In vivo self-association of the Drosophila rel-protein dorsal

Mechanism and implications of morphogen shuttling: Lessons learned from dorsal and Cactus in Drosophila

In a developing animal, morphogen gradients act to pattern tissues into distinct domains of cell types. However, despite their prevalence in development, morphogen gradient formation is a matter of debate. In our recent publication, we showed that the Dorsal/NF-κB morphogen gradient, which patterns the DV axis of the early Drosophila embryo, is partially established by a mechanism of facilitated diffusion. This mechanism, also known as "shuttling," occurs when a binding partner of the morphogen facilitates the diffusion of the morphogen, allowing it to accumulate at a given site. In this case, the inhibitor Cactus/IκB facilitates the diffusion of Dorsal/NF-κB. In the fly embryo, we used computation and experiment to not only show that shuttling occurs in the embryo, but also that it enables the viability of embryos that inherit only one copy of dorsal maternally. In this commentary, we further discuss our evidence behind the shuttling mechanism, the previous literature data explained by the mechanism, and how it may also be critical for robustness of development. Finally, we briefly provide additional experimental data pointing toward an interaction between Dorsal and BMP signaling that is likely affected by shuttling.

A facilitated diffusion mechanism establishes the Drosophila Dorsal gradient

The transcription factor NF-κB plays an important role in the immune system, apoptosis and inflammation. Dorsal, a Drosophila homolog of NF-κB, patterns the dorsal-ventral axis in the blastoderm embryo. During this stage, Dorsal is sequestered outside the nucleus by the IκB homolog Cactus. Toll signaling on the ventral side breaks the Dorsal/Cactus complex, allowing Dorsal to enter the nucleus to regulate target genes. Fluorescent data show that Dorsal accumulates on the ventral side of the syncytial blastoderm. Here, we use modeling and experimental studies to show that this accumulation is caused by facilitated diffusion, or shuttling, of the Dorsal/Cactus complex. We also show that active Toll receptors are limiting in wild-type embryos, which is a key factor in explaining global Dorsal gradient formation. Our results suggest that shuttling is necessary for viability of embryos from mothers with compromised dorsal levels. Therefore, Cactus not only has the primary role of regulating Dorsal nuclear import, but also has a secondary role in shuttling. Given that this mechanism has been found in other, independent, systems, we suggest that it might be more prevalent than previously thought.

Polydnavirus Ank proteins bind NF-κB homodimers and inhibit processing of Relish

Recent studies have greatly increased understanding of how the immune system of insects responds to infection, whereas much less is known about how pathogens subvert immune defenses. Key regulators of the insect immune system are Rel proteins that form Nuclear Factor-κB (NF-κB) transcription factors, and inhibitor κB (IκB) proteins that complex with and regulate NF-κBs. Major mortality agents of insects are parasitoid wasps that carry immunosuppressive polydnaviruses (PDVs). Most PDVs encode ank genes that share features with IκBs, while our own prior studies suggested that two ank family members from Microplitis demolitor bracovirus (MdBV) (Ank-H4 and Ank-N5) behave as IκB mimics. However, the binding affinities of these viral mimics for Rel proteins relative to endogenous IκBs remained unclear. Surface plasmon resonance (SPR) and co-immunoprecipitation assays showed that the IκB Cactus from Drosophila bound Dif and Dorsal homodimers more strongly than Relish homodimers. Ank-H4 and –N5 bound Dif, Dorsal and Relish homodimers with higher affinity than the IκB domain of Relish (Rel-49), and also bound Relish homodimers more strongly than Cactus. Ank-H4 and –N5 inhibited processing of compound Relish and reduced the expression of several antimicrobial peptide genes regulated by the Imd signaling pathway in Drosophila mbn2 cells. Studies conducted in the natural host Pseudoplusia includens suggested that parasitism by M. demolitor also activates NF-κB signaling and that MdBV inhibits this response. Overall, our data provide the first quantitative measures of insect and viral IκB binding affinities, while also showing that viral mimics disable Relish processing.

Central to the study of host-pathogen interactions is understanding how the immune system of hosts responds to infection, and reciprocally how pathogens subvert host defenses. In the case of insects, understanding of how the immune system responds to infection greatly exceeds understanding of pathogen counterstrategies. Parasitoid wasps are key mortality agents of insects. Thousands of wasp species have also evolved a symbiotic relationship with large DNA viruses in the family Polydnaviridae whose primary function is to deliver immunosuppressive virulence genes to the insect hosts that wasps parasitize. The function of most PDV-encoded virulence genes, however, remains unknown. In this article, we investigated the function of two ank gene family members from Microplitis demolitor bracovirus (MdBV). Our results indicate that Ank-H4 and Ank-N5 function as mimics of IκB proteins, which regulate a family of transcription factors called NF-κBs that control many genes of the insect immune system. IκBs and NF-κBs also function as key regulators of the mammalian immune system. Our results thus suggest that viral Ank proteins subvert the immune system of host insects by targeting conserved signaling pathways used by a diversity of organisms.

Role for sumoylation in systemic inflammation and immune homeostasis in Drosophila larvae

To counter systemic risk of infection by parasitic wasps, Drosophila larvae activate humoral immunity in the fat body and mount a robust cellular response resulting in encapsulation of the wasp egg. Innate immune reactions are tightly regulated and are resolved within hours. To understand the mechanisms underlying activation and resolution of the egg encapsulation response and examine if failure of the latter develops into systemic inflammatory disease, we correlated parasitic wasp-induced changes in the Drosophila larva with systemic chronic conditions in sumoylation-deficient mutants. We have previously reported that loss of either Cactus, the Drosophila (IκB) protein or Ubc9, the SUMO-conjugating enzyme, leads to constitutive activation of the humoral and cellular pathways, hematopoietic overproliferation and tumorogenesis. Here we report that parasite infection simultaneously activates NF-κB-dependent transcription of Spätzle processing enzyme (SPE) and cactus. Endogenous Spätzle protein (the Toll ligand) is expressed in immune cells and excessive SPE or Spätzle is pro-inflammatory. Consistent with this function, loss of Spz suppresses Ubc9⁻ defects. In contrast to the pro-inflammatory roles of SPE and Spätzle, Cactus and Ubc9 exert an anti-inflammatory effect. We show that Ubc9 maintains steady state levels of Cactus protein. In a series of immuno-genetic experiments, we demonstrate the existence of a robust bidirectional interaction between blood cells and the fat body and propose that wasp infection activates Toll signaling in both compartments via extracellular activation of Spätzle. Within each organ, the IκB/Ubc9-dependent inhibitory feedback resolves immune signaling and restores homeostasis. The loss of this feedback leads to chronic inflammation. Our studies not only provide an integrated framework for understanding the molecular basis of the evolutionary arms race between insect hosts and their parasites, but also offer insights into developing novel strategies for medical and agricultural pest control.

Parasitoid wasps are a large group of insects in which the female injects her eggs into the bodies of host caterpillars (also called larvae). When the wasp egg hatches, the parasite larva gradually eats the host alive and takes over its body. Soon after the parasite egg is laid, an arms race between the parasite and the host is initiated. In a dramatic and highly restrained reaction, the host's blood cells surround and choke the development of the parasite egg. This encapsulation reaction allows the host to resume its development. We use Drosophila and its natural parasites to identify the mechanism that is essential for proper activation and termination of the encapsulation reaction. Unchecked encapsulation-like reaction flares up into a chronic inflammatory blood cancer in uninfected sumoylation-deficient larvae. Our studies reveal the parallels between acute (egg encapsulation) and chronic (blood cancer) inflammation in the fly. Moreover, these parallels match the criteria for acute and chronic inflammation in mammals. We can now understand more clearly how virus-like particles and factors introduced into the host along with the wasp egg disable the host's immune system to win the host/parasite arms race.

NF-κB/Rel proteins and the humoral immune responses of Drosophila melanogaster

Nuclear Factor-κB (NF-κB)/Rel transcription factors form an integral part of innate immune defenses and are conserved throughout the animal kingdom. Studying the function, mechanism of activation and regulation of these factors is crucial for understanding host responses to microbial infections. The fruit fly Drosophila melanogaster has proved to be a valuable model system to study these evolutionarily conserved NF-κB mediated immune responses. Drosophila combats pathogens through humoral and cellular immune responses. These humoral responses are well characterized and are marked by the robust production of a battery of anti-microbial peptides. Two NF-κB signaling pathways, the Toll and the IMD pathways, are responsible for the induction of these antimicrobial peptides. Signal transduction in these pathways is strikingly similar to that in mammalian TLR pathways. In this chapter, we discuss in detail the molecular mechanisms of microbial recognition, signal transduction and NF-κB regulation, in both the Toll and the IMD pathways. Similarities and differences relative to their mammalian counterparts are discussed, and recent advances in our understanding of the intricate regulatory networks in these NF-κB signaling pathways are also highlighted.

The Dorsal Rel homology domain plays an active role in transcriptional regulation

The Dorsal morphogen directs formation of the Drosophila dorsoventral axis by both activating and repressing transcription. It contains an N-terminal Rel homology domain (RHD), which is responsible for DNA binding and regulated nuclear import, and a C-terminal domain (CTD) that contains activation and repression motifs. To determine if the RHD has a direct role in transcriptional control, we analyzed a series of RHD mutations in S2 cells and embryos. Two classes of mutations (termed class I and class II mutations) that alter activation without affecting DNA binding or nuclear import were identified. The two classes appear to define distinct protein interaction surfaces on opposite faces of the RHD. Class I mutations enhance an apparently inhibitory interaction between the RHD and the CTD and eliminate both activation and repression by Dorsal. In contrast, class II mutations result in increased activation in S2 cells but severely decreased activation in embryos and have little effect on repression. Analysis of the cuticles of class II mutant embryos suggests that, in the absence of Dorsal-mediated activation, Dorsal-mediated repression is not sufficient to pattern the embryo. These results provide some of the first evidence that the RHD plays an active role in transcriptional regulation in intact multicellular organisms.

Developmental arrest and physical entrapment eliminates supernumerary Ganaspis xanthopoda parasitoids in Drosophila melanogaster

Superparasitism is a common condition in which a single host is injected with more than 1 parasitoid egg, but only 1 parasite survives to adulthood, and the remaining animals are eliminated. Here, for the first time, we show that supernumerary Ganaspis xanthopoda, endoparasitoids that invade Drosophila melanogaster, are physiologically suppressed during embryonic development. Whereas the suppressed supernumerary embryos can develop to the first larval instar, their subsequent growth is blocked because they become physically trapped within a novel multicellular envelope that is formed during late embryogenesis. Supernumerary embryos can produce this envelope when cultured in vitro even if they are separated from dominant embryos. Our results suggest that physiological suppression programs supernumerary individuals for developmental arrest, starvation, and necrotic or apoptotic death.

Effects of electrical fields on cardiomyocyte differentiation of embryonic stem cells

The effects of electromagnetic fields (EMFs) on the differentiation of cardiomyocytes in embryoid bodies derived from pluripotent embryonic stem (ES) cells were investigated. A single direct current (DC) field pulse was applied to 4-day-old embryoid bodies. The electrical field induced a hyperpolarization of the anode-facing side of embryoid bodies and a depolarization at the cathode-facing side. Significant effects of a single electrical field pulse applied for 90 s on cardiomyocyte differentiation were achieved with field strengths of 250 and 500 V/m, which increased both the number of embryoid bodies differentiating beating foci of cardiomyocytes and the size of the beating foci. The 500-V/m electrical field increased intracellular reactive oxygen species (ROS), but not [Ca(2+)](i) and activated nuclear factor kappa B (NF-kappaB). A comparable increase in the number of beating embryoid bodies was achieved by an incubation for 1 h with H(2)O(2) (1-10 nM), indicating that the electrical field effect was transduced via the intracellular generation of ROS. Because the radical scavengers dehydroascorbate and pyrrolidinedithiocarbamate (APDC) and the NF-kappaB antagonist N-tosyl-L-phenylalanine chloromethyl ketone (TPCK) inhibited cardiac differentiation, we assume that ROS and NF-kappaB may play a role in early cardiac development.

Control of development and immunity by rel transcription factors in Drosophila

The Drosophila Rel/NF-kappaB transcription factors - Dorsal, Dif, and Relish - control several biological processes, including embryonic pattern formation, muscle development, immunity, and hematopoiesis. Molecular-genetic analysis of 12 mutations that cause embryonic dorsal/ventral patterning defects has defined the steps that control the formation of this axis. Regulated activation of the Toll receptor leads to the establishment of a gradient of nuclear Dorsal protein, which in turn governs the subdivision of the axis and specification of ventral, lateral and dorsal fates. Phenotypic analysis of dorsal-ventral embryonic mutants and the characterization of the two other fly Rel proteins, Dif and Relish, have shown that the intracellular portion of the Toll to Cactus pathway also controls the innate immune response in Drosophila. Innate immunity and hematopoiesis are regulated by analogous Rel/NF-kappaB-family pathways in mammals. The elucidation of the complex regulation and diverse functions of Drosophila Rel proteins underscores the relevance of basic studies in Drosophila.

Dorsal-B, a splice variant of the Drosophila factor Dorsal, is a novel Rel/NF-kappaB transcriptional activator

The Drosophila transcription factor Dorsal, a member of the Rel/NF-kappaB family of proteins, plays a key role in the establishment of dorsoventral polarity in the early embryo and is also involved in the immune response. Here, we present evidence that the primary transcript of dorsal can be alternatively spliced, generating Dorsal-B, a new Rel/NF-kappaB family member. Dorsal and Dorsal-B are identical in the N-terminal region, which comprises both a DNA-binding domain and a dimerization domain. However, Dorsal-B lacks the nuclear localization signal located at the end of the Rel domain of Dorsal and is totally divergent in the C-terminal portion. Although Dorsal-B by itself is not able to induce the expression of a kappaB-controlled Luciferase reporter gene, we demonstrate that its C-terminal portion has transactivating properties. Analysis of the dorsal-B expression pattern indicates that the splicing is tissue-specific and excludes a putative role in early embryogenesis. However, dorsal-B synthesis is enhanced upon septic injury, and this challenge induces a nuclear accumulation of the protein in fat body cells suggesting that it may be involved in the immune response.

Activities of the Sex-lethal protein in RNA binding and protein:protein interactions

The Drosophila sex determination gene Sex-lethal (Sxl) controls its own expression, and the expression of downstream target genes such as transformer , by regulating pre-mRNA splicing and mRNA translation. Sxl codes an RNA-binding protein that consists of an N-terminus of approximately 100 amino acids, two 90 amino acid RRM domains, R1 and R2, and an 80 amino acid C-terminus. In the studies reported here we have examined the functional properties of the different Sxl protein domains in RNA binding and in protein:protein interactions. The two RRM domains are responsible for RNA binding. Specificity in the recognition of target RNAs requires both RRM domains, and proteins which consist of the single domains or duplicated domains have anomalous RNA recognition properties. Moreover, the length of the linker between domains can affect RNA recognition properties. Our results indicate that the two RRM domains mediate Sxl:Sxl protein interactions, and that these interactions probably occur both in cis and trans. We speculate that cis interactions between R1 and R2 play a role in RNA recognition by the Sxl protein, while trans interactions stabilize complex formation on target RNAs that contain two or more closely spaced binding sites. Finally, we show that the interaction of Sxl with the snRNP protein Snf is mediated by the R1 RRM domain.

A class VI unconventional myosin is associated with a homologue of a microtubule-binding protein, cytoplasmic linker protein-170, in neurons and at the posterior pole of Drosophila embryos

Coordination of cellular organization requires the interaction of the cytoskeletal filament systems. Recently, several lines of investigation have suggested that transport of cellular components along both microtubules and actin filaments is important for cellular organization and function. We report here on molecules that may mediate coordination between the actin and microtubule cytoskeletons. We have identified a 195-kD protein that coimmunoprecipitates with a class VI myosin, Drosophila 95F unconventional myosin. Cloning and sequencing of the gene encoding the 195-kD protein reveals that it is the first homologue identified of cytoplasmic linker protein (CLIP)-170, a protein that links endocytic vesicles to microtubules. We have named this protein D-CLIP-190 (the predicted molecular mass is 189 kD) based on its similarity to CLIP-170 and its ability to cosediment with microtubules. The similarity between D-CLIP-190 and CLIP-170 extends throughout the length of the proteins, and they have a number of predicted sequence and structural features in common. 95F myosin and D-CLIP-190 are coexpressed in a number of tissues during embryogenesis in Drosophila. In the axonal processes of neurons, they are colocalized in the same particulate structures, which resemble vesicles. They are also colocalized at the posterior pole of the early embryo, and this localization is dependent on the actin cytoskeleton. The association of a myosin and a homologue of a microtubule-binding protein in the nervous system and at the posterior pole, where both microtubule and actin-dependent processes are known to be important, leads us to speculate that these two proteins may functionally link the actin and microtubule cytoskeletons.

A multimeric complex and the nuclear targeting of the Drosophila Rel protein Dorsal

The intracellular part of the Rel signal transduction pathway in Drosophila is encoded by Toll, tube, pelle, dorsal, and cactus, and it functions to form the dorsal-ventral axis in the Drosophila embryo. Upon activation of the transmembrane receptor Toll, Dorsal dissociates from its cytoplasmic inhibitor Cactus and enters the nucleus. Tube and Pelle are required to relay the signal from Toll to the Dorsal-Cactus complex. In a yeast two-hybrid assay, we found that both Tube and Pelle interact with Dorsal. We confirmed these interactions in an in vitro binding assay. Tube interacts with Dorsal via its C-terminal domain, whereas full-length Pelle is required for Dorsal binding. Tube and Pelle bind Dorsal in the N-terminal domain 1 of the Dorsal Rel homology region rather than at the Cactus binding site. Domain 1 has been found to be necessary for Dorsal nuclear targeting. Genetic experiments indicate that Tube-Dorsal interaction is necessary for normal signal transduction. We propose a model in which Tube, Pelle, Cactus, and Dorsal form a multimeric complex that represents an essential aspect of signal transduction.

The 18-wheeler mutation reveals complex antibacterial gene regulation in Drosophila host defense

Mammals and insects employ similar Rel/NF-kappaB signaling cascades in their humoral immune responses. The mammalian interleukin-1 type I receptor (IL-1R) is one way of activating this cascade. The Drosophila Toll protein, whose cytoplasmic domain shows striking similarity to that of the IL-1R, acts in the humoral antimicrobial response. Here we demonstrate that a second IL-1R-related Drosophila protein, 18-Wheeler (18W), is a critical component of the humoral immune response. 18-wheeler is expressed in the larval fat body, the primary organ of antimicrobial peptide synthesis. In the absence of the 18W receptor, larvae are more susceptible to bacterial infection. Nuclear translocation of the Rel protein Dorsal-like immunity factor (Dif) is inhibited, though nuclear translocation of another Rel protein, Dorsal, is unaffected. Induction of several antibacterial genes is reduced following infection, relative to wild-type: attacin is reduced by 95%, cecropin by 65% and diptericin by 12%. Finally, 18-wheeler (18w) expression is induced in response to infection and, in addition to the receptor form, four immune-specific transcripts and proteins are produced.

An activity-dependent network of interactions links the Rel protein Dorsal with its cytoplasmic regulators

A signaling pathway active on the ventral side of the Drosophila embryo defines dorsoventral polarity. A cell surface signal relayed by Toll, Tube and Pelle releases the Rel-related protein Dorsal from its cytoplasmic inhibitor Cactus; free Dorsal translocates into nuclei and directs expression of ventral fates. Using the yeast two-hybrid system and immunoprecipitation experiments, we define scaffolding and anchoring interactions among the pathway components. We show that Dorsal binds specifically to Tube, Pelle and Cactus, and that the protein kinase activity of Pelle differentially regulates its interactions with Dorsal and Tube. We also identify Drosophila Filamin as a potential adaptor linking the interaction network, via Tube, to the transmembrane receptor Toll.

Sex-lethal interacts with splicing factors in vitro and in vivo

The Drosophila sex determination gene Sex-lethal controls its own expression and the expression of downstream target genes such as transformer by regulating RNA splicing. Genetic and molecular studies have established that Sxl requires the product of another gene, snf, to autoregulate the splicing of its own transcripts. snf has recently been shown to encode a Drosophila U1 and U2 small nuclear ribonucleoprotein particle protein. In the work reported here, we demonstrate that the Sxl and Snf proteins can interact directly in vitro and that these two proteins are part of an RNase-sensitive complex in vivo which can be immunoprecipitated with the Sxl antibody. Unlike bulk Snf protein, which sediments slowly in sucrose gradients, the Snf protein associated with Sxl is in a large, rapidly sedimenting complex. Detailed characterization of the Sxl-Snf complexes from cross-linked extracts indicates that these complexes contain additional small nuclear ribonucleoprotein particle proteins and the U1 and U2 small nuclear RNAs. Finally, consistent with the RNase sensitivity of the Sxl-Snf complexes, Sxl transcripts can also be immunoprecipitated by Sxl antibodies. On the basis of the physical interactions between Sxl and Snf, we present a model for Sxl splicing regulation. This model helps explain how the Sxl protein is able to promote the sex-specific splicing of Sxl transcripts, utilizing target sequences that are distant from the regulated splice sites.

Regulated nuclear import of the Drosophila rel protein dorsal: structure-function analysis

The formation of a gradient of nuclear Dorsal protein in the early Drosophila embryo is the last step in a maternally encoded dorsal-ventral signal transduction pathway. This gradient is formed in response to a ventral signal, which leads to the dissociation of cytoplasmic Dorsal from the I kappa B homolog Cactus. Free Dorsal is then targeted to the nucleus. Dorsal is a Rel-family transcription factor. Signal-dependent nuclear localization characterizes the regulation of Rel proteins. In order to identify regions of Dorsal that are essential for its homodimerization, nuclear targeting, and interaction with Cactus, we have performed an in vivo structure-function analysis. Our results show that all these functions are carried out by regions within the conserved Rel-homology region of Dorsal. The C-terminal divergent half of Dorsal is dispensable for its selective nuclear import. A basic stretch of 6 amino acids at the C terminus of the Rel-homology region is necessary for nuclear localization. This nuclear localization signal is not required for Cactus binding. Removal of the N-terminal 40 amino acids abolished the nuclear import of Dorsal, uncovering a potentially novel function for this highly conserved region.

scute (sis-b) function in Drosophila sex determination

The primary sex determination signal, the X chromosome-to-autosome (X/A) ratio, controls the choice of sexual identity in the Drosophila melanogaster embryo by regulating the activity of the early promoter of the Sex-lethal gene, Sxl-Pe. This promoter is activated in females (2X/2A), while it remains off in males (1X/2A). Promoter activation in females is dependent upon X-linked numerator genes. One of these genes, sisterless-b (sis-b), corresponds to the scute (sc) locus of the achaete-scute complex, and it encodes a helix-loop-helix transcription factor. In the studies reported here we have used monoclonal antibodies to study the expression and functioning of the sc(sis-b) protein. Sc is first detected at nuclear cycle 6 to 7, well before Sxl-Pe is first active. At this stage, the protein is in the cytoplasm, not the nucleus. Only after the formation of the syncytial blastoderm, at nuclear cycle 10 to 11, does a substantial fraction of the protein enter the nucleus, and this nuclear import closely coincides with the initial activation of Sxl-Pe. Consistent with the idea that the dose of sc(sis-b) is critical for its function as an X-chromosome counting element, wild-type syncytial blastoderm embryos could be grouped into two classes based on the level of protein. Western blot (immunoblot) analysis demonstrates that this difference in protein level correlates directly with the activity state of the Sxl gene. Finally, we provide the first direct evidence that Sc forms heteromeric complexes in vivo in early embryos with the maternally derived helix-loop-helix protein Daughterless. This in vivo complex is likely to be critical for Sc function in Sxl-Pe activation.

Expression of a connexin 43/beta-galactosidase fusion protein inhibits gap junctional communication in NIH3T3 cells

Gap junctions contain membrane channels that mediate the cell-to-cell movement of ions, metabolites and cell signaling molecules. As gap junctions are comprised of a hexameric array of connexin polypeptides, the expression of a mutant connexin polypeptide may exert a dominant negative effect on gap junctional communication. To examine this possibility, we constructed a connexin 43 (Cx43)/beta-galactosidase (beta-gal) expression vector in which the bacterial beta-gal protein is fused in frame to the carboxy terminus of Cx43. This vector was transfected into NIH3T3 cells, a cell line which is well coupled via gap junctions and expresses high levels of Cx43. Transfectant clones were shown to express the fusion protein by northern and western analysis. X-Gal staining further revealed that all of the fusion protein containing cells also expressed beta-gal enzymatic activity. Double immunostaining with a beta-gal and Cx43 antibody demonstrated that the fusion protein is immunolocalized to the perinuclear region of the cytoplasm and also as punctate spots at regions of cell-cell contact. This pattern is similar to that of Cx43 in the parental 3T3 cells, except that in the fusion protein expressing cells, Cx43 expression was reduced at regions of cell-cell contact. Examination of gap junctional communication (GJC) with dye injection studies further showed that dye coupling was inhibited in the fusion protein expressing cells, with the largest reduction in coupling found in a clone exhibiting little Cx43 localization at regions of cell-cell contact. When the fusion protein expression vector was transfected into the communication poor C6 cell line, abundant fusion protein expression was observed, but unlike the transfected NIH3T3 cells, no fusion protein was detected at the cell surface. Nevertheless, dye coupling was inhibited in these C6 cells. Based on these observations, we propose that the fusion protein may inhibit GJC by sequestering the Cx43 protein intracellularly. Overall, these results demonstrate that the Cx43/beta-gal fusion protein can exert a dominant negative effect on GJC in two different cell types, and suggests that it may serve as a useful approach for probing the biological function of gap junctions.

RNA binding by Sxl proteins in vitro and in vivo

Sxl has been proposed to regulate splicing of specific target genes by directly interacting with their pre-mRNAs. We have therefore examined the RNA-binding properties of Sxl protein in vitro and in vivo. Gel shift and UV cross-linking assays with a purified recombinant MBP-Sxl fusion protein demonstrated preferential binding to RNAs containing poly(U) tracts, and the protein footprinted over the poly(U) region. The protein did not appear to recognize either branch point or AG dinucleotide sequences, but an adenosine residue at the 5' end of the poly(U) tract enhanced binding severalfold. MBP-Sxl formed two shifted complexes on a tra regulated acceptor site RNA; the doubly shifted form may have been stabilized by protein-protein interactions. Consistent with its proposed role in pre-mRNA processing, in nuclear extracts Sxl was found in large ribonucleoprotein (RNP) complexes which sedimented significantly faster than bulk heterogeneous nuclear RNP and small nuclear RNPs. Anti-Sxl staining of polytene chromosomes showed Sxl protein at a number of chromosomal locations, among which was the Sxl locus itself. Sxl protein could also be targeted to a new chromosomal site carrying a transgene containing splicing regulatory sequences from the Sxl gene, following transcriptional induction. After prolonged heat shock, all Sxl protein was restricted to the heat-induced puff at the hs93D locus. In contrast, a presumptive small nuclear RNP protein was observed at several heat puffs following shock.

RNA binding by Sxl proteins in vitro and in vivo

Sxl has been proposed to regulate splicing of specific target genes by directly interacting with their pre-mRNAs. We have therefore examined the RNA-binding properties of Sxl protein in vitro and in vivo. Gel shift and UV cross-linking assays with a purified recombinant MBP-Sxl fusion protein demonstrated preferential binding to RNAs containing poly(U) tracts, and the protein footprinted over the poly(U) region. The protein did not appear to recognize either branch point or AG dinucleotide sequences, but an adenosine residue at the 5' end of the poly(U) tract enhanced binding severalfold. MBP-Sxl formed two shifted complexes on a tra regulated acceptor site RNA; the doubly shifted form may have been stabilized by protein-protein interactions. Consistent with its proposed role in pre-mRNA processing, in nuclear extracts Sxl was found in large ribonucleoprotein (RNP) complexes which sedimented significantly faster than bulk heterogeneous nuclear RNP and small nuclear RNPs. Anti-Sxl staining of polytene chromosomes showed Sxl protein at a number of chromosomal locations, among which was the Sxl locus itself. Sxl protein could also be targeted to a new chromosomal site carrying a transgene containing splicing regulatory sequences from the Sxl gene, following transcriptional induction. After prolonged heat shock, all Sxl protein was restricted to the heat-induced puff at the hs93D locus. In contrast, a presumptive small nuclear RNP protein was observed at several heat puffs following shock.

Disulfide cross-linking in crude embryonic lysates reveals three complexes of the Drosophila morphogen dorsal and its inhibitor cactus

In Drosophila embryos dorsoventral polarity is determined by a concentration gradient of dorsal (dl) protein in the nuclei formed by the differential regulation of nuclear localization of dl protein. cactus (cact) represses the nuclear localization of dl protein. By introducing intermolecular disulfide bonds in homogenates of embryos, we detected three complexes of dl and/or cact proteins. Complex 1 (190 kDa) is a dl protein homodimer (dl2). Complex 2 (270 kDa) consists of one complex 1 and one cact molecule (dl2cact). Complex 3 (200 kDa) is a cact protein complex that does not contain dl protein. In wild-type embryos dl2cact was detected as the major form of dl protein, and dl2 was minor. With this assay virtually no dl monomer is detected. Analysis of the dl protein complexes in ventralized and dorsalized mutant embryos indicates that dl2cact is a cytoplasmic form, whereas dl2 is localized mainly in the nuclei. It seems that a small amount of dl2 is also present in the cytoplasm.

The Drosophila dorsal morphogen represses the tolloid gene by interacting with a silencer element

The dorsal protein (DL) regulates the transcriptional activity of several genes that determine cell fate along the dorsoventral axis of the Drosophila melanogaster embryo. DL is present at high levels in ventral nuclei, where it activates some genes (twi and sna) and represses others (zen, dpp, and tld). DL shows homology to the Rel family of proteins and interacts with specific DNA sequences in the regulatory regions of its target genes. The distal portion of the zen gene acts as a silencer that can mediate the repression of a heterologous promoter in ventral regions of the embryo. It contains four DL binding sites which alone are sufficient for activation but not repression. Here we analyze the interaction of DL with another one of its repressed targets, the tolloid (tld) gene. Approximately 800 bp of 5'-flanking sequences upstream of the tld coding region were shown to drive an expression pattern indistinguishable from the wild-type pattern. A 423-bp fragment located within these sequences contains two DL binding sites and was shown to act as a silencer to mediate ventral repression. Point mutations in the sites abolish not only DNA binding but also ventral repression. We discuss a comparison of the DNA sequences from the zen and tld promoters and the possible mechanisms of transcriptional silencing.

The Drosophila dorsal morphogen represses the tolloid gene by interacting with a silencer element

The dorsal protein (DL) regulates the transcriptional activity of several genes that determine cell fate along the dorsoventral axis of the Drosophila melanogaster embryo. DL is present at high levels in ventral nuclei, where it activates some genes (twi and sna) and represses others (zen, dpp, and tld). DL shows homology to the Rel family of proteins and interacts with specific DNA sequences in the regulatory regions of its target genes. The distal portion of the zen gene acts as a silencer that can mediate the repression of a heterologous promoter in ventral regions of the embryo. It contains four DL binding sites which alone are sufficient for activation but not repression. Here we analyze the interaction of DL with another one of its repressed targets, the tolloid (tld) gene. Approximately 800 bp of 5'-flanking sequences upstream of the tld coding region were shown to drive an expression pattern indistinguishable from the wild-type pattern. A 423-bp fragment located within these sequences contains two DL binding sites and was shown to act as a silencer to mediate ventral repression. Point mutations in the sites abolish not only DNA binding but also ventral repression. We discuss a comparison of the DNA sequences from the zen and tld promoters and the possible mechanisms of transcriptional silencing.

Relocalization of Drosophila dorsal protein can be induced by a rise in cytoplasmic calcium concentration and the expression of constitutively active but not wild-type Toll receptors

The generation of dorso-ventral polarity in Drosophila relies on the formation of a nuclear gradient of the rel/nuclear factor kappa B transcription factor dorsal in the pre-cellular syncitial embryo by a process of differential nuclear localization. It is thought that the gradient is formed by activation at ventral positions of the membrane receptor Toll that in turn causes the local dissociation of dorsal from the cytoplasmic anchor protein cactus. Although Toll is related in its cytoplasmic domain to the interleukin-1 receptor little is known about the signal transduction pathways that lead from Toll to the relocalization of dorsal. In this paper we have used immunofluorescence microscopy as a direct assay of dorsal protein nuclear localization in the Drosophila cell line Schneider 2. We find that increased cytoplasmic calcium concentration and the expression of constitutively active Toll receptors can induce the relocalization of dorsal. By contrast, we find that activation of endogenous protein kinase A and expression of wild-type Toll receptors, which activate zen-chloramphenicol acetyltransferase reporter genes in this system, have only a marginal effect on the cellular distribution of the dorsal protein. Treatment of cells with activators of protein kinase C and radical oxygen intermediates, both of which activate nuclear factor kappa B, also has little effect on dorsal protein localization. We propose that different threshold levels of dorsal activation can be established by distinctly regulated signal transduction pathways.

Dissociation of the dorsal-cactus complex and phosphorylation of the dorsal protein correlate with the nuclear localization of dorsal

The formation of dorsal-ventral polarity in Drosophila requires the asymmetric nuclear localization of the dorsal protein along the D/V axis. This process is regulated by the action of the dorsal group genes and cactus. We show that dorsal and cactus are both phosphoproteins that form a stable cytoplasmic complex, and that the cactus protein is stabilized by its interaction with dorsal. The dorsal-cactus complex dissociates when dorsal is targeted to the nucleus. While the phosphorylation of cactus remains apparently unchanged during early embryogenesis, the phosphorylation state of dorsal correlates with its release from cactus and with its nuclear localization. This differential phosphorylation event is regulated by the dorsal group pathway.

The interplay between multiple enhancer and silencer elements defines the pattern of decapentaplegic expression

The product of the zygotically active decapentaplegic (dpp) gene appears to function as a morphogen that specifies positional information in the dorsal half of the Drosophila embryo. The dorsal-specific transcription of dpp is the key step in establishing a morphogen gradient. We demonstrate here that multiple regions within the second intron of the gene cooperate with one another to generate the wild-type level and pattern of dpp transcription. These regions contain both generalized enhancer elements as well as ventral-specific repressor elements. Placed within the context of heterologous promoters, the intron retains its ability to direct general activation and ventral repression. The ventral specific repression of dpp transcription is directly mediated by binding sites for the dorsal (dl) morphogen in the repressor elements. In contrast with the zerknüllt (zen) ventral repressor element, which contains a few high-affinity dl-binding sites, dpp contains multiple relatively low-affinity sites that function together to bring about complete ventral repression. Because dpp and zen have nearly coincident early expression domains, these results indicate that the same boundary of repression can be specified by dl-binding sites of different affinity. We discuss the possibility that unknown factors interact with dl protein to determine the domain of dl-mediated repression.

Homeostatic balance between dorsal and cactus proteins in the Drosophila embryo

The maternal-effect gene dorsal encodes the ventral morphogen that is essential for elaboration of ventral and ventrolateral fates in the Drosophila embryo. Dorsal belongs to the rel family of transcription factors and controls asymmetric expression of zygotic genes along the dorsoventral axis. The dorsal protein is cytoplasmic in early embryos, possibly because of a direct interaction with cactus. In response to a ventral signal, dorsal protein becomes partitioned into nuclei of cleavage-stage syncytial blastoderms such that the ventral nuclei have the maximum amount of dorsal protein, and the lateral and dorsal nuclei have progressively less protein. Here we show that transgenic flies containing the dorsal cDNA, which is driven by the constitutively active hsp83 promoter, exhibits rescue of the dorsal- phenotype. Transformed lines were used to increase the level of dorsal protein. Females with dorsal levels roughly twice that of wild-type produced normal embryos, while a higher level of dorsal protein resulted in phenotypes similar to those observed for loss-of-function cactus mutations. By manipulating the cactus gene dose, we found that in contrast to a dorsal/cactus ratio of 2.5 which resulted in fully penetrant weak ventralization, a cactus/dorsal ratio of 3.0 was acceptable by the system. By manipulating dorsal levels in different cactus and dorsal group mutant backgrounds, we found that the relative amounts of ventral signal to that of the dorsal-cactus complex is important for the elaboration of the normal dorsoventral pattern. We propose that in a wild-type embryo, the activities of dorsal and cactus are not independently regulated; excess cactus activity is deployed only if a higher level of dorsal protein is available. Based on these results we discuss how the ventral signal interacts with the dorsal-cactus complex, thus forming a gradient of nuclear dorsal protein.