Stages of cell hair construction in Drosophila

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.

Planar cell polarity: the Dachsous/Fat system contributes differently to the embryonic and larval stages of Drosophila

The epidermal patterns of all three larval instars (L1-L3) ofDrosophilaare made by one unchanging set of cells. The seven rows of cuticular denticles of all larval stages are consistently planar polarised, some pointing forwards, others backwards. In L1 all the predenticles originate at the back of the cells but, in L2 and L3, they form at the front or the back of the cell depending on the polarity of the forthcoming denticles. We find that, to polarise all rows, the Dachsous/Fat system is differentially utilised; in L1 it is active in the placement of the actin-based predenticles but is not crucial for the final orientation of the cuticular denticles, in L2 and L3 it is needed for placement and polarity. We find Four-jointed to be strongly expressed in the tendon cells and show how this might explain the orientation of all seven rows. Unexpectedly, we find that L3 that lack Dachsous differ from larvae lacking Fat and we present evidence that this is due to differently mislocalised Dachs. We make some progress in understanding how Dachs contributes to phenotypes of wildtype and mutant larvae and adults.

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.

The Frizzled Planar Cell Polarity signaling pathway controls Drosophila wing topography

The Drosophila wing is a primary model system for studying the genetic control of epithelial Planar Cell Polarity (PCP). Each wing epithelial cell produces a distally pointing hair under the control of the Frizzled (Fz) PCP signaling pathway. Here, we show that Fz PCP signaling also controls the formation and orientation of ridges on the adult wing membrane. Ridge formation requires hexagonal cell packing, consistent with published data showing that Fz PCP signaling promotes hexagonal packing in developing wing epithelia. In contrast to hair polarity, ridge orientation differs across the wing and is primarily anteroposterior (A-P) in the anterior and proximodistal (P-D) in the posterior. We present evidence that A-P ridge specification is genetically distinct from P-D ridge organization and occurs later in wing development. We propose a two-phase model for PCP specification in the wing. P-D ridges are specified in an Early PCP Phase and both A-P ridges and distally pointing hairs in a Late PCP Phase. Our data suggest that isoforms of the Fz PCP pathway protein Prickle are differentially required for the two PCP Phases, with the Spiny-legs isoform primarily active in the Early PCP Phase and the Prickle isoform in the Late PCP Phase.

Gene expression during Drosophila wing morphogenesis and differentiation

The simple cellular composition and array of distally pointing hairs has made the Drosophila wing a favored system for studying planar polarity and the coordination of cellular and tissue level morphogenesis. We carried out a gene expression screen to identify candidate genes that functioned in wing and wing hair morphogenesis. Pupal wing RNA was isolated from tissue prior to, during, and after hair growth and used to probe Affymetrix Drosophila gene chips. We identified 435 genes whose expression changed at least fivefold during this period and 1335 whose expression changed at least twofold. As a functional validation we chose 10 genes where genetic reagents existed but where there was little or no evidence for a wing phenotype. New phenotypes were found for 9 of these genes, providing functional validation for the collection of identified genes. Among the phenotypes seen were a delay in hair initiation, defects in hair maturation, defects in cuticle formation and pigmentation, and abnormal wing hair polarity. The collection of identified genes should be a valuable data set for future studies on hair and bristle morphogenesis, cuticle synthesis, and planar polarity.

Actin filament bundles in Drosophila wing hairs: hairs and bristles use different strategies for assembly

Actin filament bundles can shape cellular extensions into dramatically different forms. We examined cytoskeleton formation during wing hair morphogenesis using both confocal and electron microscopy. Hairs elongate with linear kinetics (approximately 1 microm/h) over the course of approximately 18 h. The resulting structure is vividly asymmetric and shaped like a rose thorn--elongated in the distal direction, curved in two dimensions with an oval base and a round tip. High-resolution analysis shows that the cytoskeleton forms from microvilli-like pimples that project actin filaments into the cytoplasm. These filaments become cross-linked into bundles by the sequential use of three cross-bridges: villin, forked and fascin. Genetic loss of each cross-bridge affects cell shape. Filament bundles associate together, with no lateral membrane attachments, into a cone of overlapping bundles that matures into an oval base by the asymmetric addition of bundles on the distal side. In contrast, the long bristle cell extension is supported by equally long (up to 400 microm) filament bundles assembled together by end-to-end grafting of shorter modules. Thus, bristle and hair cells use microvilli and cross-bridges to generate the common raw material of actin filament bundles but employ different strategies to assemble these into vastly different shapes.

Bristles induce bracts via the EGFR pathway on Drosophila legs

A long-standing mystery in Drosophila has been: how do certain bristles induce adjacent cells to make bracts (a type of thick hair) on their proximal side? The apparent answer, based on loss- and gain-of-function studies, is that they emit a signal that neighbors then transduce via the epidermal growth factor receptor pathway. Suppressing this pathway removes bracts, while hyperactivating it evokes bracts indiscriminately on distal leg segments. Misexpression of the diffusible ligand Spitz (but not its membrane-bound precursor) elicits extra bracts at normal sites. What remains unclear is how a secreted signal can have effects in one specific direction.

Control of actin reorganization by Slingshot, a family of phosphatases that dephosphorylate ADF/cofilin

The ADF (actin-depolymerizing factor)/cofilin family is a stimulus-responsive mediator of actin dynamics. In contrast to the mechanisms of inactivation of ADF/cofilin by kinases such as LIM-kinase 1 (LIMK1), much less is known about its reactivation through dephosphorylation. Here we report Slingshot (SSH), a family of phosphatases that have the property of F actin binding. In Drosophila, loss of ssh function dramatically increased levels of both F actin and phospho-cofilin (P cofilin) and disorganized epidermal cell morphogenesis. In mammalian cells, human SSH homologs (hSSHs) suppressed LIMK1-induced actin reorganization. Furthermore, SSH and the hSSHs dephosphorylated P cofilin in cultured cells and in cell-free assays. Our results strongly suggest that the SSH family plays a pivotal role in actin dynamics by reactivating ADF/cofilin in vivo.

The Drosophila forked protein induces the formation of actin fiber bundles in vertebrate cells

The forked protein is an actin binding protein involved in the formation of large actin fiber bundles in developing Drosophila bristles. These are the largest example of a type of actin bundle characterized by parallel, hexagonally packed actin fibers, also found in intestinal microvilli, kidney proximal tubule microvilli, and stereocilia in the ear. Understanding how these structures are constructed and how that construction is regulated is an important question in cell and developmental biology. Because the timing of forked gene expression coincides with the formation of the actin fiber bundles, and since the forked protein is localized at the site of initiation of these bundles before they form, it has been proposed that the forked protein is an initiator of actin bundle formation. In this paper we show that the forked protein can induce the formation of bundles and increase actin polymerization in vertebrate cells. We use this system to identify regions of the forked protein which are essential for bundle formation and actin co-localization.

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

We have found that the actin and microtubule cytoskeletons have overlapping, but distinct roles in the morphogenesis of epidermal hairs during Drosophila wing development. The function of both the actin and microtubule cytoskeletons appears to be required for the growth of wing hairs, as treatment of cultured pupal wings with either cytochalasin D or vinblastine was able to slow prehair extension. At higher doses a complete blockage of hair development was seen. The microtubule cytoskeleton is also required for localizing prehair initiation to the distalmost part of the cell. Disruption of the microtubule cytoskeleton resulted in the development of multiple prehairs along the apical cell periphery. The multiple hair cells were a phenocopy of mutations in the inturned group of tissue polarity genes, which are downstream targets of the frizzled signaling/signal transduction pathway. The actin cytoskeleton also plays a role in maintaining prehair integrity during prehair development as treatment of pupal wings with cytochalasin D, which inhibits actin polymerization, led to branched prehairs. This is a phenocopy of mutations in crinkled, and suggests mutations that cause branched hairs will be in genes that encode products that interact with the actin cytoskeleton.

Developmental and genetic mosaic analysis of Drosophila m-dy mutants: tissue foci for behavioral and morphogenetic defects

Mutants of the Drosophila miniature-dusky (m-dy) gene complex display morphogenetic phenotypes (miniature or dusky) caused by a change in the size and/or shape of the epidermal cells comprising the adult wing. In addition to a dusky phenotype, certain Andante-type mutants also exhibit lengthened circadian periods for two different behavioral rhythms. If the latter phenotype results from a direct effect on the circadian pacemaker, the Andante function should be required within the brain. In order to define the tissues that require the morphogenetic and behavioral functions, we have carried out a genetic mosaic analysis. This study demonstrates that normal wing morphogenesis is entirely dependent on the genotype of wing cells. Furthermore, temperature-shift experiments with a temperature-sensitive dy mutant indicate that the morphogenetic function is required during adult development, and after the cessation of wing epidermal cell proliferation. At this time in development, a columnar epithelium in the developing wing becomes flattened into the mature wing blade, and we postulate that the cell-size defect of m-dy mutants results from an alteration of this morphogenetic process. In contrast to the wing morphogenesis phenotype, the characterization of locomotor activity in mosaic adults revealed a strong correlation between the head genotype and the Andante circadian-period phenotype. This result indicates that neural tissues mediate the rhythm function. Thus, the behavioral and morphogenetic functions require gene expression in distinct tissues.(ABSTRACT TRUNCATED AT 250 WORDS)

Altered epidermal growth factor-like sequences provide evidence for a role of Notch as a receptor in cell fate decisions

In Drosophila each neural precursor is chosen from a group of cells through cell interactions mediated by Notch and Delta which may function as receptor and ligand (signal), respectively, in a lateral signalling pathway. The cells of a group are equipotential and express both Notch and Delta. Hyperactive mutant Notch molecules, (Abruptex), probably have an enhanced affinity for the ligand. When adjacent to wild-type cells, cells bearing the Abruptex proteins are unable to produce the signal. It is suggested that in addition to the binding of Notch molecules on one cell to the Delta molecules of opposing cells, the Notch and Delta proteins on the surface of the same cell may interact. Binding between a cell's own Notch and Delta molecules would alter the availability of these proteins to interact with their counterparts on adjacent cells.

The distribution of PS integrins, laminin A and F-actin during key stages in Drosophila wing development

We first summarize wing development during metamorphosis of Drosophila and identify four critical steps in the conversion of a folded single layered wing disc to a flat bilayered wing. Each step occurs twice, once during the 12 hour prepupal period and again during the 84 hour pupal period. (1) Apposition in which basal surfaces of dorsal and ventral epithelia come close together. (2) Adhesion in which basal junctions form between the apposed basal surfaces. (3) Expansion in which wing area increases as a result of cells flattening. (4) Separation in which dorsal and ventral epithelia are separated by a bulky extracellular matrix but remain connected by slender cytoplasmic processes containing the microtubules and microfilaments of the transalar cytoskeleton. Disc ultrastructure is correlated with the distribution of the beta chain of integrin, laminin A, and filamentous actin for each key stage of pupal development. Integrin and laminin exhibit a mutually exclusive distribution from the adhesion stage onwards. Integrin is present on the basal surface of intervein cells but not on vein cells whereas laminin A is absent from the basal surfaces of intervein cells but is present on vein cells. We conclude that laminin is not a ligand for integrin in this context. During apposition and adhesion stages integrin is broadly distributed over the basal and lateral surfaces of intervein cells but subsequently becomes localized to small basal foci. These foci correspond to basal contact zones between transalar processes. The distribution of filamentous actin is dynamic, changing from an apical distribution during hair morphogenesis to a basal distribution as the transalar cytoskeleton develops. Basal adherens-type junctions are first evident during the adhesion stage and become closely associated with the transalar cytoskeleton during the separation stage. Thus, basal junction formation occurs in two discrete steps; intercellular connections are established first and junction/cytoskeletal connections are formed about 20 hours later. These observations provide a basis for future investigations of integrin mediated adhesion in vivo.

The choice of cell fate in the epidermis of Drosophila

In Drosophila, neural precursors are formed in a spaced pattern separated by intervening epidermal cells. Segregation of neural and epidermal lineages relies on cellular interactions. Failure of this cell communication, as in the mutants Notch (N), Delta, and shaggy, results in most or all of the cells becoming neural. Cells mutant for N and shaggy, but not Delta, autonomously adopt the neural fate when adjacent to wild-type cells in mosaics. Furthermore, wild-type cells adopt the epidermal fate if adjacent cells express a lower level of N activity than themselves, but produce neural precursors if adjacent cells express a higher level of N activity. This shows that there is competition between the cells and that the N protein is required for the mechanism whereby the cells choose between alternative fates. It also suggests that N acts as a receptor for an inhibitory signal emanating from the neural precursors.