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

What are those cilia doing in the neural tube?

Primary cilia are present on almost all vertebrate cells, and they have diverse functions in distinct tissues. Cilia are important for sensation in multiple capacities in contexts as different as the retina, kidney, and inner ear. In addition to these roles, cilia play a critical part in various developmental processes. Of particular importance is the development of the neural tube, where cilia are essential for the transduction of the Sonic Hedgehog (Shh) signaling pathway that specifies neuronal cell fates. This relationship is well established and is the most recognizable function for cilia in the neural tube, but it may be part of a larger picture. Here, we discuss the links between cilia and Shh signaling, as well as suggesting additional roles for cilia, and mechanisms for their placement, in the neural tube.

Alternative Wnt pathways and receptors

In addition to activating β-catenin/TCF transcriptional complexes, Wnt proteins can elicit a variety of other responses. These are often lumped together under the denominator "alternative" or "non-canonical" Wnt signaling, but they likely comprise distinct signaling events. In this article, I discuss how the use of different ligand and receptor combinations is thought to give rise to these alternative Wnt-signaling responses. Although many of the biochemical details remain to be resolved, it is evident that alternative Wnt signaling plays important roles in regulating tissue morphogenesis during embryonic development.

Planar cell polarity effector gene Intu regulates cell fate-specific differentiation of keratinocytes through the primary cilia

Genes involved in the planar cell polarity (PCP) signaling pathway are essential for a number of developmental processes in mammals, such as convergent extension and ciliogenesis. Tissue-specific PCP effector genes of the PCP signaling pathway are believed to mediate PCP signals in a tissue- and cell type-specific manner. However, how PCP signaling controls the morphogenesis of mammalian tissues remains unclear. In this study, we investigated the role of inturned (Intu), a tissue-specific PCP effector gene, during hair follicle formation in mice. Tissue-specific disruption of Intu in embryonic epidermis resulted in hair follicle morphogenesis arrest because of the failure of follicular keratinocyte to differentiate. Targeting Intu in the epidermis resulted in almost complete loss of primary cilia in epidermal and follicular keratinocytes, and a suppressed hedgehog signaling pathway. Surprisingly, the epidermal stratification and differentiation programs and barrier function were not affected. These results demonstrate that tissue-specific PCP effector genes of the PCP signaling pathway control the differentiation of keratinocytes through the primary cilia in a cell fate- and context-dependent manner, which may be critical in orchestrating the propagation and interpretation of polarity signals established by the core PCP components.

Planar cell polarity and the developmental control of cell behavior in vertebrate embryos

Planar cell polarity (PCP), the orientation and alignment of cells within a sheet, is a ubiquitous cellular property that is commonly governed by the conserved set of proteins encoded by so-called PCP genes. The PCP proteins coordinate developmental signaling cues with individual cell behaviors in a wildly diverse array of tissues. Consequently, disruptions of PCP protein functions are linked to defects in axis elongation, inner ear patterning, neural tube closure, directed ciliary beating, and left/right patterning, to name only a few. This review attempts to synthesize what is known about PCP and the PCP proteins in vertebrate animals, with a particular focus on the mechanisms by which individual cells respond to PCP cues in order to execute specific cellular behaviors.

A novel mutation causing nephronophthisis in the Lewis polycystic kidney rat localises to a conserved RCC1 domain in Nek8

Background

Nephronophthisis (NPHP) as a cause of cystic kidney disease is the most common genetic cause of progressive renal failure in children and young adults. NPHP is characterized by abnormal and/or loss of function of proteins associated with primary cilia. Previously, we characterized an autosomal recessive phenotype of cystic kidney disease in the Lewis Polycystic Kidney (LPK) rat.

Results

In this study, quantitative trait locus analysis was used to define a ~1.6Mbp region on rat chromosome 10q25 harbouring the lpk mutation. Targeted genome capture and next-generation sequencing of this region identified a non-synonymous mutation R650C in the NIMA (never in mitosis gene a)- related kinase 8 ( Nek8) gene. This is a novel Nek8 mutation that occurs within the regulator of chromosome condensation 1 (RCC1)-like region of the protein. Specifically, the R650C substitution is located within a G[QRC]LG repeat motif of the predicted seven bladed beta-propeller structure of the RCC1 domain. The rat Nek8 gene is located in a region syntenic to portions of human chromosome 17 and mouse 11. Scanning electron microscopy confirmed abnormally long cilia on LPK kidney epithelial cells, and fluorescence immunohistochemistry for Nek8 protein revealed altered cilia localisation.

Conclusions

When assessed relative to other Nek8 NPHP mutations, our results indicate the whole propeller structure of the RCC1 domain is important, as the different mutations cause comparable phenotypes. This study establishes the LPK rat as a novel model system for NPHP and further consolidates the link between cystic kidney disease and cilia proteins.

The abluminal endothelial membrane in neurovascular remodeling in health and disease

After brain injury, blood-brain barrier (BBB) integrity can be compromised as a consequence of the breakdown of cell-cell interactions in the neurovascular unit, resulting in the loss of the characteristic endothelial luminal-to-abluminal structure. During the process of restoration of the BBB and vascularization, the endothelial cells are continuously reshaped, with both the luminal and abluminal membranes serving as sites of signaling. Here, we focus on the bidirectional signaling processes that are rapidly initiated between endothelial and perivascular cells and occur in certain brain diseases or in response to injury. The goal of these processes is (i) the reemergence of endothelial cell polarity, (ii) the remodeling of extracellular matrix interactions, (iii) the realignment of pericytes and astrocytic endfeet with endothelial cells, and (iv) the restitution of a well-organized and stable BBB. This abluminal membrane exemplifies how the brain vasculature responds to stressors and may represent promising targets for therapeutic interventions of brain diseases.

Ciliary and non-ciliary expression and function of PACRG during vertebrate development

Background

Park2-co-regulated gene (PACRG) is evolutionarily highly conserved from green algae to mammals. In Chlamydomonas and trypanosomes, the PACRG protein associates with flagella. Loss of PACRG results in shortened or absent flagella. In mouse the PACRG protein is required for spermatogenesis. The purpose of the present study was to analyze (1) the expression patterns of PACRG during vertebrate embryogenesis, and (2) whether the PACRG protein was required for left-right (LR) axis specification through cilia-driven leftward flow in Xenopus laevis.

Methods

PACRG cDNAs were cloned and expression was analyzed during early embryonic development of Xenopus, mouse, rabbit and zebrafish. Antisense morpholino oligonucleotide (MO) mediated gene knockdown was applied in Xenopus to investigate LR development at the level of tissue morphology, leftward flow and asymmetric marker gene expression, using timelapse videography, scanning electron microscopy (SEM) and whole-mount in situ hybridization. Results were statistically evaluated using Wilcoxon paired and χ² tests.

Results

PACRG mRNA expression was found in cells and tissues harboring cilia throughout the vertebrates. Highly localized expression was also detected in the brain. During early development, PACRG was specifically localized to epithelia where leftward flow arises, that is, the gastrocoel roof plate (GRP) in Xenopus, the posterior notochord (PNC) in mammals and Kupffer’s vesicle (KV) in zebrafish. Besides its association with ciliary axonemes, subcellular localization of PACRG protein was found around the nucleus and in a spotty pattern in the cytoplasm. A green fluorescent protein (GFP) fusion construct preferentially labeled cilia, rendering PACRG a versatile marker for live imaging. Loss-of-function in the frog resulted dose dependently in LR, neural tube closure and gastrulation defects, representing ciliary and non-ciliary functions of PACRG.

Conclusions

The PACRG protein is a novel essential factor of cilia in Xenopus.

The evolution of land plant cilia

Eukaryotic cilia/flagella are ancient organelles with motility and sensory functions. Cilia display significant ultrastructural conservation where present across the eukaryotic phylogeny; however, diversity in ciliary biology exists and the ability to produce cilia has been lost independently on a number of occasions. Land plants provide an excellent system for the investigation of cilia evolution and loss across a broad phylogeny, because early divergent land plant lineages produce cilia, whereas most seed plants do not. This review highlights the differences in cilia form and function across land plants and discusses how recent advances in genomics are providing novel insights into the evolutionary trajectory of ciliary proteins. We propose a renewed effort to adopt ciliated land plants as models to investigate the mechanisms underpinning complex ciliary processes, such as number control, the coordination of basal body placement and the regulation of beat patterns.

Cilia, Wnt signaling, and the cytoskeleton

Primary cilia have recently been highlighted as key regulators in development and disease. This review focuses on current work demonstrating the broad role of cilia-related proteins in developmental signaling systems. Of particular consideration is the importance of the basal body region, located at the base of the cilium, in its role as a focal point for many signaling pathways and as a microtubule organizing center. As the cilium is effectively a microtubular extension of the cytoskeleton, investigating connections between the cilium and the cytoskeleton provides greater insight into signaling and cell function. Of the many signaling pathways associated with primary cilia, the most extensively studied in association with the cytoskeleton and cytoskeletal rearrangements are both canonical and non-canonical Wnt pathways. One of the key concepts currently emerging is a possible additional role for the traditionally 'cilia-related' proteins in other aspects of cellular processes. In many cases, disruption of such processes manifests at the level of the cilium. While the involvement of cilia and cilia-related proteins in signaling pathways is currently being unraveled, there is a growing body of evidence to support the notion that ciliary proteins are required not only for regulation of Wnt signaling, but also as downstream effectors of Wnt signaling. This review summarizes recent advances in our understanding of the involvement of cilia and basal body proteins in Wnt signaling pathways.

Primary cilia and graded Sonic Hedgehog signaling

Cilia are evolutionary-conserved microtubule-containing organelles protruding from the surface of cells. They are classified into two types--primary and motile cilia. Primary cilia are nearly ubiquitous, at least in vertebrate cells, and it has become apparent that they play an essential role in the intracellular transduction of a range of stimuli. Most notable among these is Sonic Hedgehog. In this article we briefly summarize the structure and biogenesis of primary cilia. We discuss the evidence implicating cilia in the transduction of extrinsic signals. We focus on the involvement and molecular mechanism of cilia in signaling by Sonic Hedgehog in embryonic tissues, specifically the neural tube, and we discuss how cilia play an active role in the interpretation of gradients of Sonic Hedgehog (Shh) signaling.

Coordinated ciliary beating requires Odf2-mediated polarization of basal bodies via basal feet

Coordinated beating of cilia in the trachea generates a directional flow of mucus required to clear the airways. Each cilium originates from a barrel-shaped basal body, from the side of which protrudes a structure known as the basal foot. We generated mice in which exons 6 and 7 of Odf2, encoding a basal body and centrosome-associated protein Odf2/cenexin, are disrupted. Although Odf2(ΔEx6,7/ΔEx6,7) mice form cilia, ciliary beating is uncoordinated, and the mice display a coughing/sneezing phenotype. Whereas residual expression of the C-terminal region of Odf2 in these mice is sufficient for ciliogenesis, the resulting basal bodies lack basal feet. Loss of basal feet in ciliated epithelia disrupted the polarized organization of apical microtubule lattice without affecting planar cell polarity. The requirement for Odf2 in basal foot formation, therefore, reveals a crucial role of this structure in the polarized alignment of basal bodies and coordinated ciliary beating.

Patterning skin by planar cell polarity: the multi-talented hair designer

In mammals, the skin can form complex global and local patterns to meet diverse functional requirements in different parts of the body. To date, the fundamental principles that underlie skin patterning remain poorly understood because of the involvement of multiple interacting processes. Genes involved in the planar cell polarity (PCP) signalling pathway, which is capable of polarizing cells within the planar plane of an epithelium, can control the orientation and differentiation of hair follicles, underlining their involvement in skin pattern formation. Here, we summarize recent progress that has been made to understand the PCP signalling pathway and its function in mammalian skin, including its role in hair follicle morphogenesis, ciliogenesis and wound healing. We argue that dissecting PCP signalling in the context of hair follicle formation might reveal many as-yet-undiscovered functions for PCP in the development, homeostasis and regeneration of skin.

An essential and highly conserved role for Zic3 in left-right patterning, gastrulation and convergent extension morphogenesis

Mutations in ZIC3 result in X-linked heterotaxy in humans, a syndrome consisting of left-right (L-R) patterning defects, midline abnormalities, and cardiac malformations. Similarly, loss of function of Zic3 in mouse results in abnormal L-R patterning and cardiac development. However, Zic3 null mice also exhibit defects in gastrulation, neural tube closure, and axial patterning, suggesting the hypothesis that Zic3 is necessary for proper convergent extension (C-E) morphogenesis. To further investigate the role of Zic3 in early embryonic development, we utilized two model systems, Xenopus laevis and zebrafish, and performed loss of function analysis using antisense morpholino-mediated gene knockdown. Both Xenopus and zebrafish demonstrated significant impairment of C-E in Zic3 morphants. L-R patterning was also disrupted, indicating that the role of Zic3 in L-R axis development is conserved across species. Correlation of L-R patterning and C-E defects in Xenopus suggests that early C-E defects may underlie L-R patterning defects at later stages, since Zic3 morphants with moderate to severe C-E defects exhibited an increase in laterality defects. Taken together, these results demonstrate a functional conservation of Zic3 in L-R patterning and uncover a previously unrecognized role for Zic3 in C-E morphogenesis during early vertebrate development.

Celsr1-3 cadherins in PCP and brain development

Cadherin EGF LAG seven-pass G-type receptors 1, 2, and 3 (Celsr1-3) form a family of three atypical cadherins with multiple functions in epithelia and in the nervous system. During the past decade, evidence has accumulated for important and distinct roles of Celsr1-3 in planar cell polarity (PCP) and brain development and maintenance. Although the role of Celsr in PCP is conserved from flies to mammals, other functions may be more distantly related, with Celsr working only with one or a subset of the classical PCP partners. Here, we review the literature on Celsr in PCP and neural development, point to several remaining questions, and consider future challenges and possible research trends.

Regulation of intraflagellar transport and ciliogenesis by small G proteins

Cilia rely on their distinctive protein compositions to function. Proteins gain access to the privileged ciliary compartment through two major routes, membrane trafficking and intraflagellar transport (IFT). Recent advances have provided two possible models for ciliary membrane transport: lateral diffusion and retention, and targeted vesicle transport. The Rab11-Rab8 cascade, which was originally discovered in the yeast's secretion pathway for bud formation, is shown to be required for cilia membrane assembly. Small GTPases, including two IFT particle subunits, and Ran, the master regulator for nuclear-cytoplasmic transport, are implicated in various aspects of IFT, a fundamental process required for the assembly of the microtubule-based backbone of cilia. This chapter reviews the key steps of ciliogenesis and possible mechanisms of IFT regulation, with emphasis on the regulatory roles of small GTPases and their regulators.

Planar cell polarity in coordinated and directed movements

Planar cell polarity is a fundamental concept to understanding the coordination of cell movements in the plane of a tissue. Since the planar cell polarity pathway was discovered in mesenchymal tissues involving cell interaction during vertebrate gastrulation, there is an emerging evidence that a variety of mesenchymal and epithelial cells utilize this genetic pathway to mediate the coordination of cells in directed movements. In this review, we focus on how the planar cell polarity pathway is mediated by migrating cells to communicate with one another in different developmental processes.

An expanding role of Vangl proteins in embryonic development

The mammalian Vangl1 and Vangl2 genes were discovered a decade ago through their association with neural tube defects, in particular the presence of Vangl2 mutations in independent alleles of the mouse mutant Loop-tail (Lp), a mouse model of the severe neural tube defect craniorachischisis. Vangl1 and Vangl2 variants have also been detected in familial and sporadic cases of spina bifida. Vangl proteins are highly conserved in evolution with relatives in flies, and distant invertebrates and vertebrates. In these organisms, they play a central role in planar cell polarity (PCP) and convergent extension (CE) movements. Over the past decade, these functional characteristics have also been established for mammalian Vangl genes. The careful analysis of mouse Vangl genes mutants has showed that these genes and the associated PCP pathway and CE movements are involved in many unexpected developmental processes, from morphogenesis of different tissues, left-right asymmetry, asymmetric cell division, and organization of many epithelial structures, as well as positioning and function of cellular appendages. Genetic studies in double mutants and biochemical studies of interacting proteins have started to elucidate the molecular pathways in which Vangl proteins participate and that regulate these complex events.

Analysis of PCP defects in mammalian eye lens

Multicellular tissues and organs often show planar cell polarity (PCP) where the constituent cells align along an axis to form coordinated patterns. Mammalian eye lenses are mainly comprised of epithelial-derived fibre cells, which exhibit highly ordered alignment that is regulated by PCP signaling. Each fibre cell has an apically situated primary cilium and in most cases this is polarized towards the lens anterior pole. Here we describe how to visualize the global cellular alignment of lens fibre cells by examining the suture pattern that is formed by the tips of fibres meeting at the anterior pole. We also describe a method for whole mount preparation, which allows observation of the polarized distribution of primary cilia at the apical surface of lens fibres. Given its relative simplicity, at least in cellular terms, and its requirement for a high degree of precision in cellular alignment and orientation, we predict that the lens will be an excellent model system to help elucidate the role of cilia and PCP components in the development of three-dimensional organization in tissues and organs.

Understanding ciliated epithelia: the power of Xenopus

Ciliated epithelia are important in a wide variety of biological contexts where they generate directed fluid flow. Here we address the fundamental advances in understanding ciliated epithelia that have been achieved using Xenopus as a model system. Xenopus embryos are covered with a ciliated epithelium that propels fluid unidirectionally across their surface. The external nature of this tissue, coupled with the molecular tools available in Xenopus and the ease of microscopic analysis on intact animals has thrust Xenopus to the forefront of ciliated epithelia biology. We discuss advances in understanding the molecular regulators of ciliated epithelia cell fate as well as basic aspects of ciliated epithelia cell biology including ciliogenesis and cell polarity.

Wnt/β-catenin signaling directly regulates Foxj1 expression and ciliogenesis in zebrafish Kupffer's vesicle

Cilia are essential for normal development. The composition and assembly of cilia has been well characterized, but the signaling and transcriptional pathways that govern ciliogenesis remain poorly studied. Here, we report that Wnt/β-catenin signaling directly regulates ciliogenic transcription factor foxj1a expression and ciliogenesis in zebrafish Kupffer's vesicle (KV). We show that Wnt signaling acts temporally and KV cell-autonomously to control left-right (LR) axis determination and ciliogenesis. Specifically, reduction of Wnt signaling leads to a disruption of LR patterning, shorter and fewer cilia, a loss of cilia motility and a downregulation of foxj1a expression. However, these phenotypes can be rescued by KV-targeted overexpression of foxj1a. In comparison to the FGF pathway that has been previously implicated in the control of ciliogenesis, our epistatic studies suggest a more downstream function of Wnt signaling in the regulation of foxj1a expression and ciliogenesis in KV. Importantly, enhancer analysis reveals that KV-specific expression of foxj1a requires the presence of putative Lef1/Tcf binding sites, indicating that Wnt signaling activates foxj1a transcription directly. We also find that impaired Wnt signaling leads to kidney cysts and otolith disorganization, which can be attributed to a loss of foxj1 expression and disrupted ciliogenesis in the developing pronephric ducts and otic vesicles. Together, our data reveal a novel role of Wnt/β-catenin signaling upstream of ciliogenesis, which might be a general developmental mechanism beyond KV. Moreover, our results also prompt a hypothesis that certain developmental effects of the Wnt/β-catenin pathway are due to the activation of Foxj1 and cilia formation.

Stages of ciliogenesis and regulation of ciliary length

Cilia and flagella are highly conserved eukaryotic microtubule-based organelles that protrude from the surface of most mammalian cells. These structures require large protein complexes and motors for distal addition of tubulin and extension of the ciliary membrane. In order for ciliogenesis to occur, coordination of many processes must take place. An intricate concert of cell cycle regulation, vesicular trafficking, and ciliary extension must all play out with accurate timing to produce a cilium. Here, we review the stages of ciliogenesis as well as regulation of the length of the assembled cilium. Regulation of ciliogenesis during cell cycle progression centers on centrioles, from which cilia extend upon maturation into basal bodies. Centriole maturation involves a shift from roles in cell division to cilium nucleation via migration to the cell surface and docking at the plasma membrane. Docking is dependent on a variety of proteinaceous structures, termed distal appendages, acquired by the mother centriole. Ciliary elongation by the process of intraflagellar transport (IFT) ensues. Direct modification of ciliary structures, as well as modulation of signal transduction pathways, play a role in maintenance of the cilium. All of these stages are tightly regulated to produce a cilium of the right size at the right time. Finally, we discuss the implications of abnormal ciliogenesis and ciliary length control in human disease as well as some open questions.

PCP effector proteins inturned and fuzzy play nonredundant roles in the patterning but not convergent extension of mammalian neural tube

PCP effector proteins Inturned (Intu) and Fuzzy (Fuz) play important roles in mammalian neural development and ciliogenesis, but the developmental defects in Intu and Fuz mutants are not the same as those with the complete loss of cilia. Furthermore, it remains unclear whether mouse Intu and Fuz play a role in convergent extension, a process regulated by PCP signaling. In the current study, we show that the functions of both Intu and Fuz in neural tube patterning are dependent on the presence of cilia. We further show that neither gene exhibits obvious genetic interaction with the core PCP regulator Vangl2 in convergent extension or patterning of the neural tube. Finally, we show in Intu; Fuz double mutants that the lack of convergent extension and more severe patterning defects in Intu and Fuz mutants does not result from a functional redundancy between these two proteins.

TMEM237 is mutated in individuals with a Joubert syndrome related disorder and expands the role of the TMEM family at the ciliary transition zone

Joubert syndrome related disorders (JSRDs) have broad but variable phenotypic overlap with other ciliopathies. The molecular etiology of this overlap is unclear but probably arises from disrupting common functional module components within primary cilia. To identify additional module elements associated with JSRDs, we performed homozygosity mapping followed by next-generation sequencing (NGS) and uncovered mutations in TMEM237 (previously known as ALS2CR4). We show that loss of the mammalian TMEM237, which localizes to the ciliary transition zone (TZ), results in defective ciliogenesis and deregulation of Wnt signaling. Furthermore, disruption of Danio rerio (zebrafish) tmem237 expression produces gastrulation defects consistent with ciliary dysfunction, and Caenorhabditis elegans jbts-14 genetically interacts with nphp-4, encoding another TZ protein, to control basal body-TZ anchoring to the membrane and ciliogenesis. Both mammalian and C. elegans TMEM237/JBTS-14 require RPGRIP1L/MKS5 for proper TZ localization, and we demonstrate additional functional interactions between C. elegans JBTS-14 and MKS-2/TMEM216, MKSR-1/B9D1, and MKSR-2/B9D2. Collectively, our findings integrate TMEM237/JBTS-14 in a complex interaction network of TZ-associated proteins and reveal a growing contribution of a TZ functional module to the spectrum of ciliopathy phenotypes.

Cilia and Hedgehog: when and how was their marriage solemnized?

Primary cilia are essential for Hedgehog (Hh) signaling in mammals, and this requirement appears to be conserved in other vertebrates as well. Here, I review recent work that has scrutinized the evolution of the link between the Hh pathway and cilia, discuss what we have learnt from these studies and speculate on how this fascinating problem can be further explored.

A meckelin-filamin A interaction mediates ciliogenesis

MKS3, encoding the transmembrane receptor meckelin, is mutated in Meckel-Gruber syndrome (MKS), an autosomal-recessive ciliopathy. Meckelin localizes to the primary cilium, basal body and elsewhere within the cell. Here, we found that the cytoplasmic domain of meckelin directly interacts with the actin-binding protein filamin A, potentially at the apical cell surface associated with the basal body. Mutations in FLNA, the gene for filamin A, cause periventricular heterotopias. We identified a single consanguineous patient with an MKS-like ciliopathy that presented with both MKS and cerebellar heterotopia, caused by an unusual in-frame deletion mutation in the meckelin C-terminus at the region of interaction with filamin A. We modelled this mutation and found it to abrogate the meckelin-filamin A interaction. Furthermore, we found that loss of filamin A by siRNA knockdown, in patient cells, and in tissues from Flna(Dilp2) null mouse embryos results in cellular phenotypes identical to those caused by meckelin loss, namely basal body positioning and ciliogenesis defects. In addition, morpholino knockdown of flna in zebrafish embryos significantly increases the frequency of dysmorphology and severity of ciliopathy developmental defects caused by mks3 knockdown. Our results suggest that meckelin forms a functional complex with filamin A that is disrupted in MKS and causes defects in neuronal migration and Wnt signalling. Furthermore, filamin A has a crucial role in the normal processes of ciliogenesis and basal body positioning. Concurrent with these processes, the meckelin-filamin A signalling axis may be a key regulator in maintaining correct, normal levels of Wnt signalling.

Cilium, centrosome and cell cycle regulation in polycystic kidney disease

Polycystic kidney disease is the defining condition of a group of common life-threatening genetic disorders characterized by the bilateral formation and progressive expansion of renal cysts that lead to end stage kidney disease. Although a large body of information has been acquired in the past years about the cellular functions that characterize the cystic cells, the mechanisms triggering the cystogenic conversion are just starting to emerge. Recent findings link defects in ciliary functions, planar cell polarity pathway, and centrosome integrity in early cystic development. Many of the signals dysregulated during cystogenesis may converge on the centrosome for its central function as a structural support for cilia formation and a coordinator of protein trafficking, polarity, and cell division. Here, we will discuss the contribution of proliferation, cilium and planar cell polarity to the cystic signal and will analyze in particular the possible role that the basal bodies/centrosome may play in the cystogenetic mechanisms. This article is part of a Special Issue entitled: Polycystic Kidney Disease.

The primary cilium as a biomarker in the hypoxic adaptation of bone marrow-derived mesenchymal stromal cells: a role for the secreted frizzled-related proteins

A pivotal role in guiding mesenchymal stem cell (MSC) differentiation has recently been attributed to the primary cilium. This solitary, non-motile microtubule-based organelle emerging from the cell surface acts as a sensorial membrane structure reflecting developmental and adaptive processes associated with pathologies including human cystic kidney disease, skeletal malformations, obesity and cancer. Given that the intrinsic hypoxic adaptation of MSC remains poorly understood within ischemic tissues or hypoxic tumours, we questioned whether the hypoxia inducible factor-1α (HIF-1α) might be a downstream effector regulating cilium maintenance. We show that murine bone marrow-derived MSC cultured under hypoxic conditions (1.2% O(2)) lose their primary cilia in a time-dependent manner. Gene silencing of HIF-1α prevented cilia loss in hypoxic cultures, and generation of MSC expressing a constitutively active HIF-1α (MSC-HIF) was found to decrease primary cilium formation. A Wnt pathway-related gene expression array was also performed on MSC-HIF and indicated that the secreted Frizzled-related proteins (sFRP)-1, -3 and -4 were down-regulated, while sFRP-2 was up-regulated. Overexpression of recombinant sFRP-2 or gene silencing of sFRP-1, -3 and -4 in MSC led to primary cilium disruption. These results indicate a molecular signalling mechanism for the hypoxic disruption of the primary cilium in MSC involving an HIF-1α/sFRP axis. This mechanism contributes to our understanding of the adaptive processes possibly involved in the oncogenic transformation and tumour-supporting potential of MSC. Our current observations also open up the possibility for the primary cilia to serve as a biomarker in MSC adaptation to low oxygen tension within (patho)physiological microenvironments.

Effect of localization, length and orientation of chondrocytic primary cilium on murine growth plate organization

The research investigates the role of the immotile chondrocytic primary cilium in the growth plate. This study was motivated by (i) the recent evidence of the mechano-sensorial function of the primary cilium in kidney tubule epithelial cells and (ii) the distinct three-dimensional orientation patterns that the chondrocytic primary cilium forms in articular cartilage in the presence or the absence of loading. For our investigation, we used the Smad1/5(CKO) mutant mouse, whose disorganized growth plate is due to the conditional deletion of Smad 1 and 5 proteins that also affect the so-called Indian Hedgehog pathway, whose physical and functional topography has been shown to be partially controlled by the primary cilium. Fluorescence and confocal microscopy on stained sections visualized ciliated chondrocytes. Morphometric data regarding position, orientation and eccentricity of chondrocytes, and ciliary localization on cell membrane, length and orientation, were collected and reconstructed from images. We established that both localization and orientation of the cilium are definite, and differently so, in the Smad1/5(CKO) and control mice. The orientation of the primary cilium, relative to the major axis of the chondrocyte, clusters at 80° with respect to the anterior-posterior direction for the Smad1/5(CKO) mice, showing loss of the additional clustering present in the control mice at 10°. We therefore hypothesized that the clustering at 10° contains information of columnar organization. To test our hypothesis, we prepared a mathematical model of relative positioning of the proliferative chondrocytic population based on ciliary orientation. Our model belongs to the category of "interactive particle system models for self-organization with birth". The model qualitatively reproduced the experimentally observed chondrocytic arrangements in growth plate of each of the Smad1/5(CKO) and control mice. Our mathematically predicted cell division process will need to be observed experimentally to advance the identification of ciliary function in the growth plate.

Fuz regulates craniofacial development through tissue specific responses to signaling factors

The planar cell polarity effector gene Fuz regulates ciliogenesis and Fuz loss of function studies reveal an array of embryonic phenotypes. However, cilia defects can affect many signaling pathways and, in humans, cilia defects underlie several craniofacial anomalies. To address this, we analyzed the craniofacial phenotype and signaling responses of the Fuz^−/− mice. We demonstrate a unique role for Fuz in regulating both Hedgehog (Hh) and Wnt/β-catenin signaling during craniofacial development. Fuz expression first appears in the dorsal tissues and later in ventral tissues and craniofacial regions during embryonic development coincident with cilia development. The Fuz^−/− mice exhibit severe craniofacial deformities including anophthalmia, agenesis of the tongue and incisors, a hypoplastic mandible, cleft palate, ossification/skeletal defects and hyperplastic malformed Meckel's cartilage. Hh signaling is down-regulated in the Fuz null mice, while canonical Wnt signaling is up-regulated revealing the antagonistic relationship of these two pathways. Meckel's cartilage is expanded in the Fuz^−/− mice due to increased cell proliferation associated with the up-regulation of Wnt canonical target genes and decreased non-canonical pathway genes. Interestingly, cilia development was decreased in the mandible mesenchyme of Fuz null mice, suggesting that cilia may antagonize Wnt signaling in this tissue. Furthermore, expression of Fuz decreased expression of Wnt pathway genes as well as a Wnt-dependent reporter. Finally, chromatin IP experiments demonstrate that β-catenin/TCF-binding directly regulates Fuz expression. These data demonstrate a new model for coordination of Hh and Wnt signaling and reveal a Fuz-dependent negative feedback loop controlling Wnt/β-catenin signaling.

Roles of planar cell polarity pathways in the development of neural [correction of neutral] tube defects

Neural tube defects (NTDs) are the second most common birth defect in humans. Despite many advances in the understanding of NTDs and the identification of many genes related to NTDs, the fundamental etiology for the majority of cases of NTDs remains unclear. Planar cell polarity (PCP) signaling pathway, which is important for polarized cell movement (such as cell migration) and organ morphogenesis through the activation of cytoskeletal pathways, has been shown to play multiple roles during neural tube closure. The disrupted function of PCP pathway is connected with some NTDs. Here, we summarize our current understanding of how PCP factors affect the pathogenesis of NTDs.

Dissecting the PCP pathway: one or more pathways?: Does a separate Wnt-Fz-Rho pathway drive morphogenesis?

Planar cell polarity (PCP), the alignment of cells within 2D tissue planes, involves a set of core molecular regulators highly conserved between animals and cell types. These include the transmembrane proteins Frizzled (Fz) and VanGogh and the cytoplasmic regulators Dishevelled (Dsh) and Prickle. It is widely accepted that this core forms part of a 'PCP pathway' for signal transduction, which can affect cell morphology through activation of an evolutionary ancient regulatory module involving Rho family GTPases and Myosin II, and/or the JNK kinase cascade. We have re-examined the evidence for interactions between the proposed PCP pathway components, and question the placing of the cell morphology regulators in the same pathway as the PCP core. While Fz and Dsh are clearly involved in both PCP and Rho-based cell morphology regulation, available evidence cannot currently discriminate whether these processes are linked mechanistically by a shared Fz/Dsh population, or pass by two distinct pathways.

Mutations in the planar cell polarity gene, Fuzzy, are associated with neural tube defects in humans

Neural tube defects (NTDs) are a heterogeneous group of common severe congenital anomalies which affect 1-2 infants per 1000 births. Most genetic and/or environmental factors that contribute to the pathogenesis of human NTDs are unknown. Recently, however, pathogenic mutations of VANGL1 and VANGL2 genes have been associated with some cases of human NTDs. Vangl genes encode proteins of the planar cell polarity (PCP) pathway that regulates cell behavior during early stages of neural tube formation. Homozygous disruption of PCP genes in mice results in a spectrum of NTDs, including defects that affect the entire neural axis (craniorachischisis), cranial NTDs (exencephaly) and spina bifida. In this paper, we report the dynamic expression of another PCP gene, Fuzzy, during neural tube formation in mice. We also identify non-synonymous Fuzzy amino acid substitutions in some patients with NTDs and demonstrate that several of these Fuzzy mutations affect formation of primary cilia and ciliary length or affect directional cell movement. Since Fuzzy knockout mice exhibit both NTDs and defective primary cilia and Fuzzy is expressed in the emerging neural tube, we propose that mutations in Fuzzy may account for a subset of NTDs in humans.

Regulation of basal body and ciliary functions by Diversin

The centrosome is essential for the formation of the cilia and has been implicated in cell polarization and signaling during early embryonic development. A number of Wnt pathway components were found to localize at the centrosome, but how this localization relates to their signaling functions is unclear. In this study, we assessed a role for Diversin, a putative Wnt pathway mediator, in developmental processes that involve cilia. We find that Diversin is specifically localized to the basal body compartment near the base of the cilium in Xenopus multi-ciliated skin cells. Overexpression of Diversin RNA disrupted basal body polarization in these cells, suggesting that tightly regulated control of Diversin levels is crucial for this process. In cells depleted of endogenous Diversin, basal body structure appeared abnormal and this was accompanied by disrupted polarity, shortened or absent cilia and defective ciliary flow. These results are consistent with the involvement of Diversin in processes that are related to the acquisition of cell polarity and require ciliary functions.

Planar cell polarity: coordinating morphogenetic cell behaviors with embryonic polarity

Planar cell polarization entails establishment of cellular asymmetries within the tissue plane. An evolutionarily conserved planar cell polarity (PCP) signaling system employs intra- and intercellular feedback interactions between its core components, including Frizzled, Van Gogh, Flamingo, Prickle, and Dishevelled, to establish their characteristic asymmetric intracellular distributions and coordinate planar polarity of cell populations. By translating global patterning information into asymmetries of cell membranes and intracellular organelles, PCP signaling coordinates morphogenetic behaviors of individual cells and cell populations with the embryonic polarity. In vertebrates, by polarizing cilia in the node/Kupffer's vesicle, PCP signaling links the anteroposterior to left-right embryonic polarity.

Planar cell polarity in Drosophila

In all multicellular organisms, epithelial cells are not only polarized along the apical-basal axis, but also within the epithelial plane, giving cells a sense of direction. Planar cell polarity (PCP) signaling regulates establishment of polarity within the plane of an epithelium. The outcomes of PCP signaling are diverse and include the determination of cell fates, the generation of asymmetric but highly aligned structures, such as the stereocilia in the human inner ear or the hairs on a fly wing, or the directional migration of cells during convergence and extension during vertebrate gastrulation. In humans, aberrant PCP signaling can result in severe developmental defects, such as open neural tubes (spina bifida), and can cause cystic kidneys. In this review, we discuss the basic mechanism and more recent findings of PCP signaling focusing on Drosophila melanogaster, the model organism in which most key PCP components were initially identified.

Planar cell polarity in the mammalian eye lens

The major role of the eye lens is to transmit and focus images onto the retina. For this function, the lens needs to develop and maintain the correct shape, notably, the precise curvature and high-level order and organization of its elements. The lens is mainly comprised of highly elongated fiber cells with hexagonal cross-sectional profiles that facilitate regular packing. Collectively, they form concentrically arranged layers around the anterior-posterior polar axis, and their convex curvature contributes to the spheroidal shape of the lens. Although the lens has been a popular system for developmental studies, little is known about the mechanism(s) that underlies the development of its exquisite three-dimensional cellular architecture. In this review, we will describe our recent work, which shows how planar cell polarity (PCP) operates in lens and contributes to its morphogenesis. We believe that the lens will be a useful model system to study PCP in general and gain insights into mechanisms that generate high-level cellular order during development.

The planar cell polarity pathway in vertebrate epidermal development, homeostasis and repair

The planar cell polarity (PCP) pathway plays a critical role in diverse developmental processes that require coordinated cellular movement, including neural tube closure and renal tubulogenesis. Recent studies have demonstrated that this pathway also has emerging relevance to the epidermis, as PCP signaling underpins many aspects of skin biology and pathology, including epidermal development, hair orientation, stem cell division and cancer. Coordinated cellular movement required for epidermal repair in mammals is also regulated by PCP signaling, and in this context, a new PCP gene encoding the developmental transcription factor Grainyhead-like 3 (Grhl3) is critical. This review focuses on the role that PCP signaling plays in the skin across a variety of epidermal functions and highlights perturbations that induce epidermal pathologies.

Principles of planar polarity in animal development

Planar polarity describes the coordinated polarisation of cells or structures in the plane of a tissue. The patterning mechanisms that underlie planar polarity are well characterised in Drosophila, where many events are regulated by two pathways: the 'core' planar polarity complex and the Fat/Dachsous system. Components of both pathways also function in vertebrates and are implicated in diverse morphogenetic processes, some of which self-evidently involve planar polarisation and some of which do not. Here, we review the molecular mechanisms and cellular consequences of planar polarisation in diverse contexts, seeking to identify the common principles across the animal kingdom.

Pointing in the right direction: new developments in the field of planar cell polarity

Planar cell polarity (PCP) is observed in an array of developmental processes that involve collective cell movement and tissue organization, and its disruption can lead to severe developmental defects. Recent studies in flies and vertebrates have identified new functions for PCP as well as new signalling components, and have proposed new mechanistic models. However, despite this progress, the search to simplify principles of understanding continues and important mechanistic uncertainties still pose formidable challenges.

Cilia in the nervous system: linking cilia function and neurodevelopmental disorders

PURPOSE OF REVIEW

Ciliopathies are genetic disorders caused by defects of primary ciliary structure and/or function and are characterized by pleiotropic clinical features. The ciliopathies include several partially overlapping syndromes such as Joubert syndrome, Bardet-Biedl syndrome and Meckel-Gruber syndrome, all of which have pronounced neurodevelopmental features. Here we focus on potential roles of cilia in central nervous system function, to explore how impairments may cause brain malformation and neurodevelopmental disease.

RECENT FINDINGS

Cilia have long been considered as 'sensory cellular antennae', responding as chemo-sensors, mechano-sensors and thermo-sensors, although their roles in development were not well understood until recently. The surprising finding that disparate syndromes are all due to defects of the primary cilia, along with the recent advances in genetics, has helped elucidate further roles of primary cilia beyond sensory functions. Several molecules that are associated with key signaling pathways have been discovered in primary cilia. These include sonic hedgehog, wingless, planar cell polarity and fibroblast growth factor, which are essential for many cellular processes. Additionally, mutations in 'ciliome' genes have largely shown developmental defects such as abnormal body axis and brain malformation, implying disrupted cilia-related signaling pathways. Accordingly, the emerging theme is that primary cilia may play roles as modulators of signal transduction to help shape cellular responses within the environmental context during both development and homeostasis.

SUMMARY

The link between cilia and signal pathways has become a framework for understanding the pathogenesis of ciliopathies. Despite recent progress in ciliary biology, fundamental questions remain about how cilia regulate neuronal function in the central nervous system. Therefore, investigation of ciliary function in the nervous system may reveal cilia-modulating mechanisms in neurodevelopmental processes, as well as suggest new treatments for disease.

Strange as it may seem: the many links between Wnt signaling, planar cell polarity, and cilia

Cilia are important cellular structures that have been implicated in a variety of signaling cascades. In this review, we discuss the current evidence for and against a link between cilia and both the canonical Wnt/β-catenin pathway and the noncanonical Wnt/planar cell polarity (PCP) pathway. Furthermore, we address the evidence implicating a role for PCP components in ciliogenesis. Given the lack of consensus in the field, we use new data on the control of ciliary protein localization as a basis for proposing new models by which cell type-specific regulation of ciliary components via differential transport, regulated entry and exit, or diffusion barriers might generate context-dependent functions for cilia.

Cell shape and contractility regulate ciliogenesis in cell cycle-arrested cells

Adhesive micropatterns show the effect of spatial confinement and actin network architecture on basal body positioning and primary cilium formation.

In most lineages, cell cycle exit is correlated with the growth of a primary cilium. We analyzed cell cycle exit and ciliogenesis in human retinal cells and found that, contrary to the classical view, not all cells exiting the cell division cycle generate a primary cilium. Using adhesive micropatterns to control individual cell spreading, we demonstrate that cell spatial confinement is a major regulator of ciliogenesis. When spatially confined, cells assemble a contractile actin network along their ventral surface and a protrusive network along their dorsal surface. The nucleus–centrosome axis in confined cells is oriented toward the dorsal surface where the primary cilium is formed. In contrast, highly spread cells assemble mostly contractile actin bundles. The nucleus–centrosome axis of spread cells is oriented toward the ventral surface, where contractility prevented primary cilium growth. These results indicate that cell geometrical confinement affects cell polarity via the modulation of actin network architecture and thereby regulates basal body positioning and primary cilium growth.

The actin nucleator Cordon-bleu is required for development of motile cilia in zebrafish

The cordon-bleu (Cobl) gene is widely conserved in vertebrates, with developmentally regulated axial and epithelial expression in mouse and chick embryos. In vitro, Cobl can bind monomeric actin and nucleate formation of unbranched actin filaments, while in cultured cells it can modulate the actin cytoskeleton. However, an essential role for Cobl in vivo has yet to be determined. We have used zebrafish as a model to assess the requirements for Cobl in embryogenesis. We find that cobl shows enriched expression in ciliated epithelial tissues during zebrafish organogenesis. Cobl protein is enriched in the apical domain of ciliated cells, in close proximity to the apical actin cap. Reduction of Cobl by antisense morpholinos reveals an essential role in development of motile cilia in organs such as Kupffer's vesicle and the pronephros. In Kupffer's vesicle, the reduction in Cobl coincides with a reduction in the amount of apical F-actin. Thus, Cobl represents a molecular activity that couples developmental patterning signals with local intracellular cytoskeletal dynamics to support morphogenesis of motile cilia.

A role for central spindle proteins in cilia structure and function

Cytokinesis and ciliogenesis are fundamental cellular processes that require strict coordination of microtubule organization and directed membrane trafficking. These processes have been intensely studied, but there has been little indication that regulatory machinery might be extensively shared between them. Here, we show that several central spindle/midbody proteins (PRC1, MKLP-1, INCENP, centriolin) also localize in specific patterns at the basal body complex in vertebrate ciliated epithelial cells. Moreover, bioinformatic comparisons of midbody and cilia proteomes reveal a highly significant degree of overlap. Finally, we used temperature-sensitive alleles of PRC1/spd-1 and MKLP-1/zen-4 in C. elegans to assess ciliary functions while bypassing these proteins' early role in cell division. These mutants displayed defects in both cilia function and cilia morphology. Together, these data suggest the conserved reuse of a surprisingly large number of proteins in the cytokinetic apparatus and in cilia.

Building the centriole

Centrioles are conserved microtubule-based organelles that lie at the core of the animal centrosome and play a crucial role in nucleating the formation of cilia and flagella in most eukaryotes. Centrioles have a complex ultrastructure with ninefold symmetry and a well-defined length. This structure is assembled from a host of proteins, including a variety of disease gene products. Over a century after the discovery of centrioles, the mechanisms underlying the assembly of these fascinating organelles, in particular the establishment of ninefold symmetry and the control of centriole length, are now starting to be uncovered.

Fuz controls the morphogenesis and differentiation of hair follicles through the formation of primary cilia

Planar cell polarity (PCP) signaling is essential in determining the polarity of cells within the plane of an epithelial sheet. Core PCP genes have been recently shown to control the global polarization of hair follicles in mice. Fuz, a homologue of the Drosophila PCP effector gene, fuzzy, is critical in ciliogenesis in vertebrates, and is required for the development of a wide range of organs in mice. Here, we report that disruption of the Fuz gene in mice severely blocked the development of hair follicles in the skin. In contrast to the loss of hair follicle polarization in mice deficient in core PCP genes, hair follicles in mice lacking the Fuz gene retained their typical anterior-posterior orientation. We show that disruption of Fuz impaired the formation of primary cilia and the hedgehog signaling pathway in the skin. In addition, using skin grafts and skin reconstitution assays we demonstrate that the expression of Fuz is required in both epidermal and dermal cells and that the formation of primary cilia is a cell-autonomous process that does not require cross talk between the epithelia and mesenchymal compartments during hair follicle formation.

Common themes in centriole and centrosome movements

Centrioles are found in nearly all eukaryotic cells and are required for growth and maintenance of the radial array of microtubules, the mitotic spindle, and cilia and flagella. Different types of microtubule structures are often required at different places in a given cell; centrioles must move around to nucleate these varied structures. Here, we draw together recent data on diverse centriole movements to decipher common themes in how centrioles move. Par proteins establish and maintain the required cellular asymmetry. The actin cytoskeleton facilitates movement of multiple basal bodies. Microtubule forces acting on the cell cortex, and nuclear-cytoskeletal links, are important for positioning individual centrosomes, and during cell division. Knowledge of these common mechanisms can inform the study of centriole movements across biology.

Planar cell polarity signaling, cilia and polarized ciliary beating

Planar cell polarity signaling governs a wide array of polarized cell behaviors in animals. Recent reports now show that PCP signaling is essential for the directional beating of motile cilia. Interestingly, PCP signaling acts in a variety of ciliated cell types that use motile cilia to generate directional fluid flow in very different ways. This review will synthesize these recent papers and place them in context with previous studies of PCP signaling in polarized cellular morphogenesis and collective cell movement.