The thrombospondin repeat containing protein MIG-21 controls a left-right asymmetric Wnt signaling response in migrating C. elegans neuroblasts

Wnt proteins are secreted signaling molecules that play a central role in development and adult tissue homeostasis. Although several Wnt signal transduction mechanisms have been described in detail, it is still largely unknown how cells are specified to adopt such different Wnt signaling responses. Here, we have used the stereotypic migration of the C. elegans Q neuroblasts as a model to study how two initially equivalent cells are instructed to activate either β-catenin dependent or independent Wnt signaling pathways to control the migration of their descendants along the anteroposterior axis. We find that the specification of this difference in Wnt signaling response is dependent on the thrombospondin repeat containing protein MIG-21, which acts together with the netrin receptor UNC-40/DCC to control an initial left-right asymmetric polarization of the Q neuroblasts. Furthermore, we show that the direction of this polarization determines the threshold for Wnt/β-catenin signaling, with posterior polarization sensitizing for activation of this pathway. We conclude that MIG-21 and UNC-40 control the asymmetry in Wnt signaling response by restricting posterior polarization to one of the two Q neuroblasts.     

Wnt signaling mediates reorientation of outer hair cell stereociliary bundles in the mammalian cochlea

In the mammalian cochlea, stereociliary bundles located on mechanosensory hair cells within the sensory epithelium are unidirectionally oriented. Development of this planar polarity is necessary for normal hearing as stereociliary bundles are only sensitive to vibrations in a single plane; however, the mechanisms governing their orientation are unknown. We report that Wnt signaling regulates the development of unidirectional stereociliary bundle orientation. In vitro application of Wnt7a protein or inhibitors of Wnt signaling, secreted Frizzled-related protein 1 or Wnt inhibitory factor 1, disrupts bundle orientation. Moreover, Wnt7a is expressed in a pattern consistent with a role in the polarization of the developing stereociliary bundles. We propose that Wnt signaling across the region of developing outer hair cells gives rise to planar polarity in the mammalian cochlea.     

分泌型卷曲相关蛋白家族在Wnt信号通路中的双向调节作用

Wnt信号通过直接参与细胞的增殖、极化和命运特化,控制胚胎发育和成体稳态,其信号异常不仅会造成发育缺陷,而且与多种癌症和代谢性疾病的发生密切相关。分泌型卷曲相关蛋白(secreted frizzled-related proteins,sFRPs)是一种可溶性蛋白质,因其结构与Wnt信号的卷曲蛋白(frizzled,Fz)受体高度同源而被认为是一类Wnt通路拮抗剂。但随着对sFRPs家族的深入研究,发现sFRPs在Wnt信号通路传导过程中并不局限于作为一种拮抗因子,还发挥激活因子的功能。最新研究还发现,sFRPs不仅作为经典的胞外因子发挥作用,在一些肿瘤干细胞中还可进入细胞核双向调节(拮抗或激活)Wnt信号传导。本文结合最新研究,全面综述了sFRPs家族蛋白在Wnt信号传导中的双向调节作用,这有助于理解sFRPs蛋白在生物体器官发育和疾病发生中的作用。     

Apical-basal polarity, Wnt signaling and vertebrate organogenesis

Wnt proteins elicit several distinct signal transduction cascades and regulate multiple cellular processes that have proven essential for embryonic development in all metazoans investigated. During embryonic development, epithelial cells become polarized along two axes: apical/basal and within the plane of the tissue. Growing evidence suggests that polarization along each axis is essential for normal embryonic development and that this polarization is regulated in part by the different branches of the Wnt pathway. Here, we review the role of A/B cell polarity in vertebrate organogenesis with a focus on the involvement of canonical Wnt signaling in this process.     

Wnt11 controls cell contact persistence by local accumulation of Frizzled 7 at the plasma membrane

Wnt11 is a key signal, determining cell polarization and migration during vertebrate gastrulation. It is known that Wnt11 functionally interacts with several signaling components, the homologues of which control planar cell polarity in Drosophila melanogaster. Although in D. melanogaster these components are thought to polarize cells by asymmetrically localizing at the plasma membrane, it is not yet clear whether their subcellular localization plays a similarly important role in vertebrates. We show that in zebrafish embryonic cells, Wnt11 locally functions at the plasma membrane by accumulating its receptor, Frizzled 7, on adjacent sites of cell contacts. Wnt11-induced Frizzled 7 accumulations recruit the intracellular Wnt signaling mediator Dishevelled, as well as Wnt11 itself, and locally increase cell contact persistence. This increase in cell contact persistence is mediated by the local interaction of Wnt11, Frizzled 7, and the atypical cadherin Flamingo at the plasma membrane, and it does not require the activity of further downstream effectors of Wnt11 signaling, such as RhoA and Rok2. We propose that Wnt11, by interacting with Frizzled 7 and Flamingo, modulates local cell contact persistence to coordinate cell movements during gastrulation.     

Wnt signals can function as positional cues in establishing cell polarity

Wnt signaling plays important roles in cell polarization in diverse organisms, and loss of cell polarity is an early event in tumorigenesis caused by mutations in Wnt pathway genes. Despite this, the precise roles of Wnt proteins in cell polarization have remained elusive. In no organism has it been shown that the asymmetric position of a Wnt signal is essential to establishing a cell's polarity. Attempts to test this by ubiquitous expression of Wnt genes have suggested that Wnt signals might act only as permissive factors in cell polarization. Here we find, by using cell manipulations and ectopic gene expression in C. elegans, that the position from which Wnt signals are presented can determine the polarity of both embryonic and postembryonic cells. Furthermore, the position from which a Wnt signal is presented can determine the polarity of Frizzled receptor localization, suggesting that the polarizing effect of Wnt is likely to be direct. These results demonstrate that Wnt proteins can function as positional cues in establishing cell polarity.     

Planar cell polarity and a potential role for a Wnt morphogen gradient in stereociliary bundle orientation in the mammalian inner ear

The planar cell polarity (PCP) pathway, a noncanonical Wnt signaling pathway, is crucial for embryonic development in all animals as it is responsible for the regulation of coordinated orientation of structures within the plane of the various epithelia. In the mammalian cochlea, one of the best examples of planar polarity in vertebrates, stereociliary bundles located on mechanosensory hair cells within the sensory epithelium are all uniformly polarized. Generation of this polarity is important for hair cell mechanotransduction and auditory perception as stereociliary bundles are only sensitive to vibrations in their single plane of polarization. We describe the two step developmental process that results in the generation of planar polarity in the mammalian inner ear. Furthermore, we review evidence for the role of Wnt signaling, and the possible generation of a Wnt gradient, in planar polarity.     

Phosphoinositide 3-kinase is required for process outgrowth and cell polarization of gastrulating mesendodermal cells

BACKGROUND: During vertebrate gastrulation, cell polarization and migration are core components in the cellular rearrangements that lead to the formation of the three germ layers, ectoderm, mesoderm, and endoderm. Previous studies have implicated the Wnt/planar cell polarity (PCP) signaling pathway in controlling cell morphology and movement during gastrulation. However, cell polarization and directed cell migration are reduced but not completely abolished in the absence of Wnt/PCP signals; this observation indicates that other signaling pathways must be involved. RESULTS: We show that Phosphoinositide 3-Kinases (PI3Ks) are required at the onset of zebrafish gastrulation in mesendodermal cells for process formation and cell polarization. Platelet Derived Growth Factor (PDGF) functions upstream of PI3K, while Protein Kinase B (PKB), a downstream effector of PI3K activity, localizes to the leading edge of migrating mesendodermal cells. In the absence of PI3K activity, PKB localization and cell polarization are strongly reduced in mesendodermal cells and are followed by slower but still highly coordinated and directed movements of these cells. CONCLUSIONS: We have identified a novel role of a signaling pathway comprised of PDGF, PI3K, and PKB in the control of morphogenetic cell movements during gastrulation. Furthermore, our findings provide insight into the relationship between cell polarization and directed cell migration at the onset of zebrafish gastrulation.     

Cell-intrinsic Wnt4 ligand regulates mitochondrial oxidative phosphorylation in macrophages

Macrophages respond to their environment by adopting a predominantly inflammatory or anti-inflammatory profile, depending on the context. The polarization of the subsequent response is regulated by a combination of intrinsic and extrinsic signals and is associated with alterations in macrophage metabolism. Although macrophages are important producers of Wnt ligands, the role of Wnt signaling in regulating metabolic changes associated with macrophage polarization remains unclear. Wnt4 upregulation has been shown to be associated with tissue repair and suppression of age-associated inflammation, which led us to generate Wnt4-deficient bone marrow–derived macrophages to investigate its role in metabolism. We show that loss of Wnt4 led to modified mitochondrial structure, enhanced oxidative phosphorylation, and depleted intracellular lipid reserves, as the cells depended on fatty acid oxidation to fuel their mitochondria. Further we found that enhanced lipolysis was dependent on protein kinase C–mediated activation of lysosomal acid lipase in Wnt4-deficient bone marrow–derived macrophages. Although not irreversible, these metabolic changes promoted parasite survival during infection with Leishmania donovani. In conclusion, our results indicate that enhanced macrophage fatty acid oxidation impairs the control of intracellular pathogens, such as Leishmania. We further suggest that Wnt4 may represent a potential target in atherosclerosis, which is characterized by lipid storage in macrophages leading to them becoming foam cells.     

Regulation of basal body and ciliary functions by Diversin

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

Multiple levels of regulation specify the polarity of an asymmetric cell division in C. elegans

Wnt signaling systems play important roles in the generation of cell and tissue polarity during development. We describe a Wnt signaling system that acts in a new way to orient the polarity of an epidermal cell division in C. elegans. In this system, the EGL-20/Wnt signal acts in a permissive fashion to polarize the asymmetric division of a cell called V5. EGL-20 regulates this polarization by counteracting lateral signals from neighboring cells that would otherwise reverse the polarity of the V5 cell division. Our findings indicate that this lateral signaling pathway also involves Wnt pathway components. Overexpression of EGL-20 disrupts both the asymmetry and polarity of lateral epidermal cell divisions all along the anteroposterior (A/P) body axis. Together our findings suggest that multiple, inter-related Wnt signaling systems may act together to polarize asymmetric cell divisions in this tissue.     

Frizzled-5 Receptor Is Involved in Neuronal Polarity and Morphogenesis of Hippocampal Neurons

The Wnt signaling pathway plays important roles during different stages of neuronal development, including neuronal polarization and dendritic and axonal outgrowth. However, little is known about the identity of the Frizzled receptors mediating these processes. In the present study, we investigated the role of Frizzled-5 (Fzd5) on neuronal development in cultured Sprague-Dawley rat hippocampal neurons. We found that Fzd5 is expressed early in cultured neurons on actin-rich structures localized at minor neurites and axonal growth cones. At 4 DIV, Fzd5 polarizes towards the axon, where its expression is detected mainly at the peripheral zone of axonal growth cones, with no obvious staining at dendrites; suggesting a role of Fzd5 in neuronal polarization. Overexpression of Fzd5 during the acquisition of neuronal polarity induces mislocalization of the receptor and a loss of polarized axonal markers. Fzd5 knock-down leads to loss of axonal proteins, suggesting an impaired neuronal polarity. In contrast, overexpression of Fzd5 in neurons that are already polarized did not alter polarity, but decreased the total length of axons and increased total dendrite length and arborization. Fzd5 activated JNK in HEK293 cells and the effects triggered by Fzd5 overexpression in neurons were partially prevented by inhibition of JNK, suggesting that a non-canonical Wnt signaling mechanism might be involved. Our results suggest that, Fzd5 has a role in the establishment of neuronal polarity, and in the morphogenesis of neuronal processes, in part through the activation of the non-canonical Wnt mechanism involving JNK.     

Wnt signaling in cartilage development and degeneration

The Wnt signaling network, which is composed of Wnt ligands, receptors, antagonists, and intracellular signaling molecules, has emerged as a powerful regulator of cell fate, proliferation, and function in multicellular organisms. Over the past two decades, the critical role of Wnt signaling in embryonic cartilage and bone development has been well established, and much has been learnt regarding the role of Wnt signaling in chondrogenesis and cartilage development. However, relatively little is known about the role of Wnt signaling in adult articular cartilage and degenerative cartilage tissue. This review will briefly summarize recent advances in Wnt regulation of chondrogenesis and hypertrophic maturation of chondrocytes, and review data concerning the role of Wnt signaling in the maintenance and degeneration of articular chondrocytes and cartilage.     

Wnt signaling and skeletal development

Wnt proteins are a family of secreted proteins that regulate many aspects of cellular functions. The discovery that mutations in low-density lipoprotein receptor-related protein 5, a putative Wnt coreceptor, could positively and negatively affect bone mass in humans generated an enormous amount of interest in the possible role of the Wnt signaling pathway in skeletal biology. Over the last decade, considerable progress has been made in determining the role of the canonical Wnt signaling pathway in various aspects of skeletal development. Furthermore, recent evidence indicates the important role of non-canonical Wnt signaling in skeletal development. In this review we discuss the current understanding of the role of Wnt signaling in chondrogenesis, osteoblastogenesis, and osteoclastogenesis.     

Cell signaling: moving to a Wnt-Rap

Non-canonical, beta-catenin-independent Wnt signaling regulates cell polarization and movements. A recent study reveals that casein kinase Iepsilon mediates an additional novel non-canonical Wnt pathway via the activation of the Rap1 GTPase during vertebrate gastrulation.     

Distinct phases of Wnt/β-catenin signaling direct cardiomyocyte formation in zebrafish

Normal heart formation requires reiterative phases of canonical Wnt/β-catenin (Wnt) signaling. Understanding the mechanisms by which Wnt signaling directs cardiomyocyte (CM) formation in vivo is critical to being able to precisely direct differentiated CMs from stem cells in vitro. Here, we investigate the roles of Wnt signaling in zebrafish CM formation using heat-shock inducible transgenes that increase and decrease Wnt signaling. We find that there are three phases during which CM formation is sensitive to modulation of Wnt signaling through the first 24 h of development. In addition to the previously recognized roles for Wnt signaling during mesoderm specification and in the pre-cardiac mesoderm, we find a previously unrecognized role during CM differentiation where Wnt signaling is necessary and sufficient to promote the differentiation of additional atrial cells. We also extend the previous studies of the roles of Wnt signaling during mesoderm specification and in pre-cardiac mesoderm. Importantly, in pre-cardiac mesoderm we define a new mechanism where Wnt signaling is sufficient to prevent CM differentiation, in contrast to a proposed role in inhibiting cardiac progenitor (CP) specification. The inability of the CPs to differentiate appears to lead to cell death through a p53/Caspase-3 independent mechanism. Together with a report for an even later role for Wnt signaling in restricting proliferation of differentiated ventricular CMs, our results indicate that during the first 3days of development in zebrafish there are four distinct phases during which CMs are sensitive to Wnt signaling.     

Lpp is involved in Wnt/PCP signaling and acts together with Scrib to mediate convergence and extension movements during zebrafish gastrulation

The zyxin-related LPP protein is localized at focal adhesions and cell–cell contacts and is involved in the regulation of smooth muscle cell migration. A known interaction partner of LPP in human is the tumor suppressor protein SCRIB. Knocking down scrib expression during zebrafish embryonic development results in defects of convergence and extension (C&E) movements, which occur during gastrulation and mediate elongation of the anterior–posterior body axis. Mediolateral cell polarization underlying C&E is regulated by a noncanonical Wnt signaling pathway constituting the vertebrate planar cell polarity (PCP) pathway. Here, we investigated the role of Lpp during early zebrafish development. We show that morpholino knockdown of lpp results in defects of C&E, phenocopying noncanonical Wnt signaling mutants. Time-lapse analysis associates the defective dorsal convergence movements with a reduced ability to migrate along straight paths. In addition, expression of Lpp is significantly reduced in Wnt11 morphants and in embryos overexpressing Wnt11 or a dominant-negative form of Rho kinase 2, which is a downstream effector of Wnt11, suggesting that Lpp expression is dependent on noncanonical Wnt signaling. Finally, we demonstrate that Lpp interacts with the PCP protein Scrib in zebrafish, and that Lpp and Scrib cooperate for the mediation of C&E.     

MIM regulates vertebrate neural tube closure

Neural tube closure is a critical morphogenetic event that is regulated by dynamic changes in cell shape and behavior. Although previous studies have uncovered a central role for the non-canonical Wnt signaling pathway in neural tube closure, the underlying mechanism remains poorly resolved. Here, we show that the missing in metastasis (MIM; Mtss1) protein, previously identified as a Hedgehog response gene and actin and membrane remodeling protein, specifically binds to Daam1 and couples non-canonical Wnt signaling to neural tube closure. MIM binds to a conserved domain within Daam1, and this interaction is positively regulated by Wnt stimulation. Spatial expression of MIM is enriched in the anterior neural plate and neural folds, and depletion of MIM specifically inhibits anterior neural fold closure without affecting convergent extension movements or mesoderm cell fate specification. Particularly, we find that MIM is required for neural fold elevation and apical constriction along with cell polarization and elongation in both the superficial and deep layers of the anterior neural plate. The function of MIM during neural tube closure requires both its membrane-remodeling domain and its actin-binding domain. Finally, we show that the effect of MIM on neural tube closure is not due to modulation of Hedgehog signaling in the Xenopus embryo. Together, our studies define a morphogenetic pathway involving Daam1 and MIM that transduces non-canonical Wnt signaling for the cytoskeletal changes and membrane dynamics required for vertebrate neural tube closure.