The Drosophila dCREB-A gene is required for dorsal/ventral patterning of the larval cuticle

Multi-faceted regulation of CREB family transcription factors

cAMP response element-binding protein (CREB) is a ubiquitously expressed nuclear transcription factor, which can be constitutively activated regardless of external stimuli or be inducibly activated by external factors such as stressors, hormones, neurotransmitters, and growth factors. However, CREB controls diverse biological processes including cell growth, differentiation, proliferation, survival, apoptosis in a cell-type-specific manner. The diverse functions of CREB appear to be due to CREB-mediated differential gene expression that depends on cAMP response elements and multi-faceted regulation of CREB activity. Indeed, the transcriptional activity of CREB is controlled at several levels including alternative splicing, post-translational modification, dimerization, specific transcriptional co-activators, non-coding small RNAs, and epigenetic regulation. In this review, we present versatile regulatory modes of CREB family transcription factors and discuss their functional consequences.

Transcriptional Control of Lipid Metabolism

Transcriptional control of lipid metabolism uses a framework that parallels the control of lipid metabolism at the protein or enzyme level, via feedback and feed-forward mechanisms. Increasing the substrates for an enzyme often increases enzyme gene expression, for example. A paucity of product can likewise potentiate transcription or stability of the mRNA encoding the enzyme or enzymes needed to produce it. In addition, changes in second messengers or cellular energy charge can act as on/off switches for transcriptional regulators to control transcript (and protein) abundance. Insects use a wide range of DNA-binding transcription factors (TFs) that sense changes in the cell and its environment to produce the appropriate change in transcription at gene promoters. These TFs work together with histones, spliceosomes, and additional RNA processing factors to ultimately regulate lipid metabolism. In this chapter, we will first focus on the important TFs that control lipid metabolism in insects. Next, we will describe non-TF regulators of insect lipid metabolism such as enzymes that modify acetylation and methylation status, transcriptional coactivators, splicing factors, and microRNAs. To conclude, we consider future goals for studying the mechanisms underlying the control of lipid metabolism in insects.

Cellular and molecular organization of the Drosophila foregut

The animal foregut is the first tissue to encounter ingested food, bacteria, and viruses. We characterized the adult Drosophila foregut using transcriptomics to better understand how it triages consumed items for digestion or immune response and manages resources. Cell types were assigned and validated using GFP-tagged and Gal4 reporter lines. Foregut-associated neuroendocrine cells play a major integrative role by coordinating gut activity with nutrition, the microbiome, and circadian cycles; some express clock genes. Multiple epithelial cell types comprise the proventriculus, the central foregut organ that secretes the peritrophic matrix (PM) lining the gut. Analyzing cell types synthesizing individual PM layers revealed abundant mucin production close to enterocytes, similar to the mammalian intestinal mucosa. The esophagus and salivary gland express secreted proteins likely to line the esophageal surface, some of which may generate a foregut commensal niche housing specific gut microbiome species. Overall, our results imply that the foregut coordinates dietary sensing, hormonal regulation, and immunity in a manner that has been conserved during animal evolution.

The transcription factor CREB3-2 regulated neutral lipase gene expression in ovary of Nilaparvata lugens

The cyclic AMP-responsive element-binding protein 3 (CREB3) members have unique regulatory roles in cellular lipid metabolism as transcription factors. Two CREB3 proteins in Nilaparvata lugens were identified and analyzed. In ovary, when silencing NlCREB3-2, triacylglycerol (TAG) content dramatically increased but glycerol and free fatty acid (FFA) significantly decreased, which implicated that NlCREB3-2 was involved in the lipase-related TAG metabolism. In N. lugens, five neutral lipases with complete features for TAG hydrolytic activity and high expression in ovary were focused. Among them, the expression levels of three neutral lipase genes were significantly down-regulated by NlCREB3-2 RNAi. The direct regulation of NlCREB3-2 towards the three neutral lipase genes was evidenced by the dual-luciferase reporter assay. After jointly silencing three neutral lipase genes, TAG and glycerol contents displayed similar changes as NlCREB3-2 RNAi. The study proved that NlCREB3-2 participated in TAG metabolism in ovary via the direct activation towards the ovary-specific neutral lipase genes.

RNAi-mediated CrebA silencing inhibits reproduction and immunity in Locusta migratoria manilensis

Locusta migratoria manilensis is a major agricultural pest that causes severe direct and indirect damage to several crops. Thus, to provide a theoretical foundation for pest control, the role of CrebA in the reproduction and immune regulation of L. migratoria was investigated. CrebA is a bZIP transcription factor that critically regulates intracellular protein secretion. In this study, CrebA was widely expressed in the brain, fat body, integument, midgut, and reproductive tissues of different maturity stages of adult locusts, especially in the female fat body. RNA interfering (RNAi)-mediated silencing of CrebA inhibited locusts ovarian development, and key reproduction gene expressions, Vgs, VgRs, Chico, and JHAMT were downregulated. After the locusts were injected with Micrococcus luteus or Escherichia coli, M. luteus activated lysozyme expression, while the E. coli activated both phenol oxidase cascade and lysozyme expression. Furthermore, both bacteria stimulated the upregulation of the antimicrobial peptide genes DEF3 and DEF4. However, CrebA silencing is fatal to locusts infection with E. coli, with a mortality rate of up to 96.3%, and resulted in a significant decrease in the expression of DEF3 and DEF4 and changes in the activities of phenol oxidase and lysozyme of locusts infected by bacteria. Collectively, CrebA may be involved in diverse biological processes, including reproduction and immunity. CrebA inhibited locusts reproduction by regulating JH signaling pathway and inhibits the expression of immune genes TLR6, IMD, and AMPs. These results demonstrate CrebA seems to play a crucial role in reproduction and innate immunity.

Transcription factor paralogs orchestrate alternative gene regulatory networks by context-dependent cooperation with multiple cofactors

In eukaryotes, members of transcription factor families often exhibit similar DNA binding properties in vitro, yet orchestrate paralog-specific gene regulatory networks in vivo. The serially homologous first (T1) and third (T3) thoracic legs of Drosophila, which are specified by the Hox proteins Scr and Ubx, respectively, offer a unique opportunity to address this paradox in vivo. Genome-wide analyses using epitope-tagged alleles of both Hox loci in the T1 and T3 leg imaginal discs, the precursors to the adult legs and ventral body regions, show that ~8% of Hox binding is paralog-specific. Binding specificity is mediated by interactions with distinct cofactors in different domains: the Hox cofactor Exd acts in the proximal domain and is necessary for Scr to bind many of its paralog-specific targets, while in the distal leg domain, the homeodomain protein Distal-less (Dll) enhances Scr binding to a different subset of loci. These findings reveal how Hox paralogs, and perhaps paralogs of other transcription factor families, orchestrate alternative downstream gene regulatory networks with the help of multiple, context-specific cofactors.

Diverse cellular morphologies during lumen maturation in Anopheles gambiae larval salivary glands

Mosquitoes are the greatest animal threat to human health, causing hundreds of millions of infections and around 1 million deaths each year. All mosquito-borne pathogens must traverse the salivary glands (SGs) to be transmitted to the next host, making this organ an ideal target for interventions. The adult SG develops from precursor cells located in the larval SG duct bud. Characterization of the larval SG has been limited. We sought to better understand larval SG architecture, secretion and gene expression. We developed an optimized method for larval SG staining and surveyed hundreds of larval stage 4 (L4) SGs using fluorescence confocal microscopy. Remarkable variation in SG cell and chromatin organization differed among individuals and across the L4 stage. Lumen formation occurred during L4 stage through secretion likely involving a coincident cellular apical lipid enrichment and extracellular vesicle-like structures. Meta-analysis of microarray data showed that larval SG gene expression is divergent from adult SGs, more similar to larval gastric cecae, but different from other larval gut compartments. This work highlights the variable cell architecture of larval Anopheles gambiae SGs and provides candidate targets for genetic strategies aiming to disrupt SGs and transmission of mosquito-borne pathogens.

Role of tbc1 in Drosophila embryonic salivary glands

BACKGROUND

CG4552/tbc1 was identified as a downstream target of Fork head (Fkh), the single Drosophila member of the FoxA family of transcription factors and a major player in salivary gland formation and homeostasis. Tbc1 and its orthologues have been implicated in phagocytosis, the innate immune response, border cell migration, cancer and an autosomal recessive form of non-degenerative Pontocerebellar hypoplasia. Recently, the mammalian Tbc1 orthologue, Tbc1d23, has been shown to bind both the conserved N-terminal domains of two Golgins (Golgin-97 and Golgin-245) and the WASH complex on endosome vesicles. Through this activity, Tbc1d23 has been proposed to link endosomally-derived vesicles to their appropriate target membrane in the trans Golgi (TGN).

RESULTS

In this paper, we provide an initial characterization of Drosophila orthologue, we call tbc1. We show that, like its mammalian orthologue, Tbc1 localizes to the trans Golgi. We show that it also colocalizes with a subset of Rabs associated with both early and recycling endosomes. Animals completely missing tbc1 survive, but females have fertility defects. Consistent with the human disease, loss of tbc1 reduces optic lobe size and increases response time to mechanical perturbation. Loss and overexpression of tbc1 in the embryonic salivary glands leads to secretion defects and apical membrane irregularities.

CONCLUSIONS

These findings support a role for tbc1 in endocytic/membrane trafficking, consistent with its activities in other systems.

A kinase-dependent feedforward loop affects CREBB stability and long term memory formation

In Drosophila, long-term memory (LTM) requires the cAMP-dependent transcription factor CREBB, expressed in the mushroom bodies (MB) and phosphorylated by PKA. To identify other kinases required for memory formation, we integrated Trojan exons encoding T2A-GAL4 into genes encoding putative kinases and selected for genes expressed in MB. These lines were screened for learning/memory deficits using UAS-RNAi knockdown based on an olfactory aversive conditioning assay. We identified a novel, conserved kinase, Meng-Po (MP, CG11221, SBK1 in human), the loss of which severely affects 3 hr memory and 24 hr LTM, but not learning. Remarkably, memory is lost upon removal of the MP protein in adult MB but restored upon its reintroduction. Overexpression of MP in MB significantly increases LTM in wild-type flies showing that MP is a limiting factor for LTM. We show that PKA phosphorylates MP and that both proteins synergize in a feedforward loop to control CREBB levels and LTM. key words: Drosophila, Mushroom bodies, SBK1, deGradFP, T2A-GAL4, MiMIC

Drosophila FoxL1 non-autonomously coordinates organ placement during embryonic development

Determining how organs attain precise positioning within an organism is a crucial facet of developmental biology. The Fox family winged-helix transcription factors are known to play key roles in development of multiple organs. Drosophila FoxL1 (aka Fd64A) is dynamically expressed in embryos but its function is completely uncharacterized. FoxL1 is expressed in a single group of body wall - muscles in the 2nd and 3rd thoracic segments, in homologous abdominal muscles at earlier stages, and in the hindgut mesoderm from early through late embryogenesis. We show that FoxL1 expression in T2 and T3 is in VIS5, which is not a single muscle spanning the entire thorax, as previously published, but is, instead, three individual muscles, each spanning a single thoracic segment. We generate mutations in foxL1 and show that, surprisingly, none of the tissues that express FoxL1 are affected by its loss. Instead, loss of foxL1 results in defects in salivary gland positioning and morphology, as well as defects in the migration of hemocytes, germ cells and Malpighian tubules. We also show that FoxL1-dependent expression of secreted Sema2a in T3 VIS5 is required for normal salivary gland positioning. Altogether, these findings suggest that Drosophila FoxL1 functions like its mammalian counterpart in non-autonomously orchestrating the behaviors of surrounding tissues.

Changes in organelle position and epithelial architecture associated with loss of CrebA

Drosophila CrebA facilitates high-level secretion by transcriptional upregulation of the protein components of the core secretory machinery. In CrebA mutant embryos, both salivary gland (SG) morphology and epidermal cuticle secretion are abnormal, phenotypes similar to those observed with mutations in core secretory pathway component genes. Here, we examine the cellular defects associated with CrebA loss in the SG epithelium. Apically localized secretory vesicles are smaller and less abundant, consistent with overall reductions in secretion. Unexpectedly, global mislocalization of cellular organelles and excess membrane accumulation in the septate junctions (SJs) are also observed. Whereas mutations in core secretory pathway genes lead to organelle localization defects similar to those of CrebA mutants, they have no effect on SJ-associated membrane. Mutations in tetraspanin genes, which are normally repressed by CrebA, have mild defects in SJ morphology that are rescued by simultaneous CrebA loss. Correspondingly, removal of several tetraspanins gives partial rescue of the CrebA SJ phenotype, supporting a role for tetraspanins in SJ organization.

Hox targets and cellular functions

Hox genes are a group of genes that specify structures along the anteroposterior axis in bilaterians. Although in many cases they do so by modifying a homologous structure with a different (or no) Hox input, there are also examples of Hox genes constructing new organs with no homology in other regions of the body. Hox genes determine structures though the regulation of targets implementing cellular functions and by coordinating cell behavior. The genetic organization to construct or modify a certain organ involves both a genetic cascade through intermediate transcription factors and a direct regulation of targets carrying out cellular functions. In this review I discuss new data from genome-wide techniques, as well as previous genetic and developmental information, to describe some examples of Hox regulation of different cell functions. I also discuss the organization of genetic cascades leading to the development of new organs, mainly using Drosophila melanogaster as the model to analyze Hox function.

Expression and localization of Luman RNA and protein during mouse implantation and decidualization

Luman (also known as LZIP and CREB3) is a basic leucine zipper transcription factor of the cAMP response element-binding protein/activating transcription factor gene family. Although Luman had specific roles near termination of Drosophila embryogenesis, the physiological functions of Luman in female mammals have apparently not been reported. Therefore, our objective was to investigate the spatiotemporal expression and regulation of Luman in the mouse uterus during the peri-implantation period. Luman protein was clearly present in the luminal and glandular epithelium on days 1 to 4 of pregnancy (day 1, presence of a vaginal plug) and was observed in decidual cells on day 6 of pregnancy. Expression had progressively increased to day 7 when the second decidual zone was formed. On day 8, apoptosis of the decidualized cells was present, and Luman protein expression was decreased (in close association with decidualization). Luman protein was also present in decidual cells of the artificially decidualized uterus. The expression of Luman was regulated by an activated embryo (according to its expression patterns during pseudopregnancy and delayed implantation). Furthermore, expression of Luman was induced by estrogen in ovariectomized mice. We have concluded that Luman might have important roles in embryo implantation and decidualization.

Organ-specific gene expression: the bHLH protein Sage provides tissue specificity to Drosophila FoxA

FoxA transcription factors play major roles in organ-specific gene expression, regulating, for example, glucagon expression in the pancreas, GLUT2 expression in the liver, and tyrosine hydroxylase expression in dopaminergic neurons. Organ-specific gene regulation by FoxA proteins is achieved through cooperative regulation with a broad array of transcription factors with more limited expression domains. Fork head (Fkh), the sole Drosophila FoxA family member, is required for the development of multiple distinct organs, yet little is known regarding how Fkh regulates tissue-specific gene expression. Here, we characterize Sage, a bHLH transcription factor expressed exclusively in the Drosophila salivary gland (SG). We show that Sage is required for late SG survival and normal tube morphology. We find that many Sage targets, identified by microarray analysis, encode SG-specific secreted cargo, transmembrane proteins, and the enzymes that modify these proteins. We show that both Sage and Fkh are required for the expression of Sage target genes, and that co-expression of Sage and Fkh is sufficient to drive target gene expression in multiple cell types. Sage and Fkh drive expression of the bZip transcription factor Senseless (Sens), which boosts expression of Sage-Fkh targets, and Sage, Fkh and Sens colocalize on SG chromosomes. Importantly, expression of Sage-Fkh target genes appears to simply add to the tissue-specific gene expression programs already established in other cell types, and Sage and Fkh cannot alter the fate of most embryonic cell types even when expressed early and continuously.

Drosophila as a model for epithelial tube formation

Epithelial tubular organs are essential for life in higher organisms and include the pancreas and other secretory organs that function as biological factories for the synthesis and delivery of secreted enzymes, hormones, and nutrients essential for tissue homeostasis and viability. The lungs, which are necessary for gas exchange, vocalization, and maintaining blood pH, are organized as highly branched tubular epithelia. Tubular organs include arteries, veins, and lymphatics, high-speed passageways for delivery and uptake of nutrients, liquids, gases, and immune cells. The kidneys and components of the reproductive system are also epithelial tubes. Both the heart and central nervous system of many vertebrates begin as epithelial tubes. Thus, it is not surprising that defects in tube formation and maintenance underlie many human diseases. Accordingly, a thorough understanding how tubes form and are maintained is essential to developing better diagnostics and therapeutics. Among the best-characterized tubular organs are the Drosophila salivary gland and trachea, organs whose relative simplicity have allowed for in depth analysis of gene function, yielding key mechanistic insight into tube initiation, remodeling and maintenance. Here, we review our current understanding of salivary gland and trachea formation - highlighting recent discoveries into how these organs attain their final form and function.

Genome-wide analysis reveals a major role in cell fate maintenance and an unexpected role in endoreduplication for the Drosophila FoxA gene Fork head

Transcription factors drive organogenesis, from the initiation of cell fate decisions to the maintenance and implementation of these decisions. The Drosophila embryonic salivary gland provides an excellent platform for unraveling the underlying transcriptional networks of organ development because Drosophila is relatively unencumbered by significant genetic redundancy. The highly conserved FoxA family transcription factors are essential for various aspects of organogenesis in all animals that have been studied. Here, we explore the role of the single Drosophila FoxA protein Fork head (Fkh) in salivary gland organogenesis using two genome-wide strategies. A large-scale in situ hybridization analysis reveals a major role for Fkh in maintaining the salivary gland fate decision and controlling salivary gland physiological activity, in addition to its previously known roles in morphogenesis and survival. The majority of salivary gland genes (59%) are affected by fkh loss, mainly at later stages of salivary gland development. We show that global expression of Fkh cannot drive ectopic salivary gland formation. Thus, unlike the worm FoxA protein PHA-4, Fkh does not function to specify cell fate. In addition, Fkh only indirectly regulates many salivary gland genes, which is also distinct from the role of PHA-4 in organogenesis. Our microarray analyses reveal unexpected roles for Fkh in blocking terminal differentiation and in endoreduplication in the salivary gland and in other Fkh-expressing embryonic tissues. Overall, this study demonstrates an important role for Fkh in determining how an organ preserves its identity throughout development and provides an alternative paradigm for how FoxA proteins function in organogenesis.

The CrebA/Creb3-like transcription factors are major and direct regulators of secretory capacity

CrebA up-regulates expression of both the general protein machinery required in all cells for secretion and genes encoding cell type–specific secreted components.

Secretion occurs in all cells, with relatively low levels in most cells and extremely high levels in specialized secretory cells, such as those of the pancreas, salivary, and mammary glands. How secretory capacity is selectively up-regulated in specialized secretory cells is unknown. Here, we find that the CrebA/Creb3-like family of bZip transcription factors functions to up-regulate expression of both the general protein machinery required in all cells for secretion and of cell type–specific secreted proteins. Drosophila CrebA directly binds the enhancers of secretory pathway genes and is both necessary and sufficient to activate expression of every secretory pathway component gene examined thus far. Microarray profiling reveals that CrebA also up-regulates expression of genes encoding cell type–specific secreted components. Finally, we found that the human CrebA orthologues, Creb3L1 and Creb3L2, have the ability to up-regulate the secretory pathway in nonsecretory cell types.

Pak1 control of E-cadherin endocytosis regulates salivary gland lumen size and shape

Generating and maintaining proper lumen size and shape in tubular organs is essential for organ function. Here, we demonstrate a novel role for p21-activated kinase 1 (Pak1) in defining the size and shape of the Drosophila embryonic salivary gland lumen by regulating the size and elongation of the apical domain of individual cells. Pak1 mediates these effects by decreasing and increasing E-cadherin levels at the adherens junctions and basolateral membrane, respectively, through Rab5- and Dynamin-dependent endocytosis. We also demonstrate that Cdc42 and Merlin act together with Pak1 to control lumen size. A role for Pak1 in E-cadherin endocytosis is supported by our studies of constitutively active Pak1, which induces the formation of multiple intercellular lumens in the salivary gland in a manner dependent on Rab5, Dynamin and Merlin. These studies demonstrate a novel and crucial role for Pak1 and E-cadherin endocytosis in determining lumen size and shape, and also identify a mechanism for multiple lumen formation, a poorly understood process that occurs in normal embryonic development and pathological conditions.

Role of en and novel interactions between msh, ind, and vnd in dorsoventral patterning of the Drosophila brain and ventral nerve cord

Subdivision of the neuroectoderm into discrete gene expression domains is essential for the correct specification of neural stem cells (neuroblasts) during central nervous system development. Here, we extend our knowledge on dorsoventral (DV) patterning of the Drosophila brain and uncover novel genetic interactions that control expression of the evolutionary conserved homeobox genes ventral nervous system defective (vnd), intermediate neuroblasts defective (ind), and muscle segment homeobox (msh). We show that cross-repression between Ind and Msh stabilizes the border between intermediate and dorsal tritocerebrum and deutocerebrum, and that both transcription factors are competent to inhibit vnd expression. Conversely, Vnd segment-specifically affects ind expression; it represses ind in the tritocerebrum but positively regulates ind in the deutocerebrum by suppressing Msh. These data provide further evidence that in the brain, in contrast to the trunc, the precise boundaries between DV gene expression domains are largely established through mutual inhibition. Moreover, we find that the segment-polarity gene engrailed (en) regulates the expression of vnd, ind, and msh in a segment-specific manner. En represses msh and ind but maintains vnd expression in the deutocerebrum, is required for down-regulation of Msh in the tritocerebrum to allow activation of ind, and is necessary for maintenance of Ind in truncal segments. These results indicate that input from the anteroposterior patterning system is needed for the spatially restricted expression of DV genes in the brain and ventral nerve cord.

Drosophila laminins act as key regulators of basement membrane assembly and morphogenesis

Laminins are heterotrimeric molecules found in all basement membranes. In mammals, they have been involved in diverse developmental processes, from gastrulation to tissue maintenance. The Drosophila genome encodes two laminin alpha chains, one beta and one Gamma, which form two distinct laminin trimers. So far, only mutations affecting one or other trimer have been analysed. In order to study embryonic development in the complete absence of laminins, we mutated the gene encoding the sole laminin beta chain in Drosophila, LanB1, so that no trimers can be made. We show that LanB1 mutant embryos develop until the end of embryogenesis. Electron microscopy analysis of mutant embryos reveals that the basement membranes are absent and the remaining extracellular material appears disorganised and diffuse. Accordingly, abnormal accumulation of major basement membrane components, such as Collagen IV and Perlecan, is observed in mutant tissues. In addition, we show that elimination of LanB1 prevents the normal morphogenesis of most organs and tissues, including the gut, trachea, muscles and nervous system. In spite of the above structural roles for laminins, our results unravel novel functions in cell adhesion, migration and rearrangement. We propose that while an early function of laminins in gastrulation is not conserved in Drosophila and mammals, their function in basement membrane assembly and organogenesis seems to be maintained throughout evolution.

A targeted gain-of-function screen identifies genes affecting salivary gland morphogenesis/tubulogenesis in Drosophila

During development individual cells in tissues undergo complex cell-shape changes to drive the morphogenetic movements required to form tissues. Cell shape is determined by the cytoskeleton and cell-shape changes critically depend on a tight spatial and temporal control of cytoskeletal behavior. We have used the formation of the salivary glands in the Drosophila embryo, a process of tubulogenesis, as an assay for identifying factors that impinge on cell shape and the cytoskeleton. To this end we have performed a gain-of-function screen in the salivary glands, using a collection of fly lines carrying EP-element insertions that allow the overexpression of downstream-located genes using the UAS-Gal4 system. We used a salivary-gland-specific fork head-Gal4 line to restrict expression to the salivary glands, in combination with reporters of cell shape and the cytoskeleton. We identified a number of genes known to affect salivary gland formation, confirming the effectiveness of the screen. In addition, we found many genes not implicated previously in this process, some having known functions in other tissues. We report the initial characterization of a subset of genes, including chickadee, rhomboid1, egalitarian, bitesize, and capricious, through comparison of gain- and loss-of-function phenotypes.

From fate to function: the Drosophila trachea and salivary gland as models for tubulogenesis

Tube formation is a ubiquitous process required to sustain life in multicellular organisms. The tubular organs of adult mammals include the lungs, vasculature, digestive and excretory systems, as well as secretory organs such as the pancreas, salivary, prostate, and mammary glands. Other tissues, including the embryonic heart and neural tube, have requisite stages of tubular organization early in development. To learn the molecular and cellular basis of how epithelial cells are organized into tubular organs of various shapes and sizes, investigators have focused on the Drosophila trachea and salivary gland as model genetic systems for branched and unbranched tubes, respectively. Both organs begin as polarized epithelial placodes, which through coordinated cell shape changes, cell rearrangement, and cell migration form elongated tubes. Here, we discuss what has been discovered regarding the details of cell fate specification and tube formation in the two organs; these discoveries reveal significant conservation in the cellular and molecular events of tubulogenesis.

Rac function in epithelial tube morphogenesis

Epithelial cell migration and morphogenesis require dynamic remodeling of the actin cytoskeleton and cell-cell adhesion complexes. Numerous studies in cell culture and in model organisms have demonstrated the small GTPase Rac to be a critical regulator of these processes; however, little is known about Rac function in the morphogenic movements that drive epithelial tube formation. Here, we use the embryonic salivary glands of Drosophila to understand the role of Rac in epithelial tube morphogenesis. We show that inhibition of Rac function, either through loss of function mutations or dominant-negative mutations, disrupts salivary gland invagination and posterior migration. In contrast, constitutive activation of Rac induces motile behavior and subsequent cell death. We further show that Rac regulation of salivary gland morphogenesis occurs through modulation of cell-cell adhesion mediated by the E-cadherin/beta-catenin complex and that shibire, the Drosophila homolog of dynamin, functions downstream of Rac in regulating beta-catenin localization during gland morphogenesis. Our results demonstrate that regulation of cadherin-based adherens junctions by Rac is critical for salivary gland morphogenesis and that this regulation occurs through dynamin-mediated endocytosis.

Organ positioning in Drosophila requires complex tissue-tissue interactions

Positioning an organ with respect to other tissues is a complex process necessary for proper anatomical development and organ function. The local environment surrounding an organ can serve both as a substrate for migration and as a source of guidance cues that direct migration. Little is known about the factors guiding Drosophila salivary gland movement or about the contacts the glands establish along their migratory path. Here, we provide a detailed description of the spatial and temporal interactions between the salivary glands and surrounding tissues during embryogenesis. The glands directly contact five other tissues: the visceral mesoderm, gastric caecae, somatic mesoderm, fat body, and central nervous system. Mutational analysis reveals that all of the tissues tested in this study are important for normal salivary gland positioning; proper differentiation of the visceral and somatic mesoderm is necessary for the glands to attain their final correct position. We also provide evidence that the segment-polarity gene, gooseberry (gsb), controls expression of signals from the developing fat body that direct posterior migration of the glands. These data further the understanding of how organ morphology and position are determined by three-dimensional constraints and guidance cues provided by neighboring tissues.

A genetic screen identifies putative targets and binding partners of CREB-binding protein in the developing Drosophila eye

Drosophila CREB-binding protein (dCBP) is a very large multidomain protein, which belongs to the CBP/p300 family of proteins that were first identified by their ability to bind the CREB transcription factor and the adenoviral protein E1. Since then CBP has been shown to bind to >100 additional proteins and functions in a multitude of different developmental contexts. Among other activities, CBP is known to influence development by remodeling chromatin, by serving as a transcriptional coactivator, and by interacting with terminal members of several signaling transduction cascades. Reductions in CBP activity are the underlying cause of Rubinstein-Taybi syndrome, which is, in part, characterized by several eye defects, including strabismus, cataracts, juvenile glaucoma, and coloboma of the eyelid, iris, and lens. Development of the Drosophila melanogaster compound eye is also inhibited in flies that are mutant for CBP. However, the vast array of putative protein interactions and the wide-ranging roles played by CBP within a single tissue such as the retina can often complicate the analysis of CBP loss-of-function mutants. Through a series of genetic screens we have identified several genes that could either serve as downstream transcriptional targets or encode for potential CBP-binding partners and whose association with eye development has hitherto been unknown. The identification of these new components may provide new insight into the roles that CBP plays in retinal development. Of particular interest is the identification that the CREB transcription factor appears to function with CBP at multiple stages of retinal development.

A Rho GTPase Signaling Pathway Is Used Reiteratively in Epithelial Folding and Potentially Selects the Outcome of Rho Activation

A single Rho GTPase family member is capable of initiating several different processes, including cell cycle regulation, cytokinesis, cell migration, and transcriptional regulation . It is not clear, however, how the Rho protein selects which of these processes to initiate. Guanine nucleotide exchange factors (GEFs), proteins that activate Rho GTPases, could be important in making this selection. We show here that in vivo, DRhoGEF2, a GEF that is ubiquitously expressed and specific for Rho1, is reiteratively required for epithelial folding and invagination, but not for other processes regulated by Rho. The limitation of DRhoGEF2 function supports the hypothesis that the GEF selects the outcome of Rho activation. DRhoGEF2 exerts its effects in gastrulation through the regulation of Myosin II to orchestrate coordinated apical cell constriction. Apical myosin localization is also regulated by Concertina (Cta), a Galpha(12/13) family member that is thought to activate DRhoGEF2 and is itself activated by a putative ligand, Folded gastrulation (Fog). Fog and Cta also play a role in the morphogenetic events requiring DRhoGEF2, suggesting the existence of a conserved signaling pathway in which Fog, Cta, and DRhoGEF2 locally activate Myosin for epithelial invagination and folding.

Insect basic leucine zipper proteins and their role in cyclic AMP-dependent regulation of gene expression

The cAMP-protein kinase A (PKA) pathway is an important intracellular signal transduction cascade that can be activated by a large variety of stimuli. Activation or inhibition of this pathway will ultimately affect the transcriptional regulation of various genes through distinct responsive sites. In vertebrates, the best- characterized nuclear targets of PKA are the cyclic AMP response element-binding (CREB) proteins. It is now well established that CREB is not only regulated by PKA, but many other kinases can exert an effect as well. Since CREB-like proteins were also discovered in invertebrates, several studies unraveling their physiological functions in this category of metazoans have been performed. This review will mainly focus on the presence and regulation of CREB proteins in insects. Differences in transcriptional responses to the PKA pathway and other CREB-regulating stimuli between cells, tissues, and even organisms can be partially attributed to the presence of different CREB isoforms. In addition, the regulation of CREB appears to show some important differences between insects and vertebrates. Since CREB is a basic leucine zipper (bZip) protein, other insect members of this important family of transcriptional regulators will be briefly discussed as well.

The role of maternal CREB in early embryogenesis of Xenopus laevis

In Xenopus embryos, body patterning and cell specification are initiated by transcription factors, which are themselves transcribed during oogenesis, and their mRNAs are stored for use after fertilization. We have previously shown that the T-box transcription factor VegT is both necessary and sufficient to initiate transcription of all endoderm, and most mesoderm genes. In the absence of maternal VegT, no mesodermal organs (including the heart) or endodermal organs form. A second maternal transcription factor XTcf3 acts as a global repressor of transcription of dorsal genes, whose repression is inactivated on the dorsal side by a maternally encoded Wnt signaling pathway. In the absence of beta-catenin, no mesodermal or endodermal organs form. We show here that the maternally encoded transcription factor CREB is also essential for development. It is required for the initiation of expression of several mesodermal genes, including Xbra, Xcad2, and -3 and also regulates the cardiogenic gene Nkx 2-5. We show that maternal CREB-depleted embryos develop gastrulation defects that are rescued by the reintroduction of activated CREB mRNA. We conclude that maternal CREB must be added to the list of essential maternal transcription factors regulating cell specification in the early embryo.

Posterior migration of the salivary gland requires an intact visceral mesoderm and integrin function

The final overall shape of an organ and its position within the developing embryo arise as a consequence of both its intrinsic properties and its interactions with surrounding tissues. Here, we focus on the role of directed cell migration in shaping and positioning the Drosophila salivary gland. We demonstrate that the salivary gland turns and migrates along the visceral mesoderm to become properly oriented with respect to the overall embryo. We show that salivary gland posterior migration requires the activities of genes that position the visceral mesoderm precursors, such as heartless, thickveins, and tinman, but does not require a differentiated visceral mesoderm. We also demonstrate a role for integrin function in salivary gland migration. Although the mutations affecting salivary gland motility and directional migration cause defects in the final positioning of the salivary gland, most do not affect the length or diameter of the salivary gland tube. These findings suggest that salivary tube dimensions may be an intrinsic property of salivary gland cells.

Epithelial tube morphology is determined by the polarized growth and delivery of apical membrane

Formation of tubes of the correct size and shape is essential for viability of most organisms, yet little is understood of the mechanisms controlling tube morphology. We identified a new allele of hairy in a mutagenesis screen and showed that hairy mutations cause branching and bulging of the normally unbranched salivary tube, in part through prolonged expression of huckebein (hkb). HKB controls polarized cell shape change and apical membrane growth during salivary cell invagination via two downstream target genes, crumbs (crb), a determinant of the apical membrane, and klarsicht (klar), which mediates microtubule-dependent organelle transport. In invaginating salivary cells, crb and klar mediate growth and delivery of apical membrane, respectively, thus regulating the size and shape of the salivary tube.

Identification of fat-cell enhancer regions in Drosophila melanogaster

The insect fat body is a dynamic tissue involved in maintaining homeostasis. It functions not only in energy storage and intermediary metabolism but also in detoxification, communication and the immune response. Some of these functions are confined to distinct groups of fat body cells. In Drosophila melanogaster, discrete precursor-cell clusters populate the fat body [Hoshizaki, D.K., Blackburn, T., Price, C., Ghosh, M., Miles, K., Ragucci, M. and Sweis, R. (1994) Embryonic fat-cell lineage in Drosophila melanogaster. Development 120: 2489-2499; Hoshizaki, D.K., Lunz, R., Ghosh, M. and Johnson, W. (1995) Identification of fat-cell enhancer activity in Drosophila melanogaster using P-element enhancer traps. Genome 38: 497-506; Riechmann, V., Rehorn, K.P., Reuter, R. and Leptin, M. (1998) The genetic control of the distinction between fat body and gonadal mesoderm in Drosophila. Development 125: 713-723]. Whether these clusters populate defined morphological regions or whether they represent the precursors to functionally similar groups of fat-body cells has not been formally demonstrated. We have identified a 2.1 kb enhancer region from serpent (srp), a GATA transcription factor gene that is sufficient to induce fat-cell formation. This enhancer region drives expression in specific groups of precursor-cell clusters, which we show give rise to defined regions of the mature embryonic fat body. We present evidence that srp expression in different precursor fat cells is controlled by independent cis-acting regulatory regions, and we have tested the role of trans-acting factors in the specification of some of these cells. We suggest that the different positional cues regulating srp expression, and therefore general fat-cell specification, might also be involved in the functional specialization of fat cells. This may be a common mechanism in insects to explain the origin of biochemically distinct regions of the larval/adult fat body.

pasilla, the Drosophila homologue of the human Nova-1 and Nova-2 proteins, is required for normal secretion in the salivary gland

From a screen for genes expressed and required in the Drosophila salivary gland, we identified pasilla (ps), which encodes a set of proteins most similar to human Nova-1 and Nova-2. Nova-1 and Nova-2 are nuclear RNA-binding proteins normally expressed in the CNS where they directly regulate splicing. In patients suffering from paraneoplastic opsoclonus myoclonus ataxia (POMA), Nova-1 and Nova-2 proteins are present as auto-antigens. Consistent with a role in splicing, PS is localized to nuclear puncta. The salivary glands of ps mutants internalize normally and maintain epithelial polarity. However, the mutant salivary glands develop irregularities in overall morphology and have defects in apical secretion. The secretory defects in ps mutants provide a potential mechanism for the loss of motor function observed in POMA patients.

Drosophila-raf acts to elaborate dorsoventral pattern in the ectoderm of developing embryos

In the early Drosophila embryo the activity of the EGF-receptor (Egfr) is required to instruct cells to adopt a ventral neuroectodermal fate. Using a gain-of-function mutation we showed that D-raf acts to transmit this and other late-acting embryonic Egfr signals. A novel role for D-raf was also identified in lateral cell development using partial loss-of-function D-raf mutations. Thus, we provide evidence that zygotic D-raf acts to specify cell fates in two distinct pathways that generate dorsoventral pattern within the ectoderm. These functional requirements for D-raf activity occur subsequent to its maternal role in organizing the anterioposterior axis. The consequences of eliminating key D-raf regulatory domains and specific serine residues in the transmission of Egfr and lateral epidermal signals were also addressed here.

Organ formation in Drosophila: specification and morphogenesis of the salivary gland

The Drosophila salivary gland has emerged as an outstanding model system for the process of organ formation. Many of the component steps, from initial regional specification through cell specialization and morphogenesis, are known and many of the genes required for these different processes have been identified. The salivary gland is a relatively simple organ; the entire gland comprises of only two major cell types, which derive from a single contiguous primordium. Salivary cells cease dividing once they are specified, and organ growth is achieved simply by an increase in size of individual cells, thus eliminating concerns about the potential unequal distribution of determinants during mitosis. Drosophila salivary glands form by the same cellular mechanisms as organs in higher organisms, including regulated cell shape changes, cell intercalation and directed cell migration. Thus, learning how these events are coordinated for tissue morphogenesis in an organism for which the genetic and molecular tools are unsurpassed should provide excellent paradigms for dissecting related processes in the more intricate organs of more complicated species.

The transcription factor Schnurri plays a dual role in mediating Dpp signaling during embryogenesis

Decapentaplegic (Dpp), a homolog of vertebrate bone morphogenic protein 2/4, is crucial for embryonic patterning and cell fate specification in Drosophila. Dpp signaling triggers nuclear accumulation of the Smads Mad and Medea, which affect gene expression through two distinct mechanisms: direct activation of target genes and relief of repression by the nuclear protein Brinker (Brk). The zinc-finger transcription factor Schnurri (Shn) has been implicated as a co-factor for Mad, based on its DNA-binding ability and evidence of signaling dependent interactions between the two proteins. A key question is whether Shn contributes to both repression of brk as well as to activation of target genes. We find that during embryogenesis, brk expression is derepressed in shn mutants. However, while Mad is essential for Dpp-mediated repression of brk, the requirement for shn is stage specific. Analysis of brk; shn double mutants reveals that upregulation of brk does not account for all aspects of the shn mutant phenotype. Several Dpp target genes are expressed at intermediate levels in double mutant embryos, demonstrating that shn also provides a brk-independent positive input to gene activation. We find that Shn-mediated relief of brk repression establishes broad domains of gene activation, while the brk-independent input from Shn is crucial for defining the precise limits and levels of Dpp target gene expression in the embryo.

Drosophila AP-1: lessons from an invertebrate

In recent years, studies in the model organism Drosophila melanogaster have contributed significant insights into the molecular and developmental biology of the AP-1 transcription factors Jun and Fos. Powerful genetic and biochemical approaches uncovered a baffling complexity and variability of the signaling connections to and from AP-1. The range of biological processes that Jun and Fos regulate in this organism is equally multi-faceted. Regulatory interactions between AP-1 and JNK, ERK, TGFbeta, Notch or other signaling systems have been implicated in the control of a multitude of embryonic and adult events, including tissue closure processes, patterning of eye, gut and wing, as well as apoptosis. Here we review the information that has been gathered on Drosophila AP-1 in signal transduction and on the developmental and cellular functions controlled by AP-1-mediated signals in the fly. Lessons learned from the studies on AP-1 in Drosophila may contribute to our general understanding, beyond species boundaries, of this fundamental class of transcriptional regulators.

Early genes required for salivary gland fate determination and morphogenesis in Drosophila melanogaster

Studies of Drosophila salivary gland formation have elucidated the regulatory pathway by which the salivary gland fate is determined and the morphogenetic processes by which the primordial cells are internalized to form the tubular glands. Both the position of the salivary primordia and the number of cells recruited to a salivary gland fate are established through a combination of the localized expression of the transcription factors SEX COMBS REDUCED (SCR), TEASHIRT (TSH) and ABDOMINAL-B (ABD-B), and localized DPP-signaling. Similarly, the distinction between the two major cell types, duct and secretory, is determined by spatially limited EGF-signaling. Salivary gland formation also requires the function of two transcription factors expressed in nearly all cells of the developing embryo, EXTRADENTICLE (EXD) and HOMOTHORAX (HTH). Once the salivary gland fate is determined, cells of the secretory primordia are internalized by an apical constriction mode of invagination. We have characterized three genes encoding transcription factors, trachealess (trh), hückebein (hkb), and fork head (fkh), that are downstream targets of the salivary gland regulators. Mutations in these transcription factors profoundly affect salivary gland morphogenesis. trh is required for the formation of the salivary duct tubes. hkb determines the order of secretory cell invagination, a regulated process critical for determining the final shape of the salivary gland. fkh has two early roles in salivary gland formation. fkh both promotes secretory cell survival and facilitates secretory cell internalization. trh, hkb, and fkh are involved in the formation of not only the salivary duct and secretory tubes, but also of other tubular structures, such as the trachea and the gut endoderm. We propose that trh, hkb, and fkh may serve as "morphogenetic cassettes" responsible for forming tubular structures in a variety of tissues.

Fork head prevents apoptosis and promotes cell shape change during formation of the Drosophila salivary glands

The secretory tubes of the Drosophila salivary glands are formed by the regulated, sequential internalization of the primordia. Secretory cell invagination occurs by a change in cell shape that includes basal nuclear migration and apical membrane constriction. In embryos mutant for fork head (fkh), which encodes a transcription factor homologous to mammalian hepatocyte nuclear factor 3beta (HNF-3beta), the secretory primordia are not internalized and secretory tubes do not form. Here, we show that secretory cells of fkh mutant embryos undergo extensive apoptotic cell death following the elevated expression of the apoptotic activator genes, reaper and head involution defective. We rescue the secretory cell death in the fkh mutants and show that the rescued cells still do not invaginate. The rescued fkh secretory cells undergo basal nuclear migration in the same spatial and temporal pattern as in wild-type secretory cells, but do not constrict their apical surface membranes. Our findings suggest at least two roles for fkh in formation of the embryonic salivary glands: an early role in promoting survival of the secretory cells, and a later role in secretory cell invagination, specifically in the constriction of the apical surface membrane.

Salivary gland development in Drosophila melanogaster

The Drosophila salivary gland is proving to be an excellent experimental system for understanding how cells commit to specific developmental programs and, once committed, how cells implement such decisions. Through genetic studies, the factors that determine where salivary glands will form, the number of cells committed to a salivary gland fate, and the distinction between the two major cell types (secretory cells and duct cells) have been discovered. Within the next few years, we will learn the molecular details of the interactions among the salivary gland regulators and salivary gland target genes. We will also learn how the early-expressed salivary gland genes coordinate their activities to mediate the morphogenetic movements required to form the salivary gland and the changes in cell physiology required for high secretory activity.

Organ shape in the Drosophila salivary gland is controlled by regulated, sequential internalization of the primordia

During Drosophila development, the salivary primordia are internalized to form the salivary gland tubes. By analyzing immuno-stained histological sections and scanning electron micrographs of multiple stages of salivary gland development, we show that internalization occurs in a defined series of steps, involves coordinated cell shape changes and begins with the dorsal-posterior cells of the primordia. The ordered pattern of internalization is critical for the final shape of the salivary gland. In embryos mutant for huckebein (hkb), which encodes a transcription factor, or faint sausage (fas), which encodes a cell adhesion molecule, internalization begins in the center of the primordia, and completely aberrant tubes are formed. The sequential expression of hkb in selected cells of the primordia presages the sequence of cell movements. We propose that hkb dictates the initial site of internalization, the order in which invagination progresses and, consequently, the final shape of the organ. We propose that fas is required for hkb-dependent signaling events that coordinate internalization.

Regulation and function of Scr, exd, and hth in the Drosophila salivary gland

Salivary gland formation in the Drosophila embryo is dependent on the homeotic gene Sex combs reduced (Scr). When Scr function is missing, salivary glands do not form, and when SCR is expressed everywhere in the embryo, salivary glands form in new places. Scr is normally expressed in all the cells that form the salivary gland. However, as the salivary gland invaginates, Scr mRNA and protein disappear. Homeotic genes, such as Scr, specify tissue identity by regulating the expression of downstream target genes. For many homeotic proteins, target gene specificity is achieved by cooperatively binding DNA with cofactors. Therefore, it is likely that SCR also requires a cofactor(s) to specifically bind to DNA and regulate salivary gland target gene expression. Here, we show that two homeodomain-containing proteins encoded by the extradenticle (exd) and homothorax (hth) genes are also required for salivary gland formation. exd and hth function at two levels: (1) exd and hth are required to maintain the expression of Scr in the salivary gland primordia prior to invagination and (2) exd and hth are required in parallel with Scr to regulate the expression of downstream salivary gland genes. We also show that Scr regulates the nuclear localization of EXD in the salivary gland primordia through repression of homothorax (hth) expression, linking the regulation of Scr activity to the disappearance of Scr expression in invaginating salivary glands.

Essential role of CREB family proteins during Xenopus embryogenesis

The leucine zipper transcription factors cAMP response element binding protein (CREB), cAMP response element modulatory protein (CREM) and activating transcription factor 1 (ATF1) bind to the cAMP response element (CRE) with the palindromic consensus sequence TGACGTCA. Their transcriptional activities are dependent on serine phosphorylation induced by various extracellular signals such as hormones, growth factors and neurotransmitters. Here we show that CREB is the predominant CRE-binding protein in Xenopus embryos and that it plays an essential role during early development. The importance of CREB for morphogenetic processes was assessed by injection of RNA encoding a dominant-negative form of CREB that is fused to a truncated progesterone receptor ligand binding domain. In this fusion protein, a dominant-negative function can be induced by application of the synthetic steroid RU486 at given developmental stages. The inhibition of CREB at blastula and early gastrula stages leads to severe posterior defects of the embryos reflected by strong spina bifida, whereas the inhibition of CREB at the beginning of neurulation resulted in stunted embryos with microcephaly. In these embryos, initial induction of neural and mesodermal tissues is not dependent on CREB function, as genes such as Otx2, Krox20, Shh and MyoD are still expressed in injected embryos. But the expression domains of Otx2 and MyoD were found to be distorted reflecting the abnormal development in both neural and somitic derivatives. In summary, our data show that CREB is essential during several developmental stages of Xenopus embryogenesis.

The Berkeley Drosophila Genome Project gene disruption project: Single P-element insertions mutating 25% of vital Drosophila genes

A fundamental goal of genetics and functional genomics is to identify and mutate every gene in model organisms such as Drosophila melanogaster. The Berkeley Drosophila Genome Project (BDGP) gene disruption project generates single P-element insertion strains that each mutate unique genomic open reading frames. Such strains strongly facilitate further genetic and molecular studies of the disrupted loci, but it has remained unclear if P elements can be used to mutate all Drosophila genes. We now report that the primary collection has grown to contain 1045 strains that disrupt more than 25% of the estimated 3600 Drosophila genes that are essential for adult viability. Of these P insertions, 67% have been verified by genetic tests to cause the associated recessive mutant phenotypes, and the validity of most of the remaining lines is predicted on statistical grounds. Sequences flanking >920 insertions have been determined to exactly position them in the genome and to identify 376 potentially affected transcripts from collections of EST sequences. Strains in the BDGP collection are available from the Bloomington Stock Center and have already assisted the research community in characterizing >250 Drosophila genes. The likely identity of 131 additional genes in the collection is reported here. Our results show that Drosophila genes have a wide range of sensitivity to inactivation by P elements, and provide a rationale for greatly expanding the BDGP primary collection based entirely on insertion site sequencing. We predict that this approach can bring >85% of all Drosophila open reading frames under experimental control.

Cell fate specification in the Drosophila salivary gland: the integration of homeotic gene function with the DPP signaling cascade

Salivary gland formation in the Drosophila embryo is linked to the expression of the homeotic gene Sex combs reduced (Scr). When Scr function is missing, salivary glands do not form, and when SCR is expressed everywhere, salivary glands form in new places. However, not every cell that expresses Scr is recruited to a salivary gland fate. Along the anterior-posterior axis, the posteriorly expressed proteins encoded by the teashirt (tsh) and Abdominal-B (Abd-B) genes block SCR activation of salivary gland genes, and along the dorsal-ventral axis, the secreted signaling molecule encoded by decapentaplegic (dpp) prevents activation of salivary gland genes by SCR in dorsal regions of parasegment 2. We have identified five downstream components in the DPP signaling cascade required to block salivary gland gene activation. These components include two known receptors, the type I receptor encoded by the thick veins (tkv) gene and the type II receptor encoded by the punt (put) gene; two of the four known Drosophila members of the Smad family of proteins which transduce signals from the receptors to the nucleus, Mothers against dpp (Mad) and Medea (Med); and, finally, a large zinc-finger transcription factor encoded by the schnurri (shn) gene. These results reveal how anterior-posterior and dorsal-ventral patterning information is integrated at the level of organ-specific gene expression.

Regulation and formation of the Drosophila salivary glands

The homeotic gene, Sex combs reduced (Scr), is a master regulator of Drosophila salivary gland formation. Embryos in which Scr function is missing do not form salivary glands, and embryos in which SCR protein is expressed everywhere form extra salivary glands. However, other known proteins, including the homeotic protein Abdominal-B, the unusual zinc finger protein Teashirt, and the secreted signaling molecule Decapentaplegic (a TGF-beta family member), limit the recruitment of SCR-expressing cells to salivary glands. To learn the molecular details of how salivary gland gene expression is controlled and as a first step toward understanding how the SCR transcription factor controls salivary gland morphogenesis, we screened for genes expressed in the developing salivary gland. Among our best candidates for potential direct downstream targets of SCR in the salivary gland are the genes trachealess (trh), dCREB-A, jalapeño, and Semaphorin II (SemaII). Our genetic studies suggest distinct and important roles for each of these genes in salivary gland morphogenesis. Current work includes studying the molecular interactions between SCR and these downstream target genes and asking how target genes coordinate their activities to effect the cell biological changes required to build functional salivary glands.

Viral mimicry: common mode of association with HCF by VP16 and the cellular protein LZIP

Upon infection of human cells, the herpes simplex virus protein VP16 associates with the endogenous cell-proliferation factor HCF. VP16 can also associate with HCFs from invertebrates, suggesting that VP16 mimics a cellular protein whose interaction with HCF has been conserved. Here, we show that VP16 mimics the human basic leucine-zipper protein LZIP, which, through association with HCF, may control cell-cycle progression. VP16 and LZIP share a tetrapeptide motif-D/EHXY-used to associate with human HCF. The LZIP-related Drosophila protein BBF-2/dCREB-A contains this HCF-binding motif, indicating that the LZIP-HCF interaction has been conserved during metazoan evolution.