Schnurri is required for Drosophila Dpp signaling and encodes a zinc finger protein similar to the mammalian transcription factor PRDII-BF1

Embryonic expression and regulation of the large zinc finger protein KRC

KRC fusion proteins bind to the kappaB enhancer motif and to the signal sequences of V(D)J recombination. Here we have characterized endogenous KRC in mouse embryos and lymphoma cell lines. Starting from midgestation, neuronal- and lymphoid-restricted expression of KRC was observed from the dorsal root ganglia, trigeminal ganglion, thymus, and cerebral cortex. Several B-cell lines produced an alternatively spliced KRC transcript of 4.5 kb and a 115-kDa DNA-binding protein isoform. Additionally, that KRC transcript was induced by lipopolysaccharide, a potent activator of cells in immunity and inflammation. In genetic-engineered B cells stably transfected with inducible expression vectors for the recombination activating genes RAG1, RAG2, or both, the avidity of KRC to DNA was markedly decreased when RAG1 and RAG2 were overexpressed. We hypothesize that KRC may function in developing thymocytes and neurons, where its role might be transcription regulation or DNA recombination.

ZAS: C2H2 zinc finger proteins involved in growth and development

A ZAS gene encodes a large protein with two separate C2H2 zinc finger pairs that independently bind to specific DNA sequences, including the kappaB motif. Three paralogous mammalian genes, ZAS1, ZAS2, and ZAS3, and a related Drosophila gene, Schnurri, have been cloned and characterized. The ZAS genes encode transcriptional proteins that activate or repress the transcription of a variety of genes involved in growth, development, and metastasis. In addition, ZAS3 associates with a TNF receptor-associated factor to inhibit NF-kappaB- and JNK/ SAPK-mediated signaling of TNF-alpha. Genetic experiments show that ZAS3 deficiency leads to proliferation of cells and tumor formation in mice. The data suggest that ZAS3 is important in controlling cell growth, apoptosis, and inflammation. The potent vasoactive hormone endothelin and transcription factor AP2 gene families also each consist of three members. The ZAS, endothelin, and transcription factor AP2 genes form several linkage groups. Knowledge of the chromosomal locations of these genes provides valuable clues to the evolution of the vertebrate genome.

Murine Schnurri-2 is required for positive selection of thymocytes

A key step in T cell development involves the positive selection of cells that recognize antigen presented by self-major histocompatibility complex. Yet, the signals that are activated by T cell receptor engagement and lead to cell survival remain unclear. We show here that mice lacking the transcription factor Schnurri-2 (Shn-2), a large metal-finger protein, had severely defective positive selection of CD4+ and CD8+ cells. Drosophila Shn acts as a cofactor of Smad homolog and is required for Decapentaplegic signaling. Vertebrates have at least three Shn orthologs (Shn-1, Shn-2 and Shn-3), which are thought to act as nuclear targets in the bone morphogenetic protein-transforming growth factor-beta-activin signaling pathways. These data raised the possibility that the Smad-Shn-2 complex is involved in the thymic selection of T cells.

A HOX complex, a repressor element and a 50 bp sequence confer regional specificity to a DPP-responsive enhancer

A central theme during development and homeostasis is the generation of cell type-specific responses to the action of a limited number of extant signaling cascades triggered by extracellular ligands. The molecular mechanisms by which information from such signals are integrated in responding cells in a cell-type specific manner remain poorly understood. We have undertaken a detailed characterization of an enhancer that is regulated by DPP signaling and by the homeotic protein Labial and its partners, Extradenticle and Homothorax. The expression driven by this enhancer (lab550) and numerous deletions and point mutants thereof was studied in wild-type and mutant Drosophila embryos as well as in cultured cells. We find that the lab550 enhancer is composed of two elements, a Homeotic Response Element (HOMRE) and a DPP Response Element (DPPRE) that synergize. None of these two elements can reproduce the expression of lab550, either with regard to expression level or with regard to spatial restriction. The isolated DPPRE of lab550 responds extremely weakly to DPP. Interestingly, we found that the inducibility of this DPPRE is weak because it is tuned down by the action of a repressor element. This repressor element and an additional 50 bp element appear to be crucial for the cooperation of the HOMRE and the DPPRE, and might tightly link the DPP response to the homeotic input. The cooperation between the different elements of the enhancer leads to the segmentally restricted activity of lab550 in the endoderm and provides a mechanism to create specific responses to DPP signaling with the help of a HOX protein complex.

Nuclear interpretation of Dpp signaling in Drosophila

Signaling by Decapentaplegic (Dpp), a member of the TGFbeta superfamily of signaling molecules similar to vertebrate BMP2 and BMP4, has been implicated in many developmental processes in Drosophila melanogaster. Notably, Dpp acts as a long-range morphogen during imaginal disc growth and patterning. Genetic approaches led to the identification of a number of gene products that constitute the core signaling pathway. In addition to the ligand-activated heteromeric receptor complex and the signal-transducing intracellular Smad proteins, Dpp signaling requires two nuclear proteins, Schnurri (Shn) and Brinker (Brk), to prime cells for Dpp responsiveness. A complex interplay between the nuclear factors involved in Dpp signaling appears to control the transcriptional readout of the Dpp morphogen gradient. It remains to be seen whether similar molecular mechanisms operate in the nucleus in vertebrate systems.

Transforming growth factor beta signalling in vitro and in vivo: activin ligand-receptor interaction, Smad5 in vasculogenesis, and repression of target genes by the deltaEF1/ZEB-related SIP1 in the vertebrate embryo

The identification and characterization of components of the transforming growth factor beta (TGFbeta) signalling pathway are proceeding at a very fast pace. To illustrate a number of our activities in this field, we first summarize our work aiming at the selection from a large collection of single residue substitution mutants of two activin A polypeptides in which D27 and K102, respectively, have been modified. This work has highlighted the importance of K102 and its positive charge for binding to activin type II receptors. Activin K102E, which did not bind to high-affinity receptor complexes, may be a valuable beta chain, when incorporated in recombinant inhibin to unambiguously detect novel inhibin binding sites at the cell surface. We then illustrate how Smad5 knockout mice and an overexpression approach with a truncated TGFbeta type II receptor in the mouse embryo can contribute to the identification of a novel TGFbeta-->TbetaRII/ALK1-->Smad5 pathway in endothelial cells in the embryo proper and the yolk sac vasculature. We conclude with a summary of our results with a Smad-interacting transcriptional repressor but focus on its biological significance in the vertebrate embryo.

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.

Direct competition between Brinker and Drosophila Mad in Dpp target gene transcription

Brinker is a nuclear protein that antagonizes Dpp signalling in Drosophila. Its expression is negatively regulated by Dpp. Here, we show that Brinker represses Ultrabithorax (Ubx) in the embryonic midgut, a HOX gene that activates, and responds to, the localized expression of Dpp during endoderm induction. We find that the functional target for Brinker repression coincides with the Dpp response sequence in the Ubx midgut enhancer, namely a tandem of binding sites for the Dpp effector Mad. We show that Brinker efficiently competes with Mad in vitro, preventing the latter from binding to these sites. Brinker also competes with activated Mad in vivo, blocking the stimulation of the Ubx enhancer in response to simultaneous Dpp signalling. These results indicate how Brinker acts as a dominant repressor of Dpp target genes, and explain why Brinker is a potent antagonist of Dpp.

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.

Asymmetric germ cell division and oocyte determination during Drosophila oogenesis

Early oogenesis in Drosophila, with a stereotypic pattern of germ cell division and differentiation, provides an attractive model for studying cell lineage and patterning. Drosophila oogenesis is initiated when a germline stem cell divides asymmetrically to produce a daughter stem cell and a cytoblast. The cystoblast then undergoes four rounds of incomplete mitoses to form a 16-cell cyst, accompanied by the formation of the fusome. Within the cyst, one of the two cells with four intercellular bridges differentiates into an oocyte while the rest become nurse cells. The oocyte then translocates within the cyst to a posterior position, which defines the anterio-posterior axis of the future embryo. Recent studies have shown that the asymmetric germline stem cell division is controlled by somatic signaling involving piwi, fs(1)Yb, and the dpp pathway as well as by intrinsic mechanisms involving pumilio, nanos, arrest, bag-of-marbles, and the spectrosome-the fusome precursor in the stem cells and the cystoblast. The spectrosome in the cystoblast appears to play an important role in polarized fusome growth during cyst formation. The fusome may guide the formation of a polarized microtubule network for the intracyst transport of certain RNAs and proteins to the cystocyte destined to become the oocyte. Genes such as egalitarian, Bicaudal D, stonewall, and encore are important for oocyte determination, while differential adhesion between the oocyte and its surrounding prefollicle cells, as mediated by armadillo, alpha-catenin, shotgun, and the spindle genes, is crucial for oocyte translocation. Early oogenesis shares many parallel features to early spermatogenesis, although distinct differences are also observed at both the phenomenological and mechanistic levels. The study of oogenesis, progressing at an exciting rate, contributes significantly to our understanding of the mechanisms underlying proliferation, differentiation, and patterning.

TGF-beta signaling by Smad proteins

Members of the transforming growth factor-beta (TGF-beta) family bind to type II and type I serine/threonine kinase receptors, which initiate intracellular signals through activation of Smad proteins. Receptor-regulated Smads (R-Smads) are anchored to the cell membrane by interaction with membrane-bound proteins, including Smad anchor for receptor activation (SARA). Upon ligand stimulation, R-Smads are phosphorylated by the receptors and form oligomeric complexes with common-partner Smads (Co-Smads). The oligomeric Smad complexes then translocate into the nucleus, where they regulate the transcription of target genes by direct binding to DNA, interaction with various DNA-binding proteins, and recruitment of transcriptional coactivators or corepressors. A third class of Smads, inhibitory Smads (I-Smads), inhibits the signals from the serine/threonine kinase receptors. Since the expression of I-Smads is induced by the TGF-beta superfamily proteins, Smads constitute an autoinhibitory signaling pathway. The functions of Smads are regulated by other signaling pathways, such as the MAP kinase pathway. Moreover, Smads interact with and modulate the functions of various transcription factors which are downstream targets of other signaling pathways. Loss of function of certain Smads is involved in tumorigenesis, e.g., pancreatic and colorectal cancers. Analyses by gene targeting revealed pivotal roles of Smads in early embryogenesis, angiogenesis, and immune functions in vivo.

Structure of the human zinc finger protein HIVEP3: molecular cloning, expression, exon-intron structure, and comparison with paralogous genes HIVEP1 and HIVEP2

Here we report the cloning and characterization of HIVEP3, the newest member in the human immunodeficiency virus type 1 enhancer-binding protein family that encodes large zinc finger proteins and regulates transcription via the kappaB enhancer motif. The largest open reading frame of HIVEP3 contains 2406 aa. and is approximately 80% identical to the mouse counterpart. The HIVEP3 gene is located in the chromosomal region 1p34 and is at least 300 kb with 10 exons. RNA studies show that multiple HIVEP3 transcripts are differentially expressed and regulated. Additionally, transcription termination occurs in the ultimate exon, exon 10, or in exon 6. Therefore, HIVEP3 may produce protein isoforms that contain or exclude the carboxyl DNA binding domain and the leucine zipper by alternative RNA splicing and differential polyadenylation. Sequence homologous to HIVEP3 exon 6 is not found in mouse nor are the paralogous genes HIVEP1 and HIVEP2. Zoo-blot analysis suggests that sequences homologous to the human exon 6 are present only in primates and cow. Therefore, a foreign DNA harboring a termination exon likely was inserted into the HIVEP3 locus relatively recently in evolution, resulting in the acquisition of novel gene regulatory mechanisms as well as the generation of structural and functional diversity.

Mutations affecting the development of the peripheral nervous system in Drosophila: a molecular screen for novel proteins

In our quest for novel genes required for the development of the embryonic peripheral nervous system (PNS), we have performed three genetic screens using MAb 22C10 as a marker of terminally differentiated neurons. A total of 66 essential genes required for normal PNS development were identified, including 49 novel genes. To obtain information about the molecular nature of these genes, we decided to complement our genetic screens with a molecular screen. From transposon-tagged mutations identified on the basis of their phenotype in the PNS we selected 31 P-element strains representing 26 complementation groups on the second and third chromosomes to clone and sequence the corresponding genes. We used plasmid rescue to isolate and sequence 51 genomic fragments flanking the sites of these P-element insertions. Database searches using sequences derived from the ends of plasmid rescues allowed us to assign genes to one of four classes: (1) previously characterized genes (11), (2) first mutations in cloned genes (1), (3) P-element insertions in genes that were identified, but not characterized molecularly (1), and (4) novel genes (13). Here, we report the cloning, sequence, Northern analysis, and the embryonic expression pattern of candidate cDNAs for 10 genes: astray, chrowded, dalmatian, gluon, hoi-polloi, melted, pebble, skittles, sticky ch1, and vegetable. This study allows us to draw conclusions about the identity of proteins required for the development of the nervous system in Drosophila and provides an example of a molecular approach to characterize en masse transposon-tagged mutations identified in genetic screens.

schnurri is required for dpp-dependent patterning of the Drosophila wing

The BMP-related ligand Decapentaplegic (Dpp) has a well-characterized role in pattern formation during Drosophila embryogenesis and in larval development. Previous work has shown that transcription of Dpp-responsive genes requires the activity of the BMP-specific Smad, Mothers against dpp (Mad). In this study we investigated the role of the zinc finger transcription factor Schnurri (Shn) in mediating the nuclear response to Dpp during adult patterning. Using clonal analysis, we show that wing imaginal disc cells mutant for shn fail to transcribe the genes spalt, optomotor blind, vestigial, and Dad, that are known to be induced by dpp signaling. shn clones also ectopically express brinker, a gene that is downregulated in response to dpp, thus implicating Shn in both activation and repression of Dpp target genes. We demonstrate that loss of shn activity affects anterior-posterior patterning and cell proliferation in the wing blade, in a manner that reflects the graded requirement for Dpp in these processes. Furthermore, we find that shn is expressed in the pupal wing and plays a distinct role in mediating dpp-dependent vein differentiation at this stage. The absence of shn activity results in defects that are similar in nature and severity to those caused by elimination of Mad, suggesting that Shn has an essential role in dpp signal transduction in the developing wing. Our data are consistent with a model in which Shn acts as a cofactor for Mad.

The zinc finger protein schnurri acts as a Smad partner in mediating the transcriptional response to decapentaplegic

In Drosophila, a BMP-related ligand Decapentaplegic (Dpp) is essential for cell fate specification during embryogenesis and in imaginal disc development. Dpp signaling culminates in the phosphorylation and nuclear translocation of Mothers against dpp (Mad), a receptor-specific Smad that can bind DNA and regulate the transcription of Dpp-responsive genes. Genetic analysis has implicated Schnurri (Shn), a zinc finger protein that shares homology with mammalian transcription factors, in the Dpp signal transduction pathway. However, a direct role for Shn in regulating the transcriptional response to Dpp has not been demonstrated. In this study we show that Shn acts as a DNA-binding Mad cofactor in the nuclear response to Dpp. Shn can bind DNA in a sequence-specific manner and recognizes sites within a well-characterized Dpp-responsive promoter element, the B enhancer of the Ultrabithorax (Ubx) gene. The Shn-binding sites are relevant for in vivo expression, since mutations in these sites affect the ability of the enhancer to respond to Dpp. Furthermore we find that Shn and Mad can interact directly through discrete domains. To examine the relative contribution of the two proteins in the regulation of endogenous Dpp target genes we developed a cell culture assay and show that Shn and Mad act synergistically to induce transcription. Our results suggest that cooperative interactions between these two transcription factors could play an important role in the regulation of Dpp target genes. This is the first evidence that Dpp/BMP signaling in flies requires the direct interaction of Mad with a partner transcription factor.

Downregulation of KRC induces proliferation, anchorage independence, and mitotic cell death in HeLa cells

The large zinc finger protein KRC regulates transcription of target genes via the kappaB gene enhancer element. As an attempt to investigate the cellular function of KRC, we have established cell lines stably transfected with KRC expression vectors. Introduction of a vector directing expression of a transcript antisense to KRC mRNAs in several mammalian cell lines resulted in accelerated proliferation. Furthermore, in HeLa cells, downregulation of KRC conferred anchorage-independent growth and promoted cell cycle progression without an intervening cytokinesis, culminating in the formation of multinucleated giant cells. Ultimately these cells died.

A niche maintaining germ line stem cells in the Drosophila ovary

Stromal cells are thought to generate specific regulatory microenviroments or "niches" that control stem cell behavior. Characterizing stem cell niches in vivo remains an important goal that has been difficult to achieve. The individual ovarioles of the Drosophila ovary each contain about two germ line stem cells that maintain oocyte production. Here we show that anterior ovariolar somatic cells comprising three cell types act as a germ line stem cell niche. Germ line stem cells lost by normal or induced differentiation are efficiently replaced, and the ability to repopulate the niche increases the functional lifetime of ovarioles in vivo. Our studies implicate one of the somatic cell types, the cap cells, as a key niche component.

Schnurri mediates Dpp-dependent repression of brinker transcription

Signalling by Decapentaplegic (Dpp), a member of the TGFbeta superfamily of signalling molecules, controls many aspects of Drosophila development by activating and repressing target genes. Several essential components of the Dpp signalling pathway have been identified, including the Dpp receptors Punt and Thick veins (Tkv) as well as the cytoplasmic mediators Mad and Medea. For target genes to be activated, Dpp signalling must suppress transcription of a repressor encoded by the brinker (brk) gene. Here we show that Schnurri (Shn), a large zinc-finger protein, is essential for Dpp-mediated repression of brk transcription; in contrast, Shn is not required for target-gene activation. Thus, the Dpp signalling pathway bifurcates, downstream of the signal-mediating SMAD proteins, into a Shn-dependent pathway leading to brk repression and a Shn-independent pathway leading to gene activation. The existence of several Shn-like proteins in vertebrates and the observation that Brk functions in BMP signalling in Xenopus indicates that a similar regulatory cascade may be conserved in higher organisms.

Interaction and functional cooperation of NF-kappa B with Smads. Transcriptional regulation of the junB promoter

The transforming growth factor-beta (TGF-beta) family of cytokines regulates diverse cellular processes through control of the expression of target genes. Smad proteins are a recently identified family of signal transducers for members of the TGF-beta family. Smads act as transcriptional regulators through binding to DNA and interacting with a variety of transcription factors. Here, we identified a kappaB site as a TGF-beta-responsive region in the 3'-downstream junB promoter region. We also demonstrate that kappaB sites alone are sufficient to mediate immediate transcriptional activation by TGF-beta. Transactivation of kappaB sites by TGF-beta requires an intact NF-kappaB pathway, cooperates with known activators of this pathway, and is mediated by Smad family members. Furthermore, we show that Smad3 interacts with p52 in vivo. These data expand the model in which Smad proteins undergo multiple interactions with several transcription factors that could induce either activation or repression of gene expression.

Regulation and function of tinman during dorsal mesoderm induction and heart specification in Drosophila

The homeobox gene tinman plays a key role in the specification of Drosophila heart progenitors and the visceral mesoderm of the midgut, both of which arise at defined positions within dorsal areas of the mesoderm. Here, we show that in addition to the heart and midgut visceral mesoderm, tinman is also required for the specification of all dorsal body wall muscles. Thus it appears that the precursors of the heart, visceral musculature, and dorsal somatic muscles are all specified within the same broad domain of dorsal mesodermal tinman expression. Locally restricted activities of tinman are also observed during its early, general mesodermal expression, where tinman is required for the activation of the homeobox gene buttonless in precursors of the "dorsal median" (DM) glial cells along the ventral midline. These observations, together with others showing only mild effects of ectopic tinman expression on heart development, indicate that tinman function is obligatory, but not sufficient to determine individual tissues within the mesoderm. Therefore, we propose that tinman has a role in integrating positional information that is provided by intersecting domains of additional regulators and signals, which may include Wingless, Sloppy Paired, and Hedgehog in the dorsal mesoderm and EGF-signaling at the ventral midline. Previous studies have shown that Dpp acts as an inductive signal from dorsal ectodermal cells to induce tinman expression in the dorsal mesoderm, which, in turn, is needed for heart and visceral mesoderm formation. In the present report, we show that Thickveins, a type I receptor of Dpp, is essential for the transmission of Dpp signals into the mesoderm. Constitutive activity of Tkv in the entire mesoderm induces ectopic tinman expression in the ventral mesoderm, and this results in the ectopic formation of heart precursors in a defined area of the ventrolateral mesoderm. We further show that Screw, a second BMP2/4-related gene product, Tolloid, a BMP1-related protein, and the zinc finger-containing protein Schnurri, are required to allow full levels of tinman induction during this process. It is likely that some of these functional and regulatory properties of tinman are shared by tinman-related genes from vertebrates that have similarly important roles in embryonic heart development.

A zinc finger transcription factor, alphaA-crystallin binding protein 1, is a negative regulator of the chondrocyte-specific enhancer of the alpha1(II) collagen gene

Transcription of the type II collagen gene (Col2a1) is regulated by multiple cis-acting sites. The enhancer element, which is located in the first intron, is necessary for high-level and cartilage-specific expression of Col2a1. A mouse limb bud cDNA expression library was screened by the Saccharomyces cerevisiae one-hybrid screening method to identify protein factors bound to the enhancer. A zinc finger protein, alphaA-crystallin binding protein 1 (CRYBP1), which had been reported to bind to the mouse alphaA-crystallin gene promoter, was isolated. We herein demonstrate that CRYBP1 is involved in the negative regulation of Col2a1 enhancer activity. CRYBP1 mRNA expression was downregulated during chondrocyte differentiation in vitro. In situ hybridization analysis of developing mouse cartilage showed that CRYBP1 mRNA was also downregulated during mesenchymal condensation and that CRYBP1 mRNA was highly expressed by hypertrophic chondrocytes, but at very low levels by resting and proliferating chondrocytes. Expression of recombinant CRYBP1 in a transfected rat chondrosarcoma cell line inhibited Col2a1 enhancer activity. Electrophoretic mobility shift assays showed that CRYBP1 bound a specific sequence within the Col2a1 enhancer and inhibited the binding of Sox9, an activator for Col2a1, to the enhancer. Cotransfection of CRYBP1 with Sox9 into BALB/c 3T3 cells inhibited activation of the Col2a1 enhancer by Sox9. These results suggest a novel mechanism that negatively regulates cartilage-specific expression of Col2a1.

Transforming growth factor beta signaling mediators and modulators

Transforming growth factor beta is a multi-functional growth and differentiation factor responsible for regulating many diverse biological processes in both vertebrate and invertebrate species. Among the most dramatic of TGFbeta's effects are those associated with specification of cell fates during development and inhibition of cell cycle progression. The core TGFbeta signaling pathway has now been described using a synergistic combination of genetic and biochemical approaches. Transmembrane receptors with intrinsic protein serine kinase activity bind ligand in the extracellular milieu and then phosphorylate intracellular proteins known as Smads. Phosphorylated Smads form heterooligomers and translocate into the nucleus where they can modulate transcriptional responses. More recent studies indicate that many other proteins serve as modulators of Smad activity, and utimately define specific cellular responses to TGFbeta. Here we describe both the simplistic core TGFbeta signaling pathway and the growing number of proteins that impinge on this pathway at the level of Smad function to either enhance or inhibit TGFbeta responses.

Schnurri interacts with Mad in a Dpp-dependent manner

BACKGROUND

Decapentaplegic (Dpp) is a member of the transforming growth factor-beta superfamily. Dpp governs various developmental processes in Drosophila through the transcriptional regulation of a variety of genes. Signals of Dpp are transmitted from the cell membrane to the nucleus by Medea and Mad, both belonging to the Smad protein family. Mad was shown to bind to the Dpp-responsive element in genes such as vestigial, labial, and Ultrabithorax. The DNA binding affinity of Smad proteins is relatively low, and requires other nuclear factor(s) to form stable DNA binding complexes. schnurri (shn) was identified as a candidate gene acting downstream of Dpp receptors, but its relevance to Mad has remained unknown.

RESULTS

We characterized the biochemical functions of Shn. Shn forms homo-oligomers. Shn is localized in the nucleus, and is likely to have multiple nuclear localizing signals. Finally, we found that Shn interacts with Mad in a Dpp-dependent manner.

CONCLUSIONS

The present results argue that Shn may act as a nuclear component of the Dpp signalling pathway through direct interaction with Mad.

Stress signaling in Drosophila

Cells commonly use multiprotein kinase cascades to signal information from the cell membrane to the nucleus. Several conserved signaling pathways related to the mitogen activated protein kinase (MAPK) pathway allow cells to respond to normal developmental signals as well as signals produced under stressful conditions. Genetic and molecular studies in Drosophila melanogaster over the last several years have related that components of stress signaling pathways, namely the Jun kinase (JNK) and p38 kinase signaling modules, are functionally conserved and participate in numerous processes during normal development. Specifically, the JNK pathway is required for morphogenetic movements in embryogenesis and generation of tissue polarity in the adult. The role of the p38 pathway in generation of axial polarity during oogenesis has been inferred from phenotypic analysis of mutations in the Drosophila homolog of DMKK3. In addition to their requirement for normal development, cell culture and genetic investigations point to a role for both the JNK and p38 pathways in regulation of the immune response in the fly. This review details the known components of stress signaling pathways in Drosophila and recent insights into how these pathways are used and regulated during development and homeostasis.

The dorsal-open group gene raw is required for restricted DJNK signaling during closure

During dorsal closure in Drosophila melanogaster, cells of the lateral epidermis migrate over the amnioserosa to encase the embryo. At least three classes of dorsal-open group gene products are necessary for this morphogenetic movement. Class I genes code for structural proteins that effect changes in epidermal cell shape and motility. Class II and III genes code for regulatory components of closure: Class II genes encode Drosophila Jun amino (N)-terminal kinase (DJNK) signaling molecules and Class III genes encode Decapentaplegic-mediated signaling molecules. All characterized dorsal-open group gene products function in the epidermis. Here we report a molecular and genetic characterization of raw, a newly defined member of the Class II dorsal-open group genes. We show that the novel protein encoded by raw is required for restriction of DJNK signaling to leading edge epidermal cells as well as for proper development of the amnioserosa. Taken together, our results demonstrate a role for Raw in restriction of epidermal signaling during closure and suggest that this effect may be mediated via the amnioserosa.

Cubitus interruptus is necessary but not sufficient for direct activation of a wing-specific decapentaplegic enhancer

In Drosophila, the imaginal discs are the primordia for adult appendages. Their proper formation is dependent upon the activation of the decapentaplegic (dpp) gene in a stripe of cells just anterior to the compartment boundary. In imaginal discs, the dpp gene has been shown to be activated by Hedgehog signal transduction. However, an initial analysis of its enhancer region suggests that its regulation is complex and depends upon additional factors. In order to understand how multiple factors regulate dpp expression, we chose to focus on a single dpp enhancer element, the dpp heldout enhancer, from the 3′ cis regulatory disc region of the dpp locus. In this report, we present a molecular analysis of this 358 bp wing- and haltere-specific dpp enhancer, which demonstrates a direct transcriptional requirement for the Cubitus interruptus (Ci) protein. The results suggest that, in addition to regulation by Ci, expression of the dpp heldout enhancer is spatially determined by Drosophila TCF (dTCF) and the Vestigial/Scalloped selector system and that temporal control is provided by dpp autoregulation. Consistent with the unexpectedly complex regulation of the dpp heldout enhancer, analysis of a Ci consensus site reporter construct suggests that Ci, a mediator of Hedgehog transcriptional activation, can only transactivate in concert with other factors.

Modes of morphogen cooperation for limb formation in vertebrates and insects

Diffusing morphogens in cooperation can control gene expression in developing limbs. Additive cooperation corresponds to the Boolean operator OR and implies the equivalent action of the (suitably scaled) concentrations of two morphogens, either by their alternative binding to the same receptor or by another way of convergence of their effects during the signal transduction procedure. This cooperation can explain the spatial and temporal collinearities of the expression of hoxd genes in the vertebrate limb bud. A multiplicative cooperation of morphogens (corresponding to the Boolean operator AND), produced at the DPP and WG domains in the Drosophila leg imaginal disc, may account for the expression domains observed for Dll and dac. A molecular interpretation of the multiplicative morphogen cooperation is proposed. Some experiments are suggested for further testing of the model.

Roles of the JNK signaling pathway in Drosophila morphogenesis

Epithelial cell differentiation and morphogenesis are crucial in many aspects of metazoan development. Recent genetic studies in Drosophila have revealed that the conserved Jun amino-terminal kinase (JNK) signaling pathway regulates epithelial morphogenesis during the process of embryonic dorsal closure and participates in the control of planar polarity in several tissues. Importantly, these studies have linked the JNK pathway to the decapentaplegic and Frizzled pathways in these processes, suggesting a high degree of integrative signaling during epithelial morphogenesis.

Mutant alleles of the Drosophila trithorax gene produce common and unusual homeotic and other developmental phenotypes

trithorax (trx) encodes chromosome-binding proteins required throughout embryogenesis and imaginal development for tissue- and cell-specific levels of transcription of many genes including homeotic genes of the ANT-C and BX-C. trx encodes two protein isoforms that contain conserved motifs including a C-terminal SET domain, central PHD fingers, an N-terminal DNA-binding homology, and two short motifs also found in the TRX human homologue, ALL1. As a first step to characterizing specific developmental functions of TRX, I examined phenotypes of 420 combinations of 21 trx alleles. Among these are 8 hypomorphic alleles that are sufficient for embryogenesis but provide different levels of trx function at homeotic genes in imaginal cells. One allele alters the N terminus of TRX, which severely impairs larval and imaginal growth. Hypomorphic alleles that alter different regions of TRX equivalently reduce function at affected genes, suggesting TRX interacts with common factors at different target genes. All hypomorphic alleles examined complement one another, suggesting cooperative TRX function at target genes. Comparative effects of hypomorphic genotypes support previous findings that TRX has tissue-specific interactions with other factors at each target gene. Some hypomorphic genotypes also produce phenotypes that suggest TRX may be a component of signal transduction pathways that provide tissue- and cell-specific levels of target gene transcription.

Dcdc42 acts in TGF-beta signaling during Drosophila morphogenesis: distinct roles for the Drac1/JNK and Dcdc42/TGF-beta cascades in cytoskeletal regulation

During Drosophila embryogenesis the two halves of the lateral epidermis migrate dorsally over a surface of flattened cells, the amnioserosa, and meet at the dorsal midline in order to form the continuous sheet of the larval epidermis. During this process of epithelial migration, known as dorsal closure, signaling from a Jun-amino-terminal-kinase cascade causes the production of the secreted transforming-growth-factor-beta-like ligand, Decapentaplegic. Binding of Decapentaplegic to the putative transforming-growth-factor-beta-like receptors Thickveins and Punt activates a transforming-growth-factor-beta-like pathway that is also required for dorsal closure. Mutations in genes involved in either the Jun-amino-terminal-kinase cascade or the transforming-growth-factor-beta-like signaling pathway can disrupt dorsal closure. Our findings show that although these pathways are linked they are not equivalent in function. Signaling by the Jun-amino-terminal-kinase cascade may be initiated by the small Ras-like GTPase Drac1 and acts to assemble the cytoskeleton and specify the identity of the first row of cells of the epidermis prior to the onset of dorsal closure. Signaling in the transforming-growth-factor-beta-like pathway is mediated by Dcdc42, and acts during the closure process to control the mechanics of the migration process, most likely via its putative effector kinase DPAK.

The Drosophila gene brinker reveals a novel mechanism of Dpp target gene regulation

decapentaplegic (dpp), a Drosophila member of the TGFbeta family of secreted molecules, functions as a long-range morphogen in patterning of the embryo and the adult appendages. Dpp signals via the SMAD proteins Mad and Medea. Here we show that in the absence of brinker (brk), Mad is not required for the activation of Dpp target genes that depend on low levels of Dpp. brk encodes a novel protein with features of a transcriptional repressor. brk itself is negatively regulated by Dpp. Dpp signaling might relieve brk's repression of low-level target genes either by transcriptional repression of brk or by antagonizing a repressor function of brk at the target gene promoters.

Regulation by phosphorylation of the zinc finger protein KRC that binds the kappaB motif and V(D)J recombination signal sequences

The DNA binding protein KRC (forkappaB binding andrecognitioncomponent of the V(D)J recombination signal sequence) belongs to a family of large zinc finger proteins that bind to the kappaB motif and contains two widely separated DNA binding structures. In addition to the kappaB motif, KRC fusion proteins bind to the signal sequences of V(D)J recombination to form highly ordered complexes. Here, we report that KRC may be regulated by post-translational modifications. Specific protein kinases present in the nucleus of pre-B cells phosphorylated a KRC fusion protein at tyrosine and serine residues. Such protein modifications increased DNA binding, thereby providing a mechanism by which KRC responds to signal transduction pathways. KRC is a substrate of epidermal growth factor receptor kinase and P34cdc2 kinase in vitro. Our results suggest that activation of the KRC family of transcription factors may provide a mechanism by which oncogenic tyrosine kinases regulate genes with kappaB-controlled gene regulatory elements.

[Dorsal closure in Drosophila. A genetic model for wound healing?]

Dorsal closure (DC) is a morphogenetic movement that establishes the dorsal ectoderm of the drosophila embryo. During this process, the two lateral epithelia stretch toward the dorsal midline, the suture line of the two leading edges. Cell migration during DC relies both on cell shape change controlled by the activity of the JNK pathway in the leading edge cells and modification of cell adhesiveness, probably dependent upon activation of the Dpp (TGF-beta) pathway. Coupling of the JNK and TGF-beta pathways is essential. The sequence of the cellular and molecular events of DC highlights interesting common features with wound healing in vertebrates. Like DC, wound healing relies on the migration of epithelia bordered by leading edges controlling the direction and speed of the movement. This review summarizes recent data concerning the control of epithelial morphogenesis during DC and the bases of wound healing. The molecular and cellular events that underlie these two analogous migratory processes are detailed, discussed and compared. We suggest that DC is a good genetic model for wound healing studying.

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.

Drosophila endoderm development requires a novel homeobox gene which is a target of Wingless and Dpp signalling

We have identified and cloned a novel type of homeobox gene that is composed of two homeodomains and is expressed in the Drosophila endoderm. Mutant analysis reveals that its activity is required at the foregut/midgut boundary for the development of the proventriculus. This organ regulates food passage from the foregut into the midgut and forms by the infolding of ectoderm and endoderm-derived tissues. The endodermal outer wall structure of the proventriculus is collapsed in the mutants leading to a failure of the ectodermal part to invaginate and build a functional multilayered organ. Lack-of-function and gain-of-function experiments show that the expression of this homeobox gene in the proventriculus endoderm is induced in response to Wingless activity emanating from the ectoderm/endoderm boundary whereas its expression in the central midgut is controlled by Dpp and Wingless signalling emanating from the overlying visceral mesoderm.

JNK, cytoskeletal regulator and stress response kinase? A Drosophila perspective

c-Jun N-terminal kinases (JNKs) are intracellular stress-activated signalling molecules, which are controlled by a highly evolutionarily conserved signalling cascade. In mammalian cells, JNKs are regulated by a wide variety of cellular stresses and growth factors and have been implicated in the regulation of remarkably diverse biological processes, such as cell shape changes, immune responses and apoptosis. How can such different stimuli activate the JNK pathway and what roles does JNK play in vivo? Molecular genetic analysis of the Drosophila JNK gene has started to provide answers to these questions, confirming the role of this molecule in development and stress responses and suggesting a conserved function for JNK signalling in processes such as wound healing. Here, we review this work and discuss how future experiments in Drosophila should reveal the cell type-specific mechanisms by which JNKs perform their diverse functions.

Direct binding between two PDZ domain proteins Canoe and ZO-1 and their roles in regulation of the jun N-terminal kinase pathway in Drosophila morphogenesis

During Drosophila embryogenesis, the ventral epidermis dorsally expands and the left and right epithelial sheets meet and fuse along the dorsal midline. For this dorsal closure to occur, two PDZ domain proteins, Cno and ZO-1, are required. The dorsal epidermis remains open when the expression of ZO-1 and Cno are reduced simultaneously by hypomorphic mutations in the relevant loci. ZO-1 and Cno colocalize at adherens junctions in embryonic epithelia, and form a protein complex upon binding to each other. Genetic analysis showed that Cno is involved in the Jun N-terminal kinase (JNK) pathway for dorsal closure, as a modulator acting upstream of, or in parallel with, the small GTPase Drac1. The ZO-1-Cno complex may be involved in dynamic changes in cytoskeletal organization and cell adhesion during morphogenetic events associated with dorsal closure in the Drosophila embryo.

A BMP responsive transcriptional region in the chicken type X collagen gene

Bone morphogenetic proteins (BMPs) were originally identified by their ability to induce ectopic bone formation and have been shown to promote both chondrogenesis and chondrocyte hypertrophy. BMPs have recently been found to activate a membrane serine/threonine kinase signaling mechanism in a variety of cell types, but the downstream effectors of BMP signaling in chondrocyte differentiation remain unidentified. We have previously reported that BMP-2 markedly stimulates type X collagen expression in prehypertrophic chick sternal chondrocytes, and that type X collagen mRNA levels in chondrocytes cultured under serum-free (SF) conditions are elevated 3- to 5-fold within 24 h. To better define the molecular mechanisms of induction of chondrocyte hypertrophy by BMPs, we examined the effect of BMPs on type X collagen production by 15-day chick embryo sternal chondrocytes cultured under SF conditions in the presence or absence of 30 ng/ml BMP-2, BMP-4, or BMP-7. Two populations of chondrocytes were used: one representing resting cartilage isolated from the caudal third of the sterna and the second representing prehypertrophic cartilage from the cephalic third of the sterna. BMP-2, BMP-4, and BMP-7 all effectively promoted chondrocyte maturation of cephalic sternal chondrocytes as measured by high levels of alkaline phosphatase, diminished levels of type II collagen, and induction of the hypertrophic chondrocyte-specific marker, type X collagen. To test whether BMP control of type X collagen expression occurs at the transcriptional level, we utilized plasmid constructs containing the chicken collagen X promoter and 5' flanking regions fused to a reporter gene. Constructs were transiently transfected into sternal chondrocytes cultured under SF conditions in the presence or absence of 30 ng/ml BMP-2, BMP-4, or BMP-7. A 533 bp region located 2.4-2.9 kb upstream from the type X collagen transcriptional start site was both necessary and sufficient for strong BMP responsiveness in cells destined for hypertrophy, but not in chondrocytes derived from the lower sterna.

A novel homeobox gene mediates the Dpp signal to establish functional specificity within target cells

Morphogen gradients of secreted molecules play critical roles in the establishment of the spatial pattern of gene expression. During midgut development in Drosophila, secreted molecules of Decapentaplegic (Dpp) and Wingless (Wg) establish unique transcriptional regulation within target cells to specify the resultant cell types. Here we report the identification of a novel homeobox gene, defective proventriculus (dve), which is required for the midgut specification under the control of Dpp and Wg. In dve mutants, two distinct parts of the midgut, the proventriculus and middle midgut, are abnormally organized. The Wg signal regulates dve expression during proventriculus development. On the other hand, dve is a downstream target of Dpp in the middle midgut and defines the functional specificity of copper cells along with another Dpp target gene, labial. Thus, the dve gene acts under the two distinct extracellular signals at distant parts of the midgut primordia.

BMPs, Smads and metalloproteases: extracellular and intracellular modes of negative regulation

Bone morphogenetic (BMPs), members of the TGF-beta superfamily, have critical functions in many biological contexts. Recent findings in Drosophila and vertebrates suggest that BMP signaling can be modulated extracellularly and intracellularly by the availability of BMP inhibitors and Smads, respectively.

Smad proteins act in combination with synergistic and antagonistic regulators to target Dpp responses to the Drosophila mesoderm

Dorsal mesoderm induction in arthropods and ventral mesoderm induction in vertebrates are closely related processes that involve signals of the BMP family. In Drosophila, induction of visceral mesoderm, dorsal muscles, and the heart by Dpp is, at least in part, effected through the transcriptional activation and function of the homeobox gene tinman in dorsal mesodermal cells during early embryogenesis. Here we present a functional dissection of a tinman enhancer that mediates the Dpp response. We provide evidence that mesoderm-specific induction of tinman requires the binding of both activators and repressors. Screens for binding factors yielded Tinman itself and the Smad4 homolog Medea. We show that the binding and synergistic activities of Smad and Tinman proteins are critical for mesodermal tinman induction, whereas repressor binding sites prevent induction in the dorsal ectoderm and amnioserosa. Thus, integration of positive and negative regulators on enhancers of target genes appears to be an important mechanism in tissue-specific induction by TGF-beta molecules.

decapentaplegic is essential for the maintenance and division of germline stem cells in the Drosophila ovary

Stem cells are thought to occupy special local environments, or niches, established by neighboring cells that give them the capability for self-renewal. Each ovariole in the Drosophila ovary contains two germline stem cells surrounded by a group of differentiated somatic cells that express hedgehog and wingless. Here we show that the BMP2/4 homolog decapentaplegic (dpp) is specifically required to maintain female germline stem cells and promote their division. Overexpression of dpp blocks germline stem cell differentiation. Conversely, mutations in dpp or its receptor (saxophone) accelerate stem cell loss and retard stem cell division. We constructed mutant germline stem cell clones to show that the dpp signal is directly received by germline stem cells. Thus, dpp signaling helps define a niche that controls germline stem cell proliferation.

Functional intertwining of Dpp and EGFR signaling during Drosophila endoderm induction

Endoderm induction in Drosophila is mediated by the extracellular signals Decapentaplegic (Dpp) and Wingless (Wg). We discovered a secondary signal with a permissive role in this process, namely Vein, a neuregulin-like ligand that stimulates the epidermal growth factor receptor (EGFR) and Ras signaling. Dpp and Wg up-regulate vein expression in the midgut mesoderm in two regions overlapping the Dpp sources. Experiments based on lack of function and ectopic stimulation of Dpp and EGFR signaling show that these two pathways are functionally interdependent and that they synergize with each other, revealing functional intertwining. The transcriptional response elements for the Dpp signal in midgut enhancers from homeotic target genes are bipartite, comprising CRE sites as well as binding sites for the Dpp signal-transducing protein Mad. Of these sites, the CRE seems to function primarily in the response to Ras, the secondary signal of Dpp. We discuss the potential significance of why an inductive process might use a secondary signal whose function is intertwined with that of the primary signal.

Proneural gene self-stimulation in neural precursors: an essential mechanism for sense organ development that is regulated by Notch signaling

To learn about the acquisition of neural fate by ectodermal cells, we have analyzed a very early sign of neural commitment in Drosophila, namely the specific accumulation of achaete-scute complex (AS-C) proneural proteins in the cell that becomes a sensory organ mother cell (SMC). We have characterized an AS-C enhancer that directs expression specifically in SMCs. This enhancer promotes Scute protein accumulation in these cells, an event essential for sensory organ development in the absence of other AS-C genes. Interspecific sequence comparisons and site-directed mutagenesis show the presence of several conserved motifs necessary for enhancer action, some of them binding sites for proneural proteins. These and other data indicate that the enhancer mediates scute self-stimulation, although only in the presence of additional activating factors, which most likely interact with conserved motifs reminiscent of NF-kappaB-binding sites. Cells neighboring the SMC do not acquire the neural fate because the Notch signaling pathway effectors, the Enhancer of split bHLH proteins, block this proneural gene self-stimulatory loop, possibly by antagonizing the action on the enhancer of the NF-kappaB-like factors or the proneural proteins. These data suggest a mechanism for SMC committment.

Identification of chromosomal regions involved in decapentaplegic function in Drosophila

Signaling molecules of the transforming growth factor beta (TGF-beta) family contribute to numerous developmental processes in a variety of organisms. However, our understanding of the mechanisms which regulate the activity of and mediate the response to TGF-beta family members remains incomplete. The product of the Drosophila decapentaplegic (dpp) locus is a well-characterized member of this family. We have taken a genetic approach to identify factors required for TGF-beta function in Drosophila by testing for genetic interactions between mutant alleles of dpp and a collection of chromosomal deficiencies. Our survey identified two deficiencies that act as maternal enhancers of recessive embryonic lethal alleles of dpp. The enhanced individuals die with weakly ventralized phenotypes. These phenotypes are consistent with a mechanism whereby the deficiencies deplete two maternally provided factors required for dpp's role in embryonic dorsal-ventral pattern formation. One of these deficiencies also appears to delete a factor required for dpp function in wing vein formation. These deficiencies remove material from the 54F-55A and 66B-66C polytene chromosomal regions, respectively. As neither of these regions has been previously implicated in dpp function, we propose that each of the deficiencies removes a novel factor or factors required for dpp function.