The IGFBP7 homolog Imp-L2 promotes insulin signaling in distinct neurons of the Drosophila brain.

In Drosophila, Insulin-like peptide 2 (Dilp-2) is expressed by insulin-producing cells in the brain, and is secreted into the hemolymph to activate insulin signaling systemically. Within the brain, however, a more local activation of insulin signaling may be required to couple behavioral and physiological traits to nutritional inputs. We show that a small subset of neurons in the larval brain has high Dilp-2-mediated insulin signaling activity. This local insulin signaling activation is accompanied by selective Dilp-2 uptake and depends on the expression of the Imaginal morphogenesis protein-late 2 (Imp-L2) in the target neurons. We suggest that Imp-L2 acts as a licensing factor for neuronal IIS activation through Dilp-2 to further increase the precision of insulin activity in the brain.

Suppression of food intake and growth by amino acids in Drosophila: the role of pumpless, a fat body expressed gene with homology to vertebrate glycine cleavage system.

We have isolated a Drosophila mutant, named pumpless, which is defective in food intake and growth at the larval stage. pumpless larvae can initially feed normally upon hatching. However, during late first instar stage, they fail to pump the food from the pharynx into the esophagus and concurrently begin moving away from the food source. Although pumpless larvae do not feed, they do not show the typical physiologic response of starving animals, such as upregulating genes involved in gluconeogenesis or lipid breakdown. The pumpless gene is expressed specifically in the fat body and encodes a protein with homology to a vertebrate enzyme involved in glycine catabolism. Feeding wild-type larvae high levels of amino acids could phenocopy the feeding and growth defects of pumpless mutants. Our data suggest the existence of an amino acid-dependent signal arising from the fat body that induces cessation of feeding in the larva. This signaling system may also mediate growth transition from larval to the pupal stage during Drosophila development.

Control of gut development by fork head and cell signaling molecules in Drosophila.

The alimentary canal of most animals can be subdivided into a fore- mid- and hindgut portion, each gut part possessing distinct physiological functions. The genetic basis underlying the formation of the different gut parts is poorly understood. Here we show that the Drosophila genes hedgehog, wingless and decapentaplegic, which encode cell signaling molecules, are required for the establishment of signaling centers that coordinate morphogenesis in the hindgut epithelium. The activation of these genes in the developing as well as in the foregut requires fork head, which encodes a transcription factor. Furthermore, we demonstrate that hedgehog and wingless activities in the gut epithelial cells are required for the expression of the homeobox gene bagpipe in the ensheathing visceral mesoderm. These results provide strong evidence that similar principles underlie Drosophila fore- and hindgut development, and that the genetic hierarchy of gut development might be conserved between Drosophila and vertebrates.

Lack of Drosophila cytoskeletal tropomyosin affects head morphogenesis and the accumulation of oskar mRNA required for germ cell formation.

Drosophila encodes five muscle and one cytoskeletal isoform of the actin-binding protein tropomyosin. We have identified a lack-of-function mutation in the cytoskeletal isoform (cTmII). Zygotic mutant embryos show a defect in head morphogenesis, while embryos lacking maternal cTmII are defective in germ cell formation but otherwise give rise to viable adults. oskar mRNA, which is required for both germ cell formation and abdominal segmentation, fails to accumulate at the posterior pole in these embryos. nanos mRNA, however, which is required exclusively for abdominal segmentation, is localized at wild-type levels. These results indicate that head morphogenesis and the accumulation of high levels of oskar mRNA necessary for germ cell formation require tropomyosin-dependent cytoskeleton.

Control of epithelial morphogenesis by cell signaling and integrin molecules in the Drosophila foregut.

Coordinated cell movements are critical for tissue and organ morphogenesis in animal development. We show that the Drosophila genes hedgehog and wingless, which encode signaling molecules, and the gene myospheroid, which encodes a beta subunit of the integrins, are required for epithelial morphogenesis during proventriculus development. In contrast, this morphogenetic process is suppressed by the decapentaplegic gene, which encodes a member of the TGF beta family of growth factors. These results identify a novel cell signaling center in the foregut that directs the formation of a multiply folded organ from a simple epithelial tube.

Identical transacting factor requirement for knirps and knirps-related Gene expression in the anterior but not in the posterior region of the Drosophila embryo.

The Drosophila genes knirps (kni) and knirps-related (knrl) are located within the 77E1,2 region on the left arm of the third chromosome. They encode nuclear hormone-like transcription factors containing almost identical Cys2/Cys2 DNA-binding zinc finger motifs which bind to the same target sequence. kni is a member of the gap class of segmentation genes, and its activity is required for the normal establishment of the abdomen. The function of knrl is still unknown; however, a possible gap gene function in the abdominal region of the embryo can be excluded. Both genes are initially expressed in three identical regions of the blastoderm embryo: in an anterior cap domain, in an anterior stripe and in a posterior broad band linked to the kni gap gene function. The transacting factor requirement for the expression of kni and knrl is identical for the two anterior domains but different, although similar, for the posterior domain of expression in the blastoderm. Both the anteroposterior morphogen bicoid and the dorsoventral morphogen dorsal are necessary but not sufficient for the activation of the two genes in the anterior cap domain, suggesting they act together to bring about its normal spatial limits.

A two-step mode of stripe formation in the Drosophila blastoderm requires interactions among primary pair rule genes.

The stripe pattern of pair rule gene expression along the anterior-posterior axis of the Drosophila blastoderm embryo represents the first sign of periodicity during the process of segmentation. Striped gene expression can be mediated by distinct cis-acting elements that give rise to individual stripe expression domains in direct response to maternal and first zygotic factors. Here we show that the expression of stripes can also be generated by a different, two-step mode which involves regulatory interactions among the primary pair rule genes hairy (h) and runt (run). Expression of h stripes 3 and 4 is directed by a common cis-acting element that results in an initial broad band of gene expression covering three stripe equivalents. Subsequently, this expression domain is split by repression in the forthcoming interstripe region, a process mediated by a separate cis-acting element that responds to run activity. This second mode of pair rule stripe formation may have evolutionary implications.

Spatial control of the gap gene knirps in the Drosophila embryo by posterior morphogen system.

The gap genes of Drosophila are the first zygotic genes to respond to the maternal positional signals and establish the body pattern along the anterior-posterior axis. The gap gene knirps, required for patterning in the posterior region of the embryo, can be activated throughout the wild-type embryo and is normally repressed from the anterior and posterior sides. These results provide direct molecular evidence that the posterior morphogen system interacts in a fundamentally different manner than do hunchback and bicoid, which are responsible for anterior pattern formation.

Transcriptional control by Drosophila gap genes.

The segmented body pattern along the longitudinal axis of the Drosophila embryo is established by a cascade of specific transcription factor activities. This cascade is initiated by maternal gene products that are localized at the polar regions of the egg. The initial long-range positional information of the maternal factors, which are transcription factors (or are factors which activate or localize transcription factors), is transferred through the activity of the zygotic segmentation genes. The gap genes act at the top of this regulatory hierarchy. Expression of the gap genes occurs in discrete domains along the longitudinal axis of the preblastoderm and defines specific, overlapping sets of segment primordia. Their protein products, which are DNA-binding transcription factors mostly of the zinc finger type, form broad and overlapping concentration gradients which are controlled by maternal factors and by mutual interactions between the gap genes themselves. Once established, these overlapping gap protein gradients provide spatial cues which generate the repeated pattern of the subordinate pair-rule gene expression, thereby blue-printing the pattern of segmental units in the blastoderm embryo. Our results show different strategies by which maternal gene products, in combination with various gap gene proteins, provide position-dependent sets of transcriptional activator/repressor systems which regulate the spatial pattern of specific gap gene expression. Region-specific combinations of different transcription factors that derive from localized gap gene expression eventually generate the periodic pattern of pair-rule gene expression by the direct interaction with individual cis-acting "stripe elements" of particular pair-rule gene promoters. Thus, the developmental fate of blastoderm cells is programmed according to their position within the anterior-posterior axis of the embryo: maternal transcription factors regulate the region-specific expression of first zygotic transcription factors which, by their specific and unique combinations, control subordinate zygotic transcription factors, thereby subdividing the embryo into increasingly smaller units later seen in the larva.

Ophthalmology. The resident's perspective.

To determine how and why current residents in ophthalmology chose their medical specialty, we formulated and distributed a questionnaire to all ophthalmology residents in accredited programs. The results of the survey gave insight into not only their decision-making processes in choosing ophthalmology, but also their backgrounds, the factors that were important in choosing their residency program, and their future plans in ophthalmology. A wide variety of factors was involved in the decision to pursue a career in ophthalmology. In addition, we found current residents to be well qualified academically and generally satisfied with their decision to enter ophthalmology.

Making stripes in the Drosophila embryo.

The striped pattern of expression of the Drosophila primary pair rule genes is controlled by independent regulatory units that give rise to individual stripes. The different stripes seem to respond in a concentration-dependent manner to the different combinations of maternal and gap protein gradients found along the anterior-posterior axis of the early embryo. Thus, the initial periodicity appears to be generated by putting together a series of nonperiodic events.

Gradients of Krüppel and knirps gene products direct pair-rule gene stripe patterning in the posterior region of the Drosophila embryo.

Abdominal segmentation of the Drosophila embryo requires the activities of the gap genes Krüppel (Kr), knirps (kni), and tailless (tll). They control the expression of the pair-rule gene hairy (h) by activating or repressing independent cis-acting units that generate individual stripes. Kr activates stripe 5 and represses stripe 6, kni activates stripe 6 and represses stripe 7, and tll activates stripe 7. Kr and kni proteins bind strongly to h control units that generate stripes in areas of low concentration of the respective gap gene products and weakly to those that generate stripes in areas of high gap gene expression. These results indicate that Kr and kni proteins form overlapping concentration gradients that generate the periodic pair-rule expression pattern.

[Pattern formation in Drosophila]. / Musterbildung bei Drosophila.

Drosophila proved an excellent system to study molecular processes in establishing the body pattern of an embryo. Genes which are active during oogenesis provide localized cues which regulate a cascade of zygotic genes that determines the developmental fate of the blastoderm cells along the longitudinal axis of the embryo.

Krüppel requirement for knirps enhancement reflects overlapping gap gene activities in the Drosophila embryo.

Segmental pattern formation in Drosophila proceeds in a hierarchical manner whereby the embryo is stepwise divided into progressively finer regions until it reaches its final metameric form. Maternal genes initiate this process by imparting on the egg a distinct antero-posterior polarity and by directing from the two polar centres the activities of the zygotic genes. The anterior system is strictly dependent on the product of the maternal gene bicoid (bcd), without which all pattern elements in the anterior region of the embryo fail to develop. The posterior system seems to lack such a morphogen. Rather, the known posterior maternal determinants simply define the boundaries within which abdominal segmentation can occur, and the process that actively generates the abdominal body pattern may be entirely due to the interactions between the zygotic genes. The most likely candidates among the zygotic genes that could fulfil the role of initiating the posterior pattern-forming process are the gap genes, as they are the first segmentation genes to be expressed in the embryo. Here we describe the interactions between the gap genes Krüppel (Kr), knirps (kni) and tailless (tll). We show that kni expression is repressed by tll activity, whereas it is directly enhanced by Kr activity. Thus, Kr activity is present throughout the domain of kni expression and forms a long-range protein gradient, which in combination with kni activity is required for abdominal segmentation of the embryo.

Abdominal segmentation of the Drosophila embryo requires a hormone receptor-like protein encoded by the gap gene knirps.

The body pattern along the anterior-posterior axis of the insect embryo is thought to be established by two organizing centres localized at the ends of the egg. Genetic analysis of the polarity-organizing centres in Drosophila has identified three distinct classes of maternal effect genes that organize the anterior, posterior and terminal pattern elements of the embryo. The factors provided by these gene classes specify the patterns of expression of the segmentation genes at defined positions along the longitudinal axis of the embryo. The system responsible for organizing the posterior segment pattern is a group of at least seven maternal genes and the zygotic gap gene knirps (kni). Their mutant phenotype has adjacent segments in the abdominal region of the embryo deleted. Genetic analysis and cytoplasmic transplantation experiments suggested that these maternal genes are required to generate a 'posterior activity' that is thought to activate the expression of kni (reviewed in ref. 2). The molecular nature of the members of the posterior group is still unknown. Here we report the molecular characterization of the kni gene that codes for a member of the steroid/thyroid receptor superfamily of proteins which in vertebrates act as ligand-dependent DNA-binding transcription regulators.