Wnt/PCP signaling controls intracellular position of MTOCs during gastrulation convergence and extension movements

In vivo cell and tissue dynamics underlying zebrafish fin fold regeneration

Background

Zebrafish (Danio rerio) has a remarkable capacity to regenerate many organs and tissues. During larval stages the fin fold allows the possibility of performing long time-lapse imaging making this system very appealing to study the relationships between tissue movements, cell migration and proliferation necessary for the regeneration process.

Results

Through the combined use of transgenic fluorescently-labeled animals and confocal microscopy imaging, we characterized in vivo the complete fin fold regeneration process. We show, for the first time, that there is an increase in the global rate of epidermal growth as a response to tissue loss. Also enhanced significantly is cell proliferation, which upon amputation happens in a broad area concerning the amputation level and not in a blastema-restricted way. This reveals a striking difference with regard to the adult fin regeneration system. Finally, an accumulation of migratory, shape-changing fibroblasts occurs proximally to the wound area, resembling a blastemal-like structure, which may act as a signaling center for the regeneration process to proceed.

Conclusions

These findings provide a novel in vivo description of fundamental mechanisms occurring during the fin fold regeneration process, thereby contributing to a better knowledge of this regenerative system and to reveal variations in the epimorphic regeneration field.

Convergent extension: using collective cell migration and cell intercalation to shape embryos

Body axis elongation represents a common and fundamental morphogenetic process in development. A key mechanism triggering body axis elongation without additional growth is convergent extension (CE), whereby a tissue undergoes simultaneous narrowing and extension. Both collective cell migration and cell intercalation are thought to drive CE and are used to different degrees in various species as they elongate their body axis. Here, we provide an overview of CE as a general strategy for body axis elongation and discuss conserved and divergent mechanisms underlying CE among different species.

Microtubules enable the planar cell polarity of airway cilia

BACKGROUND

Airway cilia must be physically oriented along the longitudinal tissue axis for concerted, directional motility that is essential for proper mucociliary clearance.

RESULTS

We show that planar cell polarity (PCP) signaling specifies directionality and orients respiratory cilia. Within all airway epithelial cells, a conserved set of PCP proteins shows interdependent, asymmetric junctional localization; nonautonomous signaling coordinates polarization between cells; and a polarized microtubule (MT) network is likely required for asymmetric PCP protein localization. We find that basal bodies dock after polarity of PCP proteins is established and are polarized nearly simultaneously, and that refinement of basal body/cilium orientation continues during airway epithelial development. Unique to mature multiciliated cells, we identify PCP-regulated, planar polarized MTs that originate from basal bodies and interact, via their plus ends, with membrane domains associated with the PCP proteins Frizzled and Dishevelled. Disruption of MTs leads to misoriented cilia.

CONCLUSIONS

A conserved PCP pathway orients airway cilia by communicating polarity information from asymmetric membrane domains at the apical junctions, through MTs, to orient the MT and actin-based network of ciliary basal bodies below the apical surface.

Wnt proteins

Wnt proteins comprise a major family of signaling molecules that orchestrate and influence a myriad of cell biological and developmental processes. Although our understanding of the role of Wnt signaling in regulating development and affecting disease, such as cancer, has been ever increasing, the study of the Wnt proteins themselves has been painstaking and slow moving. Despite advances in the biochemical characterization of Wnt proteins, many mysteries remain unsolved. In contrast to other developmental signaling molecules, such as fibroblast growth factors (FGF), transforming growth factors (TGFβ), and Sonic hedgehog (Shh), Wnt proteins have not conformed to many standard methods of protein production, such as bacterial overexpression, and analysis, such as ligand-receptor binding assays. The reasons for their recalcitrant nature are likely a consequence of the complex set of posttranslational modifications involving several highly specialized and poorly characterized processing enzymes. With the recent description of the first Wnt protein structure, the time is ripe to uncover and possibly resolve many of the remaining issues surrounding Wnt proteins and their interactions. Here we describe the process of maturation of Wnt from its initial translation to its eventual release from a cell and interactions in the extracellular environment.

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.

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

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

Centrosome movements in vivo correlate with specific neurite formation downstream of LIM homeodomain transcription factor activity

Neurons must develop complex structure to form proper connections in the nervous system. The initiation of axons in defined locations on the cell body and their extension to synaptic targets are critical steps in neuronal morphogenesis, yet the mechanisms controlling axon formation in vivo are poorly understood. The centrosome has been implicated in multiple aspects of neuronal morphogenesis; however, its function in axon development is under debate. Conflicting results from studies of centrosome function in axonogenesis suggest that its role is context dependent and underscore the importance of studying centrosome function as neurons develop in their natural environment. Using live imaging of zebrafish Rohon-Beard (RB) sensory neurons in vivo, we discovered a spatiotemporal relationship between centrosome position and the formation of RB peripheral, but not central, axons. We tested centrosome function by laser ablation and found that centrosome disruption inhibited peripheral axon outgrowth. In addition, we show that centrosome position and motility are regulated by LIM homeodomain transcription factor activity, which is specifically required for the development of RB peripheral axons. Furthermore, we show a correlation between centrosome mislocalization and ectopic axon formation in bashful (laminin alpha 1) mutants. Thus, both intrinsic transcription factor activity and extracellular cues can influence centrosome position and axon formation in vivo. This study presents the first positive association between the centrosome and axon formation in vivo and suggests that the centrosome is important for differential neurite formation in neurons with complex axonal morphologies.

Gastrulation: making and shaping germ layers

Gastrulation is a fundamental phase of animal embryogenesis during which germ layers are specified, rearranged, and shaped into a body plan with organ rudiments. Gastrulation involves four evolutionarily conserved morphogenetic movements, each of which results in a specific morphologic transformation. During emboly, mesodermal and endodermal cells become internalized beneath the ectoderm. Epibolic movements spread and thin germ layers. Convergence movements narrow germ layers dorsoventrally, while concurrent extension movements elongate them anteroposteriorly. Each gastrulation movement can be achieved by single or multiple motile cell behaviors, including cell shape changes, directed migration, planar and radial intercalations, and cell divisions. Recent studies delineate cyclical and ratchet-like behaviors of the actomyosin cytoskeleton as a common mechanism underlying various gastrulation cell behaviors. Gastrulation movements are guided by differential cell adhesion, chemotaxis, chemokinesis, and planar polarity. Coordination of gastrulation movements with embryonic polarity involves regulation by anteroposterior and dorsoventral patterning systems of planar polarity signaling, expression of chemokines, and cell adhesion molecules.

Diverse chemical scaffolds support direct inhibition of the membrane-bound O-acyltransferase porcupine

Secreted Wnt proteins constitute one of the largest families of intercellular signaling molecules in vertebrates with essential roles in embryonic development and adult tissue homeostasis. The functional redundancy of Wnt genes and the many forms of cellular responses they elicit, including some utilizing the transcriptional co-activator β-catenin, has limited the ability of classical genetic strategies to uncover their roles in vivo. We had previously identified a chemical compound class termed Inhibitor of Wnt Production (or IWP) that targets Porcupine (Porcn), an acyltransferase catalyzing the addition of fatty acid adducts onto Wnt proteins. Here we demonstrate that diverse chemical structures are able to inhibit Porcn by targeting its putative active site. When deployed in concert with small molecules that modulate the activity of Tankyrase enzymes and glycogen synthase kinase 3 β (GSK3β), additional transducers of Wnt/β-catenin signaling, the IWP compounds reveal an essential role for Wnt protein fatty acylation in eliciting β-catenin-dependent and -independent forms of Wnt signaling during zebrafish development. This collection of small molecules facilitates rapid dissection of Wnt gene function in vivo by limiting the influence of redundant Wnt gene functions on phenotypic outcomes and enables temporal manipulation of Wnt-mediated signaling in vertebrates.

Nuclear localization of Prickle2 is required to establish cell polarity during early mouse embryogenesis

The establishment of trophectoderm (TE) manifests as the formation of epithelium, and is dependent on many structural and regulatory components that are commonly found and function in many epithelial tissues. However, the mechanism of TE formation is currently not well understood. Prickle1 (Pk1), a core component of the planar cell polarity (PCP) pathway, is essential for epiblast polarization before gastrulation, yet the roles of Pk family members in early mouse embryogenesis are obscure. Here we found that Pk2(-/-) embryos died at E3.0-3.5 without forming the blastocyst cavity and not maintained epithelial integrity of TE. These phenotypes were due to loss of the apical-basal (AB) polarity that underlies the asymmetric redistribution of microtubule networks and proper accumulation of AB polarity components on each membrane during compaction. In addition, we found GTP-bound active form of nuclear RhoA was decreased in Pk2(-/-) embryos during compaction. We further show that the first cell fate decision was disrupted in Pk2(-/-) embryos. Interestingly, Pk2 localized to the nucleus from the 2-cell to around the 16-cell stage despite its cytoplasmic function previously reported. Inhibiting farnesylation blocked Pk2's nuclear localization and disrupted AB cell polarity, suggesting that Pk2 farnesylation is essential for its nuclear localization and function. The cell polarity phenotype was efficiently rescued by nuclear but not cytoplasmic Pk2, demonstrating the nuclear localization of Pk2 is critical for its function.

Planar cell polarity in coordinated and directed movements

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

Planar cell polarity: coordinating morphogenetic cell behaviors with embryonic polarity

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

Polycystic kidney disease: the complexity of planar cell polarity and signaling during tissue regeneration and cyst formation

Autosomal Dominant Polycystic Kidney Disease (ADPKD) is an inherited systemic disease with intrarenal cystogenesis as its primary characteristic. A variety of mouse models provided information on the requirement of loss of balanced polycystin levels for initiation of cyst formation, the role of proliferation in cystogenesis and the signaling pathways involved in cyst growth and expansion. Here we will review the involvement of different signaling pathways during renal development, renal epithelial regeneration and cyst formation in ADPKD, focusing on planar cell polarity (PCP) and oriented cell division (OCD). This will be discussed in context of the hypothesis that aberrant PCP signaling causes cyst formation. In addition, the role of the Hippo pathway, which was recently found to be involved in cyst growth and tissue regeneration, and well-known for regulating organ size control, will be reviewed. The fact that Hippo signaling is linked to PCP signaling makes the Hippo pathway a novel cascade in cystogenesis. The newly gained understanding of the complex signaling network involved in cystogenesis and disease progression, not only necessitates refining of the current hypothesis regarding initiation of cystogenesis, but also has implications for therapeutic intervention strategies. This article is part of a Special Issue entitled: Polycystic Kidney Disease.

The planar cell polarity pathway in vertebrate development

Directing the orientation of cells in three dimensions is a fundamental aspect of many of the processes underlying the generation of the appropriate shape and function of tissues and organs during embryonic development. In an epithelium, this requires not only the establishment of apicobasal polarity, but also cell arrangement in a specific direction in the plane of the cell sheet. The molecular pathway central to regulating this planar cell polarity (PCP) was originally discovered in the fruit fly Drosophila melanogaster and has more recently been shown to act in a highly analogous way in vertebrates, involving a strongly overlapping set of genes. Mutant studies and molecular analyses have led to insights into the role of ordered planar cell polarity in the development of a wide variety of organs and tissues. In this review, we give an overview of recent developments in the study of planar polarity signaling in vertebrates.

Non-cell-autonomous planar cell polarity propagation in the auditory sensory epithelium of vertebrates

Sensory epithelia of the inner ear require a coordinated alignment of hair cell stereociliary bundles as an essential element of mechanoreceptive function. Hair cell bundle alignment is mediated by core planar cell polarity (PCP) proteins, such as Vangl2, that localize asymmetrically to the circumference of the cell near its apical surface. During early phases of cell orientation in the chicken basilar papilla (BP), Vangl2 is present at supporting cell junctions that lie orthogonal to the polarity axis. Several days later, there is a striking shift in the Vangl2 pattern associated with hair cells that reorient towards the distal (apical) end of the organ. How the localization of PCP proteins transmits planar polarity information across the developing sensory epithelium remains unclear. To address this question, the normal asymmetric localization of Vangl2 was disrupted by overexpressing Vangl2 in clusters of cells. The BP was infected with replication-competent retrovirus encoding Vangl2 prior to hair cell differentiation. Virus-infected cells showed normal development of individual stereociliary bundles, indicating that asymmetry was established at the cellular level. Yet, bundles were misoriented in ears infected with Vangl2 virus but not Wnt5a virus. Notably, Vangl2 misexpression did not randomize bundle orientations but rather generated larger variations around a normal mean angle. Cell clusters with excess Vangl2 could induce non-autonomous polarity disruptions in wild-type neighboring cells. Furthermore, there appears to be a directional bias in the propagation of bundle misorientation that is towards the abneural edge of the epithelium. Finally, regional bundle reorientation was inhibited by Vangl2 overexpression. In conclusion, ectopic Vangl2 protein causes inaccurate local propagation of polarity information, and Vangl2 acts in a non-cell-autonomous fashion in the sensory system of vertebrates.

Zebrafish assays of ciliopathies

In light of the growing list of human disorders associated with their dysfunction, primary cilia have recently come to attention as being important regulators of developmental signaling pathways and downstream processes. These organelles, present on nearly every vertebrate cell type, are highly conserved structures allowing for study across a range of species. Zebrafish, in particular, have emerged as useful organisms in which to explore the consequences of ciliary dysfunction and to model human ciliopathies. Here, we present a range of useful techniques that allow for investigation of various aspects of ciliary function. The described assays capitalize on the hallmark gastrulation defects associated with ciliary defects as well as relative ease of visualization of cilia in whole-mount embryos. Further, we describe our recently developed assay for querying functionality of human gene variants in live developing embryos. Finally, a current catalog of known zebrafish ciliary mutant lines is included. The techniques presented here provide a basic toolkit for in vivo investigation of both the biological and genetic mechanisms underlying a growing class of human diseases.