faint sausage encodes a novel extracellular protein of the immunoglobulin superfamily required for cell migration and the establishment of normal axonal pathways in the Drosophila nervous system

Shape of my heart: Cell-cell adhesion and cytoskeletal dynamics during Drosophila cardiac morphogenesis

The fruit fly Drosophila melanogaster has recently emerged as an excellent system to investigate the genetics of cardiovascular development and disease. Drosophila provides an inexpensive and genetically-tractable in vivo system with a large number of conserved features. In addition, the Drosophila embryo is transparent, and thus amenable to time-lapse fluorescence microscopy, as well as biophysical and pharmacological manipulations. One of the conserved aspects of heart development from Drosophila to humans is the initial assembly of a tube. Here, we review the cellular behaviours and molecular dynamics important for the initial steps of heart morphogenesis in Drosophila, with particular emphasis on the cell-cell adhesion and cytoskeletal networks that cardiac precursors use to move, coordinate their migration, interact with other tissues and eventually sculpt a beating heart.

Faithful mRNA splicing depends on the Prp19 complex subunit faint sausage and is required for tracheal branching morphogenesis in Drosophila

Morphogenesis requires the dynamic regulation of gene expression, including transcription, mRNA maturation and translation. Dysfunction of the general mRNA splicing machinery can cause surprisingly specific cellular phenotypes, but the basis for these effects is not clear. Here, we show that the Drosophila faint sausage (fas) locus, which is implicated in epithelial morphogenesis and has previously been reported to encode a secreted immunoglobulin domain protein, in fact encodes a subunit of the spliceosome-activating Prp19 complex, which is essential for efficient pre-mRNA splicing. Loss of zygotic fas function globally impairs the efficiency of splicing, and is associated with widespread retention of introns in mRNAs and dramatic changes in gene expression. Surprisingly, despite these general effects, zygotic fas mutants show specific defects in tracheal cell migration during mid-embryogenesis when maternally supplied splicing factors have declined. We propose that tracheal branching, which relies on dynamic changes in gene expression, is particularly sensitive for efficient spliceosome function. Our results reveal an entry point to study requirements of the splicing machinery during organogenesis and provide a better understanding of disease phenotypes associated with mutations in general splicing factors.

Drosophila Malpighian Tubules: A Model for Understanding Kidney Development, Function, and Disease

The Malpighian tubules of insects are structurally simple but functionally important organs, and their integrity is important for the normal excretory process. They are functional analogs of human kidneys which are important physiological organs as they maintain water and electrolyte balance in the blood and simultaneously help the body to get rid of waste and toxic products after various metabolic activities. In addition, it receives early indications of insults to the body such as immune challenge and other toxic components and is essential for sustaining life. According to National Vital Statistics Reports 2016, renal dysfunction has been ranked as the ninth most abundant cause of death in the USA. This chapter provides detailed descriptions of Drosophila Malpighian tubule development, physiology, immune function and also presents evidences that Malpighian tubules can be used as a model organ system to address the fundamental questions in developmental and functional disorders of the kidney.

Functional Conservation of the Glide/Gcm Regulatory Network Controlling Glia, Hemocyte, and Tendon Cell Differentiation in Drosophila

High-throughput screens allow us to understand how transcription factors trigger developmental processes, including cell specification. A major challenge is identification of their binding sites because feedback loops and homeostatic interactions may mask the direct impact of those factors in transcriptome analyses. Moreover, this approach dissects the downstream signaling cascades and facilitates identification of conserved transcriptional programs. Here we show the results and the validation of a DNA adenine methyltransferase identification (DamID) genome-wide screen that identifies the direct targets of Glide/Gcm, a potent transcription factor that controls glia, hemocyte, and tendon cell differentiation in Drosophila. The screen identifies many genes that had not been previously associated with Glide/Gcm and highlights three major signaling pathways interacting with Glide/Gcm: Notch, Hedgehog, and JAK/STAT, which all involve feedback loops. Furthermore, the screen identifies effector molecules that are necessary for cell-cell interactions during late developmental processes and/or in ontogeny. Typically, immunoglobulin (Ig) domain-containing proteins control cell adhesion and axonal navigation. This shows that early and transiently expressed fate determinants not only control other transcription factors that, in turn, implement a specific developmental program but also directly affect late developmental events and cell function. Finally, while the mammalian genome contains two orthologous Gcm genes, their function has been demonstrated in vertebrate-specific tissues, placenta, and parathyroid glands, begging questions on the evolutionary conservation of the Gcm cascade in higher organisms. Here we provide the first evidence for the conservation of Gcm direct targets in humans. In sum, this work uncovers novel aspects of cell specification and sets the basis for further understanding of the role of conserved Gcm gene regulatory cascades.

Patterns of growth and tract formation during the early development of secondary lineages in the Drosophila larval brain

The Drosophila brain consists of a relatively small number of invariant, genetically determined lineages which provide a model to study the relationship between gene function and neuronal architecture. In following this long-term goal, we reconstruct the morphology (projection pattern and connectivity) and gene expression patterns of brain lineages throughout development. In this article, we focus on the secondary phase of lineage morphogenesis, from the reactivation of neuroblast proliferation in the first larval instar to the time when proliferation ends and secondary axon tracts have fully extended in the late third larval instar. We have reconstructed the location and projection of secondary lineages at close (4 h) intervals and produced a detailed map in the form of confocal z-projections and digital three-dimensional models of all lineages at successive larval stages. Based on these reconstructions, we could compare the spatio-temporal pattern of axon formation and morphogenetic movements of different lineages in normal brain development. In addition to wild type, we reconstructed lineage morphology in two mutant conditions. (1) Expressing the construct UAS-p35 which rescues programmed cell death we could systematically determine which lineages normally lose hemilineages to apoptosis. (2) so-Gal4-driven expression of dominant-negative EGFR ablated the optic lobe, which allowed us to conclude that the global centrifugal movement normally affecting the cell bodies of lateral lineages in the late larva is causally related to the expansion of the optic lobe, and that the central pattern of axonal projections of these lineages is independent of the presence or absence of the optic lobe.

Shaping up for action: the path to physiological maturation in the renal tubules of Drosophila

The Malpighian tubule is the main organ for excretion and osmoregulation in most insects. During a short period of embryonic development the tubules of Drosophila are shaped, undergo differentiation and become precisely positioned in the body cavity, so they become fully functional at the time of larval hatching a few hours later. In this review I explore three developmental events on the path to physiological maturation. First, I examine the molecular and cellular mechanisms that generate organ shape, focusing on the process of cell intercalation that drives tubule elongation, the roles of the cytoskeleton, the extracellular matrix and how intercalation is coordinated at the tissue level. Second, I look at the genetic networks that control the physiological differentiation of tubule cells and consider how distinctive physiological domains in the tubule are patterned. Finally, I explore how the organ is positioned within the body cavity and consider the relationship between organ position and function.

The developmental, molecular, and transport biology of Malpighian tubules

Molecular biology is reaching new depths in our understanding of the development and physiology of Malpighian tubules. In Diptera, Malpighian tubules derive from ectodermal cells that evaginate from the primitive hindgut and subsequently undergo a sequence of orderly events that culminates in an active excretory organ by the time the larva takes its first meal. Thereafter, the tubules enlarge by cell growth. Just as modern experimental strategies have illuminated the development of tubules, genomic, transcriptomic, and proteomic studies have uncovered new tubule functions that serve immune defenses and the breakdown and renal clearance of toxic substances. Moreover, genes associated with specific diseases in humans are also found in flies, some of which, astonishingly, express similar pathophenotypes. However, classical experimental approaches continue to show their worth by distinguishing between -omic possibilities and physiological reality while providing further detail about the rapid regulation of the transport pathway through septate junctions and the reversible assembly of proton pumps.

Genetic screen in Drosophila melanogaster uncovers a novel set of genes required for embryonic epithelial repair

The wound healing response is an essential mechanism to maintain the integrity of epithelia and protect all organisms from the surrounding milieu. In the "purse-string" mechanism of wound closure, an injured epithelial sheet cinches its hole closed via an intercellular contractile actomyosin cable. This process is conserved across species and utilized by both embryonic as well as adult tissues, but remains poorly understood at the cellular level. In an effort to identify new players involved in purse-string wound closure we developed a wounding strategy suitable for screening large numbers of Drosophila embryos. Using this methodology, we observe wound healing defects in Jun-related antigen (encoding DJUN) and scab (encoding Drosophila alphaPS3 integrin) mutants and performed a forward genetics screen on the basis of insertional mutagenesis by transposons that led to the identification of 30 lethal insertional mutants with defects in embryonic epithelia repair. One of the mutants identified is an insertion in the karst locus, which encodes Drosophila beta(Heavy)-spectrin. We show beta(Heavy)-spectrin (beta(H)) localization to the wound edges where it presumably exerts an essential function to bring the wound to normal closure.

Drosophila vesicular monoamine transporter mutants can adapt to reduced or eliminated vesicular stores of dopamine and serotonin

Physiologic and pathogenic changes in amine release induce dramatic behavioral changes, but the underlying cellular mechanisms remain unclear. To investigate these adaptive processes, we have characterized mutations in the Drosophila vesicular monoamine transporter (dVMAT), which is required for the vesicular storage of dopamine, serotonin, and octopamine. dVMAT mutant larvae show reduced locomotion and decreased electrical activity in motoneurons innervating the neuromuscular junction (NMJ) implicating central amines in the regulation of these activities. A parallel increase in evoked glutamate release by the motoneuron is consistent with a homeostatic adaptation at the NMJ. Despite the importance of aminergic signaling for regulating locomotion and other behaviors, adult dVMAT homozygous null mutants survive under conditions of low population density, thus allowing a phenotypic characterization of adult behavior. Homozygous mutant females are sterile and show defects in both egg retention and development; males also show reduced fertility. Homozygotes show an increased attraction to light but are mildly impaired in geotaxis and escape behaviors. In contrast, heterozygous mutants show an exaggerated escape response. Both hetero- and homozygous mutants demonstrate an altered behavioral response to cocaine. dVMAT mutants define potentially adaptive responses to reduced or eliminated aminergic signaling and will be useful to identify the underlying molecular mechanisms.

Making tubes in the Drosophila embryo

Epithelial and endothelial tubes come in various shapes and sizes and form the basic units of many tubular organs. During embryonic development, single unbranched tubes as well as highly branched networks of tubes form from simple sheets of cells by several morphogenic movements. Studies of tube formation in the Drosophila embryo have greatly advanced our understanding of the cellular and molecular mechanisms by which tubes are formed. This review highlights recent progress on formation of the hindgut, Malpighian tubules, proventriculus, salivary gland, and trachea of the Drosophila embryo, focusing on the cellular events that form each tube and their genetic requirements.

Condensation of the central nervous system in embryonic Drosophila is inhibited by blocking hemocyte migration or neural activity

Condensation is a process whereby a tissue undergoes a coordinated decrease in size and increase in cellular density during development. Although it occurs in many developmental contexts, the mechanisms underlying this process are largely unknown. Here, we investigate condensation in the embryonic Drosophila ventral nerve cord (VNC). Two major events coincide with condensation during embryogenesis: the deposition of extracellular matrix by hemocytes, and the onset of central nervous system activity. We find that preventing hemocyte migration by removing the function of the Drosophila VEGF receptor homologue, Pvr, or by disrupting Rac1 function in these cells, inhibits condensation. In the absence of hemocytes migrating adjacent to the developing VNC, the extracellular matrix components Collagen IV, Viking and Peroxidasin are not deposited around this tissue. Blocking neural activity by targeted expression of tetanus toxin light chain or an inwardly rectifying potassium channel also inhibits condensation. We find that disrupting Rac1 function in either glia or neurons, including those located in the nerve cord, causes a similar phenotype. Our data suggest that condensation of the VNC during Drosophila embryogenesis depends on both hemocyte-deposited extracellular matrix and neural activity, and allow us to propose a mechanism whereby these processes work together to shape the developing central nervous system.

Identification and characterization of two novel brain-derived immunoglobulin superfamily members with a unique structural organization

We recently used a differential display PCR screen to identify secreted and transmembrane proteins that are highly expressed in the developing rat basilar pons, a prominent ventral hindbrain nucleus used as a model for studies of neuronal migration, axon outgrowth, and axon-target recognition. Here we describe cloning and characterization of one of these molecules, now called MDGA1, and a closely related homologue, MDGA2. Analyses of the full-length coding region of MDGA1 and MDGA2 indicate that they encode proteins that comprise a novel subgroup of the Ig superfamily and have a unique structural organization consisting of six immunoglobulin (Ig)-like domains followed by a single MAM domain. Biochemical characterization demonstrates that MDGA1 and MDGA2 proteins are highly glycosylated, and that MDGA1 is tethered to the cell membrane by a GPI anchor. The MDGAs are differentially expressed by subpopulations of neurons in both the central and peripheral nervous systems, including neurons of the basilar pons, inferior olive, cerebellum, cerebral cortex, olfactory bulb, spinal cord, and dorsal root and trigeminal ganglia. Little or no MDGA expression is detected outside of the nervous system of developing rats. The similarity of MDGAs to other Ig-containing molecules and their temporal-spatial patterns of expression within restricted neuronal populations, for example migrating pontine neurons and D1 spinal interneurons, suggest a role for these novel proteins in regulating neuronal migration, as well as other aspects of neural development, including axon guidance.

Constructing an organ: the Drosophila salivary gland as a model for tube formation

Tubes are required in metazoans to transport the liquids and gases that sustain life. The conservation of molecules and mechanisms involved in tube formation suggests that what we learn by studying simple systems will apply to related processes in higher animals. Studies over the past 10 years have revealed the molecules that specify cell fate in Drosophila salivary gland and the cellular events that mediate tube morphogenesis. Here, we discuss how anterior-posterior and dorsal-ventral patterning information specifies both the position of salivary-gland primordia and how many cells they contain. We examine the transformation of a polarized epithelial sheet into an elongated, unbranched tube, and the intrinsic and extrinsic factors that influence the final position of the salivary gland.

It takes guts: the Drosophila hindgut as a model system for organogenesis

The Drosophila hindgut is fruitful territory for investigation of events common to many types of organogenesis. The development of the Drosophila hindgut provides, in microcosm, a genetic model system for studying processes such as establishment (patterning) of an epithelial primordium, its internalization by gastrulation, development of left--right asymmetric looping, patterning in both the anteroposterior and dorsoventral axes, innervation, investment of an epithelium with mesoderm, reciprocal epitheliomesenchymal interactions, cell shape change, and cell rearrangement. We review the genetic control of these processes during development of the Drosophila hindgut, and compare these to related processes in other bilaterians, particularly vertebrates. We propose that caudal/Cdx, brachyenteron/Brachyury, fork head/HNF-3, and wingless/Wnt constitute a conserved "cassette" of genes expressed in the blastopore and later in the gut, involved in posterior patterning, cell rearrangement, and gut maintenance. Elongation of the internalized Drosophila hindgut primordium is similar to elongation of the archenteron and also of the entire embryonic axis (both during and after gastrulation), as well as of various tubules (e.g., nephric ducts, Malpighian tubules), as it is driven by cell rearrangement. The genes drumstick, bowl, and lines (which encode putative transcriptional regulators) are required for this cell rearrangement, as well as for spatially localized gene expression required to establish the three morphologically distinct subregions of the hindgut. Expression of signaling molecules regulated by drumstick, bowl, and lines, in particular of the JAK/STAT activator Unpaired at the hindgut anterior, may play a role in controlling hindgut cell rearrangement. Other cell signaling molecules expressed in the hindgut epithelium are required to establish its normal size (Dpp and Hh), and to establish and maintain the hindgut visceral mesoderm (Wg and Hh). Both maternal gene activity and zygotic gene activity are required for asymmetric left--right looping of the hindgut. Some of the same genes (caudal and brachyenteron) required for embryonic hindgut development also act during pupation to construct a new hindgut from imaginal cells. Application of the plethora of genetic techniques available in Drosophila, including forward genetic screens, should identify additional genes controlling hindgut development and thus shed light on a variety of common morphogenetic processes.

Epithelial cell polarity and cell junctions in Drosophila

The polarized architecture of epithelial cells and tissues is a fundamental determinant of animal anatomy and physiology. Recent progress made in the genetic and molecular analysis of epithelial polarity and cellular junctions in Drosophila has led to the most detailed understanding of these processes in a whole animal model system to date. Asymmetry of the plasma membrane and the differentiation of membrane domains and cellular junctions are controlled by protein complexes that assemble around transmembrane proteins such as DE-cadherin, Crumbs, and Neurexin IV, or other cytoplasmic protein complexes that associate with the plasma membrane. Much remains to be learned of how these complexes assemble, establish their polarized distribution, and contribute to the asymmetric organization of epithelial cells.

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.

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.

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.

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.

Identification of genes controlling malpighian tubule and other epithelial morphogenesis in Drosophila melanogaster

The Drosophila Malpighian tubule is a model system for studying genetic mechanisms that control epithelial morphogenesis. From a screen of 1800 second chromosome lethal lines, by observing uric acid deposits in unfixed inviable embryos, we identified five previously described genes (barr, fas, flb, raw, and thr) and one novel gene, walrus (wal), that affect Malpighian tubule morphogenesis. Phenotypic analysis of these mutant embryos allows us to place these genes, along with other previously described genes, into a genetic pathway that controls Malpighian tubule development. Specifically, wal affects evagination of the Malpighian tubule buds, fas and thr affect bud extension, and barr, flb, raw, and thr affect tubule elongation. In addition, these genes were found to have different effects on development of other epithelial structures, such as foregut and hindgut morphogenesis. Finally, from the same screen, we identified a second novel gene, drumstick, that affects only foregut and hindgut morphogenesis.

Posterior gut development in Drosophila: a model system for identifying genes controlling epithelial morphogenesis

The posterior gut of the Drosophila embryo, consisting of hindgut and Malpighian tubules, provides a simple, well-defined system where it is possible to use a genetic approach to define components essential for epithelial morphogenesis. We review here the advantages of Drosophila as a model genetic organism, the morphogenesis of the epithelial structures of the posterior gut, and what is known about the genetic requirements to form these structures. In overview, primordia are patterned by expression of hierarchies of transcription factors; this leads to localized expression of cell signaling molecules, and finally, to the least understood step: modulation of cell adhesion and cell shape. We describe approaches to identify additional genes that are required for morphogenesis of these simple epithelia, particularly those that might play a structural role by affecting cell adhesion and cell shape.