Structure–function analysis of the Drosophila retinal determination protein Dachshund

Mapping Gene Expression in Whole Larval Brains of Bicyclus anynana Butterflies

Butterfly larvae display intricate cognitive capacities and behaviors, but relatively little is known about how those behaviors alter their brains at the molecular level. Here, we optimized a hybridization chain reaction 3.0 (HCR v3.0) protocol to visualize the expression of multiple RNA molecules in fixed larval brains of the African butterfly Bicyclus anynana. We optimized the polyacrylamide gel mounting, fixation, and sample permeabilization steps, and mapped the expression domains of ten genes in whole larval brain tissue at single-cell resolution. The genes included optomotor blind (omb), yellow-like, zinc finger protein SNAI2-like (SNAI2), weary (wry), extradenticle (exd), Synapsin, Distal-less (Dll), bric-à-brac 1 (bab1), dachshund (dac), and acetyl coenzyme A acetyltransferase B (AcatB). This method can be used alongside single-cell sequencing to visualize the spatial location of brain cells that change in gene expression or splicing patterns in response to specific behaviors or cognitive experiences.

Something old, something new: the origins of an unusual renal cell underpinning a beetle water-conserving mechanism

Tenebrionid beetles have been highly successful in colonising environments where water is scarce, underpinned by their unique osmoregulatory adaptations. These include a cryptonephridial arrangement of their organs, in which part of their renal/Malpighian tubules are bound to the surface of the rectum. Within the cryptonephridial tubules, an unusual cell type, the leptophragmata, plays a key physiological role underpinning water conservation. Nothing was known about the developmental mechanisms or evolution of these unusual renal cells. Here, we investigate mechanisms underpinning leptophragmata development in Tribolium castaneum. We find that leptophragmata express and require the Tiptop transcription factor, similar to secondary renal cells in Drosophila melanogaster, which express Teashirt and Tiptop, despite Drosophila lacking a crypronephridial arrangement. An additional transcription factor, Dachshund, is required to establish leptophragmata identity and to distinguish them from the secondary cells in the non-cryptonephridial region of renal tubule of Tribolium. Dachshund is also expressed in a sub-population of secondary cells in Drosophila. Leptophragmata, which are unique to the beetle lineage, appear to have originated from a specific renal cell type present ancestrally and to be specified by a conserved repertoire of transcription factors.

Knockout of crustacean leg patterning genes suggests that insect wings and body walls evolved from ancient leg segments

The origin of insect wings has long been debated. Central to this debate is whether wings are a novel structure on the body wall resulting from gene co-option, or evolved from an exite (outgrowth; for example, a gill) on the leg of an ancestral crustacean. Here, we report the phenotypes for the knockout of five leg patterning genes in the crustacean Parhyale hawaiensis and compare these with their previously published phenotypes in Drosophila and other insects. This leads to an alignment of insect and crustacean legs that suggests that two leg segments that were present in the common ancestor of insects and crustaceans were incorporated into the insect body wall, moving the proximal exite of the leg dorsally, up onto the back, to later form insect wings. Our results suggest that insect wings are not novel structures, but instead evolved from existing, ancestral structures.

Transcriptional cofactors Ski and SnoN are major regulators of the TGF-β/Smad signaling pathway in health and disease

The transforming growth factor-β (TGF-β) family plays major pleiotropic roles by regulating many physiological processes in development and tissue homeostasis. The TGF-β signaling pathway outcome relies on the control of the spatial and temporal expression of >500 genes, which depend on the functions of the Smad protein along with those of diverse modulators of this signaling pathway, such as transcriptional factors and cofactors. Ski (Sloan-Kettering Institute) and SnoN (Ski novel) are Smad-interacting proteins that negatively regulate the TGF-β signaling pathway by disrupting the formation of R-Smad/Smad4 complexes, as well as by inhibiting Smad association with the p300/CBP coactivators. The Ski and SnoN transcriptional cofactors recruit diverse corepressors and histone deacetylases to repress gene transcription. The TGF-β/Smad pathway and coregulators Ski and SnoN clearly regulate each other through several positive and negative feedback mechanisms. Thus, these cross-regulatory processes finely modify the TGF-β signaling outcome as they control the magnitude and duration of the TGF-β signals. As a result, any alteration in these regulatory mechanisms may lead to disease development. Therefore, the design of targeted therapies to exert tight control of the levels of negative modulators of the TGF-β pathway, such as Ski and SnoN, is critical to restore cell homeostasis under the specific pathological conditions in which these cofactors are deregulated, such as fibrosis and cancer.

Proteins that repress molecular signaling through the transforming growth factor-beta (TGF-β) pathway offer promising targets for treating cancer and fibrosis. Marina Macías-Silva and colleagues from the National Autonomous University of Mexico in Mexico City review the ways in which a pair of proteins, called Ski and SnoN, interact with downstream mediators of TGF-β to inhibit the effects of this master growth factor. Aberrant levels of Ski and SnoN have been linked to diverse range of diseases involving cell proliferation run amok, and therapies that regulate the expression of these proteins could help normalize TGF-β signaling to healthier physiological levels. For decades, drug companies have tried to target the TGF-β pathway, with limited success. Altering the activity of these repressors instead could provide a roundabout way of remedying pathogenic TGF-β activity in fibrosis and oncology.

The APC/C Coordinates Retinal Differentiation with G1 Arrest through the Nek2-Dependent Modulation of Wingless Signaling

The cell cycle is coordinated with differentiation during animal development. Here we report a cell-cycle-independent developmental role for a master cell-cycle regulator, the anaphase-promoting complex or cyclosome (APC/C), in the regulation of cell fate through modulation of Wingless (Wg) signaling. The APC/C controls both cell-cycle progression and postmitotic processes through ubiquitin-dependent proteolysis. Through an RNAi screen in the developing Drosophila eye, we found that partial APC/C inactivation severely inhibits retinal differentiation independently of cell-cycle defects. The differentiation inhibition coincides with hyperactivation of Wg signaling caused by the accumulation of a Wg modulator, Drosophila Nek2 (dNek2). The APC/C degrades dNek2 upon synchronous G1 arrest prior to differentiation, which allows retinal differentiation through local suppression of Wg signaling. We also provide evidence that decapentaplegic signaling may posttranslationally regulate this APC/C function. Thus, the APC/C coordinates cell-fate determination with the cell cycle through the modulation of developmental signaling pathways.

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APC/C inactivation disrupts retinal differentiation in the Drosophila eye

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APC/C inactivation causes the ectopic activation of Wg signaling

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APC/C^Fzr downregulates a Wg modulator, dNek2, by proteolysis upon G1 arrest

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Local dNek2 degradation ensures the coordination of retinal differentiation

dachshund Potentiates Hedgehog Signaling during Drosophila Retinogenesis

Proper organ patterning depends on a tight coordination between cell proliferation and differentiation. The patterning of Drosophila retina occurs both very fast and with high precision. This process is driven by the dynamic changes in signaling activity of the conserved Hedgehog (Hh) pathway, which coordinates cell fate determination, cell cycle and tissue morphogenesis. Here we show that during Drosophila retinogenesis, the retinal determination gene dachshund (dac) is not only a target of the Hh signaling pathway, but is also a modulator of its activity. Using developmental genetics techniques, we demonstrate that dac enhances Hh signaling by promoting the accumulation of the Gli transcription factor Cubitus interruptus (Ci) parallel to or downstream of fused. In the absence of dac, all Hh-mediated events associated to the morphogenetic furrow are delayed. One of the consequences is that, posterior to the furrow, dac- cells cannot activate a Roadkill-Cullin3 negative feedback loop that attenuates Hh signaling and which is necessary for retinal cells to continue normal differentiation. Therefore, dac is part of an essential positive feedback loop in the Hh pathway, guaranteeing the speed and the accuracy of Drosophila retinogenesis.

Molecules of the Hedgehog (Hh) family are involved in the control of many developmental processes in both vertebrates and invertebrates. One of these processes is the formation of the retina in the fruitfly Drosophila. Here, Hh orchestrates a differentiation wave that allows the fast and precise differentiation of the fly retina, by controlling cell cycle, fate and morphogenesis. In this work we identify the gene dachshund (dac) as necessary to potentiate Hh signaling. In its absence, all Hh-dependent processes are delayed and retinal differentiation is severely impaired. Using genetic analysis, we find that dac, a nuclear factor that can bind DNA, is required for the stabilization of the nuclear transducer of the Hh signal, the Gli transcription factor Ci. dac expression is activated by Hh signaling and therefore is a key element in a positive feedback loop within the Hh signaling pathway that ensures a fast and robust differentiation of the retina. The vertebrate dac homologues, the DACH1 and 2 genes, are also important developmental regulators and cancer genes and a potential link between DACH genes and the Hh pathway in vertebrates awaits investigation.

Argonaute: The executor of small RNA function

The discovery of small non-coding RNAs - microRNA (miRNA), short interfering RNA (siRNA) and PIWI-interacting RNA (piRNA) - represents one of the most exciting frontiers in biology specifically on the mechanism of gene regulation. In order to execute their functions, these small RNAs require physical interactions with their protein partners, the Argonaute (AGO) family proteins. Over the years, numerous studies have made tremendous progress on understanding the roles of AGO in gene silencing in various organisms. In this review, we summarize recent progress of AGO-mediated gene silencing and other cellular processes in which AGO proteins have been implicated with a particular focus on progress made in flies, humans and other model organisms as compliment.

The retinal determination gene dachshund restricts cell proliferation by limiting the activity of the Homothorax-Yorkie complex

The Drosophila transcriptional co-activator protein Yorkie and its vertebrate orthologs YAP and TAZ are potent oncogenes, whose activity is normally kept in check by the upstream Hippo kinase module. Upon its translocation into the nucleus, Yorkie forms complexes with several tissue-specific DNA-binding partners, which help to define the tissue-specific target genes of Yorkie. In the progenitor cells of the eye imaginal disc, the DNA-binding transcription factor Homothorax is required for Yorkie-promoted proliferation and survival through regulation of the bantam microRNA (miRNA). The transit from proliferating progenitors to cell cycle quiescent precursors is associated with the progressive loss of Homothorax and gain of Dachshund, a nuclear protein related to the Sno/Ski family of co-repressors. We have identified Dachshund as an inhibitor of Homothorax-Yorkie-mediated cell proliferation. Loss of dachshund induces Yorkie-dependent tissue overgrowth. Conversely, overexpressing dachshund inhibits tissue growth, prevents Yorkie or Homothorax-mediated cell proliferation of disc epithelia and restricts the transcriptional activity of the Yorkie-Homothorax complex on the bantam enhancer in Drosophila cells. In addition, Dachshund collaborates with the Decapentaplegic receptor Thickveins to repress Homothorax and Cyclin B expression in quiescent precursors. The antagonistic roles of Homothorax and Dachshund in Yorkie activity, together with their mutual repression, ensure that progenitor and precursor cells are under distinct proliferation regimes. Based on the crucial role of the human dachshund homolog DACH1 in tumorigenesis, our work suggests that DACH1 might prevent cellular transformation by limiting the oncogenic activity of YAP and/or TAZ.

Nemo phosphorylates Eyes absent and enhances output from the Eya-Sine oculis transcriptional complex during Drosophila retinal determination

The retinal determination gene network comprises a collection of transcription factors that respond to multiple signaling inputs to direct Drosophila eye development. Previous genetic studies have shown that nemo (nmo), a gene encoding a proline-directed serine/threonine kinase, can promote retinal specification through interactions with the retinal determination gene network, although the molecular point of cross-talk was not defined. Here, we report that the Nemo kinase positively and directly regulates Eyes absent (Eya). Genetic assays show that Nmo catalytic activity enhances Eya-mediated ectopic eye formation and potentiates induction of the Eya-Sine oculis (So) transcriptional targets dachshund and lozenge. Biochemical analyses demonstrate that Nmo forms a complex with and phosphorylates Eya at two consensus mitogen-activated protein kinase (MAPK) phosphorylation sites. These same sites appear crucial for Nmo-mediated activation of Eya function in vivo. Thus, we propose that Nmo phosphorylation of Eya potentiates its transactivation function to enhance transcription of Eya-So target genes during eye specification and development.

Conserved role for the Dachshund protein with Drosophila Pax6 homolog Eyeless in insulin expression

Members of the insulin family peptides have conserved roles in the regulation of growth and metabolism in a wide variety of metazoans. The Drosophila genome encodes seven insulin-like peptide genes, dilp1-7, and the most prominent dilps (dilp2, dilp3, and dilp5) are expressed in brain neurosecretory cells known as "insulin-producing cells" (IPCs). Although these dilps are expressed in the same cells, the expression of each dilp is regulated independently. However, the molecular mechanisms that regulate the expression of individual dilps in the IPCs remain largely unknown. Here, we show that Dachshund (Dac), which is a highly conserved nuclear protein, is a critical transcription factor that specifically regulates dilp5 expression. Dac was strongly expressed in IPCs throughout development. dac loss-of-function analyses revealed a severely reduced dilp5 expression level in young larvae. Dac interacted physically with the Drosophila Pax6 homolog Eyeless (Ey), and these proteins synergistically promoted dilp5 expression. In addition, the mammalian homolog of Dac, Dach1/2, facilitated the promoting action of Pax6 on the expression of islet hormone genes in cultured mammalian cells. These observations indicate the conserved role of Dac/Dach in controlling insulin expression in conjunction with Ey/Pax6.

Dorsal eye selector pannier (pnr) suppresses the eye fate to define dorsal margin of the Drosophila eye

Axial patterning is crucial for organogenesis. During Drosophila eye development, dorso-ventral (DV) axis determination is the first lineage restriction event. The eye primordium begins with a default ventral fate, on which the dorsal eye fate is established by expression of the GATA-1 transcription factor pannier (pnr). Earlier, it was suggested that loss of pnr function induces enlargement in the dorsal eye due to ectopic equator formation. Interestingly, we found that in addition to regulating DV patterning, pnr suppresses the eye fate by downregulating the core retinal determination genes eyes absent (eya), sine oculis (so) and dacshund (dac) to define the dorsal eye margin. We found that pnr acts downstream of Ey and affects the retinal determination pathway by suppressing eya. Further analysis of the "eye suppression" function of pnr revealed that this function is likely mediated through suppression of the homeotic gene teashirt (tsh) and is independent of homothorax (hth), a negative regulator of eye. Pnr expression is restricted to the peripodial membrane on the dorsal eye margin, which gives rise to head structures around the eye, and pnr is not expressed in the eye disc proper that forms the retina. Thus, pnr has dual function, during early developmental stages pnr is involved in axial patterning whereas later it promotes the head specific fate. These studies will help in understanding the developmental regulation of boundary formation of the eye field on the dorsal eye margin.

The retinal determination gene eyes absent is regulated by the EGF receptor pathway throughout development in Drosophila

Members of the Eyes absent (Eya) protein family play important roles in tissue specification and patterning by serving as both transcriptional activators and protein tyrosine phosphatases. These activities are often carried out in the context of complexes containing members of the Six and/or Dach families of DNA binding proteins. eyes absent, the founding member of the Eya family is expressed dynamically within several embryonic, larval, and adult tissues of the fruit fly, Drosophila melanogaster. Loss-of-function mutations are known to result in disruptions of the embryonic head and central nervous system as well as the adult brain and visual system, including the compound eyes. In an effort to understand how eya is regulated during development, we have carried out a genetic screen designed to identify genes that lie upstream of eya and govern its expression. We have identified a large number of putative regulators, including members of several signaling pathways. Of particular interest is the identification of both yan/anterior open and pointed, two members of the EGF Receptor (EGFR) signaling cascade. The EGFR pathway is known to regulate the activity of Eya through phosphorylation via MAPK. Our findings suggest that this pathway is also used to influence eya transcriptional levels. Together these mechanisms provide a route for greater precision in regulating a factor that is critical for the formation of a wide range of diverse tissues.

The molecular circuitry governing retinal determination

The developing eye of the fruit fly, Drosophila melanogaster, has become a premier model system for studying the genetic and molecular mechanisms that govern tissue determination. Over the last fifteen years a regulatory circuit consisting of the members of the Pax, Six, Eya and Dach gene families has been identified and shown to govern the specification of a wide range of tissues including the retina of both insects and mammals. These genes are not organized in a simple developmental pathway or cascade in which there is a unidirectional flow of information. Rather, there are multiple feedback loops built into the system rendering its appearance and functionality more in line with the workings of a network. In this review I will attempt to describe the genetic, molecular and biochemical interactions that govern the specification of the Drosophila compound eye. In particular, the primary focus will be on the interactions that have been experimentally verified at the molecular and biochemical levels. During the course of this description I will also attempt to place each discovery in its own historical context. While a number of signaling pathways play significant roles in early eye development this review will focus on the network of nuclear factors that promote retinal determination.

CLOCK expression identifies developing circadian oscillator neurons in the brains of Drosophila embryos

BACKGROUND

The Drosophila circadian oscillator is composed of transcriptional feedback loops in which CLOCK-CYCLE (CLK-CYC) heterodimers activate their feedback regulators period (per) and timeless (tim) via E-box mediated transcription. These feedback loop oscillators are present in distinct clusters of dorsal and lateral neurons in the adult brain, but how this pattern of expression is established during development is not known. Since CLK is required to initiate feedback loop function, defining the pattern of CLK expression in embryos and larvae will shed light on oscillator neuron development.

RESULTS

A novel CLK antiserum is used to show that CLK expression in the larval CNS and adult brain is limited to circadian oscillator cells. CLK is initially expressed in presumptive small ventral lateral neurons (s-LNvs), dorsal neurons 2 s (DN2s), and dorsal neuron 1 s (DN1s) at embryonic stage (ES) 16, and this CLK expression pattern persists through larval development. PER then accumulates in all CLK-expressing cells except presumptive DN2s during late ES 16 and ES 17, consistent with the delayed accumulation of PER in adult oscillator neurons and antiphase cycling of PER in larval DN2s. PER is also expressed in non-CLK-expressing cells in the embryonic CNS starting at ES 12. Although PER expression in CLK-negative cells continues in ClkJrk embryos, PER expression in cells that co-express PER and CLK is eliminated.

CONCLUSION

These data demonstrate that brain oscillator neurons begin development during embryogenesis, that PER expression in non-oscillator cells is CLK-independent, and that oscillator phase is an intrinsic characteristic of brain oscillator neurons. These results define the temporal and spatial coordinates of factors that initiate Clk expression, imply that circadian photoreceptors are not activated until the end of embryogenesis, and suggest that PER functions in a different capacity before oscillator cell development is initiated.

Evidence for a novel role for dachshund in patterning the proximal arthropod leg

The branchiopod crustacean Triops longicaudatus has paddlelike thoracic appendages with few joints and multiple marginal lobes. Here, we explore the degree to which the Triops limb is patterned by the same network of genes known to pattern the uniramous, multi-jointed insect appendage. Insect leg patterning proceeds through a process of subdividing the leg into proximal, intermediate, and distal regions by the activity of the transcription factors hth/exd, dac, and Dll. The immature Triops limb is subdivided into large, discrete regional domains (proximal and distal) as defined by nuclear-EXD and DLL. We show that HTH expression in Triops overlaps cell-to-cell with n-EXD expression. In addition, dac is expressed in two domains: (1) adjacent to and partially overlapping the distal Dll domain and (2) along the medial margin of the developing leg. The DAC domain adjacent to the distal Dll domain supports the early establishment of the expected intermediate domain of DAC expression. The medial expression domain resolves over time into a series of reiterated stripes located on the lower side of each medial lobe. Later, this expression pattern correlates with the sclerotized regions associated with limb flexion. We propose that these stripes of DAC expression play a role in forming reiterated medial lobes. Unlike Drosophila, where the proximal distal patterning of the leg is coincident with patterning of reiterated structures (segments), we hypothesize that the patterning in Triops may reflect an ancestral state where the patterning of reiterated medial structures was not coincident with proximodistal limb patterning.

How Drosophila change their combs: the Hox gene Sex combs reduced and sex comb variation among Sophophora species

Identification of the events responsible for rapid morphological variation during evolution can help understand how developmental processes are changed by genetic modifications and thus produce diverse body features and shapes. Sex combs, a sexually dimorphic structure, show considerable variation in morphology and numbers among males from related species of Sophophora, a subgenus of Drosophila. To address which evolutionary changes in developmental processes underlie this diversity, we first analyzed the genetic network that controls morphogenesis of a single sex comb in the model D. melanogaster. We show that it depends on positive and negative regulatory inputs from proximo-distal identity specifying genes, including dachshund, bric à brac, and sex combs distal. All contribute to spatial regulation of the Hox gene Sex combs reduced (Scr), which is crucial for comb formation. We next analyzed the expression of these genes in sexually dimorphic species with different comb numbers. Only Scr shows considerable expression plasticity, which is correlated with comb number variation in these species. We suggest that differences in comb numbers reflect changes of Scr expression in tarsus primordia, and discuss how initial comb formation could have occurred in an ancestral Sophophora fly following regulatory modifications of developmental programs both parallel to and downstream of Scr.

The eyes absent family of phosphotyrosine phosphatases: properties and roles in developmental regulation of transcription

Integration of multiple signaling pathways at the level of their transcriptional effectors provides an important strategy for fine-tuning gene expression and ensuring a proper program of development. Posttranslational modifications, such as phosphorylation, play important roles in modulating transcription factor activity. The discovery that the transcription factor Eyes absent (Eya) possesses protein phosphatase activity provides an interesting new paradigm. Eya may regulate the phosphorylation state of either itself or its transcriptional cofactors, thereby directly affecting transcriptional output. The identification of a growing number of transcription factors with enzymic activity suggests that such dual-function proteins exert greater control of signaling events than previously imagined. Given the conservation of both its phosphatase and transcription factor activity across mammalian species, Eya provides an excellent model for studying how a single protein integrates these two functions under the influence of multiple signaling pathways to promote development.

Ancient mechanisms of visual sense organ development based on comparison of the gene networks controlling larval eye, ocellus, and compound eye specification in Drosophila

Key mechanisms of development are strongly constrained, and hence often shared in the formation of highly diversified homologous organs. This diagnostic is applied to uncovering ancient gene activities in the control of visual sense organ development by comparing the gene networks, which regulate larval eye, ocellus and compound eye specification in Drosophila. The comparison reveals a suite of shared aspects that are likely to predate the diversification of arthropod visual sense organs and, consistent with this, have notable similarities in the developing vertebrate visual system: (I) Pax-6 genes participate in the patterning of primordia of complex visual organs. (II) Primordium determination and differentiation depends on formation of a transcription factor complex that contains the products of the selector genes Eyes absent and Sine oculis. (III) The TGF-beta signaling factor Decapentaplegic exerts transcriptional activation of eyes absent and sine oculis. (IV) Canonical Wnt signaling contributes to primordium patterning by repression of eyes absent and sine oculis. (V) Initiation of determination and differentiation is controlled by hedgehog signaling. (VI) Egfr signaling drives retinal cell fate specification. (VII) The proneural transcription factor atonal regulates photoreceptor specification. (VII) The zinc finger gene glass regulates photoreceptor specification and differentiation.

Conservation, innovation, and the evolution of horned beetle diversity

Beetle horns represent an evolutionary novelty exhibiting remarkable diversity above and below the species level. Here, we show that four typical appendage patterning genes, extradenticle (exd), homothorax (hth), dachshund (dac), and Distal-less (Dll) are expressed in the context of the development of sexually dimorphic thoracic horns in three Onthophagus species. At least two of these genes, Dll and hth, exhibited expression patterns consistent with a conservation of patterning function during horn development relative to their known roles in the development of insect legs. exd, hth, and dac expression patterns during horn development were largely invariable across species or sexes within species. In contrast, Dll expression was far more discrete and exhibited consistent differences between sexes and species. Most importantly, differences in location and domain size of Dll expression tightly correlated with the degree to which prepupal horn primordia were retained or resorbed before the final adult molt. Our results lend further support to the hypothesis that the origin of beetle horns relied, at least in part, on the redeployment of already existing developmental mechanisms, such as appendage patterning processes and that changes in the exact location and domain size of Dll expression may represent important modifier mechanisms that modulate horn expression in different species or sexes. If correct, this would imply that certain components of genetic basis of horn development may be able to diversify rapidly within lineages and largely independent of phylogenetic distance. We present a first model that integrates presently available data on the genetic regulation of horn development and diversity.

Mouse Dach2 mutants do not exhibit gross defects in eye development or brain function

Drosophila dachshund is a critical regulator of eye, brain, and limb formation. Vertebrate homologs, Dach1 and Dach2, are expressed in the developing retina, brain, and limbs, suggesting functional conservation of the dachshund/Dach gene family. Dach1 mutants die postnatally, but exhibit grossly normal development. Here we report the generation of Dach2 mutant mice. Although deletion of Dach2 exon 1 results in abrogation of RNA expression, Dach2 mutants are viable and fertile. Histochemical analysis reveals grossly normal Dach2 mutant eye development. In addition, a battery of neurological assays failed to yield significant differences in behavior between Dach2 mutants and controls. We discuss these findings in the light of published observations of DACH2 mutations in the human population. Finally, to test the functional conservation hypothesis, we generated Dach2; Dach1 double mutant mice. Dach double mutants die after birth, similar to Dach1 homozygotes. However, unlike Drosophila dachshund mutants that lack eyes and exhibit leg truncations, the eyes and limbs of Dach double mutants are present, suggesting differences between Dach and dachshund gene function during embryonic eye and limb formation.

glial cells missing and gcm2 cell autonomously regulate both glial and neuronal development in the visual system of Drosophila

The transcription factors Glial cells missing (Gcm) and Gcm2 are known to play a crucial role in promoting glial-cell differentiation during Drosophila embryogenesis. Our findings reveal a central function for gcm genes in regulating neuronal development in the postembryonic visual system. We demonstrate that Gcm and Gcm2 are expressed in both glial and neuronal precursors within the optic lobe. Removal of gcm and gcm2 function shows that the two genes act redundantly and are required for the formation of a subset of glial cells. They also cell-autonomously control the differentiation and proliferation of specific neurons. We show that the transcriptional regulator Dachshund acts downstream of gcm genes and is required to make lamina precursor cells and lamina neurons competent for neuronal differentiation through regulation of epidermal growth factor receptor levels. Our findings further suggest that gcm genes regulate neurogenesis through collaboration with the Hedgehog-signaling pathway.

Dual regulation and redundant function of two eye-specific enhancers of the Drosophila retinal determination gene dachshund

Drosophila eye development is controlled by a conserved network of retinal determination (RD) genes. The RD genes encode nuclear proteins that form complexes and function in concert with extracellular signal-regulated transcription factors. Identification of the genomic regulatory elements that govern the eye-specific expression of the RD genes will allow us to better understand how spatial and temporal control of gene expression occurs during early eye development. We compared conserved non-coding sequences (CNCSs) between five Drosophilids along the approximately 40 kb genomic locus of the RD gene dachshund (dac). Our analysis uncovers two separate eye enhancers in intron eight and the 3' non-coding regions of the dac locus defined by clusters of highly conserved sequences. Loss- and gain-of-function analyses suggest that the 3' eye enhancer is synergistically activated by a combination of eya, so and dpp signaling, and only indirectly activated by ey, whereas the 5' eye enhancer is primarily regulated by ey, acting in concert with eya and so. Disrupting conserved So-binding sites in the 3' eye enhancer prevents reporter expression in vivo. Our results suggest that the two eye enhancers act redundantly and in concert with each other to integrate distinct upstream inputs and direct the eye-specific expression of dac.

Identification of thermosensory and olfactory neuron-specific genes via expression profiling of single neuron types

Most C. elegans sensory neuron types consist of a single bilateral pair of neurons, and respond to a unique set of sensory stimuli. Although genes required for the development and function of individual sensory neuron types have been identified in forward genetic screens, these approaches are unlikely to identify genes that when mutated result in subtle or pleiotropic phenotypes. Here, we describe a complementary approach to identify sensory neuron type-specific genes via microarray analysis using RNA from sorted AWB olfactory and AFD thermosensory neurons. The expression patterns of subsets of these genes were further verified in vivo. Genes identified by this analysis encode 7-transmembrane receptors, kinases, and nuclear factors including dac-1, which encodes a homolog of the highly conserved Dachshund protein. dac-1 is expressed in a subset of sensory neurons including the AFD neurons and is regulated by the TTX-1 OTX homeodomain protein. On thermal gradients, dac-1 mutants fail to suppress a cryophilic drive but continue to track isotherms at the cultivation temperature, representing the first genetic separation of these AFD-mediated behaviors. Expression profiling of single neuron types provides a rapid, powerful, and unbiased method for identifying neuron-specific genes whose functions can then be investigated in vivo.

Signaling circuitries in development: insights from the retinal determination gene network

Context-specific integration of information received from the Notch, Transforming growth factor beta, Wingless/Wnt, Hedgehog and Epidermal growth factor receptor signaling pathways sets the stage for deployment of the retinal determination gene network (RDGN), a group of transcription factors that collectively directs the formation of the eye and other tissues. Recent investigations have revealed how these transcription factors are regulated by their interactions with each other and with effectors of the above signaling pathways. Further study of the RDGN may provide insights into how common cues can generate context-specific responses, a key aspect of developmental regulation that remains poorly understood.