XGAP, an ArfGAP, is required for polarized localization of PAR proteins and cell polarity in Xenopus gastrulation

Models of convergent extension during morphogenesis

Convergent extension (CE) is a fundamental and conserved collective cell movement that forms elongated tissues during embryonic development. Thus far, studies have demonstrated two different mechanistic models of collective cell movements during CE. The first, termed the crawling mode, was discovered in the process of notochord formation in Xenopus laevis embryos, and has been the established model of CE for decades. The second model, known as the contraction mode, was originally reported in studies of germband extension in Drosophila melanogaster embryos and was recently demonstrated to be a conserved mechanism of CE among tissues and stages of development across species. This review summarizes the two modes of CE by focusing on the differences in cytoskeletal behaviors and relative expression of cell adhesion molecules. The upstream molecules regulating these machineries are also discussed. There are abundant studies of notochord formation in X. laevis embryos, as this was one of the pioneering model systems in this field. Therefore, the present review discusses these findings as an approach to the fundamental biological question of collective cell regulation. WIREs Dev Biol 2018, 7:e293. doi: 10.1002/wdev.293

This article is categorized under: 1

Early Embryonic Development > Gastrulation and Neurulation

2

Comparative Development and Evolution > Model Systems

Filopodial-Tension Model of Convergent-Extension of Tissues

In convergent-extension (CE), a planar-polarized epithelial tissue elongates (extends) in-plane in one direction while shortening (converging) in the perpendicular in-plane direction, with the cells both elongating and intercalating along the converging axis. CE occurs during the development of most multicellular organisms. Current CE models assume cell or tissue asymmetry, but neglect the preferential filopodial activity along the convergent axis observed in many tissues. We propose a cell-based CE model based on asymmetric filopodial tension forces between cells and investigate how cell-level filopodial interactions drive tissue-level CE. The final tissue geometry depends on the balance between external rounding forces and cell-intercalation traction. Filopodial-tension CE is robust to relatively high levels of planar cell polarity misalignment and to the presence of non-active cells. Addition of a simple mechanical feedback between cells fully rescues and even improves CE of tissues with high levels of polarity misalignments. Our model extends easily to three dimensions, with either one converging and two extending axes, or two converging and one extending axes, producing distinct tissue morphologies, as observed in vivo.

The development of an embryo from a fertilized egg to an adult organism requires not only cell proliferation and differentiation, but also numerous types of tissue restructuring. The development of a relatively round initial embryo into one elongated along its rostral-caudal axis involves coordinated tissue elongation and cell reorganization in one or more groups of cells or tissues. Counterintuitively, in many organisms, cells in elongating tissues elongate and increase their protrusive activity in the direction perpendicular to the axis of elongation (convergent extension). Experimental and theoretical studies have not determined how this cell-level oriented protrusive activity leads to observed tissue-level changes in morphology. We propose a filopodial-tension model that shows how tension from oriented cell protrusions leads to observed patterns of tissue CE.

Nuclear fallout provides a new link between aPKC and polarized cell trafficking

Background

Cell polarity, essential for cell physiology and tissue coherence, emerges as a consequence of asymmetric localization of protein complexes and directional trafficking of cellular components. Although molecules required in both processes are well known their relationship is still poorly understood.

Results

Here we show a molecular link between Nuclear Fallout (Nuf), an adaptor of Rab11-GTPase to the microtubule motor proteins during Recycling Endosome (RE) trafficking, and aPKC, a pivotal kinase in the regulation of cell polarity. We demonstrate that aPKC phosphorylates Nuf modifying its subcellular distribution. Accordingly, in aPKC mutants Nuf and Rab11 accumulate apically indicating altered RE delivery. We show that aPKC localization in the apico-lateral cortex is dynamic. When we block exocytosis, by means of exocyst-sec mutants, aPKC accumulates inside the cells. Moreover, apical aPKC concentration is reduced in nuf mutants, suggesting aPKC levels are maintained by recycling.

Conclusions

We demonstrate that active aPKC interacts with Nuf, phosphorylating it and, as a result, modifying its subcellular distribution. We propose a regulatory loop by which Nuf promotes aPKC apical recycling until sufficient levels of active aPKC are reached. Thus, we provide a novel link between cell polarity regulation and traffic control in epithelia.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-016-0253-6) contains supplementary material, which is available to authorized users.

Influence of cell polarity on early development of the sea urchin embryo

BACKGROUND

Establishment and maintenance of cell polarity is critical for normal embryonic development. Previously, it was thought that the echinoderm embryo remained relatively unpolarized until the first asymmetric division at the 16-cell stage. Here, we analyzed roles of the cell polarity regulators, the PAR complex proteins, and how their disruption in early development affects later developmental milestones.

RESULTS

We found that PAR6, aPKC, and CDC42 localize to the apical cortex as early as the 2-cell stage and that this localization is retained through the gastrula stage. Of interest, PAR1 also colocalizes with these apical markers through the gastrula stage. Additionally, PAR1 was found to be in complex with aPKC, but not PAR6. PAR6, aPKC, and CDC42 are anchored in the cortical actin cytoskeleton by assembled myosin. Furthermore, assembled myosin was found to be necessary to maintain proper PAR6 localization through subsequent cleavage divisions. Interference with myosin assembly prevented the embryos from reaching the blastula stage, while transient disruptions of either actin or microtubules did not have this effect.

CONCLUSIONS

These observations suggest that disruptions of the polarity in the early embryo can have a significant impact on the ability of the embryo to reach later critical stages in development.

Polycystin-1 binds Par3/aPKC and controls convergent extension during renal tubular morphogenesis

Several organs, including lungs and kidneys, are formed by epithelial tubes whose proper morphogenesis ensures correct function. This is best exemplified by the kidney, where defective establishment or maintanance of tubular diameter results in polycystic kidney disease, a common genetic disorder. Most polycystic kidney disease cases result from loss-of-function mutations in the PKD1 gene, encoding Polycystin-1 (PC-1), a large receptor of unknown function. Here we demonstrate that PC-1 plays an essential role in establishment of correct tubular diameter during nephron development. PC-1 associates with Par3 favoring the assembly of a pro-polarizing Par3/aPKC complex and it regulates a program of cell polarity important for oriented cell migration and for a convergent extension-like process during tubular morphogenesis. Par3 inactivation in the developing kidney results in defective convergent extension and tubular morphogenesis and in renal cyst formation. Our data define PC-1 as central to cell polarization and to epithelial tube morphogenesis and homeostasis.

Cell intercalation from top to bottom

Animal development requires a carefully orchestrated cascade of cell fate specification events and cellular movements. A surprisingly small number of choreographed cellular behaviours are used repeatedly to shape the animal body plan. Among these, cell intercalation lengthens or spreads a tissue at the expense of narrowing along an orthogonal axis. Key steps in the polarization of both mediolaterally and radially intercalating cells have now been clarified. In these different contexts, intercalation seems to require a distinct combination of mechanisms, including adhesive changes that allow cells to rearrange, cytoskeletal events through which cells exert the forces needed for cell neighbour exchange, and in some cases the regulation of these processes through planar cell polarity.

Ofd1 controls dorso-ventral patterning and axoneme elongation during embryonic brain development

Oral-facial-digital type I syndrome (OFDI) is a human X-linked dominant-male-lethal developmental disorder caused by mutations in the OFD1 gene. Similar to other inherited disorders associated to ciliary dysfunction OFD type I patients display neurological abnormalities. We characterized the neuronal phenotype that results from Ofd1 inactivation in early phases of mouse embryonic development and at post-natal stages. We determined that Ofd1 plays a crucial role in forebrain development, and in particular, in the control of dorso-ventral patterning and early corticogenesis. We observed abnormal activation of Sonic hedgehog (Shh), a major pathway modulating brain development. Ultrastructural studies demonstrated that early Ofd1 inactivation results in the absence of ciliary axonemes despite the presence of mature basal bodies that are correctly orientated and docked. Ofd1 inducible-mediated inactivation at birth does not affect ciliogenesis in the cortex, suggesting a developmental stage-dependent role for a basal body protein in ciliogenesis. Moreover, we showed defects in cytoskeletal organization and apical-basal polarity in Ofd1 mutant embryos, most likely due to lack of ciliary axonemes. Thus, the present study identifies Ofd1 as a developmental disease gene that is critical for forebrain development and ciliogenesis in embryonic life, and indicates that Ofd1 functions after docking and before elaboration of the axoneme in vivo.

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.

Punctuated actin contractions during convergent extension and their permissive regulation by the non-canonical Wnt-signaling pathway

Actomyosin networks linked to the micro-environment through the plasma membrane are thought to be key players in regulating cell behaviors within multicellular tissues, such as converging and extending mesoderm. Here, we observe the dynamics of actin contractions called 'punctuated actin contractions' in the mid-cell body of embryonic mesenchymal cells in the mesoderm. These contraction dynamics are a common feature of Xenopus embryonic tissues and are important for cell shape changes during morphogenesis. Quantitative morphological analysis of these F-actin dynamics indicates that frequent and aligned movements of multiple actin contractions accompany mesoderm cells as they intercalate and elongate. Using inhibitors combined with fluorescence recovery after photobleaching (FRAP) analysis, we find that the dynamics of actin contractions are regulated by both myosin contractility and F-actin polymerization. Furthermore, we find that the non-canonical Wnt-signaling pathway permissively regulates levels of punctuated actin contractions. Overexpression of Xfz7 (Fzd7) can induce early maturation of actin contractions in mesoderm and produce mesoderm-like actin contractions in ectoderm cells. By contrast, expression of the dominant-negative Xenopus disheveled construct Xdd1 blocks the progression of actin contractions into their late mesoderm dynamics but has no effect in ectoderm. Our study reveals punctuated actin contractions within converging and extending mesoderm and uncovers a permissive role for non-canonical Wnt-signaling, myosin contractility and F-actin polymerization in regulating these dynamics.

Planar cell polarity signaling: the developing cell's compass

Cells of many tissues acquire cellular asymmetry to execute their physiologic functions. The planar cell polarity system, first characterized in Drosophila, is important for many of these events. Studies in Drosophila suggest that an upstream system breaks cellular symmetry by converting tissue gradients to subcellular asymmetry, whereas a downstream system amplifies subcellular asymmetry and communicates polarity between cells. In this review, we discuss apparent similarities and differences in the mechanism that controls PCP as it has been adapted to a broad variety of morphological cellular asymmetries in various organisms.

Widely conserved signaling pathways in the establishment of cell polarity

How are the asymmetric distributions of proteins, lipids, and RNAs established and maintained in various cell types? Studies from diverse organisms show that Par proteins, GTPases, kinases, and phosphoinositides participate in conserved signaling pathways to establish and maintain cell polarity.

xGit2 and xRhoGAP 11A regulate convergent extension and tissue separation in Xenopus gastrulation

In a microarray-based screen for genes that are involved in tissue separation downstream of Paraxial Protocadherin (PAPC) and Frizzled-7 (Fz7)-mediated signaling we identified xGit2 and xRhoGAP 11A, two GTPase-activating proteins (GAP) for small GTPases. xGit2 and xRhoGAP 11A are expressed in the dorsal ectoderm, and their transcription is downregulated in the involuting dorsal mesoderm by PAPC and Fz7. Overexpression of xGit2 and xRhoGAP 11A inhibits Rho activity and impairs convergent extension movements as well as tissue separation behaviour. We propose that Rho activity in the involuting mesoderm is enhanced through inhibition of xGit2 and xRhoGAP 11A transcription by PAPC and Fz7. By this mechanism xRhoGAP 11A and xGit2 are restricted to the dorsal ectoderm, while Rho signaling is inhibited.

PKC and the control of localized signal dynamics

Networks of signal transducers determine the conversion of environmental cues into cellular actions. Among the main players in these networks are protein kinases, which can acutely and reversibly modify protein functions to influence cellular events. One group of kinases, the protein kinase C (PKC) family, have been increasingly implicated in the organization of signal propagation, particularly in the spatial distribution of signals. Examples of where and how various PKC isoforms direct this tier of signal organization are becoming more evident.

Multi-scale mechanics from molecules to morphogenesis

Dynamic mechanical processes shape the embryo and organs during development. Little is understood about the basic physics of these processes, what forces are generated, or how tissues resist or guide those forces during morphogenesis. This review offers an outline of some of the basic principles of biomechanics, provides working examples of biomechanical analyses of developing embryos, and reviews the role of structural proteins in establishing and maintaining the mechanical properties of embryonic tissues. Drawing on examples we highlight the importance of investigating mechanics at multiple scales from milliseconds to hours and from individual molecules to whole embryos. Lastly, we pose a series of questions that will need to be addressed if we are to understand the larger integration of molecular and physical mechanical processes during morphogenesis and organogenesis.

An adhesion-independent, aPKC-dependent function for cadherins in morphogenetic movements

Cadherin shedding affects migration and occurs in development and cancer progression. By examining the in vivo biological function of the extracellular cadherin domain (CEC1-5) independently of the shedding process itself, we identified a novel function for cadherins in convergent extension (CE) movements in Xenopus. CEC1-5 interfered with CE movements during gastrulation. Unexpectedly, CEC1-5 did not alter cell aggregation or adhesion to cadherin substrates. Instead, gastrulation defects were rescued by a membrane-anchored cadherin cytoplasmic domain, the polarity protein atypical PKC (aPKC) or constitutive active Rac, indicating that CEC1-5 modulates a cadherin-dependent signalling pathway. We found that the cadherin interacts with aPKC and, more importantly, that the extracellular domain alters this association as well as the phosphorylation status of aPKC. This suggests that CE movements require a dynamic regulation of cadherin-aPKC interaction. Our results show that cadherins play a dual role in CE movements: a previously identified adhesive activity and an adhesion-independent function that requires aPKC and Rac, thereby directly connecting cadherins with polarity. Our results also suggest that increased cadherin shedding, often observed in cancer progression, can regulate migration and invasion by modulating polarity protein activity.

Regulation of planar cell polarity by Smurf ubiquitin ligases

Planar cell polarity (PCP) is critical for morphogenesis in metazoans. PCP in vertebrates regulates stereocilia alignment in neurosensory cells of the cochlea and closure of the neural tube through convergence and extension movements (CE). Noncanonical Wnt morphogens regulate PCP and CE in vertebrates, but the molecular mechanisms remain unclear. Smurfs are ubiquitin ligases that regulate signaling, cell polarity and motility through spatiotemporally restricted ubiquitination of diverse substrates. Here, we report an unexpected role for Smurfs in controlling PCP and CE. Mice mutant for Smurf1 and Smurf2 display PCP defects in the cochlea and CE defects that include a failure to close the neural tube. Further, we show that Smurfs engage in a noncanonical Wnt signaling pathway that targets the core PCP protein Prickle1 for ubiquitin-mediated degradation. Our work thus uncovers ubiquitin ligases in a mechanistic link between noncanonical Wnt signaling and PCP/CE.

Noncanonical Wnt/PCP signaling during vertebrate gastrulation

The branch of the Wnt pathway, related to planar cell polarity signaling in Drosophila, is fundamental not only to the establishment of tissue polarity but also to a variety of morphogenetic processes in vertebrates. The genetic pathway has been noted for its similarity as well as divergence of between vertebrates and Drosophila. This review focuses on issues related to the complexity of the output of the planar cell polarity pathway during gastrulation in zebrafish and Xenopus and, to a lesser extent, during gastrulation/neurulation in mice.

Regulated adhesion as a driving force of gastrulation movements

Recent data have reinforced the fundamental role of regulated cell adhesion as a force that drives morphogenesis during gastrulation. As we discuss, cell adhesion is required for all modes of gastrulation movements in all organisms. It can even be instructive in nature, but it must be tightly and dynamically regulated. The picture that emerges from the recent findings that we review here is that different modes of gastrulation movements use the same principles of adhesion regulation, while adhesion molecules themselves coordinate the intra- and extracellular changes required for directed cell locomotion.

Multicellular dynamics during epithelial elongation

The reorganization of multicellular populations to produce an elongated tissue structure is a conserved mechanism for shaping the body axis and several organ systems. In the Drosophila germband epithelium, this process is accompanied by the formation of a planar polarized network of junctional and cytoskeletal proteins in response to striped patterns of gene expression. Actomyosin cables and adherens junctions are dynamically remodeled during intercalation, providing the basis for polarized cell behavior. Quantitative analysis of cell behavior in living embryos reveals unexpected cell population dynamics that include the formation of multicellular rosette structures as well as local neighbor exchange.

Coordination of cell polarity during Xenopus gastrulation

Cell polarity is an essential feature of animal cells contributing to morphogenesis. During Xenopus gastrulation, it is known that chordamesoderm cells are polarized and intercalate each other allowing anterior-posterior elongation of the embryo proper by convergent extension (CE). Although it is well known that the cellular protrusions at both ends of polarized cells exert tractive force for intercalation and that PCP pathway is known to be essential for the cell polarity, little is known about what triggers the cell polarization and what the polarization causes to control intracellular events enabling the intercalation that leads to the CE. In our research, we used EB3 (end-binding 3), a member of +TIPs that bind to the plus end of microtubule (MT), to visualize the intracellular polarity of chordamesoderm cells during CE to investigate the trigger of the establishment of cell polarity. We found that EB3 movement is polarized in chordamesoderm cells and that the notochord-somite tissue boundary plays an essential role in generating the cell polarity. This polarity was generated before the change of cell morphology and the polarized movement of EB3 in chordamesoderm cells was also observed near the boundary between the chordamesoderm tissue and naïve ectoderm tissue or lateral mesoderm tissues induced by a low concentration of nodal mRNA. These suggest that definitive tissue separation established by the distinct levels of nodal signaling is essential for the chordamesodermal cells to acquire mediolateral cell polarity.

Expression patterns of three Par-related genes in sea urchin embryos

Partitioning-defective (Par) genes were originally identified in Caenorhabditis elegans and are involved in asymmetric divisions of the egg. Recently, the expression and function of Par orthologs have been elucidated in deuterostomes, including vertebrates. In this study, we isolated three Par-related genes, Par-1, Par-6, and atypical protein kinase C (aPKC), from the sea urchin Hemicentrotus pulcherrimus and examined their temporal and spatial expression patterns during embryogenesis up to the pluteus stage. All three transcripts existed maternally in eggs and were uniformly expressed in cleavage-stage embryos. From the blastula to early gastrula stages, HpPar-1 expression was transiently restricted to the vegetal plate, including the primary mesenchyme cells (PMCs); this transient reduction was followed by uniform expression. HpPar-6 was expressed uniformly throughout development. In contrast, HpaPKC expression changed dramatically during development. At the blastula stage, HpaPKC expression was restricted to the vegetal region, including PMCs and the vegetal plate. During gastrulation, expression was maintained in PMCs and the archenteron tip, but expression declined at the late gastrula stage. From the prism stage, two cell types started to express HpaPKC: ectoderm cells interspersed in the ciliary band and skeletogenic cells at the posterior end of the larva. At the pluteus stage, the stomach began to express HpaPKC, in addition to the interspersed ciliary band and skeletogenic cells.

Planar cell polarity signaling: from fly development to human disease

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

Integrating morphogenesis with underlying mechanics and cell biology

Morphogenesis integrates a wide range of cellular processes into a self-organizing, self-deforming tissue. No single molecular "magic bullet" controls morphogenesis. Wide ranging cellular processes, often without parallels in conventional cell culture systems, work together to generate the architecture and modulate forces that produce and guide shape changes in the embryo. In this review we summarize the early development of the frog Xenopus laevis from a biomechanical perspective. We describe processes operating in the embryo from whole embryo scale, the tissue-scale, to the cellular and extracellular matrix scale. We focus on describing cells, their behaviors and the unique microenvironments they traverse during gastrulation and discuss the role of tissue mechanics in these processes.

Lrp6 is required for convergent extension during Xenopus gastrulation

Wnt signaling regulates beta-catenin-mediated gene transcription and planar cell polarity (PCP). The Wnt co-receptor, Lrp6, is required for signaling along the beta-catenin arm. We show that Lrp6 downregulation (by morpholino injection) or overexpression in Xenopus embryos disrupts convergent extension, a hallmark feature of Wnt/PCP components. In embryos with decreased Lrp6 levels, cells of the dorsal marginal zone (DMZ), which undergoes extensive cellular rearrangements during gastrulation, exhibit decreased length:width ratios, decreased migration, and increased numbers of transient cytoplasmic protrusions. We show that Lrp6 opposes Wnt11 activity and localizes to the posterior edge of migrating DMZ cells and that Lrp6 downregulation enhances cortical and nuclear localization of Dsh and phospho-JNK, respectively. Taken together, these data suggest that Lrp6 inhibits Wnt/PCP signaling. Finally, we identify the region of the Lrp6 protein with Wnt/PCP activity to a stretch of 36 amino acids, distinct from regions required for Wnt/beta-catenin signaling. We propose a model in which Lrp6 plays a critical role in the switch from Wnt/PCP to Wnt/beta-catenin signaling.

The PAR proteins: fundamental players in animal cell polarization

The par genes were discovered in genetic screens for regulators of cytoplasmic partitioning in the early embryo of C. elegans, and encode six different proteins required for asymmetric cell division by the worm zygote. Some of the PAR proteins are localized asymmetrically and form physical complexes with one another. Strikingly, the PAR proteins have been found to regulate cell polarization in many different contexts in diverse animals, suggesting they form part of an ancient and fundamental mechanism for cell polarization. Although the picture of how the PAR proteins function remains incomplete, cell biology and biochemistry are beginning to explain how PAR proteins polarize cells.

Convergent extension and the hexahedral cell

During development, embryonic cells sculpt three-dimensional tissues. Although cell polarity is commonly analysed along one, and sometimes two, dimensions, this perspective illustrates how higher-order cell polarity regulates convergent extension - the coordinated cell rearrangement that produces solid tissue elongation.

Wnt5a functions in planar cell polarity regulation in mice

Planar cell polarity (PCP) refers to the polarization of cells within the plane of a cell sheet. A distinctive epithelial PCP in vertebrates is the uniform orientation of stereociliary bundles of the sensory hair cells in the mammalian cochlea. In addition to establishing epithelial PCP, planar polarization is also required for convergent extension (CE); a polarized cellular movement that occurs during neural tube closure and cochlear extension. Studies in Drosophila and vertebrates have revealed a conserved PCP pathway, including Frizzled (Fz) receptors. Here we use the cochlea as a model system to explore the involvement of known ligands of Fz, Wnt morphogens, in PCP regulation. We show that Wnt5a forms a reciprocal expression pattern with a Wnt antagonist, the secreted frizzled-related protein 3 (Sfrp3 or Frzb), along the axis of planar polarization in the cochlear epithelium. We further demonstrate that Wnt5a antagonizes Frzb in regulating cochlear extension and stereociliary bundle orientation in vitro, and that Wnt5a(-/-) animals have a shortened and widened cochlea. Finally, we show that Wnt5a is required for proper subcellular distribution of a PCP protein, Ltap/Vangl2, and that Wnt5a interacts genetically with Ltap/Vangl2 for uniform orientation of stereocilia, cochlear extension, and neural tube closure. Together, these findings demonstrate that Wnt5a functions in PCP regulation in mice.

Planar Polarity and Tissue Morphogenesis

Planar polarity is a global, tissue-level phenomenon that coordinates cell behavior in a two-dimensional plane. The Frizzled/planar cell polarity (PCP) and anterior-posterior (AP) patterning systems for planar polarity operate in a variety of cell types and provide direction to cells with different morphologies and behaviors. These two systems involve different sets of proteins but both use directional cues provided locally by communication between neighboring cells. This review describes our current understanding of the mechanisms that transmit directional signals from cell to cell and compares the strategies for generating global systems of spatial information in stationary and dynamic cell populations.

Essential role for beta-arrestin 2 in the regulation of Xenopus convergent extension movements

beta-Arrestin 2 (betaarr2) is a multifunctional protein that regulates numerous aspects of G-protein-coupled receptor function. However, its possible involvement in developmental processes is poorly understood. In this work, we examined the potential role of betaarr2 during Xenopus early development. Gain- and loss-of-function studies showed that Xenopus betaarr2 (xbetaarr2) is required for proper convergent extension (CE) movements, and normal cell polarization and intercalation without affecting cell fate. Moreover, for CE movements, betaarr2 acts as an essential regulator of dishevelled-mediated PCP (planar cell polarity) signaling, but not G-protein-mediated Ca(2+) signaling. Notably, xbetaarr2 is localized with the same distribution as the dishevelled protein, which is reasonable, as xbetaarr2 is required for dishevelled activation of RhoA. Furthermore, xbetaarr2 interacts with the N-terminal quarter of Daam1 and RhoA proteins, but not Rac1, and regulates RhoA activation through Daam1 activation for CE movements. We provide evidence that the endocytic activity of xbetaarr2 is essential for control of CE movements. Taken together, our results suggest that betaarr2 has a pivotal role in the regulation of Xenopus CE movements.

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

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

Planar cell polarity signaling in vertebrates

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

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

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

Multicellular rosette formation links planar cell polarity to tissue morphogenesis

Elongation of the body axis is accompanied by the assembly of a polarized cytoarchitecture that provides the basis for directional cell behavior. We find that planar polarity in the Drosophila embryo is established through a sequential enrichment of actin-myosin cables and adherens junction proteins in complementary surface domains. F-actin accumulation at AP interfaces represents the first break in planar symmetry and occurs independently of proper junctional protein distribution at DV interfaces. Polarized cells engage in a novel program of locally coordinated behavior to generate multicellular rosette structures that form and resolve in a directional fashion. Actin-myosin structures align across multiple cells during rosette formation, and adherens junction proteins assemble in a stepwise fashion during rosette resolution. Patterning genes essential for axis elongation selectively affect the frequency and directionality of rosette formation. We propose that the generation of higher-order rosette structures links local cell interactions to global tissue reorganization during morphogenesis.

A GAP in convergent extension scores PAR

The process of "convergence and extension" regulates cellular intercalation during gastrulation. An ArfGAP-PAR protein complex is required for the associated cellular polarization. Potential interactions between this complex and relevant planar cell polarity factors in this context are discussed.