T-Box Genes in Drosophila Limb Development.

T-box genes are essential for limb development in vertebrates and arthropods. The Drosophila genome encodes eight T-box genes, six of which are expressed in limb ontogenesis. The Tbx20-related gene pair midline and H15 is essential for dorso-ventral patterning of the Drosophila legs. The three Tbx6-related Dorsocross genes are required for epithelial remodeling during wing development. The Drosophila gene optomotor-blind (omb) is the only member of the Tbx2 subfamily in the fly and is predominantly involved in wing development. Omb is essential for wing development and is sufficient to promote the development of a second wing pair. Targeted manipulations of omb expression have shown that the bulk omb requirement for wing development can be deconstructed into a number of individual functions. Even though omb expression in the wing disc is symmetrical with regard to the anterior/posterior (A/P) compartment boundary, anterior and posterior knockdowns have distinct consequences: Anterior Omb is required for the maintenance of a straight A/P lineage restriction boundary. Posterior Omb suppresses formation of an apical epithelial fold along the A/P boundary. Drosophila T-box gene expression is not confined to the ectoderm-derived epithelia of the imaginal discs. Both Doc and Omb are prominently expressed in leg disc muscle precursor cells. Omb is also strongly expressed in a tracheal branch that invades the extracellular matrix of the wing disc. The function of Doc and Omb in the latter tissues is not known, indicative of the many questions still open in the field.

omb and circumstance.

Since its isolation in a behavioral screen in 1971, work on the gene, optomotor-blind (omb, FlyBase: bifid, bi), has proceeded in many laboratories. The Drosophila T-box transcription factor, Omb, is active in several developmental processes, in particular in those of wing, eye, and abdominal tergites and optic lobes. The orthologous genes, TBX2 and TBX3, are found overexpressed or amplified in an increasing number of human cancers, emphasizing the importance of Tbx genes also in postdevelopmental processes. This review summarizes recent findings pertaining to omb and orthologous genes.

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.

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.