Biochemical characterization of the Drosophila dpp protein, a member of the transforming growth factor beta family of growth factors

Regulated delivery controls Drosophila Hedgehog, Wingless, and Decapentaplegic signaling

Morphogen signaling proteins disperse across tissues to activate signal transduction in target cells. We investigated dispersion of Hedgehog (Hh), Wnt homolog Wingless (Wg), and Bone morphogenic protein homolog Decapentaplegic (Dpp) in the Drosophila wing imaginal disc. We discovered that delivery of Hh, Wg, and Dpp to their respective targets is regulated. We found that <5% of Hh and <25% of Wg are taken up by disc cells and activate signaling. The amount of morphogen that is taken up and initiates signaling did not change when the level of morphogen expression was varied between 50 and 200% (Hh) or 50 and 350% (Wg). Similar properties were observed for Dpp. We analyzed an area of 150 μm×150 μm that includes Hh-responding cells of the disc as well as overlying tracheal cells and myoblasts that are also activated by disc-produced Hh. We found that the extent of signaling in the disc was unaffected by the presence or absence of the tracheal and myoblast cells, suggesting that the mechanism that disperses Hh specifies its destinations to particular cells, and that target cells do not take up Hh from a common pool.

Drosophila ML-DmD17-c3 cells respond robustly to Dpp and exhibit complex transcriptional feedback on BMP signaling components

Background

BMP signaling is involved in myriad metazoan developmental processes, and study of this pathway in Drosophila has contributed greatly to our understanding of its molecular and genetic mechanisms. These studies have benefited not only from Drosophila’s advanced genetic tools, but from complimentary in vitro culture systems. However, the commonly-used S2 cell line is not intrinsically sensitive to the major BMP ligand Dpp and must therefore be augmented with exogenous pathway components for most experiments.

Results

Herein we identify and characterize the responses of Drosophila ML-DmD17-c3 cells, which are sensitive to Dpp stimulation and exhibit characteristic regulation of BMP target genes including Dad and brk. Dpp signaling in ML-DmD17-c3 cells is primarily mediated by the receptors Put and Tkv, with additional contributions from Wit and Sax. Furthermore, we report complex regulatory feedback on core pathway genes in this system.

Conclusions

Native ML-DmD17-c3 cells exhibit robust transcriptional responses to BMP pathway induction. We propose that ML-DmD17-c3 cells are well-suited for future BMP pathway analyses.

Electronic supplementary material

The online version of this article (10.1186/s12861-019-0181-0) contains supplementary material, which is available to authorized users.

The BMP2/4 ortholog Dpp can function as an inter-organ signal that regulates developmental timing

Increased local trapping of morphogens within tissues as they grow would reduce circulating levels and can therefore provide a systemic readout of the status of their growth and maturation.

Developmental transitions are often triggered by a neuroendocrine axis and can be contingent upon multiple organs achieving sufficient growth and maturation. How the neurodendocrine axis senses the size and maturity of peripheral organs is not known. In Drosophila larvae, metamorphosis is triggered by a sharp increase in the level of the steroid hormone ecdysone, secreted by the prothoracic gland (PG). Here, we show that the BMP2/4 ortholog Dpp can function as a systemic signal to regulate developmental timing. Dpp from peripheral tissues, mostly imaginal discs, can reach the PG and inhibit ecdysone biosynthesis. As the discs grow, reduced Dpp signaling in the PG is observed, consistent with the possibility that Dpp functions in a checkpoint mechanism that prevents metamorphosis when growth is insufficient. Indeed, upon starvation early in the third larval instar, reducing Dpp signaling in the PG abrogates the critical-weight checkpoint which normally prevents pupariation under these conditions. We suggest that increased local trapping of morphogen within tissues as they grow would reduce circulating levels and hence provide a systemic readout of their growth status.

Anchor negatively regulates BMP signalling to control Drosophila wing development

G protein-coupled receptors play particularly important roles in many organisms. The novel Drosophila gene anchor is an orthologue of vertebrate GPR155. However, the roles of anchor in molecular functions and biological processes, especially in wing development, remain unknown. Knockdown of anchor resulted in an increased wing size and additional and thickened veins. These abnormal wing phenotypes were similar to those observed in BMP signalling gain-of-function experiments. We observed that the BMP signalling indicator p-Mad was significantly increased in wing discs in which anchor RNAi was induced in larvae and accumulated abnormally in intervein regions in pupae. Furthermore, the expression of target genes of the BMP signalling pathway was examined using a lacZ reporter, and the results indicated that omb and sal were substantially increased in anchor-knockdown wing discs. An investigation of genetic interactions between Anchor and the BMP signalling pathway revealed that the thickened and ectopic vein tissues were rescued by knocking down BMP levels. These results suggested that Anchor functions to negatively regulate BMP signalling during wing development and vein formation.

Cloning and characterization of microbial activated Aedes aegypti MEK4 (AaMEK4): influences of noncatalytic domains on enzymatic activity

Protein kinases are known to be involved in a number of signal transduction cascades. Both the stress-activated Jun N-terminal kinase (JNK) and mitogen-activated protein kinase (MAPK) p38 pathways have been shown to correlate with the insect immune response to microbial infection. MAP kinase kinase 4 (MEK4) is an upstream kinase of JNK and p38 kinase. The cDNA of AaMEK4 was cloned and characterized. AaMEK4 was activated by microbial lysates of Gram-positive, Gram-negative bacteria and yeast. The conserved lysine (K112 ) and the putative phosphorylation sites (S238 and T242 ) were shown to be important for kinase activity by site-directed mutagenesis. A common MAPK docking site (MAPK_dsA) was found and in addition, a new nearby docking site, MAPK_dsB, was identified in the N-terminal noncatalytic domain of AaMEK4. MAPK_dsB was shown to be a unique element in the MEK4 family. In this study, both MAPK_dsA and _dsB were demonstrated to be important to AaMEK4 enzymatic activity for the downstream protein kinase, Aap38.

Strategies for exploring TGF-β signaling in Drosophila

The TGF-β pathway is an evolutionarily conserved signal transduction module that mediates diverse biological processes in animals. In Drosophila, both the BMP and Activin branches are required for viability. Studies rooted in classical and molecular genetic approaches continue to uncover new developmental roles for TGF-β signaling. We present an overview of the secreted ligands, transmembrane receptors and cellular Smad transducer proteins that compose the core pathway in Drosophila. An assortment of tools have been developed to conduct tissue-specific loss- and gain-of-function experiments for these pathway components. We discuss the deployment of these reagents, with an emphasis on appropriate usage and limitations of the available tools. Throughout, we note reagents that are in need of further improvement or development, and signaling features requiring further study. A general theme is that comparison of phenotypes for ligands, receptors, and Smads can be used to map tissue interactions, and to separate canonical and non-canonical signaling activities. Core TGF-β signaling components are subject to multiple layers of regulation, and are coupled to context-specific inputs and outputs. In addition to fleshing out how TGF-β signaling serves the fruit fly, we anticipate that future studies will uncover new regulatory nodes and modes and will continue to advance paradigms for how TGF-β signaling regulates general developmental processes.

Morphogen transport

The graded distribution of morphogens underlies many of the tissue patterns that form during development. How morphogens disperse from a localized source and how gradients in the target tissue form has been under debate for decades. Recent imaging studies and biophysical measurements have provided evidence for various morphogen transport models ranging from passive mechanisms, such as free or hindered extracellular diffusion, to cell-based dispersal by transcytosis or cytonemes. Here, we analyze these transport models using the morphogens Nodal, fibroblast growth factor and Decapentaplegic as case studies. We propose that most of the available data support the idea that morphogen gradients form by diffusion that is hindered by tortuosity and binding to extracellular molecules.

The initiation of Hox gene expression in Xenopus laevis is controlled by Brachyury and BMP-4

Hox genes encode a family of transcription factors that specify positional identities along the anterior-posterior (AP) axis during the development of vertebrate embryos. The earliest Hox expression in vertebrates is during gastrulation, at a position distant from the organiser or its equivalent. However, the mechanism that initiates this early expression is still not clear. Guided by the expression pattern, we identified upstream regulators in Xenopus laevis. The mesodermal transcription factor brachyury (Xbra) controls the early Hox expression domain in the animal-vegetal direction and the secreted growth factor BMP-4 limits it in the organiser/non-organiser direction. The overlap of these two signals, indicated by a Cartesian coordinate system, defines the initial Hox expression domain. We postulate that this system is a general mechanism for the activation of all Hox genes expressed during gastrulation.

Early patterning of the spider embryo: a cluster of mesenchymal cells at the cumulus produces Dpp signals received by germ disc epithelial cells

In early embryogenesis of spiders, the cumulus is characteristically observed as a cellular thickening that arises from the center of the germ disc and moves centrifugally. This cumulus movement breaks the radial symmetry of the germ disc morphology, correlating with the development of the dorsal region of the embryo. Classical experiments on spider embryos have shown that a cumulus has the capacity to induce a secondary axis when transplanted ectopically. In this study, we have examined the house spider, Achaearanea tepidariorum, on the basis of knowledge from Drosophila to characterize the cumulus at the cellular and molecular level. In the cumulus, a cluster of about 10 mesenchymal cells, designated the cumulus mesenchymal (CM) cells, is situated beneath the epithelium, where the CM cells migrate to the rim of the germ disc. Germ disc epithelial cells near the migrating CM cells extend cytoneme-like projections from their basal side onto the surface of the CM cells. Molecular cloning and whole-mount in situ hybridization showed that the CM cells expressed a spider homolog of Drosophila decapentaplegic (dpp), which encodes a secreted protein that functions as a dorsal morphogen in the Drosophila embryo. Furthermore, the spider Dpp signal appeared to induce graded levels of the phosphorylated Mothers against dpp (Mad) protein in the nuclei of germ disc epithelial cells. Adding data from spider homologs of fork head, orthodenticle and caudal, we suggest that, in contrast to the Drosophila embryo, the progressive mesenchymal-epithelial cell interactions involving the Dpp-Mad signaling cascade generate dorsoventral polarity in accordance with the anteroposterior axis formation in the spider embryo. Our findings support the idea that the cumulus plays a central role in the axial pattern formation of the spider embryo.

Lumenal transmission of decapentaplegic in Drosophila imaginal discs

Drosophila imaginal discs are sac-like appendage primordia comprising apposed peripodial and columnar cell layers. Cell survival in disc columnar epithelia requires the secreted signal Decapentaplegic (DPP), which also acts as a gradient morphogen during pattern formation. The distribution mechanism by which secreted DPP mediates global cell survival and graded patterning is poorly understood. Here we report detection of DPP in the lumenal cavity between apposed peripodial and columnar cell layers of both wing and eye discs. We show that peripodial cell survival hinges upon DPP signal reception and implicate DPP-dependent viability of the peripodial epithelium in growth of the entire disc. These results are consistent with lumenal transmission of the DPP survival signal during imaginal disc development.

A Delta-Notch signaling border regulated by Engrailed/Invected repression specifies boundary cells in the Drosophila hindgut

The Drosophila hindgut develops three morphologically distinct regions along its anteroposterior axis: small intestine, large intestine and rectum. Single-cell rings of 'boundary cells' delimit the large intestine from the small intestine at the anterior, and the rectum at the posterior. The large intestine also forms distinct dorsal and ventral regions; these are separated by two single-cell rows of boundary cells. Boundary cells are distinguished by their elongated morphology, high level of both apical and cytoplasmic Crb protein, and gene expression program. During embryogenesis, the boundary cell rows arise at the juxtaposition of a domain of Engrailed (En)- plus Invected (Inv)-expressing cells with a domain of Delta (Dl)-expressing cells. Analysis of loss-of-function and ectopic expression phenotypes shows that the domain of Dl-expressing cells is defined by En/Inv repression. Further, Notch pathway signaling, specifically the juxtaposition of Dl-expressing and Dl-non-expressing cells, is required to specify the rows of boundary cells. This Notch-induced cell specification is distinguished by the fact that it does not appear to utilize the ligand Serrate and the modulator Fringe.

Gradient formation of the TGF-beta homolog Dpp

Secreted morphogens such as the Drosophila TGF-beta homolog Decapentaplegic (Dpp) are thought to spread through target tissues and form long-range concentration gradients providing positional information. Using a GFP-Dpp fusion, we monitored a TGF-beta family member trafficking in situ throughout the target tissue and forming a long-range concentration gradient. Evidence is presented that long-range Dpp movement involves Dpp receptor and Dynamin functions. We also show that the rates of endocytic trafficking and degradation determine Dpp signaling range. We propose a model where the gradient is formed via intracellular trafficking initiated by receptor-mediated endocytosis of the ligand in receiving cells with the gradient slope controlled by endocytic sorting of Dpp toward recycling versus degradation.

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.

Regulation of body length and male tail ray pattern formation of Caenorhabditis elegans by a member of TGF-beta family

We have identified a new member of the TGF-beta superfamily, CET-1, from Caenorhabditis elegans, which is expressed in the ventral nerve cord and other neurons. cet-1 null mutants have shortened bodies and male tail abnormal phenotype resembling sma mutants, suggesting cet-1, sma-2, sma-3 and sma-4 share a common pathway. Overexpression experiments demonstrated that cet-1 function requires wild-type sma genes. Interestingly, CET-1 appears to affect body length in a dose-dependent manner. Heterozygotes for cet-1 displayed body lengths ranging between null mutant and wild type, and overexpression of CET-1 in wild-type worms elongated body length close to lon mutants. In male sensory ray patterning, lack of cet-1 function results in ray fusions. Epistasis analysis revealed that mab-21 lies downstream and is negatively regulated by the cet-1/sma pathway in the male tail. Our results show that cet-1 controls diverse biological processes during C. elegans development probably through different target genes.

Regulation of bone morphogenetic protein activity by pro domains and proprotein convertases

Bone morphogenetic proteins (BMPs) are derived from inactive precursor proteins by endoproteolytic cleavage. Here we show that processing of Nodal and Myc-tagged BMP4 is significantly enhanced by SPC1/Furin or SPC4/PACE4, providing direct evidence that regulation of BMP signaling is likely to be controlled by subtilisin-like proprotein convertase (SPC) activities. Nodal processing is dramatically enhanced if two residues adjacent to the precursor cleavage site are substituted with amino acids found at the equivalent positions of Activin, demonstrating that structural constraints at the precursor cleavage site limit the processing efficiency. However, in transfection assays, mature Nodal is undetectable either in culture supernatants or in cell lysates, despite efficient cleavage of the precursor protein, suggesting that mature Nodal is highly unstable. Domain swap experiments support this conclusion since mature BMP4 or Dorsalin are also destabilized when expressed in conjunction with the Nodal pro domain. By contrast, mature Nodal is stabilized by the Dorsalin pro domain, which mediates the formation of stable complexes. Collectively, these data show that the half-life of mature BMPs is greatly influenced by the identity of their pro regions.

Biochemical and biophysical characterization of refolded Drosophila DPP, a homolog of bone morphogenetic proteins 2 and 4

The mature C-terminal signaling domain of the Drosophila Decapentaplegic proprotein (DPP) can be efficiently refolded from chaotrope-solubilized inclusion bodies with the aid of a membrane protein-solubilizing detergent, high concentrations (0.75-2 M) of NaCl, and low temperatures (5-15 degreesC). The disulfide-linked homodimeric product contains N-terminal heparin-binding sites that were utilized as intrinsic affinity tags to obtain a highly enriched preparation in one chromatographic step. A subsequent C4 reverse phase high pressure liquid chromatography step provides high purity, salt-free protein that is amenable to biophysical and structural studies at a yield of approximately 3 mg/liter of bacterial culture. The dimeric protein is correctly folded as determined by electrophoretic, spectroscopic, chemical, and proteolytic analyses. Refolded DPP is also bioactive as shown by induction of chondrogenesis in embryonic chick limb bud cells and by high affinity binding to Noggin, an antagonist of bone morphogenetic protein signaling. In contrast to bone morphogenetic proteins extracted from demineralized bone or overexpressed in cell culture, the refolded Escherichia coli-expressed protein is not glycosylated at a conserved N-linked site and is therefore homogeneous. The C-terminal domain dimer is more hydrophobic and thus less soluble than its unfolded or partially folded forms, necessitating highly solubilizing conditions for recovery after folding in vitro. Hence solubilization of the mature ligand may be one of the principal roles of the large (250-400 amino acids) N-terminal prodomains of transforming growth factor-beta superfamily members, shown to act as intramolecular chaperones in vivo.

Ventral mesoderm induction and patterning by bone morphogenetic protein heterodimers in Xenopus embryos

Bone morphogenetic proteins (BMPs) perform diverse functions in vertebrate development. Here we demonstrate that the heterodimeric BMP-4/7 protein directly induces ventral mesoderm and blood in Xenopus animal caps, and BMP-2/7 heterodimers may function similarly. We also provide indirect evidence that BMP heterodimers function in embryos, using assays with dominant-negative BMP ligands. Homodimeric BMP-2 and BMP-4 proteins do not induce mesoderm, but they ventralize mesoderm induction by activin. In contrast, BMP-7 protein interferes with mesoderm induction by activin, but BMP-7 stimulates ventral mesoderm induction by the heterodimer, BMP-4/7. This novel property of BMP-7 distinguishes it from other BMPs. BMP-7 may therefore function in early embryogenesis to antagonize activin signals and potentiate BMP signals. We propose that BMP heterodimers convey signals for ventral mesoderm induction and patterning in Xenopus development.

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.

PLAB, a novel placental bone morphogenetic protein

Bone morphogenetic proteins (BMP) constitute a sub-group of the large transforming growth factor-beta (TGF-beta) family. They play important roles in the embryonic development of multiple structures and in adult bone modeling. We have recently isolated a novel member of the BMP family from placenta, termed PLAB. PLAB is expressed highly in placenta, but can be found upon stringent analysis in low levels in most other tissues. At the amino acid level, PLAB is most closely related to BMP-8/OP-2, another member of the BMP family. Like TGF-beta, PLAB inhibits the proliferation of primitive hematopoietic progenitors. The high expression of PLAB by placenta raises the possibility that it may be a mediator of placental control of embryonic development.

Limb morphogenesis: connections between patterning and growth

Limb development is a complex process involving precise control of both patterning and growth. Great strides have been made in understanding limb morphogenesis and identifying essential patterning genes in Drosophila. Differential expression of these genes divides the future limb into territories, which will give rise to different regions of the adult appendage. Recent analyses have defined the role of territorial boundaries as organizers of both patterning and growth, highlighting the connection between these two processes. The organizing activity of territorial boundaries seems to be mediated through the activity of two locally produced morphogens: Wingless and Decapentaplegic. We propose a model in which these two molecules, distributed in a graded fashion, act in synergy to promote growth of the entire appendage. We also suggest that existence of growth inhibitors that counteract the action of Wingless and Decapentaplegic; by opposing the gradient of these growth factors, the inhibitors guide the near-uniform proliferation that shapes the imaginal discs from which the adult appendages are formed in Drosophila.

Drosophila brachyenteron regulates gene activity and morphogenesis in the gut

Chromosomal region 68D/E is required for various aspects of Drosophila gut development; within this region maps the Brachyury homolog T-related gene (Trg), DNA of which rescues the hindgut defects of deficiency 68D/E. From a screen of 13,000 mutagenized chromosomes we identified six non-complementing alleles that are lethal over deficiencies of 68D/E and show a hindgut phenotype. These mutations constitute an allelic series and are all rescued to viability by a Trg transgene. We have named the mutant alleles and the genetic locus they define brachyenteron (byn); phenotypic characterization of the strongest alleles allows determination of the role of byn in embryogenesis. byn expression is activated by tailless, but byn does not regulate itself. byn expression in the hindgut and anal pad primordia is required for the regulation of genes encoding transcription factors (even-skipped, engrailed, caudal, AbdominalB and orthopedia) and cell signaling molecules (wingless and decapentaplegic). In byn mutant embryos, the defective program of gene activity in these primordia is followed by apoptosis (initiated by reaper expression and completed by macrophage engulfment), resulting in severely reduced hindgut and anal pads. Although byn is not expressed in the midgut or the Malpighian tubules, it is required for the formation of midgut constrictions and for the elongation of the Malpighian tubules.

smoothened encodes a receptor-like serpentine protein required for hedgehog signalling

Members of the Hedgehog family of secreted proteins control a number of important inductive interactions in the development of both vertebrates and Drosophila, but little is known about the ways in which their signalling activities are transduced. In Drosophila, hedgehog is one of the segment-polarity genes, mutations of which disrupt the pattern and polarity of individual embryonic segments and their adult derivatives; several of these genes have been implicated in transduction of the hedgehog signal. Here we show that the segment-polarity gene smoothened is required for the response of cells to hedgehog signalling during the development of both the embryonic segments and imaginal discs. Sequence analysis of the smoothened transcription unit reveals a single open reading frame encoding a protein with seven putative transmembrane domains. This structure is typical of G-protein-coupled receptors, suggesting that the Smoothened protein may act as a receptor for the Hedgehog ligand.

Purification, characterization, and cDNA cloning of a novel growth factor from the conditioned medium of NIH-Sape-4, an embryonic cell line of Sarcophaga peregrina (flesh fly)

A growth factor from the conditioned medium of NIH-Sape-4, an embryonic cell line of the flesh fly, was purified to homogeneity. This growth factor, termed IDGF, stimulated the proliferation of NIH-Sape-4 cells in an autocrine manner; it was a homodimer of a protein with a molecular mass of 52 kDa, and its specific activity was comparable with those of mammalian growth factors. Immunoblotting experiments revealed that unfertilized mature eggs of the flesh fly contained this growth factor, a certain level of which was maintained throughout embryonic development. Analysis of cDNA for this growth factor showed that this factor is a novel protein consisting of 553 amino acid residues. No significant sequence similarity was found between this factor and other proteins except atrial gland granule-specific antigen of Aplysia californica.

Identification and characterization of autosomal genes that interact with glass in the developing Drosophila eye

The glass gene encodes a zinc finger, DNA-binding protein that is required for photoreceptor cell development in Drosophila melanogaster. In the developing compound eye, glass function is regulated at two points: (1) the protein is expressed in all cells' nuclei posterior to the morphogenetic furrow and (2) the ability of the Glass protein to regulate downstream genes is largely limited to the developing photoreceptor cells. We conducted a series of genetic screens for autosomal dominant second-site modifiers of the weal allele glass3, to discover genes with products that may regulate glass function at either of these levels. Seventy-six dominant enhancer mutations were recovered (and no dominant suppressors). Most of these dominant mutations are in essential genes and are associated with recessive lethality. We have assigned these mutations to 23 complementation groups that include multiple alleles of Star and hedgehog as well as single alleles of Delta, roughened eye, glass and hairy. Mutations in 18 of the complementation groups are embryonic lethals, and of these, 13 show abnormal adult retinal phenotypes in homozygous clones, usually with altered numbers of photoreceptor cells in some of the ommatidia.

Sequential organizing activities of engrailed, hedgehog and decapentaplegic in the Drosophila wing

The Drosophila wing is formed by two cell populations, the anterior and posterior compartments, which are distinguished by the activity of the selector gene engrailed (en) in posterior cells. Here, we show that en governs growth and patterning in both compartments by controlling the expression of the secreted proteins hedgehog (hh) and decapentaplegic (dpp) as well as the response of cells to these signaling molecules. First, we demonstrate that en activity programs wing cells to express hh whereas the absence of en activity programs them to respond to hh by expressing dpp. As a consequence, posterior cells secrete hh and induce a stripe of neighboring anterior cells across the compartment boundary to secrete dpp. Second, we demonstrate that dpp can exert a long-range organizing influence on surrounding wing tissue, specifying anterior or posterior pattern depending on the compartmental provenance, and hence the state of en activity, of the responding cells. Thus, dpp secreted by anterior cells along the compartment boundary has the capacity to organize the development of both compartments. Finally, we report evidence suggesting that dpp may exert its organizing influence by acting as a gradient morphogen in contrast to hh which appears to act principally as a short range inducer of dpp.

Identification of two Drosophila TGF-beta family members in the grasshopper Schistocerca americana

Intercellular signaling molecules of the transforming growth factor-beta (TGF-beta) superfamily are required for pattern formation in many multicellular organisms. The decapentaplegic (dpp) gene of Drosophila melanogaster has several developmental roles. To improve our understanding of the evolutionary diversification of this large family we identified dpp in the grasshopper Schistocerca americana. S. americana diverged from D. melanogaster approximately 350 million years ago, utilizes a distinct developmental program, and has a 60-fold-larger genome than D. melanogaster. Our analyses indicate a single dpp locus in D. melanogaster and S. americana, suggesting that dpp copy number does not correlate with increasing genome size. Another TGF-beta superfamily member, the D. melanogaster gene 60A, is also present in only one copy in each species. Comparison of homologous sequences from D. melanogaster, S. americana, and H. sapiens, representing roughly 900 million years of evolutionary distance, reveals significant constraint on sequence divergence for both dpp and 60A. In the signaling portion of the dpp protein, the amino acid identity between these species exceeds 74%. Our results for the TGF-beta superfamily are consistent with current hypotheses describing gene duplication and diversification as a frequent response to high levels of selective pressure on individual family members.

The Dfur2 gene of Drosophila melanogaster: genetic organization, expression during embryogenesis, and pro-protein processing activity of its translational product Dfurin2

The gene structure and expression of the Dfur2 gene of Drosophila melanogaster, which encodes the subtilisin-like serine endoprotease Dfurin2, was studied. The Dfur2 gene is very compact in contrast to the related Dfur1 gene, which has an estimated size of over 100 kbp. The 6-kb Dfur2 mRNA is encoded by 16 exons dispersed over a genomic region of about 9 kbp. The exon/intron organization shows conservation of intron positions not only in comparison with Dfur1, but also with the related mammalian genes FUR, PC1/PC3, PC2, and PC4. This conservation supports the hypothesis that all genes belonging to the family of subtilisin-like pro-protein processing enzymes are evolutionary related by descent from a common ancestral gene. In primer extension experiments, Dfur2 transcription initiation sites were identified in the presumed Dfur2 promoter region. This region was found to contain general RNA polymerase II promoter elements like a potential TATA box, a potential CAP signal, and several potential CCAAT boxes. Also, several sequence motifs putatively corresponding to binding sites for Drosophila transcription factors like zeste, bicoid, and engrailed were found to be present. RNA in situ hybridization experiments on Drosophila embryos revealed presumably maternal Dfur2 expression until the syncytial blastoderm (stage 5 of embryogenesis), no expression during gastrulation (stage 9), transient expression in a subset of neurons in the central nervous system of stage 12-13 embryos, and, from stage 13 onwards, expression in the developing tracheal tree. In a vaccinia expression system, the endoprotease Dfurin2 not only cleaved wild-type precursor of von Willebrand factor (pro-vWF) with pro-region cleavage site R-S-K-R decreases, but also, although to a lesser extent, pro-vWF mutants in which the P2 (vWFK-2A) or P4 (vWFR-4A) basic residue with respect to the pro-region cleavage site had been mutated. This cleavage specificity resembles that of human furin. The cleavage of pro-vWF by Dfurin2 shows that the previously reported lack of cleavage of the precursor of the beta A-chain of activin-A by Dfurin2 in this vaccinia expression system is substrate determined.

Chondrocyte differentiation

Data obtained while investigating growth plate chondrocyte differentiation during endochondral bone formation both in vivo and in vitro indicate that initial chondrogenesis depends on positional signaling mediated by selected homeobox-containing genes and soluble mediators. Continuation of the process strongly relies on interactions of the differentiating cells with the microenvironment, that is, other cells and extracellular matrix. Production of and response to different hormones and growth factors are observed at all times and autocrine and paracrine cell stimulations are key elements of the process. Particularly relevant is the role of the TGF-beta superfamily, and more specifically of the BMP subfamily. Other factors include retinoids, FGFs, GH, and IGFs, and perhaps transferrin. The influence of local microenvironment might also offer an acceptable settlement to the debate about whether hypertrophic chondrocytes convert to bone cells and live, or remain chondrocytes and die. We suggest that the ultimate fate of hypertrophic chondrocytes may be different at different microanatomical sites.

Regulation of a decapentaplegic midgut enhancer by homeotic proteins

The clustered homeotic genes encode transcription factors that regulate pattern formation in all animals, conferring cell fates by coordinating the activities of downstream ‘target’ genes. In the Drosophila midgut, the Ultrabithorax (Ubx) protein activates and the abdominalA (abd-A) protein represses transcription of the decapentaplegic (dpp) gene, which encodes a secreted signalling protein of the TGF beta class. We have identified an 813 bp dpp enhancer which is capable of driving expression of a lacZ gene in a correct pattern in the embryonic midgut. The enhancer is activated ectopically in the visceral mesoderm by ubiquitous expression of Ubx or Antennapedia but not by Sex combs reduced protein. Ectopic expression of abd-A represses the enhancer. Deletion analysis reveals regions required for repression and activation. A 419 bp subfragment of the 813 bp fragment also drives reporter gene expression in an appropriate pattern, albeit more weakly. Evolutionary sequence conservation suggests other factors work with homeotic proteins to regulate dpp. A candidate cofactor, the extradenticle protein, binds to the dpp enhancer in close proximity to homeotic protein binding sites. Mutation of either this site or another conserved motif compromises enhancer function. A 45 bp fragment of DNA from within the enhancer correctly responds to both UBX and ABD-A in a largely tissue-specific manner, thus representing the smallest in vivo homeotic response element (HOMRE) identified to date.

Role of the teashirt gene in Drosophila midgut morphogenesis: secreted proteins mediate the action of homeotic genes

Homeotic genes control the development of embryonic structure by coordinating the activities of downstream ‘target’ genes. The identities and functions of target genes must be understood in order to learn how homeotic genes control morphogenesis. Drosophila midgut development is regulated by homeotic genes expressed in the visceral mesoderm, where two of their target genes have been identified. Both encode secreted proteins. The Ultrabithorax (Ubx) homeotic gene activates transcription of the decapentaplegic (dpp) gene, which encodes a TGF beta class protein, while in adjacent mesoderm cells the abdominal-A (abd-A) homeotic gene activates transcription of the wingless (wg) gene, which encodes a Wnt class protein. The homeotic genes Antennapedia (Antp) and Sex combs reduced (Scr) act in more anterior midgut regions. Here we report the identification of another homeotic gene target in the midgut mesoderm, the teashirt (tsh) gene, which encodes a protein with zinc finger motifs. tsh is necessary for proper formation of anterior and central midgut structures. Antp activates tsh in anterior midgut mesoderm. In the central midgut mesoderm Ubx, abd-A, dpp, and wg are required for proper tsh expression. The control of tsh by Ubx and abd-A, and probably also by Antp, is mediated by secreted signaling molecules. By responding to signals as well as localized transcription regulators, the tsh transcription factor is produced in a spatial pattern distinct from any of the homeotic genes.

TGF-beta related genes in development

Embryonic induction is the process by which signals from one cell population change the developmental fate of another. Polypeptides related to growth factors are one group of molecules mediating many inductive events. Recent data on the embryonic expression and function of signaling proteins related to transforming growth factor beta, in both vertebrate and invertebrate systems, have shown that these molecules play important roles in both pattern formation and tissue specification during embryogenesis.

Ectopic decapentaplegic in the Drosophila midgut alters the expression of five homeotic genes, dpp, and wingless, causing specific morphological defects

The patterns of homeotic gene expression in the Drosophila midgut visceral mesoderm are instrumental in several morphogenetic events, including the formation of the gastric caeca and the positioning of the three midgut constrictions. We demonstrate that a potent regulator of homeotic gene expression in the visceral mesoderm is the secreted growth factor-like molecule encoded by the decapentaplegic (dpp) gene. Ectopic dpp in the visceral mesoderm caused changes in the gene expression of Sex combs reduced, Antennapedia, Ultrabithorax (Ubx), and abdominal-A (abd-A) and disrupted the formation of the gastric caeca and the first and third midgut constrictions. Ectopic dpp also induced expression of teashirt, wingless (wg) and the endogenous dpp gene in the visceral mesoderm and enhanced labial expression in the adjacent endoderm. The patterns of gene expression and the formation of the second midgut constriction in the presence of ectopic dpp are most consistent with a dpp-induced transformation of virtually the entire midgut to cell fates normally seen only in the parasegment (ps)7 and ps8 regions of the midgut. We conclude that dpp is a primary signal in maintaining Ubx expression in the visceral mesoderm in a pattern different from Ubx expression in the embryonic ectoderm and in providing a cell-cell communication mechanism by which Ubx expression influences gene expression across germlayers and across the ps7 to ps8 parasegment boundary in the visceral mesoderm.

Identification of two bone morphogenetic protein type I receptors in Drosophila and evidence that Brk25D is a decapentaplegic receptor

Drosophila sequences at chromosomal positions 25D (Brk25D) and 43E (Brk43E) are similar to the TGF beta type I receptor serine/threonine kinases and are expressed broadly during embryogenesis. Brk25D binds dpp protein and bone morphogenetic protein 2 with high affinity. Mutations affecting Brk25D map to the gene thick veins and block the expression of two decapentaplegic-responsive (dpp-responsive) genes, dpp and labial, in the embryonic midgut. Defects in Brk25D receptor function combined with reduced expression of dpp ligand produce mutant phenotypes in the embryo and adult. Brk43E is the product of the gene saxophone, which also interacts with dpp. We conclude that dpp signaling in vivo is mediated by at least two receptors, Brk25D and Brk43E.

Interactions of decapentaplegic, wingless, and Distal-less in the Drosophila leg

The genes decapentaplegic, wingless, and Distalless appear to be instrumental in constructing the anatomy of the adult Drosophila leg. In order to investigate how these genes function and whether they act coordinately, we analyzed the leg phenotypes of the single mutants and their inter se double mutant compounds. In decapentaplegic the tarsi frequently exhibit dorsal deficiencies which suggest that the focus of gene action may reside dorsally rather than distally. In wingless the tarsal hinges are typically duplicated along with other dorsal structures, confirming that the hinges arise dorsally. The plane of symmetry in double-ventral duplications caused by decapentaplegic is virtually the same as the plane in double-dorsal duplications caused by wingless. It divides the fate map into two parts, each bisected by the dorsoventral axis. In the double mutant decapentaplegic wingless the most ventral and dorsal tarsal structures are missing, consistent with the notion that both gene products function as morphogens. In wingless Distal-less compounds the legs are severely truncated, indicating an important interaction between these genes. Distal-less and decapentaplegic manifest a relatively mild synergism when combined.

Compartment boundaries and the control of Drosophila limb pattern by hedgehog protein

Drosophila limbs are subdivided into anterior and posterior compartments which derive from adjacent cell populations founded early in development. Evidence is now provided that posterior cells organize growth and cell patterning in both compartments by secreting hedgehog protein and that hedgehog protein acts indirectly by inducing neighbouring anterior cells to secrete decapentaplegic or wingless protein.

Embryonic expression patterns of the Drosophila decapentaplegic gene: separate regulatory elements control blastoderm expression and lateral ectodermal expression

Patterns of decapentaplegic (dpp) transcripts derived from the intact gene were compared to the patterns of transcripts generated by partial dpp transgenes in Drosophila embryos. Sequences closest to the dpp coding regions, the dpp hin region, were sufficient to express lacZ-tagged mRNA in patterns indistinguishable from the patterns of endogenous dpp expression in the dorsal and terminal cells at the blastoderm stage, in the dorsal ectoderm during germ band elongation, and in narrow stripes of ectodermal cells along the dorsal edge of the ectoderm and at the boundary between the lateral and ventral neurogenic regions during germ band shortening. The latter pattern of expression responded to the segment polarity genes naked and wingless. However, these dpp sequences were not sufficient to drive lacZ-tagged mRNA expression in other cells normally expressing dpp, including cells in the gnathal segments, the clypeolabrum, the foregut, the midgut visceral mesoderm, and the hindgut. Two separate regulatory regions were found in the dpp hin region. A 479 bp region upstream of the promoter was necessary for the segmented pattern of expression in the lateral ectoderm and for expression in the midgut endoderm. Cis-acting elements in the 2 kbp second intron directed expression in the dorsal and terminal regions of the blastoderm, acted on a heterologous promoter, the P-element promoter, and responded to pattern information derived from the maternal effect dorsal/ventral patterning genes.

The daf-4 gene encodes a bone morphogenetic protein receptor controlling C. elegans dauer larva development

The bone morphogenetic protein (BMP) family is a conserved group of signalling molecules within the transforming growth factor-beta (TGF-beta) superfamily. This group, including the Drosophila decapentaplegic (dpp) protein and the mammalian BMPs, mediates cellular interactions and tissue differentiation during development. Here we show that a homologue of human BMPs controls a developmental switch in the life cycle of the free-living soil nematode Caenorhabditis elegans. Starvation and overcrowding induce C. elegans to form a developmentally arrested, third-stage dauer larva. The daf-4 gene, which acts to inhibit dauer larva formation and promote growth, encodes a receptor protein kinase similar to the daf-1, activin and TGF-beta receptor serine/threonine kinases. When expressed in monkey COS cells, the daf-4 receptor binds human BMP-2 and BMP-4. The daf-4 receptor is the first to be identified for any growth factor in the BMP family.

Cloning of a novel TGF-beta related cytokine, the vgr, from rat brain: cloning of and comparison to homologous human cytokines

Here we describe cloning of a TGF-beta related cytokine from a rat brain cDNA library. This novel cytokine, the vgr (vegetal related), is homologous to the vegetal (Vg1) gene of Xenopus (DL Weeks and DA Melton, Cell, 51:861-867, 1987). In rat brain mRNA a single 3.5 kb RNA could be detected by Northern blot analysis. Thus, this new cytokine is constitutively expressed in the central nervous system. A monoclonal antibody reactive with a synthetic peptide of vgr revealed a faint vgr-like immunoreactivity throughout the CNS, with more pronounced staining of hippocampal neurons, ependymal cells, cells of the choroid plexus, and hypophysis. Using the rat cDNA, two homologous human cytokine cDNAs encoding the human vgr and op-1 were cloned.

Genes for bone morphogenetic proteins are differentially transcribed in early amphibian embryos

We have previously demonstrated that activin, a member of the TGF-beta family, has a potent mesoderm-inducing activity in Xenopus embryos. In the course of screening for activin-related genes from Xenopus, we have cloned cDNAs for Xenopus homologue of BMP-2, -4 and -7. Northern blot analysis revealed that these BMP genes are maternally encoded and differentially regulated after fertilization. Alkaline phosphatase-inducing assay using the recombinant BMP proteins has shown that at least BMP-2 and -4 have similar activity to mammalian counterparts.

TGF-? family factors inDrosophila morphogenesis

Many Drosophila genes have now been identified with substantial sequence similarity to vertebrate protooncogenes and growth factors. Some of these have been isolated directly by cross-hybridization with vertebrate probes and some have been recognized in the sequences of genes cloned because of their intiguing mutant phenotypes. An example of a gene isolated for its interesting development functions but with homology to a vertebrate growth factor is the Drosophila decapentaplegic gene (dpp). An example of a Drosophila gene isolated by virtue of its sequence conservation is the vgr/60A gene. Both dpp and vgr/60A are members of the transforming growth factor-beta family and are most similar to the human bone morphogenetic proteins. The regulation of the dpp gene by several different groups of pattern formation genes including the dorsal/ventral group, the terminal group, the segment polarity genes, and the homeotic genes indicates that many events in embryogenesis require the cell to cell communication mediated by the secreted dpp protein. The temporal and spatial pattern of vgr/60A expression differs from that of dpp indicating that it may be regulated by different pattern information genes. The experimental advantages of the Drosophila system should permit a better understanding of the importance of growth factor homologs in specific developmental events, aid in establishing the functional interactions between these regulatory molecules, and identify new genes that are important for the biological functions of growth factors. It is likely that some of the newly identified genes will have vertebrate homologs and the analysis of these may be helpful in studies on vertebrate development and tumor biology.

Sequence, biochemical characterization, and developmental expression of a new member of the TGF-beta superfamily in Drosophila melanogaster

More than 20 members of the transforming growth factor-beta (TGF-beta) superfamily of growth and differentiation factors have been implicated in development. One member of the TGF-beta family has been previously reported from Drosophila, the decapentaplegic (dpp) gene which is involved in embryonic dorsal/ventral polarity, embryonic gut formation, and imaginal disk development. Using PCR methods, we have identified a second Drosophila gene in the TGF-beta family. It encodes a protein product that is more similar to the TGF-beta-related human bone morphogenetic proteins (BMPs) 5, 6, and 7 than it is to the Drosophila dpp gene product. Because of its localization on the polytene chromosome map, we refer to this gene as 60A. Expression of a 60A cDNA in Drosophila S2 cells was used to determine that 60A encodes a preproprotein that is processed to yield secreted amino- and carboxy-terminal polypeptides. The carboxy-terminal peptides are recovered as disulfide-linked homodimers. The 60A transcripts and protein are first detected at the onset of gastrulation, primarily in the mesoderm of the extending germ band. As the germ band retracts, and throughout later stages of embryonic development, the 60A transcript and protein are most readily detected in cells of the developing foregut and hindgut.

The vegetalizing factor. A member of the evolutionarily highly conserved activin family

The mesoderm and endoderm inducing vegetalizing factor was partially sequenced after BrCN cleavage. A sequence which is highly conserved in activin A near the C-terminal end was identified. This shows that the factor belongs to the activin family. The activins are not confined to embryos and gonads, but widely distributed in other tissues like calf kidney and calf liver. Functional aspects are discussed.

Expression and characterization of bone morphogenetic protein-2 in Chinese hamster ovary cells

Bone is a dynamic tissue that responds to many factors including vitamin D, parathyroid hormone, estrogen, calcitonin, and bone morphogenetic proteins (BMPs). The ability to stimulate new bone growth would permit novel therapies for situations where bone mass has been lost due to accident or disease. Purified BMP-2, in conjunction with a suitable matrix, is sufficient to stimulate the synthesis of new bone (Wang et al., 1990). We have expressed recombinant human BMP-2 at high levels in Chinese hamster ovary cells using methotrexate-mediated gene amplification. Several forms of BMP-2 are secreted from CHO cells: (1) an amino-terminal propeptide of 40-45 kDa, (23) a mature active 30 kDa homodimer consisting of 18-22 kDa subunits, and (3) a small amount of uncleaved 60 kDa precursor protein. The mature, active protein is predominantly a 30 kDa homodimer consisting of subspecies of 18 and 22 kDa which differ by proteolytic processing at their amino termini. Mature BMP-2 and propeptide contain high mannose and complex N-linked oligosaccharides, respectively. The molar amount of secreted, processed propeptide is approximately 5-fold higher than mature BMP-2 in conditioned medium. BMP-2 associates with both the extracellular matrix and the surface of CHO cells, which may in part account for the unequal levels of extracellular propeptide and mature forms of the molecule in the conditioned medium. Recombinant BMP-2 can be expressed in sufficient quantities to assess its therapeutic potential for bone regeneration.

Transforming growth factor-beta-related genes in Drosophila and vertebrate development

The Drosophila decapentaplegic gene, the Xenopus activin genes and the genes encoding the mouse bone morphogenetic proteins are transforming growth factor-beta-related genes whose roles in development are the focus of current studies. They exhibit elaborate patterns of expression during development, and the protein products have potent effects on the differentiation of specific cell types.

Murine osteogenic protein (OP-1): high levels of mRNA in kidney

The murine OP-1 gene (EMBL accession No. X56906) encoding the homolog of human osteogenic protein-1 was isolated from cDNA and genomic libraries using human OP-1 cDNA as probe. The deduced murine OP-1 amino acid sequence revealed 11 amino acids changes, three of them in the mature protein. Murine OP-1 probes were used for analysis of OP-1 mRNA in mouse embryo and organ tissues. Northern blot hybridization revealed multiple mRNA species: the major species of 2.2 kb, minor species of 1.8 and 2.4 kb and a large 4 kb species, which may represent alternative splices. Tissue specific expression was studied in brain, lung, heart, liver, spleen, kidney, adrenal and bladder tissue. Maximal levels of OP-1 mRNA were found in kidney which may explain the phenomenon of epithelial osteogenesis, first described by Huggins in 1931 using epithelium from the urinary tract. Moreover, our data suggest that kidneys may be the main site of OP-1 synthesis, even though it is distant from its physiological site of action, skeletal bone.

The vegetalizing factor from chicken embryos: its EDF (activin A)-like activity

The erythroid differentiation capacity of the HPLC-purified mesoderm- and endoderm-inducing vegetalizing factor from chicken embryos and of recombinant erythroid differentiation factor (EDF = activin A), an evolutionary highly conserved member of the TGF-beta protein superfamily have been compared. Both factors stimulate the synthesis of hemoglobin in erythroleukemia cells in the same concentration range. The EDF-activity of the mesoderm-inducing HPLC-fractions is inhibited by follistatin, an EDF-binding protein. The factor induces in ectoderm of Triturus taeniatus all kinds of mesodermal organs. The wide spectrum of organs is very likely to be induced by secondary interactions. At higher concentration (15 ng/ml), notochord- and endoderm-like tissues are induced in a high percentage.