dachsous and frizzled contribute separately to planar polarity in the Drosophila ventral epidermis

Cells that comprise tissues often need to coordinate cytoskeletal events to execute morphogenesis properly. For epithelial tissues, some of that coordination is accomplished by polarization of the cells within the plane of the epithelium. Two groups of genes--the Dachsous (Ds) and Frizzled (Fz) systems--play key roles in the establishment and maintenance of such polarity. There has been great progress in uncovering the how these genes work together to produce planar polarity, yet fundamental questions remain unanswered. Here, we study the Drosophila larval ventral epidermis to begin to address several of these questions. We show that ds and fz contribute independently to polarity and that they do so over spatially distinct domains. Furthermore, we find that the requirement for the Ds system changes as field size increases. Lastly, we find that Ds and its putative receptor Fat (Ft) are enriched in distinct patterns in the epithelium during embryonic development.     

Cell biology of planar polarity transmission in the Drosophila wing

The coordination of epithelial planar polarization is a critical step in the formation of well-ordered tissues. The process has been extensively studied in Drosophila, where genetic analysis has identified a set of "tissue polarity" genes that serve to coordinate planar polarity of cells in the developing wings, bristles and eyes. In the last several years, it has emerged that six of these genes encode junctional proteins. In the wing epithelium, these proteins undergo a polarized redistribution, forming separate proximal and distal cortical domains within each cell. The mechanisms that mediate cortical polarization and cue its direction have been the subject of intense investigation. Cuing the orientation of cortical polarization appears to depend on the atypical Cadherins Fat and Dachsous, although these proteins do not become polarized themselves, nor do they colocalize with components of polarized cortical domains. Interestingly, these Cadherins also act at earlier developmental stages to polarize tissue growth along the proximal-distal axis and it will be interesting to see whether these processes are mechanistically related. Once the axis of polarization is determined, cortical polarity seems to be propagated, at least locally, by a cascade of direct cell-cell interactions mediated by the proximal and distal domains. The cell biological mechanisms leading to polarization are still unclear, but the process depends on the control of Protein Phosphatase 2A activity by its regulatory subunit, Widerborst. Interestingly, Widerborst is found on a planar web of microtubules with connections to apical junctions, suggesting that these microtubules may have an important function in polarizing the cortex.     

Intertissue mechanical stress affects Frizzled-mediated planar cell polarity in the Drosophila notum epidermis

Frizzled/planar cell polarity (Fz/PCP) signaling controls the orientation of sensory bristles and cellular hairs (trichomes) along the anteroposterior axis of the Drosophila thorax (notum). A subset of the trichome-producing notum cells differentiate as "tendon cells," serving as attachment sites for the indirect flight muscles (IFMs) to the exoskeleton. Through the analysis of chascon (chas), a gene identified by its ability to disrupt Fz/PCP signaling under overexpression conditions, and jitterbug (jbug)/filamin, we show that maintenance of anteroposterior planar polarization requires the notum epithelia to balance mechanical stress generated by the attachment of the IFMs. chas is expressed in notum tendon cells, and its loss of function disturbs cellular orientation at and near the regions where IFMs attach to the epidermis. This effect is independent of the Fz/PCP and fat/dachsous systems. The chas phenotype arises during normal shortening of the IFMs and is suppressed by genetic ablation of the IFMs. chas acts through jbug/filamin and cooperates with MyosinII to modulate the mechanoresponse of notum tendon cells. These observations support the notion that the ability of epithelia to respond to mechanical stress generated by one or more interactions with other tissues during development and organogenesis influences the maintenance of its shape and PCP features.     

Developmental compartments and planar polarity in Drosophila

BACKGROUND: Planar polarity refers to the asymmetry of a cell within the plane of the epithelium; for example, cells may form hairs that point in a posterior direction, or cilia may beat in one way. This property implies that cells have information about their orientation; we wish to understand the nature of this information. Relevant also is the body plan of insects, which, in the ectoderm and somatic mesoderm, consists of a chain of alternating anterior and posterior compartments - basic units of development with independent cell lineage and subject to independent genetic control.RESULTS: Using the abdomen of adult Drosophila, we have taken genes required for normal polarity and either removed the gene or constitutively expressed it in small clones of cells and observed the effects on polarity. Hitherto, all such studies of polarity genes have not found any difference of behavior between the different compartments. We report here that the three genes, four-jointed, dachsous, and fat, cause opposite effects in anterior and posterior compartments. For example, in anterior compartments, clones ectopically expressing four-jointed reverse the polarity of cells in front of the clone, while, in posterior compartments, they reverse behind the clone. These three genes have been reported by others to be functionally linked.CONCLUSIONS: This discovery impacts on models of how cells read polarity. At the heart of one class of models is the hypothesis that cell polarity is determined by the vector of a morphogen gradient. Here, we present evidence that cell polarity in the abdomen depends on at least two protein gradients (Fj and Ds), each of which is reflected at compartment borders. Consequently, these gradients have opposing slopes in the two compartments. Because all polarized structures made by abdominal cells point posteriorly, we surmise that cells in each compartment are programmed to interpret these protein gradients with opposite signs, pointing up the gradient in one compartment and down the gradient in the other.     

Planar polarity and actin dynamics in the epidermis of Drosophila

Dorsal closure is a morphogenetic process involving the coordinated convergence of two epithelial sheets to enclose the Drosophila melanogaster embryo. Specialized populations of cells at the edges of each epithelial sheet, the dorsal-most epidermal cells, emit actin-based processes that are essential for the proper enclosure of the embryo. Here we show that actin dynamics at the leading edge is preceded by a planar polarization of the dorsal-most epidermal cells associated with a reorganization of the cytoskeleton. An important consequence of this planar polarization is the formation of actin-nucleating centres at the leading edge, which are important in the dynamics of actin. We show that Wingless (Wg) signalling and Jun amino-terminal kinase (JNK) signalling have overlapping but different roles in these events.     

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.     

The WD40 repeat protein fritz links cytoskeletal planar polarity to frizzled subcellular localization in the Drosophila epidermis

Much of our understanding of the genetic mechanisms that control planar cell polarity (PCP) in epithelia has derived from studies of the formation of polarized cell hairs during Drosophila wing development. The correct localization of an F-actin prehair to the distal vertex of the pupal wing cell has been shown to be dependent upon the polarized subcellular localization of Frizzled and other core PCP proteins. However, the core PCP proteins do not organize actin cytoskeletal polarity directly but require PCP effector proteins such as Fuzzy and Inturned to mediate this process. Here we describe the characterization of a new PCP effector gene, fritz, that encodes a novel but evolutionarily conserved coiled-coil WD40 protein. We show that the fritz gene product functions cell-autonomously downstream of the core PCP proteins to regulate both the location and the number of wing cell prehair initiation sites.     

Frizzled signaling and cell-cell interactions in planar polarity.

The function of the Frizzled pathway is essential for the formation of the array of distally pointing hairs found on the Drosophila wing. Previous research found that regulating the subcellular location for hair initiation controlled hair polarity. Recent work argues a graded Frizzled-dependent signal results in the accumulation of the Frizzled, Dishevelled and Flamingo proteins along the distal edge of the wing cells. This cortical mark leads to the local activation of downstream gene products and the subsequent activation of the cytoskeleton to form a hair.     

Patterned gene expression directs bipolar planar polarity in Drosophila

During convergent extension in Drosophila, polarized cell movements cause the germband to narrow along the dorsal-ventral (D-V) axis and more than double in length along the anterior-posterior (A-P) axis. This tissue remodeling requires the correct patterning of gene expression along the A-P axis, perpendicular to the direction of cell movement. Here, we demonstrate that A-P patterning information results in the polarized localization of cortical proteins in intercalating cells. In particular, cell fate differences conferred by striped expression of the even-skipped and runt pair-rule genes are both necessary and sufficient to orient planar polarity. This polarity consists of an enrichment of nonmuscle myosin II at A-P cell borders and Bazooka/PAR-3 protein at the reciprocal D-V cell borders. Moreover, bazooka mutants are defective for germband extension. These results indicate that spatial patterns of gene expression coordinate planar polarity across a multicellular population through the localized distribution of proteins required for cell movement.     

Planar cell polarity in the larval epidermis of Drosophila and the role of microtubules

We investigate planar cell polarity (PCP) in the Drosophila larval epidermis. The intricate pattern of denticles depends on only one system of PCP, the Dachsous/Fat system. Dachsous molecules in one cell bind to Fat molecules in a neighbour cell to make intercellular bridges. The disposition and orientation of these Dachsous–Fat bridges allows each cell to compare two neighbours and point its denticles towards the neighbour with the most Dachsous. Measurements of the amount of Dachsous reveal a peak at the back of the anterior compartment of each segment. Localization of Dachs and orientation of ectopic denticles help reveal the polarity of every cell. We discuss whether these findings support our gradient model of Dachsous activity. Several groups have proposed that Dachsous and Fat fix the direction of PCP via oriented microtubules that transport PCP proteins to one side of the cell. We test this proposition in the larval cells and find that most microtubules grow perpendicularly to the axis of PCP. We find no meaningful bias in the polarity of microtubules aligned close to that axis. We also reexamine published data from the pupal abdomen and find no evidence supporting the hypothesis that microtubular orientation draws the arrow of PCP.     

Cell polarity during wound healing in an insect epidermis

The insect integument displays uniform posterior orientation of cuticular denticles or bristles formed by the epidermal cells. We want to understand how cell polarities become uniformly oriented in the plane of the epidermal sheet. Here we test whether directed cell migration disturbs the orientation of denticles. Burning a circular area of epidermal cells beneath the cuticle causes cells to migrate into the resulting wound and the cuticle pattern observed after the subsequent moult depends on the time interval between burning and ecdysis. After a short wound-healing period cuticular protrusions tend to point away from the wound. With increasing would healing periods they tend to point more and more towards the wound centre. These results suggest that the migrating cells tend to orient cuticular protrusions in the direction of cell movement while continued cell movement will bend nascent cuticular protrusions outwards. Cell shape may also determine denticle orientation. I propose that the asymmetric localization of cell components known to determine the orientation of cell migration may also determine denticle orientation in insect epidermal cells.     

Cell division orientation and planar cell polarity pathways

The orientation of cell division has a crucial role in early embryo body plan specification, axis determination and cell fate diversity generation, as well as in the morphogenesis of tissues and organs. In many instances, cell division orientation is regulated by the planar cell polarity (PCP) pathways: the Wnt/Frizzled non-canonical pathway or the Fat/Dachsous/Four-jointed pathway. Firstly, using asymmetric cell division in both Drosophila and C. elegans, we describe the central role of the Wnt/Frizzled pathway in the regulation of asymmetric cell division orientation, focusing on its cooperation with either the Src kinase pathway or the heterotrimeric G protein pathway. Secondly, we describe our present understanding of the mechanisms by which the planar cell polarity pathways drive tissue morphogenesis by regulating the orientation of symmetric cell division within a field of cells. Finally, we will discuss the important avenues that need to be explored in the future to better understand how planar cell polarity pathways control embryo body plan determination, cell fate specification or tissue morphogenesis by mitotic spindle orientation.     

Cell polarity and oocyte determination in Drosophila melanogaster

During early oogenesis, one cell from a cyst of 16 germ cells is selected to become the oocyte. Recent data suggest that the choice of this cell within the cyst is strongly biased as early as the cyst itself forms. However, it was further shown that, although selected, the oocyte fate needs to be maintained. The maintenance of the oocyte identity requires the activity of the Drosophila homologues of the Caenorhabditis elegans par genes. It was shown that the par genes are required for the first polarisation of the oocyte as early as in region 3 of the germarium. This reveals a striking conservation between the polarisation along the antero-posterior axis of the Caenorhabditis elegans one-cell embryo and the Drosophila oocyte.     

Cell shape and epithelial patterning in the Drosophila embryonic epidermis

The development of denticle rows on the ventral Drosophila embryo is a valuable system for studying the genetic control of epithelial patterning. During late embryogenesis, the apical surfaces of denticle-producing cells acquire a distinctive rectangular morphology with long anteroposterior boundaries, along which the denticles form, and short ventrolateral boundaries that stain strongly for adherens junction proteins. We observe that ventrolateral denticle cell boundaries are also convoluted, suggesting that the strong adherens staining results, at least in part, from the additional membrane in these regions. Embryos mutant for the Planar Cell Polarity (PCP) Effector gene multiple wing hairs (mwh), or expressing dominant negative form of the small GTPase Rac1, have cells present between the normal denticle cell rows. These 'Interloper Cells' do not have convoluted ventrolateral boundaries with strong adherens protein staining, but have normal denticle placement, suggesting that adherens protein localization is not critical for denticle cell PCP. Based on these and other observations, we propose that denticle cell morphology arises from an epithelial stretch without junction remodeling. A crude mechanical model suggests that this mechanism can generate both the straight anteroposterior boundaries and the compacted ventrolateral boundaries typical of denticle cells. We discuss the significance of cell adhesion for denticle cell morphogenesis, especially given the established role for Rac1 in cell adhesion.     

Nonautonomous planar polarity patterning in Drosophila: dishevelled-independent functions of frizzled

The frizzled (fz) gene of Drosophila is required for planar polarity establishment in the adult cuticle, acting both cell autonomously and nonautonomously. We demonstrate that these two activities of fz in planar polarity are temporally separable in both the eye and wing. The nonautonomous function is dishevelled (dsh) independent, and its loss results in polarity phenotypes that resemble those seen for mutations in dachsous (ds). Genetic interactions and epistasis analysis suggest that fz, ds, and fat (ft) act together in the long-range propagation of polarity signals in the eye and wing. We also find evidence that polarity information may be propagated by modulation of the binding affinities of the cadherins encoded by the ds and ft loci.     

Differential activities of the core planar polarity proteins during Drosophila wing patterning

During planar polarity patterning of the Drosophila wing, a "core" group of planar polarity genes has been identified which acts downstream of global polarity cues to locally coordinate cell polarity and specify trichome production at distal cell edges. These genes encode protein products that assemble into asymmetric apicolateral complexes that straddle the proximodistal junctional region between adjacent cells. We have carried out detailed genetic analysis experiments, analysing the requirements of each complex component for planar polarity patterning. We find that the three transmembrane proteins at the core of the complex, Frizzled, Strabismus and Flamingo, are required earliest in development and are the only components needed for intercellular polarity signalling. Notably, cells that lack both Frizzled and Strabismus are unable to signal, revealing an absolute requirement for both proteins in cell-cell communication. In contrast the cytoplasmic components Dishevelled, Prickle and Diego are not needed for intercellular communication. These factors contribute to the cell-cell propagation of polarity, most likely by promotion of intracellular asymmetry. Interestingly, both local polarity propagation and trichome placement occur normally in mutant backgrounds where asymmetry of polarity protein distribution is undetectable, suggesting such asymmetry is not an absolute requirement for any of the functions of the core complex.     

Cell polarity and asymmetric division in the peripheral nervous system of Drosophila

During metazoan development, cell fate diversity is generated in part by asymmetric cell divisions, in which mother cells divide to produce two daughter cells with distinct developmental potentials. Adoption of different cell fates often relies on the polarised distribution and unequal segregation of cell-fate determinants. Unequal segregation of cell-fate determinants requires that the mother cell becomes polarised prior to mitosis. In response to this polarisation, cell-fate determinants localise asymmetrically and the mitotic spindle lines up with the pole to which cell-fate determinants accumulate, thereby leading to their unequal partitioning upon cytokinesis. I review here the regulatory mechanisms that establish cell asymmetry and orient this asymmetry relative to the body axis in the sensory organ lineages of Drosophila.     

Temporal regulation of planar cell polarity: insights from the Drosophila eye

In this issue of Cell, identify a first regulatory link between planar cell polarity (PCP) signaling and apical-basal polarity. The authors propose that a component of the apical Crumbs complex regulates the phosphorylation of the Frizzled (Fz) PCP receptor, thus modulating PCP in the Drosophila eye.