The PCP pathway instructs the planar orientation of ciliated cells in the Xenopus larval skin

Revealing non-trivial information structures in aneural biological tissues via functional connectivity

A central challenge in the progression of a variety of open questions in biology, such as morphogenesis, wound healing, and development, is learning from empirical data how information is integrated to support tissue-level function and behavior. Information-theoretic approaches provide a quantitative framework for extracting patterns from data, but so far have been predominantly applied to neuronal systems at the tissue-level. Here, we demonstrate how time series of Ca2+ dynamics can be used to identify the structure and information dynamics of other biological tissues. To this end, we expressed the calcium reporter GCaMP6s in an organoid system of explanted amphibian epidermis derived from the African clawed frog Xenopus laevis, and imaged calcium activity pre- and post- a puncture injury, for six replicate organoids. We constructed functional connectivity networks by computing mutual information between cells from time series derived using medical imaging techniques to track intracellular Ca2+. We analyzed network properties including degree distribution, spatial embedding, and modular structure. We find organoid networks exhibit potential evidence for more connectivity than null models, with our models displaying high degree hubs and mesoscale community structure with spatial clustering. Utilizing functional connectivity networks, our model suggests the tissue retains non-random features after injury, displays long range correlations and structure, and non-trivial clustering that is not necessarily spatially dependent. In the context of this reconstruction method our results suggest increased integration after injury, possible cellular coordination in response to injury, and some type of generative structure of the anatomy. While we study Ca2+ in Xenopus epidermal cells, our computational approach and analyses highlight how methods developed to analyze functional connectivity in neuronal tissues can be generalized to any tissue and fluorescent signal type. We discuss expanded methods of analyses to improve models of non-neuronal information processing highlighting the potential of our framework to provide a bridge between neuroscience and more basal modes of information processing.

The bidirectional relationship between cilia and PCP signaling pathway core protein Vangl2

Vangl2, a core component of the PCP signaling pathway, serves as a scaffold protein on the cell membrane, playing a crucial role in organizing protein complexes. Cilia, dynamic structures on the cell surface, carry out a wide range of functions. Research has highlighted a bidirectional regulatory interaction between Vangl2 and cilia, underscoring their interconnected roles in cellular processes. This relationship is demonstrated by the localization of Vangl2 at the base and proximal regions of cilia, where it plays essential roles in ciliary positioning, asymmetric distribution, and ciliogenesis. In contrast, the absence of cilia can disrupt Vangl2-mediated signal transduction processes. This review offers a narrative review of recent research on Vangl2's function in cilia and examines the regulatory effects of cilia on Vangl2-mediated signaling.

Proliferation associated 2G4 is required for the ciliation of vertebrate motile cilia

Motile cilia are critical structures that regulate early embryonic development and tissue homeostasis through synchronized ciliary motility. The formation of motile cilia is dependent on precisely controlled sequential processes including the generation, migration, and docking of centrioles/basal bodies as well as ciliary growth. Using the published proteomics data from various organisms, we identified proliferation-associated 2G4 as a novel regulator of ciliogenesis. Loss-of-function studies using Xenopus laevis as a model system reveal that Pa2G4 is essential for proper ciliogenesis and synchronized movement of cilia in multiciliated cells (MCCs) and the gastrocoel roof plate (GRP). Pa2G4 morphant MCCs exhibit defective basal body docking to the surface as a result of compromised Rac1 activity, apical actin network formation, and immature distal appendage generation. Interestingly, the regions that include the RNA-binding domain and the C-terminus of Pa2G4 are necessary for ciliogenesis in both MCCs and GRP cells. Our findings may provide insights into motile cilia-related genetic diseases such as Primary Ciliary Dyskinesia.

Centriole Translational Planar Polarity in Monociliated Epithelia

Ciliated epithelia are widespread in animals and play crucial roles in many developmental and physiological processes. Epithelia composed of multi-ciliated cells allow for directional fluid flow in the trachea, oviduct and brain cavities. Monociliated epithelia play crucial roles in vertebrate embryos, from the establishment of left-right asymmetry to the control of axis curvature via cerebrospinal flow motility in zebrafish. Cilia also have a central role in the motility and feeding of free-swimming larvae in a variety of marine organisms. These diverse functions rely on the coordinated orientation (rotational polarity) and asymmetric localization (translational polarity) of cilia and of their centriole-derived basal bodies across the epithelium, both being forms of planar cell polarity (PCP). Here, we review our current knowledge on the mechanisms of the translational polarity of basal bodies in vertebrate monociliated epithelia from the molecule to the whole organism. We highlight the importance of live imaging for understanding the dynamics of centriole polarization. We review the roles of core PCP pathways and of apicobasal polarity proteins, such as Par3, whose central function in this process has been recently uncovered. Finally, we emphasize the importance of the coordination between polarity proteins, the cytoskeleton and the basal body itself in this highly dynamic process.

Planar Cell Polarity in the Multiciliated Epithelial Lining of the Mouse Eustachian Tube

OBJECTIVES

Planar cell polarity (PCP) signaling, essential for uniform alignment and directional beating of motile cilia, has been investigated in multiciliated epithelia. As a complex structure connecting the middle ear to the nasopharynx, the eustachian tube (ET) is important in the onset of ear-nose-throat diseases. However, PCP signaling, including the orientation that is important for ciliary motility and clearance function in the ET, has not been studied. We evaluated PCP in the ET epithelium.

STUDY DESIGN

Morphometric examination of the mouse ET.

METHODS

We performed electron microscopy to assess ciliary polarity in the mouse ET, along with immunohistochemical analysis of PCP protein localization in the ET epithelium.

RESULTS

We discovered PCP in the ET epithelium. Motile cilia were aligned in the same direction in individual and neighboring cells; this alignment manifested as ciliary polarity in multiciliated cells. Additionally, PCP proteins were asymmetrically localized between adjacent cells in the plane of the ET.

CONCLUSIONS

The multiciliated ET epithelium exhibits polarization, suggesting novel structural features that may be critical for ET function.

LEVEL OF EVIDENCE

NA Laryngoscope, 134:3795-3801, 2024.

A vertebrate Vangl2 translational variant required for planar cell polarity

First described in the milkweed bug Oncopeltus fasciatus, planar cell polarity (PCP) is a developmental process essential for embryogenesis and development of polarized structures in Metazoans. This signaling pathway involves a set of evolutionarily conserved genes encoding transmembrane (Vangl, Frizzled, Celsr) and cytoplasmic (Prickle, Dishevelled) molecules. Vangl2 is of major importance in embryonic development as illustrated by its pivotal role during neural tube closure in human, mouse, Xenopus, and zebrafish embryos. Here, we report on the molecular and functional characterization of a Vangl2 isoform, Vangl2-Long, containing an N-terminal extension of about 50 aa, which arises from an alternative near-cognate AUA translation initiation site, lying upstream of the conventional start codon. While missing in Vangl1 paralogs and in all invertebrates, including Drosophila, this N-terminal extension is conserved in all vertebrate Vangl2 sequences. We show that Vangl2-Long belongs to a multimeric complex with Vangl1 and Vangl2. Using morpholino oligonucleotides to specifically knockdown Vangl2-Long in Xenopus, we found that this isoform is functional and required for embryo extension and neural tube closure. Furthermore, both Vangl2 and Vangl2-Long must be correctly expressed for the polarized distribution of the PCP molecules Pk2 and Dvl1 and for centriole rotational polarity in ciliated epidermal cells. Altogether, our study suggests that Vangl2-Long significantly contributes to the pool of Vangl2 molecules present at the plasma membrane to maintain PCP in vertebrate tissues.

Dual-color live imaging unveils stepwise organization of multiple basal body arrays by cytoskeletons

For mucociliary clearance of pathogens, tracheal multiciliated epithelial cells (MCCs) organize coordinated beating of cilia, which originate from basal bodies (BBs) with basal feet (BFs) on one side. To clarify the self-organizing mechanism of coordinated intracellular BB-arrays composed of a well-ordered BB-alignment and unidirectional BB-orientation, determined by the direction of BB to BF, we generated double transgenic mice with GFP-centrin2-labeled BBs and mRuby3-Cep128-labeled BFs for long-term, high-resolution, dual-color live-cell imaging in primary-cultured tracheal MCCs. At early timepoints of MCC differentiation, BB-orientation and BB-local alignment antecedently coordinated in an apical microtubule-dependent manner. Later during MCC differentiation, fluctuations in BB-orientation were restricted, and locally aligned BB-arrays were further coordinated to align across the entire cell (BB-global alignment), mainly in an apical intermediate-sized filament-lattice-dependent manner. Thus, the high coordination of the BB-array was established for efficient mucociliary clearance as the primary defense against pathogen infection, identifying apical cytoskeletons as potential therapeutic targets.

Bidirectional multiciliated cell extrusion is controlled by Notch-driven basal extrusion and Piezo1-driven apical extrusion

Xenopus embryos are covered with a complex epithelium containing numerous multiciliated cells (MCCs). During late-stage development, there is a dramatic remodeling of the epithelium that involves the complete loss of MCCs. Cell extrusion is a well-characterized process for driving cell loss while maintaining epithelial barrier function. Normal cell extrusion is typically unidirectional, whereas bidirectional extrusion is often associated with disease (e.g. cancer). We describe two distinct mechanisms for MCC extrusion, a basal extrusion driven by Notch signaling and an apical extrusion driven by Piezo1. Early in the process there is a strong bias towards basal extrusion, but as development continues there is a shift towards apical extrusion. Importantly, response to the Notch signal is age dependent and governed by the maintenance of the MCC transcriptional program such that extension of this program is protective against cell loss. In contrast, later apical extrusion is regulated by Piezo1, such that premature activation of Piezo1 leads to early extrusion while blocking Piezo1 leads to MCC maintenance. Distinct mechanisms for MCC loss underlie the importance of their removal during epithelial remodeling.

Zebrafish crocc2 mutants exhibit divergent craniofacial shape, misregulated variability, and aberrant cartilage morphogenesis

BACKGROUND

Phenotypic variation is of paramount importance in development, evolution, and human health; however, the molecular mechanisms that influence organ shape and shape variability are not well understood. During craniofacial development, the behavior of skeletal precursors is regulated by both biochemical and environmental inputs, and the primary cilia play critical roles in transducing both types of signals. Here, we examine a gene that encodes a key constituent of the ciliary rootlets, crocc2, and its role in cartilage morphogenesis in larval zebrafish.

RESULTS

Geometric morphometric analysis of crocc2 mutants revealed altered craniofacial shapes and expanded variation. At the cellular level, we observed altered chondrocyte shapes and planar cell polarity across multiple stages in crocc2 mutants. Notably, cellular defects were specific to areas that experience direct mechanical input. Cartilage cell number, apoptosis, and bone patterning were not affected in crocc2 mutants.

CONCLUSIONS

Whereas "regulatory" genes are widely implicated in patterning the craniofacial skeleton, genes that encode "structural" aspects of the cell are increasingly implicated in shaping the face. Our results add crocc2 to this list, and demonstrate that it affects craniofacial geometry and canalizes phenotypic variation. We propose that it does so via mechanosensing, possibly through the ciliary rootlet. If true, this would implicate a new organelle in skeletal development and evolution.

The histone H4K20 methyltransferase SUV4-20H1/KMT5B is required for multiciliated cell differentiation in Xenopus

H4 lysine 20 dimethylation (H4K20me2) is the most abundant histone modification in vertebrate chromatin. It arises from sequential methylation of unmodified histone H4 proteins by the mono-methylating enzyme PR-SET7/KMT5A, followed by conversion to the dimethylated state by SUV4-20H (KMT5B/C) enzymes. We have blocked the deposition of this mark by depleting Xenopus embryos of SUV4-20H1/H2 methyltransferases. In the larval epidermis, this results in a severe loss of cilia in multiciliated cells (MCC), a key component of mucociliary epithelia. MCC precursor cells are correctly specified, amplify centrioles, but ultimately fail in ciliogenesis because of the perturbation of cytoplasmic processes. Genome-wide transcriptome profiling reveals that SUV4-20H1/H2-depleted ectodermal explants preferentially down-regulate the expression of several hundred ciliogenic genes. Further analysis demonstrated that knockdown of SUV4-20H1 alone is sufficient to generate the MCC phenotype and that its catalytic activity is needed for axoneme formation. Overexpression of the H4K20me1-specific histone demethylase PHF8/KDM7B also rescues the ciliogenic defect in a significant manner. Taken together, this indicates that the conversion of H4K20me1 to H4K20me2 by SUV4-20H1 is critical for the formation of cilia tufts.

Novel analytical tools reveal that local synchronization of cilia coincides with tissue-scale metachronal waves in zebrafish multiciliated epithelia

Motile cilia are hair-like cell extensions that beat periodically to generate fluid flow along various epithelial tissues within the body. In dense multiciliated carpets, cilia were shown to exhibit a remarkable coordination of their beat in the form of traveling metachronal waves, a phenomenon which supposedly enhances fluid transport. Yet, how cilia coordinate their regular beat in multiciliated epithelia to move fluids remains insufficiently understood, particularly due to lack of rigorous quantification. We combine experiments, novel analysis tools, and theory to address this knowledge gap. To investigate collective dynamics of cilia, we studied zebrafish multiciliated epithelia in the nose and the brain. We focused mainly on the zebrafish nose, due to its conserved properties with other ciliated tissues and its superior accessibility for non-invasive imaging. We revealed that cilia are synchronized only locally and that the size of local synchronization domains increases with the viscosity of the surrounding medium. Even though synchronization is local only, we observed global patterns of traveling metachronal waves across the zebrafish multiciliated epithelium. Intriguingly, these global wave direction patterns are conserved across individual fish, but different for left and right noses, unveiling a chiral asymmetry of metachronal coordination. To understand the implications of synchronization for fluid pumping, we used a computational model of a regular array of cilia. We found that local metachronal synchronization prevents steric collisions, i.e., cilia colliding with each other, and improves fluid pumping in dense cilia carpets, but hardly affects the direction of fluid flow. In conclusion, we show that local synchronization together with tissue-scale cilia alignment coincide and generate metachronal wave patterns in multiciliated epithelia, which enhance their physiological function of fluid pumping.

Bidirectional multiciliated cell extrusion is controlled by Notch driven basal extrusion and Piezo 1 driven apical extrusion

Xenopus embryos are covered with a complex epithelium containing numerous multiciliated cells (MCCs). During late stage development there is a dramatic remodeling of the epithelium that involves the complete loss of MCCs. Cell extrusion is a well-characterized process for driving cell loss while maintaining epithelial barrier function. Normal cell extrusion is typically unidirectional whereas bidirectional extrusion is often associated with disease (e.g. cancer). We describe two distinct mechanisms for MCC extrusion, a basal extrusion driven by Notch signaling and an apical extrusion driven by Piezo1. Early in the process there is a strong bias towards basal extrusion, but as development continues there is a shift towards apical extrusion. Importantly, receptivity to the Notch signal is age-dependent and governed by the maintenance of the MCC transcriptional program such that extension of this program is protective against cell loss. In contrast, later apical extrusion is regulated by Piezo 1 such that premature activation of Piezo 1 leads to early extrusion while blocking Piezo 1 leads to MCC maintenance. Distinct mechansms for MCC loss underlie the importance of their removal during epithelial remodeling.

Summay Statement

Cell extrusion typically occurs unidirectionally. We have identified a single population of multiciliated cells that extrudes bidirectionally: Notch-driven basal extrusion and Piezo 1-mediated apical extrusion.

Structures and functions of cilia during vertebrate embryo development

Cilia are hair-like structures that project from the surface of cells. In vertebrates, most cells have an immotile primary cilium that mediates cell signaling, and some specialized cells assemble one or multiple cilia that are motile and beat synchronously to move fluids in one direction. Gene mutations that alter cilia structure or function cause a broad spectrum of disorders termed ciliopathies that impact virtually every system in the body. A wide range of birth defects associated with ciliopathies underscores critical functions for cilia during embryonic development. In many cases, the mechanisms underlying cilia functions during development and disease remain poorly understood. This review describes different types of cilia in vertebrate embryos and discusses recent research results from diverse model systems that provide novel insights into how cilia form and function during embryo development. The work discussed here not only expands our understanding of in vivo cilia biology, but also opens new questions about cilia and their roles in establishing healthy embryos.

Coordination of Cilia Movements in Multi-Ciliated Cells

Multiple motile cilia are formed at the apical surface of multi-ciliated cells in the epithelium of the oviduct or the fallopian tube, the trachea, and the ventricle of the brain. Those cilia beat unidirectionally along the tissue axis, and this provides a driving force for directed movements of ovulated oocytes, mucus, and cerebrospinal fluid in each of these organs. Furthermore, cilia movements show temporal coordination between neighboring cilia. To establish such coordination of cilia movements, cilia need to sense and respond to various cues, including the organ's orientation and movements of neighboring cilia. In this review, we discuss the mechanisms by which cilia movements of multi-ciliated cells are coordinated, focusing on planar cell polarity and the cytoskeleton, and highlight open questions for future research.

Computational Modeling of Motile Cilia-Driven Cerebrospinal Flow in the Brain Ventricles of Zebrafish Embryo

Motile cilia are hair-like microscopic structures which generate directional flow to provide fluid transport in various biological processes. Ciliary beating is one of the sources of cerebrospinal flow (CSF) in brain ventricles. In this study, we investigated how the tilt angle, quantity, and phase relationship of cilia affect CSF flow patterns in the brain ventricles of zebrafish embryos. For this purpose, two-dimensional computational fluid dynamics (CFD) simulations are performed to determine the flow fields generated by the motile cilia. The cilia are modeled as thin membranes with prescribed motions. The cilia motions were obtained from a two-day post-fertilization zebrafish embryo previously imaged via light sheet fluorescence microscopy. We observed that the cilium angle significantly alters the generated flow velocity and mass flow rates. As the cilium angle gets closer to the wall, higher flow velocities are observed. Phase difference between two adjacent beating cilia also affects the flow field as the cilia with no phase difference produce significantly lower mass flow rates. In conclusion, our simulations revealed that the most efficient method for cilia-driven fluid transport relies on the alignment of multiple cilia beating with a phase difference, which is also observed in vivo in the developing zebrafish brain.

Vangl as a Master Scaffold for Wnt/Planar Cell Polarity Signaling in Development and Disease

The establishment of polarity within tissues and dynamic cellular morphogenetic events are features common to both developing and adult tissues, and breakdown of these programs is associated with diverse human diseases. Wnt/Planar cell polarity (Wnt/PCP) signaling, a branch of non-canonical Wnt signaling, is critical to the establishment and maintenance of polarity in epithelial tissues as well as cell motility events critical to proper embryonic development. In epithelial tissues, Wnt/PCP-mediated planar polarity relies upon the asymmetric distribution of core proteins to establish polarity, but the requirement for this distribution in Wnt/PCP-mediated cell motility remains unclear. However, in both polarized tissues and migratory cells, the Wnt/PCP-specific transmembrane protein Vangl is required and appears to serve as a scaffold upon which the core pathway components as well as positive and negative regulators of Wnt/PCP signaling assemble. The current literature suggests that the multiple interaction domains of Vangl allow for the binding of diverse signaling partners for the establishment of context- and tissue-specific complexes. In this review we discuss the role of Vangl as a master scaffold for Wnt/PCP signaling in epithelial tissue polarity and cellular motility events in developing and adult tissues, and address how these programs are dysregulated in human disease.

Mechanical regulation of early vertebrate embryogenesis

Embryonic cells grow in environments that provide a plethora of physical cues, including mechanical forces that shape the development of the entire embryo. Despite their prevalence, the role of these forces in embryonic development and their integration with chemical signals have been mostly neglected, and scrutiny in modern molecular embryology tilted, instead, towards the dissection of molecular pathways involved in cell fate determination and patterning. It is now possible to investigate how mechanical signals induce downstream genetic regulatory networks to regulate key developmental processes in the embryo. Here, we review the insights into mechanical control of early vertebrate development, including the role of forces in tissue patterning and embryonic axis formation. We also highlight recent in vitro approaches using individual embryonic stem cells and self-organizing multicellular models of human embryos, which have been instrumental in expanding our understanding of how mechanics tune cell fate and cellular rearrangements during human embryonic development.

Lrrcc1 and Ccdc61 are conserved effectors of multiciliated cell function

Ciliated epithelia perform essential functions across animal evolution, ranging from locomotion of marine organisms to mucociliary clearance of airways in mammals. These epithelia are composed of multiciliated cells (MCCs) harbouring myriads of motile cilia, which rest on modified centrioles called basal bodies (BBs), and beat coordinately to generate directed fluid flows. Thus, BB biogenesis and organization is central to MCC function. In basal eukaryotes, the coiled-coil domain proteins Lrrcc1 and Ccdc61 were shown to be required for proper BB construction and function. Here, we used the Xenopus embryonic ciliated epidermis to characterize Lrrcc1 and Ccdc61 in vertebrate MCCs. We found that they both encode BB components, localized proximally at the junction with striated rootlets. Knocking down either gene caused defects in BB docking, spacing, and polarization. Moreover, their depletion impaired the apical cytoskeleton, and altered ciliary beating. Consequently, cilia-powered fluid flow was greatly reduced in morphant tadpoles, which displayed enhanced mortality when exposed to pathogenic bacteria. This work illustrates how integration across organizational scales make elementary BB components essential for the emergence of the physiological function of ciliated epithelia.

Imaging Planar Cell Polarity Proteins in Xenopus Neuroectoderm

Planar cell polarity (PCP) refers to coordinated cell polarization in the plane of the tissue. Genetic studies in Drosophila identified several core PCP genes, whose products function together in a signaling pathway that regulates cell shape, epithelial tissue organization and remodeling during morphogenesis. PCP is detected by the asymmetric distribution of core PCP proteins at different borders of epithelial cells. Believed to be critical for signaling, this segregation is studied by a variety of techniques, such as direct immunostaining and imaging of fluorescent PCP protein fusions or fluorescence recovery after photobleaching (FRAP). All of the above techniques can be applied to the analysis of the Xenopus neural plate to study the dynamics of tissue polarization, making this system one of the best vertebrate PCP models. This chapter describes how to image PCP proteins in Xenopus neuroectoderm for both fixed and live samples. These robust cellular techniques will contribute to mechanistic studies of PCP in vertebrate embryos.

Conserved and Divergent Principles of Planar Polarity Revealed by Hair Cell Development and Function

Planar polarity describes the organization and orientation of polarized cells or cellular structures within the plane of an epithelium. The sensory receptor hair cells of the vertebrate inner ear have been recognized as a preeminent vertebrate model system for studying planar polarity and its development. This is principally because planar polarity in the inner ear is structurally and molecularly apparent and therefore easy to visualize. Inner ear planar polarity is also functionally significant because hair cells are mechanosensors stimulated by sound or motion and planar polarity underlies the mechanosensory mechanism, thereby facilitating the auditory and vestibular functions of the ear. Structurally, hair cell planar polarity is evident in the organization of a polarized bundle of actin-based protrusions from the apical surface called stereocilia that is necessary for mechanosensation and when stereociliary bundle is disrupted auditory and vestibular behavioral deficits emerge. Hair cells are distributed between six sensory epithelia within the inner ear that have evolved unique patterns of planar polarity that facilitate auditory or vestibular function. Thus, specialized adaptations of planar polarity have occurred that distinguish auditory and vestibular hair cells and will be described throughout this review. There are also three levels of planar polarity organization that can be visualized within the vertebrate inner ear. These are the intrinsic polarity of individual hair cells, the planar cell polarity or coordinated orientation of cells within the epithelia, and planar bipolarity; an organization unique to a subset of vestibular hair cells in which the stereociliary bundles are oriented in opposite directions but remain aligned along a common polarity axis. The inner ear with its complement of auditory and vestibular sensory epithelia allows these levels, and the inter-relationships between them, to be studied using a single model organism. The purpose of this review is to introduce the functional significance of planar polarity in the auditory and vestibular systems and our contemporary understanding of the developmental mechanisms associated with organizing planar polarity at these three cellular levels.

New mouse models for high resolution and live imaging of planar cell polarity proteins in vivo

The collective polarization of cellular structures and behaviors across a tissue plane is a near universal feature of epithelia known as planar cell polarity (PCP). This property is controlled by the core PCP pathway, which consists of highly conserved membrane-associated protein complexes that localize asymmetrically at cell junctions. Here, we introduce three new mouse models for investigating the localization and dynamics of transmembrane PCP proteins: Celsr1, Fz6 and Vangl2. Using the skin epidermis as a model, we characterize and verify the expression, localization and function of endogenously tagged Celsr1-3xGFP, Fz6-3xGFP and tdTomato-Vangl2 fusion proteins. Live imaging of Fz6-3xGFP in basal epidermal progenitors reveals that the polarity of the tissue is not fixed through time. Rather, asymmetry dynamically shifts during cell rearrangements and divisions, while global, average polarity of the tissue is preserved. We show using super-resolution STED imaging that Fz6-3xGFP and tdTomato-Vangl2 can be resolved, enabling us to observe their complex localization along junctions. We further explore PCP fusion protein localization in the trachea and neural tube, and discover new patterns of PCP expression and localization throughout the mouse embryo.

A 'tad' of hope in the fight against airway disease

Xenopus tadpoles have emerged as a powerful in vivo model system to study mucociliary epithelia such as those found in the human airways. The tadpole skin has mucin-secreting cells, motile multi-ciliated cells, ionocytes (control local ionic homeostasis) and basal stem cells. This cellular architecture is very similar to the large airways of the human lungs and represents an easily accessible and experimentally tractable model system to explore the molecular details of mucociliary epithelia. Each of the cell types in the tadpole skin has a human equivalent and a conserved network of genes and signalling pathways for their differentiation has been discovered. Great insight into the function of each of the cell types has been achieved using the Xenopus model and this has enhanced our understanding of airway disease. This simple model has already had a profound impact on the field but, as molecular technologies (e.g. gene editing and live imaging) continue to develop apace, its use for understanding individual cell types and their interactions will likely increase. For example, its small size and genetic tractability make it an ideal model for live imaging of a mucociliary surface especially during environmental challenges such as infection. Further potential exists for the mimicking of human genetic mutations that directly cause airway disease and for the pre-screening of drugs against novel therapeutic targets.

The dorsal blastopore lip is a source of signals inducing planar cell polarity in the Xenopus neural plate

Coordinated polarization of cells in the tissue plane, known as planar cell polarity (PCP), is associated with a signaling pathway critical for the control of morphogenetic processes. Although the segregation of PCP components to opposite cell borders is believed to play a critical role in this pathway, whether PCP derives from egg polarity or preexistent long-range gradient, or forms in response to a localized cue, remains a challenging question. Here we investigate the Xenopus neural plate, a tissue that has been previously shown to exhibit PCP. By imaging Vangl2 and Prickle3, we show that PCP is progressively acquired in the neural plate and requires a signal from the posterior region of the embryo. Tissue transplantations indicated that PCP is triggered in the neural plate by a planar cue from the dorsal blastopore lip. The PCP cue did not depend on the orientation of the graft and was distinct from neural inducers. These observations suggest that neuroectodermal PCP is not instructed by a preexisting molecular gradient but induced by a signal from the dorsal blastopore lip.

A cellular platform for the development of synthetic living machines

Robot swarms have, to date, been constructed from artificial materials. Motile biological constructs have been created from muscle cells grown on precisely shaped scaffolds. However, the exploitation of emergent self-organization and functional plasticity into a self-directed living machine has remained a major challenge. We report here a method for generation of in vitro biological robots from frog (Xenopus laevis) cells. These xenobots exhibit coordinated locomotion via cilia present on their surface. These cilia arise through normal tissue patterning and do not require complicated construction methods or genomic editing, making production amenable to high-throughput projects. The biological robots arise by cellular self-organization and do not require scaffolds or microprinting; the amphibian cells are highly amenable to surgical, genetic, chemical, and optical stimulation during the self-assembly process. We show that the xenobots can navigate aqueous environments in diverse ways, heal after damage, and show emergent group behaviors. We constructed a computational model to predict useful collective behaviors that can be elicited from a xenobot swarm. In addition, we provide proof of principle for a writable molecular memory using a photoconvertible protein that can record exposure to a specific wavelength of light. Together, these results introduce a platform that can be used to study many aspects of self-assembly, swarm behavior, and synthetic bioengineering, as well as provide versatile, soft-body living machines for numerous practical applications in biomedicine and the environment.

Intercellular and intracellular cilia orientation is coordinated by CELSR1 and CAMSAP3 in oviduct multi-ciliated cells

The molecular mechanisms by which cilia orientation is coordinated within and between multi-ciliated cells (MCCs) are not fully understood. In the mouse oviduct, MCCs exhibit a characteristic basal body (BB) orientation and microtubule gradient along the tissue axis. The intracellular polarities were moderately maintained in cells lacking CELSR1 (cadherin EGF LAG seven-pass G-type receptor 1), a planar cell polarity (PCP) factor involved in tissue polarity regulation, although the intercellular coordination of the polarities was disrupted. However, CAMSAP3 (calmodulin-regulated spectrin-associated protein 3), a microtubule minus-end regulator, was found to be critical for determining the intracellular BB orientation. CAMSAP3 localized to the base of cilia in a polarized manner, and its mutation led to the disruption of intracellular coordination of BB orientation, as well as the assembly of microtubules interconnecting BBs, without affecting PCP factor localization. Thus, both CELSR1 and CAMSAP3 are responsible for BB orientation but in distinct ways; their cooperation should therefore be critical for generating functional multi-ciliated tissues.

Planar cell polarity governs the alignment of the nasopharyngeal epithelium in mammals

Planar cell polarity (PCP) signalling specifies the orientation of epithelial cells and regulates directional beating of motile cilia of multiciliated epithelial cells. Clinically, defects in cilia function are associated with nasopharyngeal symptoms. The polarity of the nasopharyngeal epithelium is poorly understood. Here, we demonstrated PCP in the nasopharyngeal epithelium. Multiciliated cells (MCCs) were uniformly aligned with their long axis parallel to the tissue axis of the nasopharynx (NP). In addition, PCP proteins exhibited an asymmetrical localisation between adjacent cells. Motile cilia were uniformly aligned in the same direction within both individual cells and neighbouring cells, which manifested as cilial polarity in MCCs. Mutation of Vangl2, a mammalian homologue of the Drosophila PCP gene, resulted in significant disruption of the orientation of epithelial cells. Finally, keratin-5-positive basal cells constantly replenished the luminal ciliated cells; the new dynamic ciliated cells were also oriented parallel to the tissue axis. These results indicate a role for the PCP pathway in the uniform orientation of dynamically replenished epithelial cells in the NP.

Aquatic models of human ciliary diseases

Cilia are microtubule-based structures that either transmit information into the cell or move fluid outside of the cell. There are many human diseases that arise from malfunctioning cilia. Although mammalian models provide vital insights into the underlying pathology of these diseases, aquatic organisms such as Xenopus and zebrafish provide valuable tools to help screen and dissect out the underlying causes of these diseases. In this review we focus on recent studies that identify or describe different types of human ciliopathies and outline how aquatic organisms have aided our understanding of these diseases.

Super-Resolution Microscopy and FIB-SEM Imaging Reveal Parental Centriole-Derived, Hybrid Cilium in Mammalian Multiciliated Cells

Motile cilia are cellular beating machines that play a critical role in mucociliary clearance, cerebrospinal fluid movement, and fertility. In the airways, hundreds of motile cilia present on the surface of a multiciliated epithelia cell beat coordinately to protect the epithelium from bacteria, viruses, and harmful particulates. During multiciliated cell differentiation, motile cilia are templated from basal bodies, each extending a basal foot-an appendage linking motile cilia together to ensure coordinated beating. Here, we demonstrate that among the many motile cilia of a multiciliated cell, a hybrid cilium with structural features of both primary and motile cilia is harbored. The hybrid cilium is conserved in mammalian multiciliated cells, originates from parental centrioles, and its cellular position is biased and dependent on ciliary beating. Furthermore, we show that the hybrid cilium emerges independently of other motile cilia and functions in regulating basal body alignment.

Building a ciliated epithelium: Transcriptional regulation and radial intercalation of multiciliated cells

The epidermis of the Xenopus embryo has emerged as a powerful tool for studying the development of a ciliated epithelium. Interspersed throughout the epithelium are multiciliated cells (MCCs) with 100+ motile cilia that beat in a coordinated manner to generate fluid flow over the surface of the cell. MCCs are essential for various developmental processes and, furthermore, ciliary dysfunction is associated with numerous pathologies. Therefore, understanding the cellular mechanisms involved in establishing a ciliated epithelium are of particular interest. MCCs originate in the inner epithelial layer of Xenopus skin, where Notch signaling plays a critical role in determining which progenitors will adopt a ciliated cell fate. Then, activation of various transcriptional regulators, such as GemC1 and MCIDAS, initiate the MCC transcriptional program, resulting in centriole amplification and the formation of motile cilia. Following specification and differentiation, MCCs undergo the process of radial intercalation, where cells apically migrate from the inner layer to the outer epithelial layer. This process involves the cooperation of various cytoskeletal networks, activation of various signaling molecules, and changes in cell-ECM and cell-cell adhesion. Coordination of these cellular processes is required for complete incorporation into the outer epithelial layer and generation of a functional ciliated epithelium. Here, we highlight recent advances made in understanding the transcriptional cascades required for MCC specification and differentiation and the coordination of cellular processes that facilitate radial intercalation. Proper regulation of these signaling pathways and processes are the foundation for developing a ciliated epithelium.

From biomechanics to mechanobiology: Xenopus provides direct access to the physical principles that shape the embryo

Features of amphibian embryos that have served so well to elucidate the genetics of vertebrate development also enable detailed analysis of the physics that shape morphogenesis and regulate development. Biophysical tools are revealing how genes control mechanical properties of the embryo. The same tools that describe and control mechanical properties are being turned to reveal how dynamic mechanical information and feedback regulate biological programs of development. In this review we outline efforts to explore the various roles of mechanical cues in guiding cilia biology, axonal pathfinding, goblet cell regeneration, epithelial-to-mesenchymal transitions in neural crest, and mesenchymal-to-epithelial transitions in heart progenitors. These case studies reveal the power of Xenopus experimental embryology to expose pathways integrating mechanical cues with programs of development, organogenesis, and regeneration.

Dynamic Polarization of the Multiciliated Planarian Epidermis between Body Plan Landmarks

Polarity is a universal design principle of biological systems that manifests at all organizational scales, yet its coordination across scales remains poorly understood. Here, we make use of the extreme anatomical plasticity of planarian flatworms to probe the interplay between global body plan polarity and local cell polarity. Our quantitative analysis of ciliary rootlet orientation in the epidermis reveals a dynamic polarity field with head and tail as independent determinants of anteroposterior (A/P) polarization and the body margin as determinant of mediolateral (M/L) polarization. Mathematical modeling rationalizes the global polarity field and its response to experimental manipulations as superposition of separate A/P and M/L fields, and we identify the core PCP and Ft/Ds pathways as their molecular mediators. Overall, our study establishes a framework for the alignment of cellular polarity vectors relative to planarian body plan landmarks and establishes the core PCP and Ft/Ds pathways as evolutionarily conserved 2D-polarization module.

CAMSAP3 facilitates basal body polarity and the formation of the central pair of microtubules in motile cilia

Cilia are composed of hundreds of proteins whose identities and functions are far from being completely understood. In this study, we determined that calmodulin-regulated spectrin-associated protein 3 (CAMSAP3) plays an important role for the function of motile cilia in multiciliated cells (MCCs). Global knockdown of CAMSAP3 protein expression in mice resulted in defects in ciliary structures, polarity, and synchronized beating in MCCs. These animals also displayed signs and symptoms reminiscent of primary ciliary dyskinesia (PCD), including a mild form of hydrocephalus, subfertility, and impaired mucociliary clearance that leads to hyposmia, anosmia, rhinosinusitis, and otitis media. Functional characterization of CAMSAP3 enriches our understanding of the molecular mechanisms underlying the generation and function of motile cilia in MCCs.

Synchronized beating of cilia on multiciliated cells (MCCs) generates a directional flow of mucus across epithelia. This motility requires a “9 + 2” microtubule (MT) configuration in axonemes and the unidirectional array of basal bodies of cilia on the MCCs. However, it is not fully understood what components are needed for central MT-pair assembly as they are not continuous with basal bodies in contrast to the nine outer MT doublets. In this study, we discovered that a homozygous knockdown mouse model for MT minus-end regulator calmodulin-regulated spectrin-associated protein 3 (CAMSAP3), Camsap3^tm1a/tm1a, exhibited multiple phenotypes, some of which are typical of primary ciliary dyskinesia (PCD), a condition caused by motile cilia defects. Anatomical examination of Camsap3^tm1a/tm1a mice revealed severe nasal airway blockage and abnormal ciliary morphologies in nasal MCCs. MCCs from different tissues exhibited defective synchronized beating and ineffective generation of directional flow likely underlying the PCD-like phenotypes. In normal mice, CAMSAP3 localized to the base of axonemes and at the basal bodies in MCCs. However, in Camsap3^tm1a/tm1a, MCCs lacked CAMSAP3 at the ciliary base. Importantly, the central MT pairs were missing in the majority of cilia, and the polarity of the basal bodies was disorganized. These phenotypes were further confirmed in MCCs of Xenopus embryos when CAMSAP3 expression was knocked down by morpholino injection. Taken together, we identified CAMSAP3 as being important for the formation of central MT pairs, proper orientation of basal bodies, and synchronized beating of motile cilia.

Noncanonical Wnt planar cell polarity signaling in lung development and disease

The planar cell polarity (PCP) signaling pathway is a potent developmental regulator of directional cell behaviors such as migration, asymmetric division and morphological polarization that are critical for shaping the body axis and the complex three-dimensional architecture of tissues and organs. PCP is considered a noncanonical Wnt pathway due to the involvement of Wnt ligands and Frizzled family receptors in the absence of the beta-catenin driven gene expression observed in the canonical Wnt cascade. At the heart of the PCP mechanism are protein complexes capable of generating molecular asymmetries within cells along a tissue-wide axis that are translated into polarized actin and microtubule cytoskeletal dynamics. PCP has emerged as an important regulator of developmental, homeostatic and disease processes in the respiratory system. It acts along other signaling pathways to create the elaborately branched structure of the lung by controlling the directional protrusive movements of cells during branching morphogenesis. PCP operates in the airway epithelium to establish and maintain the orientation of respiratory cilia along the airway axis for anatomically directed mucociliary clearance. It also regulates the establishment of the pulmonary vasculature. In adult tissues, PCP dysfunction has been linked to a variety of chronic lung diseases such as cystic fibrosis, chronic obstructive pulmonary disease, and idiopathic pulmonary arterial hypertension, stemming chiefly from the breakdown of proper tissue structure and function and aberrant cell migration during regenerative wound healing. A better understanding of these (impaired) PCP mechanisms is needed to fully harness the therapeutic opportunities of targeting PCP in chronic lung diseases.

The Emerging Role of Sperm-Associated Antigen 6 Gene in the Microtubule Function of Cells and Cancer

Accumulated evidence shows that sperm-associated antigen 6 (SPAG6) gene has multiple biological functions. It maintains the normal function of a variety of cells including ciliary/flagellar biogenesis and polarization, neurogenesis, and neuronal migration. Moreover, SPAG6 is found to be critically involved in auditory transduction and the fibroblast life cycle. Furthermore, SPAG6 plays an essential role in immuno-regulation. Notably, SPAG6 has been demonstrated to participate in the occurrence and progression of a variety of human cancers. New evidence shows that SPAG6 gene regulates tumor cell proliferation, apoptosis, invasion, and metastasis. Therefore, in this review, we describe the physiological function and mechanism of SPAG6 in human normal cells and cancer cells. We also highlight that SPAG6 gene may be an effective biomarker for the diagnosis of human cancer. Taken together, targeting SPAG6 could be a novel strategy for the treatment of human diseases including cancer.

Frizzled3 and Frizzled6 Cooperate with Vangl2 to Direct Cochlear Innervation by Type II Spiral Ganglion Neurons

Type II spiral ganglion neurons provide afferent innervation to outer hair cells of the cochlea and are proposed to have nociceptive functions important for auditory function and homeostasis. These neurons are anatomically distinct from other classes of spiral ganglion neurons because they extend a peripheral axon beyond the inner hair cells that subsequently makes a distinct 90 degree turn toward the cochlear base. As a result, patterns of outer hair cell innervation are coordinated with the tonotopic organization of the cochlea. Previously, it was shown that peripheral axon turning is directed by a nonautonomous function of the core planar cell polarity (PCP) protein VANGL2. We demonstrate using mice of either sex that Fzd3 and Fzd6 similarly regulate axon turning, are functionally redundant with each other, and that Fzd3 genetically interacts with Vangl2 to guide this process. FZD3 and FZD6 proteins are asymmetrically distributed along the basolateral wall of cochlear-supporting cells, and are required to promote or maintain the asymmetric distribution of VANGL2 and CELSR1. These data indicate that intact PCP complexes formed between cochlear-supporting cells are required for the nonautonomous regulation of axon pathfinding. Consistent with this, in the absence of PCP signaling, peripheral axons turn randomly and often project toward the cochlear apex. Additional analyses of Porcn mutants in which WNT secretion is reduced suggest that noncanonical WNT signaling establishes or maintains PCP signaling in this context. A deeper understanding of these mechanisms is necessary for repairing auditory circuits following acoustic trauma or promoting cochlear reinnervation during regeneration-based deafness therapies.SIGNIFICANCE STATEMENT Planar cell polarity (PCP) signaling has emerged as a complementary mechanism to classical axon guidance in regulating axon track formation, axon outgrowth, and neuronal polarization. The core PCP proteins are also required for auditory circuit assembly, and coordinate hair cell innervation with the tonotopic organization of the cochlea. This is a non-cell-autonomous mechanism that requires the formation of PCP protein complexes between cochlear-supporting cells located along the trajectory of growth cone navigation. These findings are significant because they demonstrate how the fidelity of auditory circuit formation is ensured during development, and provide a mechanism by which PCP proteins may regulate axon outgrowth and guidance in the CNS.

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.

Desmoplakin is required for epidermal integrity and morphogenesis in the Xenopus laevis embryo

Desmoplakin (Dsp) is a unique and critical desmosomal protein, that is integral to epidermal development. However, it is unclear whether this protein is required specifically for epidermal morphogenesis. Using morpholinos or Crispr/Cas9 mutagenesis we decreased the function of Dsp in frog embryos to better understand its role during epidermal development. Dsp morphant and mutant embryos had developmental defects such as epidermal fragility that mimicked what has been reported in mammals. Most importantly, we also uncovered a novel function for Dsp in the morphogenesis of the epidermis in X. laevis. In particular, Dsp is required during the process of radial intercalation where basally located cells move into the outer epidermal layer. Once inserted these newly intercalated cells expand their apical surface and then they differentiate into specific epidermal cell types. Decreased levels of Dsp resulted in the failure of the radially intercalating cells to expand their apical surface, thereby reducing the number of differentiated multiciliated and secretory cells. Such defects correlate with changes in E-cadherin levels and actin and microtubule localization which could explain the defects in apical expansion. A mutated form of Dsp that maintains cell-cell adhesion but eliminates the connections to the cytoskeleton results in the same epidermal morphogenesis defect. These results suggest a specific role for Dsp in the apical expansion of cells during radial intercalation. We have developed a novel system, in the frog, to demonstrate for the first time that desmosomes not only protect against mechanical stress but are also critical for epidermal morphogenesis.

Vangl2 coordinates cell rearrangements during gut elongation

BACKGROUND

The embryonic gut tube undergoes extensive lengthening to generate the surface area required for nutrient absorption across the digestive epithelium. In Xenopus, narrowing and elongation of the tube is driven by radial rearrangements of its core of endoderm cells, a process that concomitantly opens the gut lumen and facilitates epithelial morphogenesis. How endoderm rearrangements are properly oriented and coordinated to achieve this complex morphogenetic outcome is unknown.

RESULTS

We find that, prior to gut elongation, the core Wnt/PCP component Vangl2 becomes enriched at both the anterior and apical aspects of individual endoderm cells. In Vangl2-depleted guts, the cells remain unpolarized, down-regulate cell-cell adhesion proteins, and, consequently, fail to rearrange, leading to a short gut with an occluded lumen and undifferentiated epithelium. In contrast, endoderm cells with ectopic Vangl2 protein acquire abnormal polarity and adhesive contacts. As a result, endoderm cells also fail to rearrange properly and undergo ectopic differentiation, resulting in guts with multiple torturous lumens, irregular epithelial architecture, and variable intestinal topologies.

CONCLUSIONS

Asymmetrical enrichment of Vangl2 in individual gut endoderm cells orients polarity and adhesion during radial rearrangements, coordinating digestive epithelial morphogenesis and lumen formation with gut tube elongation.

The ciliary GTPase Arl3 maintains tissue architecture by directing planar spindle orientation during epidermal morphogenesis

Arl/ARF GTPases regulate ciliary trafficking, but their tissue-specific functions are unclear. Here, we demonstrate that ciliary GTPase Arl3 is required for mitotic spindle orientation of mouse basal stem cells during skin development. Arl3 loss diminished cell divisions within the plane of the epithelium, leading to increased perpendicular divisions, expansion of progenitor cells and loss of epithelial integrity. These observations suggest that an Arl3-dependent mechanism maintains cell division polarity along the tissue axis, and disruption of planar spindle orientation has detrimental consequences for epidermal architecture. Defects in planar cell polarity (PCP) can disrupt spindle positioning during tissue morphogenesis. Upon Arl3 loss, the PCP signaling molecules Celsr1 and Vangl2 failed to maintain planar polarized distributions, resulting in defective hair follicle angling, a hallmark of disrupted PCP. In the absence of Celsr1 polarity, frizzled 6 lost its asymmetrical distribution and abnormally segregated to the apical cortex of basal cells. We propose that Arl3 regulates polarized endosomal trafficking of PCP components to compartmentalized membrane domains. Cell-cell communication via ciliary GTPase signaling directs mitotic spindle orientation and PCP signaling, processes that are crucial for the maintenance of epithelial architecture.

Primary Cilium-Mediated Retinal Pigment Epithelium Maturation Is Disrupted in Ciliopathy Patient Cells

Primary cilia are sensory organelles that protrude from the cell membrane. Defects in the primary cilium cause ciliopathy disorders, with retinal degeneration as a prominent phenotype. Here, we demonstrate that the retinal pigment epithelium (RPE), essential for photoreceptor development and function, requires a functional primary cilium for complete maturation and that RPE maturation defects in ciliopathies precede photoreceptor degeneration. Pharmacologically enhanced ciliogenesis in wild-type induced pluripotent stem cells (iPSC)-RPE leads to fully mature and functional cells. In contrast, ciliopathy patient-derived iPSC-RPE and iPSC-RPE with a knockdown of ciliary-trafficking protein remain immature, with defective apical processes, reduced functionality, and reduced adult-specific gene expression. Proteins of the primary cilium regulate RPE maturation by simultaneously suppressing canonical WNT and activating PKCδ pathways. A similar cilium-dependent maturation pathway exists in lung epithelium. Our results provide insights into ciliopathy-induced retinal degeneration, demonstrate a developmental role for primary cilia in epithelial maturation, and provide a method to mature iPSC epithelial cells for clinical applications.

May-Simera et al. show that primary cilia regulate the maturation and polarization of human iPSC-RPE, mouse RPE, and human iPSC-lung epithelium through canonical WNT suppression and PKCδ activation. RPE cells derived from ciliopathy patients exhibit defective structure and function. These results provide insights into ciliopathy-induced retinal degeneration.

Biophysics in oviduct: Planar cell polarity, cilia, epithelial fold and tube morphogenesis, egg dynamics

Organs and tissues in multi-cellular organisms exhibit various morphologies. Tubular organs have multi-scale morphological features which are closely related to their functions. Here we discuss morphogenesis and the mechanical functions of the vertebrate oviduct in the female reproductive tract, also known as the fallopian tube. The oviduct functions to convey eggs from the ovary to the uterus. In the luminal side of the oviduct, the epithelium forms multiple folds (or ridges) well-aligned along the longitudinal direction of the tube. In the epithelial cells, cilia are formed orienting toward the downstream of the oviduct. The cilia and the folds are supposed to be involved in egg transportation. Planar cell polarity (PCP) is developed in the epithelium, and the disruption of the Celsr1 gene, a PCP related-gene, causes randomization of both cilia and fold orientations, discontinuity of the tube, inefficient egg transportation, and infertility. In this review article, we briefly introduce various biophysical and biomechanical issues in the oviduct, including physical mechanisms of formation of PCP and organized cilia orientation, epithelial cell shape regulation, fold pattern formation generated by mechanical buckling, tubulogenesis, and egg transportation regulated by fluid flow. We also mention about possible roles of the oviducts in egg shape formation and embryogenesis, sinuous patterns of tubes, and fold and tube patterns observed in other tubular organs such as the gut, airways, etc.

Centriole Positioning: Not Just a Little Dot in the Cell

Organelle positioning as many other morphological parameters in a cell is not random. Centriole positioning as centrosomes or ciliary basal bodies is not an exception to this rule in cell biology. Indeed, centriole positioning is a tightly regulated process that occurs during development, and it is critical for many organs to function properly, not just during development but also in the adulthood. In this book chapter, we overview our knowledge on centriole positioning in different and highly specialized animal cells like photoreceptor or ependymal cells. We will also discuss recent advances in the discovery of molecular pathways involved in this process, mostly related to the cytoskeleton and the cell polarity pathways. And finally, we present quantitative methods that have been used to assess centriole positioning in different cell types although mostly in epithelial cells.

Centriole planar polarity assessment in Drosophila wings

In vertebrates, planar polarization of ciliary basal bodies has been associated with actin polymerization that occurs downstream of the Frizzled-planar cell polarity (Fz-PCP) pathway. In Drosophila wing epithelial cells, which do not have cilia, centrioles also polarize in a Fz-PCP-dependent manner, although the relationship with actin polymerization remains unknown. By combining existing and new quantitative methods, we unexpectedly found that known PCP effectors linked to actin polymerization phenotypes affect neither final centriole polarization nor apical centriole distribution. But actin polymerization is required upstream of Fz-PCP to maintain the centrioles in restricted areas in the apical-most planes of those epithelial cells before and after the actin-based hair is formed. Furthermore, in the absence of proper core Fz-PCP signalling, actin polymerization is insufficient to drive this off-centred centriole migration. Altogether, the results reveal that there are at least two pathways controlling centriole positioning in Drosophila pupal wings - an upstream actin-dependent mechanism involved in centriole distribution that is PCP independent, and an unknown mechanism that links core Fz-PCP and centriole polarization.

The small molecule AMBMP disrupts microtubule growth, ciliogenesis, cell polarity, and cell migration

2-Amino-4-(3,4-[methylenedioxy]benzylamino)-6-(3-methoxyphenyl)pyrimidine (AMBMP) is a small molecule that has been previously reported to be both a Wnt agonist and a microtubule (MT) regulator. Here we report a detailed analysis of AMBMPs effects on MTs and on MT associated cellular processes including cell polarity, ciliogenesis, and cell migration. Specifically, treatment of Xenopus embryos with AMBMP leads to defects similar to the MT depolymerizing drug nocodazole, including a failure to generate or polarize cilia (depending on the timing of treatment) and a loss of the cell movements associated with radial intercalation. The dramatic effect AMBMP has on basic MT based cellular functions suggests that its usefulness as a Wnt regulator is questionable. Moreover, it may be an important new tool for experimental or pharmacological manipulation of MTs.

Na+/H+ Exchangers Are Required for the Development and Function of Vertebrate Mucociliary Epithelia

Na+/H+ exchangers (NHEs) represent a highly conserved family of ion transporters that regulate pH homeostasis. NHEs as well as other proton transporters were previously linked to the regulation of the Wnt signaling pathway, cell polarity signaling, and mucociliary function. Furthermore, mutations in the gene SLC9A3 (encoding NHE3) were detected as additional risk factors for airway infections in cystic fibrosis patients. Here, we used the Xenopus embryonic mucociliary epidermis as well as human airway epithelial cells (HAECs) as models to investigate the functional roles of NHEs in mucociliary development and regeneration. In Xenopus embryos, NHEs 1-3 were expressed during epidermal development, and loss of NHE function impaired mucociliary clearance in tadpoles. Clearance defects were caused by reduced cilia formation, disrupted alignment of basal bodies in multiciliated cells (MCCs), and dysregulated mucociliary gene expression. These data also suggested that NHEs may contribute to the activation of Wnt signaling in mucociliary epithelia. In HAECs, pharmacological inhibition of NHE function also caused defective ciliation and regeneration in airway MCCs. Collectively, our data revealed a requirement for NHEs in vertebrate mucociliary epithelia and linked NHE activity to cilia formation and function in differentiating MCCs. Our results provide an entry point for the understanding of the contribution of NHEs to signaling, development, and pathogenesis in the human respiratory tract.

WDR5 Stabilizes Actin Architecture to Promote Multiciliated Cell Formation

The actin cytoskeleton is critical to shape cells and pattern intracellular organelles, which collectively drives tissue morphogenesis. In multiciliated cells (MCCs), apical actin drives expansion of the cell surface necessary to host hundreds of cilia. The apical actin also forms a lattice to uniformly distribute basal bodies. This apical actin network is dynamically remodeled, but the molecules that regulate its architecture remain poorly understood. We identify the chromatin modifier, WDR5, as a regulator of apical F-actin in MCCs. Unexpectedly in MCCs, WDR5 has a function independent of chromatin modification. We discover a scaffolding role for WDR5 between the basal body and F-actin. Specifically, WDR5 binds to basal bodies and migrates apically, where F-actin organizes around WDR5. Using a monomer trap for G-actin, we show that WDR5 stabilizes F-actin to maintain lattice architecture. In summary, we identify a non-chromatin role for WDR5 in stabilizing F-actin in MCCs.

CLAMP/Spef1 regulates planar cell polarity signaling and asymmetric microtubule accumulation in the Xenopus ciliated epithelia

Kim et al. show that CLAMP regulates planar cell polarity (PCP) signaling. Its depletion causes a loss of the atypical cadherin Celsr2, a loss of PCP protein asymmetry, and a defect in cilia polarity and oriented cell division. CLAMP also, via its role in PCP, regulates the accumulation of an asymmetric pool of microtubules.

Most epithelial cells polarize along the axis of the tissue, a feature known as planar cell polarity (PCP). The initiation of PCP requires cell–cell signaling via the noncanonical Wnt/PCP pathway. Additionally, changes in the cytoskeleton both facilitate and reflect this polarity. We have identified CLAMP/Spef1 as a novel regulator of PCP signaling. In addition to decorating microtubules (MTs) and the ciliary rootlet, a pool of CLAMP localizes at the apical cell cortex. Depletion of CLAMP leads to the loss of PCP protein asymmetry, defects in cilia polarity, and defects in the angle of cell division. Additionally, depletion of CLAMP leads to a loss of the atypical cadherin-like molecule Celrs2, suggesting that CLAMP facilitates the stabilization of junctional interactions responsible for proper PCP protein localization. Depletion of CLAMP also affects the polarized organization of MTs. We hypothesize that CLAMP facilitates the establishment of cell polarity and promotes the asymmetric accumulation of MTs downstream of the establishment of proper PCP.

Domineering non-autonomy in Vangl1;Vangl2 double mutants demonstrates intercellular PCP signaling in the vertebrate inner ear

The organization of polarized stereociliary bundles is critical for the function of the inner ear sensory receptor hair cells that detect sound and motion, and these cells present a striking example of Planar Cell Polarity (PCP); the coordinated orientation of polarized structures within the plane of an epithelium. PCP is best understood in Drosophila where the essential genes regulating PCP were first discovered, and functions for the core PCP proteins encoded by these genes have been deciphered through phenotypic analysis of core PCP gene mutants. One illuminating phenotype is the domineering non-autonomy that is observed where abrupt disruptions in PCP signaling impacts the orientation of neighboring wild type cells, because this demonstrates local intercellular signaling mediated by the core PCP proteins. Using Emx2-Cre to generate an analogous mutant boundary in the mouse inner ear, we disrupted vertebrate PCP signaling in Vangl1;Vangl2 conditional knockouts. Due to unique aspects of vestibular anatomy, core PCP protein distribution along the mutant boundary generated in the utricle resembles the proximal side of vang mutant clones in the Drosophila wing, while the boundary in the saccule resembles and the distal side. Consistent with these protein distributions, a domineering non-autonomy phenotype occurs along the Emx2-Cre boundary in the mutant utricle that does not occur in the saccule. These results further support the hypothesis that core PCP function is conserved in vertebrates by demonstrating intercellular PCP signaling in the sensory epithelia of the mouse ear.