A molecular aspect of hematopoiesis and endoderm development common to vertebrates and Drosophila

Genetic mosaic analysis reveals that GATA-4 is required for proper differentiation of mouse gastric epithelium

During mouse embryogenesis GATA-4 is expressed first in primitive endoderm and then in definitive endoderm derivatives, including glandular stomach and intestine. To explore the role of GATA-4 in specification of definitive gastric endoderm, we generated chimeric mice by introducing Gata4(-/-) ES cells into ROSA26 morulae or blastocysts. In E14.5 chimeras, Gata4(-/-) cells were represented in endoderm lining the proximal and distal stomach. These cells expressed early cytodifferentiation markers, including GATA-6 and ApoJ. However, by E18.5, only rare patches of Gata4(-/-) epithelium were evident in the distal stomach. This heterotypic epithelium had a squamous morphology and did not express markers associated with differentiation of gastric epithelial cell lineages. Sonic Hedgehog, an endoderm-derived signaling molecule normally down-regulated in the distal stomach, was overexpressed in Gata4(-/-) cells. We conclude that GATA-4-deficient cells have an intrinsic defect in their ability to differentiate. Similarities in the phenotypes of Gata4(-/-) chimeras and mice with other genetically engineered mutations that affect gut development suggest that GATA-4 may be involved in the gastric epithelial response to members of the TGF-beta superfamily.

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

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

Functional conservation of hematopoietic factors in Drosophila and vertebrates

The GATA, Friend of GATA, and Runt homology domain protein families function during hematopoiesis to promote progenitor cell development and regulate lineage commitment and differentiation. The hematopoietic functions of these factors have been remarkably conserved across taxonomic groups, ranging from flies to humans. Furthermore, aspects of hematopoiesis and hemocyte function appear to be conserved. Thus, comparative studies using Drosophila and vertebrate models should enhance our understanding of blood cell development.

The Drosophila VEGF receptor homolog is expressed in hemocytes

Several signalling pathways have been defined by studies of genes originally characterised in Drosophila. However, some mammalian signalling systems have so far escaped discovery in the fly. Here, we describe the identification and characterisation of fly homologs for the mammalian vascular endothelial growth factor/platelet derived growth factor (VEGF/PDGF) and the VEGF receptor. The Drosophila factor (DmVEGF-1) gene has two splice variants and is expressed during all stages, the signal distribution during embryogenesis being ubiquitous. The receptor (DmVEGFR) gene has several splice variants; the variations affecting only the extracellular domain. The most prominent form is expressed in cells of the embryonic haematopoietic cell lineage, starting in the mesodermal area of the head around stage 10 of embryogenesis. Expression persists in hemocytes as embryonic development proceeds and the cells migrate posteriorly. In a fly strain carrying a deletion uncovering the DmVEGFR gene, hemocytes are still present, but their migration is hampered and the hemocytes remain mainly in the anterior end close to their origin. These data suggest that the VEGF/PDGF signalling system may regulate the migration of the Drosophila embryonic haemocyte precursor cells.

Hematopoietic development: a balancing act

Over the past year, significant new insights have been gained in our understanding of the lineage determination of red blood cells. In particular, evidence has emerged demonstrating that cross-antagonism of lineage-specific transcription factors plays an important role in determining cell phenotype by actively repressing alternate lineage gene programs.

Hemangioblast commitment in the avian allantois: cellular and molecular aspects

We recently identified the allantois as a site producing hemopoietic and endothelial cells capable of colonizing the bone marrow of an engrafted host. Here, we report a detailed investigation of some early cytological and molecular processes occurring in the allantoic bud, which are probably involved in the production of angioblasts and hemopoietic cells. We show that the allantois undergoes a program characterized by the prominent expression of several "hemangioblastic" genes in the mesoderm accompanied by other gene patterns in the associated endoderm. VEGF-R2, at least from stage HH17 onward, is expressed and is shortly followed by transcription factors GATA-2, SCL/tal-1, and GATA-1. Blood island-like structures differentiate that contain both CD45(+) cells and cells accumulating hemoglobin; these structures look exactly like blood islands in the yolk sac. This hemopoietic process takes place before the establishment of a vascular network connecting the allantois to the embryo. As far as the endoderm is concerned, GATA-3 mRNA is found in the region where allantois will differentiate before the posterior instestinal portal becomes anatomically distinct. Shortly before the bud grows out, GATA-2 was expressed in the endoderm and, at the same time, the hemangioblastic program became initiated in the mesoderm. GATA-3 is detected at least until E8 and GATA-2 until E3 the latest stage examined for this factor. Using in vitro cultures, we show that allantoic buds, dissected out before the establishment of circulation between the bud and the rest of the embryo, produced erythrocytes of the definitive lineage. Moreover, using heterospecific grafts between chick and quail embryos, we demonstrate that the allantoic vascular network develops from intrinsic progenitors. Taken together, these results extend our earlier findings about the commitment of mesoderm to the endothelial and hemopoietic lineages in the allantois. The detection of a prominent GATA-3 expression restricted to the endoderm of the preallantoic region and allantoic bud, followed by that of GATA-2, is an interesting and novel information, in the context of organ formation and endoderm specification in the emergence of hemopoietic cells.

Core binding factor and its role in normal hematopoietic development

The core binding factors are a small family of transcription factors comprising a DNA binding CBFalpha subunit and a non-DNA binding CBFbeta subunit. One gene encoding a CBFalpha subunit, RUNX1 (also known as AML1, CBFA2, and PEBPA2A), and the gene encoding CBFbeta (CBFB) are essential for hematopoiesis and are frequently mutated in human leukemias. Both genes are required for the generation of hematopoietic stem cells (HSCs) during embryonic development. Expression studies in fish and frogs and functional analyses in flies indicate that a role for these genes in hematopoiesis is evolutionarily conserved.

The Friend of GATA proteins U-shaped, FOG-1, and FOG-2 function as negative regulators of blood, heart, and eye development in Drosophila

Friend of GATA (FOG) proteins regulate GATA factor-activated gene transcription. During vertebrate hematopoiesis, FOG and GATA proteins cooperate to promote erythrocyte and megakaryocyte differentiation. The Drosophila FOG homologue U-shaped (Ush) is expressed similarly in the blood cell anlage during embryogenesis. During hematopoiesis, the acute myeloid leukemia 1 homologue Lozenge and Glial cells missing are required for the production of crystal cells and plasmatocytes, respectively. However, additional factors have been predicted to control crystal cell proliferation. In this report, we show that Ush is expressed in hemocyte precursors and plasmatocytes throughout embryogenesis and larval development, and the GATA factor Serpent is essential for Ush embryonic expression. Furthermore, loss of ush function results in an overproduction of crystal cells, whereas forced expression of Ush reduces this cell population. Murine FOG-1 and FOG-2 also can repress crystal cell production, but a mutant version of FOG-2 lacking a conserved motif that binds the corepressor C-terminal binding protein fails to affect the cell lineage. The GATA factor Pannier (Pnr) is required for eye and heart development in Drosophila. When Ush, FOG-1, FOG-2, or mutant FOG-2 is coexpressed with Pnr during these developmental processes, severe eye and heart phenotypes result, consistent with a conserved negative regulation of Pnr function. These results indicate that the fly and mouse FOG proteins function similarly in three distinct cellular contexts in Drosophila, but may use different mechanisms to regulate genetic events in blood vs. cardial or eye cell lineages.

serpent, a GATA-like transcription factor gene, induces fat-cell development in Drosophila melanogaster

The GATA-like transcription factor gene serpent is necessary for embryonic fat-cell differentiation in Drosophila (Sam, S., Leise, W. and Hoshizaki, D. K. (1996) Mech. Dev. 60, 197–205) and has been proposed to function in a cell-fate choice between fat cell and somatic gonadal precursors (Moore, L. A., Broihier, H. T., Van Doren, M. and Lehmann, R. (1998) Development 125, 837–44; Riechmann, V., Irion, U., Wilson, R., Grosskortenhaus, R. and Leptin, M. (1997) Development 124, 2915–22). Here, we report that deregulated expression of serpent in the mesoderm induces the formation of ectopic fat cells and prevents the migration and coalescence of the somatic gonadal precursors. The ectopic fat cells do not arise from hyperproliferation of the primary fat-cell clusters but they do associate with the endogenous fat cells to form a fat body that is expanded in both the dorsal/ventral and anterior/posterior axes. Misexpression of serpent also affects the differentiation of muscle cells. Few body-wall muscle precursors are specified and there is a loss of most body-wall muscle fibers. The precursors of the visceral mesoderm are also absent and concomitantly the visceral muscle is absent. We suggest that the ectopic fat cells might originate from cells that have the potential, but do not normally, differentiate into fat cells or from cells that have acquired a fat-cell fate. In light of our results, we discuss the role of serpent in fat-cell specification and in cell fate choices.

The GATA factor Serpent is required for the onset of the humoral immune response in Drosophila embryos

Innate immunity in Drosophila is characterized by the inducible expression of antimicrobial peptides. We have investigated the development and regulation of immune responsiveness in Drosophila embryos after infection. Immune competence, as monitored by the induction of Cecropin A1-lacZ constructs, was observed first in the embryonic yolk. This observation suggests that the yolk plays an important role in the humoral immune response of the developing embryo by synthesizing antimicrobial peptides. Around midembryogenesis, the response in the yolk was diminished. Simultaneously, Cecropin expression became inducible in a large number of cells in the epidermis, demonstrating that late-stage embryos can synthesize their own antibiotics in the epidermis. This production likely serves to provide the hatching larva with an active antimicrobial barrier and protection against systemic infections. Cecropin expression in the yolk required the presence of a GATA site in the promoter as well as the involvement of the GATA-binding transcription factor Serpent (dGATAb). In contrast, neither the GATA site nor Serpent were necessary for Cecropin expression in the epidermis. Thus, the inducible immune responses in the yolk and in the epidermis can be uncoupled and call for distinct sets of transcription factors. Our data suggest that Serpent is involved in the distinction between a systemic response in the yolk/fat body and a local immune response in epithelial cells. In addition, the present study shows that signal transduction pathways controlling innate and epithelial defense reactions can be dissected genetically in Drosophila embryos.

Initiation and early patterning of the endoderm

We review the early stages of endoderm formation in the major animal models. In Amphibia maternal molecules are important in initiating endoderm formation. This is followed by successive signaling events that establish and then pattern the endoderm. In other organisms there are differences in endodermal development, particularly in the initial, prephylotypic stages. Later many of the same key families of transcription factors and signaling cassettes are used in all animals, but more work will be needed to establish exact evolutionary homologies.

Bone morphogenetic protein function is required for terminal differentiation of the heart but not for early expression of cardiac marker genes

To examine potential roles for bone morphogenetic proteins (BMPs) in cardiogenesis, we used intracellular BMP inhibitors to disrupt this signaling cascade in Xenopus embryos. BMP-deficient embryos showed endodermal defects, a reduction in cardiac muscle-specific gene expression, a decrease in the number of cardiomyocytes and cardia bifida. Early expression of markers of endodermal and precardiac fate, however, was not perturbed. Heart defects were observed even when BMP signal transduction was blocked only in cells that contribute primarily to endodermal, and not cardiac fates, suggesting a non-cell autonomous function. Our results suggest that BMPs are not required for expression of early transcriptional regulators of cardiac fate but are essential for migration and/or fusion of the heart primordia and cardiomyocyte differentiation.

Maternal VegT is the initiator of a molecular network specifying endoderm in Xenopus laevis

During cleavage stages, maternal VegT mRNA and protein are localized to the Xenopus embryo's vegetal region from which the endoderm will arise and where several zygotic gene transcripts will be localized. Previous loss-of-function experiments on this T-box transcription factor suggested a role for VegT in Xenopus endoderm formation. Here, we test whether VegT is required to initiate endoderm formation using a loss of function approach. We find that the endodermal genes, Bix1, Bix3, Bix4, Milk (Bix2), Mix.1, Mix.2, Mixer, Xsox17 alpha, Gata4, Gata5, Gata6 and endodermin, as well as the anterior endodermal genes Xhex and cerberus, and the organizer specific gene, Xlim1, are downstream of maternal VegT. We also find that the TGF beta s, Xnr1, Xnr2, Xnr4 and derriere rescue expression of these genes, supporting the idea that cell interactions are critical for proper endoderm formation. Additionally, inhibitory forms of Xnr2 and Derriere blocked the ability of VegT mRNA injection to rescue VegT-depleted embryos. Furthermore, a subset of endodermal genes was rescued in VegT-depleted vegetal masses by induction from an uninjected vegetal mass. Finally, we begin to establish a gene hierarchy downstream of VegT by testing the ability of Mixer and Gata5 to rescue the expression of other endodermal genes. These results identify VegT as the maternal regulator of endoderm initiation and illustrate the complexity of zygotic pathways activated by VegT in the embryo's vegetal region.

Multiple roles for Gata5 in zebrafish endoderm formation

Previous studies have indicated that gata5, a zinc-finger transcription factor gene, is required for the development of the zebrafish gut tube. Here, we show that gata5 mutants also display defects in the development of other endodermal organs such as the liver, pancreas, thyroid and thymus. gata5 is expressed in the endodermal progenitors from late blastula stages, suggesting that it functions early during endoderm development. We indeed find that during gastrulation stages, gata5 mutants form fewer endodermal cells than their wild-type siblings. In addition, the endodermal cells that form in gata5 mutants appear to express lower than wild-type levels of endodermal genes such as sox17 and axial/foxA2. Conversely, overexpression of gata5 leads to expanded endodermal gene expression. These data indicate that Gata5 is involved both in the generation of endodermal cells at late blastula stages and in the maintenance of endodermal sox17 expression during gastrulation. We have also analyzed the relationship of Gata5 to other factors involved in endoderm formation. Using complementary mutant and overexpression analyses, we show that Gata5 regulates endoderm formation in cooperation with the Mix-type transcription factor Bon, that Gata5 and Bon function downstream of Nodal signaling, and that cas function is usually required for the activity of Gata5 in endoderm formation. Finally, we show that fau/gata5, bon and cas exhibit dominant genetic interactions providing additional support that they function in the same pathway. Together, these data demonstrate that Gata5 plays multiple roles in endoderm development in zebrafish, and position Gata5 relative to other regulators of endoderm formation.

Drosophila grain encodes a GATA transcription factor required for cell rearrangement during morphogenesis

The genetic mechanisms controlling organ shape are largely unknown. We show that the Drosophila grain gene is required during development for shaping the adult legs and the larval posterior spiracles. Mutant legs are short and wide rather than long and thin, while the spiracles are flat instead of dome-shaped. We demonstrate that grain encodes the GATAc transcription factor. Analysis of loss-of-function mutations at the cellular level indicates that grain affects organ shape by locally controlling cell rearrangement. Ectopic grain expression causes major morphogenetic movements, resulting in the invagination of the posterior segments into the embryo. This is the first gene that has been shown to affect epithelial morphogenesis by controlling cell rearrangements, and suggests a novel function for GATA transcription factors.

Repressive and restrictive mesodermal interactions with gut endoderm: possible relation to Meckel's Diverticulum

The midgut and hindgut endoderm of the mouse embryo give rise to the intestinal epithelium, yet it is not known how the intestinal program is chosen in contrast to other endoderm-derived cell types. Previous tissue explant studies with embryos at 8.5 to 11.5 days gestation (d) showed that when the gut mesoderm is removed from the prospective intestinal endoderm, the endoderm activates the expression of liver-specific genes such as serum albumin, demonstrating the endoderm's pluripotence. This reversible repression of liver genes does not affect the expression of the endodermal transcription factors HNF3 and GATA4, nor these factors' ability to engage target sites in chromatin. We have now found that at 13.5 d, the mesoderm gains a second inhibitory activity, resulting in the irreversible loss of expression of HNF3 (Foxa2) and GATA factors in the endoderm and the absence of factors binding to their target sites in chromatin. The second inhibitory activity causes the endoderm to lose the potential to activate a liver gene, and this restriction precedes the normal cytodifferentiation of the intestinal epithelium. In summary, two inhibitory interactions with mesoderm successively restrict the developmental potential of the gut endoderm, leading to intestinal differentiation. We also observed rare gut bud structures in midgestation embryos that appear to represent murine examples of Meckel's Diverticulum, a congenital abnormality in human development. The absence of restrictive mesodermal interactions could explain how Meckel's diverticula express diverse non-intestinal, endoderm-derived cell types.

Molecular mechanisms of cell death and phagocytosis in Drosophila

The genetic tools available in Drosophila have facilitated our understanding of how apoptosis is regulated and executed in the context of the developing organism. All embryonic apoptosis is initiated by the activity of three genes, rpr, grim and hid. Each of these genes is independently regulated, allowing developmental apoptosis to be finely controlled. These initiators in turn activate the core apoptotic machinery, including the caspases. Drosophila counterparts to other conserved components of the apoptotic machinery have been recently identified, and we discuss how these may be integrated into the process of normal developmentally regulated cell death. We also outline the role that phagocytosis plays in the final stages of apoptosis and consider the molecular mechanisms guiding the elimination of apoptotic corpses.

Function of GATA transcription factors in preadipocyte-adipocyte transition

Genes that control the early stages of adipogenesis remain largely unknown. Here, we show that murine GATA-2 and GATA-3 are specifically expressed in white adipocyte precursors and that their down-regulation sets the stage for terminal differentiation. Constitutive GATA-2 and GATA-3 expression suppressed adipocyte differentiation and trapped cells at the preadipocyte stage. This effect is mediated, at least in part, through the direct suppression of peroxisome proliferator-activated receptor gamma. GATA-3-deficient embryonic stem cells exhibit an enhanced capacity to differentiate into adipocytes, and defective GATA-2 and GATA-3 expression is associated with obesity. Thus, GATA-2 and GATA-3 regulate adipocyte differentiation through molecular control of the preadipocyte-adipocyte transition.

Runt and Lozenge function in Drosophila development

Runt and Lozenge (LZ) are members of the Runt domain family of transcriptional regulators and control a large number of developmental processes in Drosophila. Runt is a pair-rule gene, and is part of the network of genes that control pattern formation in the embryo. In the central nervous system, Runt function is necessary for the development of a subset of neurons. Runt is also a key regulator of sex determination, and directly controls Sex-lethal, a master gene that determines sex of the animal and controls dosage compensation. The LZ protein also participates in several key processes. LZ controls pre-patterning and cell-fate choices in the development of the visual system by regulating the expression of several fate-specifying transcription factors, and works in conjunction with general signaling pathways. LZ function is also required in hematopoiesis for the specification of a Drosophila blood cell lineage.

Potential roles for RUNX1 and its orthologs in determining hematopoietic cell fate

Runx1 (also known as AML1, Cbfa2 and Pebpa2b) and Cbfb encode a DNA-binding alpha subunit and the non-DNA-binding beta subunit of a mammalian core-binding factor (CBF). The discovery of RUNX1 and CBFB as genes rearranged in human leukemias prompted predictions that both genes would play important roles in normal hematopoiesis. These predictions were borne out, as indeed Runx1 and its Xenopus and Drosophila homologs, Xaml and lozenge (lz), appear to determine hematopoietic cell fate during development. We will review what is known about Runx1 function in hematopoiesis in three model organisms, mouse, frog and fly, focusing on the earliest events of hematopoietic cell emergence in the embryo.

The gene encoding the mitogen-responsive phosphoprotein Dab2 is differentially regulated by GATA-6 and GATA-4 in the visceral endoderm

Gene targeting studies have demonstrated that the zinc finger transcription factor GATA-6 lies upstream in a transcriptional cascade that controls differentiation of the visceral endoderm. To understand the function of GATA-6 in the visceral endoderm and to identify genes regulated by GATA-6 in this tissue, subtractive hybridization was performed using template cDNAs derived from differentiated wild-type embryonic stem (ES) cells and GATA-6(-/-) ES cells, respectively. These analyses revealed that the gene encoding Dab2, a mitogen-responsive phosphoprotein, is differentially expressed in wild-type and GATA-6-deficient ES cells. Consistent with these findings, Dab2 is expressed in the visceral endoderm of wild-type embryos but not in the visceral endoderm of GATA-6-deficient embryos. Cotransfection experiments demonstrate that the human Dab2 promoter can be transactivated by forced expression of GATA-6 in NIH-3T3 cells. In contrast, forced expression of GATA-4 does not transactivate the human Dab2 promoter and Dab2 is expressed in the visceral endoderm of GATA-4 null embryos. Surprisingly, the specificity of GATA-6-induced transactivation of the Dab2 promoter is not mediated through its zinc finger DNA-binding domain. Taken together, these data demonstrate that the mitogen-responsive phosphoprotein Dab2 is a downstream target of GATA-6 in the visceral endoderm. Moreover, these data demonstrate that molecular mechanisms have evolved that direct, and distinguish, the functional specificity of GATA family members when they are developmentally coexpressed.

Origins and functions of phagocytes in the embryo

To review the data on the origins, phenotype, and function of embryonic phagocytes that has accumulated over past decade. Most of the relevant articles were selected based on the PubMed database entries. In additional, the Interactive Fly database (http://sdb.bio. purdue.edu/fly/aimain/1aahome.htm), FlyBase (http://flybase.bio. indiana.edu:82/), and TBase (http://tbase.jax.org/) were used to search for relevant information and articles. Phagocytes in a vertebrate embryo develop in two sites (yolk sac and liver) and contribute to organogenesis in part through their ability to recognize and clear apoptotic cells. Yolk sac-derived phagocytes differ in differentiation pathway and marker gene expression from macrophages produced via classic hematopoietic progenitors in the liver. We argue that yolk sac-derived phagocytes constitute a separate cell lineage. This conclusion raises the question of whether primitive phagocytes persist into the adulthood.

The Drosophila melanogaster sex determination gene sisA is required in yolk nuclei for midgut formation

During sex determination, the sisterlessA (sisA) gene functions as one of four X:A numerator elements that set the alternative male or female regulatory states of the switch gene Sex-lethal. In somatic cells, sisA functions specifically in sex determination, but its expression pattern also hints at a role in the yolk cell, a syncytial structure believed to provide energy and nutrients to the developing embryo. Previous studies of sisA have been limited by the lack of a null allele, leaving open the possibility that sisA has additional functions. Here we report the isolation and molecular characterization of four new sisA alleles including two null mutations. Our findings highlight key aspects of sisA structure-function and reveal important qualitative differences between the effects of sisA and the other strong X:A numerator element, sisterlessB, on Sex-lethal expression. We use genetic, expression, clonal, and phenotypic analyses to demonstrate that sisA has an essential function in the yolk nuclei of both sexes. In the absence of sisA, endoderm migration and midgut formation are blocked, suggesting that the yolk cell may have a direct role in larval gut development. To our knowledge, this is the first report of a requirement for the yolk nuclei in Drosophila development.

Action of the Caenorhabditis elegans GATA factor END-1 in Xenopus suggests that similar mechanisms initiate endoderm development in ecdysozoa and vertebrates

In ecdysozoan protostomes, including arthropods and nematodes, transcription factors of the GATA family specify the endoderm: Drosophila dGATAb (ABF/Serpent) and Caenorhabditis elegans END-1 play important roles in generating this primary germ layer. end-1 is the earliest expressed endoderm-specific gene known in C. elegans and appears to initiate the program of gene expression required for endoderm differentiation, including a cascade of GATA factors required for development and maintenance of the intestine. Among vertebrate GATA proteins, the GATA-4/5/6 subfamily regulates aspects of late endoderm development, but a role for GATA factors in establishing the endoderm is unknown. We show here that END-1 binds to the canonical target DNA sequence WGATAR with specificity similar to that of vertebrate GATA-1 and GATA-4, and that it functions as a transcriptional activator. We exploited this activity of END-1 to demonstrate that establishment of the vertebrate endoderm, like that of invertebrate species, also appears to involve GATA transcriptional activity. Like the known vertebrate endoderm regulators Mixer and Sox17, END-1 is a potent activator of endoderm differentiation in isolated Xenopus ectoderm. Moreover, a dominant inhibitory GATA-binding fusion protein abrogates endoderm differentiation in intact embryos. By examining these effects in conjunction with those of Mixer- and Sox17beta-activating and dominant inhibitory constructs, we further establish the likely relationships between GATA activity and these regulators in early development of the vertebrate endoderm. These results suggest that GATA factors may function sequentially to regulate endoderm differentiation in both protostomes and deuterostomes.

Specification of Drosophila hematopoietic lineage by conserved transcription factors

Two major classes of cells observed within the Drosophila hematopoietic repertoire are plasmatocytes/macrophages and crystal cells. The transcription factor Lz (Lozenge), which resembles human AML1 (acute myeloid leukemia- 1) protein, is necessary for the development of crystal cells during embryonic and larval hematopoiesis. Another transcription factor, Gcm (glial cells missing), has previously been shown to be required for plasmatocyte development. Misexpression of Gcm causes crystal cells to be transformed into plasmatocytes. The Drosophila GATA protein Srp (Serpent) is required for both Lz and Gcm expression and is necessary for the development of both classes of hemocytes, whereas Lz and Gcm are required in a lineage-specific manner. Given the similarities of Srp and Lz to mammalian GATA and AML1 proteins, observations in Drosophila are likely to have broad implications for understanding mammalian hematopoiesis and leukemias.

Endoderm development: from patterning to organogenesis

Although the ectoderm and mesoderm have been the focus of intensive work in the recent era of studies on the molecular control of vertebrate development, the endoderm has received less attention. Because signaling must occur between germ layers in order to achieve a properly organized body, our understanding of the coordinated development of all organs requires a more thorough consideration of the endoderm and its derivatives. This review focuses on present knowledge and perspectives concerning endoderm patterning and organogenesis. Some of the classical embryology of the endoderm is discussed and the progress and deficiencies in cellular and molecular studies are noted.

The evolutionary ecology of resistance to parasitoids by Drosophila

Parasitoids are the most important natural enemies of many insect species. Larvae of many Drosophila species can defend themselves against attack by parasitoids through a cellular immune response called encapsulation. The paper reviews recent studies of the evolutionary biology and ecological genetics of resistance in Drosophila, concentrating on D. melanogaster. The physiological basis of encapsulation, and the genes known to interfere with resistance are briefly summarized. Evidence for within- and between-population genetic variation in resistance from isofemale line, artificial selection and classical genetic studies are reviewed. There is now firm evidence that resistance is costly to Drosophila, and the nature of this cost is discussed, and the possibility that it may involve a reduction in metabolic rate considered. Comparative data on encapsulation and metabolic rates across seven Drosophila species provides support for this hypothesis. Finally, the possible population and community ecological consequences of evolution in the levels of host resistance are examined.

The zinc finger proteins Pannier and GATA4 function as cardiogenic factors in Drosophila

The regulation of cardiac gene expression by GATA zinc finger transcription factors is well documented in vertebrates. However, genetic studies in mice have failed to demonstrate a function for these proteins in cardiomyocyte specification. In Drosophila, the existence of a cardiogenic GATA factor has been implicated through the analysis of a cardial cell enhancer of the muscle differentiation gene D-mef2. We show that the GATA gene pannier is expressed in the dorsal mesoderm and required for cardial cell formation while repressing a pericardial cell fate. Ectopic expression of Pannier results in cardial cell overproduction, while co-expression of Pannier and the homeodomain protein Tinman synergistically activate cardiac gene expression and induce cardial cells. The related GATA4 protein of mice likewise functions as a cardiogenic factor in Drosophila, demonstrating an evolutionarily conserved function between Pannier and GATA4 in heart development.

Regulation of metallothionein gene transcription. Identification of upstream regulatory elements and transcription factors responsible for cell-specific expression of the metallothionein genes from Caenorhabditis elegans

Metallothioneins are small, cysteine-rich proteins that function in metal detoxification and homeostasis. Metallothionein transcription is controlled by cell-specific factors, as well as developmentally modulated and metal-responsive pathways. By using the nematode Caenorhabditis elegans as a model system, the mechanism that controls cell-specific metallothionein transcription in vivo was investigated. The inducible expression of the C. elegans metallothionein genes, mtl-1 and mtl-2, occurs exclusively in intestinal cells. Sequence comparisons of these genes with other C. elegans intestinal cell-specific genes identified multiple repeats of GATA transcription factor-binding sites (i.e. GATA elements). In vivo deletion and site-directed mutation analyses confirm that one GATA element in mtl-1 and two in mtl-2 are required for transcription. Electrophoretic mobility shift assays show that the C. elegans GATA transcription factor ELT-2 specifically binds to these elements. Ectopic expression of ELT-2 in non-intestinal cells of C. elegans activates mtl-2 transcription in these cells. Likewise, mtl-2 is not expressed in nematodes in which elt-2 has been disrupted. These results indicate that cell-specific transcription of the C. elegans metallothionein genes is regulated by the binding of ELT-2 to GATA elements in these promoters. Furthermore, a model is proposed where ELT-2 constitutively activates metallothionein expression; however, a second metal-responsive factor prevents transcription in the absence of metals.

Role of the amnioserosa in germ band retraction of the Drosophila melanogaster embryo

As the germ band shortens in Drosophila melanogaster embryos, cell shape changes cause segments to narrow anteroposteriorly and to lengthen dorsoventrally. One of the genes required for this retraction process is the hindsight (hnt) gene. hnt encodes a nuclear Zinc-finger protein that is expressed in the extraembryonic amnioserosa and the endodermal midgut prior to and during germ band retraction (M. L. R. Yip, M. L. Lamka, and H. D. Lipshitz, 1997, Development 124, 2129-2141). Here we show, through analysis of hnt genetic mosaic embryos, that hnt activity in the amnioserosa-particularly in those cells that are adjacent to the epidermis-is necessary for germ band retraction. In hnt mutant embryos the amnioserosa undergoes premature cell death (L. C. Frank and C. Rushlow, 1996, Development 122, 1343-1352). We demonstrate that prevention of premature apoptosis in hnt mutants does not rescue retraction. Thus, failure of this process is not an indirect consequence of premature amnioserosal apoptosis; instead, hnt must function in a pathway that controls germ band retraction. We show that the Krüppel gene is activated by hnt in the amnioserosa while the Drosophila insulin receptor (INR) functions downstream of hnt in the germ band. We present evidence against a physical model in which the amnioserosa "pushes" the germ band during retraction. Rather, it is likely that the amnioserosa functions in production, activation, or presentation of a diffusible signal required for retraction.

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.

Huckebein repressor activity in Drosophila terminal patterning is mediated by Groucho

The Groucho corepressor mediates negative transcriptional regulation in association with various DNA-binding proteins in diverse developmental contexts. We have previously implicated Groucho in Drosophila embryonic terminal patterning, showing that it is required to confine tailless and huckebein terminal gap gene expression to the pole regions of the embryo. Here we reveal an additional requirement for Groucho in this developmental process by establishing that Groucho mediates repressor activity of the Huckebein protein. Putative Huckebein target genes are derepressed in embryos lacking maternal groucho activity and biochemical experiments demonstrate that Huckebein physically interacts with Groucho. Using an in vivo repression assay, we identify a functional repressor domain in Huckebein that contains an FRPW tetrapeptide, similar to the WRPW Groucho-recruitment domain found in Hairy-related repressor proteins. Mutations in Huckebein's FRPW motif abolish Groucho binding and in vivo repression activity, indicating that binding of Groucho through the FRPW motif is required for the repressor function of Huckebein. Taken together with our earlier results, these findings show that Groucho-repression regulates sequential aspects of terminal patterning in Drosophila.

Dual requirement for the EcR/USP nuclear receptor and the dGATAb factor in an ecdysone response in Drosophila melanogaster

The EcR/USP nuclear receptor controls Drosophila metamorphosis by activating complex cascades of gene transcription in response to pulses of the steroid hormone ecdysone at the end of larval development. Ecdysone release provides a ubiquitous signal for the activation of the receptor, but a number of its target genes are induced in a tissue- and stage-specific manner. Little is known about the molecular mechanisms involved in this developmental modulation of the EcR/USP-mediated pathway. Fbp1 is a good model of primary ecdysone response gene expressed in the fat body for addressing this question. We show here that the dGATAb factor binds to three target sites flanking an EcR/USP binding site in a 70-bp enhancer that controls the tissue and stage specificity of Fbp1 transcription. We demonstrate that one of these sites and proper expression of dGATAb are required for specific activation of the enhancer in the fat body. In addition, we provide further evidence that EcR/USP plays an essential role as a hormonal timer. Our study provides a striking example of the integration of molecular pathways at the level of a tissue-specific hormone response unit.

Induction and regulation of antimicrobial peptides in Drosophila

Activation of the innate immune response involves recognition of the infectious agent and the subsequent activation of cellular and humoral reactions. In insects, a number of immunity genes are activated at the level of transcription leading to the synthesis of antimicrobial peptides. Genetic analyses in Drosophila have identified several signal transduction pathways that promote activation of these immunity genes. Recent data suggest that the insect immune system is able to discriminate between a bacterial and a fungal infection, and responds by higher levels of activation of the appropriate peptides to repel the infection. These and other recent data on transcription factors and regulation of antimicrobial genes are integrated into a model to suggest how differential activation of antifungal and antibacterial peptides can occur in response to fungal and bacterial infection.

Developmental competence of the gut endoderm: genetic potentiation by GATA and HNF3/fork head proteins

A long-standing problem in developmental biology has been to understand how the embryonic germ layers gain the competence to differentiate into distinct cell types. Genetic studies have shown that members of the GATA and HNF3/fork head transcription factor families are essential for the formation and differentiation of gut endoderm tissues in worms, flies, and mammals. Recent in vivo footprinting studies have shown that GATA and HNF3 binding sites in chromatin are occupied on a silent gene in endoderm that has the potential to be activated solely in that germ layer. These and other data indicate that these evolutionarily conserved factors help impart the competence of a gene to be activated in development, a phenomenon called genetic potentiation. The mechanistic implications of genetic potentiation and its general significance are discussed.

Development of the human gastrointestinal tract: twenty years of progress

A combination of approaches has begun to elucidate the mechanisms of gastrointestinal development. This review describes progress over the last 20 years in understanding human gastrointestinal development, including data from both human and experimental animal studies that address molecular mechanisms. Rapid progress is being made in the identification of genes regulating gastrointestinal development. Genes directing initial formation of the endoderm as well as organ-specific patterning are beginning to be identified. Signaling pathways regulating the overall right-left asymmetry of the gastrointestinal tract and epithelial-mesenchymal interactions are being clarified. In searching for extrinsic developmental regulators, numerous candidate trophic factors have been proposed, but compelling evidence remains elusive. A critical gene that initiates pancreas development has been identified, as well as a number of genes regulating liver, stomach, and intestinal development. Mutations in genes affecting neural crest cell migration have been shown to give rise to Hirschsprung's disease. Considerable progress has been achieved in understanding specific phenomena, such as the transcription factors regulating expression of sucrase-isomaltase and fatty acid-binding protein. The challenge for the future is to integrate these data into a more complete understanding of the physiology of gastrointestinal development.

Molecular cloning of FOG-2: a modulator of transcription factor GATA-4 in cardiomyocytes

GATA transcription factors are important regulators of both hematopoiesis (GATA-1/2/3) and cardiogenesis (GATA-4) in mammals. The transcriptional activities of the GATA proteins are modulated by their interactions with other transcription factors and with transcriptional coactivators and repressors. Recently, two related zinc finger proteins, U-shaped (USH) and Friend of GATA-1 (FOG) have been reported to interact with the GATA proteins Pannier and GATA-1, respectively, and to modulate their transcriptional activities in vitro and in vivo. In this report, we describe the molecular cloning and characterization of a third FOG-related protein, FOG-2. FOG-2 is an 1,151 amino acid nuclear protein that contains eight zinc finger motifs that are structurally related to those of both FOG and USH. FOG-2 is first expressed in the mouse embryonic heart and septum transversum at embryonic day 8.5 and is subsequently expressed in the developing neuroepithelium and urogenital ridge. In the adult, FOG-2 is expressed predominately in the heart, brain, and testis. FOG-2 associates physically with the N-terminal zinc finger of GATA-4 both in vitro and in vivo. This interaction appears to modulate specifically the transcriptional activity of GATA-4 because overexpression of FOG-2 in both NIH 3T3 cells and primary rat cardiomyocytes represses GATA-4-dependent transcription from multiple cardiac-restricted promoters. Taken together, these results implicate FOG-2 as a novel modulator of GATA-4 function during cardiac development and suggest a paradigm in which tissue-specific interactions between different FOG and GATA proteins regulate the differentiation of distinct mesodermal cell lineages.

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.

Reprogramming of early embryonic blastomeres into endodermal progenitors by a Caenorhabditis elegans GATA factor

The END-1 GATA factor has been implicated in specifying endoderm in Caenorhabditis elegans and is the earliest known zygotic protein expressed in the lineage of E, the clonal endoderm progenitor. We report that ubiquitous end-1 expression during a critical period in embryogenesis causes all non-endodermal lineages to produce endoderm instead of ectoderm and/or mesoderm. END-1 expression bypasses the requirement for maternal SKN-1 and the maternal Wnt signaling pathway in endoderm formation. This suggests that a primary function of these maternal factors is to regulate zygotic end-1 expression, which is then sufficient to initiate the entire program for endoderm development.

GATA transcription factors as potentiators of gut endoderm differentiation

Gene inactivation studies have shown that members of the GATA family of transcription factors are critical for endoderm differentiation in mice, flies and worms, yet how these proteins function in such a conserved developmental context has not been understood. We use in vivo footprinting of mouse embryonic endoderm cells to show that a DNA-binding site for GATA factors is occupied on a liver-specific, transcriptional enhancer of the serum albumin gene. GATA site occupancy occurs in gut endoderm cells at their pluripotent stage: the cells have the potential to initiate tissue development but they have not yet been committed to express albumin or other tissue-specific genes. The GATA-4 isoform accounts for about half of the nuclear GATA-factor-binding activity in the endoderm. GATA site occupancy persists during hepatic development and is necessary for the activity of albumin gene enhancer. Thus, GATA factors in the endoderm are among the first to bind essential regulatory sites in chromatin. Binding occurs prior to activation of gene expression, changes in cell morphology or functional commitment that would indicate differentiation. We suggest that GATA factors at target sites in chromatin may generally help potentiate gene expression and tissue specification in metazoan endoderm development.

Key residues characteristic of GATA N-fingers are recognized by FOG

Protein-protein interactions play significant roles in the control of gene expression. These interactions often occur between small, discrete domains within different transcription factors. In particular, zinc fingers, usually regarded as DNA-binding domains, are now also known to be involved in mediating contacts between proteins. We have investigated the interaction between the erythroid transcription factor GATA-1 and its partner, the 9 zinc finger protein, FOG (Friend Of GATA). We demonstrate that this interaction represents a genuine finger-finger contact, which is dependent on zinc-coordinating residues within each protein. We map the contact domains to the core of the N-terminal zinc finger of GATA-1 and the 6th zinc finger of FOG. Using a scanning substitution strategy we identify key residues within the GATA-1 N-finger which are required for FOG binding. These residues are conserved in the N-fingers of all GATA proteins known to bind FOG, but are not found in the respective C-fingers. This observation may, therefore, account for the particular specificity of FOG for N-fingers. Interestingly, the key N-finger residues are seen to form a contiguous surface, when mapped onto the structure of the N-finger of GATA-1.

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.

Involvement of the N-finger in the self-association of GATA-1

Zinc fingers are recognized as small protein domains that bind to specific DNA sequences. Recently however, zinc fingers from a number of proteins, in particular the GATA family of transcription factors, have also been implicated in specific protein-protein interactions. The erythroid protein GATA-1 contains two zinc fingers: the C-finger, which is sufficient for sequence-specific DNA-binding, and the N-finger, which appears both to modulate DNA-binding and to interact with other transcription factors. We have expressed and purified the N-finger domain and investigated its involvement in the self-association of GATA-1. We demonstrate that this domain does not homodimerize but instead makes intermolecular contacts with the C-finger, suggesting that GATA dimers are maintained by reciprocal N-finger-C-finger contacts. Deletion analysis identifies a 25-residue region, C-terminal to the core N-finger domain, that is sufficient for interaction with intact GATA-1. A similar subdomain exists C-terminal to the C-finger, and we show that self-association is substantially reduced when both subdomains are disrupted by mutation. Moreover, mutations that impair GATA-1 self-association also interfere with its ability to activate transcription in transfection studies.

Early liver differentiation: genetic potentiation and multilevel growth control

Hepatocytes undergo distinct phases of differentiation as they arise from the gut endoderm, coalesce to form the liver, and mature by birth. Gene inactivation and in vivo footprinting studies in mouse embryos have identified regulatory transcription factors and cell signaling molecules that control some but not all of these transitions. The latest studies reveal DNA-binding proteins that appear to potentiate gene activation during liver specification and the importance of signals between early hepatocytes and other cell types that promote early liver growth.

Mutational analysis of AREA, a transcriptional activator mediating nitrogen metabolite repression in Aspergillus nidulans and a member of the "streetwise" GATA family of transcription factors

The transcriptional activator AREA is a member of the GATA family of transcription factors and mediates nitrogen metabolite repression in the fungus Aspergillus nidulans. The nutritional versatility of A. nidulans and its amenability to classical and reverse genetic manipulations make the AREA DNA binding domain (DBD) a useful model for analyzing GATA family DBDs, particularly as structures of two AREA-DNA complexes have been determined. The 109 extant mutant forms of the AREA DBD surveyed here constitute one of the highest totals of eukaryotic transcription factor DBD mutants, are discussed in light of the roles of individual residues, and are compared to corresponding mutant sequence changes in other fungal GATA factor DBDs. Other topics include delineation of the DBD using both homology and mutational truncation, use of frameshift reversion to detect regions of tolerance to mutational change, the finding that duplication of the DBD can apparently enhance AREA function, and use of the AREA system to analyze a vertebrate GATA factor DBD. Some major points to emerge from work on the AREA DBD are (i) tolerance to sequence change (with retention of function) is surprisingly great, (ii) mutational changes in a transcription factor can have widely differing, even opposing, effects on expression of different structural genes so that monitoring expression of one or even several structural genes can be insufficient and possibly misleading, and (iii) a mutational change altering local hydrophobic packing and DNA binding target specificity can markedly influence the behavior of mutational changes elsewhere in the DBD.

The maternal-to-zygotic transition in embryonic patterning of Caenorhabditis elegans

Maternal factors laid down in the oocyte regulate blastomere identities in the early Caenorhabditis elegans embryo by activating zygotic patterning genes and restricting their expression to the appropriate lineages. A number of early-acting zygotic genes that specify various cell fates have been identified recently and their temporal and spatial regulation by maternal factors has begun to be elucidated.

pha-4, an HNF-3 homolog, specifies pharyngeal organ identity in Caenorhabditis elegans

To build complex organs, embryos have evolved mechanisms that integrate the development of cells unrelated to one another by cell type or ancestry. Here we show that the pha-4 locus establishes organ identity for the Caenorhabditis elegans pharynx. In pha-4 mutants, pharyngeal cells are transformed into ectoderm. Conversely, ectopic pha-4 expression produces excess pharyngeal cells. pha-4 encodes an HNF-3 homolog selectively expressed in the nascent digestive tract, including all pharynx precursors at the time they are restricted to a pharyngeal fate. We suggest that pha-4 is a key component of a transcription-based mechanism to endow cells with pharyngeal organ identity.