Cellular interactions in the developing Drosophila eye

Proteome-wide quantitative analysis of redox cysteine availability in the Drosophila melanogaster eye reveals oxidation of phototransduction machinery during blue light exposure and age

The retina is one of the highest oxygen-consuming tissues because visual transduction and light signaling processes require large amounts of ATP. Thus, because of the high energy demand, oxygen-rich environment, and tissue transparency, the eye is susceptible to excess production of reactive oxygen species (ROS) resulting in oxidative stress. Oxidative stress in the eye is associated with the development and progression of ocular diseases including cataracts, glaucoma, age-related macular degeneration, and diabetic retinopathy. ROS can modify and damage cellular proteins, but can also be involved in redox signaling. In particular, the thiol groups of cysteines can undergo reversible or irreversible oxidative post-translational modifications (PTMs). Identifying the redox-sensitive cysteines on a proteome-wide scale provides insight into those proteins that act as redox sensors or become irreversibly damaged upon exposure to oxidative stress. In this study, we profiled the redox proteome of the Drosophila eye under prolonged, high intensity blue light exposure and age using iodoacetamide isobaric label sixplex reagents (iodo-TMT) to identify changes in cysteine availability. Although redox metabolite analysis of the major antioxidant, glutathione, revealed similar ratios of its oxidized and reduced form in aged or light-stressed eyes, we observed different changes in the redox proteome under these conditions. Both conditions resulted in significant oxidation of proteins involved in phototransduction and photoreceptor maintenance but affected distinct targets and cysteine residues. Moreover, redox changes induced by blue light exposure were accompanied by a large reduction in light sensitivity that did not arise from a reduction in the photopigment level, suggesting that the redox-sensitive cysteines we identified in the phototransduction machinery might contribute to light adaptation. Our data provide a comprehensive description of the redox proteome of Drosophila eye tissue under light stress and aging and suggest how redox signaling might contribute to light adaptation in response to acute light stress.

Molecular Chaperone Hsp70 and Its Constitutively Active Form Hsc70 Play an Indispensable Role During Eye Development of Drosophila melanogaster

In the present study, we demonstrate that molecular chaperone Hsp70 and Hsc70 is essential for normal organization and development of ommatidial cells in Drosophila melanogaster eye. An exogenously expressed dominant negative mutant of Hsp70 (K71E) and Hsc70.4 (K71S and D206S) in an eye-specific manner resulted in eye degeneration that includes loss of eye pigment, disorganized ommatidia, abnormality in bristle cell arrangement and reduction in the eye size. The developmental organization of ommatidial cells (cone, photoreceptor, pigment, and bristle cell complex) was disturbed in Hsp70 and Hsc70 mutants. Acridine orange (AO) and caspase 3 staining showed an increased cell death in Hsp70 and Hsc70 mutant eyes. Genetic interaction study of Hsp70 and Hsc70 mutants with candidate genes of JNK signaling pathway and immunocytochemistry study using phospho-JNK antibody suggested that mutation in Hsp70 and Hsc70 results in ectopic activation of JNK signaling in fly eye. Further, anti-PH3 staining in Hsp70 and Hsc70 mutant eyes revealed a reduced number of mitotic cells in second mitotic wave (SMW) of developing eye and anti-Rh1 staining showed reduced Rh1 expression, accumulation of Rh1 in the cytoplasm, and rhabdomere degeneration. Thus, on the basis of results, it was concluded that molecular chaperone Hsp70 and Hsc70 play an indispensable role during Drosophila eye development.

Evolutionarily conserved transcription factor Apontic controls the G1/S progression by inducing cyclin E during eye development

During Drosophila eye development, differentiation initiates in the posterior region of the eye disk and progresses anteriorly as a wave marked by the morphogenetic furrow (MF), which demarcates the boundary between anterior undifferentiated cells and posterior differentiated photoreceptors. However, the mechanism underlying the regulation of gene expression immediately before the onset of differentiation remains unclear. Here, we show that Apontic (Apt), which is an evolutionarily conserved transcription factor, is expressed in the differentiating cells posterior to the MF. Moreover, it directly induces the expression of cyclin E and is also required for the G1-to-S phase transition, which is known to be essential for the initiation of cell differentiation at the MF. These observations identify a pathway crucial for eye development, governed by a mechanism in which Cyclin E promotes the G1-to-S phase transition when regulated by Apt.

Stochastic de-repression of Rhodopsins in single photoreceptors of the fly retina

The photoreceptors of the Drosophila compound eye are a classical model for studying cell fate specification. Photoreceptors (PRs) are organized in bundles of eight cells with two major types – inner PRs involved in color vision and outer PRs involved in motion detection. In wild type flies, most PRs express a single type of Rhodopsin (Rh): inner PRs express either Rh3, Rh4, Rh5 or Rh6 and outer PRs express Rh1. In outer PRs, the K₅₀ homeodomain protein Dve is a key repressor that acts to ensure exclusive Rh expression. Loss of Dve results in de-repression of Rhodopsins in outer PRs, and leads to a wide distribution of expression levels. To quantify these effects, we introduce an automated image analysis method to measure Rhodopsin levels at the single cell level in 3D confocal stacks. Our sensitive methodology reveals cell-specific differences in Rhodopsin distributions among the outer PRs, observed over a developmental time course. We show that Rhodopsin distributions are consistent with a two-state model of gene expression, in which cells can be in either high or basal states of Rhodopsin production. Our model identifies a significant role of post-transcriptional regulation in establishing the two distinct states. The timescale for interconversion between basal and high states is shown to be on the order of days. Our results indicate that even in the absence of Dve, the Rhodopsin regulatory network can maintain highly stable states. We propose that the role of Dve in outer PRs is to buffer against rare fluctuations in this network.

Complex networks of genetic interactions govern the development of multicellular organisms. One of the best-characterized networks governs the development of the fruit-fly retina, a highly organized, three-dimensional organ composed of a hexagonal grid of eight types of photoreceptor neurons. Each photoreceptor responds to a particular wavelength of light depending on the Rhodopsin protein it expresses. We present novel computational methods to quantify cell-specific Rhodopsin levels from confocal microscopy images. We apply these methods to study the effect of the loss of a key repressor that ensures each photoreceptor expresses only one Rhodopsin. We show that this perturbation has cell-specific effects. Our measurement of the cell-type specific Rhodopsin distributions reveals differences between photoreceptor cells, which could not otherwise be detected. Using mathematical models of gene expression, we attribute this variability to stochastic events that activate Rhodopsin production.

Switch and template pattern formation in a discrete reaction-diffusion system inspired by the Drosophila eye

We examine a spatially discrete reaction-diffusion model based on the interactions that create a periodic pattern in the Drosophila eye imaginal disc. This model is known to be capable of generating a regular hexagonal pattern of gene expression behind a moving front, as observed in the fly system. In order to better understand the novel "switch and template" mechanism behind this pattern formation, we present here a detailed study of the model's behavior in one dimension, using a combination of analytic methods and numerical searches of parameter space. We find that patterns are created robustly, provided that there is an appropriate separation of timescales and that self-activation is sufficiently strong, and we derive expressions in this limit for the front speed and the pattern wavelength. Moving fronts in pattern-forming systems near an initial linear instability generically select a unique pattern, but our model operates in a strongly nonlinear regime where the final pattern depends on the initial conditions as well as on parameter values. Our work highlights the important role that cellularization and cell-autonomous feedback can play in biological pattern formation.

Building a fly eye: terminal differentiation events of the retina, corneal lens, and pigmented epithelia

In the past, vast differences in ocular structure, development, and physiology throughout the animal kingdom led to the widely accepted notion that eyes are polyphyletic, that is, they have independently arisen multiple times during evolution. Despite the dissimilarity between vertebrate and invertebrate eyes, it is becoming increasingly evident that the development of the eye in both groups shares more similarity at the genetic level than was previously assumed, forcing a reexamination of eye evolution. Understanding the molecular underpinnings of cell type specification during Drosophila eye development has been a focus of research for many labs over the past 25 years, and many of these findings are nicely reviewed in Chapters 1 and 4. A somewhat less explored area of research, however, considers how these cells, once specified, develop into functional ocular structures. This review aims to summarize the current knowledge related to the terminal differentiation events of the retina, corneal lens, and pigmented epithelia in the fly eye. In addition, we discuss emerging evidence that the different functional components of the fly eye share developmental pathways and functions with the vertebrate eye.

Drosophila melanogaster as a model organism of brain diseases

Drosophila melanogaster has been utilized to model human brain diseases. In most of these invertebrate transgenic models, some aspects of human disease are reproduced. Although investigation of rodent models has been of significant impact, invertebrate models offer a wide variety of experimental tools that can potentially address some of the outstanding questions underlying neurological disease. This review considers what has been gleaned from invertebrate models of neurodegenerative diseases, including Alzheimer’s disease, Parkinson’s disease, metabolic diseases such as Leigh disease, Niemann-Pick disease and ceroid lipofuscinoses, tumor syndromes such as neurofibromatosis and tuberous sclerosis, epilepsy as well as CNS injury. It is to be expected that genetic tools in Drosophila will reveal new pathways and interactions, which hopefully will result in molecular based therapy approaches.

Septate junctions are required for ommatidial integrity and blood-eye barrier function in Drosophila

The anatomical organization of the Drosophila ommatidia is achieved by specification and contextual placement of photoreceptors, cone and pigment cells. The photoreceptors must be sealed from high ionic concentrations of the hemolymph by a barrier to allow phototransduction. In vertebrates, a blood-retinal barrier (BRB) is established by tight junctions (TJs) present in the retinal pigment epithelium and endothelial membrane of the retinal vessels. In Drosophila ommatidia, the junctional organization and barrier formation is poorly understood. Here we report that septate junctions (SJs), the vertebrate analogs of TJs, are present in the adult ommatidia and are formed between and among the cone and pigment cells. We show that the localization of Neurexin IV (Nrx IV), a SJ-specific protein, coincides with the location of SJs in the cone and pigment cells. Somatic mosaic analysis of nrx IV null mutants shows that loss of Nrx IV leads to defects in ommatidial morphology and integrity. nrx IV hypomorphic allelic combinations generated viable adults with defective SJs and displayed a compromised blood-eye barrier (BEB) function. These findings establish that SJs are essential for ommatidial integrity and in creating a BEB around the ion and light sensitive photoreceptors. Our studies may provide clues towards understanding the vertebrate BEB formation and function.

Hair cell regeneration: the identities of progenitor cells, potential triggers and instructive cues

Hair cells are produced and accumulate in the ears of fish and amphibians as they grow during postembryonic life; hair cell regeneration occurs in lateral line organs in those groups and in the cochlea in birds. Continuous time-lapse microscopy has directly demonstrated that supporting cells divide to give rise to hair cells during regeneration in lateral line neuromasts. Supporting cells also appear to give rise to hair cells during regeneration in the avian ear, but additional cell types have been proposed as hair cell progenitors. Alternative interpretations of current evidence are discussed in relation to the possibility that supporting cells may be the common progenitor in all cases of hair cell regeneration. The regenerative proliferation of hair cells in birds occurs in populations of cells that are mitotically quiescent in undamaged ears. Evidence suggests that the extrusion of damaged hair cells and the breaking of intercellular junctional adhesions may be a trigger for regenerative proliferation. The potential triggering influence of phagocytes is also discussed. The differentiation of replacement cells during regeneration in the cochlea may be regulated by surface interactions between cells. A model that could account for the reconstitution of the mosaic pattern of hair cells and supporting cells is proposed.

Identification of ras targets using a genetic approach

The Sevenless receptor tyrosine kinase is required for the development of the R7 photoreceptor cell in the Drosophila eye. Several components of the Sevenless signal transduction pathway have been identified in genetic screens for enhancers/suppressors of the sevenless phenotype. These studies suggest that activation of Sevenless leads to stimulation of Ras1 activity, whereas Gap1 appears to act as a negative regulator of the pathway. Inactivation of the Gap1 locus causes transformation of non-neuronal cone cells into supernumerary R7 cells. This same mutant phenotype is observed when activated Ras1 is expressed under the control of the sevenless promoter. While studies in other organisms have demonstrated a role for ras gene products in signal transduction, the effectors of Ras activity have not yet been identified. We are carrying out genetic screens for enhancers and suppressors of the Gap1 and activated Ras1 phenotypes in the hope of identifying genes encoding some of these effectors. We are conducting chemical mutagenesis screens and have also screened existing collections of P element lines. A molecular characterization of the most promising mutations is in progress.

Pattern formation in the Drosophila eye

The insect compound eye is one of the most precise and highly ordered patterns in the living world. It develops from an unpatterned simple epithelium by a series of cell fate decisions and complex morphogenetic movements. In the first days of metamorphosis, this interplay is particularly noticeable. Recent insights have revealed how interactions between neighboring cells drive the process. Interaction between Delta on cone cells and Notch proteins on the surface of their neighbors induces the first pigment cells to differentiate. The primary pigment cells then express a Nephrin protein, Hibris, that interacts with a different Nephrin, Roughest, on their neighbors. Heterophilic adhesion between Hibris and Roughest results in remodeling contacts between cells to favor their contact with the pigment cells. In conjunction, the primary pigment cells signal to their neighbors through the EGF receptor to survive, rather than undergo apoptosis. This sorting and culling process results in a sculpted pattern with a precise number and position of cells that is repeated hundreds of times in each compound eye.

The Drosophila ramshackle gene encodes a chromatin-associated protein required for cell morphology in the developing eye

We have identified ramshackle (ram) as a dominant suppressor of hedgehog loss-of-function in the developing Drosophila eye. We have characterized the gene and it encodes a double bromodomain protein with eight WD40 repeats. The Ram protein is localized predominantly to polytene chromosome interbands and is required for the transcription of some genes. ram is an essential gene and null mutants die during larval life. In the developing retina, ram mutant cells have morphological defects including disrupted apical junctions, disorganized actin cytoskeletons and mislocalized nuclei, which are followed by delays in cell-cycle transitions and the expression of differentiation markers. ram is a conserved gene: its vertebrate homolog (WDR9), which lies in Down's Syndrome Critical region 2 (DCR2) is also known to be associated with Brahma-Related-Gene 1 (BRG1).

Microscopy of the Drosophila facet eye: vademecum for standardized fixation, embedding, and sectioning

We describe here a standardized method for histological processing of the Drosophila compound eye. Primary fixation with 2.5% glutaraldehyde, obligatorily supplemented with 0.1% household detergent regularly yielded the best structural preservation, as compared with that of other, more complicated fixation protocols tested. Notably, it proved indispensable not only to cut off the fly's head to facilitate the penetration of the reagents but also to open the chitinous head capsule. For this, we locally pierced the cuticle between the eyes, leaving the head structurally almost intact, a prerequisite for precisely aligning the head for microtomy. We developed a two-step re-embedding procedure allowing for exact and reproducible orientation of the fly heads. Thus, highly comparable series of cross sections through a representative number of ommatidia were obtained. The feasibility of our embedding and sectioning approach is finally demonstrated by three-dimensional reconstructions of the middle segments of the R1, R7, and R8 photoreceptor cells. We present reconstructions from structurally modified ommatidia, as seen after RNAi-mediated depletion of the endosomal adaptor protein p14, and from normal ommatidia corresponding to the wildtype.

Characterization of Drosophila mini-me, a gene required for cell proliferation and survival

In the developing Drosophila eye, the morphogenetic furrow is a developmental organizing center for patterning and cell proliferation. The furrow acts both to limit eye size and to coordinate the number of cells to the number of facets. Here we report the molecular and functional characterization of Drosophila mini-me (mnm), a potential regulator of cell proliferation and survival in the developing eye. We first identified mnm as a dominant modifier of hedgehog loss-of-function in the developing eye. We report that mnm encodes a conserved protein with zinc knuckle and RING finger domains. We show that mnm is dispensable for patterning of the eye disc, but required in the eye for normal cell proliferation and survival. We also show that mnm null mutant cells exhibit altered cell cycle profiles and contain excess nucleic acid. Moreover, mnm overexpression can induce cells to proliferate and incorporate BrdU. Thus, our data implicate mnm as a regulator of mitotic progression during the proliferative phase of eye development, possibly through the control of nucleic acid metabolism.

Smoothened, thickveins and the genetic control of cell cycle and cell fate in the developing Drosophila eye

The Hedgehog and Decapentaplegic pathways have several well-characterized functions in the developing Drosophila compound eye, including initiation and progression of the morphogenetic furrow. Other functions involve control of cell cycle and cell survival as well as cell type specification. Here we have used the mosaic clone analysis of null mutations of the smoothened and thickveins genes (which encode the receptors for these two signals) both alone and in combination, to study cell cycle and cell fate in the developing eye. We conclude that both pathways have several, but differing roles in furrow induction and cell fate and survival, but that neither directly affects cell type specification.

Slingshot cofilin phosphatase localization is regulated by receptor tyrosine kinases and regulates cytoskeletal structure in the developing Drosophila eye

Animal development requires that positional information act on the genome to control cell fate and cell shape. The primary determinant of animal cell shape is the cytoskeleton and thus the mechanisms by which extracellular signals influence the cytoskeleton are crucial for morphogenesis. In the developing Drosophila compound eye, localized polymerization of actin functions to constrict the apical surface of epithelial cells, both at the morphogenetic furrow and later to maintain the coherence of the nascent ommatidia. As elsewhere, actin polymerization in the developing eye is regulated by ADF/cofilin ('Twinstar', or 'Tsr' in Drosophila), which is activated by Slingshot (Ssh), a cofilin phosphatase. Here we show that Ssh does act in the developing eye to limit actin polymerization in the assembling ommatidia, but not in the morphogenetic furrow. While Ssh does control cell shape, surprisingly there are no direct or immediate consequences for cell type. Ssh protein becomes apically concentrated in cells that express elevated levels of the Sevenless (Sev) receptor-tyrosine kinase (RTK), even those which receive no ligand. We interpret this as a non-signal driven, RTK-dependent localization of Ssh to allow for locally increased actin filament turnover. We suggest that there are two modes of actin remodeling in the developing eye: a non-RTK, non-Ssh mediated mechanism in the morphogenetic furrow, and an RTK and Ssh-dependent mode during ommatidial assembly.

Drosophila eye disc margin is a center for organizing long-range planar polarity

Planar polarity patterning involves long-range signaling and signal transduction. In Drosophila eye, Dishevelled (Dsh) is not only crucial for cell-autonomous transduction of a polarity signal(s) but is also involved in nonautonomous signaling function. To identify the sites for long-range polarity signaling in eye disc, we examined spatial and temporal conditions for nonautonomous Dsh function. Here we show that Dsh and its downstream factor Armadillo (Arm) are required in the border region of eye disc between the peripodial membrane (PM) and the disc proper (DP) for nonautonomous signaling. Conditional misexpression of Dsh or Arm at the posterior margin of the disc was sufficient to induce nonautonomous polarity reversals. A critical time window for the induction of such changes was approximately coincident with the timing of morphogenetic furrow initiation. Our data suggest that the disc margin is an essential site for organizing planar polarity during the initial stage of retinal morphogenesis.

Genetic regions that interact with loss- and gain-of-function phenotypes of deltex implicate novel genes in Drosophila Notch signaling

The Notch signaling pathway is an evolutionarily conserved mechanism that regulates many cell fate decisions. The deltex (dx) gene encodes an E3-ubiquitin ligase that binds to the intracellular domain of the Notch protein and regulates Notch signaling in a positive manner. However, it is still not clear how Dx does this. We generated a transgenic line, GMR-dx, which overexpresses dx in the developing Drosophila eye disc. The GMR-dx line showed a rough-eye phenotype, specific transformation of a photoreceptor cell (R3 to R4), and a rotation defect in the ommatidia. This phenotype was suppressed in combination with a dx loss-of-function mutant, indicating that it was due to a dx gain-of-function. We previously reported that overexpression of Dx results in the stabilization of Notch in late endosomes. Here, we found that three motifs in Dx, a region that binds to Notch, a proline-rich motif and a RING-H2 finger, were required for this stabilization, although the relative activity of these variants in this assay did not always correspond to the severity of the rough-eye phenotype. In an attempt to identify novel genes of the Notch pathway, we tested a large collection of chromosomal deficiencies for the ability to modify the eye phenotypes of the GMR-dx line. Twelve genomic segments that enhanced the rough-eye phenotype of GMR-dx were identified. To evaluate the specificity of these interactions, we then determined whether the deletions also interacted with the wing phenotypes associated with a loss-of-function mutation of dx, dx24. Analyses based on whole-genome information allowed us to conclude that we have identified two novel loci that probably include uncharacterized genes involved in Dx-mediated Notch signaling.

Molecular characterization, functional expression and tissue distribution of a second NCKX Na+/Ca2+-K+ exchanger from Drosophila

The Na+/Ca2+ -K+ exchanger (NCKX) utilizes the inward Na+ gradient and the outward K+ gradient to promote Ca2+ extrusion from cells. Here, we have characterized a second NCKX from Drosophila. Based on its chromosomal location (X chromosome) we have named it Ncxk-x. Three splice variants were isolated with three distinct N-terminal sequences. NCKX-X differs from NCKX proteins described so far in other species by lacking an N-terminal signal peptide. Heterologous expression of the respective cDNA's resulted in NCKX-X protein expression and K+ -dependent Na+/Ca2+ exchange activity for two of the three splice variants. Transcript localization of Nckx-x was investigated and compared with that previously described by us for Nckx30C.

Spatial pattern of nonmuscle myosin-II distribution during the development of the Drosophila compound eye and implications for retinal morphogenesis

Nonmuscle myosin-II is a motor protein that drives cell movement and changes in cell shape during tissue and organ development. This study has determined the dynamic changes in myosin-II distribution during Drosophila compound eye morphogenesis. In photoreceptor neurons, myosin-II is undetectable at the apical domain throughout the first half of pupal life, at which time this membrane domain is involuted into the epithelium and progresses toward the retinal floor. Myosin-II is deployed at the apical surface at about 60% of pupal development, once the developing rhabdomeres reach the retinal floor. Subsequently, myosin-II becomes restricted to two stripes at the sides of the developing rhabdomere, adopting its final position within the visual cells R1-6; here, myosin-II is associated with a set of actin filaments that extend alongside the rhabdomeres. At the midpupal stage, myosin-II is also incorporated into stress-fiber-like arrays within the basal endfeet of the pigment cells that then change their shape. This spatiotemporal pattern of myosin-II localization and the morphological defects observed in the eyes of a myosin-II mutant suggest that the myosin-II/F-actin system is involved in the alignment of the rhabdomeres within the retina and in the flattening of the retinal floor. The observation that the myosin-II/F-actin arrays are incomplete or disorganized in R7/R8 and in rhodopsin-1-null R1-6 suggests further that the establishment and stability of this cytoskeletal system depend on rhodopsin-1 expression.

Cell-cycle regulation and cell-type specification in the developing Drosophila compound eye

During nervous system development stem cell daughters must exit the proliferative cycle to adopt specific neural and glial fates and they must do so in the correct positions. Cell proliferation in the central nervous system occurs in neuroepithelia such as the neural retina and the ventricular zones. As cells are assigned specific fates they migrate out of the plane of the epithelium to form higher layers. Recent evidence from the Drosophila compound eye suggests that a novel mode of Ras pathway regulation may be crucial in both cell-cycle exit and neural patterning: "MAP Kinase cytoplasmic hold".

Hedgehog Signaling in the Drosophila Eye and Head: An Analysis of the Effects of Differentpatched Trans-heterozygotes

Characterization of different alleles of the Hedgehog receptor patched (ptc) indicates that they can be grouped into several classes. Most mutations result in complete loss of Ptc function. However, missense mutations located within the putative sterol-sensing domain (SSD) or C terminus of ptc encode antimorphic proteins that are unable to repress Smo activity and inhibit wild-type Ptc from doing so, but retain the ability to bind and sequester Hh. Analysis of the eye and head phenotypes of Drosophila melanogaster in various ptc/ptctuf1 heteroallelic combinations shows that these two classes of ptc allele can be easily distinguished by their eye phenotype, but not by their head phenotype. Adult eye size is inversely correlated with head vertex size, suggesting an alteration of cell fate within the eye-antennal disc. A balance between excess cell division and cell death in the mutant eye discs may also contribute to final eye size. In addition, contrary to results reported recently, the role of Hh signaling in the Drosophila head vertex appears to be primarily in patterning rather than in proliferation, with Ptc and Smo having opposing effects on formation of medial structures.

Determination of human myosin III as a motor protein having a protein kinase activity

The class III myosin is the most divergent member of the myosin superfamily, having a domain with homology to a protein kinase. However, the function of class III myosin at a molecular level is not known at all, and it has been questioned whether it is actually an actin-based motor molecule. Here, we showed that human myosin III has an ATPase activity that is significantly activated by actin (20-fold) with Kactin of 112 microm and Vmax of 0.34 s-1, indicating the mechanoenzymatic activity of myosin III. Furthermore, we found that human myosin III has actin translocating activity (0.11 +/- 0.05 microm/s) using an in vitro actin gliding assay, and it moves toward the plus end of actin filaments. Myosin III containing calmodulin as the light chain subunit showed a protein kinase activity and underwent autophosphorylation. The autophosphorylation was the intramolecular process, and the sites were at the C-terminal end of the motor domain. Autophosphorylation significantly activated the kinase activity, although it did not affect the ATPase activity. The present study is the first report that clearly demonstrates that the class III myosin is an actin-based motor protein having a protein kinase activity.

Requirement of the roughest gene for differentiation and time of death of interommatidial cells during pupal stages of Drosophila compound eye development

The roughest locus of Drosophila melanogaster encodes a transmembrane protein of the immunoglobulin superfamily required for several developmental processes, including axonal pathfinding in the developing optic lobe, mechanosensory bristle differentiation and myogenesis. In the compound eye, rst was previously shown to be required for establishing the correct number and spacing of secondary and tertiary pigment cells during the final steps of ommatidial assembly. We have further investigated its function in the developing pupal retina by performing a developmental and molecular analysis of a novel dominant rst allele, rst(D). In addition to showing evidence that rst(D) is a regulatory mutant, the results strongly suggest a previously unnoticed role of the rst gene in the differentiation of secondary/tertiary pigment cell fate as well as establishing the correct timing of surplus cell removal by programmed cell death in the compound eye.

Potassium-dependent sodium-calcium exchange through the eye of the fly

In this review, we describe the characterization of a Drosophila sodium/calcium-potassium exchanger, Nckx30C. Sodium/calcium (-potassium) exchangers (NCX and NCKX) are required for the rapid removal of calcium in excitable cells. The deduced protein topology for NCKX30C is similar to that of mammalian NCKX, with 5 hydrophobic domains in the amino terminus separated from 6 at the carboxy-terminal end by a large intracellular loop. NCKX30C functions as a potassium-dependent sodium-calcium exchanger and is expressed in adult neurons and during ventral nerve cord development in the embryo. Nckx30C is expressed in a dorsal/ventral pattern in the eye-antennal disc, suggesting that large fluxes of calcium may be occurring during imaginal disc development in the larvae. NCKX30C may play a critical role in modulating calcium during development as well as in the removal of calcium and maintenance of calcium homeostasis in adults.

Loss of the phospholipase C gene product induces massive endocytosis of rhodopsin and arrestin in Drosophila photoreceptors

Previously we have shown that a subset of visual transduction mutants in Drosophila melanogaster induce the formation of stable complexes between rhodopsin and arrestin. One such mutant is in a visual system-specific phospholipase C (PLC). The rhodopsin/arrestin complexes generated in PLC mutants induce massive retinal degeneration. Here we demonstrate that both arrestin and rhodopsin undergo light-dependent endocytosis in a PLC mutant background. Interestingly, the internalized rhodopsin is rapidly degraded, but the arrestin is fully stable. The data are discussed with respect to mechanisms of arrestin-mediated endocytosis and human retinal disease.

Signalling pathways in Drosophila and vertebrate retinal development

The near-catholic conservation of paired box gene 6 (Pax6) and its supporting cast of retinal determination genes throughout the animal kingdom has sparked a scientific war over the evolutionary origins of the eye. The battle pits those who support a polyphyletic history for the eye against those who argue for a common ancestor for all 'seeing' animals. Recent papers have shed light on how eyes in both vertebrates and invertebrates are patterned. New insights into the roles that signal-transduction cascades might have in determining the Drosophila melanogaster eye indicate that, like many developmental processes, eye specification is an inductive process.

The role of the Drosophila TAK homologue dTAK during development

The TAK kinases belong to the MAPKKK group and have been implicated in a variety of signaling events. Originally described as a TGF-beta activated kinase (TAK) it has, however, subsequently been demonstrated to signal through p38, Jun N-terminal kinase (JNK) and Nemo types of MAP kinases, and the NFkappaB inducing kinase. Despite these multiple proposed functions, the in vivo role of TAK family kinases remains unclear. Here we report the isolation and genetic characterization of the Drosophila TAK homologue (dTAK). By employing overexpression and double-stranded RNA interference (RNAi) techniques we have analyzed its function during embryogenesis and larval development. Overexpression of dTAK in the embryonic epidermis is sufficient to induce the transcription of the JNK target genes decapentaplegic and puckered. Furthermore, overexpression of dominant negative (DN) or wild-type forms of dTAK in wing and eye imaginal discs, respectively, results in defects in thorax closure and ommatidial planar polarity, two well described phenotypes associated with JNK signaling activity. Surprisingly, RNAi and DN-dTAK expression studies in the embryo argue for a differential requirement of dTAK during developmental processes controlled by JNK signaling, and a redundant or minor role of dTAK in dorsal closure. In addition, dTAK-mediated activation of JNK in the Drosophila eye imaginal disc leads to an eye ablation phenotype due to ectopically induced apoptotic cell death. Genetic analyses in the eye indicate that dTAK can also act through the p38 and Nemo kinases in imaginal discs. Our results suggest that dTAK can act as a JNKKK upstream of JNK in multiple contexts and also other MAPKs in the eye. However, the loss-of-function RNAi studies indicate that it is not strictly required and thus either redundant or playing only a minor role in the context of embryonic dorsal closure.

A primary role for the epidermal growth factor receptor in ommatidial spacing in the Drosophila eye

BACKGROUND

The differentiation of regularly spaced structures within an epithelium is a common feature of developmental pattern formation. The regular spacing of ommatidia in the Drosophila eye imaginal disc provides a good model for this phenomenon. The correct spacing of ommatidia is a central event in establishing the precise hexagonal pattern of ommatidia in the Drosophila compound eye. The R8 photoreceptors are the founder cells of each of the ommatidia that comprise the adult eye and are specified by a bHLH transcription factor, Atonal.

RESULTS

We find that the epidermal growth factor receptor (Egfr) has a primary function in regulating R8 spacing. The receptor's activation within nascent ommatidia induces the expression of a secreted inhibitor that blocks atonal expression, and therefore ommatidial initiation, in nearby cells. The identity of the secreted inhibitor remains elusive but, contrary to previous suggestions, we show that it is not Argos. This Egfr-dependent inhibition acts in parallel to the inhibition of atonal by the secreted protein Scabrous. The activation of the Egfr pathway is dependent on Atonal function via the expression of Rhomboid-1. Our results also allow us to conclude that Egfr's role in promoting cell survival is largely independent of its role in photoreceptor recruitment; even when cell death is blocked, most photoreceptors fail to form.

CONCLUSIONS

Based on our data and those of others, we propose a model for R8 spacing that comprises a self-organizing network of signaling molecules. This model describes how successive rows of ommatidia form out of phase with each other, leading to the hexagonal array of facets in the compound eye.

R7 photoreceptor specification requires Notch activity

The eight photoreceptors in each ommatidium of the Drosophila eye are assembled by a process of recruitment [1,2]. First, the R8 cell is singled out, and then subsequent photoreceptors are added in pairs (R2 and R5, R3 and R4, R1 and R6) until the final R7 cell acquires a neuronal fate. R7 development requires the Sevenless receptor tyrosine kinase which is activated by a ligand from R8 [3]. Here, we report that the specification of R7 requires a second signal that activates Notch. We found that a Notch target gene is expressed in R7 shortly after recruitment. When Notch activity was reduced, the cell was misrouted to an R1/R6 fate. Conversely, when activated Notch was present in the R1/R6 cells, it caused them to adopt R7 fates or, occasionally, cone cell fates. In this context, Notch activity appears to act co-operatively, rather than antagonistically, with the receptor tyrosine kinase/Ras pathway in R7 photoreceptor specification. We propose two models: a ratchet model in which Notch would allow cells to remain competent to respond to sequential rounds of Ras signalling, and a combinatorial model in which Notch and Ras signalling would act together to regulate genes that determine cell fate.

Nuclear signaling by Rac and Rho GTPases is required in the establishment of epithelial planar polarity in the Drosophila eye

BACKGROUND

The small GTPases Rac and Rho act as cellular switches in many important biological processes. In the fruit fly Drosophila, RhoA participates in the establishment of planar polarity, a process mediated by the receptor Frizzled (Fz). Thus far, analysis of Rac in this process has not been possible because of the absence of mutant Rac alleles. Here, we have investigated the role of Rac and Rho in establishing the polarity of ommatidia in the Drosophila eye.

RESULTS

By expressing a dominant negative or a constitutively activated form of Rac1, we interfered specifically with Rac signaling and disrupted ommatidial polarity. The resulting defects were similar to the loss/gain-of-function phenotypes typical of tissue-polarity genes. Through genetic interaction and rescue experiments involving a polarity-specific, loss-of-function dishevelled (dsh) allele, we found that Rac1 acts downstream of Dsh in the Fz signaling pathway, but upstream of, or in parallel to, RhoA. Rac signaled to the nucleus through the Jun N-terminal kinase (JNK) cascade in this process. By generating point mutations in the effector loop of RhoA, we found that RhoA also signals to the nucleus during the establishment of ommatidial polarity. Nevertheless, Rac and RhoA activated transcription of distinct target genes.

CONCLUSIONS

Rac is specifically required downstream of Dsh in the Fz pathway. It functions upstream or in parallel to RhoA and both signal to the nucleus, through distinct effectors, to establish planar polarity in the Drosophila eye.

Jun mediates Frizzled-induced R3/R4 cell fate distinction and planar polarity determination in the Drosophila eye

Jun acts as a signal-regulated transcription factor in many cellular decisions, ranging from stress response to proliferation control and cell fate induction. Genetic interaction studies have suggested that Jun and JNK signaling are involved in Frizzled (Fz)-mediated planar polarity generation in the Drosophila eye. However, simple loss-of-function analysis of JNK signaling components did not show comparable planar polarity defects. To address the role of Jun and JNK in Fz signaling, we have used a combination of loss- and gain-of-function studies. Like Fz, Jun affects the bias between the R3/R4 photoreceptor pair that is critical for ommatidial polarity establishment. Detailed analysis of jun(−) clones reveals defects in R3 induction and planar polarity determination, whereas gain of Jun function induces the R3 fate and associated polarity phenotypes. We find also that affecting the levels of JNK signaling by either reduction or overexpression leads to planar polarity defects. Similarly, hypomorphic allelic combinations and overexpression of the negative JNK regulator Puckered causes planar polarity eye phenotypes, establishing that JNK acts in planar polarity signaling. The observation that Dl transcription in the early R3/R4 precursor cells is deregulated by Jun or Hep/JNKK activation, reminiscent of the effects seen with Fz overexpression, suggests that Jun is one of the transcription factors that mediates the effects of fz in planar polarity generation.

A novel method for the parallel monitoring of mitotic recombination and clastogenicity in somatic cells in vivo

Both homologous mitotic recombination (HMR), causing loss of heterozygosity (LOH) of the wild-type allele, and structural chromosome aberrations (CA) involve the formation of double-strand breaks in DNA. Whether the induction of CAs is always accompanied by HMR, or whether there exist DNA lesions specifically forming only one of the two end-points is unknown. Answering this fundamental question requires a system for the parallel detection of CAs and HMR, because only then is their analysis under strictly identical condition (dose, repair, genetic background) possible. We describe here a novel system for the parallel detection of HMR and loss of a whole chromosome as a measure of CA, utilizing somatic cells of Drosophila. In haploid germ cells of Drosophila, loss of a ring-shaped X-chromosome (rX) constitutes a frequent event providing an efficient method for measuring clastogenicity. For somatic cells, however, it was unclear whether the development of such a system would be feasible. The generally accepted notion has been that in XX female genotypes, loss of an entire X-chromosome acts as a cell lethal when generated at or shortly after blastoderm stage. However, here we show that rX-loss, if induced in pre-ommatidia cells of 3rd instar larvae, generates viable clones visible as small white patches in the red compound eye. To set up optimal conditions for the detection and quantification of rX-loss compared to HMR, several protocols were developed and tested against model carcinogens (methyl methanesulfonate, cisplatin and 7,12-dimethylbenz[a]anthracene). Generally, we find striking differences in the efficiency of these carcinogens for recombination when compared with clastogenicity. The cross-linking agent cisplatin is 4- to 6-fold more clastogenic than recombinagenic. 7,12-Dimethylbenz[a]-anthracene, on the contrary, produced less than a doubling effect for rX-loss but was highly active (20-times the background) for HMR. It appears therefore that both processes can be separated from each other. To the best of our knowledge, this is the first report suggesting, in terms of DNA adducts involved, qualitative differences between homologous recombination and clastogenic effects. Application of our system for studies on DNA repair may therefore provide new insight into the linkage of repair pathways in either of the two mechanisms.

Cloning and characterization of a potassium-dependent sodium/calcium exchanger in Drosophila

Sodium/calcium(-potassium) exchangers (NCX and NCKX) are critical for the rapid extrusion of calcium, which follows the stimulation of a variety of excitable cells. To further understand the mechanisms of calcium regulation in signaling, we have cloned a Drosophila sodium/calcium-potassium exchanger, Nckx30C. The overall deduced protein topology for NCKX30C is similar to that of mammalian NCKX, having five membrane-spanning domains in the NH(2) terminus separated from six at the COOH-terminal end by a large intracellular loop. We show that NCKX30C functions as a potassium-dependent sodium/calcium exchanger, and is not only expressed in adult neurons as was expected, but is also expressed during ventral nerve cord development in the embryo and in larval imaginal discs. Nckx30C is expressed in a dorsal-ventral pattern in the eye-antennal disc in a pattern that is similar to, but broader than that of wingless, suggesting that large fluxes of calcium may be occurring during imaginal disc development. Nckx30C may not only function in the removal of calcium and maintenance of calcium homeostasis during signaling in the adult, but may also play a critical role in signaling during development.

The role of long range, local and direct signalling molecules during chick feather bud development involving the BMPs, follistatin and the Eph receptor tyrosine kinase Eph-A4

The development of the feather buds during avian embryogenesis is a classic example of a spacing pattern. The regular arrangement of feather buds is achieved by a process of lateral inhibition whereby one developing feather bud prevents the formation of similar buds in the immediate vicinity. Lateral inhibition during feather formation implicates a role of long range signalling during this process. Recent work has shown that BMPs are able to enforce lateral inhibition during feather bud formation. However these results do not explain how the feather bud escapes the inhibition itself. We show that this could be achieved by the expression of the BMP antagonist, Follistatin. Furthermore we show that local application of Follistatin leads to the development of ectopic feather buds. We suggest that Follistatin locally antagonises the action of the BMPs and so permits the cellular changes associated with feather placode formation. We also provide evidence for the role of short range signalling during feather formation. We have correlated changes in cellular morphology in feather placodes with the expression of the gene Eph-A4 which encodes a receptor tyrosine kinase that requires direct cell-cell contact for activation. We show that the expression of this gene precedes cellular reorganisation required for feather bud formation.

Frizzled regulation of Notch signalling polarizes cell fate in the Drosophila eye

The Drosophila eye, a paradigm for epithelial organization, is highly polarized with mirror-image symmetry about the equator. The R3 and R4 photoreceptors in each ommatidium are vital in this polarity; they adopt asymmetrical positions in adult ommatidia and are the site of action for several essential genes. Two such genes are frizzled (fz) and dishevelled (dsh), the products of which are components of a signalling pathway required in R3, and which are thought to be activated by a diffusible signal. Here we show that the transmembrane receptor Notch is required downstream of dsh in R3/R4 for them to adopt distinct fates. By using an enhancer for the Notch target gene Enhancer of split mdelta, we show that Notch becomes activated specifically in R4. We propose that Fz/Dsh promotes activity of the Notch ligand Delta and inhibits Notch receptor activity in R3, creating a difference in Notch signalling capacity between R3 and R4. Subsequent feedback in the Notch pathway ensures that this difference becomes amplified. This interplay between Fz/Dsh and Notch indicates that polarity is established through local comparisons between two cells and explains how a signal from one position (for example, the equator in the eye) could be interpreted by all ommatidia in the field.

Asymmetric Notch activation specifies photoreceptors R3 and R4 and planar polarity in the Drosophila eye

Planar polarity is seen in epidermally derived structures throughout the animal kingdom. In the Drosophila eye, planar polarity is reflected in the mirror-symmetric arrangement of ommatidia (eye units) across the dorsoventral midline or equator; ommatidia on the dorsal and ventral sides of the equator exhibit opposite chirality. Photoreceptors R3 and R4 are essential in the establishment of the polarity of ommatidia. The R3 cell is thought to receive the polarizing signal, through the receptor Frizzled (Fz), before or at higher levels then the R4 cell, generating a difference between neighbouring R3 and R4 cells. Both loss-of-function and overexpression of Fz in the R3/R4 pair result in polarity defects and loss of mirror-image symmetry. Here we identify Notch and Delta (Dl) as dominant enhancers of the phenotypes produced by overexpression of fz and dishevelled (dsh), which encodes a signalling component downstream of Fz, and we show that D1-mediated activation of Notch is required for establishment of ommatidial polarity. Whereas fz signalling is required to specify R3, Notch signalling induces the R4 fate. Our data indicate that Dl is a transcriptional target of Fz/Dsh signalling in R3, and activates Notch in the neighbouring R4 precursor. This two-tiered mechanism explains how small differences in the level and/or timing of Fz activation reliably generate a binary cell-fate decision, leading to specification of R3 and R4 and ommatidial chirality.

Dominant effects of the bcr-abl oncogene on Drosophila morphogenesis

We targeted expression of human/fly chimeric Bcr-Abl proteins to the developing central nervous system (CNS) and eye imaginal disc of Drosophila melanogaster. Neural expression of human/fly chimeric P210 Bcr-Abl or P185 Bcr-Abl rescued abl mutant flies from pupal lethality, indicating that P210 and P185 Bcr-Abl can substitute functionally for Drosophila Abl during axonogenesis. However, increased levels of neurally expressed P210 or P185 Bcr-Abl but not Drosophila Abl produced CNS defects and lethality. Expression of P210 or P185 in the eye imaginal disc produced a dominant rough eye phenotype that was dependent on dosage of the transgene. Drosophila Enabled, previously identified as a suppressor of the abl mutant phenotype and substrate for Drosophila Abl kinase, had markedly increased phosphotyrosine levels in Bcr-Abl expressing Drosophila, indicating that it is a substrate for Bcr-Abl as well. Drosophila, therefore, is a suitable model system to identify Bcr-Abl interactions important for signal transduction and oncogenesis.

Dissecting the roles of the Drosophila EGF receptor in eye development and MAP kinase activation

A new conditional Egfr allele was used to dissect the roles of the receptor in eye development and to test two published models. EGFR function is necessary for morphogenetic furrow initiation, is not required for establishment of the founder R8 cell in each ommatidium, but is necessary to maintain its differentiated state. EGFR is required subsequently for recruitment of all other neuronal cells. The initial EGFR-dependent MAP kinase activation occurs in the furrow, but the active kinase (dp-ERK) is observed only in the cytoplasm for over 2 hours. Similarly, SEVENLESS-dependent activation results in cytoplasmic appearance of dp-ERK for 6 hours. These results suggest an additional regulated step in this pathway and we discuss models for this.

Dorsal-ventral signaling in the Drosophila eye

The development of the Drosophila eye has served as a model system for investigations of tissue patterning and cell-cell communication; however, early eye development has not been well understood. The results presented here indicate that specialized cells are established along the dorsal-ventral midline of the developing eye by Notch-mediated signaling between dorsal and ventral cells, and that Notch activation at the midline plays an essential role both in promoting the growth of the eye primordia and in regulating eye patterning. These observations imply that the developmental homology between Drosophila wings and vertebrate limbs extends to Drosophila eyes.

Linking cell-fate specification to planar polarity: determination of the R3/R4 photoreceptors is a prerequisite for the interpretation of the Frizzled mediated polarity signal

The adult eye of Drosophila is a highly ordered structure composed of about 800 ommatidia, each displaying precise polarity. The planar polarity is reflected in the mirror-symmetric arrangement of ommatidia relative to the dorso-ventral midline, the equator. This arrangement is generated when ommatidia rotate towards the equator and the photoreceptor R3 displaces R4 creating different chiral forms in each half. Analysis of ommatidia mosaic for the tissue polarity gene frizzled (fz) has shown that the presence of a single Fz+ photoreceptor cell within the R3/ R4 pair is critical for the direction of rotation and chirality. By analysing clones mutant for seven-up (svp), in which R3/R4 precursors reside in their normal positions and become photoreceptor neurones but fail to adopt the normal R3/R4 fate, we find that the R3/R4 photoreceptor subtype specification is a prerequisite for planar polarisation in the eye. Moreover, in mosaic R3/R4 pairs we find that the svp- cell always adopts the R4 position. This bias is reminiscent of what happens in fz mosaic R3/R4 pairs, where the fz- cell also almost always adopts the R4 position. In addition, we find that in genotypes where too many cells adopt the R3/R4 fate, ommatidial polarity is also disturbed. Taken together, these data imply that correct specification of a single R3 cell per ommatidium is essential for the normal interpretation of the Fz-mediated polarity signal.

Distribution of Na+,K(+)-ATPase in photoreceptor cells of insects

Light stimulation of insect photoreceptors causes opening of cation channels and an inward current that is partially carried by Na+ ions. There is also an efflux of K+ ions upon photostimulation. Na+ and K+ gradients across the photoreceptor membrane are reestablished by the activity of the enzyme Na+,K(+)-ATPase. About two-thirds of the total amount of ATP consumed in response to a light stimulus is attributed to the activity of this ion pump, demonstrating the importance of this enzyme for photoreceptor function. Insect photoreceptor cells are polarized epithelial cells; their plasma membrane is organized into two domains having a distinct morphology, molecular composition, and function. The visual pigment rhodopsin and the molecular components of the transduction machinery are localized in the rhabdomere, an array of densely packed microvilli, whereas Na+,K(+)-ATPase resides in the nonrhabdomeric membrane. Comparative immunolocalization studies on compound eyes of diverse insect species have demonstrated subtle variations in the distribution patterns of Na+,K(+)-ATPase. These may be accounted for by differences in the mechanisms responsible for Na+,K(+)-ATPase positioning.

muscleblind, a gene required for photoreceptor differentiation in Drosophila, encodes novel nuclear Cys3His-type zinc-finger-containing proteins

We have isolated the embryonic lethal gene muscleblind (mbl) as a suppressor of the sev-svp2 eye phenotype. Analysis of clones mutant for mbl during eye development shows that it is autonomously required for photoreceptor differentiation. Mutant cells are recruited into developing ommatidia and initiate neural differentiation, but they fail to properly differentiate as photoreceptors. Molecular analysis reveals that the mbl locus is large and complex, giving rise to multiple different proteins with common 5′ sequences but different carboxy termini. Mbl proteins are nuclear and share a Cys3His zinc-finger motif which is also found in the TIS11/NUP475/TTP family of proteins and is highly conserved in vertebrates and invertebrates. Functional analysis of mbl, the observation that it also dominantly suppresses the sE-Jun(Asp) gain-of-function phenotype and the phenotypic similarity to mutants in the photoreceptor-specific glass gene suggest that mbl is a general factor required for photoreceptor differentiation.