Distinct roles for the actin and microtubule cytoskeletons in the morphogenesis of epidermal hairs during wing development in Drosophila

Osiris17 is essential for stable integrin localization and function during insect wing epithelia remodeling

The dynamic adhesion between cells and their extracellular matrix is essential for the development and function of organs. During insect wing development, two epithelial sheets contact each other at their basal sites through the interaction of βPS integrins with the extracellular matrix. We report that Osiris17 contributes to the maintenance of βPS integrins localization and function in developing wing of Drosophila and locust. In flies with reduced Osiris17 expression the epithelia sheets fail to maintain the integrity of basal cytoplasmic junctional bridges and basal adhesion. In contrast to the continuous basal integrin localization in control wings, this localization is disrupted during late stages of wing development in Osiris17 depleted flies. In addition, the subcellular localization revealed that Osiris17 co-localizes with the endosomal markers Rab5 and Rab11. This observation suggests an involvement of Osiris17 in endosomal recycling of integrins. Indeed, Osiris17 depletion reduced the numbers of Rab5 and Rab11 positive endosomes. Moreover, overexpression of Osiris17 increased co-localization of Rab5 and βPS integrins and partially rescued the detachment phenotype in flies with reduced βPS integrins. Taken together, our data suggest that Osiris17 is an endosome related protein that contributes to epithelial remodeling and morphogenesis by assisting basal integrins localization in insects.

Dusky-like Is Critical for Morphogenesis of the Cellular Protuberances and Formation of the Cuticle in Henosepilachna vigintioctopunctata

Dusky-like (Dyl) is a transmembrane protein containing a zona pellucida domain. Its physiological roles during metamorphosis have been well explored in Drosophila melanogaster and have also been documented in Tribolium castaneum. However, Dyl has undergone a functional shift between Diptera and Coleoptera insects. Further investigation of Dyl in other insects will be helpful to further clarify its function in insect growth and development. Henosepilachna vigintioctopunctata is an important Coleoptera that causes enormous economic losses in agriculture in China. In this study, we found that the expression of Hvdyl was detectable in embryos, larvae, prepupae, pupae, and adults. We knocked down Hvdyl in third- and fourth-instar larvae and pupae with RNA interference (RNAi). RNAi of Hvdyl mainly caused two phenotypic defects. Firstly, the growth of epidermal cellular protuberances was suppressed. Injection of dsdyl (double-stranded dusky-like RNA) at the third-instar larval stage truncated the scoli throughout the thorax and abdomen and shortened the setae on the head capsules and mouthparts of the fourth-instar larvae. Introduction of dsdyl at the third- and fourth-instar stages led to misshapen pupal setae. The setae were shortened or became black nodules. Treatment with dsdyl at the larval and pupal stages resulted in deformed adults with completely suppressed wing hairs. Moreover, the knockdown of Hvdyl at the third-instar stage caused deformed larval mouthparts at the fourth-instar period. As a result, foliage consumption was inhibited, and larval growth was slowed. The results indicate that Dyl is associated with the growth of cellular protuberances throughout development and with the formation of the cuticle in H. vigintioctopunctata.

RNAi screen in the Drosophila wing of genes encoding proteins related to cytoskeleton organization and cell division

Cell division and cytoskeleton organization are fundamental processes participating in the development of Drosophila imaginal discs. In this manuscript we describe the phenotypes in the adult fly wing generated by knockdowns of 85% of Drosophila genes encoding proteins likely related to the regulation of cell division and cytoskeleton organization. We also compile a molecular classification of these proteins into classes that describe their expected or known main biochemical characteristics, as well as mRNA expression in the wing disc and likely protein subcellular localization for a subset of these genes. Finally, we analyze in more detail one protein family of cytoskeleton genes (Arp2/3 complex), and define the consequences of interfering with cell division for wing growth and patterning.

The conserved Pelado/ZSWIM8 protein regulates actin dynamics by promoting linear actin filament polymerization

Actin filament polymerization can be branched or linear, which depends on the associated regulatory proteins. Competition for actin monomers occurs between proteins that induce branched or linear actin polymerization. Cell specialization requires the regulation of actin filaments to allow the formation of cell type-specific structures, like cuticular hairs in Drosophila, formed by linear actin filaments. Here, we report the functional analysis of CG34401/pelado, a gene encoding a SWIM domain-containing protein, conserved throughout the animal kingdom, called ZSWIM8 in mammals. Mutant pelado epithelial cells display actin hair elongation defects. This phenotype is reversed by increasing actin monomer levels or by either pushing linear actin polymerization or reducing branched actin polymerization. Similarly, in hemocytes, Pelado is essential to induce filopodia, a linear actin-based structure. We further show that this function of Pelado/ZSWIM8 is conserved in human cells, where Pelado inhibits branched actin polymerization in a cell migration context. In summary, our data indicate that the function of Pelado/ZSWIM8 in regulating actin cytoskeletal dynamics is conserved, favoring linear actin polymerization at the expense of branched filaments.

Mutations in Drosophila crinkled/Myosin VIIA disrupt denticle morphogenesis

Actin filament crosslinking, bundling and molecular motor proteins are necessary for the assembly of epithelial projections such as microvilli, stereocilia, hairs, and bristles. Mutations in such proteins cause defects in the shape, structure, and function of these actin - based protrusions. One protein necessary for stereocilia formation, Myosin VIIA, is an actin - based motor protein conserved throughout phylogeny. In Drosophila melanogaster, severe mutations in the MyoVIIA homolog crinkled (ck) are "semi - lethal" with only a very small percentage of flies surviving to adulthood. Such survivors show morphological defects related to actin bundling in hairs and bristles. To better understand ck/MyoVIIA's function in bundled - actin structures, we used dominant female sterile approaches to analyze the loss of maternal and zygotic (M/Z) ck/MyoVIIA in the morphogenesis of denticles, small actin - based projections on the ventral epidermis of Drosophila embryos. M/Z ck mutants displayed severe defects in denticle morphology - actin filaments initiated in the correct location, but failed to elongate and bundle to form normal projections. Using deletion mutant constructs, we demonstrated that both of the C - terminal MyTH4 and FERM domains are necessary for proper denticle formation. Furthermore, we show that ck/MyoVIIA interacts genetically with dusky - like (dyl), a member of the ZPD family of proteins that links the extracellular matrix to the plasma membrane, and when mutated also disrupts normal denticle formation. Loss of either protein alone does not alter the localization of the other; however, loss of the two proteins together dramatically enhances the defects in denticle shape observed when either protein alone was absent. Our data indicate that ck/MyoVIIA plays a key role in the formation and/or organization of actin filament bundles, which drive proper shape of cellular projections.

Chitinase 10 controls chitin amounts and organization in the wing cuticle of Drosophila

Wings are essential for insect fitness. A number of proteins and enzymes have been identified to be involved in wing terminal differentiation, which is characterized by the formation of the wing cuticle. Here, we addressed the question whether chitinase 10 (Cht10) may play an important role in chitin organization in the wings of the fruit fly Drosophila melanogaster. Initially, we first found that Cht10 expression coincides with the expression of the chitin synthase coding gene kkv. This suggests that the respective proteins may cooperate during wing differentiation. In tissue-specific RNA interference experiments, we demonstrate that suppression of Cht10 causes an excess in chitin amounts in the wing cuticle. Chitin organization is severely disrupted in these wings. Based on these data, we hypothesize that Cht10 restricts chitin amounts produced by Kkv in order to ensure normal chitin organization and wing cuticle formation. In addition, we found by scanning electron microscopy that Cht10 suppression also affects the cuticle surface. In turn, cuticle inward permeability is enhanced in Cht10-less wings. Moreover, flies with reduced Cht10 function are unable to fly. In conclusion, Cht10 is essential for wing terminal differentiation and function.

Actin bundles play a different role in shaping scales compared to bristles in the mosquito Aedes aegypti

Insect epithelial cells contain cellular extensions such as bristles, hairs, and scales. These cellular extensions are homologous structures that differ in morphology and function. They contain actin bundles that dictate their cellular morphology. While the organization, function, and identity of the major actin-bundling proteins in bristles and hairs are known, this information on scales is unknown. In this study, we characterized the development of scales and the role of actin bundles in the mosquito, Aedes aegypti. We show that scales undergo drastic morphological changes during development, from a cylindrical to flat shape with longer membrane invagination. Scale actin-bundle distribution changes from the symmetrical organization of actin bundles located throughout the bristle membrane to an asymmetrical organization. By chemically inhibiting actin polymerization and by knocking out the forked gene in the mosquito (Ae-Forked; a known actin-bundling protein) by CRISPR-Cas9 gene editing, we showed that actin bundles are required for shaping bristle, hair, and scale morphology. We demonstrated that actin bundles and Ae-Forked are required for bristle elongation, but not for that of scales. In scales, actin bundles are required for width formation. In summary, our results reveal, for the first time, the developmental process of mosquito scale formation and also the role of actin bundles and actin-bundle proteins in scale morphogenesis. Moreover, our results reveal that although scale and bristle are thought to be homologous structures, actin bundles have a differential requirement in shaping mosquito scales compared to bristles.

Membrane trafficking in morphogenesis and planar polarity

Our understanding of how membrane trafficking pathways function to direct morphogenetic movements and the planar polarization of developing tissues is a new and emerging field. While a central focus of developmental biology has been on how protein asymmetries and cytoskeletal force generation direct cell shaping, the role of membrane trafficking in these processes has been less clear. Here, we review recent advances in Drosophila and vertebrate systems in our understanding of how trafficking events are coordinated with planar cytoskeletal function to drive lasting changes in cell and tissue topologies. We additionally explore the function of trafficking pathways in guiding the complex interactions that initiate and maintain core PCP (planar cell polarity) asymmetries and drive the generation of systematically oriented cellular projections during development.

A cytoskeletal activator and inhibitor are downstream targets of the frizzled/starry night planar cell polarity pathway in the Drosophila epidermis

The frizzled pathway regulates the planar polarity of epithelial cells. In insects this is manifested by the polarity of cuticular structures such as hairs (trichomes) and sensory bristles. A variety of evidence has established that this is achieved by regulating the subcellular location for activating the cytoskeleton in the epithelial cells. How this is accomplished is still poorly understood. In the best-studied tissue, the Drosophila pupal wing two important cytoskeletal regulators have been identified. One, shavenoid (sha), appears to be an activator while the second multiple wing hairs (mwh), appears to be an inhibitor. In vitro biochemistry has confirmed that the Multiple Wing Hairs protein inhibits the elongation of F-actin chains and surprisingly that it also bundles F-actin. These two activities can explain the multifaceted mwh mutant phenotype.

Crumbs, Moesin and Yurt regulate junctional stability and dynamics for a proper morphogenesis of the Drosophila pupal wing epithelium

The Crumbs (Crb) complex is a key epithelial determinant. To understand its role in morphogenesis, we examined its function in the Drosophila pupal wing, an epithelium undergoing hexagonal packing and formation of planar-oriented hairs. Crb distribution is dynamic, being stabilized to the subapical region just before hair formation. Lack of crb or stardust, but not DPatj, affects hexagonal packing and delays hair formation, without impairing epithelial polarities but with increased fluctuations in cell junctions and perimeter length, fragmentation of adherens junctions and the actomyosin cytoskeleton. Crb interacts with Moesin and Yurt, FERM proteins regulating the actomyosin network. We found that Moesin and Yurt distribution at the subapical region depends on Crb. In contrast to previous reports, yurt, but not moesin, mutants phenocopy crb junctional defects. Moreover, while unaffected in crb mutants, cell perimeter increases in yurt mutant cells and decreases in the absence of moesin function. Our data suggest that Crb coordinates proper hexagonal packing and hair formation, by modulating junction integrity via Yurt and stabilizing cell perimeter via both Yurt and Moesin. The Drosophila pupal wing thus appears as a useful system to investigate the functional diversification of the Crb complex during morphogenesis, independently of its role in polarity.

Centriole positioning in epithelial cells and its intimate relationship with planar cell polarity

Planar cell polarity (PCP)-signaling and associated tissue polarization are evolutionarily conserved. A well documented feature of PCP-signaling in vertebrates is its link to centriole/cilia positioning, although the relationship of PCP and ciliogenesis is still debated. A recent report in Drosophila established that Frizzled (Fz)-PCP core signaling has an instructive input to polarized centriole positioning in non-ciliated Drosophila wing epithelia as a PCP read-out. Here, we review the impact of this observation in the context of recent descriptions of the relationship(s) of core Fz-PCP signaling and cilia/centriole positioning in epithelial and non-epithelial cells. All existing data are consistent with a model where Fz-PCP signaling functions upstream of centriole/cilia positioning, independent of ciliogenesis. The combined data sets indicate that the Fz-Dsh PCP complex is instructive for centriole/ciliary positioning via an actin-based mechanism. Thereby, centriole/cilia/centrosome positioning can be considered an evolutionarily conserved readout and common downstream effect of PCP-signaling from flies to mammals.

The Gene Expression Program for the Formation of Wing Cuticle in Drosophila

The cuticular exoskeleton of insects and other arthropods is a remarkably versatile material with a complex multilayer structure. We made use of the ability to isolate cuticle synthesizing cells in relatively pure form by dissecting pupal wings and we used RNAseq to identify genes expressed during the formation of the adult wing cuticle. We observed dramatic changes in gene expression during cuticle deposition, and combined with transmission electron microscopy, we were able to identify candidate genes for the deposition of the different cuticular layers. Among genes of interest that dramatically change their expression during the cuticle deposition program are ones that encode cuticle proteins, ZP domain proteins, cuticle modifying proteins and transcription factors, as well as genes of unknown function. A striking finding is that mutations in a number of genes that are expressed almost exclusively during the deposition of the envelope (the thin outermost layer that is deposited first) result in gross defects in the procuticle (the thick chitinous layer that is deposited last). An attractive hypothesis to explain this is that the deposition of the different cuticle layers is not independent with the envelope instructing the formation of later layers. Alternatively, some of the genes expressed during the deposition of the envelope could form a platform that is essential for the deposition of all cuticle layers.

Insects and other arthropods are an extremely successful group of animals. A unique and key feature of their lifestyle is their chitin containing cuticular exoskeleton, a complex layered material, which remains rather poorly understood for so prominent of a biological material. We have characterized the gene expression pattern of wing epithelial cells over the period of cuticle formation and also carried out transmission electron microscopy, which allows us to identify genes that likely play a role in the formation of different cuticle layers. Functional studies suggest that the deposition of the earliest layer influences the deposition of the later ones.

Cooperative functions of the two F-BAR proteins Cip4 and Nostrin in the regulation of E-cadherin in epithelial morphogenesis

F-BAR proteins are prime candidates to regulate membrane curvature and dynamics during different developmental processes. Here, we analyzed nostrin, a so-far-unknown Drosophila melanogaster F-BAR protein related to Cip4. Genetic analyses revealed a strong synergism between nostrin and cip4 functions.Whereas single mutant flies are viable and fertile, combined loss of nostrin and cip4 results in reduced viability and fertility. Double mutant escaper flies show enhanced wing polarization defects and females exhibit strong egg chamber encapsulation defects. Live imaging analysis suggests that the observed phenotypes are caused by an impaired turnover of E-cadherin at the membrane. Simultaneous knockdown of Cip4 and Nostrin strongly increases the formation of tubular E-cadherin vesicles at adherens junctions. Cip4 and Nostrin localize at distinct membrane subdomains. Both proteins prefer similar membrane curvatures but seem to form distinct membrane coats and do not heterooligomerize. Our data suggest an important synergistic function of both F-BAR proteins in membrane dynamics. We propose a cooperative recruitment model, in which Cip4 initially promotes membrane invagination and early-actin-based endosomal motility, and Nostrin makes contacts with microtubules through the kinesin Khc-73 for trafficking of recycling endosomes.

Positioning of centrioles is a conserved readout of Frizzled planar cell polarity signalling

Planar cell polarity (PCP) signalling is a well-conserved developmental pathway regulating cellular orientation during development. An evolutionarily conserved pathway readout is not established and, moreover, it is thought that PCP mediated cellular responses are tissue-specific. A key PCP function in vertebrates is to regulate coordinated centriole/cilia positioning, a function that has not been associated with PCP in Drosophila. Here we report instructive input of Frizzled-PCP (Fz/PCP) signalling into polarized centriole positioning in Drosophila wings. We show that centrioles are polarized in pupal wing cells as a readout of PCP signalling, with both gain and loss-of-function Fz/PCP signalling affecting centriole polarization. Importantly, loss or gain of centrioles does not affect Fz/PCP establishment, implicating centriolar positioning as a conserved PCP-readout, likely downstream of PCP-regulated actin polymerization. Together with vertebrate data, these results suggest a unifying model of centriole/cilia positioning as a common downstream effect of PCP signalling from flies to mammals.

Planar cell polarity (PCP) contributes to cellular orientation during development but how this is regulated in Drosophila is unclear. Here, the authors identify Frizzled-PCP signalling as regulating polarised centriole positioning in the wing disc.

The Drosophila planar polarity gene multiple wing hairs directly regulates the actin cytoskeleton

The evolutionarily conserved frizzled/starry night (fz/stan) pathway regulates planar cell polarity (PCP) in vertebrates and invertebrates. This pathway has been extensively studied in the Drosophila wing, where it is manifested by an array of distally pointing cuticular hairs. Using in vivo imaging we found that, early in hair growth, cells have multiple actin bundles and hairs that subsequently fuse into a single growing hair. The downstream PCP gene multiple wing hairs (mwh) plays a key role in this process and acts to antagonize the actin cytoskeleton. In mwh mutants hair initiation is not limited to a small region at the distal edge of pupal wing cells as in wild type, resulting in multiple hairs with aberrant polarity. Extra actin bundles/hairs are formed and do not completely fuse, in contrast to wild type. As development proceeded additional hairs continued to form, further increasing hair number. We identified a fragment of Mwh with in vivo rescue activity and that bound and bundled F-actin filaments and inhibited actin polymerization in in vitro actin assays. The loss of these activities can explain the mwh mutant phenotype. Our data suggest a model whereby, prior to hair initiation, proximally localized Mwh inhibits actin polymerization resulting in polarized activation of the cytoskeleton and hair formation on the distal side of wing cells. During hair growth Mwh is found in growing hairs, where we suggest it functions to promote the fusion of actin bundles and inhibit the formation of additional actin bundles that could lead to extra hairs.

Polarized trafficking provides spatial cues for planar cell polarization within a tissue

Planar cell polarity, the polarization of cells within the plane of the epithelium, orthogonal to the apical-basal axis, is essential for a growing list of developmental events, and - over the last 15 years - has evolved from a little-studied curiosity in Drosophila to the subject of a substantial research enterprise. In that time, it has been recognized that two molecular systems are responsible for polarization of most tissues: Both the "core" Frizzled system and the "global" Fat/Dachsous/Four-jointed system produce molecular asymmetry within cells, and contribute to morphological polarization. In this review, we discuss recent findings on the molecular mechanism that links "global" directional signals with local coordinated polarity.

The diaphanous gene of Drosophila interacts antagonistically with multiple wing hairs and plays a key role in wing hair morphogenesis

The Drosophila wing is covered by an array of distally pointing hairs that has served as a key model system for studying planar cell polarity (PCP). The adult cuticular hairs are formed in the pupae from cell extensions that contain extensive actin filaments and microtubules. The importance of the actin cytoskeleton for hair growth and morphogenesis is clear from the wide range of phenotypes seen in mutations in well-known actin regulators. Formin proteins promote the formation of long actin filaments of the sort thought to be important for hair growth. We report here that the formin encoding diaphanous (dia) gene plays a key role in hair morphogenesis. Both loss of function mutations and the expression of a constitutively active Dia led to cells forming both morphologically abnormal hairs and multiple hairs. The conserved frizzled (fz)/starry night (stan) PCP pathway functions to restrict hair initiation and activation of the cytoskeleton to the distal most part of wing cells. It also ensures the formation of a single hair per cell. Our data suggest that the localized inhibition of Dia activity may be part of this mechanism. We found the expression of constitutively active Dia greatly expands the region for activation of the cytoskeleton and that dia functions antagonistically with multiple wing hairs (mwh), the most downstream member of the fz/stan pathway. Further we established that purified fragments of Dia and Mwh could be co-immunoprecipitated suggesting the genetic interaction could reflect a direct physical interaction.

Regions within a single epidermal cell of Drosophila can be planar polarised independently

Planar cell polarity (PCP), the coordinated and consistent orientation of cells in the plane of epithelial sheets, is a fundamental and conserved property of animals and plants. Up to now, the smallest unit expressing PCP has been considered to be an entire single cell. We report that, in the larval epidermis of Drosophila, different subdomains of one cell can have opposite polarities. In larvae, PCP is driven by the Dachsous/Fat system; we show that the polarity of a subdomain within one cell is its response to levels of Dachsous/Fat in the membranes of contacting cells. During larval development, cells rearrange (Saavedra et al., 2014) and when two subdomains of a single cell have different types of neighbouring cells, then these subdomains can become polarised in opposite directions. We conclude that polarisation depends on a local comparison of the amounts of Dachsous and Fat within opposing regions of a cell's membrane.

Sperm-associated antigen 6 (SPAG6) deficiency and defects in ciliogenesis and cilia function: polarity, density, and beat

SPAG6, an axoneme central apparatus protein, is essential for function of ependymal cell cilia and sperm flagella. A significant number of Spag6-deficient mice die with hydrocephalus, and surviving males are sterile because of sperm motility defects. In further exploring the ciliary dysfunction in Spag6-null mice, we discovered that cilia beat frequency was significantly reduced in tracheal epithelial cells, and that the beat was not synchronized. There was also a significant reduction in cilia density in both brain ependymal and trachea epithelial cells, and cilia arrays were disorganized. The orientation of basal feet, which determines the direction of axoneme orientation, was apparently random in Spag6-deficient mice, and there were reduced numbers of basal feet, consistent with reduced cilia density. The polarized epithelial cell morphology and distribution of intracellular mucin, α-tubulin, and the planar cell polarity protein, Vangl2, were lost in Spag6-deficient tracheal epithelial cells. Polarized epithelial cell morphology and polarized distribution of α-tubulin in tracheal epithelial cells was observed in one-week old wild-type mice, but not in the Spag6-deficient mice of the same age. Thus, the cilia and polarity defects appear prior to 7 days post-partum. These findings suggest that SPAG6 not only regulates cilia/flagellar motility, but that in its absence, ciliogenesis, axoneme orientation, and tracheal epithelial cell polarity are altered.

Myosin VIIA regulates microvillus morphogenesis and interacts with cadherin Cad99C in Drosophila oogenesis

Microvilli and related actin-based protrusions permit multiple interactions between cells and their environment. How the shape, length and arrangement of microvilli are determined remains largely unclear. To address this issue and explore the cooperation of the two main components of a microvillus, the central F-actin bundle and the enveloping plasma membrane, we investigated the expression and function of Myosin VIIA (Myo7A), which is encoded by crinkled (ck), and its interaction with cadherin Cad99C in the microvilli of the Drosophila follicular epithelium. Myo7A is present in the microvilli and terminal web of follicle cells, and associates with several other F-actin-rich structures in the ovary. Loss of Myo7A caused brush border defects and a reduction in the amount of the microvillus regulator Cad99C. We show that Myo7A and Cad99C form a molecular complex and that the cytoplasmic tail of Cad99C recruits Myo7A to microvilli. Our data indicate that Myo7A regulates the structure and spacing of microvilli, and interacts with Cad99C in vivo. A comparison of the mutant phenotypes suggests that Myo7A and Cad99C have co-dependent and independent functions in microvilli.

Combover/CG10732, a novel PCP effector for Drosophila wing hair formation

The polarization of cells is essential for the proper functioning of most organs. Planar Cell Polarity (PCP), the polarization within the plane of an epithelium, is perpendicular to apical-basal polarity and established by the non-canonical Wnt/Fz-PCP signaling pathway. Within each tissue, downstream PCP effectors link the signal to tissue specific readouts such as stereocilia orientation in the inner ear and hair follicle orientation in vertebrates or the polarization of ommatidia and wing hairs in Drosophila melanogaster. Specific PCP effectors in the wing such as Multiple wing hairs (Mwh) and Rho Kinase (Rok) are required to position the hair at the correct position and to prevent ectopic actin hairs. In a genome-wide screen in vitro, we identified Combover (Cmb)/CG10732 as a novel Rho kinase substrate. Overexpression of Cmb causes the formation of a multiple hair cell phenotype (MHC), similar to loss of rok and mwh. This MHC phenotype is dominantly enhanced by removal of rok or of other members of the PCP effector gene family. Furthermore, we show that Cmb physically interacts with Mwh, and cmb null mutants suppress the MHC phenotype of mwh alleles. Our data indicate that Cmb is a novel PCP effector that promotes to wing hair formation, a function that is antagonized by Mwh.

Microtubules provide directional information for core PCP function

Planar cell polarity (PCP) signaling controls the polarization of cells within the plane of an epithelium. Two molecular modules composed of Fat(Ft)/Dachsous(Ds)/Four-jointed(Fj) and a ‘PCP-core’ including Frizzled(Fz) and Dishevelled(Dsh) contribute to polarization of individual cells. How polarity is globally coordinated with tissue axes is unresolved. Consistent with previous results, we find that the Ft/Ds/Fj-module has an effect on a MT-cytoskeleton. Here, we provide evidence for the model that the Ft/Ds/Fj-module provides directional information to the core-module through this MT organizing function. We show Ft/Ds/Fj-dependent initial polarization of the apical MT-cytoskeleton prior to global alignment of the core-module, reveal that the anchoring of apical non-centrosomal MTs at apical junctions is polarized, observe that directional trafficking of vesicles containing Dsh depends on Ft, and demonstrate the feasibility of this model by mathematical simulation. Together, these results support the hypothesis that Ft/Ds/Fj provides a signal to orient core PCP function via MT polarization.

DOI:http://dx.doi.org/10.7554/eLife.02893.001

Almost all cells exhibit some sort of polarity: the epithelial cells that line the digestive tract, for example, have an apical domain, which faces out, and a basal domain, which faces the tissue underneath. Some epithelial cells also exhibit planar cell polarity: this involves key structures within the cell being oriented along an axis within the plane of an epithelium. Disruption of planar cell polarity is associated with various developmental defects.

It is known that the planar polarity of epithelial cells relies on two molecular complexes—a ‘core’ complex and a signaling complex called the Ft/Ds/Fj system—working together. While each of these complexes contributes to whole tissues having the correct polarity, the way they interact to achieve this is not fully understood.

Now, by studying epithelial cells in the wings of fruit flies, Matis et al. have provided evidence for a specific model for this interaction. The process starts with the Ft/Ds/Fj signaling complex, which orients structures called microtubules inside the cell. Microtubules are involved in providing structural support for cells, and also in the transport of organelles within cells.

Once the microtubules are oriented in the correct direction, they help to orient the core complex by moving some of the proteins that make up this complex in a specified direction. An important future challenge will be to understand how the proteins in the Ft/Ds/Fj system interact with microtubules to give them their orientation.

DOI:http://dx.doi.org/10.7554/eLife.02893.002

Prickle/spiny-legs isoforms control the polarity of the apical microtubule network in planar cell polarity

Microtubules (MTs) are substrates upon which plus- and minus-end directed motors control the directional movement of cargos that are essential for generating cell polarity. Although centrosomal MTs are organized with plus-ends away from the MT organizing center, the regulation of non-centrosomal MT polarity is poorly understood. Increasing evidence supports the model that directional information for planar polarization is derived from the alignment of a parallel apical network of MTs and the directional MT-dependent trafficking of downstream signaling components. The Fat/Dachsous/Four-jointed (Ft/Ds/Fj) signaling system contributes to orienting those MTs. In addition to previously defined functions in promoting asymmetric subcellular localization of 'core' planar cell polarity (PCP) proteins, we find that alternative Prickle (Pk-Sple) protein isoforms control the polarity of this MT network. This function allows the isoforms of Pk-Sple to differentially determine the direction in which asymmetry is established and therefore, ultimately, the direction of tissue polarity. Oppositely oriented signals that are encoded by oppositely oriented Fj and Ds gradients produce the same polarity outcome in different tissues or compartments, and the tissue-specific activity of alternative Pk-Sple protein isoforms has been observed to rectify the interpretation of opposite upstream directional signals. The control of MT polarity, and thus the directionality of apical vesicle traffic, by Pk-Sple provides a mechanism for this rectification.

Dynamics of F-actin prefigure the structure of butterfly wing scales

The wings of butterflies and moths consist of dorsal and ventral epidermal surfaces that give rise to overlapping layers of scales and hairs (Lepidoptera, "scale wing"). Wing scales (average length ~200 µm) are homologous to insect bristles (macrochaetes), and their colors create the patterns that characterize lepidopteran wings. The topology and surface sculpture of wing scales vary widely, and this architectural complexity arises from variations in the developmental program of the individual scale cells of the wing epithelium. One of the more striking features of lepidopteran wing scales are the longitudinal ridges that run the length of the mature (dead) cell, gathering the cuticularized scale cell surface into pleats on the sides of each scale. While also present around the periphery of other insect bristles and hairs, longitudinal ridges in lepidopteran wing scales gain new significance for their creation of iridescent color through microribs and lamellae. Here we show the dynamics of the highly organized F-actin filaments during scale cell development, and present experimental manipulations of actin polymerization that reveal the essential role of this cytoskeletal component in wing scale elongation and the positioning of longitudinal ribs.

Mathematical modeling of sub-cellular asymmetry of fat-dachsous heterodimer for generation of planar cell polarity

Planar Cell Polarity (PCP) is an evolutionarily conserved characteristic of animal tissues marked by coordinated polarization of cells or structures in the plane of a tissue. In insect wing epithelium, for instance, PCP is characterized by en masse orientation of hairs orthogonal to its apical-basal axis and pointing along the proximal-distal axis of the organ. Directional cue for PCP has been proposed to be generated by complex sets of interactions amongst three proteins - Fat (Ft), Dachsous (Ds) and Four-jointed (Fj). Ft and Ds are two atypical cadherins, which are phosphorylated by Fj, a Golgi kinase. Ft and Ds from adjacent cells bind heterophilically via their tandem cadherin repeats, and their binding affinities are regulated by Fj. Further, in the wing epithelium, sub-cellular levels of Ft-Ds heterodimers are seen to be elevated at the distal edges of individual cells, prefiguring their PCP. Mechanisms generating this sub-cellular asymmetry of Ft-Ds heterodimer in proximal and distal edges of cells, however, have not been resolved yet. Using a mathematical modeling approach, here we provide a framework for generation of this sub-cellular asymmetry of Ft-Ds heterodimer. First, we explain how the known interactions within Ft-Ds-Fj system translate into sub-cellular asymmetry of Ft-Ds heterodimer. Second, we show that this asymmetric localization of Ft-Ds heterodimer is lost when tissue-level gradient of Fj is flattened, or when phosphorylation of Ft by Fj is abolished, but not when tissue-level gradient of Ds is flattened or when phosphorylation of Ds is abrogated. Finally, we show that distal enrichment of Ds also amplifies Ft-Ds asymmetry. These observations reveal that gradient of Fj expression, phosphorylation of Ft by Fj and sub-cellular distal accumulation of Ds are three critical elements required for generating sub-cellular asymmetry of Ft-Ds heterodimer. Our model integrates the known experimental data and presents testable predictions for future studies.

Mechanisms of regulating tissue elongation in Drosophila wing: impact of oriented cell divisions, oriented mechanical forces, and reduced cell size

Regulation of cell growth and cell division plays fundamental roles in tissue morphogenesis. However, the mechanisms of regulating tissue elongation through cell growth and cell division are still not well understood. The wing imaginal disc of Drosophila provides a model system that has been widely used to study tissue morphogenesis. Here we use a recently developed two-dimensional cellular model to study the mechanisms of regulating tissue elongation in Drosophila wing. We simulate the effects of directional cues on tissue elongation. We also computationally analyze the role of reduced cell size. Our simulation results indicate that oriented cell divisions, oriented mechanical forces, and reduced cell size can all mediate tissue elongation, but they function differently. We show that oriented cell divisions and oriented mechanical forces act as directional cues during tissue elongation. Between these two directional cues, oriented mechanical forces have a stronger influence than oriented cell divisions. In addition, we raise the novel hypothesis that reduced cell size may significantly promote tissue elongation. We find that reduced cell size alone cannot drive tissue elongation. However, when combined with directional cues, such as oriented cell divisions or oriented mechanical forces, reduced cell size can significantly enhance tissue elongation in Drosophila wing. Furthermore, our simulation results suggest that reduced cell size has a short-term effect on cell topology by decreasing the frequency of hexagonal cells, which is consistent with experimental observations. Our simulation results suggest that cell divisions without cell growth play essential roles in tissue elongation.

Planar cell polarity signaling: coordination of cellular orientation across tissues

Establishment of Planar Cell Polarity (PCP) in epithelia, in the plane of an epithelium, is an important feature of the development and homeostasis of most organs. Studies in different model organisms have contributed a wealth of information regarding the mechanisms that govern PCP regulation. Genetic studies in Drosophila have identified two signaling systems, the Fz/PCP and Fat/Dachsous system, which are both required for PCP establishment in many different tissues in a largely non-redundant manner. Recent advances in vertebrate PCP studies have added novel factors of PCP regulation and also new cellular features requiring PCP-signaling input, including the positioning and orientation of the primary cilium of many epithelial cells. This review focuses mostly on several recent advances made in the Drosophila and vertebrate PCP field and integrates these within the existing PCP-signaling framework.

Filopodia as sensors

Filopodia are sensors on both excitable and non-excitable cells. The sensing function is well documented in neurons and blood vessels of adult animals and is obvious during dorsal closure in embryonic development. Nerve cells extend neurites in a bidirectional fashion with growth cones at the tips where filopodia are concentrated. Their sensing of environmental cues underpins the axon's ability to "guide," bypassing non-target cells and moving toward the target to be innervated. This review focuses on the role of filopodia structure and dynamics in the detection of environmental cues, including both the extracellular matrix (ECM) and the surfaces of neighboring cells. Other protrusions including the stereocilia of the inner ear and epididymus, the invertebrate Type I mechanosensors, and the elongated processes connecting osteocytes, share certain principles of organization with the filopodia. Actin bundles, which may be inside or outside of the excitable cell, function to transduce stress from physical perturbations into ion signals. There are different ways of detecting such perturbations. Osteocyte processes contain an actin core and are physically anchored on an extracellular structure by integrins. Some Type I mechanosensors have bridge proteins that anchor microtubules to the membrane, but bundles of actin in accessory cells exert stress on this complex. Hair cells of the inner ear rely on attachments between the actin-based protrusions to activate ion channels, which then transduce signals to afferent neurons. In adherent filopodia, the focal contacts (FCs) integrated with ECM proteins through integrins may regulate integrin-coupled ion channels to achieve signal transduction. Issues that are not understood include the role of Ca(2+) influx in filopodia dynamics and how integrins coordinate or gate signals arising from perturbation of channels by environmental cues.

Shaping the nervous system: role of the core planar cell polarity genes

Planar cell polarity (PCP) is complementary to the intrinsic polarization of single cells and refers to the global coordination of cell behaviour in the plane of a tissue and, by extension, to the signalling pathways that control it. PCP is most evident in cell sheets, and research into PCP was for years confined to studies in Drosophila melanogaster. However, PCP has more recently emerged as an important phenomenon in vertebrates, in which it regulates various developmental processes and is associated with multiple disorders. In particular, core PCP genes are crucial for the development and function of the nervous system. They are involved in neural tube closure, ependymal polarity, neuronal migration, dendritic growth and axon guidance.

dusky-like is required to maintain the integrity and planar cell polarity of hairs during the development of the Drosophila wing

The cuticular hairs and sensory bristles that decorate the adult Drosophila epidermis and the denticles found on the embryo have been used in studies on planar cell polarity and as models for the cytoskeletal mediated morphogenesis of cellular extensions. ZP domain proteins have recently been found to be important for the morphogenesis of both denticles and bristles. Here we show that the ZP domain protein Dusky-like is a key player in hair morphogenesis. As is the case in bristles, in hairs dyl mutants display a dramatic phenotype that is the consequence of a failure to maintain the integrity of the extension after outgrowth. Hairs lacking dyl function are split, thinned, multipled and often very short. dyl is required for normal chitin deposition in hairs, but chitin is not required for the normal accumulation of Dyl, hence dyl acts upstream of chitin. A lack of chitin however, does not mimic the dyl hair phenotype, thus Dyl must have other targets in hair morphogenesis. One of these appears to be the actin cytoskeleton. Interestingly, dyl mutants also display a unique planar cell polarity phenotype that is distinct from that seen with mutations in the frizzled/starry night or dachsous/fat pathway genes. Rab11 was previously found to be essential for Dyl plasma membrane localization in bristles. Here we found that the expression of a dominant negative Rab11 can mimic the dyl hair morphology phenotype consistent with Rab11 also being required for Dyl function in hairs. We carried out a small directed screen to identify genes that might function with dyl and identified Chitinase 6 (Cht6) as a strong candidate, as knocking down Cht6 function led to weak versions of all of the dyl hair phenotypes.

Microtubules enable the planar cell polarity of airway cilia

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

Drosophila CK1-γ, gilgamesh, controls PCP-mediated morphogenesis through regulation of vesicle trafficking

CK1-γ/gilgamesh spatially limits the planar cell polarity–regulated process of trichome formation in Drosophila through its effect on polarized vesicle recycling.

Cellular morphogenesis, including polarized outgrowth, promotes tissue shape and function. Polarized vesicle trafficking has emerged as a fundamental mechanism by which protein and membrane can be targeted to discrete subcellular domains to promote localized protrusions. Frizzled (Fz)/planar cell polarity (PCP) signaling orchestrates cytoskeletal polarization and drives morphogenetic changes in such contexts as the vertebrate body axis and external Drosophila melanogaster tissues. Although regulation of Fz/PCP signaling via vesicle trafficking has been identified, the interplay between the vesicle trafficking machinery and downstream terminal PCP-directed processes is less established. In this paper, we show that Drosophila CK1-γ/gilgamesh (gish) regulates the PCP-associated process of trichome formation through effects on Rab11-mediated vesicle recycling. Although the core Fz/PCP proteins dictate prehair formation broadly, CK1-γ/gish restricts nucleation to a single site. Moreover, CK1-γ/gish works in parallel with the Fz/PCP effector multiple wing hairs, which restricts prehair formation along the perpendicular axis to Gish. Our findings suggest that polarized Rab11-mediated vesicle trafficking regulated by CK1-γ is required for PCP-directed processes.

Dusky-like functions as a Rab11 effector for the deposition of cuticle during Drosophila bristle development

The morphogenesis of Drosophila sensory bristles is dependent on the function of their actin and microtubule cytoskeleton. Actin filaments are important for bristle shape and elongation, while microtubules are thought to mediate protein and membrane trafficking to promote growth. We have identified an essential role for the bristle cuticle in the maintenance of bristle structure and shape at late stages of bristle development. We show that the small GTPase Rab11 mediates the organized deposition of chitin, a major cuticle component in bristles, and disrupting Rab11 function leads to phenotypes that result from bristle collapse rather than a failure to elongate. We further establish that Rab11 is required for the plasma membrane localization of the ZP domain-containing Dusky-like (Dyl) protein and that Dyl is also required for cuticle formation in bristles. Our data argue that Dyl functions as a Rab11 effector for mediating the attachment of the bristle cell membrane to chitin to establish a stable cuticle. Our studies also implicate the exocyst as a Rab11 effector in this process and that Rab11 trafficking along the bristle shaft is mediated by microtubules.

The frizzled/stan pathway and planar cell polarity in the Drosophila wing

Drosophila has been the key model system for studies on planar cell polarity (PCP). The rich morphology of the insect exoskeleton contains many structures that display PCP. Among these are the trichomes (cuticular hairs) that cover much of the exoskeleton, sensory bristles, and ommatidia. Many genes have been identified that must function for the development of normal PCP. Among these are the genes that comprise the frizzled/starry night (fz/stan) and dachsous/fat pathways. The mechanisms that underlie the function of the fz/stan pathway are best understood. All of the protein products of these genes accumulate asymmetrically in wing cells and there is good evidence that this involves local intercellular signaling between protein complexes on the distal edge of one cell and the juxtaposed proximal edge of its neighbor. It is thought that a feedback system, directed transport, and stabilizing protein-protein interactions mediate the formation of distal and proximal protein complexes. These complexes appear to recruit downstream proteins that function to spatially restrict the activation of the cytoskeleton in wing cells. This leads to the formation of the array of distally pointing hairs found on wings.

Translating cell polarity into tissue elongation

Planar cell polarity, the orientation of single-cell asymmetries within the plane of a multicellular tissue, is essential to generating the shape and dimensions of organs and organisms. Planar polarity systems align cell behavior with the body axes and orient the cellular processes that lead to tissue elongation. Using Drosophila as a model system, significant progress has been made toward understanding how planar polarity is generated by biochemical and mechanical signals. Recent studies using time-lapse imaging reveal that cells engage in a number of active behaviors whose orientation and dynamics translate planar cell polarity into tissue elongation. Here we review recent progress in understanding the cellular mechanisms that link planar polarity to large-scale changes in tissue structure.

Planar cell polarity in Drosophila

In all multicellular organisms, epithelial cells are not only polarized along the apical-basal axis, but also within the epithelial plane, giving cells a sense of direction. Planar cell polarity (PCP) signaling regulates establishment of polarity within the plane of an epithelium. The outcomes of PCP signaling are diverse and include the determination of cell fates, the generation of asymmetric but highly aligned structures, such as the stereocilia in the human inner ear or the hairs on a fly wing, or the directional migration of cells during convergence and extension during vertebrate gastrulation. In humans, aberrant PCP signaling can result in severe developmental defects, such as open neural tubes (spina bifida), and can cause cystic kidneys. In this review, we discuss the basic mechanism and more recent findings of PCP signaling focusing on Drosophila melanogaster, the model organism in which most key PCP components were initially identified.

Planar cell polarity signaling, cilia and polarized ciliary beating

Planar cell polarity signaling governs a wide array of polarized cell behaviors in animals. Recent reports now show that PCP signaling is essential for the directional beating of motile cilia. Interestingly, PCP signaling acts in a variety of ciliated cell types that use motile cilia to generate directional fluid flow in very different ways. This review will synthesize these recent papers and place them in context with previous studies of PCP signaling in polarized cellular morphogenesis and collective cell movement.

Atypical cadherins Dachsous and Fat control dynamics of noncentrosomal microtubules in planar cell polarity

How global organ asymmetry and individual cell polarity are connected to each other is a central question in studying planar cell polarity (PCP). In the Drosophila wing, which develops PCP along its proximal-distal (P-D) axis, we previously proposed that the core PCP mediator Frizzled redistributes distally in a microtubule (MT)-dependent manner. Here, we performed organ-wide analysis of MT dynamics by introducing quantitative in vivo imaging. We observed MTs aligning along the P-D axis at the onset of redistribution and a small but significant excess of + ends-distal MTs in the proximal region of the wing. This characteristic alignment and asymmetry of MT growth was controlled by atypical cadherins Dachsous (Ds) and Fat (Ft). Furthermore, the action of Ft was mediated in part by PAR-1. All these data support the idea that the active reorientation of MT growth adjusts cell polarity along the organ axis.

Mical links semaphorins to F-actin disassembly

How instructive cues present on the cell surface have their precise effects on the actin cytoskeleton is poorly understood. Semaphorins are one of the largest families of these instructive cues and are widely studied for their effects on cell movement, navigation, angiogenesis, immunology and cancer. Semaphorins/collapsins were characterized in part on the basis of their ability to drastically alter actin cytoskeletal dynamics in neuronal processes, but despite considerable progress in the identification of semaphorin receptors and their signalling pathways, the molecules linking them to the precise control of cytoskeletal elements remain unknown. Recently, highly unusual proteins of the Mical family of enzymes have been found to associate with the cytoplasmic portion of plexins, which are large cell-surface semaphorin receptors, and to mediate axon guidance, synaptogenesis, dendritic pruning and other cell morphological changes. Mical enzymes perform reduction-oxidation (redox) enzymatic reactions and also contain domains found in proteins that regulate cell morphology. However, nothing is known of the role of Mical or its redox activity in mediating morphological changes. Here we report that Mical directly links semaphorins and their plexin receptors to the precise control of actin filament (F-actin) dynamics. We found that Mical is both necessary and sufficient for semaphorin-plexin-mediated F-actin reorganization in vivo. Likewise, we purified Mical protein and found that it directly binds F-actin and disassembles both individual and bundled actin filaments. We also found that Mical utilizes its redox activity to alter F-actin dynamics in vivo and in vitro, indicating a previously unknown role for specific redox signalling events in actin cytoskeletal regulation. Mical therefore is a novel F-actin-disassembly factor that provides a molecular conduit through which actin reorganization-a hallmark of cell morphological changes including axon navigation-can be precisely achieved spatiotemporally in response to semaphorins.

Rho1 has multiple functions in Drosophila wing planar polarity

The frizzled (fz) signaling/signal transduction pathway controls planar cell polarity in both vertebrates and invertebrates. Previous data implicated Rho1 as a component of the fz pathway in Drosophila but it was unclear how it functioned. The existence of a G Protein Binding-Formin Homology 3 (GBD-FH3) domain in Multiple Wing Hairs, a downstream component of the pathway suggested that Rho1 might function by binding to and activating Mwh. We re-examined the role of Rho1 in wing planar polarity and found that it had multiple functions. Aberrant Rho1 activity led to changes in the number of hairs formed, changes in cell shape and F-actin and changes in cellular junctions. Experiments that utilized Rho effector loop mutations argued that these phenotypes were mediated by effects of Rho1 on the cytoskeleton and not by effects on transcription. We found strong positive genetic interactions between Rho1 and mwh, that Rho1 regulated the accumulation of Mwh protein and that these two proteins could be co-immunoprecipitated. The Mwh GBD:FH3 domain was sufficient for co-immunoprecipitation with Rho1, consistent with this domain mediating the interaction. However, further experiments showed that Rho1 function in wing differentiation was not limited to interacting with Mwh. We established by genetic experiments that Rho1 could influence hair morphogenesis in the absence of mwh and that the disruption of Rho1 activity could interfere with the zig zag accumulation pattern of upstream fz pathway proteins. Thus, our results argue that in addition to its interaction with Mwh Rho1 has functions in wing planar polarity that are parallel to and upstream of fz. The upstream function may be an indirect one and associated with the requirement for normal apical basal polarity and adherens junctions for the accumulation of PCP protein complexes.

Planar polarity genes in the Drosophila wing regulate the localisation of the FH3-domain protein Multiple Wing Hairs to control the site of hair production

The core planar polarity proteins play important roles in coordinating cell polarity, in part by adopting asymmetric subcellular localisations that are likely to serve as cues for cell polarisation by as yet uncharacterised pathways. Here we describe the role of Multiple Wing Hairs (Mwh), a novel formin homology 3 (FH3)-domain protein, which acts downstream of the core polarity proteins to restrict the production of actin-rich prehairs to distal cell edges in the Drosophila pupal wing. Mwh appears to function as a repressor of actin filament formation and, in its absence, ectopic actin bundles are seen across the entire apical surface of cells. We show that the proximally localised core polarity protein Strabismus acts via the downstream effector proteins Inturned, Fuzzy and Fritz to stabilise Mwh in apico-proximal cellular regions. In addition, the distally localised core polarity protein Frizzled positively promotes prehair initiation, suggesting that both proximal and distal cellular cues act together to ensure accurate prehair placement.

Primary cilia in planar cell polarity regulation of the inner ear

Primary cilia are essential components of diverse cellular processes. Many of the requirements can be linked to the apparent signaling function of primary cilia. Recent studies have also uncovered a role for primary cilia in planar cell polarity (PCP) signaling. PCP refers to the coordinated orientation of cells along an axis parallel to the plane of the cell sheet. In vertebrates, the inner ear sensory organs display distinctive forms of PCP. One of the inner ear PCP characteristics is the coordinated positioning of a primary cilium eccentrically in every sensory hair cell within each organ. The inner ear, therefore, provides an opportunity to explore the cellular role of primary cilia in PCP signaling. In this chapter, we will introduce the PCP of the inner ear sensory organs, describe the conserved mechanism underlying the establishment of the planar polarity axis in invertebrates and vertebrates, and highlight a unique requirement for primary cilia in PCP regulation in vertebrates. Additionally, we will discuss a potentially ubiquitous role for cilia in cellular polarization in general.

The flare gene, which encodes the AIP1 protein of Drosophila, functions to regulate F-actin disassembly in pupal epidermal cells

Adult Drosophila are decorated with several types of polarized cuticular structures, such as hairs and bristles. The morphogenesis of these takes place in pupal cells and is mediated by the actin and microtubule cytoskeletons. Mutations in flare (flr) result in grossly abnormal epidermal hairs. We report here that flr encodes the Drosophila actin interacting protein 1 (AIP1). In other systems this protein has been found to promote cofilin-mediated F-actin disassembly. In Drosophila cofilin is encoded by twinstar (tsr). We show that flr mutations result in increased levels of F-actin accumulation and increased F-actin stability in vivo. Further, flr is essential for cell proliferation and viability and for the function of the frizzled planar cell polarity system. All of these phenotypes are similar to those seen for tsr mutations. This differs from the situation in yeast where cofilin is essential while aip1 mutations result in only subtle defects in the actin cytoskeleton. Surprisingly, we found that mutations in flr and tsr also result in greatly increased tubulin staining, suggesting a tight linkage between the actin and microtubule cytoskeleton in these cells.

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.

The balance between the novel protein target of wingless and the Drosophila Rho-associated kinase pathway regulates planar cell polarity in the Drosophila wing

Planar cell polarity (PCP) signaling is mediated by the serpentine receptor Frizzled (Fz) and transduced by Dishevelled (Dsh). Wingless (Wg) signaling utilizes Drosophila Frizzled 2 (DFz2) as a receptor and also requires Dsh for transducing signals to regulate cell proliferation and differentiation in many developmental contexts. Distinct pathways are activated downstream of Dsh in Wg- and Fz-signaling pathways. Recently, a number of genes, which have essential roles as downstream components of PCP signaling, have been identified in Drosophila. They include the small GTPase RhoA/Rho1, its downstream effector Drosophila rho-associated kinase (Drok), and a number of genes such as inturned (in) and fuzzy (fy), whose biochemical functions are unclear. RhoA and Drok provide a link from Fz/Dsh signaling to the modulation of actin cytoskeleton. Here we report the identification of the novel gene target of wingless (tow) by enhancer trap screening. tow expression is negatively regulated by Wg signaling in wing imaginal discs, and the balance between tow and the Drok pathway regulates wing-hair morphogenesis. A loss-of-function mutation in tow does not result in a distinct phenotype. Genetic interaction and gain-of-function studies provide evidence that Tow acts downstream of Fz/Dsh and plays a role in restricting the number of hairs that wing cells form.

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.

Planar cell polarity, ciliogenesis and neural tube defects

Cilia are microtubule-based protrusions that are found on the surface of most vertebrate cells. Long studied by cell biologists, these organelles have recently caught the attention of developmental biologists and human geneticists. In this review, I will discuss recent findings suggesting a link between cilia and the planar cell polarity signaling cascade. In particular, I will focus on how this interaction may influence the process of neural tube closure and how these results may be relevant to our understanding of common human birth defects in which neural tube closure is compromised.

Twinstar, the Drosophila homolog of cofilin/ADF, is required for planar cell polarity patterning

Planar cell polarity (PCP) is a level of tissue organization in which cells adopt a uniform orientation within the plane of an epithelium. The process of tissue polarization is likely to be initiated by an extracellular gradient. Thus, determining how cells decode and convert this graded information into subcellular asymmetries is key to determining how cells direct the reorganization of the cytoskeleton to produce uniformly oriented structures. Twinstar (Tsr), the Drosophila homolog of Cofilin/ADF (actin depolymerization factor), is a component of the cytoskeleton that regulates actin dynamics. We show here that various alleles of tsr produce PCP defects in the wing, eye and several other epithelia. In wings mutant for tsr, Frizzled (Fz) and Flamingo (Fmi) proteins do not properly localize to the proximodistal boundaries of cells. The correct asymmetric localization of these proteins instructs the actin cytoskeleton to produce one actin-rich wing hair at the distal-most vertex of each cell. These results argue that actin remodeling is not only required in the manufacture of wing hairs, but also in the PCP read-out that directs where a wing hair will be secreted.

Cytoskeletal dynamics and cell signaling during planar polarity establishment in theDrosophilaembryonic denticle

Many epithelial cells are polarized along the plane of the epithelium, a property termed planar cell polarity. The Drosophila wing and eye imaginal discs are the premier models of this process. Many proteins required for polarity establishment and its translation into cytoskeletal polarity were identified from studies of those tissues. More recently, several vertebrate tissues have been shown to exhibit planar cell polarity. Striking similarities and differences have been observed when different tissues exhibiting planar cell polarity are compared. Here we describe a new tissue exhibiting planar cell polarity – the denticles, hair-like projections of the Drosophila embryonic epidermis. We describe in real time the changes in the actin cytoskeleton that underlie denticle development, and compare this with the localization of microtubules, revealing new aspects of cytoskeletal dynamics that may have more general applicability. We present an initial characterization of the localization of several actin regulators during denticle development. We find that several core planar cell polarity proteins are asymmetrically localized during the process. Finally, we define roles for the canonical Wingless and Hedgehog pathways and for core planar cell polarity proteins in denticle polarity.

Polarized Transport of Frizzled along the Planar Microtubule Arrays in Drosophila Wing Epithelium

Cells in a variety of developmental contexts sense extracellular cues that are given locally on their surfaces, and subsequently amplify the initial signal to achieve cell polarization. Drosophila wing cells acquire planar polarity along the proximal-distal (P-D) axis, in which the amplification of the presumptive cue involves assembly of a multiprotein complex that spans distal and proximal boundaries of adjacent cells. Here we pursue the mechanisms that place one of the components, Frizzled (Fz), at the distal side. Intracellular particles of GFP-tagged Fz moved preferentially toward distal boundaries before Fz::GFP and other components were tightly localized at the P/D cortex. Arrays of microtubules (MTs) were approximately oriented along the P-D axis and these MTs contributed to the formation of the cortical complex. Furthermore, there appeared to be a bias in the P-D MTs, with slightly more plus ends oriented distally. The hypothesis of polarized vesicular trafficking of Fz is discussed.