Early decisions in Drosophila eye morphogenesis

Cytonemes, Their Formation, Regulation, and Roles in Signaling and Communication in Tumorigenesis

Increasing evidence during the past two decades shows that cells interconnect and communicate through cytonemes. These cytoskeleton-driven extensions of specialized membrane territories are involved in cell–cell signaling in development, patterning, and differentiation, but also in the maintenance of adult tissue homeostasis, tissue regeneration, and cancer. Brain tumor cells in glioblastoma extend ultralong membrane protrusions (named tumor microtubes, TMs), which contribute to invasion, proliferation, radioresistance, and tumor progression. Here we review the mechanisms underlying cytoneme formation, regulation, and their roles in cell signaling and communication in epithelial cells and other cell types. Furthermore, we discuss the recent discovery of glial cytonemes in the Drosophila glial cells that alter Wingless (Wg)/Frizzled (Fz) signaling between glia and neurons. Research on cytoneme formation, maintenance, and cell signaling mechanisms will help to better understand not only physiological developmental processes and tissue homeostasis but also cancer progression.

A global survey identifies novel upstream components of the Ath5 neurogenic network

Regulators of vertebrate Ath5 expression were identified by high-throughput screening; extending the current gene regulatory model network controlling retinal neurogenesis.

Background

Investigating the architecture of gene regulatory networks (GRNs) is essential to decipher the logic of developmental programs during embryogenesis. In this study we present an upstream survey approach, termed trans-regulation screen, to comprehensively identify the regulatory input converging on endogenous regulatory sequences.

Results

Our dual luciferase-based screen queries transcriptome-scale collections of cDNAs. Using this approach we study the regulation of Ath5, the central node in the GRN controlling retinal ganglion cell (RGC) specification in vertebrates. The Ath5 promoter integrates the input of upstream regulators to enable the transient activation of the gene, which is an essential step for RGC differentiation. We efficiently identified potential Ath5 regulators that were further filtered for true positives by an in situ hybridization screen. Their regulatory activity was validated in vivo by functional assays in medakafish embryos.

Conclusions

Our analysis establishes functional groups of genes controlling different regulatory phases, including the onset of Ath5 expression at cell-cycle exit and its down-regulation prior to terminal RGC differentiation. These results extent the current model of the GRN controlling retinal neurogenesis in vertebrates.

Evolution of eye development in arthropods: phylogenetic aspects

The architecture of the adult arthropod visual system for many decades has contributed important character sets that are useful for reconstructing the phylogenetic relationships within this group. In the current paper we explore whether aspects of eye development can also contribute new arguments to the discussion of arthropod phylogeny. We review the current knowledge on eye formation in Trilobita, Xiphosura, Myriapoda, Hexapoda, and Crustacea. All euarthropod taxa share the motif of a proliferation zone at the side of the developing eye field that contributes new eye elements. Two major variations of this common motif can be distinguished: 1. The "row by row type" of Trilobita, Xiphosura, and Diplopoda. In this type, the proliferation zone at the side of the eye field generates new single, large elements with a high and variable cell number, which are added to the side of the eye and extend rows of existing eye elements. Cell proliferation, differentiation and ommatidial assembly seem to be separated in time but spatially confined within the precursors of the optic units which grow continuously once they are formed (intercalary growth). 2. The "morphogenetic front type" of eye formation in Crustacea+Hexapoda (Tetraconata). In this type, there is a clear temporal and spatial separation of the formation and differentiation processes. Proliferation and the initial steps of pattern formation take place in linear and parallel mitotic and morphogenetic fronts (the mitotic waves and the morphogenetic furrow/transition zone) and numerous but small new elements with a strictly fixed set of cells are added to the eye field. In Tetraconata, once formed, the individual ommatidia do not grow any more. Scutigeromorph chilopods take an intermediate position between these two major types. We suggest that the "row by row type" as seen in Trilobita, Xiphosura and Diplopoda represents the plesiomorphic developmental mode of eye formation from the euarthropod ground pattern whereas the "morphogenetic front type" is apomorphic for the Tetraconata. Our data are discussed with regard to two competing hypotheses on arthropod phylogeny, the "Tracheata" versus "Tetraconata" concept. The modes of eye development in Myriapoda is more parsimonious to explain in the Tetraconata hypothesis so that our data raise the possibility that myriapod eyes may not be secondarily reconstructed insect eyes as the prevailing hypothesis suggests.

Evolution of arthropod visual systems: Development of the eyes and central visual pathways in the horseshoe crab Limulus polyphemus Linnaeus, 1758 (Chelicerata, Xiphosura)

Despite ongoing interest into the architecture, biochemistry, and physiology of the visual systems of the xiphosuran Limulus polyphemus, their ontogenetic aspects have received little attention. Thus, we explored the development of the lateral eyes and associated neuropils in late embryos and larvae of these animals. The first external evidence of the lateral eyes was the appearance of white pigment spots-guanophores associated with the rudimentary photoreceptors-on the dorsolateral side of the late embryos, suggesting that these embryos can perceive light. The first brown pigment emerges in the eyes during the last (third) embryonic molt to the trilobite stage. However, ommatidia develop from this field of pigment toward the end of the larval trilobite stage so that the young larvae at hatching do not have object recognition. Double staining with the proliferation marker bromodeoxyuridine (BrdU) and an antibody against L. polyphemus myosin III, which is concentrated in photoreceptors of this species, confirmed previous reports that, in the trilobite larvae, new cellular material is added to the eye field from an anteriorly located proliferation zone. Pulse-chase experiments indicated that these new cells differentiate into new ommatidia. Examining larval eyes labeled for opsin showed that the new ommatidia become organized into irregular rows that give the eye field a triangular appearance. Within the eye field, the ommatidia are arranged in an imperfect hexagonal array. Myosin III immunoreactivity in trilobite larvae also revealed the architecture of the central visual pathways associated with the median eye complex and the lateral eyes. Double labeling with myosin III and BrdU showed that neurogenesis persists in the larval brain and suggested that new neurons of both the lamina and the medulla originate from a single common proliferation zone. These data are compared with eye development in Drosophila melanogaster and are discussed with regard to new ideas on eye evolution in the Euarthropoda.

Regulation of the retinal determination gene dachshund in the embryonic head and developing eye of Drosophila

The retinal determination gene dachshund is distantly related to the family of Ski/Sno proto-oncogenes and influences the development of a wide range of tissues including the embryonic head, optic lobes, brain, central nervous system as well as the post-embryonic leg, wing, genital and eye-antennal discs. We were interested in the regulatory mechanisms that control the dynamic expression pattern of dachshund and in this report we set out to ascertain how the transcription of dachshund is modulated in the embryonic head and developing eye-antennal imaginal disc. We demonstrate that the TGFbeta signaling cascade, the transcription factor zerknullt and several other patterning genes prevent dachshund from being expressed inappropriately within the embryonic head. Additionally, we show that several members of the eye specification cascade influence the transcription of dachshund during normal and ectopic eye development. Our results suggest that dachshund is regulated by a complex combinatorial code of transcription factors and signaling pathways. Unraveling this code may lead to an understanding of how dachshund regulates the development of many diverse tissue types including the eye.

Novel dominant-negative mutation within the six domain of the conserved eye specification gene sine oculis inhibits eye development in Drosophila

The development of the compound eye of Drosophila is controlled, in part, by the concerted actions of several nuclear proteins that form an intricate regulatory system. One member of this network is sine oculis (so), the founding member of the Six gene family. Mutations within so affect the entire visual system, including the compound eye. The vertebrate homologs Six3 and Six6 also appear to play crucial roles in retinal formation. Mutations in Six3 inhibit retinal formation in chickens and fish, whereas those in Six6 are the underlying cause of bilateral anophthalmia in humans. Together, these phenotypes suggest a conserved role for the Six genes in eye development. In this report, we describe the effects of a dominant-negative mutation of sine oculis on the development of the compound eye of Drosophila. The mutation resides within the Six domain and may have implications for eye development and disease.

Cellular patterns in the inner retina of adult zebrafish: quantitative analyses and a computational model of their formation

The mechanisms that control cellular pattern formation in the growing vertebrate central nervous system are poorly understood. In an effort to reveal mechanistic rules of cellular pattern formation in the central nervous system, quantitative spatial analysis and computational modeling techniques were applied to cellular patterns in the inner retina of the adult zebrafish. All the analyzed cell types were arrayed in nonrandom patterns tending toward regularity; specifically, they were locally anticlustered. Over relatively large spatial scales, only one cell type exhibited consistent evidence for pattern regularity, suggesting that cellular pattern formation in the inner retina is dominated by local anticlustering mechanisms. Cross-correlation analyses revealed independence between the patterns of different cell types, suggesting that cellular pattern formation may involve multiple, independent, homotypic anticlustering mechanisms. A computational model of cellular pattern formation in the growing zebrafish retina was developed, which featured an inhibitory, homotypic signaling mechanism, arising from differentiated cells, that controlled the spatial profile of cell fate decisions. By adjusting the spatial profile of this decaying-exponential signal, the model provided good estimates of all the cellular patterns that were observed in vivo, as objectively judged by quantitative spatial pattern analyses. The results support the hypothesis that cellular pattern formation in the inner retina of zebrafish is dominated by a set of anticlustering mechanisms that may control events at, or near, the spatiotemporal point of cell fate decision.

Control of Drosophila eye specification by Wingless signalling

Organ formation requires early specification of the groups of cells that will give rise to specific structures. The Wingless protein plays an important part in this regional specification of imaginal structures in Drosophila, including defining the region of the eye-antennal disc that will become retina. We show that Wingless signalling establishes the border between the retina and adjacent head structures by inhibiting the expression of the eye specification genes eyes absent, sine oculis and dachshund. Ectopic Wingless signalling leads to the repression of these genes and the loss of eyes, whereas loss of Wingless signalling has the opposite effects. Wingless expression in the anterior of wild-type discs is complementary to that of these eye specification genes. Contrary to previous reports, we find that under conditions of excess Wingless signalling, eye tissue is transformed not only into head cuticle but also into a variety of inappropriate structures.

Misexpression of the eyes absent family triggers the apoptotic program

Genetic studies in Drosophila and mice have shown that eyes absent (eya) is an important and conserved transcriptional regulator of development. Along with eyeless/Pax6, sine oculis, and dachshund, eya genes function as master regulators in eye development and can induce ectopic eye formation. Furthermore, the loss-of-function mutants of these genes in the fly causes partial or complete loss of the compound eye, and this is associated with inappropriate apoptosis. Conversely, ectopic eyeless expression in the context of eyes absent or sine oculis mutations results in apoptosis, suggesting that the proper ratio of these factors regulates apoptosis. Here we report that enforced expression of fly eya or of one of its mammalian homologs, Eya2, triggers rapid apoptosis in interleukin-3-dependent 32D.3 murine myeloid cells, which express Eya family members but not Pax6. Eya-induced cell death overrides survival factors and has many features typical of apoptosis, including plasma and mitochondrial membrane changes and caspase activation. Eya-induced apoptosis is blocked by Bcl-2 overexpression but not by the broad-spectrum caspase inhibitor z-VAD.fmk, suggesting that mitochondria are a major target in Eya-induced apoptosis. These results support the concept that inappropriate changes in the steady state levels of Eya proteins may trigger programmed cell deaths during development.

Eye specification in Drosophila: perspectives and implications

The discovery that Drosophila eyeless is homologous to vertebrate Pax6 produced enormous interest in eye specification and a reappraisal of eye evolution. While the transcription factor Eyeless/Pax6 is necessary and in some circumstances sufficient to induce eye development the simple story of eye specification has become more epic than haiku. At least seven other nuclear proteins act with Eyeless/Pax6 to induce the eye and, furthermore, extrinsic developmental signals are required. Some striking similarities between later events of retinal patterning in vertebrates and insects have led to a deeper debate on the evolutionary path to these apparently quite different organs.

The alpha subunits of Gz and Gi interact with the eyes absent transcription cofactor Eya2, preventing its interaction with the six class of homeodomain-containing proteins

Yeast two-hybrid techniques were used to identify possible effectors for the heterotrimeric G protein G(z) in human bone marrow cells. Eya2, a human homologue of the Drosophila Eya transcription co-activator, was identified. Eya2 interacts with activated Galpha(z) and at least one other member of the Galpha(i) family, Galpha(i2). Interactions were confirmed in mammalian two-hybrid and glutathione S-transferase fusion protein pull-down assays. Regions of Eya2-mediating interaction were mapped to the C-terminal Eya consensus domain. Eya2 is an intrinsically cytosolic protein that is translocated to the nucleus by members of the Six homeodomain-containing family of proteins. Activated Galpha(z) and Galpha(i2) prevent Eya2 translocation and inhibit Six/Eya2-mediated activation of a reporter gene controlled through the MEF3/TATA promoter. Although G proteins are known to regulate the activity of numerous transcription factors, this regulation is normally achieved indirectly via one or more intermediates. We show here a novel functional regulation of a co-activator directly by G protein subunits.

Perspectives on eye development

The lens of the vertebrate eye was the classic model used to demonstrate the concepts of inductive interactions controlling development. However, it is in the Drosophila model that the greatest progress in understanding molecular mechanisms of eye development have most recently been mode. This progress can be attributed to the power of molecular genetics, an approach that was once confined to simpler systems like worms and flies, but is now becoming possible in vertebrates. Thus, the use of transgenic and knock-out gene technology, coupled with the availability of new positional cloning methods, has recently initiated a surge of progress in the mouse genetic model and has also led to the identification of genes involved in human inherited disorders. In addition, gene transfer techniques have opened up opportunities for progress using chick, Xenopus, and other classic developmental systems. Finally, a new vertebrate genetic model, zebrafish, appears very promising for molecular studies. As a result of the opportunities presented by these new approaches, eye development has come into the limelight, hence the timeliness of this focus issue of Developmental Genetics. In this introductory review, we discuss three areas of current work arising through the use of these newer genetic approaches, and pertinent to research articles presented herein. We also touch on related studies reported at the first Keystone Meeting on Ocular Cell and Molecular Biology, recently held in Tamarron Springs, Colorado, January 7-12, 1997.

Morphogenetic furrow initiation and progression during eye development in Drosophila: the roles of decapentaplegic, hedgehog and eyes absent

The Drosophila signaling factor decapentaplegic (dpp) mediates the effects of hedgehog (hh) in tissue patterning by regulating the expression of tissue-specific genes. In the eye disc, the transcription factors eyeless (ey), eyes absent (eya), sine oculis (so) and dachshund (dac) participate with these signaling molecules in a complex regulatory network that results in the initiation of eye development. Our analysis of functional relationships in the early eye disc indicates that hh and dpp play no role in regulating ey, but are required for eya, so and dac expression. We show that restoring expression of eya in loss-of-function dpp mutant backgrounds is sufficient to induce so and dac expression and to rescue eye development. Thus, once expressed, eya can carry out its functions in the absence of dpp. These experiments indicate that dpp functions downstream of or in parallel with ey, but upstream of eya, so and dac. Additional control is provided by a feedback loop that maintains expression of eya and so and includes dpp. The fact that exogenous overexpression of ey, eya, so and dac interferes with wild-type eye development demonstrates the importance of such a complicated mechanism for maintaining proper levels of these factors during early eye development. Whereas initiation of eye development fails in either Hh or Dpp signaling mutants, the subsequent progression of the morphogenetic furrow is only slowed down. However, we find that clones that are simultaneously mutant for Hh and Dpp signaling components completely block furrow progression and eye differentiation, suggesting that Hh and Dpp serve partially redundant functions in this process. Interestingly, furrow-associated expression of eya, so and dac is not affected by double mutant tissue, suggesting that some other factor(s) regulates their expression during furrow progression.

mus304 encodes a novel DNA damage checkpoint protein required during Drosophila development

Checkpoints block cell cycle progression in eukaryotic cells exposed to DNA damaging agents. We show that several Drosophilahomologs of checkpoint genes, mei-41, grapes, and14-3-3ε, regulate a DNA damage checkpoint in the developing eye. We have used this assay to show that the mutagen-sensitive gene mus304 is also required for this checkpoint. mus304 encodes a novel coiled-coil domain protein, which is targeted to the cytoplasm. Similar to mei-41,mus304 is required for chromosome break repair and for genomic stability. mus304 animals also exhibit three developmental defects, abnormal bristle morphology, decreased meiotic recombination, and arrested embryonic development. We suggest that these phenotypes reflect distinct developmental consequences of a single underlying checkpoint defect. Similar mechanisms may account for the puzzling array of symptoms observed in humans with mutations in the ATM tumor suppressor gene.

Cellular aspects of trophic actions in the nervous system

During the past three decades the number of molecules exhibiting trophic actions in the brain has increased drastically. These molecules promote and/or control proliferation, differentiation, migration, and survival (sometimes even the death) of their target cells. In this review a comprehensive overview of small diffusible factors showing trophic actions in the central nervous system (CNS) is given. The factors discussed are neurotrophins, epidermal growth factor, fibroblast growth factor, platelet-derived growth factor, insulin-like growth factors, ciliary neurotrophic factor and related molecules, glial-derived growth factor and related molecules, transforming growth factor-beta and related molecules, neurotransmitters, and hormones. All factors are discussed with respect to their trophic actions, their expression patterns in the brain, and molecular aspects of their receptors and intracellular signaling pathways. It becomes evident that there does not exist "the" trophic factor in the CNS but rather a multitude of them interacting with each other in a complicated network of trophic actions forming and maintaining the adult nervous system.

Mouse Dac, a novel nuclear factor with homology to Drosophila dachshund shows a dynamic expression in the neural crest, the eye, the neocortex, and the limb bud

Dac is a novel nuclear factor in mouse and humans that shares homology with Drosophila dachshund (dac). Alignment with available sequences defines a conserved box of 117 amino acids that shares weak homology with the proto-oncogene Ski and Sno. Dac expression is found in various neuroectodermal and mesenchymal tissues. At early developmental stages Dac is expressed in lateral mesoderm and in neural crest cells. In the neural plate/tube Dac expression is initially seen in the prosencephalon and gets gradually restricted to the presumptive neocortex and the distal portion of the outgrowing optic vesicle. Furthermore, Dac transcripts are detected in the mesenchyme underlying the Apical Ectodermal Ridge (AER) of the extending limb bud, the dorsal root ganglia and chain ganglia, and the mesenchyme of the growing genitalia. Dac expression in the Gli 3 mutant extra toes (Xt/Xt) shows little difference compared to the expression in wild-type limb buds. In contrast, a significant expansion of Dac expression are observed in the anterior mesenchyme of the limb buds of hemimelic extra toes (Hx/+) mice. FISH analysis reveals that human DAC maps to chromosome 13q22.3-23 and further fine-mapping defined a position of the DAC gene at 54cM or 13q21.1, a locus that associates with mental retardation and skeletal abnormalities.

Eyeless initiates the expression of both sine oculis and eyes absent during Drosophila compound eye development

The Drosophila Pax-6 gene eyeless acts high up in the genetic hierarchy involved in compound eye development and can direct the formation of extra eyes in ectopic locations. Here we identify sine oculis and eyes absent as two mediators of the eye-inducing activity of eyeless. We show that eyeless induces and requires the expression of both genes independently during extra eye development. During normal eye development, eyeless is expressed earlier than and is required for the expression of sine oculis and eyes absent, but not vice versa. Based on the results presented here and those of others, we propose a model in which eyeless induces the initial expression of both sine oculis and eyes absent in the eye disc. sine oculis and eyes absent then appear to participate in a positive feedback loop that regulates the expression of all three genes. In contrast to the regulatory interactions that occur in the developing eye disc, we also show that in the embryonic head, sine oculis acts in parallel to eyeless and twin of eyeless, a second Pax-6 gene from Drosophila. Recent studies in vertebrate systems indicate that the epistatic relationships among the corresponding vertebrate homologs are very similar to those observed in Drosophila.

Local induction of patterning and programmed cell death in the developing Drosophila retina

Local cell signaling can pattern the nervous system by directing cell fates, including programmed cell death. In the developing Drosophila retina, programmed cell death is used to remove excess cells between ommatidia. Cell ablation revealed the source and position of signals required for regulating the pattern of programmed cell death among these interommatidial cells. Two types of signals regulate this patterning event. Notch-mediated signals between interommatidial precursors result in removal of unneeded cells. Cone cells and primary pigment cells oppose this signal by supplying a ‘life’-promoting activity; evidence is provided that this signal occurs through localized activation of the EGF Receptor/Ras pathway. Together, these signals refine the highly regular pattern observed in the adult retina.

Dual functions of the Drosophila eyes absent gene in the eye and embryo

In eyes absent (eya) mutants, eye progenitor cells undergo cell death early in development. Whereas the phenotype of eya1 is limited to the eye, other mutations are lethal. Genetic and molecular analysis reveals that mutations in one region of the gene cause embryonic lethality, whereas mutations throughout the gene cause defects in eye development. Mosaic analysis indicates that the eya requirement is cell autonomous. In eye-specific mutants, expression in the eye disc is lacking while embryonic expression is normal. Both the type I and type II transcripts are expressed in the developing eye, and expression of either can rescue the eye phenotype. These data indicate a specific requirement for eya function in eye progenitor cells that is normally fulfilled by both transcripts.

Multiple roles of the eyes absent gene in Drosophila

The eyes absent (eya) gene plays an essential role in the events that lead to formation of the Drosophila eye; without expression of eya in retinal progenitor cells, they undergo programmed cell death just prior to the morphogenetic furrow, leading to an eyeless or reduced eye phenotype. The eya gene has recently been found to be highly conserved to humans, defining a new gene family. Insights into the gene's function in the fly, therefore, are likely to be relevant to the role of its homologs in vertebrates. Detailed studies at the subcellular level indicate that the Eya protein is localized to the nucleoplasm, suggesting a role in control of nuclear events. The eya gene shows expression and roles in tissues other than the eye, including subsets of cells of the adult visual system, brain, and ovary, as well as an elaborate expression pattern in the embryo. Various mutations in the eya gene cause loss of ocelli, female sterility, or lethality. Analysis of the embryonic lethal phenotype indicates that mutant alleles show defects in head morphogenesis. These data indicate that eya has critical roles in morphogenesis of a number of tissues in the animal, in addition to its role in early eye formation. Despite multiple roles at multiple stages of development of the fly, both the type I and type II forms of the protein, when expressed ectopically during larval development, can direct eye formation.

The Homothorax homeoprotein activates the nuclear localization of another homeoprotein, extradenticle, and suppresses eye development in Drosophila

The Extradenticle (Exd) protein in Drosophila acts as a cofactor to homeotic proteins. Its nuclear localization is regulated. We report the cloning of the Drosophila homothorax (hth) gene, a homolog of the mouse Meis1 proto-oncogene that has a homeobox related to that of exd. Comparison with Meis1 finds two regions of high homology: a novel MH domain and the homeodomain. In imaginal discs, hth expression coincides with nuclear Exd. hth and exd also have virtually identical, mutant clonal phenotypes in adults. These results suggest that hth and exd function in the same pathway. We show that hth acts upstream of exd and is required and sufficient for Exd protein nuclear localization. We also show that hth and exd are both negative regulators of eye development; their mutant clones caused ectopic eye formation. Targeted expression of hth, but not of exd, in the eye disc abolished eye development completely. We suggest that hth acts with exd to delimit the eye field and prevent inappropriate eye development.

Dachshund and eyes absent proteins form a complex and function synergistically to induce ectopic eye development in Drosophila

The eyeless, dachshund, and eyes absent genes encode conserved, nuclear proteins that are essential for eye development in Drosophila. Misexpression of eyeless or dachshund is also sufficient to induce the formation of ectopic compound eyes. Here we show that the dachshund and eyes absent genes act synergistically to induce ectopic retinal development and positively regulate the expression of each other. Moreover, we show that the Dachshund and Eyes Absent proteins can physically interact through conserved domains, suggesting a molecular basis for the genetic synergy observed and that a similar complex may function in mammals. We propose that a conserved regulatory network, rather than a linear hierarchy, controls retinal specification and involves multiple protein complexes that function during distinct steps of eye development.

Hedgehog directly controls initiation and propagation of retinal differentiation in the Drosophila eye

Patterning of the compound eye begins at the posterior edge of the eye imaginal disc and progresses anteriorly toward the disc margin. The advancing front of ommatidial differentiation is marked by the morphogenetic furrow (MF). Here we show by clonal analysis that Hedgehog (Hh), secreted from two distinct populations of cells has two distinct functions: It was well documented that Hh expression in the differentiating photoreceptor cells drives the morphogenetic furrow. Now we show that, in addition, Hh, secreted from cells at the posterior disc margin, is absolutely required for the initiation of patterning and predisposes ommatidial precursor cells to enter ommatidial assembly later. These two functions of Hh in eye patterning are similar to the biphasic requirement for Sonic Hh in patterning of the ventral neural tube in vertebrates. We show further that Hh induces ommatidial development in the absence of its secondary signals Wingless (Wg) and Dpp and that the primary function of Dpp in MF initiation is the repression of wg, which prevents ommatidial differentiation. Our results show that the regulatory relationships between Hh, Dpp, and Wg in the eye are similar to those found in other imaginal discs such as the leg disc despite obvious differences in their modes of development.

Current views on eye development

Several genes involved in the regulation of eye development in different species have been identified. Structural and functional conservation have been found between some of these genes in organisms as diverse as Drosophila and mouse. One notable example is the relationship between the mouse Pax6 gene and eyeless of Drosophila. Ectopic expression of eyeless or mouse Pax6 in Drosophila results in the formation of additional eyes. Recently, another homeobox gene, Six3, was found to promote ectopic lens formation in fish embryos. The next step will be to unravel the associated regulatory pathways of these genes and assess the degree to which they display evolutionary conservation. This will be important in order to assimilate these findings with current anatomical and embryological models. It seems reasonable to believe that in the near future the characterization of the whole framework required for vertebrate eye development will be accomplished.

Eyes absent: a gene family found in several metazoan phyla

Genes related to the Drosophila eyes absent gene were identified in vertebrates (mouse and human), mollusks (squid), and nematodes (C. elegans). Proteins encoded by these genes consist of conserved C-terminal and variable N-terminal domains. In the conserved 271-amino acid C-terminal region, Drosophila and vertebrate proteins are 65-67% identical. A vertebrate homolog of eyes absent, designated Eya2, was mapped to Chromosome (Chr) 2 in the mouse and to Chr 20q13.1 in human. Eya2 shows a dynamic pattern of expression during development. In the mouse, expression of Eya2 was first detected in 8.5-day embryos in the region of head ectoderm fated to become the forebrain. At later stages of development, Eya2 is expressed in the olfactory placode and in a variety of neural crest derivatives. In the eye, expression of Eya2 was first detected after formation of the lens vesicle. At day 17.5, the highest level of Eya2 mRNA was observed in primary lens fibers. Low levels of Eya2 expression was detected in retina, sclera, and cornea. By postnatal day 10, Eya2 was expressed in secondary lens fibers, cornea, and retina. Although Eya2 is expressed relatively late in eye development, it belongs to the growing list of factors that may be essential for eye development across metazoan phyla. Like members of the Pax-6 gene family, eyes absent gene family members were probably first involved in functions not related to vision, with recruitment for visual system formation and function occurring later.

Bone morphogenetic proteins in the nervous system

Bone morphogenetic proteins (BMPs) are a rapidly expanding subclass of the transforming growth factor superfamily. BMP ligands and receptor subunits are present throughout neural development within discrete regions of the embryonic brain and within neural crest-derived pre- and post-migratory zones. BMPs initially inhibit the formation of neuroectoderm during gastrulation while, within the neural tube, they act as gradient morphogens to promote the differentiation of dorsal cell types and intermediate cell types throughout co-operative signaling. In the peripheral nervous system, BMPs act as instructive signals for neuronal lineage commitment and promote graded stages of neuronal differentiation. By contrast, within the CNS, these same factors promote astroglial lineage elaboration from embryonic subventricular zone progenitor cells, with concurrent suppression of the neuronal or oligodendroglial lineages, or both. In addition, BMPs act on more lineage-restricted embryonic CNS progenitor cells to promote regional neuronal survival and cellular differentiation. Furthermore, these versatile cytokines induce selective apoptosis of discrete rhombencephalic neural crest-associated cellular populations. These observations suggest that the BMPs exhibit a broad range of cellular and context-specific effects during multiple stages of neural development.

The genetics of visual system development in Drosophila: specification, connectivity and asymmetry

Encoding visual information requires a complex neuronal network. Recently, genes regulating early tissue specification, the growth of retinal target structures, the connectivity of photoreceptor axons, and mirror-image retinal symmetry in Drosophila have been identified. The insights gained from studying visual system development in flies promise to inform our understanding of similar processes in vertebrates.

Cloning and characterization of two vertebrate homologs of the Drosophila eyes absent gene

The Drosophila eyes absent (eya) gene plays an essential role in the events that lead to proper development of the fly eye and embryo. Here we report the analysis of two human and two mouse homologs of the fly eya gene. Sequence comparison reveals a large domain of approximately 270 amino acids in the carboxyl terminus of the predicted mammalian proteins that shows 53% identity between the fly sequence and all of the vertebrate homologs. This Eya-homology domain is of novel sequence, with no previously identified motifs. RNA hybridization studies indicate that the mouse genes are expressed during embryogenesis and in select tissues of the adult. Both mouse Eya genes are expressed in the eye, suggesting that these genes may function in eye development in vertebrates as eya does in the fly. The mouse Eya2 gene maps to chromosome 2 in the region syntenic with human chromosome 20q13, and the mouse Eya2 gene maps to chromosome 4 in the region syntenic with human chromosome 1p36. Our findings support the notion that several families of genes (Pax-6/eyeless, Six-3/sine oculis, and Eya) play related and critical roles in the eye for both files and vertebrates.

Cell determination strategies in the Drosophila eye

Cells in the Drosophila eye are determined by inductive signalling. Here I describe a new model of eye development that explains how simple intercellular signals could specify the diverse cell types that constitute the ommatidium. This model arises from the recent observation that the Drosophila homologue of the EGF receptor (DER) is used reiteratively to trigger the differentiation of each of the cell types--successive rounds of DER activation recruit first the photoreceptors, then cone and finally pigment cells. It seems that a cell's identity is not determined by the specific signal that induces it, but is instead a function of the state of the cell when it receives the signal. DER signalling is activated by the ligand, Spitz, and inhibited by the secreted protein, Argos. Spitz is initially produced by the central cells in the ommatidium and diffuses over a small distance. Argos has a longer range, allowing it to block more distal cells from being activated by low levels of Spitz; I have termed this interplay between a short-range activator and a long-range inhibitor ‘remote inhibition’. Since inductive signalling is common in many organisms and its components have been conserved, it is possible that the logic of signalling may also be conserved.

Ectopic eye development in Drosophila induced by directed dachshund expression

The dachshund gene encodes a nuclear protein that is required for normal eye development in Drosophila. In the absence of dachshund function, flies develop with severely reduced or no eyes. We show that targeted expression of dachshund is sufficient to direct ectopic retinal development in a variety of tissues, including the adult head, thorax and legs. This result is similar to that observed with the highly conserved Drosophila gene eyeless, which can induce ectopic eye formation on all major appendages. Here, we show that dachshund and eyeless induce the expression of each other and that dachshund is required for ectopic retinal development driven by eyeless misexpression. These results suggest that the control of eye development requires the complex interaction of multiple genes, even at the very highest regulatory levels.

Mouse Eya homologues of the Drosophila eyes absent gene require Pax6 for expression in lens and nasal placode

We have identified and mapped three members of a new family of vertebrate genes, designated Eya1, Eya2 and Eya3, which share high sequence similarity with the Drosophila eyes absent (eya) gene. Comparison of all three murine Eya gene products and that encoded by the Drosophila eya gene defines a 271 amino acid carboxyl terminal Eya domain, which has been highly conserved during evolution. Eya1 and Eya2, which are closely related, are extensively expressed in cranial placodes, in the branchial arches and CNS and in complementary or overlapping patterns during organogenesis. Eya3 is also expressed in the branchial arches and CNS, but lacks cranial placode expression. All three Eya genes are expressed in the developing eye. Eyal is expressed in developing anterior chamber structures, including the lens placode, the iris and ciliary region and the prospective corneal ectoderm. Eyal is also expressed in retinal pigment epithelium and optic nerve. Eya2 is expressed in neural retina, sclera and optic nerve sheath. Moreover, Eya1 and Eya2 expressions in the lens and nasal placode overlap with and depend upon expression of Pax6. The high sequence similarity with Drosophila eya, the conserved developmental expression of Eya genes in the eye and the Pax6 dependence of Eya expression in the lens and nasal placode indicates that these genes likely represent functional homologues of the Drosophila eya gene. These results suggest that members of the Eya gene family play critical roles downstream of Pax genes in specifying placodal identity and support the idea that despite enormous morphological differences, the early development of insect and mammalian eyes is controlled by a conserved regulatory hierarchy.

Bone morphogenetic proteins: multifunctional regulators of vertebrate development

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Pax proteins and eye development

Homologous members of the Pax gene family are required for eye development in Drosophila and vertebrates. Despite superficial similarities in the phenotypes of vertebrates with mutations in pax-6 and Drosophila eyeless mutants, it remains uncertain whether the two proteins encoded by these genes have comparable functions. The genetic cascade triggered by eyeless leads to eye formation, whereas pax-6 is not necessary for optic vesicle formation, but is required at other stages of eye development. A second vertebrate Pax gene, pax-2, is also required during eye development and appears to play a role during closure of the choroid fissure.