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

A genetic screen for modifiers of Drosophila decapentaplegic signaling identifies mutations in punt, Mothers against dpp and the BMP-7 homologue, 60A

decapentaplegic (dpp) is a Transforming Growth Factor beta (TGF-beta)-related growth factor that controls multiple developmental processes in Drosophila. To identify components involved in dpp signaling, we carried out a genetic screen for dominant enhancer mutations of a hypomorphic allele of thick veins (tkv), a type I receptor for dpp. We recovered new alleles of tkv, punt, Mothers against dpp (Mad) and Medea (Med), all of which are known to mediate dpp signaling. We also recovered mutations in the 60A gene which encodes another TGF-beta-related factor in Drosophila. DNA sequence analysis established that all three 60A alleles were nonsense mutations in the prodomain of the 60A polypeptide. These mutations in 60A caused defects in midgut morphogenesis and fat body differentiation. We present evidence that when dpp signaling is compromised, lowering the level of 60A impairs several dpp-dependent developmental processes examined, including the patterning of the visceral mesoderm, the embryonic ectoderm and the imaginal discs. These results provide the first in vivo evidence for the involvement of 60A in the dpp pathway. We propose that 60A activity is required to maintain optimal signaling capacity of the dpp pathway, possibly by forming biologically active heterodimers with Dpp proteins.

Xvent-1 mediates BMP-4-induced suppression of the dorsal-lip-specific early response gene XFD-1' in Xenopus embryos

Ectopic expression of the ventralizing morphogen BMP-4 (bone morphogenetic protein-4) in the dorsal lip (Spemann organizer) of Xenopus embryos blocks transcription of dorsal-lip-specific early response genes. We investigated the molecular mechanism underlying the BMP-4-induced inhibition of the fork head gene XFD-1'. The promoter of this gene contains a BMP-triggered inhibitory element (BIE) which prevents activation of this gene at the ventral/vegetal side of the embryo in vivo. In the present study, we show that BMP-4-induced inhibition is not direct but indirect, and is mediated by Xvent homeobox proteins. Micro-injections of Xvent-1 RNA and XFD-1' promoter deletion mutants demonstrate that Xvent-1 mimics the effect of BMP-4 signalling not only by suppression of the XFD-1' gene, but also by utilizing the BIE. Suppression could be reverted using a dominant-negative Xvent-1 mutant. The repressor domain was localized to the N-terminal region of the protein. Gel-shift and footprint analyses prove that Xvent-1 binds to the BIE. Moreover, PCR-based target-site selection for the Xvent-1 homeodomain confirms distinct motifs within the BIE as preferential binding sites. Thus, biological and molecular data suggest that Xvent-1 acts as direct repressor for XFD-1' transcription and mediates BMP-4-induced inhibition.

The Drosophila Medea gene is required downstream of dpp and encodes a functional homolog of human Smad4

The Transforming Growth Factor-beta superfamily member decapentaplegic (dpp) acts as an extracellular morphogen to pattern the embryonic ectoderm of the Drosophila embryo. To identify components of the dpp signaling pathway, we screened for mutations that act as dominant maternal enhancers of a weak allele of the dpp target gene zerknLllt. In this screen, we recovered new alleles of the Mothers against dpp (Mad) and Medea genes. Phenotypic analysis of the new Medea mutations indicates that Medea, like Mad, is required for both embryonic and imaginal disc patterning. Genetic analysis suggests that Medea may have two independently mutable functions in patterning the embryonic ectoderm. Complete elimination of maternal and zygotic Medea activity in the early embryo results in a ventralized phenotype identical to that of null dpp mutants, indicating that Medea is required for all dpp-dependent signaling in embryonic dorsal-ventral patterning. Injection of mRNAs encoding DPP or a constitutively activated form of the DPP receptor, Thick veins, into embryos lacking all Medea activity failed to induce formation of any dorsal cell fates, demonstrating that Medea acts downstream of the thick veins receptor. We cloned Medea and found that it encodes a protein with striking sequence similarity to human SMAD4. Moreover, injection of human SMAD4 mRNA into embryos lacking all Medea activity conferred phenotypic rescue of the dorsal-ventral pattern, demonstrating conservation of function between the two gene products.

Smads are the central component in transforming growth factor-beta signaling

Until recently, little was known about how transforming growth factor (TGF)-beta signals are transduced to the nucleus. With the discovery of the Smad proteins initially in Drosophila and C. elegans, the unraveling of the pathway has begun. Nine different vertebrate members also have been reported, indicating that Smads are a conserved component of the TGF-beta pathway. Currently, there are three functional classes of Smads. Class I Smads are phosphorylated by TGF-beta receptors and move to the nucleus. The Class II Smads function with Class I Smads, while Class III Smads antagonize the function of Class I Smads. New evidence shows that Smads bind specific DNA sequences and induce transcription of downstream target genes, thus placing the Smads at the center of the TGF-beta signaling pathway.

Regulation of cellular and system function by activin

Activin is an important molecule that regulates hormonogenesis, cellular homeostasis (divide or die pathways), and differentiation programs (developmentally and in adult cells). The cellular mechanisms that integrate an activin signal into a physiological response include a binary receptor complex and tandem serine threonine kinases, intracellular signal mediators, and nuclear transcription factors. Activin antagonists (inhibins) and bioneutralizing binding proteins (follistatins) act as gating molecules to ensure accurate delivery of activin signals to cellular machinery. Correct execution of an activin cue intracellularly permits actions as fundamental as embryonic mesoderm development, neuronal survival, hematopoietic function, and reproductive cyclicity. Absent or incorrect activin signaling results in phenotypes as catastrophic as embryonic lethality, tumor formation, and infertility. The general ways in which a cell senses and responds to an activin signal will be reviewed in the first part of this paper. The role of this ligand in reproductive function will also be examined as a specific example of activin activity.

Signal transduction by bone morphogenetic proteins

Bone morphogenetic proteins (BMPs) are multifunctional cytokines, which are members of the transforming growth factor-beta (TGF-beta) superfamily. Activities of BMPs are extracellularly regulated by BMP-binding proteins, Noggin and Chordin. BMPs bind to two different types of serine-threonine kinase receptors, type I and type II. Two BMP type I receptors and a BMP type II receptor have been identified in mammals. Intracellular signals are transduced by Smad proteins. Smad1, Smad5 and probably MADH6, are activated by BMP receptors, form heteromeric complexes with Smad4, and translocate into the nucleus where they may activate transcription of various genes. Smad6 and Smad7 are inhibitory Smads, and may act as autocrine switch-off signals. In Drosophila melanogaster, Decapentaplegic (Dpp) is a homologue of mammalian BMPs. In this review, mechanism of action of Dpp will be discussed in comparison with that of BMPs.

Genetic analysis of the actin cytoskeleton in the Drosophila ovary

The Drosophila ovary provides a favorable model system in which to study cellular morphogenesis. The development of a mature egg involves a syncytium of 16 germline cells and over 1000 somatically derived follicle cells. Intercellular transport, stable intercellular bridges, cell migrations, cell shape changes, and specific subcellular localization of many embryonic patterning determinants contribute to egg development and require a dynamic cytoskeleton. We discuss many of the recent genetic and cell biological studies that have led to insights into how the actin cytoskeleton is assembled and regulated during the morphogenesis of the Drosophila egg.

The tao of stem cells in the germline

Germline stem cells (GSCs) are the self-renewing population of germ cells that serve as the source for gametogenesis. GSCs exist in diverse forms, from those that undergo strict self-renewing asymmetric divisions in Drosophila to those that maintain their population by balancing between mitosis and differentiation in Caenorhabditis elegans. Most vertebrate spermatogonial GSCs appear to adopt an intermediate strategy. In most animals, GSCs are established during preadult gonadogenesis following the proliferation and migration of embryonic primordial germ cells. GSCs produce numerous gametes throughout the sexually active period of adult life. The establishment and self-renewing division of GSCs are controlled by extracellular signals such as hormones from the hypothalamic-pituitary axis and local interactions between GSCs and their neighboring cells. These extracellular signals may then influence differential gene expression, cell cycle machinery, and cytoskeletal organization of GSCs for their formation and/or divisional asymmetry. In addition, the GSC mechanism is related to that for germline and sex determination. Current knowledge has provided a solid framework for further study of GSCs and stem cells in general.

TGF-beta signal transduction

The transforming growth factor beta (TGF-beta) family of growth factors control the development and homeostasis of most tissues in metazoan organisms. Work over the past few years has led to the elucidation of a TGF-beta signal transduction network. This network involves receptor serine/threonine kinases at the cell surface and their substrates, the SMAD proteins, which move into the nucleus, where they activate target gene transcription in association with DNA-binding partners. Distinct repertoires of receptors, SMAD proteins, and DNA-binding partners seemingly underlie, in a cell-specific manner, the multifunctional nature of TGF-beta and related factors. Mutations in these pathways are the cause of various forms of human cancer and developmental disorders.

Synergistic activation of a Drosophila enhancer by HOM/EXD and DPP signaling

The homeotic proteins encoded by the genes of the Drosophila HOM and the vertebrate HOX complexes do not bind divergent DNA sequences with a high selectivity. In vitro, HOM (HOX) specificity can be increased by the formation of heterodimers with Extradenticle (EXD) or PBX homeodomain proteins. We have identified a single essential Labial (LAB)/EXD-binding site in a Decapentaplegic (DPP)-responsive enhancer of the homeotic gene lab which drives expression in the developing midgut. We show that LAB and EXD bind cooperatively to the site in vitro, and that the expression of the enhancer in vivo requires exd and lab function. In addition, point mutations in either the EXD or the LAB subsite compromise enhancer function, strongly suggesting that EXD and LAB bind to this site in vivo. Interestingly, we found that the activity of the enhancer is only stimulated by DPP signaling significantly upon binding of LAB and EXD. Thus, the enhancer appears to integrate positional information via the homeotic gene lab, and spatiotemporal information via DPP signaling; only when these inputs act in concert in an endodermal cell is the enhancer fully active. Our results illustrate how a tissue-specific response to DPP can be generated through synergistic effects on an enhancer carrying both DPP- and HOX-responsive sequences.

TGF-beta signalling from cell membrane to nucleus through SMAD proteins

The recent identification of the SMAD family of signal transducer proteins has unravelled the mechanisms by which transforming growth factor-beta (TGF-beta) signals from the cell membrane to the nucleus. Pathway-restricted SMADs are phosphorylated by specific cell-surface receptors that have serine/threonine kinase activity, then they oligomerize with the common mediator Smad4 and translocate to the nucleus where they direct transcription to effect the cell's response to TGF-beta. Inhibitory SMADs have been identified that block the activation of these pathway-restricted SMADs.

Developmental biology of hematopoiesis

Hematopoietic stem cells are at the top of a hierarchy that regulates the generation of a vast repertoire of blood cells during the lifetime of a vertebrate. Recent experiments, using a vast variety of embryonic systems, shed new light on the origin of stem cells and the genes that function to regulate and maintain hematopoietic differentiation programs. Two distinct populations of stem cells develop--derived initially from transient, extraembryonic source and later from a stable, intraembryonic source; it is possible that both are generated from a pro-HSC able to respond differentially to local inductions. The initial blood cells develop from ventral mesoderm. The blood-forming region develops as a result of signaling from specific, secreted, embryonic growth factors, including the bone morphogenetic proteins. Stem cells give rise to progenitors that are restricted progressively in their ability to contribute to specific lineages. This process is regulated by transcription factors, whose functions are confirmed through genetic analyses. The identification of highly conserved, embryonic signaling pathways and transcription regulatory genes illustrates the enormous utility of analyzing embryonic hematopoiesis in frog, chick, fish, and mouse systems to further our understanding of human stem cells.

u-shaped encodes a zinc finger protein that regulates the proneural genes achaete and scute during the formation of bristles in Drosophila

The pattern of the large sensory bristles on the notum of Drosophila arises as a consequence of the expression of the achaete and scute genes. The gene u-shaped encodes a novel zinc finger that acts as a transregulator of achaete and scute in the dorsal region of the notum. Viable hypomorphic u-shaped mutants display additional dorsocentral and scutellar bristles that result from overexpression of achaete and scute. In contrast, overexpression of u-shaped causes a loss of achaete-scute expression and consequently a loss of dorsal bristles. The effects on the dorsocentral bristles appear to be mediated through the enhancer sequences that regulate achaete and scute at this site. The effects of u-shaped mutants are similar to those of a class of dominant alleles of the gene pannier with which they display allele-specific interactions, suggesting that the products of both genes cooperate in the regulation of achaete and scute. A study of the sites at which the dorsocentral bristles arise in mosaic u-shaped nota, suggests that the levels of the u-shaped protein are crucial for the precise positioning of the precursors of these bristles.

punt and schnurri regulate a somatically derived signal that restricts proliferation of committed progenitors in the germline

To identify regulators of stem cell lineages, we are focusing on spermatogenesis in Drosophila. In spermatogenesis, each germline stem cell divides asymmetrically, renewing itself and producing a transiently amplifying daughter, which divides four times. By screening for mutants in which daughter cells fail to stop dividing, we find that the TGF-beta signal transducers schnurri and punt are required to limit transient amplification of germ cells. Mosaic analysis demonstrates that punt and schnurri act within somatic cyst cells that surround germ cells, rather than in germ cells. Thus, a cyst-cell-derived signal restricts germ cell proliferation and this signal is initiated by input from a member of the TGF-beta superfamily. Thus, a signal relay regulates progression through the germline stem cell lineage.

D-Fos, a target gene of Decapentaplegic signalling with a critical role during Drosophila endoderm induction

The Drosophila endoderm is patterned by the signals Decapentaplegic and Wingless secreted from the visceral mesoderm. This induction culminates in a precise pattern of spatially restricted expression of labial, a selector gene with a role in cell type specification in the larval midgut. Here, we show that Decapentaplegic signalling induces elevated expression of the Drosophila AP-1 transcription factor D-Fos in a slightly broader endodermal region than labial. This induction occurs in parallel to, and independently of, that of labial. Furthermore, we present evidence that D-Fos is required for labial induction in the embryo as well as for maintenance of labial expression through larval stages; and that D-Fos is critical for cellular differentiation in the larval gut. We propose that Decapentaplegic, by inducing D-Fos, broadly defines an endodermal region which thus becomes predisposed to express labial, and that D-Fos cooperates with signal-activated response factors to confer the precise pattern of labial expression in this region.

Mothers against dpp participates in a DDP/TGF-beta responsive serine-threonine kinase signal transduction cascade

Mothers against dpp (Mad) is the prototype of a family of genes required for signaling by TGF-beta related ligands. In Drosophila, Mad is specifically required in cells responding to Decapentaplegic (DPP) signals. We further specify the role of Mad in DPP-mediated signaling by utilizing tkvQ199D, an activated form of the DPP type I receptor serine-threonine kinase thick veins (tkv). In the embryonic midgut, tkvQ199D mimics DPP-mediated inductive interactions. Homozygous Mad mutations block signaling by tkvQ199D. Appropriate responses to signaling by tkvQ199D are restored by expression of MAD protein in DPP-target cells. Endogenous MAD is phosphorylated in a ligand-dependent manner in Drosophila cell culture. DPP overexpression in the embryonic midgut induces MAD nuclear accumulation; after withdrawal of the overexpressed DPP signal, MAD is detected only in the cytoplasm. However, in three different tissues and developmental stages actively responding to endogenous DPP, MAD protein is detected in the cytoplasm but not in the nucleus. From these observations, we discuss possible roles for MAD in a DPP-dependent serine-threonine kinase signal transduction cascade integral to the proper interpretation of DPP signals.

Drosophila Jun kinase regulates expression of decapentaplegic via the ETS-domain protein Aop and the AP-1 transcription factor DJun during dorsal closure

During Drosophila embryogenesis, ectodermal cells of the lateral epithelium stretch in a coordinated fashion to internalize the amnioserosa cells and close the embryo dorsally. This process, dorsal closure, requires two signaling pathways: the Drosophila Jun-amino-terminal kinase (DJNK) pathway and the Dpp pathway. We have identified mutations in DJun and show that DJNK controls dorsal closure by activating DJun and inactivating the ETS repressor Aop/Yan by phosphorylation. DJun and Aop regulate dpp expression in the most dorsal row of cells. Secreted Dpp then instructs more ventrally located cells to stretch. Our results provide a causal link between the DJNK and Dpp pathways during dorsal closure. Interestingly, in vertebrates, transforming growth factor-beta and c-Jun regulate collagenase gene expression during wound healing, a process that also involves the closing of an epithelial sheath.

Drosophila Jun relays the Jun amino-terminal kinase signal transduction pathway to the Decapentaplegic signal transduction pathway in regulating epithelial cell sheet movement

We have characterized mutations in the Drosophila homolog of the mammalian proto-oncogene c-Jun gene (Djun). We demonstrate that DJUN in the embryo is a downstream target of the JNK signal transduction pathway during dorsal closure formation, and that the function of the JNK/DJUN pathway is to control the localized expression of decapentalegic (dpp), a member of the TGF-beta growth factor family. In contrast to previous observations, we find that both in the embryo and during photoreceptor cell determination, DJUN is not regulated by a pathway that involves MAPK.

Bone morphogenetic proteins in the nervous system

Bone morphogenetic proteins (BMPs) are a rapidly expanding subclass of the transforming growth factor superfamily. BMP ligands and receptor subunits are present throughout neural development within discrete regions of the embryonic brain and within neural crest-derived pre- and post-migratory zones. BMPs initially inhibit the formation of neuroectoderm during gastrulation while, within the neural tube, they act as gradient morphogens to promote the differentiation of dorsal cell types and intermediate cell types throughout co-operative signaling. In the peripheral nervous system, BMPs act as instructive signals for neuronal lineage commitment and promote graded stages of neuronal differentiation. By contrast, within the CNS, these same factors promote astroglial lineage elaboration from embryonic subventricular zone progenitor cells, with concurrent suppression of the neuronal or oligodendroglial lineages, or both. In addition, BMPs act on more lineage-restricted embryonic CNS progenitor cells to promote regional neuronal survival and cellular differentiation. Furthermore, these versatile cytokines induce selective apoptosis of discrete rhombencephalic neural crest-associated cellular populations. These observations suggest that the BMPs exhibit a broad range of cellular and context-specific effects during multiple stages of neural development.

Coupling of Jun amino-terminal kinase and Decapentaplegic signaling pathways in Drosophila morphogenesis

Dorsal closure in Drosophila embryos involves the migration of two lateral epithelia toward the dorsal midline to establish the dorsal ectoderm. Previous work showed that this morphogenetic movement depends on the activities of a Jun amino (N)-terminal kinase kinase (JNKK) encoded by the hemipterous (hep) gene, and of a JNK encoded by basket. Hep is required for cell determination in the leading edge of migrating epithelia, by controlling specific expression of the puckered (puc) gene in these cells. During dorsal closure, decapentaplegic (dpp), a member of the transforming growth factor-beta (TGF-beta) superfamily, is expressed in the row of cells making up the leading edge of the epithelia. Here, we show that the small GTPases Dcdc42, Drac1, and the Hep JNKK control dpp expression in this migratory process. Appropriate dpp and puc expression in the leading edge also depends on the inhibitory function of the puc gene. Further, our data suggest that the leading edge is the source of a JNK autocrine signal, and exclude a role of Dpp as such a ligand. Dorsal closure couples JNK and dpp signaling pathways, a situation that may be conserved in vertebrate development.

A CREB-binding site as a target for decapentaplegic signalling during Drosophila endoderm induction

Decapentaplegic (Dpp) is an extracellular signal of the transforming growth factor-beta family with multiple functions during Drosophila development. For example, it plays a key role in the embryo during endoderm induction. During this process, Dpp stimulates transcription of the homeotic genes Ultrabithorax in the visceral mesoderm and labial in the subjacent endoderm. Here, we show that a cAMP response element (CRE) from an Ultrabithorax enhancer mediates Dpp-responsive transcription in the embryonic midgut, and that endoderm expression from a labial enhancer depends on multiple CREs. Furthermore, the Drosophila CRE-binding protein dCREB-B binds to the Ultrabithorax CRE, and ubiquitous expression of a dominant-negative form of dCREB-B suppresses CRE-mediated reporter gene expression and reduces labial expression in the endoderm. Therefore, a CREB protein may act as a nuclear target, or as a partner of a nuclear target, for Dpp signalling in the embryonic midgut.

Genetic and biochemical analysis of TGF beta signal transduction

TGF beta-like ligands are involved in many different developmental processes that pattern a variety of tissues in invertebrates and vertebrates. In the last few years, rapid progress has been made toward elucidating the developmental roles of the TGF beta-like pathways and identifying the novel components involved in transducing their signals, particularly the newly discovered Smads. This rapid progress has been the result of a synergy between classical genetic approaches and biochemical approaches, and this combined approach is likely to propel future understanding of the signaling pathway used by TGF beta.

Regulation of a dpp target gene in the Drosophila embryo

In Drosophila, two TGF-beta growth factors, dpp and screw, function synergistically to subdivide the dorsal ectoderm into two embryonic tissues, the amnioserosa and dorsal epidermis. Previous studies have shown that peak dpp activity is required for the localized expression of zerknullt (zen), which encodes a homeodomain transcription factor. We present evidence that zen directly activates the amnioserosa-specific expression of a downstream target gene, Race (Related to angiotensin converting enzyme). A 533 bp enhancer from the Race promoter region is shown to mediate selective expression in the amnioserosa, as well as the anterior and posterior midgut rudiments. This enhancer contains three zen protein binding sites, and mutations in these sites virtually abolish the expression of an otherwise normal Race-lacZ fusion gene in the amnioserosa, but not in the gut. Genetic epistasis experiments suggest that zen is not the sole activator of Race, although a hyperactivated form of zen (a zen-VP16 fusion protein) can partially complement reduced levels of dpp activity. These results suggest that dpp regulates multiple transcription factors, which function synergistically to specify the amnioserosa.

Role of decapentaplegic in initiation and progression of the morphogenetic furrow in the developing Drosophila retina

Morphogenesis in the Drosophila retina initiates at the posterior margin of the eye imaginal disc by an unknown mechanism. Upon initiation, a wave of differentiation, its forward edge marked by the morphogenetic furrow (MF), proceeds anteriorly across the disc. Progression of the MF is driven by hedgehog (hh), expressed by differentiating photoreceptor cells. The TGF-beta homolog encoded by decapentaplegic (dpp) is expressed at the disc's posterior margin prior to initiation and in the furrow, under the control of hh, during MF progression. While dpp has been implicated in eye disc growth and morphogenesis, its precise role in retinal differentiation has not been determined. To address the role of dpp in initiation and progression of retinal differentiation we analyzed the consequences of reduced and increased dpp function during eye development. We find that dpp is not only required for normal MF initiation, but is sufficient to induce ectopic initiation of differentiation. Inappropriate initiation is normally inhibited by wingless (wg). Loss of dpp function is accompanied by expansion of wg expression, while increased dpp function leads to loss of wg transcription. In addition, dpp is required to maintain, and sufficient to induce, its own expression along the disc's margins. We postulate that dpp autoregulation and dpp-mediated inhibition of wg expression are required for the coordinated regulation of furrow initiation and progression. Finally, we show that in the later stages of retinal differentiation, reduction of dpp function leads to an arrest in MF progression.

Specification of the embryonic limb primordium by graded activity of Decapentaplegic

Two thoracic limbs of Drosophila, the leg and the wing, originate from a common cluster of cells that include the source of two secreted signaling molecules, Decapentaplegic and Wingless. We show that Wingless, but not Decapentaplegic, is responsible for initial specification of the limb primordia with a distal identity. Limb formation is restricted to the lateral position of the embryo by negative control of the early function of Decapentaplegic and the EGF receptor homolog that determine the global dorsoventral pattern. Late function of Decapentaplegic locally determines two additional cell identities in a dosage dependent manner. Loss of Decapentaplegic activity results in a deletion of the proximal structures of the limb, which is in contrast to the consequence of decapentaplegic mutations in the imaginal disc, which cause a deletion of distal structures. The results indicate that the limb pattern elements are added in a distal to proximal direction in the embryo, which is opposite to what is happening in the growing imaginal disc. We propose that Wingless and Decapentaplegic act sequentially to initiate the proximodistal axis.

Synthetic inhibitors of regulatory proteins involved in the signaling pathway of the replication of human immunodeficiency virus 1

NF-kappa B, HIV-EP1, Sp1, and E1A are transcriptional proteins involved in the long terminal repeat-directed expression of HIV-1. The inhibitory effect of 18 dimethylaminopyridine-based compounds against these regulatory proteins was studied. Experiments using NF-kappa B-beads showed that histidine-pyridine-histidine compounds and their zinc complexes are inhibitory not only for the NF-kappa B-DNA binding, but also for the binding of NF-kappa B with the inhibitory protein I kappa B. Discriminative inhibition of the DNA binding of two distinct C2H2 type zinc finger proteins HIV-EP1 and Sp1 was also attempted using the synthetic compounds. Whereas some compounds inhibited the DNA binding of both HIV-EP1 and Sp1 at 300 microM, others preferentially and completely inhibited HIV-EP1 without much suppression of Sp1. Mercapto compounds were more potent and uniformly inhibitory against both HIV-EP1 and Sp1 at 30 microM. Disulfide compounds were also remarkably inhibitory against HIV-EP1 and Sp1 also at 30 microM whereas the shorter-chain disulfides 7 and 9 were effective only for HIV-EP1. S-Alkyl derivatives preferentially inhibited HIV-EP1 at 300 microM. The dimethylamino compound was the sole compound inhibitory only against Sp1, being non-inhibitory against HIV-EP1. Relevant combinations of these inhibitors would allow us to inhibit NF-kappa B, HIV-EP1, and Sp1 in any combinations. Inhibition of the TBP binding of C4 type zinc finger protein adenovirus E1A was also examined. It was found that two compounds induced, at 50 mM concentration, effective inhibition of the TBP binding of E1A, demonstrating that it is possible in principle to inhibit the protein-protein interaction of zinc finger proteins.

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

We report on the characterization of the first loss-of-function mutation in a Drosophila CREB gene, dCREB-A. In the epidermis, dCREB-A is required for patterning cuticular structures on both dorsal and ventral surfaces since dCREB-A mutant larvae have only lateral structures around the entire circumference of each segment. Based on results from epistasis tests with known dorsal/ventral patterning genes, we propose that dCREB-A encodes a transcription factor that functions near the end of both the DPP- and SPI-signaling cascades to translate the corresponding extracellular signals into changes in gene expression. The lateralizing phenotype of dCREB-A mutants reveals a much broader function for CREB proteins than previously thought.

Developmental control of cell cycle regulators: a fly's perspective

During early development in many species, maternally supplied gene products permit the cell cycle to run at maximum velocity, subdividing the fertilized egg into smaller and smaller cells. As development proceeds, zygotic controls are activated that first limit divisions to defined spatial and temporal domains, coordinating them with morphogenesis, and then halt proliferation altogether, to allow cell differentiation. Analysis of the regulation of cyclin-dependent kinases (Cdks) in Drosophila has provided insights into how this embryonic program of cell proliferation is controlled at the molecular level and how it is linked to developmental cues. Recent studies have also begun to reveal how cell proliferation is controlled during the second phase of Drosophila development, which occurs in imaginal tissues. In contrast to their embryonic progenitors, imaginal cells proliferate with a cycle that requires cell growth and is linked to patterning processes controlled by secreted cell signaling molecules. The functions of these signaling molecules appear to be nearly as conserved between vertebrates and invertebrates as the cell cycle control apparatus itself, suggesting that the mechanisms that coordinate growth, patterning, and cell proliferation in developing tissues have ancient origins.

Zygotic lethal mutations with maternal effect phenotypes in Drosophila melanogaster. II. Loci on the second and third chromosomes identified by P-element-induced mutations

Screens for zygotic lethal mutations that are associated with specific maternal effect lethal phenotypes have only been conducted for the X chromosome. To identify loci on the autosomes, which represent four-fifths of the Drosophila genome, we have used the autosomal "FLP-DFS" technique to screen a collection of 496 P element-induced mutations established by the Berkeley Drosophila Genome Project. We have identified 64 new loci whose gene products are required for proper egg formation or normal embryonic development.

Signal transduction by members of the transforming growth factor-beta superfamily

Transforming growth factor-beta (TGF beta) superfamily members exert their diverse biological effects through their interaction with heteromeric receptor complexes of transmembrane serine/threonine kinases. Both components of the receptor complex, known as receptor I and receptor II are essential for signal transduction. The composition of these complexes can vary significantly due to the promiscuous nature of the ligands and the receptors, and this diversity of interactions can yield a variety of biological responses. Several receptor interacting proteins and potential mediators of signal transduction have now been identified. Recent advances, particularly in our understanding of the function of Mothers against dpp-related (MADR) proteins, are providing new insights into how the TGF beta superfamily signals its diverse biological activities.

The Drosophila decapentaplegic and short gastrulation genes function antagonistically during adult wing vein development

TGF-beta-related signaling pathways play diverse roles during vertebrate and invertebrate development. A common mechanism for regulating the activity of TGF-beta family members is inhibition by extracellular antagonists. Recently, the Drosophila short gastrulation (sog) gene was shown to encode a predicted diffusible factor which antagonizes signaling mediated by the TGF-beta-like Decapentaplegic (Dpp) pathway in the early blastoderm embryo. sog and dpp, which are among the earliest zygotic genes to be activated, are expressed in complementary dorsal-ventral domains. The opposing actions of sog and dpp in the early embryo have been highly conserved during evolution as their vertebrate counterparts, chordin and BMP-4, function homologously to define neural versus non-neural ectoderm in Xenopus. Here we exploit the genetically sensitive adult wing vein pattern to investigate the generality of the antagonistic relationship between sog and dpp. We show that dpp is expressed in vein primordia during pupal wing development and functions to promote vein formation. In contrast, sog is expressed in complementary intervein cells and suppresses vein formation. sog and dpp function during the same phenocritical periods (i.e. 16–28 hours after pupariation) to influence the vein versus intervein cell fate choice. The conflicting activities of dpp and sog are also revealed by antagonistic dosage-sensitive interactions between these two genes during vein development. Analysis of vein and intervein marker expression in dpp and sog mutant wings suggests that dpp promotes vein fates indirectly by activating the vein gene rhomboid (rho), and that sog functions by blocking an autoactivating Dpp feedback loop. These data support the view that Sog is a dedicated Dpp antagonist.

araucan and caupolican provide a link between compartment subdivisions and patterning of sensory organs and veins in the Drosophila wing

The homeo box prepattern genes araucan (ara) and caupolican (caup) are coexpressed near the anterior-posterior (AP) compartment border of the developing Drosophila wing in two symmetrical patches located one at each side of the dorsoventral (DV) compartment border. ara-caup expression at these patches is necessary for the specification of the prospective vein L3 and associated sensory organs through the transcriptional activation, in smaller overlapping domains, of rhomboid/veinlet and the proneural genes achaete and scute. We show that ara-caup expression at those patches is mediated by the Hedgehog signal through its induction of high levels of Cubitus interruptus (Ci) protein in anterior cells near to the AP compartment border. The high levels of Ci activate decapentaplegic (dpp) expression, and, together, Ci and Dpp positively control ara-caup. The posterior border of the patches is apparently defined by repression by engrailed. Wingless accumulation at the DV border sets, also by repression, the gap between the two patches. Thus, ara and caup integrate the inputs of genes effecting the primary subdivisions of the wing disc into compartments to define two smaller territories. These in turn help create the even smaller domains of rhomboid/veinlet and achaete-scute expression.

Regulation of transforming growth factor beta- and activin-induced transcription by mammalian Mad proteins

Members of the transforming growth factor beta (TGF-beta) superfamily are involved in diverse physiological activities including development, tissue repair, hormone regulation, bone formation, cell growth, and differentiation. At the cellular level, these functions are initiated by the interaction of ligands with specific transmembrane receptors with intrinsic serine/threonine kinase activity. The signaling pathway that links receptor activation to the transcriptional regulation of the target genes is largely unknown. Recent work in Drosophila and Xenopus signaling suggested that Mad (Mothers against dpp) functions downstream of the receptors of the TGF-beta family. Mammalian Mad1 has been reported to respond to bone morphogenetic protein (BMP), but not to TGF-beta or activin. We report here the cloning and functional studies of a novel mammalian Mad molecule, Mad3, as well as a rat Mad1 homologue. Overexpression of Mad3 in a variety of cells stimulated basal transcriptional activity of the TGF-beta/activin-responsive reporter construct, p3TP-Lux. Furthermore, expression of Mad3 could potentiate the TGF-beta- and activin-induced transcriptional stimulation of p3TP-Lux. By contrast, overexpression of Mad1 inhibited the basal as well as the TGF-beta/activin induced p3TP-Lux activity. These findings, therefore, support the hypothesis that Mad3 may serve as a mediator linking TGF-beta/activin receptors to transcriptional regulation.

A JNK signal transduction pathway that mediates morphogenesis and an immune response in Drosophila

The Drosophila MAP kinase DJNK is a homolog of the mammalian c-Jun amino-terminal kinase (JNK). Mutations in the DJNK gene correspond to the complementation group basket. DJNK is phosphorylated and activated by the Drosophila MAP kinase kinase HEP. Substrates of DJNK include the transcription factor DJun. DJNK participates in multiple physiological processes. Exposure to endotoxic lipopolysaccharide initiates an insect immune response and leads to DJNK activation. In addition, embryos lacking DJNK are defective in dorsal closure, a process in which the lateral epithelial cells migrate over the embryo and join at the dorsal midline. These data demonstrate that the DJNK signal transduction pathway mediates an immune response and morphogenesis in vivo.

A transcriptional partner for MAD proteins in TGF-beta signalling

The transforming-growth-factor-beta (TGF-beta) superfamily is critical for establishing mesoderm during early embryogenesis in Xenopus. The transcriptional activation of Mix.2, an immediate-early response gene specific to activin-like members of the TGF-beta superfamily, is associated with the rapid appearance of a site-specific DNA-binding activity that recognizes a fifty-base-pair regulatory element known as ARE in the Mix.2 promoter. Cloning of the site-specific DNA-binding component of this activity revealed it to be a new winged-helix transcription factor and a direct target for signalling by the TGF-beta superfamily. XMAD2, a recently identified TGF-beta signal transducer, forms a complex with the transcription factor in an activin-dependent fashion to generate an activated ARE-binding complex. A model is proposed to explain how TGF-beta superfamily signals might regulate the expression of specific genes in the early embryo.

Intracellular signalling: the mad way to do it

A family of proteins has been identified that transduce a signal inside cells following binding of a TGF-beta family ligand to its cognate receptor. Like the prototype Drosophila protein Mad, many members of the family play key roles in developmental signalling.

Regulation of differentiation by TGF-beta

Recent experiments in neural, skeletal, endothelial, and hematopoietic tissues have provided new insights into the way members of the transforming growth factor-beta (TGF-beta) superfamily regulate cellular differentiation. TGF-betas regulate the fate of multipotential stem cells instructively (in the neural crest) by regulating the expression or function of tissue-specific transcription factors, as well as selectively (in the mesenchyme) by regulating the expression of required growth factors and their receptors. During skeletal development, TGF-betas have unique functions and act sequentially to modulate chondrocyte and osteoblast differentiation. Responsiveness to TGF-betas changes as cells differentiate and evidence now suggests that changes in TGF-beta receptor profile may account for some of these differences. Drosophila and transgenic mouse models are now providing useful insights into mechanisms of TGF-beta action in vivo.

The Xvent-2 homeobox gene is part of the BMP-4 signalling pathway controlling [correction of controling] dorsoventral patterning of Xenopus mesoderm

We describe a novel Xenopus homeobox gene, Xvent-2, which together with the previously identified homeobox gene Xvent-1, defines a novel class of homeobox genes. vent genes are related by sequence homology, expression pattern and gain-of-function phenotype. Evidence is presented for a role of Xvent-2 in the BMP-4 pathway involved in dorsoventral patterning of mesoderm. (1) Xvent-2 is expressed in regions that also express BMP-4. (2) Xvent-2 and BMP-4 interact in a positive feedback loop. (3) Xvent-2 ventralizes dorsal mesoderm in a dose-dependent manner resulting in phenoytpes ranging from microcephaly to Bauchstuck pieces, as does BMP-4. (4) Like BMP-4 and gsc, Xvent-2 and gsc are able to interact in a crossregulatory loop to suppress each other. (5) Microinjection of Xvent-2 mRNA can rescue dorsalization by a dominant-negative BMP-4 receptor. The results suggest that Xvent-2 functions in the BMP-4 signalling pathway that antagonizes the Spemann organizer.

Receptor-associated Mad homologues synergize as effectors of the TGF-beta response

Transforming growth factor-beta TGF-beta is the prototype for a family of extracellular proteins that affect cell proliferation and tissue differentiation. TGF-beta-related factors, including BMP-2/4, Dpp and activin, act through two types of serine/threonine kinase receptors which can form a heteromeric complex. However, the mechanism of signal transduction by these receptors is largely unknown. In Drosophila, Mad is required for signalling by Dpp. We have isolated complementary DNAs for four human Mad homologues, one of which, hMAD-4, is identical to DPC-4, a candidate tumour suppressor. hMAD-3 and -4 synergized to induce strong ligand-independent TGF-beta-like responses. When truncated at their carboxy termini, hMAD-3 and -4 act as dominant-negative inhibitors of the normal TGF-beta response. The activity of hMAD-3 and -4 was regulated by the TGF-beta receptors, and hMAD-3 but not hMAD-4 was phosphorylated and associated with the ligand-bound receptor complex. These results define hMAD-3 and -4 as effectors of the TGF-beta response and demonstrate a function for DPCA-4/hMAD-4 as a tumour suppressor.

Characterization of the interaction of FKBP12 with the transforming growth factor-beta type I receptor in vivo

The type I transforming growth factor-beta receptor (TbetaR-I) is the efferent component of the receptor complex, which presumably phosphorylates intracellular targets. FKBP12, a binding protein for FK506 and rapamycin, is shown to associate with the cytoplasmic region of TbetaR-I in vitro. In this report, we investigated the interaction of FKBP12 with TbetaR-I in vivo. FKBP12 interacts with TbetaR-I in mammalian cells as well as in yeast. Ligand addition does not affect the interaction, and both constitutively active and kinase-negative mutants of TbetaR-I bind FKBP12. FKBP12 dissociates from TbetaR-I in the presence of a high concentration of FK506. The juxtamembrane region of TbetaR-I, containing the major phosphorylation sites by the type II receptor, is required for the interaction. One of the deletion mutants in this region, which was shown to mediate transcriptional response, does not bind FKBP12, suggesting that FKBP12 is not directly involved in TGF-beta signaling. Furthermore TbetaR-I does not phosphorylate FKBP12 in vitro. FKBP12 may not be a direct substrate of TbetaR-I but possibly modulates the TbetaR-I function through its interaction with the regulatory domain of the kinase.

The Drosophila Serum Response Factor gene is required for the formation of intervein tissue of the wing and is allelic to blistered

The adult Drosophila wing is formed by an epithelial sheet, which differentiates into two non-neural tissues, vein or intervein. A large number of genes, many of them encoding components of an EGF-receptor signaling pathway, have previously been shown to be required for differentiation of vein tissue. Much less is known about the molecular control of intervein differentiation. Here we report that the Drosophila homolog of the mammalian Serum Response Factor gene (DSRF), which encodes a MADS-box containing transcriptional regulator, is expressed in the future intervein tissue of wing imaginal discs. In adult flies carrying only one functional copy of the DSRF gene, additional vein tissue develops in the wing, indicating that DSRF is required to spatially restrict the formation of veins. In mitotic clones lacking DSRF, intervein tissue fails to differentiate and becomes vein-like in appearance. Genetic and molecular evidence demonstrates that DSRF is encoded by the blistered locus, which produces ectopic veins and blistered wings when mutant. Our results show that DSRF plays a dual role during wing differentiation. It acts in a dosage-dependent [correction of dosage-dependant] manner to suppress the formation of wing veins and is required cell-autonomously to promote the development of intervein cells. We propose that DSRF acts at a key step between regulatory genes that define the early positional values in the developing wing disc and the subsequent localized expression of intervein-specific structural genes.

Bone morphogenetic proteins in development

The bone morphogenetic proteins (BMPs) constitute a large family of cytokines related to members of the transforming growth factor-beta superfamily. Recent evidence, in particular from gene targeting experiments in the mouse, indicates that BMPs are required for mesoderm formation and for the development and patterning of many different organ systems. Significant progress has also been made in understanding the role of BMPs in gastrulation and neurulation in Xenopus and in identifying genes regulating BMP expression and components of the downstream signaling pathways. Extracellular modifiers of BMP activity may constitute an opposing morphogenetic system.