Genetic basis of planar polarity

Several epitheliums exhibit a clear polarity that lies within the plane of the epithelium. This polarity, referred to as planar polarity or tissue polarity, is oriented perpendicular to the apical-basal polarity of the epithelium. Over the last two decades, the genetic and molecular bases of planar polarity have been intensively investigated in Drosophila. Recent studies have shown that establishment of planar polarity relies on the unipolar distribution of a small number of signaling molecules localizing at the apical cortex. Unipolar localization of planar polarity proteins defines two opposite and complementary cortical domains. These domains show a stereotyped orientation at the tissue level. Positioning of these cortical domains is coordinated at the tissue level by a second class of signaling molecules that form an activity gradient across the epithelium. Together these data have led to a general model of planar polarity establishment. Considering that planar polarity genes have been conserved from flies to vertebrates, this model may be useful for our understanding of epithelium biology in mammals.     

Epithelial cell polarity: what flies can teach us about cancer

Epithelial cells are polarized along their apical-basal axis. Much of the cellular machinery that goes into establishing and maintaining epithelial cell polarity is evolutionarily conserved. Model organisms, including the fruit fly, Drosophila melanogaster, are thus particularly useful for the study of cell polarity. Work in Drosophila has identified several important components of the polarity machinery and has also established the surprising existence of a secondary cell polarity pathway required only under conditions of energetic stress. This work has important implications for the understanding of human cancer. Most cancers are epithelial in origin, and the loss of cell polarity is a critical step towards malignancy. Thus a better understanding of how polarity is established and maintained in epithelial cells will help us to understand the process of malignant transformation and may lead to improved therapies. In the present chapter we discuss the current understanding of how epithelial cell polarity is regulated and the known associations between polarity factors and cancer.     

Lethal (2) Giant Larvae: An Indispensable Regulator of Cell Polarity and Cancer Development

Cell polarity is one of the most basic properties of all normal cells and is essential for regulating numerous biological processes. Loss of polarity is considered a hallmark for cancer. Multiple polarity proteins are implicated in maintenance of cell polarity. Lethal (2) giant larvae (Lgl) is one of polarity proteins that plays an important role in regulating cell polarity, asymmetric division as well as tumorigenesis. Lgl proteins in different species have similar structures and conserved functions. Lgl acts as an indispensable regulator of cell biological function, including cell polarity and asymmetric division, through interplaying with other polarity proteins, regulating exocytosis, mediating cytoskeleton and being involved in signaling pathways. Furthermore, Lgl plays a role of a tumor suppressor, and the aberrant expression of Hugl, a human homologue of Lgl, contributes to multiple cancers. However, the exact functions of Lgl and the underlying mechanisms remain enigmatic. In this review, we will give an overview of the Lgl functions in cell polarity and cancer development, discuss the potential mechanisms underlying these functions, and raise our conclusion of previous studies and points of view about the future studies.     

The tumor-suppressor and cell adhesion molecule Fat controls planar polarity via physical interactions with Atrophin,a transcriptional co-repressor

Fat is an atypical cadherin that controls both cell growth and planar polarity. Atrophin is a nuclear co-repressor that is also essential for planar polarity; however, it is not known what genes Atrophin controls in planar polarity, or how Atrophin activity is regulated during the establishment of planar polarity. We show that Atrophin binds to the cytoplasmic domain of Fat and that Atrophin mutants show strong genetic interactions with fat. We find that both Atrophin and fat clones in the eye have non-autonomous disruptions in planar polarity that are restricted to the polar border of clones and that there is rescue of planar polarity defects on the equatorial border of these clones. Both fat and Atrophin are required to control four-jointed expression. In addition our mosaic analysis demonstrates an enhanced requirement for Atrophin in the R3 photoreceptor. These data lead us to a model in which fat and Atrophin act twice in the determination of planar polarity in the eye: first in setting up positional information through the production of a planar polarity diffusible signal, and later in R3 fate determination.     

PAR‐3 controls endothelial planar polarity and vascular inflammation under laminar flow

Impaired cell polarity is a hallmark of diseased tissue. In the cardiovascular system, laminar blood flow induces endothelial planar cell polarity, represented by elongated cell shape and asymmetric distribution of intracellular organelles along the axis of blood flow. Disrupted endothelial planar polarity is considered to be pro‐inflammatory, suggesting that the establishment of endothelial polarity elicits an anti‐inflammatory response. However, a causative relationship between polarity and inflammatory responses has not been firmly established. Here, we find that a cell polarity protein, PAR‐3, is an essential gatekeeper of GSK3β activity in response to laminar blood flow. We show that flow‐induced spatial distribution of PAR‐3/aPKCλ and aPKCλ/GSK3β complexes controls local GSK3β activity and thereby regulates endothelial planar polarity. The spatial information for GSK3β activation is essential for flow‐dependent polarity to the flow axis, but is not necessary for flow‐induced anti‐inflammatory response. Our results shed light on a novel relationship between endothelial polarity and vascular homeostasis highlighting avenues for novel therapeutic strategies.     

Kinetics of polar auxin transport

The movement of auxin in the basipetal and acropetal directions is compared for 4 types of tissue. It is observed that the transport may proceed in either a linear or a non-linear manner with time. The polarity of transport through any given type of tissue increases exponentially with increasing lengths of tissue traversed, suggesting that the polarity of transport is developed as a consequence of the repeated passage through cells. Using the mathematical model of Leopold and Hall, the extent of polarity for individual cells is estimated, and a very small polarity of individual cells is found to be capable of accounting for the marked polarity of whole tissues. It is suggested that transport polarity may be functionally a property of the multicellular structure, being amplified from very small differences in activities at the 2 ends of individual cells.     

Apico-basal polarity complex and cancer

Apico-basal polarity is a cardinal molecular feature of adult eukaryotic epithelial cells and appears to be involved in several key cellular processes including polarized cell migration and maintenance of tissue architecture. Epithelial cell polarity is maintained by three well-conserved polarity complexes, namely, PAR, Crumbs and SCRIB. The location and interaction between the components of these complexes defines distinct structural domains of epithelial cells. Establishment and maintenance of apico-basal polarity is regulated through various conserved cell signalling pathways including TGFβ, Integrin and WNT signalling. Loss of cell polarity is a hallmark for carcinoma, and its underlying molecular mechanism is beginning to emerge from studies on model organisms and cancer cell lines. Moreover, deregulated expression of apico-basal polarity complex components has been reported in human tumours. In this review, we provide an overview of the apico-basal polarity complexes and their regulation, their role in cell migration, and finally their involvement in carcinogenesis.     

Reinterpreting polarity and cancer: The changing landscape from tumor suppression to tumor promotion

Cell polarity is a fundamental property used to generate asymmetry and structure in all cells. Cancer is associated with loss of cell and tissue structure. While observations made in model system such as Drosophila, identify polarity regulators as tumor suppressors that cause inappropriate cell division, studies in mammalian epithelia do not always support such a causative contribution. Our analysis of published cancer dataset shows that many polarity genes, including PARD6B, SCRIB, PRKCI, DLG1, DLG2, DLG5 and LLGL2, are frequently amplified in multiple cancers raising the possibility that mammalian epithelia may have evolved to use polarity proteins in multiple ways where they may have tumor promoting functions. In this review, we reinterpret the published results and propose a modified perspective for the role of polarity regulators in cancer biology. In addition to the traditional form of cell polarity, which is involved establishment of maintenance of normal cell structure and asymmetry, we propose that some mammalian polarity proteins also regulate subcellular polarity (intracellular asymmetry), which can improve cellular fitness to carry out functions such as proliferation, apoptosis, stress adaptation, stemness and organelle biology. Here, we define subcellular polarity and discuss evidence that supports a role for subcellular polarity in biology.     

VE-cadherin interacts with cell polarity protein Pals1 to regulate vascular lumen formation

Blood vessel tubulogenesis requires the formation of stable cell-to-cell contacts and the establishment of apicobasal polarity of vascular endothelial cells. Cell polarity is regulated by highly conserved cell polarity protein complexes such as the Par3-aPKC-Par6 complex and the CRB3-Pals1-PATJ complex, which are expressed by many different cell types and regulate various aspects of cell polarity. Here we describe a functional interaction of VE-cadherin with the cell polarity protein Pals1. Pals1 directly interacts with VE-cadherin through a membrane-proximal motif in the cytoplasmic domain of VE-cadherin. VE-cadherin clusters Pals1 at cell–cell junctions. Mutating the Pals1-binding motif in VE-cadherin abrogates the ability of VE-cadherin to regulate apicobasal polarity and vascular lumen formation. In a similar way, deletion of the Par3-binding motif at the C-terminus of VE-cadherin impairs apicobasal polarity and vascular lumen formation. Our findings indicate that the biological activity of VE-cadherin in regulating endothelial polarity and vascular lumen formation is mediated through its interaction with the two cell polarity proteins Pals1 and Par3.     

Tiam1 takes PARt in cell polarity

Cell polarity is an essential requirement for the proper tissue development of complex organisms. This is underscored by in vivo studies showing that loss of cell polarity contributes to the formation and progression of tumours. Evolutionary conserved multiprotein complexes, such as the Par3-Par6-aPKC or, in short, the Par polarity complex, regulate the establishment of cell polarity. The small Rho GTPases CDC42 and Rac control the activation of the Par polarity complex. Evidence now implicates the Rac activator Tiam1 as a crucial component of the Par complex in regulating neuronal (axonal) and epithelial (apical-basal) polarity. Our current knowledge places Tiam1 at the centre of a pivotal biological process, the establishment and maintenance of cell polarity, and suggests that deregulation of the Tiam1-Par complex contributes to tumourigenicity.     

Long-range coordination of planar polarity in Drosophila

The mechanisms by which cells become polarised in the plane of an epithelium have been studied in Drosophila for many years. Work has focussed on two key questions: firstly, how individual cells adopt a defined polarity, and secondly how the polarity of each cell within a tissue is aligned with its neighbours. It has been established that asymmetric subcellular localisation of a number of polarity proteins is an essential mechanism underlying polarisation of single cells. The process by which this polarity is coordinated between cells however is less well understood, but is thought to involve gradients of activity of the atypical cadherins Dachsous and Fat. Subsequently, this long-range polarity signal is refined by local cell-cell interactions involving the transmembrane molecules Frizzled, Strabismus and Flamingo. The role of these factors in coordinating polarity will be discussed.     

Ups and downs of tissue and planar polarity in plants

The polar orientation of cells within a tissue is an intensively studied research area in animal cells. The term planar polarity refers to the common polar arrangement of cells within the plane of an epithelium. In plants, the subcellular analysis of tissue polarity has been limited by the lack of appropriate markers. Recently, research on plant tissue polarity has come of age. Advances are based on studies of Arabidopsis patterning, cell polarity and auxin transport mutants employing the coordinated, polar localization of auxin transporters and the planar polarity of root epidermal hairs as markers. These approaches have revealed auxin transport and response, vesicular trafficking, membrane sterol and cytoskeletal requirements of tissue polarity. This review summarizes recent progress in research on vascular tissue and planar epidermal polarity in the Arabidopsis root and compares it to findings on planar polarity in animals and cell polarity in yeast.     

A Safeguard Mechanism Regulates Rho GTPases to Coordinate Cytokinesis with the Establishment of Cell Polarity

The spatiotemporal control of cell polarity is crucial for the development of multicellular organisms and for reliable polarity switches during cell cycle progression in unicellular systems. A tight control of cell polarity is especially important in haploid budding yeast, where the new polarity site (bud site) is established next to the cell division site after cell separation. How cells coordinate the temporal establishment of two adjacent polarity sites remains elusive. Here, we report that the bud neck associated protein Gps1 (GTPase-mediated polarity switch 1) establishes a novel polarity cue that concomitantly sustains Rho1-dependent polarization and inhibits premature Cdc42 activation at the site of cytokinesis. Failure of Gps1 regulation leads to daughter cell death due to rebudding inside the old bud site. Our findings provide unexpected insights into the temporal control of cytokinesis and describe the importance of a Gps1-dependent mechanism for highly accurate polarity switching between two closely connected locations.     

Strabismus requires Flamingo and Prickle function to regulate tissue polarity in the Drosophila eye

Tissue polarity in Drosophila is regulated by a number of genes that are thought to function in a complex, many of which interact genetically and/or physically, co-localize, and require other tissue polarity proteins for their localization. We report the enhancement of the strabismus tissue polarity phenotype by mutations in two other tissue polarity genes, flamingo and prickle. Flamingo is autonomously required for the establishment of ommatidial polarity. Its localization is dynamic throughout ommatidial development and is dependent on Frizzled and Notch. Flamingo and Strabismus co-localize for several rows posterior to the morphogenetic furrow and subsequently diverge. While neither of these proteins is required for the other's localization, Prickle localization is influenced by Strabismus function. Our data suggest that Strabismus, Flamingo and Prickle function together to regulate the establishment of tissue polarity in the Drosophila eye.     

Polarized Organization of the Cytoskeleton: Regulation by Cell Polarity Proteins

Polarity is one of the fundamental properties displayed by living organisms. In metazoans, cell polarity governs developmental processes and plays an essential role during maintenance of forms of tissues as well as their functions. The mechanisms of establishment and maintenance of cell polarity have been investigated extensively in the last two decades. This has resulted in identification of “core cell polarity modules” that control anterior–posterior, front–rear and apical–basal polarity across various cell types. Here, we review how these polarity modules interact closely with the cytoskeleton during establishment and maintenance of cytoskeletal polarity. We further suggest that reciprocal interactions between cell polarity modules and the cytoskeleton consolidate the initial weaker polarity, arising from an external cue, into a committed polarized system.     

Kif3a regulates planar polarization of auditory hair cells through both ciliary and non-ciliary mechanisms

Auditory hair cells represent one of the most prominent examples of epithelial planar polarity. In the auditory sensory epithelium, planar polarity of individual hair cells is defined by their V-shaped hair bundle, the mechanotransduction organelle located on the apical surface. At the tissue level, all hair cells display uniform planar polarity across the epithelium. Although it is known that tissue planar polarity is controlled by non-canonical Wnt/planar cell polarity (PCP) signaling, the hair cell-intrinsic polarity machinery that establishes the V-shape of the hair bundle is poorly understood. Here, we show that the microtubule motor subunit Kif3a regulates hair cell polarization through both ciliary and non-ciliary mechanisms. Disruption of Kif3a in the inner ear led to absence of the kinocilium, a shortened cochlear duct and flattened hair bundle morphology. Moreover, basal bodies are mispositioned along both the apicobasal and planar polarity axes of mutant hair cells, and hair bundle orientation was uncoupled from the basal body position. We show that a non-ciliary function of Kif3a regulates localized cortical activity of p21-activated kinases (PAK), which in turn controls basal body positioning in hair cells. Our results demonstrate that Kif3a-PAK signaling coordinates planar polarization of the hair bundle and the basal body in hair cells, and establish Kif3a as a key component of the hair cell-intrinsic polarity machinery, which acts in concert with the tissue polarity pathway.     

Lethal (2) giant larvae regulates pleural mesothelial cell polarity in pleural fibrosis

Pleural fibrosis is barely reversible and the underlying mechanisms are poorly understood. Pleural mesothelial cells (PMCs) which have apical-basal polarity play a key role in pleural fibrosis. Loss of cell polarity is involved in the development of fibrotic diseases. Partition defective protein (PAR) complex is a key regulator of cell polarity. However, changes of PMC polarity and PAR complex in pleural fibrosis are still unknown. In this study, we observed that PMC polarity was lost in fibrotic pleura. Next we found increased Lethal (2) giant larvae (Lgl) bound with aPKC and PAR-6B competing against PAR-3A in PAR complex, which led to cell polarity loss. Then we demonstrated that Lgl1 siRNA prevented cell polarity loss in PMCs, and Lgl1 conditional knockout (ER-Cre^+/?Lgl1^flox/flox) attenuated pleural fibrosis in a mouse model. Our data indicated that Lgl1 regulates cell polarity of PMCs, inhibition of Lgl1 and maintenance of cell polarity in PMCs could be a potential therapeutic treatment approach for pleural fibrosis.     

The aPKC/Par3/Par6 Polarity Complex and Membrane Order Are Functionally Interdependent in Epithelia During Vertebrate Organogenesis

The differential distribution of lipids between apical and basolateral membranes is necessary for many epithelial cell functions, but how this characteristic membrane organization is integrated within the polarity network during ductal organ development is poorly understood. Here we quantified membrane order in the gut, kidney and liver ductal epithelia in zebrafish larvae at 3–11 days post fertilization (dpf) with Laurdan 2‐photon microscopy. We then applied a combination of Laurdan imaging, antisense knock‐down and analysis of polarity markers to understand the relationship between membrane order and apical‐basal polarity. We found a reciprocal relationship between membrane order and the cell polarity network. Reducing membrane condensation by exogenously added oxysterol or depletion of cholesterol reduced apical targeting of the polarity protein, aPKC. Conversely, using morpholino knock down in zebrafish, we found that membrane order was dependent upon the Crb3 and Par3 polarity protein expression in ductal epithelia. Hence our data suggest that the biophysical property of membrane lipid packing is a regulatory element in apical basal polarity.     

A Highlights from MBoC Selection: Exploratory polarization facilitates mating partner selection in Saccharomyces cerevisiae

Yeast decode pheromone gradients to locate mating partners, providing a model for chemotropism. How yeast polarize toward a single partner in crowded environments is unclear. Initially, cells often polarize in unproductive directions, but then they relocate the polarity site until two partners’ polarity sites align, whereupon the cells “commit” to each other by stabilizing polarity to promote fusion. Here we address the role of the early mobile polarity sites. We found that commitment by either partner failed if just one partner was defective in generating, orienting, or stabilizing its mobile polarity sites. Mobile polarity sites were enriched for pheromone receptors and G proteins, and we suggest that such sites engage in an exploratory search of the local pheromone landscape, stabilizing only when they detect elevated pheromone levels. Mobile polarity sites were also enriched for pheromone secretion factors, and simulations suggest that only focal secretion at polarity sites would produce high pheromone concentrations at the partner’s polarity site, triggering commitment.     

Novel insights into epithelial polarity proteins in Drosophila

Apical-basal polarity is a basic organizing principle of epithelial cells. Consequently, defects in polarity are associated with numerous human pathologies, including many forms of cancer. Recent work in Drosophila has identified novel roles for, or has greatly enhanced our understanding of, functional modules within the epithelial polarity network. A series of recent papers have highlighted the key function of the scaffolding protein Bazooka/Par3 as an early polarity landmark, and its crucial role in dynamic segregation of the apical membrane from the adherens junction. Moreover, novel polarity modules have recently been discovered; the Yurt/Coracle group supports the basolateral membrane during a defined time window of development, while a second module, including the kinases LKB1 and AMP-activated protein kinase, is required for polarity when epithelial cells experience metabolic stress. These new findings emphasize unforeseen complexities in the regulation of epithelial polarity, and raise new questions about the mechanisms of epithelial tissue organization and function.