fj). In Drosophila, Fat represses fj transcription, and Ds represses Fat activity in PCP. Here we show that Fat4 is an essential gene that has a key role in vertebrate PCP. Loss of Fat4 disrupts oriented cell divisions and tubule elongation during kidney development, leading to cystic kidney disease. Fat4 genetically interacts with the PCP genes Vangl2 and Fjx1 in cyst formation. In addition, Fat4 represses Fjx1 expression, indicating that Fat signaling is conserved. Together, these data suggest that Fat4 regulates vertebrate PCP and that loss of PCP signaling may underlie some cystic diseases in humans.

Dishevelled controls apical docking and planar polarization of basal bodies in ciliated epithelial cells

The planar cell polarity (PCP) signaling system governs many aspects of polarized cell behavior. Here, we use an in vivo model of vertebrate mucociliary epithelial development to show that Dishevelled (Dvl) is essential for the apical positioning of basal bodies. We find that Dvl and Inturned mediate the activation of the Rho GTPase specifically at basal bodies, and that these three proteins together mediate the docking of basal bodies to the apical plasma membrane. Moreover, we find that this docking involves a Dvl-dependent association of basal bodies with membrane-bound vesicles and the vesicle-trafficking protein, Sec8. Once docked, basal bodies again require Dvl and Rho for the planar polarization that underlies directional beating of cilia. These results demonstrate previously undescribed functions for PCP signaling components and suggest that a common signaling apparatus governs both apical docking and planar polarization of basal bodies.

Cystic kidney gene seahorse regulates cilia-mediated processes and Wnt pathways

Recently the cilium has emerged as an important sensory organelle for a wide range of cell types in vertebrates. However, the signaling cascade that links ciliary signals to cellular events remains poorly understood. Here, we show that the zebrafish cystic kidney gene seahorse is closely associated with ciliary functions: seahorse is required for establishing left-right asymmetry and for preventing kidney cyst formation; seahorse transcript is highly enriched in heavily ciliated tissues; and seahorse genetically interacts with the ciliary gene inversin. Yet seahorse is dispensable for cilia assembly or motility and the Seahorse protein is cytoplasmic. We provide evidence that Seahorse associates with Dishevelled. Finally, we show that seahorse constrains the canonical Wnt pathway and promotes the noncanonical Wnt pathway during gastrulation. Together, these data suggest that Seahorse may provide a link between ciliary signals and Wnt pathways.

Acute kidney injury and aberrant planar cell polarity induce cyst formation in mice lacking renal cilia

Polycystic kidney disease (PKD) is an inherited disorder that is characterized by the accumulation of cysts in the renal parenchyma and progressive decline in renal function. Recent studies suggest that PKD arises from abnormalities of the primary cilium. We have previously shown that kidney-specific inactivation of the ciliogenic gene Kif3a during embryonic development produces kidney cysts and renal failure. Here, we used tamoxifen-inducible, kidney-specific gene targeting to inactivate Kif3a in the postnatal mouse kidney. Kidney-specific inactivation of Kif3a in newborn mice resulted in the loss of primary cilia and produced kidney cysts primarily in the loops of Henle, whereas inactivation in adult mice did not lead to the rapid development of cysts despite a comparable loss of primary cilia. The age-dependence and locations of the cysts suggested that cyst formation required increased rates of cell proliferation. To test this possibility, we stimulated cell proliferation in the adult kidney by inducing acute kidney injury and tubular regeneration. Acute kidney injury induced cyst formation in adult Kif3a mutant mice. Analysis of pre-cystic tubules in Kif3a mutant mice showed that the loss of cilia did not stimulate cell proliferation but instead resulted in aberrant planar cell polarity as manifested by abnormalities in the orientation of cell division. We conclude that primary cilia are required for the maintenance of planar cell polarity in the mammalian kidney and that acute kidney injury exacerbates cystic disease.

Cilia orientation and the fluid mechanics of development

Motile cilia produce large-scale fluid flows crucial for development and physiology. Defects in ciliary motility cause a range of disease symptoms including bronchiectasis, hydrocephalus, and situs inversus. However, it is not enough for cilia to be motile and generate a flow -- the flow must be driven in the proper direction. Generation of properly directed coherent flow requires that the cilia are properly oriented relative to tissue axes. Genetic, molecular, and ultrastructural studies have begun to suggest pathways linking cilia orientation to planar cell polarity (PCP) and other long-range positional cues and also suggest that cilia-driven flow can itself play a causal role in orienting the cilia that create it. Errors in cilia orientation have been observed in human ciliary disease patients, suggesting that orientation defects may constitute a novel class of ciliopathies with a distinct etiology at the cell biological level.

Ciliary function and Wnt signal modulation

With the increase in complexity of morphogenetic signaling cascades over the course of evolution and the emergence of broadly ciliated organisms, the cilium seems to have acquired a role as regulator of paracrine signal transduction. Recently, several lines of evidence have provided a link between basal body and ciliary proteins and Wnt signaling. In this chapter, we will evaluate the evidence linking the basal body and cilium with the regulation of beta-catenin-dependent (canonical) and beta-catenin-independent (noncanonical) signaling processes as well as which role(s) Wnt signaling might play in ciliogenesis. In addition, we will discuss aberrant Wnt signaling could contribute to phenotypes common to most ciliopathies and why these phenotypes might be driven by loss of noncanonical rather than gain of noncanonical Wnt signaling.

Planar cell polarity signaling: from fly development to human disease

Most, if not all, cell types and tissues display several aspects of polarization. In addition to the ubiquitous epithelial cell polarity along the apical-basolateral axis, many epithelial tissues and organs are also polarized within the plane of the epithelium. This is generally referred to as planar cell polarity (PCP; or historically, tissue polarity). Genetic screens in Drosophila pioneered the discovery of core PCP factors, and subsequent work in vertebrates has established that the respective pathways are evolutionarily conserved. PCP is not restricted only to epithelial tissues but is also found in mesenchymal cells, where it can regulate cell migration and cell intercalation. Moreover, particularly in vertebrates, the conserved core PCP signaling factors have recently been found to be associated with the orientation or formation of cilia. This review discusses new developments in the molecular understanding of PCP establishment in Drosophila and vertebrates; these developments are integrated with new evidence that links PCP signaling to human disease.

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

Cell cycle control and migration are critical processes during development and maintenance of tissue functions. Recently, primary cilia were shown to take part in coordination of the signaling pathways that control these cellular processes in human health and disease. In this review, we present an overview of the function of primary cilia and the centrosome in the signaling pathways that regulate cell cycle control and migration with focus on ciliary signaling via platelet-derived growth factor receptor alpha (PDGFRalpha). We also consider how the primary cilium and the centrosome interact with the extracellular matrix, coordinate Wnt signaling, and modulate cytoskeletal changes that impinge on both cell cycle control and cell migration.

Primary cilia in planar cell polarity regulation of the inner ear

Primary cilia are essential components of diverse cellular processes. Many of the requirements can be linked to the apparent signaling function of primary cilia. Recent studies have also uncovered a role for primary cilia in planar cell polarity (PCP) signaling. PCP refers to the coordinated orientation of cells along an axis parallel to the plane of the cell sheet. In vertebrates, the inner ear sensory organs display distinctive forms of PCP. One of the inner ear PCP characteristics is the coordinated positioning of a primary cilium eccentrically in every sensory hair cell within each organ. The inner ear, therefore, provides an opportunity to explore the cellular role of primary cilia in PCP signaling. In this chapter, we will introduce the PCP of the inner ear sensory organs, describe the conserved mechanism underlying the establishment of the planar polarity axis in invertebrates and vertebrates, and highlight a unique requirement for primary cilia in PCP regulation in vertebrates. Additionally, we will discuss a potentially ubiquitous role for cilia in cellular polarization in general.

Modeling ciliopathies: Primary cilia in development and disease

Primary (nonmotile) cilia are currently enjoying a renaissance in light of novel ascribed functions ranging from mechanosensory to signal transduction. Their importance for key developmental pathways such as Sonic Hedgehog (Shh) and Wnt is beginning to emerge. The function of nodal cilia, for example, is vital for breaking early embryonic symmetry, Shh signaling is important for tissue morphogenesis and successful Wnt signaling for organ growth and differentiation. When ciliary function is perturbed, photoreceptors may die, kidney tubules develop cysts, limb digits multiply and brains form improperly. The etiology of several uncommon disorders has recently been associated with cilia dysfunction. The causative genes are often similar and their cognate proteins certainly share cellular locations and/or pathways. Animal models of ciliary gene ablation such as Ift88, Kif3a, and Bbs have been invaluable for understanding the broad function of the cilium. Herein, we describe the wealth of information derived from the study of the ciliopathies and their animal models.

Cilia involvement in patterning and maintenance of the skeleton

Although the expression of cilia on chondrocytes was described over 40 years ago, the importance of this organelle in skeletal development and maintenance has only recently been recognized. Primary cilia are found on most mammalian cells and have been shown to play a role in chemosensation and mechanosensation. A growing number of human pleiotropic syndromes have been shown to be associated with ciliary or basal body dysfunction. Skeletal phenotypes, including alterations in limb patterning, endochondral bone formation, craniofacial development, and dentition, have been described in several of these syndromes. Additional insights into the potential roles and mechanisms of cilia action in the mammalian skeleton have been provided by research in model organisms including mouse and zebrafish. In this article we describe what is currently known about the localization of cilia in the skeleton as well as the roles and underlying molecular mechanisms of cilia in skeletal development.

Fish and frogs: models for vertebrate cilia signaling

The presence of cilia in many vertebrate cell types and its function has been ignored for many years. Only in the past few years has its importance been rediscovered. In part, this was triggered by the realization that many gene products mutated in polycystic kidney diseases are localized to cilia and dysfunctional cilia result in kidney disease. Another breakthrough was the observation that the establishment of the left-right body axis is dependent on cilia function. Since then, many other developmental paradigms have been shown to rely on cilia-dependent signaling. In addition to mouse and Chlamydomonas, lower vertebrate model systems such as zebrafish, medaka and Xenopus have provided important new insights into cilia signaling and its role during embryonic development. This review will summarize those studies. We will also illustrate how these lower vertebrates are promising model systems for future studies defining the physiological function of cilia during organogenesis and disease pathophysiology.

Ciliary proteins link basal body polarization to planar cell polarity regulation

Planar cell polarity (PCP) refers to coordinated polarization of cells within the plane of a cell sheet. A conserved signaling pathway is required for the establishment of PCP in epithelial tissues and for polarized cellular rearrangements known as convergent extension. During PCP signaling, core PCP proteins are sorted asymmetrically along the polarization axis; this sorting is thought to direct coordinated downstream morphogenetic changes across the entire tissue. Here, we show that a gene encoding a ciliary protein (a 'ciliary gene'), Ift88, also known as Polaris, is required for establishing epithelial PCP and for convergent extension of the cochlear duct of Mus musculus. We also show that the proper positioning of ciliary basal bodies and the formation of polarized cellular structures are disrupted in mice with mutant ciliary proteins ('ciliary mutants'), whereas core PCP proteins are partitioned normally along the polarization axis. Thus, our data uncover a distinct requirement for ciliary genes in basal body positioning and morphological polarization during PCP regulation.

WNTers in La Jolla

A 'traditional' Wnt meeting, the first of which occurred over two decades ago as a meeting of the laboratories of Harold Varmus and Roel Nusse, was held at the University of California, San Diego, in June 2007. Organized by Karl Willert, Anthony Wynshaw-Boris and Katherine Jones, the meeting was attended by nearly 400 scientists interested in ;all things Wnt', including Wnt signal transduction mechanisms, and Wnt signaling in evolutionary and developmental biology, stem cell biology, regeneration and disease. Themes that dominated the meeting included the need for precise control over each step of the signal transduction mechanism and developing therapeutics for diseases caused by altered Wnt-signaling.

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

Mucociliary epithelia are essential for homeostasis of many organs and consist of mucus-secreting goblet cells and ciliated cells. Here, we present the ciliated epidermis of Xenopus embryos as a facile model system for in vivo molecular studies of mucociliary epithelial development. Using an in situ hybridization-based approach, we identified numerous genes expressed differentially in mucus-secreting cells or in ciliated cells. Focusing on genes expressed in ciliated cells, we have identified new candidate ciliogenesis factors, including several not present in the current ciliome. We find that TTC25-GFP is localized to the base of cilia and to ciliary axonemes, and disruption of TTC25 function disrupts ciliogenesis. Mig12-GFP localizes very strongly to the base of cilia and confocal imaging of this construct allows for simple visualization of the planar polarity of basal bodies that underlies polarized ciliary beating. Knockdown of Mig12 disrupts ciliogenesis. Finally, we show that ciliogenesis factors identified in the Xenopus epidermis are required in the midline to facilitate neural tube closure. These results provide further evidence of a requirement for cilia in neural tube morphogenesis and suggest that genes identified in the Xenopus epidermis play broad roles in ciliogenesis. The suites of genes identified here will provide a foundation for future studies, and may also contribute to our understanding of pathological changes in mucociliary epithelia that accompany diseases such as asthma.

When cilia go bad: cilia defects and ciliopathies

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.

Two oppositely localised frizzled RNAs as axis determinants in a cnidarian embryo

In phylogenetically diverse animals, including the basally diverging cnidarians, “determinants” localised within the egg are responsible for directing development of the embryonic body plan. Many such determinants are known to regulate the Wnt signalling pathway, leading to regionalised stabilisation of the transcriptional coregulator β-catenin; however, the only strong molecular candidate for a Wnt-activating determinant identified to date is the ligand Wnt11 in Xenopus. We have identified embryonic “oral–aboral” axis determinants in the cnidarian Clytia hemisphaerica in the form of RNAs encoding two Frizzled family Wnt receptors, localised at opposite poles of the egg. Morpholino-mediated inhibition of translation showed that CheFz1, localised at the animal pole, activates the canonical Wnt pathway, promotes oral fates including gastrulation, and may also mediate global polarity in the ectoderm. CheFz3, whose RNA is localised at the egg vegetal cortex, was found to oppose CheFz1 function and to define an aboral territory. Active downregulation mechanisms maintained the reciprocal localisation domains of the two RNAs during early development. Importantly, ectopic expression of either CheFz1 or CheFz3 was able to redirect axis development. These findings identify Frizzled RNAs as axis determinants in Clytia, and have implications for the evolution of embryonic patterning mechanisms, notably that diverse Wnt pathway regulators have been adopted to initiate asymmetric Wnt pathway activation.

How do different animal body parts form in the correct arrangement during development? Often, the explanation is provided by “determinant” molecules, prepositioned in the egg cell before it is fertilised. These determinant molecules initiate spatially localized programmes of gene expression, causing the various body parts to form in the appropriate place. Many determinants work by activating the Wnt signalling pathway; however, few concrete examples of determinant molecules have yet been discovered. We have found a new example of such a molecule by studying embryos of a jellyfish called Clytia. This molecule, found on one side of the egg, belongs to the “Frizzled” group of membrane proteins that activate Wnt signalling. Unexpectedly, we also found a second type of Frizzled molecule on the other side of the egg, which has a counterbalancing role in the embryo. Comparison of our findings in Clytia with those in other animals suggests that the molecular mechanisms responsible for body patterning via asymmetric Wnt pathway activation have not been tightly constrained during evolution.

The axis of a cnidarian is specified by the location of two maternal Frizzled mRNAs, revising the view that cnidarian axes are specified by the first cleavage initiation site.

Bardet-Biedl syndrome: beyond the cilium

The Bardet-Biedl syndrome (BBS) is a significant genetic cause of chronic and end-stage renal failure in children. Despite being a relatively rare recessive condition, BBS has come to prominence during the past few years owing to revelations of primary cilia dysfunction underlying pathogenesis. The study of this multi-system disorder, which includes obesity, cognitive impairment, genito-urinary tract malformations and limb deformities, is beginning to reveal insights into several aspects of mammalian development and organogenesis. Involvement of BBS proteins in disparate pathways such as the non-canonical Wnt and Sonic Hedgehog pathways is highlighting their interplay in disease pathogenesis. Here we review the recent developments in this emerging field, with the emphasis on the renal component of the syndrome and potential future directions.

Toward understanding the genetic basis of neural tube defects

Neural tube defects (NTDs) represent a common group of severe congenital malformations that result from failure of neural tube closure during early development. Their etiology is quite complex involving environmental and genetic factors and their underlying molecular and cellular pathogenic mechanisms remain poorly understood. Animal studies have recently demonstrated an essential role for the planar cell polarity pathway (PCP) in mediating a morphogenetic process called convergent extension during neural tube formation. Alterations in members of this pathway lead to NTDs in vertebrate models, representing novel and exciting candidates for human NTDs. Genetic studies in NTDs have focused mainly on folate-related genes based on the finding that perinatal folic acid supplementation reduces the risk of NTDs by 60-70%. A few variants in these genes have been found to be significantly associated with an increased risk for NTDs. The candidate gene approach investigating genes involved in neurulation has failed to identify major causative genes in the etiology of NTDs. Despite this history of generally negative findings, we are achieving a rapid and impressive progress in understanding the genetic basis of NTDs, based mainly on the powerful tool of animal models.

Tissue/planar cell polarity in vertebrates: new insights and new questions

This review focuses on the tissue/planar cell polarity (PCP) pathway and its role in generating spatial patterns in vertebrates. Current evidence suggests that PCP integrates both global and local signals to orient diverse structures with respect to the body axes. Interestingly, the system acts on both subcellular structures, such as hair bundles in auditory and vestibular sensory neurons, and multicellular structures, such as hair follicles. Recent work has shown that intriguing connections exist between the PCP-based orienting system and left-right asymmetry, as well as between the oriented cell movements required for neural tube closure and tubulogenesis. Studies in mice, frogs and zebrafish have revealed that similarities, as well as differences, exist between PCP in Drosophila and vertebrates.

Conditional Kif3a ablation causes abnormal hedgehog signaling topography, growth plate dysfunction, and excessive bone and cartilage formation during mouse skeletogenesis

The motor protein Kif3a and primary cilia regulate important developmental processes, but their roles in skeletogenesis remain ill-defined. Here we created mice deficient in Kif3a in cartilage and focused on the cranial base and synchondroses. Kif3a deficiency caused cranial base growth retardation and dysmorphogenesis, which were evident in neonatal animals by anatomical and micro-computed tomography (microCT) inspection. Kif3a deficiency also changed synchondrosis growth plate organization and function, and the severity of these changes increased over time. By postnatal day (P)7, mutant growth plates lacked typical zones of chondrocyte proliferation and hypertrophy, and were instead composed of chondrocytes with an unusual phenotype characterized by strong collagen II (Col2a1) gene expression but barely detectable expression of Indian hedgehog (Ihh), collagen X (Col10a1), Vegf (Vegfa), MMP-13 (Mmp13) and osterix (Sp7). Concurrently, unexpected developmental events occurred in perichondrial tissues, including excessive intramembranous ossification all along the perichondrial border and the formation of ectopic cartilage masses. Looking for possible culprits for these latter processes, we analyzed hedgehog signalling topography and intensity by monitoring the expression of the hedgehog effectors Patched 1 and Gli1, and of the hedgehog-binding cell-surface component syndecan 3. Compared with controls, hedgehog signaling was quite feeble within mutant growth plates as early as P0, but was actually higher and was widespread all along mutant perichondrial tissues. Lastly, we studied postnatal mice deficient in Ihh in cartilage; their cranial base defects only minimally resembled those in Kif3a-deficient mice. In summary, Kif3a and primary cilia make unique contributions to cranial base development and synchondrosis growth plate function. Their deficiency causes abnormal topography of hedgehog signaling, growth plate dysfunction, and un-physiologic responses and processes in perichondrial tissues, including ectopic cartilage formation and excessive intramembranous ossification.

The mother centriole plays an instructive role in defining cell geometry

Centriole positioning is a key step in establishment and propagation of cell geometry, but the mechanism of this positioning is unknown. The ability of pre-existing centrioles to induce formation of new centrioles at a defined angle relative to themselves suggests they may have the capacity to transmit spatial information to their daughters. Using three-dimensional computer-aided analysis of cell morphology in Chlamydomonas, we identify six genes required for centriole positioning relative to overall cell polarity, four of which have known sequences. We show that the distal portion of the centriole is critical for positioning, and that the centriole positions the nucleus rather than vice versa. We obtain evidence that the daughter centriole is unable to respond to normal positioning cues and relies on the mother for positional information. Our results represent a clear example of "cytotaxis" as defined by Sonneborn, and suggest that centrioles can play a key function in propagation of cellular geometry from one generation to the next. The genes documented here that are required for proper centriole positioning may represent a new class of ciliary disease genes, defects in which would be expected to cause disorganized ciliary position and impaired function.

RhoA-mediated apical actin enrichment is required for ciliogenesis and promoted by Foxj1

Programs that direct cellular differentiation are dependent on the strict temporal expression of regulatory factors that can be provided by Rho GTPases. Ciliogenesis is a complex sequence of events involving the generation and docking of basal bodies at the apical membrane, followed by ciliary axoneme generation. Although a cilia proteome has been assembled, programs that direct ciliated cell differentiation are not well established, particularly in mammalian systems. Using mouse primary culture airway epithelial cells, we identified a critical stage of ciliogenesis requiring the temporal establishment of an apical web-like structure of actin for basal body docking and subsequent axoneme growth. Apical web formation and basal body docking were prevented by interruption of actin remodeling and were dependent on RhoA activation. Additional evidence for this program was provided by analysis of Foxj1-null mice that failed to dock basal bodies and lacked apical actin. Foxj1 expression coincided with actin web formation, activated RhoA and RhoB, and persisted despite RhoA inhibition, suggesting that Foxj1 promoted RhoA during ciliogenesis. Apical ezrin localization was also dependent on Foxj1, actin remodeling, and RhoA, but was not critical for ciliogenesis. Thus, temporal Foxj1 and RhoA activity are essential regulatory events for cytoskeletal remodeling during mammalian ciliogenesis.

Overview of structure and function of mammalian cilia

Cilia are membrane-bounded, centriole-derived projections from the cell surface that contain a microtubule cytoskeleton, the ciliary axoneme, surrounded by a ciliary membrane. Axonemes in multiciliated cells of mammalian epithelia are 9 + 2, possess dynein arms, and are motile. In contrast, single nonmotile 9 + 0 primary cilia are found on epithelial cells, such as those of the kidney tubule, but also on nonepithelial cells, such as chondrocytes, fibroblasts, and neurons. The ciliary membranes of all cilia contain specific receptors and ion channel proteins that initiate signaling pathways controlling motility and/or linking mechanical or chemical stimuli, including sonic hedgehog and growth factors, to intracellular transduction cascades regulating differentiation, migration, and cell growth during development and in adulthood. Unique motile 9 + 0 cilia, found during development at the embryonic node, determine left-right asymmetry of the body.

Wnt signaling in polycystic kidney disease

Wnt signaling cascades activate morphogenetic programs that range from cell migration and proliferation to cell fate determination and stem cell renewal. These pathways enable cells to translate environmental cues into the complex cellular programs that are needed to organize tissues and build organs. Wnt signaling is essential for renal development; however, the specific molecular underpinnings involved are poorly understood. Recent research has revealed an unexpected intersection between Wnt signaling and polycystic kidney disease. Some polycystic kidney disease proteins, such as Inversin and Bardet-Biedl syndrome family members, were found to use components of the Wnt signaling cascade to orient cells along a secondary polarity axis within the plane of the epithelium. These spatial cues may be needed to position nascent tubules with a defined geometry.

Centriole/basal body morphogenesis and migration during ciliogenesis in animal cells

Cilia, either motile or immotile, exist on most cells in the human body. There are several different mechanisms of ciliogenesis, which enable the production of many kinds of cilia and flagella: motile and immotile, transient and long-lived. These can be linked to the cell cycle or associated with differentiation. A primary cilium is extended from a basal body analogous to the mitotic centrioles, whereas the several hundred centrioles needed to form the cilia of a multi-ciliated cell can be generated by centriolar or acentriolar pathways. Little is known about the molecular control of these pathways and most of our knowledge comes from ultrastructural studies. The increasing number of genetic diseases linked to dysfunctional cilia and basal bodies has renewed interest in this area, and recent proteomic and cell biological studies in model organisms have helped to shed light on the molecular components of these enigmatic organelles.

Convergence and extension movements mediate the specification and fate maintenance of zebrafish slow muscle precursors

During vertebrate gastrulation, concurrent inductive events and cell movements fashion the body plan. Convergence and extension (C&E) gastrulation movements narrow the vertebrate embryonic body mediolaterally while elongating it rostrocaudally. Segmented somites are shaped and positioned by C&E alongside the notochord and differentiate into skeleton, fast, and slow muscles during somitogenesis. In zebrafish, simultaneous inactivation of non-canonical Wnt signaling components Knypek and Trilobite strongly impairs C&E gastrulation movements. Here we show that knypek;trilobite double mutants exhibit a severe deficit in slow muscles and their precursor, adaxial cells, revealing essential roles of C&E movements in adaxial cell development. Adaxial cells become distinguishable in the presomitic mesoderm during late gastrulation by their expression of myogenic factors and axial-adjacent position. Using cell tracing analyses and genetic manipulations, we demonstrate that C&E movements regulate the number of prospective adaxial cells specified during gastrulation by determining the size of the interface between the inductive axial and target presomitic tissues. During segmentation, when the range of Hedgehog signaling from the axial tissue declines, tight apposition of prospective adaxial cells to the notochord, which is achieved by convergence movements, is necessary for their continuous Hedgehog reception and fate maintenance. We provide direct evidence to show that the deficiency of adaxial cells in knypek;trilobite double mutants is due to impaired C&E movements, rather than an alteration in Hedgehog signal and its reception, or a cell-autonomous requirement for Knypek and Trilobite in adaxial cell development. Our results underscore the significance of precise coordination between cell movements and inductive tissue interactions during cell fate specification.

Sensory cilia and integration of signal transduction in human health and disease

The primary cilium is a hallmark of mammalian tissue cells. Recent research has shown that these organelles display unique sets of selected signal transduction modules including receptors, ion channels, effector proteins and transcription factors that relay chemical and physical stimuli from the extracellular environment in order to control basic cellular processes during embryonic and postnatal development, as well as in tissue homeostasis in adulthood. Consequently, defects in building of the cilium or in transport or function of ciliary signal proteins are associated with a series of pathologies, including developmental disorders and cancer. In this review, we highlight recent examples of the mechanisms by which signal components are selectively targeted and transported to the ciliary membrane and we present an overview of the signal transduction pathways associated with primary and motile cilia in vertebrate cells, including platelet-derived growth factor receptor-alpha (PDGFRalpha), hedgehog and Wnt signaling pathways. Finally, we discuss the functions of these cilia-associated signal transduction pathways and their role in human health and development.

Frizzled/PCP signalling: a conserved mechanism regulating cell polarity and directed motility

Signalling through Frizzled (Fz)/planar cell polarity (PCP) is a conserved mechanism that polarizes cells along specific axes in a tissue. Genetic screens in Drosophila melanogaster pioneered the discovery of core PCP factors, which regulate the orientation of hairs on wings and facets in eyes. Recent genetic evidence shows that the Fz/PCP pathway is conserved in vertebrates and is crucial for disparate processes as gastrulation and sensory cell orientation. Fz/PCP signalling depends on complex interactions between core components, leading to their asymmetric distribution and ultimately polarized activity in a cell. Whereas several mechanistic aspects of PCP have been uncovered, the global coordination of this polarization remains debated.

Planar cell polarity signaling in vertebrates

Planar cell polarity (PCP) refers to the polarization of a field of cells within the plane of a cell sheet. This form of polarization is required for diverse cellular processes in vertebrates, including convergent extension (CE), the establishment of PCP in epithelial tissues and ciliogenesis. Perhaps the most distinct example of vertebrate PCP is the uniform orientation of stereociliary bundles at the apices of sensory hair cells in the mammalian auditory sensory organ. The establishment of PCP in the mammalian cochlea occurs concurrently with CE in this ciliated epithelium, therefore linking three cellular processes regulated by the vertebrate PCP pathway in the same tissue and emerging as a model system for dissecting PCP signaling. This review summarizes the morphogenesis of this model system to assist the interpretation of the emerging data and proposes molecular mechanisms underlying PCP signaling in vertebrates.

Development of the post-natal growth plate requires intraflagellar transport proteins

In the post-natal growth plate, chondrocytes are arranged in columns parallel to the long axis of the bone. Chondrocytes divide perpendicular to this axis and then move into position one on top of another in a process called "rotation" that maintains columnar organization. Primary cilia are non-motile microtubule base appendages extending from the surface of almost all vertebrate cells. Primary cilia were described on chondrocytes almost 40 years ago but the function of these structures in cartilage biology is not known. Intraflagellar transport (IFT) is the process by which primary cilia are generated and maintained. This study tested the hypothesis that IFT plays an important role in post-natal skeletal development. Kif3a, a subunit of the Kinesin II motor complex, that is required for intraflagellar transport and the formation of cilia, was deleted in mouse chondrocytes via Col2a-Cre-mediated recombination. Disruption of IFT resulted in subsequent depletion of cilia and post-natal dwarfism due to premature loss of the growth plate likely a result of reduced proliferation and accelerated hypertrophic differentiation of chondrocytes. Cell shape and columnar orientation in the growth plate were also disrupted suggesting a defect in the process of rotation. Alterations in chondrocyte rotation were accompanied by disruption of the actin cytoskeleton and alterations in the localization of activated FAK to focal adhesion-like structures on chondrocytes. This is the first report indicating a role for IFT and primary cilia in the development of the post-natal growth plate. The results suggest a model in which IFT/cilia act to maintain the columnar organization of the growth plate via the process of chondrocyte rotation.

The Meckel-Gruber Syndrome proteins MKS1 and meckelin interact and are required for primary cilium formation

Meckel-Gruber syndrome (MKS) is an autosomal recessive lethal malformation syndrome characterized by renal cystic dysplasia, central nervous system malformations (typically, posterior occipital encephalocele), and hepatic developmental defects. Two MKS genes, MKS1 and MKS3, have been identified recently. The present study describes the cellular, sub-cellular and functional characterization of the novel proteins, MKS1 and meckelin, encoded by these genes. In situ hybridization studies for MKS3 in early human embryos showed transcript localizations in agreement with the tissue phenotype of MKS patients. Both MKS proteins predominantly localized to epithelial cells, including proximal renal tubules and biliary epithelial cells. MKS1 localized to basal bodies, while meckelin localized both to the primary cilium and to the plasma membrane in ciliated cell-lines and primary cells. Meckelin protein with the Q376P missense mutation was unable to localize at the cell membrane. siRNA-mediated reduction of Mks1 and Mks3 expression in a ciliated epithelial cell-line blocked centriole migration to the apical membrane and consequent formation of the primary cilium. Co-immunoprecipitation experiments show that wild-type meckelin and MKS1 interact and, in three-dimensional tissue culture assays, epithelial branching morphogenesis was severely impaired. These results suggest that MKS proteins mediate a fundamental developmental stage of ciliary formation and epithelial morphogenesis.

Shaping the mammalian auditory sensory organ by the planar cell polarity pathway

The human ear is capable of processing sound with a remarkable resolution over a wide range of intensity and frequency. This ability depends largely on the extraordinary feats of the hearing organ, the organ of Corti and its sensory hair cells. The organ of Corti consists of precisely patterned rows of sensory hair cells and supporting cells along the length of the snail-shaped cochlear duct. On the apical surface of each hair cell, several rows of actin-containing protrusions, known as stereocilia, form a "V"-shaped staircase. The vertices of all the "V"-shaped stereocilia point away from the center of the cochlea. The uniform orientation of stereocilia in the organ of Corti manifests a distinctive form of polarity known as planar cell polarity (PCP). Functionally, the direction of stereociliary bundle deflection controls the mechanical channels located in the stereocilia for auditory transduction. In addition, hair cells are tonotopically organized along the length of the cochlea. Thus, the uniform orientation of stereociliary bundles along the length of the cochlea is critical for effective mechanotransduction and for frequency selection. Here we summarize the morphological and molecular events that bestow the structural characteristics of the mammalian hearing organ, the growth of the snail-shaped cochlear duct and the establishment of PCP in the organ of Corti. The PCP of the sensory organs in the vestibule of the inner ear will also be described briefly.

Cilia and developmental signaling

Recent studies have revealed unexpected connections between the mammalian Hedgehog (Hh) signal transduction pathway and the primary cilium, a microtubule-based organelle that protrudes from the surface of most vertebrate cells. Intraflagellar transport proteins, which are required for the construction of cilia, are essential for all responses to mammalian Hh proteins, and proteins required for Hh signal transduction are enriched in primary cilia. The phenotypes of different mouse mutants that affect ciliary proteins suggest that cilia may act as processive machines that organize sequential steps in the Hh signal transduction pathway. Cilia on vertebrate cells are likely to be important in additional developmental signaling pathways and are required for PDGF receptor alpha signaling in cultured fibroblasts. Cilia are not essential for either canonical or noncanonical Wnt signaling, although cell-type-specific modulation of cilia components may link cilia and Wnt signaling in some tissues. Because ciliogenesis in invertebrates is limited to a very small number of specialized cell types, the role of cilia in developmental signaling pathways is likely a uniquely vertebrate phenomenon.

Zebrafish Gastrulation: Cell Movements, Signals, and Mechanisms

Gastrulation is a morphogenetic process that results in the formation of the embryonic germ layers. Here we detail the major cell movements that occur during zebrafish gastrulation: epiboly, internalization, and convergent extension. Although gastrulation is known to be regulated by signaling pathways such as the Wnt/planar cell polarity pathway, many questions remain about the underlying molecular and cellular mechanisms. Key factors that may play a role in gastrulation cell movements are cell adhesion and cytoskeletal rearrangement. In addition, some of the driving force for gastrulation may derive from tissue interactions such as those described between the enveloping layer and the yolk syncytial layer. Future exploration of gastrulation mechanisms relies on the development of sensitive and quantitative techniques to characterize embryonic germ-layer properties.

Understanding pathogenic mechanisms in polycystic kidney disease provides clues for therapy

PURPOSE OF REVIEW

Polycystic kidney diseases are a group of inherited disorders that result in tubular dilatation and/or the development of fluid-filled cysts in the kidney. Identification and analysis of the primary defective protein in many of these diseases are providing insights into a common pathogenesis to polycystic kidney disease. This review explores this pathogenesis and determines the role that this knowledge is playing in the development of potential therapies.

RECENT FINDINGS

Study of simple and syndromic forms of polycystic kidney disease has revealed that the defective proteins are localized to the primary cilia/basal body and that the pleiotropic phenotypes are often associated with defective ciliogenesis. Data indicating that the polycystins are involved in ciliary mechanosensation, and cellular changes in intracellular Ca and cAMP, have provided clues for possible therapeutic approaches that have proved highly effective in pre-clinical trials.

SUMMARY

Polycystic kidney diseases are associated with defects to proteins involved in developing functional, sensory cilia in the kidney. While the primary defects in these disorders cannot be corrected at present, downsteam cellular changes can be targeted. Potential therapeutic agents are now being tested in patients, moving polycystic kidney disease research into a new and exciting phase.

Planar cell polarity, ciliogenesis and neural tube defects

Cilia are microtubule-based protrusions that are found on the surface of most vertebrate cells. Long studied by cell biologists, these organelles have recently caught the attention of developmental biologists and human geneticists. In this review, I will discuss recent findings suggesting a link between cilia and the planar cell polarity signaling cascade. In particular, I will focus on how this interaction may influence the process of neural tube closure and how these results may be relevant to our understanding of common human birth defects in which neural tube closure is compromised.