Functional identity of hypothalamic melanocortin neurons depends on Tbx3

Heterogeneous populations of hypothalamic neurons orchestrate energy balance via the release of specific signatures of neuropeptides. However, how specific intracellular machinery controls peptidergic identities and function of individual hypothalamic neurons remains largely unknown. The transcription factor T-box 3 (Tbx3) is expressed in hypothalamic neurons sensing and governing energy status, whereas human TBX3 haploinsufficiency has been linked with obesity. Here, we demonstrate that loss of Tbx3 function in hypothalamic neurons causes weight gain and other metabolic disturbances by disrupting both the peptidergic identity and plasticity of Pomc/Cart and Agrp/Npy neurons. These alterations are observed after loss of Tbx3 in both immature hypothalamic neurons and terminally differentiated mouse neurons. We further establish the importance of Tbx3 for body weight regulation in Drosophila melanogaster and show that TBX3 is implicated in the differentiation of human embryonic stem cells into hypothalamic Pomc neurons. Our data indicate that Tbx3 directs the terminal specification of neurons as functional components of the melanocortin system and is required for maintaining their peptidergic identity. In summary, we report the discovery of a key mechanistic process underlying the functional heterogeneity of hypothalamic neurons governing body weight and systemic metabolism.

Fold formation at the compartment boundary of Drosophila wing requires Yki signaling to suppress JNK dependent apoptosis

Compartment boundaries prevent cell populations of different lineage from intermingling. In many cases, compartment boundaries are associated with morphological folds. However, in the Drosophila wing imaginal disc, fold formation at the anterior/posterior (A/P) compartment boundary is suppressed, probably as a prerequisite for the formation of a flat wing surface. Fold suppression depends on optomotor-blind (omb). Omb mutant animals develop a deep apical fold at the A/P boundary of the larval wing disc and an A/P cleft in the adult wing. A/P fold formation is controlled by different signaling pathways. Jun N-terminal kinase (JNK) and Yorkie (Yki) signaling are activated in cells along the fold and are necessary for the A/P fold to develop. While JNK promotes cell shape changes and cell death, Yki target genes are required to antagonize apoptosis, explaining why both pathways need to be active for the formation of a stable fold.

Putative Breast Cancer Driver Mutations in TBX3 Cause Impaired Transcriptional Repression

The closely related T-box transcription factors TBX2 and TBX3 are frequently overexpressed in melanoma and various types of human cancers, in particular, breast cancer. The overexpression of TBX2 and TBX3 can have several cellular effects, among them suppression of senescence, promotion of epithelial-mesenchymal transition, and invasive cell motility. In contrast, loss of function of TBX3 and most other human T-box genes causes developmental haploinsufficiency syndromes. Stephens and colleagues (1), by exome sequencing of breast tumor samples, identified five different mutations in TBX3, all affecting the DNA-binding T-domain. One in-frame deletion of a single amino acid, p.N212delN, was observed twice. Due to the clustering of these mutations to the T-domain and for statistical reasons, TBX3 was inferred to be a driver gene in breast cancer. Since mutations in the T-domain generally cause loss of function and because the tumorigenic action of TBX3 has generally been attributed to overexpression, we determined whether the putative driver mutations had loss- or gain-of-function properties. We tested two in-frame deletions, one missense, and one frameshift mutant protein for DNA-binding in vitro, and for target gene repression in cell culture. In addition, we performed an in silico analysis of somatic TBX mutations in breast cancer, collected in The Cancer Genome Atlas (TCGA). Both the experimental and the in silico analysis indicate that the observed mutations predominantly cause loss of TBX3 function.

The orthologous Tbx transcription factors Omb and TBX2 induce epithelial cell migration and extrusion in vivo without involvement of matrix metalloproteinases

The transcription factors TBX2 and TBX3 are overexpressed in various human cancers. Here, we investigated the effect of overexpressing the orthologous Tbx genes Drosophila optomotor-blind (omb) and human TBX2 in the epithelium of the Drosophila wing imaginal disc and observed two types of cell motility. Omb/TBX2 overexpressing cells could move within the plane of the epithelium. Invasive cells migrated long-distance as single cells retaining or regaining normal cell shape and apico-basal polarity in spite of attenuated apical DE-cadherin concentration. Inappropriate levels of DE-cadherin were sufficient to drive cell migration in the wing disc epithelium. Omb/TBX2 overexpression and reduced DE-cadherin-dependent adhesion caused the formation of actin-rich lateral cell protrusions. Omb/TBX2 overexpressing cells could also delaminate basally, penetratingthe basal lamina, however, without degradation of extracellular matrix. Expression of Timp, an inhibitor of matrix metalloproteases, blocked neither intraepithelial motility nor basal extrusion. Our results reveal an MMP-independent mechanism of cell invasion and suggest a conserved role of Tbx2-related proteins in cell invasion and metastasis-related processes.

Optomotor-blind negatively regulates Drosophila eye development by blocking Jak/STAT signaling

Organ formation requires a delicate balance of positive and negative regulators. In Drosophila eye development, wingless (wg) is expressed at the lateral margins of the eye disc and serves to block retinal development. The T-box gene optomotor-blind (omb) is expressed in a similar pattern and is regulated by Wg. Omb mediates part of Wg activity in blocking eye development. Omb exerts its function primarily by blocking cell proliferation. These effects occur predominantly in the ventral margin. Our results suggest that the primary effect of Omb is the blocking of Jak/STAT signaling by repressing transcription of upd which encodes the Jak receptor ligand Unpaired.

Dpp signaling inhibits proliferation in the Drosophila wing by Omb-dependent regional control of bantam

The control of organ growth is a fundamental aspect of animal development but remains poorly understood. The morphogen Dpp has long been considered as a general promoter of cell proliferation during Drosophila wing development. It is an ongoing debate whether the Dpp gradient is required for the uniform cell proliferation observed in the wing imaginal disc. Here, we investigated how the Dpp signaling pathway regulates proliferation during wing development. By systematic manipulation of Dpp signaling we observed that it controls proliferation in a region-specific manner: Dpp, via omb, promoted proliferation in the lateral and repressed proliferation in the medial wing disc. Omb controlled the regional proliferation rate by oppositely regulating transcription of the microRNA gene bantam in medial versus lateral wing disc. However, neither the Dpp nor Omb gradient was essential for uniform proliferation along the anteroposterior axis.

Enhancer trap infidelity in Drosophila optomotor-blind

Reporter gene activity in enhancer trap lines is often implicitly assumed to mirror quite faithfully the endogenous expression of the "trapped" gene, even though there are numerous examples of enhancer trap infidelity. optomotor-blind (omb) is a 160 kb gene in which 16 independent P-element enhancer trap insertions of three different types have been mapped in a range of more than 60 kb. We have determined the expression pattern of these elements in wing, eye-antennal and leg imaginal discs as well as in the pupal tergites. We noted that one pGawB insertion (omb (P4) ) selectively failed to report parts of the omb pattern even though the missing pattern elements were apparent in all other 15 lines. We ruled out that omb (P4) was defective in the Gal4 promoter region or had inactivated genomic enhancers in the integration process. We propose that the Gal4 reporter gene in pGawB may be sensitive to orientation or promoter proximity effects.

The Dorsocross T-box transcription factors promote tissue morphogenesis in the Drosophila wing imaginal disc

The Drosophila wing imaginal disc is subdivided into notum, hinge and blade territories during the third larval instar by formation of several deep apical folds. The molecular mechanisms of these subdivisions and the subsequent initiation of morphogenic processes during metamorphosis are poorly understood. Here, we demonstrate that the Dorsocross (Doc) T-box genes promote the progression of epithelial folds that not only separate the hinge and blade regions of the wing disc but also contribute to metamorphic development by changing cell shapes and bending the wing disc. We found that Doc expression was restricted by two inhibitors, Vestigial and Homothorax, leading to two narrow Doc stripes where the folds separating hinge and blade are forming. Doc mutant clones prevented the lateral extension and deepening of these folds at the larval stage and delayed wing disc bending in the early pupal stage. Ectopic Doc expression was sufficient to generate deep apical folds by causing a basolateral redistribution of the apical microtubule web and a shortening of cells. Cells of both the endogenous blade/hinge folds and of folds elicited by ectopic Doc expression expressed Matrix metalloproteinase 2 (Mmp2). In these folds, integrins and extracellular matrix proteins were depleted. Overexpression of Doc along the blade/hinge folds caused precocious wing disc bending, which could be suppressed by co-expressing MMP2RNAi.

Spatial discontinuity of optomotor-blind expression in the Drosophila wing imaginal disc disrupts epithelial architecture and promotes cell sorting

Background

Decapentaplegic (Dpp) is one of the best characterized morphogens, required for dorso-ventral patterning of the Drosophila embryo and for anterior-posterior (A/P) patterning of the wing imaginal disc. In the larval wing pouch, the Dpp target gene optomotor-blind (omb) is generally assumed to be expressed in a step function above a certain threshold of Dpp signaling activity.

Results

We show that the transcription factor Omb forms, in fact, a symmetrical gradient on both sides of the A/P compartment boundary. Disruptions of the Omb gradient lead to a re-organization of the epithelial cytoskeleton and to a retraction of cells toward the basal membrane suggesting that the Omb gradient is required for correct epithelial morphology. Moreover, by analysing the shape of omb gain- and loss-of-function clones, we find that Omb promotes cell sorting along the A/P axis in a concentration-dependent manner.

Conclusions

Our findings show that Omb distribution in the wing imaginal disc is described by a gradient rather than a step function. Graded Omb expression is necessary for normal cell morphogenesis and cell affinity and sharp spatial discontinuities must be avoided to allow normal wing development.

Null mutations in Drosophila Optomotor-blind affect T-domain residues conserved in all Tbx proteins

The T-box transcription factors TBX2 and TBX3 are overexpressed in many human cancers raising the need for a thorough understanding of the cellular function of these proteins. In Drosophila, there is one corresponding ortholog, Optomotor-blind (Omb). Currently, only two missense mutations are known for the two human proteins. Making use of the developmental defects caused by inactivation of omb, we have isolated and molecularly characterized four new omb mutations, three of them are missense mutations of amino acids fully conserved in all Tbx proteins. We interpret the functional defects in the framework of the known structure of the human TBX3 protein and provide evidence for loss of Omb DNA-binding activity in all three newly identified missense mutations.

Optomotor-blind expression in glial cells is required for correct axonal projection across the Drosophila inner optic chiasm

In the Drosophila adult visual system, photoreceptor axons and their connecting interneurons are tied into a retinotopic pattern throughout the consecutive neuropil regions: lamina, medulla and lobula complex. Lamina and medulla are joined by the first or outer optic chiasm (OOC). Medulla, lobula and lobula plate are connected by the second or inner optic chiasm (IOC). In the regulatory mutant In(1)omb(H31) of the T-box gene optomotor-blind (omb), fibers were found to cross aberrantly through the IOC into the neuropil of the lobula complex. Here, we show that In(1)omb(H31) causes selective loss of OMB expression from glial cells within the IOC previously identified as IOC giant glia (ICg-glia). In the absence of OMB, ICg-glia retain their glial cell identity and survive until the adult stage but tend to be displaced into the lobula complex neuropil leading to a misprojection of axons through the IOC. In addition, adult mutant glia show an aberrant increase in length and frequency of glial cell processes. We narrowed down the onset of the IOC defect to the interval between 48 h and 72 h of pupal development. Within the 40 kb of regulatory DNA lacking in In(1)omb(H31), we identified an enhancer element (ombC) with activity in the ICg-glia. ombC-driven expression of omb in ICg-glia restored proper axonal projection through the IOC in In(1)omb(H31) mutant flies, as well as proper glial cell positioning and morphology. These results indicate that expression of the transcription factor OMB in ICg-glial cells is autonomously required for glial cell migration and morphology and non-autonomously influences axonal pathfinding.

Glial and neuronal expression of polyglutamine proteins induce behavioral changes and aggregate formation in Drosophila

Patients with polyglutamine expansion diseases, like Huntington's disease or several spinocerebellar ataxias, first present with neurological symptoms that can occur in the absence of neurodegeneration. Behavioral symptoms thus appear to be caused by neuronal dysfunction, rather than cell death. Pathogenesis in polyglutamine expansion diseases is largely viewed as a cell-autonomous process in neurons. It is likely, however, that this process is influenced by changes in glial physiology and, at least in the case of DRPLA glial inclusions and glial cell death, seems to be an important part in the pathogenesis. To investigate these aspects in a Drosophila model system, we expressed polyglutamine proteins in the adult nervous system. Glial-specific expression of a polyglutamine (Q)-expanded (n=78) and also a nonexpanded (n=27) truncated version of human ataxin-3 led to the formation of protein aggregates and glial cell death. Behavioral changes were observed prior to cell death. This reveals that glia is susceptible to the toxic action of polyglutamine proteins. Neuronal expression of the same constructs resulted in behavioral changes similar to those resulting from glial expression but did not cause neurodegeneration. Behavioral deficits were selective and affected two analyzed fly behaviors differently. Both glial and neuronal aggregates of Q78 and Q27 appeared early in pathogenesis and, at the electron microscopic resolution, had a fibrillary substructure. This shows that a nonexpanded stretch can cause similar histological and behavioral symptoms as the expanded stretch, however, with a significant delay.

The relative role of the T-domain and flanking sequences for developmental control and transcriptional regulation in protein chimeras of Drosophila OMB and ORG-1

optomotor-blind (omb) and optomotor-blind related-1 (org-1) encode T-domain DNA binding proteins in Drosophila. Members of this family of transcription factors play widely varying roles during early development and organogenesis in both vertebrates and invertebrates. Functional specificity differs in spite of similar DNA binding preferences of all family members. Using a series of domain swap chimeras, in which different parts of OMB and ORG-1 were mutually exchanged, we investigated the relevance of individual domains in vitro and in vivo. In cell culture transfection assays, ORG-1 was a strong transcriptional activator, whereas OMB appeared neutral. The main transcriptional activation function was identified in the C-terminal part of ORG-1. Also in vivo, OMB and ORG-1 showed qualitative differences when the proteins were ectopically expressed during development. Gain-of-function expression of OMB is known to counteract eye formation and resulted in the loss of the arista, whereas ORG-1 had little effect on eye development but caused antenna-to-leg transformations and shortened legs in the corresponding gain-of-function situations. The functional properties of OMB/ORG-1 chimeras in several developmental contexts was dominated by the origin of the C-terminal region, suggesting that the transcriptional activation potential can be one major determinant of developmental specificity. In late eye development, we observed, however, a strong influence of the T-domain on ommatidial differentiation. The specificity of chimeric omb/org-1transgenes, thus, depended on the cellular context in which they were expressed. This suggests that both transcriptional activation/repression properties as well as intrinsic DNA binding specificity can contribute to the functional characteristics of T-domain factors.

Glia in development, function, and neurodegeneration of the adult insect brain

Glial cells have long been viewed as a passive framework for neurons but in the meanwhile were shown to play a much more active role in brain function and development. Several reviews have described the function of glia in the insect embryo. The focus of this review is the role of glial cells in the development and function of the normal and diseased adult brain. In different insect species, a considerable variety of central nervous system glia has been described indicating adaptation to different functional requirements. In the development of the adult visual and olfactory system, glial cells guide incoming axons acting as intermediate targets. Glia are part of the insect blood-brain barrier, provide nourishment for neurons, and help to regulate the extracellular concentration of ions and neurotransmitters. To fulfill these tasks insect glial cells, like vertebrate glia, interact with each other and with neurons, thus influencing neural activity. The examples presented suggest that crosstalk between all brain cells is necessary not only to develop and maintain the complex insect brain but also to endow it with the capacity to respond and adapt to the changing environment.

Defective pigment granule biogenesis and aberrant behavior caused by mutations in the Drosophila AP-3beta adaptin gene ruby

Lysosomal protein trafficking is a fundamental process conserved from yeast to humans. This conservation extends to lysosome-like organelles such as mammalian melanosomes and insect eye pigment granules. Recently, eye and coat color mutations in mouse (mocha and pearl) and Drosophila (garnet and carmine) were shown to affect subunits of the heterotetrameric adaptor protein complex AP-3 involved in vesicle trafficking. Here we demonstrate that the Drosophila eye color mutant ruby is defective in the AP-3beta subunit gene. ruby expression was found in retinal pigment and photoreceptor cells and in the developing central nervous system. ruby mutations lead to a decreased number and altered size of pigment granules in various cell types in and adjacent to the retina. Humans with lesions in the related AP-3betaA gene suffer from Hermansky-Pudlak syndrome, which is caused by defects in a number of lysosome-related organelles. Hermansky-Pudlak patients have a reduced skin pigmentation and suffer from internal bleeding, pulmonary fibrosis, and visual system malfunction. The Drosophila AP-3beta adaptin also appears to be involved in processes other than eye pigment granule biogenesis because all ruby allele combinations tested exhibited defective behavior in a visual fixation paradigm.

Wide distribution of the cysteine string proteins in Drosophila tissues revealed by targeted mutagenesis

The "cysteine string protein" (CSP) genes of higher eukaryotes code for a novel family of proteins characterized by a "J" domain and an unusual cysteine-rich region. Previous studies had localized the proteins in neuropil and synaptic terminals of larval and adult Drosophila and linked the temperature-sensitive paralysis of the mutants described here to conditional failure of synaptic transmission. We now use the null mutants as negative controls in order to reliably detect even low concentrations of CSPs by immunohistochemistry, employing three monoclonal antibodies. In wild-type flies high levels of cysteine string proteins are found not only in apparently all synaptic terminals of the embryonic, larval, and adult nervous systems, but also in the "tall cells" of the cardia, in the follicle cells of the ovary, in specific structures of the female spermatheca, and in the male testis and ejaculatory bulb. In addition, low levels of CSPs appear to be present in all tissues examined, including neuronal perikarya, axons, muscles, Malpighian tubules, and salivary glands. Western blots of isolated tissues demonstrate that of the four isoforms expressed in heads only the largest is found in non-neural organs. The wide expression of CSPs suggests that at least some of the various phenotypes of the null mutants observed at permissive temperatures, such as delayed development, short adult lifespan, modified electroretinogram, and optomotor behavior, may be caused by the lack of CSPs outside synaptic terminals.

Genetic lesions in Drosophila behavioural mutants

Over the last 30 years, several hundred behavioural mutants have been isolated in Drosophila. Only a fraction of these are well characterized genetically, behaviourally, and structurally. From six areas of behaviour a set of 24 well-studied mutants was chosen, in which the behavioural defect is probably caused by a central dysfunction and not by an impairment of sensory input or motor output. In all cases, the affected genes can be mutated to more than just a behavioural phenotype. Most genes in the sample are essential. Thus, phenotypic specificity is caused by the specificity of the mutation and not by the gene being a 'behavioural gene'. This study investigates how partial functional inactivation in these loci is brought about genetically. In particular, an attempt is made to discern whether behavioural mutations affect part of a protein's functional repertoire, a subset of protein isoforms, or the spatio-temporal expression of a gene. Not unexpectedly, in view of the predominant use of ethyl methanesulfonate (EMS) as mutagen, the majority of sampled mutations fall into the first two categories. The potentially richest source of genetic versatility, the spatio-temporal modulation of promoter activity by enhancers and silencers, has thus been insufficiently exploited for obtaining behavioural mutants. Various mutagens are reviewed as to their suitability in inducing selective regulatory mutations.

Isolation of a Drosophila T-box gene closely related to human TBX1

T-box genes, in all metazoans studied from nematode to man, exist in small gene families. They encode transcription factors with a novel, large, and highly conserved DNA binding domain termed the T-domain. In all cases studied, T-box genes have important developmental roles. Two familial diseases, Holt-Oram syndrome and ulnar-mammary syndrome, were recently shown to be caused by mutations in the human T-box genes TBX5 and TBX3, respectively. T-box genes were first identified in Drosophila and mouse. Two of the three known Drosophila T-box genes show a close sequence homology to mammalian genes. Similarities in the phenotypes of fly and mammalian mutants can be taken as evidence of functional conservation. We report here the isolation of a fourth Drosophila T-box gene, optomotor-blind-related gene-1 (org-1), closely related to mouse and human TBX1. We localized TBX1 to chromosomal band 22q11, confirming a recent report, and discuss TBX1 as a candidate gene for DiGeorge and related syndromes.

Gypsy retrotransposon as a tool for the in vivo analysis of the regulatory region of the optomotor-blind gene in Drosophila

We report here a method for the in vivo dissection of the regulatory region of a gene in the Drosophila genome. Our system includes (i) the reporter genes lacZ and white to detect transcriptional enhancer and silencer activities in a target gene, (ii) an efficient way to induce integration of gypsy elements in the genome, and (iii) unidirectional blocking of regulatory activities by the gypsy element, which is dependent on the su(Hw) protein. The optomotor-blind (omb) gene was analyzed. In the omb(P1) line, a P[lacW] construct is inserted about 1.4 kb upstream of the omb transcription start site. The lacZ reporter gene within P[lacW] exhibits the same expression pattern as omb. The white reporter gene is expressed in a "bipolar" pattern. We induced high frequency gypsy mobilization in omb(P1) and identified two lines (D11 and D13-1) with altered eye pigmentation pattern, which is dependent on su(Hw) activity. A gypsy element was found inserted in the first intron of omb in D13-1 and in P[lacW] in D11. These results indicate that it is the blocking of regulatory activities by gypsy that caused the changes in the white reporter gene expression. The effect of these gypsy insertions on the expression patterns allowed us to predict several aspects of the organization of the regulatory elements in the omb locus.

Invertebrate synapsins: a single gene codes for several isoforms in Drosophila

Vertebrate synapsins constitute a family of synaptic proteins that participate in the regulation of neurotransmitter release. Information on the presence of synapsin homologs in invertebrates has been inconclusive. We have now cloned a Drosophila gene coding for at least two inferred proteins that both contain a region with 50% amino acid identity to the highly conserved vesicle- and actin-binding "C" domain of vertebrate synapsins. Within the C domain coding sequence, the positions of two introns have been conserved exactly from fly to human. The positions of three additional introns within this domain are similar. The Drosophila synapsin gene (Syn) is widely expressed in the nervous system of the fly. The gene products are detected in all or nearly all conventional synaptic terminals. A single amber (UAG) stop codon terminates the open reading frame (ORF1) of the most abundant transcript of the Syn gene 140 amino acid codons downstream of the homology domain. Unexpectedly, the stop codon is followed by another 443 in-frame amino acid codons (ORF2). Using different antibodies directed against ORF1 or ORF2, we demonstrate that in the adult fly small and large synapsin isoforms are generated. The small isoforms are only recognized by antibodies against ORF1; the large isoforms bind both kinds of antibodies. We suggest that the large synapsin isoform in Drosophila may be generated by UAG read-through. Implications of such an unconventional mechanism for the generation of protein diversity from a single gene are discussed.

Control of the gene optomotor-blind in Drosophila wing development by decapentaplegic and wingless

Diffusible factors of several protein families control appendage outgrowth and patterning in both insects and vertebrates. In Drosophila wing development, the gene decapentaplegic (dpp) is expressed along the anteroposterior compartment boundary. Early wingless (wg) expression is involved in setting up the dorsoventral boundary. Interaction between dpp- and wg-expressing cells promotes appendage outgrowth. Here, it is shown that optomotor-blind (omb) expression is required for distal wing development and is controlled by both dpp and wg. Ectopic omb expression can lead to the growth of additional wings. Thus, omb is essential for wing development and is controlled by two signaling pathways.

The sap47 gene of Drosophila melanogaster codes for a novel conserved neuronal protein associated with synaptic terminals

Proteins expressed specifically in neurons and transported to synaptic terminals are likely to constitute important molecular elements of nervous system function. In an effort to characterize synapse-associated proteins (SAPs) of Drosophila, we have isolated from a hybridoma library several monoclonal antibodies (MABs) that selectively stain synaptic terminals in immunohistochemical preparations. MAB nc46 binds to most but not all synaptic terminals of the Drosophila nervous system, it also recognizes a protein with homologous distribution in other dipteran flies and binds to large parts of fish CNS. In Western blots the antibody labels a Drosophila brain protein of 47 kDa and cross-reacts with brain proteins from several species including insects, fish, mouse and man. From these data we conclude that the corresponding gene has been conserved in evolution at least among diptera. Using MAB nc46 and expression cloning we have identified the 'sap47' gene coding for the 'synapse-associated protein of 47 kDa' of Drosophila melanogaster. Sequence analysis of genomic and cDNA clones reveals the intron-exon structure of the gene and characterizes the complete open reading frames of two alternatively spliced transcripts. The sap47 gene is located in 89A8-B3 on chromosome 3R and codes for two almost identical inferred polypeptides of 347 and 351 amino acids with no significant sequence homology to known proteins.

Optomotor-blind of Drosophila melanogaster: a neurogenetic approach to optic lobe development and optomotor behaviour

The gene optomotor-blind (omb) plays a crucial role in Drosophila optic lobe development. Various mutations in omb lead to different structural defects in the adult optic lobes with correlated behavioural phenotypes. Molecular analysis of omb allows one to trace back behavioural defects to the spatio-temporal misexpression of the gene in mutant development.

Facilitated isolation of rare recombinants by ligase chain reaction: selection for intragenic crossover events in the Drosophila optomotor-blind gene

Ligase chain reaction (LCR) was evaluated as a tool for the detection of point mutations. For the mutation studied, the specificity of the method is sufficient to detect the mutant allele in the presence of a 200-fold molar excess of the wild-type sequence. LCR was therefore employed in a genetic recombination experiment as a probe for a recessive lethal point mutation. LCR greatly facilitated the isolation of a rare recombinant originating from a crossover event in the 40 kb interval separating the lethal mutation and an enhancer trap insertion in the optomotor-blind locus. The recombinant will allow the study of gene control in situ, in a largely unperturbed regulatory environment.

An invertebrate calcium-binding protein of the calbindin subfamily: protein structure, genomic organization, and expression pattern of the calbindin-32 gene of Drosophila

Antisera against vertebrate calcium-binding proteins cross-react with Drosophila nervous and muscle tissue. We have used an antiserum against carp parvalbumin to isolate from a Drosophila head cDNA library immunopositive expression clones. Tissue in situ hybridization identified a clone that labeled specific neurons and muscles similar to the parvalbumin-like immunohistochemical staining pattern. Five independent cDNAs derive from an mRNA whose open reading frame codes for a 310 amino acid polypeptide. Sequence analysis identifies six EF-hand calcium-binding domains and reveals 42% and 37% homology to chicken calretinin and calbindin D-28k, respectively. Since the positions of 9 out of 10 introns within the ORF are conserved from the Drosophila gene to both vertebrate genes, we conclude that we have identified the first invertebrate member of the calbindin sub-family of calcium-binding protein genes of the EF-hand homolog family. The calbindin-32 gene (cbn) maps to 53E on the second chromosome. It is expressed through most of ontogenesis with a selective distribution in the nervous system and in a few small adult thoracic muscles. The cloning of a Drosophila homolog to vertebrate neuronal Ca(2+)-binding proteins opens new routes to study the so far largely elusive function of these brain molecules.

Transcript identification in the optomotor-blind locus of Drosophila melanogaster by intragenic recombination mapping and PCR-aided sequence analysis of lethal point mutations

The optomotor-blind gene of Drosophila melanogaster is large and genetically complex. Five partly independent complementation groups are uncovered by several viable and lethal mutations at the locus. At least 15 RNA signals have been detected by Northern blot analysis. One of them, T3, derived from a 75 kb primary transcript, has been proposed as the carrier of optomotor-blind function, based on the large size of its precursor and its tissue distribution. We here provide direct evidence that T3 is the optomotor-blind transcript. A facile and generally applicable selection scheme for the isolation of intragenic meiotic recombinants was applied to map two lethal optomotor-blind point mutations to exons of the T3 transcript. Amplification of mutant DNA by the polymerase chain reaction (PCR) and sequencing of the amplified exons revealed the presence of mutations that lead to truncation of the T3 open reading frame. The recombination rate observed in the optomoter-blind locus is within the range of rates that have been determined in a few other Drosophila loci.

Expression of the Drosophila optomotor-blind gene transcript in neuronal and glial cells of the developing nervous system

Mutations in the complex gene locus optomotor-blind (omb) can lead to defects in the development of both the optic lobes and external features of the adult fly. We describe here the expression of omb in the developing and adult nervous system using in situ hybridization. During embryogenesis, omb expression is first observed in the optic lobe anlagen. It later expands to a larger part of the developing larval brain and to the gnathal lobes. Cells in the ventral and peripheral nervous systems begin to express omb after completion of germ band extension. Later in embryonic development, expression declines and only persists in the antennomaxillary complex and in part of the brain hemispheres. During the larval and pupal stages, omb expression in the brain is confined to the developing optic lobes and contiguous regions of the central brain. At these stages, only a few cells show expression in the ventral ganglion. In the eye imaginal disc, transcript accumulation is most conspicuous in a group of presumptive glia precursor cells posterior to the morphogenetic furrow and in the optic stalk. In the adult brain, expression is prominent in several regions of the optic lobe cortex and along the border between central brain and optic lobes. In the mutation In(1)ombH31, 40 kb of regulatory DNA, downstream from the transcription unit, are removed from the omb gene. In(1)ombH31 is characterized by the lack of a set of giant interneurons from the lobula plate of the adult optic lobes. We find that, already during embryogenesis, there is a drastic difference between wild type and In(1)ombH31 in the level of the omb transcript in the optic lobe primordia. The adult mutant phenotype may thus be caused by omb misexpression during embryonic development.

A homology domain shared between Drosophila optomotor-blind and mouse Brachyury is involved in DNA binding

The distribution of sequence elements divides the optomotor-blind protein into three regions and is suggestive of a transcriptional regulatory role of this protein. The central region of Omb is homologous to the N-terminal half of the Brachyury protein. The conserved domain of Omb is here shown to possess general DNA binding affinity but has no significant similarity to recognized DNA binding motifs.

Giant lens, a gene involved in cell determination and axon guidance in the visual system of Drosophila melanogaster

Mutations in the Drosophila gene giant lens (gil) affect ommatidial development, photoreceptor axon guidance and optic lobe development. We have cloned the gene using an enhancer trap line. Molecular analysis of gil suggests that it encodes a secreted protein with an epidermal-growth-factor-like motif. We have generated mutations at the gil locus by imprecise excision of the enhancer trap P-element. In the absence of gil, additional photoreceptors develop at the expense of pigment cells, suggesting an involvement of gil in cell determination during eye development. In addition, gil mutants show drastic effects on photoreceptor axon guidance and optic lobe development. In wildtype flies, photoreceptor axons grow from the eye disc through the optic stalk into the larval brain hemisphere, where retinal innervation is required for the normal development of the lamina and distal medulla. The projection pattern of these axons in the developing lamina and medulla is highly regular and reproducible. In gil, photoreceptor axons enter the larval brain but fail to establish proper connections in the lamina or medulla. We propose that gil encodes a new type of signalling molecule involved in the process of axon pathfinding and cell determination in the visual system of Drosophila.

The lethal(1)optomotor-blind gene of Drosophila melanogaster is a major organizer of optic lobe development: isolation and characterization of the gene

The X-chromosomal complementation unit lethal(1)optomotor-blind [l(1)omb] is defined by lack of complementation among over a dozen recessive lethal mutations that map to the omb gene locus. Mutations in l(1)omb also fail to complement viable mutations of three seemingly unrelated functions in this region: bifid (bi), manifesting defective wings, Quadroon (Qd), a semi-dominant mutation expressing abnormal tergite pigmentation, and In(1)ombH31, giving rise to a normal external morphology but with discrete defects in the optic lobes and behavior. The locus encodes a 70-kilobase primary transcript that is spliced into a 6-kilobase mature RNA. cDNAs for this transcript were isolated and sequenced and the derived amino acid sequence was analyzed. Certain features of this sequence suggest that the l(1)omb gene product is a nuclear regulatory protein. The lethal phase of various apparent null mutants was determined and found to occur mainly in the pupal stage. A large proportion of all hemizygous mutant males develop to pharate adults that eclose only rarely but can be rescued from the pupal case. These animals show a severe maldevelopment of the optic lobes. In addition they have only rudimentary wings as well as a Quadroon-like abdominal pigmentation. Thus, in the lethal mutants those parts of the body are affected for which independent viable mutations have been previously described in the omb locus, such as optomotor-blind, bifid, and Quadroon.

Mutations in the proximal region of the optomotor-blind locus of Drosophila melanogaster reveal a gradient of neuroanatomical and behavioral phenotypes

Mutations in the complex optomotor-blind (omb) gene locus (4C4-6) lead to a number of different phenotypes in various tissues of the adult Drosophila melanogaster fly. At the core of the locus lies a lethal complementation group, named l(1)omb, whose mutations cause larval and pupal lethality. Some 40% of all males hemizygous for lethal omb alleles develop to the pharate adult stage. These flies can be rescued from the pupal case and show a severe disturbance in optic lobe development. The recessive viable allele In(1)ombH31 reduces the optomotor response in walking flies and during stationary flight of tethered flies. At the neuroanatomical level, these animals lack a subset of lobula plate giant neurons (LPGNs), which are thought to mediate optomotor behavior. Chromosomal aberrations deleting the proximal, non transcribed part of the locus complement the lethality, but still cause neuroanatomical and optomotor defects. Analysis of different allelic combinations of such mutations, in which increasing amounts of DNA downstream of the transcribed region are removed, reveals a step gradient of increasing severity of the neuroanatomical defects and behavioral phenotypes. On this basis the 3'-regulatory region is divided into three domains each having specific effects on optic lobe development.

A cysteine-string protein is expressed in retina and brain of Drosophila

Antibodies can be used to identify tissue- and stage-specifically expressed genes. A monoclonal antibody MAB ab49 from a hybridoma library screened for immunohistochemical staining in the adult nervous system of Drosophila melanogaster was found to selectively bind to all neuropil regions and to synaptic boutons of motor neurons. In Western blots of homogenized brains the antibody recognizes two proteins of 32 and 34 kD. Using this antibody we have isolated seven cDNA clones that derive from two polyadenylated mRNA splice variants of a gene located at 79E1-2 on polytene chromosomes. The two mRNAs code for two inferred proteins of 249 and 223 amino acids, respectively, which are identical except for their C-terminals and a central deletion of 21 amino acids in the second protein. Both contain a contiguous string of 11 cysteine residues. In situ hybridization to frozen head sections detects expression of this gene in retina and neuronal perikarya. The 32 and 34 kD brain proteins that presumably are localized predominantly in synaptic terminals of photoreceptors and most if not all neurons may correspond to two variant cysteine-string proteins as they are of similar molecular weight and share an antigenic binding site for MAB ab49.

Genetic and molecular characterization of the optomotor-blind gene locus in Drosophila melanogaster

The Drosophila gene optomotor-blind (omb) is involved in the development of a set of giant neurons in the optic lobes and possibly other structures in the imaginal brain. Adult flies have discrete defects in optomotor behavior. The gene has previously been mapped in chromomeres 4C5-6, together with three other genes, bifid, Quadroon and lacqueredgls. We have localized the gene in a genomic walk of 340 kb of DNA. By mapping seven chromosome breakpoints with omb phenotype we determined its minimum size to about 80 kb. From this region more than 20 RNAs of different size and temporal expression pattern are transcribed. Three of them (T3, T7 and T7') stem from primary transcripts of 40-80 kb in length. In its distal part the omb gene overlaps in at least 19 kb with four other complementation units, bifid, l(1)bifid, Quadroon and lacqueredgls. The three nonlethals affect the external appearance of the fly and seem to be unrelated to brain development.