Sensory Organ Formation Research Articles

Emc has a role in dorsal appendage fate formation in Drosophila oogenesis

extramacrochaetae ( emc) functions during many developmental processes in Drosophila, such as sensory organ formation, sex determination, wing vein differentiation, regulation of eye photoreceptor differentiation, cell proliferation and development of the Malpighian tubules, trachea and muscles in the embryo. It encodes a Helix-Loop-Helix transcription factor that negatively regulates bHLH proteins. We show here that emc mRNA and protein are present throughout oogenesis in a dynamic expression pattern and that emc is involved in the regulation of chorionic appendage formation during late oogenesis. Expression of sense and antisense emc constructs as well as emc follicle cell clones leads to eggs with shorter, thicker dorsal appendages that are closer together at base than in the wild type. We demonstrate that emc lies downstream of fs( 1) K10, gurken and EGFR in the Grk/EGFR signalling pathway and that it participates in controlling Broad-Complex expression at late stages of oogenesis.

The three‐sided romance of the lateral line: Glia love axons love precursors love glia

The lateral line system of fish and amphibians is closely related to the inner ear in terms of evolution, morphology and physiology. Several recent papers have shed new light on the postembryonic development of this system, and have revealed an unexpected triangular relationship where migrating sensory precursors guide axons, axons guide glia and glia, in turn, control the formation of sensory organs. They have also revealed the crucial importance of controlled cell migration not only for patterning the system, but also for determining polarity (and therefore directional sensitivity) of the mechanosensory hair cells. The remarkable accessibility of the lateral line system may allow a detailed analysis of cell migration and polarization, and may help us better understand the complex interactions between sensory precursor cells, neurons and glia during development.

Charlatan, a Zn-finger transcription factor, establishes a novel level of regulation of the proneuralachaete/scutegenes ofDrosophila

The proneural genes achaete (ac) and scute (sc) are necessary for the formation of the external sensory organs (SOs) of Drosophila. ac and sc are expressed in proneural clusters and impart their cells with neural potential. For this potential to be realized, and the SO precursor cell (SOP) to arise within a cluster, sufficient proneural protein must accumulate in the cluster. Here we describe a novel gene, charlatan (chn), which encodes a zinc finger transcription factor that facilitates this accumulation by forming a stimulatory loop with ac/sc. We find that loss of function of chn decreases the accumulation of Sc in proneural clusters and partially removes notum macrochaetae, while overexpression of chn enhances ac/sc expression and the formation of extra SOs. Moreover, chn is activated by ac/sc in proneural clusters. Chn apparently stimulates ac/sc by physically interacting with the proneural cluster-specific enhancers and increasing enhancer efficiency, thus acting as a stimulator of ac/sc expression in proneural clusters. chn is also required for the proper development of the embryonic peripheral nervous system, as its absence leads to loss of neurons and causes aberrant development of chordotonal organs.

Cullin-3 regulates pattern formation, external sensory organ development and cell survival during Drosophila development

Ubiquitin-mediated proteolysis regulates the steady-state abundance of proteins and controls cellular homoeostasis by abrupt elimination of key effector proteins. A multienzyme system targets proteins for destruction through the covalent attachment of a multiubiquitin chain. The specificity and timing of protein ubiquitination is controlled by ubiquitin ligases, such as the Skp1-Cullin-F box protein complex. Cullins are major components of SCF complexes, and have been implicated in degradation of key regulatory molecules including Cyclin E, β-catenin and Cubitus interruptus. Here, we describe the genetic identification and molecular characterisation of the Drosophila Cullin-3 homologue. Perturbation of Cullin-3 function has pleiotropic effects during development, including defects in external sensory organ development, pattern formation and cell growth and survival. Loss or overexpression of Cullin-3 causes an increase or decrease, respectively, in external sensory organ formation, implicating Cullin-3 function in regulating the commitment of cells to the neural fate. We also find that Cullin-3 function modulates Hedgehog signalling by regulating the stability of full-length Cubitus interruptus (Ci155). Loss of Cullin-3 function in eye discs but not other imaginal discs promotes cell-autonomous accumulation of Ci155. Conversely, overexpression of Cullin-3 results in a cell-autonomous stabilisation of Ci155 in wing, haltere and leg, but not eye, imaginal discs suggesting tissue-specific regulation of Cullin-3 function. The diverse nature of Cullin-3 phenotypes highlights the importance of targeted proteolysis during Drosophila development.

A conserved developmental program for sensory organ formation in Drosophila melanogaster.

Different sensory organs, such the eye and ear, are widely thought to have separate origins, guided by distinct organ-specific factors that direct all aspects of their development. Previous studies of the D. melanogaster gene eyeless (ey) and its vertebrate homolog Pax6 suggested that this gene acts in such a manner and specifically drives eye development. But diverse sensory organs might instead arise by segment-specific modification of a developmental program that is involved more generally in sensory organ formation. In D. melanogaster, a common proneural gene called atonal (ato) functions in the initial process of development of a number of segment-specific organs, including the compound eye, the auditory organ and the stretch receptor, suggesting that these organs share an evolutionary origin. Here we show that D. melanogaster segment-specific sensory organs form through the integration of decapentaplegic (dpp), wingless (wg) and ecdysone signals into a single cis-regulatory element of ato. The induction of ectopic eyes by ey also depends on these signals for ato expression, and the ey mutant eye imaginal disc allows ato expression if cell death is blocked. These results imply that ey does not induce the entire eye morphogenetic program but rather modifies ato-dependent neuronal development. Our findings strongly suggest that various sensory organs evolved from an ato-dependent protosensory organ through segment specification by ey and Hox genes.

Homologs of wingless and decapentaplegic display a complex and dynamic expression profile during appendage development in the millipede Glomeris marginata (Myriapoda: Diplopoda)

BackgroundThe Drosophila genes wingless (wg) and decapentaplegic (dpp) comprise the top level of a hierarchical gene cascade involved in proximal-distal (PD) patterning of the legs. It remains unclear, whether this cascade is common to the appendages of all arthropods. Here, wg and dpp are studied in the millipede Glomeris marginata, a representative of the Myriapoda.ResultsGlomeris wg (Gm-wg) is expressed along the ventral side of the appendages compatible with functioning during the patterning of both the PD and dorsal-ventral (DV) axes. Gm-wg may also be involved in sensory organ formation in the gnathal appendages by inducing the expression of Distal-less (Dll) and H15 in the organ primordia. Expression of Glomeris dpp (Gm-dpp) is found at the tip of the trunk legs as well as weakly along the dorsal side of the legs in early stages. Taking data from other arthropods into account, these results may be interpreted in favor of a conserved mode of WG/DPP signaling. Apart from the main PD axis, many arthropod appendages have additional branches (e.g. endites). It is debated whether these extra branches develop their PD axis via the same mechanism as the main PD axis, or whether branch-specific mechanisms exist. Gene expression in possible endite homologs in Glomeris argues for the latter alternative.ConclusionAll available data argue in favor of a conserved role of WG/DPP morphogen gradients in guiding the development of the main PD axis. Additional branches in multibranched (multiramous) appendage types apparently do not utilize the WG/DPP signaling system for their PD development. This further supports recent work on crustaceans and insects, that lead to similar conclusions.

Mild mutations in the pan neural gene prospero affect male-specific behaviour in Drosophila melanogaster.

The fruitfly Drosophila melanogaster is one of the most appropriate model organisms to study the genetics of behaviour. Here, we focus on prospero (pros), a key gene for the development of the nervous system which specifies multiple aspects from the early formation of the embryonic central nervous system to the formation of larval and adult sensory organs. We studied the effects on locomotion, courtship and mating behaviour of three mild pros mutations. These newly isolated pros mutations were induced after the incomplete excision of a transposable genomic element that, before excision, caused a lethal phenotype during larval development. Strikingly, these mutant strains, but not the strains with a clean excision, produced a high frequency of heterozygous flies, after more than 50 generations in the lab. We investigated the factors that could decrease the fitness of homozygotes relatively to heterozygous pros mutant flies. Flies of both genotypes had slightly different levels of fertility. More strikingly, homozygous mutant males had a lower sexual activity than heterozygous males and failed to mate in a competitive situation. No similar effect was detected in mutant females. These findings suggest that mild mutations in pros did not alter vital functions during development but drastically changed adult male behaviour and reproductive fitness.

Regulation of dally, an Integral Membrane Proteoglycan, and Its Function during Adult Sensory Organ Formation of Drosophila

In Drosophila, imaginal wing discs, Wg and Dpp, play important roles in the development of sensory organs. These secreted growth factors govern the positions of sensory bristles by regulating the expression of achaete-scute (ac-sc), genes affecting neuronal precursor cell identity. Earlier studies have shown that Dally, an integral membrane, heparan sulfate-modified proteoglycan, affects both Wg and Dpp signaling in a tissue-specific manner. Here, we show that dally is required for the development of specific chemosensory and mechanosensory organs in the wing and notum. dally enhancer trap is expressed at the anteroposterior and dorsoventral boundaries of the wing pouch, under the control of hh and wg, respectively. dally affects the specification of proneural clusters for dally-sensitive bristles and shows genetic interactions with either wg or dpp signaling components for distinct sensory bristles. These findings suggest that dally can differentially regulate Wg- or Dpp-directed patterning during sensory organ assembly. We have also determined that, for pSA, a bristle on the lateral notum, dally shows genetic interactions with iroquois complex (IRO-C), a gene complex affecting ac-sc expression. Consistent with this interaction, dally mutants show markedly reduced expression of an iro::lacZ reporter. These findings establish dally as an important regulator of sensory organ formation via Wg- and Dpp-mediated specification of proneural clusters.

Confocal imaging of sensory organ formation during Xenopus inner ear development

During Xenopus larval development, the otic vesicle gradually gives rise to several auditory and vestibular sensory organs that are innervated by the eighth cranial nerve. Confocal imaging methods were used to examine the appearance of stereociliary bundles in the various sensory epithelia during Xenopus development (stages 28–50), and to gather comparative developmental data that show how the inner ear gradually forms distinct auditory and vestibular compartments discernable by larval stage 50. A BioRad MRC-1024 confocal microscope was used to collect digitized images from inner ears labeled with Alexa 488 phalloidin to detect the actin cytoskeleton, and propidium iodide to highlight DNA in cell nuclei. Confocal images from whole inner ears clearly show the developmental emergence of uniquely organized sensory epithelia populated by hair cell bundles of varying morphology, as well as the position of the sensory ganglia relative to the endorgans. Images from sectioned tissue provide additional information about cell morphology and the arrangement of the sensory epithelium in relation to neural innervation. Taken together, the image data illustrate the complex structural changes that underlie inner ear organogenesis and morphogenesis. [Work supported by grants to EES (NIH NIGMS, NASA) and awards to QQ (NASA NMSGC Fellowship, NIGMS RISE).]

Senseless, a Zn Finger Transcription Factor, Is Necessary and Sufficient for Sensory Organ Development in Drosophila

The senseless ( sens) gene is required for proper development of most cell types of the embryonic and adult peripheral nervous system (PNS) of Drosophila. Sens is a nuclear protein with four Zn fingers that is expressed and required in the sensory organ precursors (SOP) for proper proneural gene expression. Ectopic expression of Sens in many ectodermal cells causes induction of PNS external sensory organ formation and is able to recreate an ectopic proneural field. Hence, sens is both necessary and sufficient for PNS development. Our data indicate that proneural genes activate sens expression. Sens is then in turn required to further activate and maintain proneural gene expression. This feedback mechanism is essential for selective enhancement and maintenance of proneural gene expression in the SOPs.

Identification of a novel mouse Iroquois homeobox gene,Irx5, and chromosomal localisation of all members of the mouse Iroquois gene family

The Drosophila genes of the Iroquois-Complex encode homeodomain containing transcription factors that positively regulate the activity of certain proneural Achaete/Scute-C (AS-C) genes during the formation of external sensory organs (J. L. Gomez-Skarmeta and J. Modolell, EMBO J 17:181-190, 1996). Previously, we have identified three highly-related genes of the mouse Iroquois gene family that exert specific expression patterns in the central nervous system (A. Bosse et al., Mech Dev 69:169-181, 1997). In the present paper, we report the identification of a novel member of the Iroquois gene family, Irx5, that shows a restricted spatio/temporal expression during early mouse embryogenesis, distinct from the expression of Irx1-3. An extensive sequence analysis of 20 Iroquois-like genes from seven organisms reveals a high conservation of the homeodomain. Phylogenetic tree reconstruction showed a clustering of the members of the Iroquois gene family into groups of orthologous genes. Together, with the data obtained from the chromosomal mapping analysis, the results indicate that these genes have appeared in vertebrates during evolution as a result of gene duplication.

Two murine and human homologs of mab-21, a cell fate determination gene involved in Caenorhabditis elegans neural development.

We report the cloning and genetic characterization of one human and two murine homologs of the mab-21 cell fate specification gene. mab-21 participates in the formation of sensory organs in the male nematode tail, and is essential for other developmental functions elsewhere in the Caenorhabditis elegans embryo. The expanding mab-21 gene family, which is strikingly conserved in evolution, includes two putative Drosophila members. The two mammalian genes, encoding 41 kDa nuclear basic proteins, are expressed in partially overlapping territories in the embryonic brain, eye and limbs, as well as in neural crest derivatives. Recent genetic data implicating mab-21 as a downstream target of TGF-beta signaling, together with the distribution of mab-21 transcripts in the mouse embryo, propose these novel genes as relevant factors in various aspects of vertebrate neural development.

Mirror, a Drosophila homeobox gene in the iroquois complex, is required for sensory organ and alula formation

The Drosophila notum, the dorsal body wall of the thorax, is subdivided genetically into longitudinal domains (Calleja, M., Moreno, E., Pelaz, S. and Morata, G. (1996) Science 274, 252-255). Two homeobox genes clustered in the iroquois complex, araucan and caupolican, regulate proneural genes and are required for development of sensory bristles in the lateral notum (Gomez-Skarmeta, J. L., del Corral, R. D., de la Calle-Mustienes, E., Ferres-Marco, D. and Modolell, J. (1996) Cell 85, 95-105). An iroquois-related homeobox gene, mirror, was recently isolated and is localized close to the iroquois complex region (McNeil, H., Yang, C.-H., Brodsky, M., Ungos, J. and Simon, M. A. (1997) Genes and Development 11, 1073-1082; this study). We show that mirror is required for the formation of the alula and a subset of sensory bristles in the lateral domain of the notum. Genetic analysis suggests that mirror and the other iroquois genes interact to form the alula as well as the sensory organs. Based on similarities between mirror and the iroquois genes in their genetic map positions, expression, protein structure and function, mirror is considered a new member of the iroquois complex and is involved in prepatterning sensory precursor cells in the lateral notum.

Axial specification for sensory organs versus non-sensory structures of the chicken inner ear.

A mature inner ear is a complex labyrinth containing multiple sensory organs and nonsensory structures in a fixed configuration. Any perturbation in the structure of the labyrinth will undoubtedly lead to functional deficits. Therefore, it is important to understand molecularly how and when the position of each inner ear component is determined during development. To address this issue, each axis of the otocyst (embryonic day 2.5, E2.5, stage 16-17) was changed systematically at an age when axial information of the inner ear is predicted to be fixed based on gene expression patterns. Transplanted inner ears were analyzed at E4.5 for gene expression of BMP4 (bone morphogenetic protein), SOHo-1 (sensory organ homeobox-1), Otx1 (cognate of Drosophila orthodenticle gene), p75NGFR (nerve growth factor receptor) and Msx1 (muscle segment homeobox), or at E9 for their gross anatomy and sensory organ formation. Our results showed that axial specification in the chick inner ear occurs later than expected and patterning of sensory organs in the inner ear was first specified along the anterior/posterior (A/P) axis, followed by the dorsal/ventral (D/V) axis. Whereas the A/P axis of the sensory organs was fixed at the time of transplantation, the A/P axis for most non-sensory structures was not and was able to be re-specified according to the new axial information from the host. The D/V axis for the inner ear was not fixed at the time of transplantation. The asynchronous specification of the A/P and D/V axes of the chick inner ear suggests that sensory organ formation is a multi-step phenomenon, rather than a single inductive event.

Identification of the vertebrate Iroquois homeobox gene family with overlapping expression during early development of the nervous system

In Drosophila the decision processes between the neural and epidermal fate for equipotent ectodermal cells depend on the activity of proneural genes. Members of the Drosophila Iroquois-Complex (Iro-C) positively regulate the activity of certain proneural AS-C genes during the formation of external sensory organs. We have identified and characterized three mouse Iroquois-related genes: Irx1, -2 and -3, which have a homeodomain very similar to that of the Drosophila Iro-C genes. The sequence similarity implies that these three genes represent a separate homeobox family. All three genes are expressed with distinct spatio/temporal patterns during early mouse embryogenesis. These patterns implicate them in a number of embryonic developmental processes: the A/P and D/V patterning of specific regions of the central nervous system (CNS), and regionalization of the otic vesicle, branchial epithelium and limbs.

Neuronal type information encoded in the basic-helix-loop-helix domain of proneural genes.

The proneural genes encode basic-helix-loop-helix (bHLH) proteins and promote the formation of distinct types of sensory organs. In Drosophila, two sets of proneural genes, atonal (ato) and members of the achaete-scute complex (ASC), are required for the formation of chordotonal (ch) organs and external sensory (es) organs, respectively. We assayed the production of sensory organs in transgenic flies expressing chimeric genes of ato and scute (sc), a member of ASC, and found that the information that specifies ch organs resides in the bHLH domain of ato; chimeras containing the b domain of ato and the HLH domain of sc also induced ch organ formation, but to a lesser extent than those containing the bHLH domain of ato. The b domains of ato and sc differ in seven residues. Mutations of these seven residues in the b domain of ato suggest that most or perhaps all of these residues are required for induction of ch organs. None of these seven residues is predicted to contact DNA directly by computer simulation using the structure of the myogenic factor MyoD as a model, implying that interaction of ato with other cofactors is likely to be involved in neuronal type specification.

Differential Expression of Bone Morphogenetic Proteins in the Developing Vestibular and Auditory Sensory Organs

The genes responsible for the formation of various sensory organs in the inner ear are not known. There are eight sensory organs in the chick inner ear, and our previous study showed that all presumptive sensory organs initially express bone morphogenetic protein 4 (BMP4), a member of the transforming growth factor (TGF)-beta gene family. To address the potential role of BMPs in the patterning of different sensory organ structures, we investigated the expression of BMP4, BMP5, and BMP7 during sensory organ differentiation in the chick inner ear. The gene expression pattern of BMP5, although similar to that of BMP4, was transient and disappeared by embryonic day 3.5 (E3.5). In contrast, BMP7 gene expression was quite extensive, starting in the otic placode. By E5, gene expression patterns of BMP4 and BMP7 differed among vestibular and auditory sensory organs. In the vestibular sensory organs, BMP7 gene expression segregated from the main sensory tissue areas at the onset of differentiation, whereas BMP4 expression concentrated in supporting cells. In the cochlea, however, BMP7 gene expression became restricted to sensory tissue over time and eventually concentrated in supporting cells, whereas BMP4 gene expression was localized to hair cells. The different BMP expression patterns in developing auditory and vestibular sensory organs may help to shape each respective sensory structure. Furthermore, the expression of BMP4 in the cochlea also revealed an interesting pattern of sensory cell differentiation: the distal portion of the cochlea differentiates first, and the tall hair cells develop before the short hair cells.

Sensory organ formation and the equivalence group in ascidian embryos.

The cell fate determination in ascidian embryos is mediated by various cell interactions. Two sensory pigment cell precursors constitute an equivalence group, in which the ocellus (eye) pathway is the dominant fate. It is suggested that the unique cell interaction, which is called lateral inhibition, is involved in determination events of these equivalent cells. It is likely that the mode of specification which utilizes the lateral inhibition within equivalent cells plays a fundamental role during early embryogenesis in both invertebrates and vertebrates.

Evolutionary conservation of a cell fate specification gene: the Hydra achaete-scute homolog has proneural activity in Drosophila

Members of the Achaete-scute family of basic helix-loop-helix transcription factors are involved in cell fate specification in vertebrates and invertebrates. We have isolated and characterized a cnidarian achaete-scute homolog, CnASH, from Hydra vulgaris, a representative of an evolutionarily ancient branch of metazoans. There is a single achaete-scute gene in Hydra, and the bHLH domain of the predicted gene product shares a high degree of amino acid sequence similarity with those of vertebrate and Drosophila Achaete-scute proteins. In Hydra, CnASH is expressed in a subset of the interstitial cells as well as differentiation intermediates of the nematocyte pathways. In vitro translated CnASH protein can form heterodimers with the Drosophila bHLH protein Daughterless, and these dimers bind to consensus Achaete-scute DNA binding sites in a sequence-specific manner. Ectopic expression of CnASH in wild-type late third instar Drosophila larvae and early pupae leads to the formation of ectopic sensory organs, mimicking the effect of ectopic expression of the endogenous achaete-scute genes. Expression of CnASH in flies that are achaete and scute double mutants gives partial rescue of the mutant phenotype, comparable to the degree of rescue obtained by ectopic expression of the Drosophila genes. These results indicate that the achaete-scute type of bHLH genes for cell fate specification, as well as their mode of action, arose early and have been conserved during metazoan evolution.

Hairy function as a DNA-binding helix-loop-helix repressor of Drosophila sensory organ formation.

Sensory organ formation in Drosophila is activated by proneural genes that encode basic-helix-loop-helix (bHLH) transcription factors. These genes are antagonized by hairy and other proline-bHLH proteins. hairy has not been shown to bind to DNA and has been proposed to form inactive heterodimers with proneural activator proteins. Here, we show that hairy does bind to DNA and has novel DNA-binding activity: hairy prefers a noncanonical site, CACGCG, although it also binds to related sites. Mutation of a single CACGCG site in the achaete (ac) proneural gene blocks hairy-mediated repression of ac transcription in cultured Drosophila cells. Moreover, the same CACGCG mutation in an ac minigene transformed into Drosophila creates ectopic sensory hair organs like those seen in hairy mutants. Together these results indicate that hairy represses sensory organ formation by directly repressing transcription of the ac proneural gene.