Regulation of ciliary polarity by the APC/C

Systems genetics analysis of human body fat distribution genes identifies adipocyte processes

Excess abdominal fat is a sexually dimorphic risk factor for cardio-metabolic disease and is approximated by the waist-to-hip ratio adjusted for body mass index (WHRadjBMI). Whereas this trait is highly heritable, few causal genes are known. We aimed to identify novel drivers of WHRadjBMI using systems genetics. We used two independent cohorts of adipose tissue gene expression and constructed sex- and depot-specific Bayesian networks to model gene-gene interactions from 8,492 genes. Using key driver analysis, we identified genes that, in silico and putatively in vitro, regulate many others. 51-119 key drivers in each network were replicated in both cohorts. In other cell types, 23 of these genes are found in crucial adipocyte pathways: Wnt signaling or mitochondrial function. We overexpressed or down-regulated seven key driver genes in human subcutaneous pre-adipocytes. Key driver genes ANAPC2 and RSPO1 inhibited adipogenesis, whereas PSME3 increased adipogenesis. RSPO1 increased Wnt signaling activity. In differentiated adipocytes, MIGA1 and UBR1 down-regulation led to mitochondrial dysfunction. These five genes regulate adipocyte function, and we hypothesize that they regulate fat distribution.

Identification of the principal neuropeptide MIP and its action pathway in larval settlement of the echiuran worm Urechis unicinctus

BACKGROUND

Larval settlement and metamorphosis represent critical events in the life history of marine benthic animals. Myoinhibitory peptide (MIP) plays a pivotal role in larval settlement of marine invertebrates. However, the molecular mechanisms of MIP involved in this process are not well understood.

RESULTS

In this study, we evaluated the effects of thirteen MIP mature peptides on triggering the larval settlement of Urechis unicinctus (Xenopneusta, Urechidae), and determined that MIP2 was the principal neuropeptide. Transcriptomic analysis was employed to identify differentially expressed genes (DEGs) between the MIP2-treated larvae and normal early-segmentation larvae. Both cAMP and calcium signaling pathways were enriched in the DEGs of the MIP2-treated larvae, and two neuropeptide receptor genes (Spr, Fmrfar) were up-regulated in the MIP2-treated larvae. The activation of the SPR-cAMP pathway by MIP2 was experimentally validated in HEK293T cells. Furthermore, fourteen cilia-related genes, including Tctex1d2, Cfap45, Ift43, Ift74, Ift22, Cav1 and Mns1, etc. exhibited down-regulated expression in the MIP2-treated larvae. Whole-mount in situ hybridization identified two selected ciliary genes, Tctex1d2 and Cfap45, were specially expressed in circumoral ciliary cells of the early-segmentation larvae. Knocking down Tctex1d2 mRNA levels by in vivo RNA interference significantly increased the larval settlement rate.

CONCLUSION

Our findings suggest that MIP2 inhibits the function of the cilia-related genes, such as Tctex1d2, through the SPR-cAMP-PKA pathway, thereby inducing larval settlement in U. unicinctus. The study contributes important data to the understanding of neuropeptide regulation in larval settlement.

Systems genetics analysis of human body fat distribution genes identifies Wnt signaling and mitochondrial activity in adipocytes

BACKGROUND

Excess fat in the abdomen is a sexually dimorphic risk factor for cardio-metabolic disease. The relative storage between abdominal and lower-body subcutaneous adipose tissue depots is approximated by the waist-to-hip ratio adjusted for body mass index (WHRadjBMI). Genome-wide association studies (GWAS) identified 346 loci near 495 genes associated with WHRadjBMI. Most of these genes have unknown roles in fat distribution, but many are expressed and putatively act in adipose tissue. We aimed to identify novel sex- and depot-specific drivers of WHRadjBMI using a systems genetics approach.

METHODS

We used two independent cohorts of adipose tissue gene expression with 362 - 444 males and 147 - 219 females, primarily of European ancestry. We constructed sex- and depot- specific Bayesian networks to model the gene-gene interactions from 8,492 adipose tissue genes. Key driver analysis identified genes that, in silico and putatively in vitro, regulate many others, including the 495 WHRadjBMI GWAS genes. Key driver gene function was determined by perturbing their expression in human subcutaneous pre-adipocytes using lenti-virus or siRNA.

RESULTS

51 - 119 key drivers in each network were replicated in both cohorts. We used single-cell expression data to select replicated key drivers expressed in adipocyte precursors and mature adipocytes, prioritized genes which have not been previously studied in adipose tissue, and used public human and mouse data to nominate 53 novel key driver genes (10 - 21 from each network) that may regulate fat distribution by altering adipocyte function. In other cell types, 23 of these genes are found in crucial adipocyte pathways: Wnt signaling or mitochondrial function. We selected seven genes whose expression is highly correlated with WHRadjBMI to further study their effects on adipogenesis/Wnt signaling (ANAPC2, PSME3, RSPO1, TYRO3) or mitochondrial function (C1QTNF3, MIGA1, PSME3, UBR1).Adipogenesis was inhibited in cells overexpressing ANAPC2 and RSPO1 compared to controls. RSPO1 results are consistent with a positive correlation between gene expression in the subcutaneous depot and WHRadjBMI, therefore lower relative storage in the subcutaneous depot. RSPO1 inhibited adipogenesis by increasing β-catenin activation and Wnt-related transcription, thus repressing PPARG and CEBPA. PSME3 overexpression led to more adipogenesis than controls. In differentiated adipocytes, MIGA1 and UBR1 downregulation led to mitochondrial dysfunction, with lower oxygen consumption than controls; MIGA1 knockdown also lowered UCP1 expression.

SUMMARY

ANAPC2, MIGA1, PSME3, RSPO1, and UBR1 affect adipocyte function and may drive body fat distribution.

Proteolytic control in ciliogenesis: Temporal restriction or early initiation?

Cellular processes are highly dependent on a dynamic proteome that undergoes structural and functional rearrangements to allow swift conversion between different cellular states. By inducing proteasomal degradation of inhibitory or stimulating factors, ubiquitylation is particularly well suited to trigger such transitions. One prominent example is the remodelling of the centrosome upon cell cycle exit, which is required for the formation of primary cilia - antenna-like structures on the surface of most cells that act as integrative hubs for various extracellular signals. Over the last decade, many reports on ubiquitin-related events involved in the regulation of ciliogenesis have emerged. Very often, these processes are considered to be initiated ad hoc, that is, directly before its effect on cilia biogenesis becomes evident. While such a temporal restriction may hold true for the majority of events, there is evidence that some of them are initiated earlier during the cell cycle. Here, we provide an overview of ubiquitin-dependent processes in ciliogenesis and discuss available data that indicate such an early onset of proteolytic regulation within preceding cell cycle stages.

Regulation of Dishevelled protein activity and stability by post-translational modifications and autophagy

As a key component of Wnt signaling, Dishevelled (Dvl/Dsh) plays essential roles in development processes and adult tissue homeostasis in multicellular organisms, and its deregulation results in human development disorders and other diseases. Dvl integrates and relays complex Wnt signals by acting as a branch-point of β-catenin-dependent canonical and β-catenin-independent noncanonical pathways. It dynamically interacts with multiple proteins to modulate Wnt signaling, while its activity and stability are tightly controlled by other proteins. This Review summarizes the current understanding of regulation of Dvl activity, localization, and stability by post-translational modifications, aggregation, and autophagy, and the impacts on Dvl function in both Wnt signaling and biological processes.

Moonlighting of mitotic regulators in cilium disassembly

Correct timing of cellular processes is essential during embryological development and to maintain the balance between healthy proliferation and tumour formation. Assembly and disassembly of the primary cilium, the cell’s sensory signalling organelle, are linked to cell cycle timing in the same manner as spindle pole assembly and chromosome segregation. Mitotic processes, ciliary assembly, and ciliary disassembly depend on the centrioles as microtubule-organizing centres (MTOC) to regulate polymerizing and depolymerizing microtubules. Subsequently, other functional protein modules are gathered to potentiate specific protein–protein interactions. In this review, we show that a significant subset of key mitotic regulator proteins is moonlighting at the cilium, among which PLK1, AURKA, CDC20, and their regulators. Although ciliary assembly defects are linked to a variety of ciliopathies, ciliary disassembly defects are more often linked to brain development and tumour formation. Acquiring a better understanding of the overlap in regulators of ciliary disassembly and mitosis is essential in finding therapeutic targets for the different diseases and types of tumours associated with these regulators.

Expression Pattern of α-Tubulin, Inversin and Its Target Dishevelled-1 and Morphology of Primary Cilia in Normal Human Kidney Development and Diseases

The spatiotemporal expression of α-tubulin, inversin and dishevelled-1 (DVL-1) proteins associated with the Wnt-signaling pathway, and primary cilia morphology were analyzed in developing kidneys (14th–38th developmental weeks), healthy postnatal (1.5- and 7-years old) and pathologically changed human kidneys, including multicystic dysplastic kidneys (MCDK), focal segmental glomerulosclerosis (FSGS) and nephrotic syndrome of the Finnish type (CNF). The analysis was performed by double immunofluorescence, electron microscopy, semiquantitative and statistical methods. Cytoplasmic co-expression of α-tubulin, inversin and DVL-1 was observed in the proximal convoluted tubules (pct), distal convoluted tubules (dct) and glomeruli (g) of analyzed tissues. During kidney development, the overall expression of α-tubulin, inversin and DVL-1 decreased, while in the postnatal period slightly increased. The highest expressions of α-tubulin and inversin characterized dct and g, while high DVL-1 characterized pct. α-tubulin, inversin and DVL-1 expression pattern in MCDK, FSGS and CNF kidneys significantly differed from the healthy control. Compared to healthy kidneys, pathologically changed kidneys had dysmorphic primary cilia. Different expression dynamics of α-tubulin, inversin and DVL-1 during kidney development could indicate that switch between the canonical and noncanonical Wnt-signaling is essential for normal kidney morphogenesis. In contrast, their disturbed expression in pathological kidneys might be associated with abnormal primary cilia, leading to chronic kidney diseases.

HECT (Homologous to the E6-AP Carboxyl Terminus)-Type Ubiquitin E3 Ligase ITCH Attenuates Cardiac Hypertrophy by Suppressing the Wnt/β-Catenin Signaling Pathway

The HECT (homologous to the E6-AP carboxyl terminus)-type ubiquitin E3 ligase ITCH is an enzyme that plays an important role in ubiquitin-proteasomal protein degradation. Disheveled proteins (Dvl1 [disheveled protein 1], Dvl2, and Dvl3) are the main components of the Wnt/β-catenin signaling pathway, which is involved in cardiac hypertrophy. The aim of this study was to examine the role of ITCH during development of cardiac hypertrophy. Thoracic transverse aortic constriction (TAC) was performed in transgenic mice with cardiac-specific overexpression of ITCH (ITCH-Tg) and wild-type mice. Cardiac hypertrophy after TAC was attenuated in ITCH-Tg mice, and the survival rate was higher for ITCH-Tg mice than for wild-type mice. Protein interaction between ITCH and Dvls was confirmed with immunoprecipitation in vivo and in vitro. Expression of key molecules of the Wnt/β-catenin signaling pathway (Dvl1, Dvl2, GSK3β [glycogen synthase kinase 3β], and β-catenin) was inhibited in ITCH-Tg mice compared with wild-type mice. Notably, the ubiquitination level of Dvl proteins increased in ITCH-Tg mice. Protein and mRNA expression levels of ITCH increased in response to Wnt3a stimulation in neonatal rat cardiomyocytes. Knockdown of ITCH using small-interfering RNA increased cardiomyocyte size and augmented protein expression levels of Dvl proteins, phospho-GSK3β, and β-catenin after Wnt3a stimulation in cardiomyocytes. Conversely, overexpression of ITCH attenuated cardiomyocyte hypertrophy and decreased protein expression levels of Dvl proteins, phospho-GSK3β and β-catenin. In conclusion, ITCH targets Dvl proteins for ubiquitin-proteasome degradation in cardiomyocytes and attenuates cardiac hypertrophy by suppressing the Wnt/β-catenin signaling pathway.

Emerging roles of metazoan cell cycle regulators as coordinators of the cell cycle and differentiation

In multicellular organisms, cell proliferation must be tightly coordinated with other developmental processes to form functional tissues and organs. Despite significant advances in our understanding of how the cell cycle is controlled by conserved cell-cycle regulators (CCRs), how the cell cycle is coordinated with cell differentiation in metazoan organisms and how CCRs contribute to this process remain poorly understood. Here, we review the emerging roles of metazoan CCRs as intracellular proliferation-differentiation coordinators in multicellular organisms. We illustrate how major CCRs regulate cellular events that are required for cell fate acquisition and subsequent differentiation. To this end, CCRs employ diverse mechanisms, some of which are separable from those underpinning the conventional cell-cycle-regulatory functions of CCRs. By controlling cell-type-specific specification/differentiation processes alongside the progression of the cell cycle, CCRs enable spatiotemporal coupling between differentiation and cell proliferation in various developmental contexts in vivo. We discuss the significance and implications of this underappreciated role of metazoan CCRs for development, disease and evolution.

Developmentally regulated GTP-binding protein 1 modulates ciliogenesis via an interaction with Dishevelled

Our study reveals Drg1 as a new binding partner of Dishevelled. The Drg1–Dishevelled association regulates Daam1 and RhoA interactions and activity, leading to polymerization and stability of the actin cytoskeleton, a process that is essential for proper multiciliation.

Cilia are critical for proper embryonic development and maintaining homeostasis. Although extensively studied, there are still significant gaps regarding the proteins involved in regulating ciliogenesis. Using the Xenopus laevis embryo, we show that Dishevelled (Dvl), a key Wnt signaling scaffold that is critical to proper ciliogenesis, interacts with Drg1 (developmentally regulated GTP-binding protein 1). The loss of Drg1 or disruption of the interaction with Dvl reduces the length and number of cilia and displays defects in basal body migration and docking to the apical surface of multiciliated cells (MCCs). Moreover, Drg1 morphants display abnormal rotational polarity of basal bodies and a decrease in apical actin and RhoA activity that can be attributed to disruption of the protein complex between Dvl and Daam1, as well as between Daam1 and RhoA. These results support the concept that the Drg1–Dvl interaction regulates apical actin polymerization and stability in MCCs. Thus, Drg1 is a newly identified partner of Dvl in regulating ciliogenesis.

PLEKHA4/kramer Attenuates Dishevelled Ubiquitination to Modulate Wnt and Planar Cell Polarity Signaling

Wnt signaling pathways direct key physiological decisions in development. Here, we establish a role for a pleckstrin homology domain-containing protein, PLEKHA4, as a modulator of signaling strength in Wnt-receiving cells. PLEKHA4 oligomerizes into clusters at PI(4,5)P2-rich regions of the plasma membrane and recruits the Cullin-3 (CUL3) E3 ubiquitin ligase substrate adaptor Kelch-like protein 12 (KLHL12) to these assemblies. This recruitment decreases CUL3-KLHL12-mediated polyubiquitination of Dishevelled, a central intermediate in canonical and non-canonical Wnt signaling. Knockdown of PLEKHA4 in mammalian cells demonstrates that PLEKHA4 positively regulates canonical and non-canonical Wnt signaling via these effects on the Dishevelled polyubiquitination machinery. In vivo knockout of the Drosophila melanogaster PLEKHA4 homolog, kramer, selectively affects the non-canonical, planar cell polarity (PCP) signaling pathway. We propose that PLEKHA4 tunes the sensitivities of cells toward the stimulation of Wnt or PCP signaling by sequestering a key E3 ligase adaptor controlling Dishevelled polyubiquitination within PI(4,5)P2-rich plasma membrane clusters.

Primary Cilia-An Underexplored Topic in Major Mental Illness

Though much progress has been made in recent years towards understanding the function and physiology of primary cilia, they remain a somewhat elusive organelle. Some studies have explored the role of primary cilia in the developing nervous system, and their dysfunction has been linked with several neurosensory deficits. Yet, very little has been written on their potential role in psychiatric disorders. This article provides an overview of some of the functions of primary cilia in signalling pathways, and demonstrates that they are a worthy candidate in psychiatric research. The links between primary cilia and major mental illness have been demonstrated to exist at several levels, spanning genetics, signalling pathways, and pharmacology as well as cell division and migration. The primary focus of this review is on the sensory role of the primary cilium and the neurodevelopmental hypothesis of psychiatric disease. As such, the primary cilium is demonstrated to be a key link between the cellular environment and cell behaviour, and hence of key importance in the considerations of the nature and nurture debate in psychiatric research.

March2 is required for head formation by mediating Dishevelled degradation in concert with Dapper1

Dishevelled (Dvl/Dsh) is a key scaffold protein that propagates Wnt signaling essential for embryogenesis and homeostasis. However, whether antagonism of Wnt signaling necessary for vertebrate head formation can be achieved through regulation of Dsh protein stability is unclear. Here we show that membrane-associated RING-CH2 (March2), a RING-type E3 ubiquitin ligase, antagonizes Wnt signaling by regulating the turnover of Dsh protein via ubiquitin-mediated lysosomal degradation in prospective head region of Xenopus. We further found that March2 acquires regional and functional specificities for head formation from the Dsh-interacting protein Dapper1 (Dpr1). Dpr1 stabilizes interaction between March2 and Dsh for mediating ubiquitination and subsequent degradation of Dsh protein only in the dorso-animal region of Xenopus embryo. These results suggest that March2 restricts cytosolic pools of Dsh protein and leads to subsequent limitation of Wnt signaling for the precise vertebrate head development.

The connections of Wnt pathway components with cell cycle and centrosome: side effects or a hidden logic?

Wnt signaling cascade has developed together with multicellularity to orchestrate the development and homeostasis of complex structures. Wnt pathway components - such as β-catenin, Dishevelled (DVL), Lrp6, and Axin-- are often dedicated proteins that emerged in evolution together with the Wnt signaling cascade and are believed to function primarily in the Wnt cascade. It is interesting to see that in recent literature many of these proteins are connected with cellular functions that are more ancient and not limited to multicellular organisms - such as cell cycle regulation, centrosome biology, or cell division. In this review, we summarize the recent literature describing this crosstalk. Specifically, we attempt to find the answers to the following questions: Is the response to Wnt ligands regulated by the cell cycle? Is the centrosome and/or cilium required to activate the Wnt pathway? How do Wnt pathway components regulate the centrosomal cycle and cilia formation and function? We critically review the evidence that describes how these connections are regulated and how they help to integrate cell-to-cell communication with the cell and the centrosomal cycle in order to achieve a fine-tuned, physiological response.

Multiciliated Cells in Animals

Many animal cells assemble single cilia involved in motile and/or sensory functions. In contrast, multiciliated cells (MCCs) assemble up to 300 motile cilia that beat in a coordinate fashion to generate a directional fluid flow. In the human airways, the brain, and the oviduct, MCCs allow mucus clearance, cerebrospinal fluid circulation, and egg transportation, respectively. Impairment of MCC function leads to chronic respiratory infections and increased risks of hydrocephalus and female infertility. MCC differentiation during development or repair involves the activation of a regulatory cascade triggered by the inhibition of Notch activity in MCC progenitors. The downstream events include the simultaneous assembly of a large number of basal bodies (BBs)-from which cilia are nucleated-in the cytoplasm of the differentiating MCCs, their migration and docking at the plasma membrane associated to an important remodeling of the actin cytoskeleton, and the assembly and polarization of motile cilia. The direction of ciliary beating is coordinated both within cells and at the tissue level by a combination of planar polarity cues affecting BB position and hydrodynamic forces that are both generated and sensed by the cilia. Herein, we review the mechanisms controlling the specification and differentiation of MCCs and BB assembly and organization at the apical surface, as well as ciliary assembly and coordination in MCCs.

Ciliogenesis and the DNA damage response: a stressful relationship

Both inherited and sporadic mutations can give rise to a plethora of human diseases. Through myriad diverse cellular processes, sporadic mutations can arise through a failure to accurately replicate the genetic code or by inaccurate separation of duplicated chromosomes into daughter cells. The human genome has therefore evolved to encode a large number of proteins that work together with regulators of the cell cycle to ensure that it remains error-free. This is collectively known as the DNA damage response (DDR), and genome stability mechanisms involve a complex network of signalling and processing factors that ensure redundancy and adaptability of these systems. The importance of genome stability mechanisms is best illustrated by the dramatic increased risk of cancer in individuals with underlying disruption to genome maintenance mechanisms. Cilia are microtubule-based sensory organelles present on most vertebrate cells, where they facilitate transduction of external signals into the cell. When not embedded within the specialised ciliary membrane, components of the primary cilium's basal body help form the microtubule organising centre that controls cellular trafficking and the mitotic segregation of chromosomes. Ciliopathies are a collection of diseases associated with functional disruption to cilia function through a variety of different mechanisms. Ciliopathy phenotypes can vary widely, and although some cellular overgrowth phenotypes are prevalent in a subset of ciliopathies, an increased risk of cancer is not noted as a clinical feature. However, recent studies have identified surprising genetic and functional links between cilia-associated proteins and genome maintenance factors. The purpose of this mini-review is to therefore highlight some of these discoveries and discuss their implications with regards to functional crosstalk between the DDR and ciliogenesis pathways, and how this may impact on the development of human disease.

The polarity protein Inturned links NPHP4 to Daam1 to control the subapical actin network in multiciliated cells

Inturned-mediated complex formation of NPHP4 and DAAM1 is important for ciliogenesis and ciliary function in multiciliated cells, presumably because of its requirement for the local rearrangement of actin cytoskeleton.

Motile cilia polarization requires intracellular anchorage to the cytoskeleton; however, the molecular machinery that supports this process remains elusive. We report that Inturned plays a central role in coordinating the interaction between cilia-associated proteins and actin-nucleation factors. We observed that knockdown of nphp4 in multiciliated cells of the Xenopus laevis epidermis compromised ciliogenesis and directional fluid flow. Depletion of nphp4 disrupted the subapical actin layer. Comparison to the structural defects caused by inturned depletion revealed striking similarities. Furthermore, coimmunoprecipitation assays demonstrated that the two proteins interact with each other and that Inturned mediates the formation of ternary protein complexes between NPHP4 and DAAM1. Knockdown of daam1, but not formin-2, resulted in similar disruption of the subapical actin web, whereas nphp4 depletion prevented the association of Inturned with the basal bodies. Thus, Inturned appears to function as an adaptor protein that couples cilia-associated molecules to actin-modifying proteins to rearrange the local actin cytoskeleton.

Genome-wide screen identifies novel machineries required for both ciliogenesis and cell cycle arrest upon serum starvation

Biogenesis of the primary cilium, a cellular organelle mediating various signaling pathways, is generally coordinated with cell cycle exit/re-entry. Although the dynamic cell cycle-associated profile of the primary cilium has been largely accepted, the mechanism governing the link between ciliogenesis and cell cycle progression has been poorly understood. Using a human genome-wide RNAi screen, we identify genes encoding subunits of the spliceosome and proteasome as novel regulators of ciliogenesis. We demonstrate that 1) the mRNA processing-related hits are essential for RNA expression of molecules acting in cilia disassembly, such as AURKA and PLK1, and 2) the ubiquitin-proteasome systems (UPS)-involved hits are necessary for proteolysis of molecules acting in cilia assembly, such as IFT88 and CPAP. In particular, we show that these screen hit-associated mechanisms are crucial for both cilia assembly and cell cycle arrest in response to serum withdrawal. Finally, our data suggest that the mRNA processing mechanism may modulate the UPS-dependent decay of cilia assembly regulators to control ciliary resorption-coupled cell cycle re-entry.

Current topics of functional links between primary cilia and cell cycle

Primary cilia, microtubule-based sensory structures, orchestrate various critical signals during development and tissue homeostasis. In view of the rising interest into the reciprocal link between ciliogenesis and cell cycle, we discuss here several recent advances to understand the molecular link between the individual step of ciliogenesis and cell cycle control. At the onset of ciliogenesis (the transition from centrosome to basal body), distal appendage proteins have been established as components indispensable for the docking of vesicles at the mother centriole. In the initial step of axonemal extension, CP110, Ofd1, and trichoplein, key negative regulators of ciliogenesis, are found to be removed by a kinase-dependent mechanism, autophagy, and ubiquitin–proteasome system, respectively. Of note, their disposal functions as a restriction point to decide that the axonemal nucleation and extension begin. In the elongation step, Nde1, a negative regulator of ciliary length, is revealed to be ubiquitylated and degraded by CDK5-SCF^Fbw7 in a cell cycle-dependent manner. With regard to ciliary length control, it has been uncovered in flagellar shortening of Chlamydomonas that cilia itself transmit a ciliary length signal to cytoplasm. At the ciliary resorption step upon cell cycle re-entry, cilia are found to be disassembled not only by Aurora A-HDAC6 pathway but also by Nek2-Kif24 and Plk1-Kif2A pathways through their microtubule-depolymerizing activity. On the other hand, it is becoming evident that the presence of primary cilia itself functions as a structural checkpoint for cell cycle re-entry. These data suggest that ciliogenesis and cell cycle intimately link each other, and further elucidation of these mechanisms will contribute to understanding the pathology of cilia-related disease including cancer and discovering targets of therapeutic interventions.

Disinhibition of the HECT E3 ubiquitin ligase WWP2 by polymerized Dishevelled

Dishevelled is a pivot in Wnt signal transduction, controlling both β-catenin-dependent transcription to specify proliferative cell fates, and cell polarity and other non-nuclear events in post-mitotic cells. In response to Wnt signals, or when present at high levels, Dishevelled forms signalosomes by dynamic polymerization. Its levels are controlled by ubiquitylation, mediated by various ubiquitin ligases, including NEDD4 family members that bind to a conserved PPxY motif in Dishevelled (mammalian Dvl1-3). Here, we show that Dvl2 binds to the ubiquitin ligase WWP2 and unlocks its ligase activity from autoinhibition. This disinhibition of WWP2 depends on several features of Dvl2 including its PPxY motif and to a lesser extent its DEP domain, but crucially on the ability of Dvl2 to polymerize, indicating that WWP2 is activated in Wnt signalosomes. We show that Notch intracellular domains are substrates for Dvl-activated WWP2 and their transcriptional activity is consequently reduced, providing a molecular mechanism for cross-talk between Wnt and Notch signalling. These regulatory interactions are conserved in Drosophila whose WWP2 orthologue, Suppressor-of-deltex, downregulates Notch signalling upon activation by Dishevelled in developing wing tissue. Attentuation of Notch signalling by Dishevelled signalosomes could be important during the transition of cells from the proliferative to the post-mitotic state.

A human interactome in three quantitative dimensions organized by stoichiometries and abundances

The organization of a cell emerges from the interactions in protein networks. The interactome is critically dependent on the strengths of interactions and the cellular abundances of the connected proteins, both of which span orders of magnitude. However, these aspects have not yet been analyzed globally. Here, we have generated a library of HeLa cell lines expressing 1,125 GFP-tagged proteins under near-endogenous control, which we used as input for a next-generation interaction survey. Using quantitative proteomics, we detect specific interactions, estimate interaction stoichiometries, and measure cellular abundances of interacting proteins. These three quantitative dimensions reveal that the protein network is dominated by weak, substoichiometric interactions that play a pivotal role in defining network topology. The minority of stable complexes can be identified by their unique stoichiometry signature. This study provides a rich interaction dataset connecting thousands of proteins and introduces a framework for quantitative network analysis.

New frontiers: discovering cilia-independent functions of cilia proteins

In most vertebrates, mitotic spindles and primary cilia arise from a common origin, the centrosome. In non-cycling cells, the centrosome is the template for primary cilia assembly and, thus, is crucial for their associated sensory and signaling functions. During mitosis, the duplicated centrosomes mature into spindle poles, which orchestrate mitotic spindle assembly, chromosome segregation, and orientation of the cell division axis. Intriguingly, both cilia and spindle poles are centrosome-based, functionally distinct structures that require the action of microtubule-mediated, motor-driven transport for their assembly. Cilia proteins have been found at non-cilia sites, where they have distinct functions, illustrating a diverse and growing list of cellular processes and structures that utilize cilia proteins for crucial functions. In this review, we discuss cilia-independent functions of cilia proteins and re-evaluate their potential contributions to "cilia" disorders.

The cellular story of dishevelleds

Dishevelled (DVL) proteins, three of which have been identified in humans, are highly conserved components of canonical and noncanonical Wnt signaling pathways. These multifunctional proteins, originally discovered in the fruit fly, through their different domains mediate complex signal transduction: DIX (dishevelled, axin) and PDZ (postsynaptic density 95, discs large, zonula occludens-1) domains serve for canonical beta-catenin signaling, while PDZ and DEP (dishevelled, Egl-10, pleckstrin) domains serve for non-canonical signaling. In canonical or beta-catenin signaling, DVL forms large molecular supercomplexes at the plasma membrane consisting of Wnt-Fz-LRP5/6-DVL-AXIN. This promotes the disassembly of the beta-catenin destruction machinery, beta-catenin accumulation, and consequent activation of Wnt signaling. Therefore, DVLs are considered to be key regulators that rescue cytoplasmic beta-catenin from degradation. The potential medical importance of DVLs is in both human degenerative disease and cancer. The overexpression of DVL has been shown to potentiate the activation of Wnt signaling and it is now apparent that up-regulation of DVLs is involved in several types of cancer.

The Microtubule-Depolymerizing Activity of a Mitotic Kinesin Protein KIF2A Drives Primary Cilia Disassembly Coupled with Cell Proliferation

The primary cilium is an antenna-like, microtubule-based organelle on the surface of most vertebrate cells for receiving extracellular information. Although primary cilia form in the quiescent phase, ciliary disassembly occurs when quiescent cells re-enter the proliferative phase. It was shown that a mitotic kinase, Polo-like kinase 1 (PLK1), is required for cell-proliferation-coupled primary cilia disassembly. Here, we report that kinesin superfamily protein 2A (KIF2A), phosphorylated at T554 by PLK1, exhibits microtubule-depolymerizing activity at the mother centriole to disassemble the primary cilium in a growth-signal-dependent manner. KIF2A-deficient hTERT-RPE1 cells showed the impairment of primary cilia disassembly following growth stimulation. It was also found that the PLK1-KIF2A pathway is constitutively active in cells from patients with premature chromatid separation (PCS) syndrome and is responsible for defective ciliogenesis in this syndrome. These findings provide insights into the roles of the PLK1-KIF2A pathway in physiological cilia disassembly and cilia-associated disorders.

The master cell cycle regulator APC-Cdc20 regulates ciliary length and disassembly of the primary cilium

The primary cilium has an important role in signaling; defects in structure are associated with a variety of human diseases. Much of the most basic biology of this organelle is poorly understood, even basic mechanisms, such as control of growth and resorption. We show that the activity of the anaphase-promoting complex (APC), an E3 that regulates the onset of anaphase, destabilizes axonemal microtubules in the primary cilium. Furthermore, the metaphase APC co-activator, Cdc20, is specifically recruited to the basal body of primary cilia. Inhibition of APC-Cdc20 activity increases the ciliary length, while overexpression of Cdc20 suppresses cilium formation. APC-Cdc20 activity is required for the timely resorption of the cilium after serum stimulation. In addition, APC regulates the stability of axonemal microtubules through targeting Nek1, the ciliary kinase, for proteolysis. These data demonstrate a novel function of APC beyond cell cycle control and implicate critical role of ubiquitin-mediated proteolysis in ciliary disassembly.

DOI:http://dx.doi.org/10.7554/eLife.03083.001

The majority of cells in the human body have small hair-like structures that project from the cell surface. These structures, known as primary cilia, are involved in sensing light and touch, and they are also required for an organism to develop normally. Defects in cilia result in a wide range of human diseases that are collectively known as ciliopathies. These include polycystic kidney disease and Bardet–Biedl syndrome. Ciliary disorders can also affect almost every organ in the body leading to blindness, obesity, diabetes, and cancer.

Cilia are dynamic structures that are dis-assembled when cells start to divide and are then re-assembled when cells are quiescent. The anaphase promoting complex (APC) has a critical role during cell division and targets key proteins that need to be degraded at specific times during this process. APC is localized in the basal body, which is found at the bottom of cilia, and it works together with a number of proteins which assist its function.

Wang et al. now report that a complex formed by APC and its co-activator protein Cdc20 has two functions at the basal body: it is needed to maintain the optimal length of the cilia in quiescent cells and to shorten the cilia when cells exit from quiescent stage.

Wang et al. also investigated the role of Nek1, an enzyme that is localised in the basal body. It was found that reducing the level of Nek1 in quiescent cells resulted in the formation of defective cilia, suggesting that this enzyme controls the stability and integrity of cilia. Moreover, when cells undergo division, the APC-Cdc20 complex targets the Nek1 enzyme, causing it to be degraded and allowing the cilia to be disassembled. A detailed understanding of how cells maintain the length of cilia could lead to the development of new approaches for the treatment of human ciliopathies.

DOI:http://dx.doi.org/10.7554/eLife.03083.002

The nephronophthisis gene product NPHP2/Inversin interacts with Aurora A and interferes with HDAC6-mediated cilia disassembly

BACKGROUND

Nephronophthisis (NPH) is a rare recessive disease caused by several different gene mutations. Most gene products localize to the cilium, and thus, the various NPH manifestations including kidney cysts and situs inversus have been linked to ciliary defects.

RESULTS

Here, we describe that targeted knockdown of NPHP2 significantly reduced the number of cilia on polarized MDCK cells. As one of the underlying molecular mechanisms, we identified a direct interaction between NPHP2 and Aurora A, a cell cycle kinase that promotes ciliary disassembly after activation by Hef1. NPHP2 inhibited the phosphorylation and activation of Aurora A, and reduced its kinase activity in vitro. Aurora A and histone deacetylase inhibitors ameliorated the ciliogenesis defect in NPHP2-deficient MDCK cells, supporting our hypothesis that NPHP2 is involved in the control of ciliary disassembly. Furthermore, we observed that nephrocystin (NPHP1), an interaction partner of NPHP2, also binds Aurora A, exerting very similar inhibitory effects on Hef1-mediated Aurora A activation.

CONCLUSIONS

Taken together, these findings suggest that NPHP gene products can interfere with ciliary disassembly through interaction with the Hef1/Aurora A module, thereby modulating cell cycle control and cell proliferation.

ANKS6 is a central component of a nephronophthisis module linking NEK8 to INVS and NPHP3

Nephronophthisis (NPH) is an autosomal recessive cystic kidney disease that leads to renal failure in childhood or adolescence. Most NPHP gene products form molecular networks. We have identified ANKS6 as a new NPHP family member that connects NEK8 (NPHP9) to INVERSIN (INVS, NPHP2) and NPHP3 to form a distinct NPHP module. ANKS6 localizes to the proximal cilium and knockdown experiments in zebrafish and Xenopus confirmed a role in renal development. Genetic screening identified six families with ANKS6 mutations and NPH, including severe cardiovascular abnormalities, liver fibrosis and situs inversus. The oxygen sensor HIF1AN (FIH) hydroxylates ANKS6 and INVS, while knockdown of Hif1an in Xenopus resembled the loss of other NPHP proteins. HIF1AN altered the composition of the ANKS6/INVS/NPHP3 module. Network analyses, uncovering additional putative NPHP-associated genes, placed ANKS6 at the center of the NPHP module, explaining the overlapping disease manifestation caused by mutations of either ANKS6, NEK8, INVS or NPHP3.

Wnt/β-catenin signalling: from plasma membrane to nucleus

Wnt/β-catenin signalling plays essential roles in embryonic development as well as tissue homoeostasis in adults. Thus abnormal regulation of Wnt/β-catenin signalling is linked to a variety of human diseases, including cancer, osteoporosis and Alzheimer's disease. Owing to the importance of Wnt signalling in a wide range of biological fields, a better understanding of its precise mechanisms could provide fundamental insights for therapeutic applications. Although many studies have investigated the regulation of Wnt/β-catenin signalling, our knowledge remains insufficient due to the complexity and diversity of Wnt signalling. It is generally accepted that the identification of novel regulators and their functions is a prerequisite to fully elucidating the regulation of Wnt/β-catenin signalling. Recently, several novel modulators of Wnt signalling have been determined through multiple genetic and proteomic approaches. In the present review, we discuss the mechanistic regulation of Wnt/β-catenin signalling by focusing on the roles of these novel regulators.

HECT domain-containing E3 ubiquitin ligase NEDD4L negatively regulates Wnt signaling by targeting dishevelled for proteasomal degradation

Wnt signaling plays a pivotal role in embryogenesis and tissue homeostasis. Dishevelled (Dvl) is a central mediator for both Wnt/β-catenin and Wnt/planar cell polarity pathways. NEDD4L, an E3 ubiquitin ligase, has been shown to regulate ion channel activity, cell signaling, and cell polarity. Here, we report a novel role of NEDD4L in the regulation of Wnt signaling. NEDD4L induces Dvl2 polyubiquitination and targets Dvl2 for proteasomal degradation. Interestingly, the NEDD4L-mediated ubiquitination of Dvl2 is Lys-6, Lys-27, and Lys-29 linked but not typical Lys-48-linked ubiquitination. Consistent with the role of Dvl in both Wnt/β-catenin and Wnt/planar cell polarity signaling, NEDD4L regulates the cellular β-catenin level and Rac1, RhoA, and JNK activities. We have further identified a hierarchical regulation that Wnt5a induces JNK-mediated phosphorylation of NEDD4L, which in turn promotes its ability to degrade Dvl2. Finally, we show that NEDD4L inhibits Dvl2-induced axis duplication in Xenopus embryos. Our work thus demonstrates that NEDD4L is a negative feedback regulator of Wnt signaling.

Genomic analysis of Meckel-Gruber syndrome in Arabs reveals marked genetic heterogeneity and novel candidate genes

Meckel-Gruber syndrome (MKS, OMIM #249000) is a multiple congenital malformation syndrome that represents the severe end of the ciliopathy phenotypic spectrum. Despite the relatively common occurrence of this syndrome among Arabs, little is known about its genetic architecture in this population. This is a series of 18 Arab families with MKS, who were evaluated clinically and studied using autozygome-guided mutation analysis and exome sequencing. We show that autozygome-guided candidate gene analysis identified the underlying mutation in the majority (n=12, 71%). Exome sequencing revealed a likely pathogenic mutation in three novel candidate MKS disease genes. These include C5orf42, Ellis-van-Creveld disease gene EVC2 and SEC8 (also known as EXOC4), which encodes an exocyst protein with an established role in ciliogenesis. This is the largest and most comprehensive genomic study on MKS in Arabs and the results, in addition to revealing genetic and allelic heterogeneity, suggest that previously reported disease genes and the novel candidates uncovered by this study account for the overwhelming majority of MKS patients in our population.

Context-dependent regulation of Wnt signaling through the primary cilium

The primary cilium is a highly conserved environmental sensor and modulator of fluid movement in tubular structures. The growing recognition of mutations among its many components has led to the discovery of new disorders collectively called ciliopathies. Ciliary dysfunction disturbs a variety of signaling pathways along its basal body and axoneme that are critical for embryonic development and cell and organ homeostasis. Among the many pathways, here we discuss the emerging role of Wnt proteins in morphogenic signaling and ciliary biology during health and disease.

Dishevelled stabilization by the ciliopathy protein Rpgrip1l is essential for planar cell polarity

Rpgrip1l is required for planar localization of the basal body and acts within a ciliopathy protein complex by stabilizing dishevelled.

Cilia are at the core of planar polarity cellular events in many systems. However, the molecular mechanisms by which they influence the polarization process are unclear. Here, we identify the function of the ciliopathy protein Rpgrip1l in planar polarity. In the mouse cochlea and in the zebrafish floor plate, Rpgrip1l was required for positioning the basal body along the planar polarity axis. Rpgrip1l was also essential for stabilizing dishevelled at the cilium base in the zebrafish floor plate and in mammalian renal cells. In rescue experiments, we showed that in the zebrafish floor plate the function of Rpgrip1l in planar polarity was mediated by dishevelled stabilization. In cultured cells, Rpgrip1l participated in a complex with inversin and nephrocystin-4, two ciliopathy proteins known to target dishevelled to the proteasome, and, in this complex, Rpgrip1l prevented dishevelled degradation. We thus uncover a ciliopathy protein complex that finely tunes dishevelled levels, thereby modulating planar cell polarity processes.

The kidney and planar cell polarity

Planar cell polarity (PCP) or tissue polarity describes a coordinated polarity at the plane of the tissue where most or all cells within a tissue are polarized in one direction. It is perpendicular to the apical-basal polarity of the cell. PCP is manifested readily in the Drosophila wing and cuticle bristles, Drosophila eye ommatidia, and mammalian hair and inner ear hair bundles, and less evidently, in cellular processes such as in the coordinated, directional cell movements, and oriented cell divisions that are important for tissue morphogenesis. Several distinct molecular and cellular processes have been implicated in the regulation of PCP. Here, we review potential roles for PCP during mouse kidney development and maintenance, including ureteric bud branching morphogenesis, renal medulla elongation, tubule diameter establishment/maintenance, glomerulogenesis, and response to injury. The potential mechanisms underlying these processes, including oriented cell division and coordinated cell migration/cell intercalation, are discussed. In addition, we discuss some unaddressed research topics related to PCP in the kidney that we hope will spur further discussion and investigation.

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.

Functional dissection of phosphorylation of Disheveled in Drosophila

Disheveled/Dsh proteins (Dvl in mammals) are core components of both Wnt/Wg-signaling pathways: canonical β-catenin signaling and Frizzled (Fz)-planar cell polarity (PCP) signaling. Although Dsh is a key cytoplasmic component of both Wnt/Fz-pathways, regulation of its signaling specificity is not well understood. Dsh is phosphorylated, but the functional significance of its phosphorylation remains unclear. We have systematically investigated the phosphorylation of Dsh by combining mass-spectrometry analyses, biochemical studies, and in vivo genetic methods in Drosophila. Our approaches identified multiple phospho-residues of Dsh in vivo. Our data define three novel and unexpected conclusions: (1) strikingly and in contrast to common assumptions, all conserved serines/threonines are non-essential for Dsh function in either pathway; (2) phosphorylation of conserved Tyrosine473 in the DEP domain is critical for PCP-signaling - Dsh(Y473F) behaves like a PCP-specific allele; and (3) defects associated with the PCP specific dsh(1) allele, Dsh(K417M), located within a putative Protein Kinase C consensus site, are likely due to a post-translational modification requirement of Lys417, rather than phosphorylation nearby. In summary, our combined data indicate that while many Ser/Thr and Tyr residues are indeed phosphorylated in vivo, strikingly most of these phosphorylation events are not critical for Dsh function with the exception of DshY473.

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.

Processive ubiquitin chain formation by the anaphase-promoting complex

Progression through mitosis requires the sequential ubiquitination of cell cycle regulators by the anaphase-promoting complex, resulting in their proteasomal degradation. Although several mechanisms contribute to APC/C regulation during mitosis, the APC/C is able to discriminate between its many substrates by exploiting differences in the processivity of ubiquitin chain assembly. Here, we discuss how the APC/C achieves processive ubiquitin chain formation to trigger the sequential degradation of cell cycle regulators during mitosis.

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.

Insufficiency of BUBR1, a mitotic spindle checkpoint regulator, causes impaired ciliogenesis in vertebrates

Budding uninhibited by benzimidazole-related 1 (BUBR1) is a central molecule of the spindle assembly checkpoint. Germline mutations in the budding uninhibited by benzimidazoles 1 homolog beta gene encoding BUBR1 cause premature chromatid separation (mosaic variegated aneuploidy) [PCS (MVA)] syndrome, which is characterized by constitutional aneuploidy and a high risk of childhood cancer. Patients with the syndrome often develop Dandy-Walker complex and polycystic kidneys; implying a critical role of BUBR1 in morphogenesis. However, little is known about the function of BUBR1 other than mitotic control. Here, we report that BUBR1 is essential for the primary cilium formation, and that the PCS (MVA) syndrome is thus a novel ciliopathy. Morpholino knockdown of bubr1 in medaka fish also caused ciliary dysfunction characterized by defects in cerebellar development and perturbed left-right asymmetry of the embryo. Biochemical analyses demonstrated that BUBR1 is required for ubiquitin-mediated proteasomal degradation of cell division cycle protein 20 in the G0 phase and maintains anaphase-promoting complex/cyclosome-CDC20 homolog 1 activity that regulates the optimal level of dishevelled for ciliogenesis.

Signal transmission, conversion and multiplication by polar molecules confined in nanochannels

The mechanism of signal transmission, conversion and multiplication at molecular level has been of great interest lately, due to its wide applications in nanoscience and nanotechnology. The interferences between authentic signals and thermal noises at the nanoscale make it difficult for molecular signal transduction. Here we review some of our recent progress on the signal transduction mediated by water and other polar molecules confined in nanochannels, such as Y-shaped carbon nanotubes. We also explore possible future directions in this emerging field. These studies on molecular signal conduction might have significance in future designs and applications of nanoscale electronic devices, and might also provide useful insights for a better understanding of signal conduction in both physical and biological systems.

The various roles of ubiquitin in Wnt pathway regulation

Wnt signaling mediates key developmental and homeostatic processes including stem cell maintenance, growth and cell fate specification, cell polarity and migration. Inappropriate activation of Wnt signaling is linked to a range of human disorders, most notably cancer and neurodegenerative diseases. In the Wnt/β-catenin cascade, signaling events converge on the regulation of ubiquitin-mediated degradation of the crucial transcriptional regulator β-catenin. The emerging mechanisms by which ubiquitin modification of proteins controls cellular pathways comprise both proteolytic and nonproteolytic functions. In nonproteolytic functions, ubiquitin acts as a signaling device in the control of protein activity, subcellular localization and complex formation. Here, we review and discuss recent developments that implicate ubiquitin-mediated mechanisms at multiple steps of Wnt pathway activation.

Planar polarity of multiciliated ependymal cells involves the anterior migration of basal bodies regulated by non-muscle myosin II

Motile cilia generate constant fluid flow over epithelial tissue, and thereby influence diverse physiological processes. Such functions of ciliated cells depend on the planar polarity of the cilia and on their basal bodies being oriented in the downstream direction of fluid flow. Recently, another type of basal body planar polarity, characterized by the anterior localization of the basal bodies in individual cells, was reported in the multiciliated ependymal cells that line the surface of brain ventricles. However, little is known about the cellular and molecular mechanisms by which this polarity is established. Here, we report in mice that basal bodies move in the apical cell membrane during differentiation to accumulate in the anterior region of ependymal cells. The planar cell polarity signaling pathway influences basal body orientation, but not their anterior migration, in the neonatal brain. Moreover, we show by pharmacological and genetic studies that non-muscle myosin II is a key regulator of this distribution of basal bodies. This study demonstrates that the orientation and distribution of basal bodies occur by distinct mechanisms.

Dishevelled: The hub of Wnt signaling

Wnt signaling controls a variety of developmental and homeostatic events. As a key component of Wnt signaling, Dishevelled (Dvl/Dsh) protein relays Wnt signals from receptors to downstream effectors. In the canonical Wnt pathway that depends on the nuclear translocation of beta-catenin, Dvl is recruited by the receptor Frizzled and prevents the constitutive destruction of cytosolic beta-catenin. In the non-canonical Wnt pathways such as Wnt-Frizzled/PCP (planar cell polarity) signaling, Dvl signals via the Daam1-RhoA axis and the Rac1 axis. In addition, Dvl plays important roles in Wnt-GSK3beta-microtubule signaling, Wnt-calcium signaling, Wnt-RYK signaling, Wnt-atypical PKC signaling, etc. Dvl also functions to mediate receptor endocytosis. To fulfill its multifaceted functions, it is not surprising that Dvl associates with various kinds of proteins. Its activity is also modulated dynamically by phosphorylation, ubiquitination and degradation. In this review, we summarize the current understanding of Dvl functions in Wnt signal transduction and its biological functions in mouse development, and also discuss the molecular mechanisms of its actions.