Pinning down positional information: dorsal-ventral polarity in the Drosophila embryo

Clip-SP1 cleavage activates downstream prophenoloxidase activating protease (PAP) in Plutella xylostella

Melanization is a component of the humoral immune defense of insects and is induced by serine protease-mediated phenoloxidase (PO) catalysis. Prophenoloxidase (PPO) in the midgut of Plutella xylostella is activated by the CLIP domain serine protease (clip-SP) in response to Bacillus thuringiensis (Bt) infection, but the detailed signaling cascade following this activation is unknown. Here, we report that activation of clip-SP enhances PO activity in the P. xylostella midgut by cleaving three downstream PPO-activating proteases (PAPs). First, the expression level of clip-SP1 was increased in the midgut after Bt8010 infection of P. xylostella. Then, purified recombinant clip-SP1 was able to activate three PAPs - PAPa, PAPb and PAP3 - which in turn enhanced their PO activity in the hemolymph. Furthermore, clip-SP1 showed a dominant effect on PO activity compared to the individual PAPs. Our results indicate that Bt infection induces the expression of clip-SP1, which is upstream of a signaling cascade, to efficiently activate PO catalysis and mediate melanization in the midgut of P. xylostella. And it provides a basis for studying the complex PPO regulatory system in the midgut during Bt infection.

Homeodomain proteins POU-M2, antennapedia and abdominal-B are involved in regulation of the segment-specific expression of the clip-domain serine protease gene CLIP13 in the silkworm, Bombyx mori

Clip-domain serine proteases (CLIPs) play important roles in insect innate immunity and development. Our previous studies indicated that CLIP13, an epidermis-specific gene, was involved in cuticle remodeling during molting and metamorphosis in the silkworm, Bombyx mori. However, the transcriptional regulatory mechanism and regulatory pathways of CLIP13 remained unclear. In the present study, we investigated CLIP13 expression and the regulation pathway controlled by 20-hydroxyecdysone (20E) in the silkworm. At the transcriptional level, expression of CLIP13 exhibited pronounced spatial and temporal specificity in different regions of the epidermis; homeodomain transcription factors POU-M2, antennapedia (Antp), and abdominal-B (Abd-B) showed similar expression change trends as CLIP13 in the head capsule, thorax, and abdomen, respectively. Furthermore, results of cell transfection assays, electrophoretic mobility shift assays, and chromatin immunoprecipitation demonstrated that POU-M2, Antp, and Abd-B were involved in the transcriptional regulation of CLIP13 by directly binding to their cis-response elements in CLIP13 promoter. RNA interference-mediated silencing of POU-M2, Antp, and Abd-B led to a decrease of CLIP13 expression in the head capsule, the epidermis of the 1st to 3rd thoracic segments and the 7th to 10th abdominal segments, respectively. Consistent with CLIP13, 20E treatment significantly upregulated expression of POU-M2, Antp, and Abd-B in the silkworm epidermis. Taken together, these data suggest that 20E positively regulates transcription of CLIP13 via homeodomain proteins POU-M2, Antp, and Abd-B in different regions of the silkworm epidermis during metamorphosis, thus affecting the molting process. Our findings provide new insight into the functions of homeodomain transcription factors in insect molting.

Characterization and functional analysis of a clip domain serine protease (MncSP) and its alternative transcript (MncSP-isoform) from Macrobrachium nipponense

Clip domain serine protease (cSPs) play an important role in the innate immune defense of crustaceans. In this study, a clip domain serine protease (MncSP) and its alternative transcript (MncSP-isoform) were identified from Macrobrachium nipponense. The full-length cDNA sequences of MncSP and MncSP-isoform were 2447 and 2351 bp with open reading frames comprising 1497 and 1401 bp nucleotides and encoding 498 and 466 amino acids, respectively. The genome of MncSP had 10 exons and 9 introns. MncSP contained all 10 exons, whereas MncSP-isoform lacked the second exon. MncSP and MncSP-isoform contained a signal peptide, a clip domain, and a Tryp_SPc domain. Phylogenetic tree analysis showed that MncSP and MncSP-isoform clustered with cSPs from Palaemonidae. MncSP and MncSP-isoform were widely distributed in hemocytes, heart, hepatopancreas, gills, stomach, and intestine. The expression profiles of MncSP and MncSP-isoform in the hemocytes of M. nipponense changed after simulation by Vibrio parahaemolyticus or Staphylococcus aureus. The RNAi of MncSP could inhibit the expression of antimicrobial peptides (AMPs), including crustins and anti-lipopolysaccharide factors. Phenoloxidase activity was also down-regulated in MncSP-silenced prawns. This study indicated that MncSP participated in the synthesis of AMPs and the activation of prophenoloxidase.

Roles of the clip domains of two protease zymogens in the coagulation cascade in horseshoe crabs

The lipopolysaccharide (LPS)-triggered coagulation cascade in horseshoe crabs comprises three protease zymogens: prochelicerase C (proC), prochelicerase B (proB), and the proclotting enzyme (proCE). The presence of LPS results in autocatalytic activation of proC to α-chelicerase C, which, in turn, activates proB to chelicerase B, converting proCE to the clotting enzyme (CE). ProB and proCE contain an N-terminal clip domain, but the roles of these domains in this coagulation cascade remain unknown. Here, using recombinant proteins and kinetics and binding assays, we found that five basic residues in the clip domain of proB are required to maintain its LPS-binding activity and activation by α-chelicerase C. Moreover, an amino acid substitution at a potential hydrophobic cavity in proB's clip domain (V55A-proB) reduced both its LPS-binding activity and activation rate. WT proCE exhibited no LPS-binding activity, and the WT chelicerase B-mediated activation of a proCE variant with a substitution at a potential hydrophobic cavity (V53A-proCE) was ∼4-fold slower than that of WT proCE. The k _cat/K_m value of the interaction of WT chelicerase B with V53A-proCE was 7-fold lower than that of the WT chelicerase B-WT proCE interaction. The enzymatic activities of V55A-chelicerase B and V53A-CE against specific peptide substrates were indistinguishable from those of the corresponding WT proteases. In conclusion, the clip domain of proB recruits it to a reaction center composed of α-chelicerase C and LPS, where α-chelicerase C is ready to activate proB, leading to chelicerase B-mediated activation of proCE via its clip domain.

A Clip Domain Serine Protease Involved in Egg Production in Nilaparvata lugens: Expression Patterns and RNA Interference

Clip domain serine proteases play vital roles in various innate immune functions and in embryonic development. Nilaparvata lugens proclotting enzymes (NlPCEs) belong to this protease family. NlPCE1 was reported to be involved in innate immunity, whereas the role of other NlPCEs is unclear. In the present study, N. lugens proclotting enzyme-3 (NlPCE3) was cloned and characterized. NlPCE3 contains a signal peptide, a clip domain, and a trypsin-like serine protease domain. NlPCE3 was expressed in all tissues examined (gut, fat body, and ovary), and at all developmental stages. Immunofluorescence staining showed that NlPCE3 was mainly expressed in the cytoplasm and cytomembrane of follicular cells. Double stranded NlPCE3 RNA interference clearly inhibited the expression of NlPCE3, resulting in abnormal egg formation and obstruction of ovulation. These results indicate that NlPCE3 plays an important role in egg production in N. lugens.

Functional characterization of two clip-domain serine proteases in the swimming crab Portunus trituberculatus

Clip domain serine proteases (cSPs), a family of multifunctional proteins, play a crucial role in innate immune system. Here, we report the functional characterization of two clip domain serine proteases (PtcSP1 and PtcSP3) from the swimming crab Portunus trituberculatus. The recombinant N-terminal clip domains and the C-terminal SP-like domains of PtcSP1 and PtcSP3 were expressed in Escherichia coli system, and assayed for various biological functions: protease activity, antimicrobial activity, bacterial clearance and microbial-binding activity. The recombinant SP-like domains of PtcSP1 and PtcSP3 exhibited trypsin-like protease activity, while their recombinant clip domains showed strong antibacterial activity and could bind to bacteria and yeast, suggesting the potential roles of PtcSP1 and PtcSP3 in immune defense and pattern recognition. Unlike PtcSP3, PtcSP1 revealed the opsonic activity as shown by a higher bacterial clearance rate of Vibrio alginolyticus coated with the combination of the recombinant clip domain and SP-like domain of PtcSP1 as compared with V. alginolyticus only. Knockdown of PtcSP1 or PtcSP3 by RNA interference resulted in a significant decrease of total phenoloxidase (PO) activity in crab, suggesting that PtcSP1 and PtcSP3 are involved in the proPO system. In addition, suppression of PtcSP1 or PtcSP3 changed the expression of PtALFs and complement-like components. All these findings suggest that PtcSP1 and PtcSP3 are multifunctional immune molecules and perform different protective functions in crab defense.

Robust and local positional information within a fin ray directs fin length during zebrafish regeneration

It has been proposed that cells are regulated to form specific morphologies and sizes according to positional information. However, the entity and nature of positional information have not been fully understood yet. The zebrafish caudal fin has a characteristic V-shape; dorsal and ventral fin rays are longer than the central ones. This fin shape regenerates irrespective of the sites or shape of fin amputation. It is thought that reformation of tissue occurs according to positional information. In this study, we developed a novel transplantation procedure for grafting a whole fin ray to an ectopic position and examined whether the information that specifies fin length exists within each fin ray. Intriguingly, when long and short fin rays were swapped, they regenerated to form longer or shorter fin rays than the adjacent host fin rays, respectively. Further, the abnormal fin ray lengths were maintained for a long time, more than 5 months, and after further re-amputation. In contrast to intra-fin grafting, when fin ray grafting was performed between fish, cells in the grafts disappeared due to immune rejection, and the grafted fin rays adapted to the host position to form a normal fin. Together, our data suggest that the information that directs fin length does exist in cells within a single fin ray and that it has a robust property-it is stable for a long time and is hard to rewrite. Our study highlighted a novel positional information mechanism for directing regenerating fin length.

Modeling Renal Disease "On the Fly"

Detoxification is a fundamental function for all living organisms that need to excrete catabolites and toxins to maintain homeostasis. Kidneys are major organs of detoxification that maintain water and electrolyte balance to preserve physiological functions of vertebrates. In insects, the renal function is carried out by Malpighian tubules and nephrocytes. Due to differences in their circulation, the renal systems of mammalians and insects differ in their functional modalities, yet carry out similar biochemical and physiological functions and share extensive genetic and molecular similarities. Evolutionary conservation can be leveraged to model specific aspects of the complex mammalian kidney function in the genetic powerhouse Drosophila melanogaster to study how genes interact in diseased states. Here, we compare the human and Drosophila renal systems and present selected fly disease models.

A clip domain serine protease involved in moulting in the silkworm, Bombyx mori: cloning, characterization, expression patterns and functional analysis

Clip domain serine proteases (CLIPs), characterized by one or more conserved clip domains, are essential components of extracellular signalling cascades in various biological processes, especially in innate immunity and the embryonic development of insects. Additionally, CLIPs may have additional non-immune functions in insect development. In the present study, the clip domain serine protease gene Bombyx mori serine protease 95 (BmSP95), which encodes a 527-residue protein, was cloned from the integument of B. mori. Bioinformatics analysis indicated that BmSP95 is a typical CLIP of the subfamily D and possesses a clip domain at the N terminus, a trypsin-like serine protease (tryp_spc) domain at the C terminus and a conserved proline-rich motif between these two domains. At the transcriptional level, BmSP95 is expressed in the integument during moulting and metamorphosis, and the expression pattern is consistent with the fluctuating 20-hydroxyecdysone (20E) titre in B. mori. At the translational level, BmSP95 protein is synthesized in the epidermal cells, secreted as a zymogen and activated in the moulting fluid. Immunofluorescence revealed that BmSP95 is distributed into the old endocuticle in the moulting stage. The expression of BmSP95 was upregulated by 20E. Moreover, expression of BmSP95 was downregulated by pathogen infection. RNA interference-mediated silencing of BmSP95 led to delayed moulting from pupa to moth. These results suggest that BmSP95 is involved in integument remodelling during moulting and metamorphosis.

A chymotrypsin-like serine protease from Portunus trituberculatus involved in pathogen recognition and AMP synthesis but not required for prophenoloxidase activation

Clip domain serine proteases (clip-SPs) play critical roles in various immune responses in arthropods, such as hemolymph coagulation, antimicrobial peptide (AMP) synthesis, cell adhesion and melanization. In the present study, we report the molecular and functional characterization of a clip domain serine protease (PtcSP2) from the swimming crab Portunus trituberculatus. The N-terminal clip domain and the C-terminal SP-like domain of PtcSP2 were expressed in Escherichia coli system, and assayed for their activities. Sequence similarity and phylogenetic analysis revealed that PtcSP2 may belong to the chymotrypsin family, which was confirmed by protease activity assay of the recombinant SP-like domain. The clip domain of PtcSP2 exhibited strong antibacterial activity and microbial-binding activity, suggesting the potential role in immune defense and recognition. Knockdown of PtcSP2 by RNA interference could significantly reduce PtcSP2 transcript levels, but neither decrease the total phenoloxidase (PO) activity in crab nor significantly alter the expression levels of serine protease inhibitors PtPLC and PtSerpin. These results indicate that PtcSP2 is not involved in the proPO system. However, suppression of PtcSP2 led to a significant change in the expression of AMP genes PtALFs and PtCrustin but not PtALF5. All these findings suggest that PtcSP2 is a multifunctional chymotrypsin-like serine protease and may participate in crab innate immunity by its antibacterial activity, immune recognition or regulation of AMP expression.

A snake-like serine proteinase (PmSnake) activates prophenoloxidase-activating system in black tiger shrimp Penaeus monodon

Clip domain serine proteinases (ClipSPs) play critical roles in the activation of proteolytic cascade in invertebrate immune systems including the prophenoloxidase (proPO) activating system. In this study, we characterized a snake-like serine protease, namely PmSnake, from the shrimp Penaeus monodon which has previously been identified based on the subtractive cDNA library of proPO double-stranded RNA (dsRNA)-treated hemocytes. An open reading frame of PmSnake contains 1068 bp encoding a predicted protein of 355 amino acid residues with a putative signal peptide of 22 amino acids and two conserved domains (N-terminal clip domain and C-terminal trypsin-like serine proteinase domain). Sequence analysis revealed that PmSnake was closest to the AeSnake from ant Acromyrmex echinatior (53% similarity), but was quite relatively distant from other shrimp PmclipSPs. PmSnake transcript was mainly expressed in shrimp hemocytes and up-regulated after systemic Vibrio harveyi infection indicating that it is an immune-responsive gene. Suppression of PmSnake expression by dsRNA interference reduced both transcript and protein levels leading to a reduction of the hemolymph phenoloxidase (PO) activity (36%), compared to the control, suggesting that the PmSnake functions as a clip-SP in shrimp proPO system. Western blot analysis using anti-PmSnake showed that the PmSnake was detected in hemocytes but not in cell-free plasma. In vitro PO activity and serine proteinase activity assays showed that adding rPmSnake into the shrimp hemolymph could increase PO activity as well as serine proteinase activity suggesting that the rPmSnake activates the proPO system via serine proteinase cascade.

Identification of a novel clip domain serine proteinase (Sp-cSP) and its roles in innate immune system of mud crab Scylla paramamosain

Clip domain serine proteinases and their homologs are involved in the innate immunity of invertebrates. To identify the frontline defense molecules against pathogenic infection, we isolated a novel clip domain serine proteinase (Sp-cSP) from the hemocytes of mud crab Scylla paramamosain. The full-length 1362 bp Sp-cSP contains a 1155 bp open reading frame (ORF) encoding 384 amino acids. Multiple alignment analysis showed that the putative amino acid sequence of Sp-cSP has about 52% and 51% identity with Pt-cSP2 (AFA42360) and Pt-cSP3 (AFA42361) from Portunus trituberculatus, respectively, while the similarity with other cSP sequences was lower than 30%. However, all cSP sequences possess a conserved clip domain at the N-terminal and a Tryp-SPc domain at the C-terminal. The genomic organization of Sp-cSP consists of nine exons and eight introns, with some introns containing one or more tandem repeats. RT-PCR results indicated that Sp-cSP transcripts were predominantly expressed in the subcuticular epidermis, muscle and mid-intestine, but barely detectable in the brain and heart. Further, Sp-cSP transcripts were significantly up-regulated after challenge with lipopolysaccharides (LPS), Vibrio parahaemolyticus, polyinosinic polycytidylic acid (PolyI:C) or white spot syndrome virus (WSSV). Moreover, in vitro, the recombinant Sp-cSP revealed a strong antimicrobial activity against a Gram-positive (Staphylococcus aureus) and four Gram-negative (V. parahaemolyticus, Vibrio alginolyticus, Escherichia coli, Aeromonas hydrophila) bacteria in a dose-dependent manner. Taken together, the acute-phase response to immune challenges and the antimicrobial activity assay indicate that Sp-cSP is a potent immune protector and plays an important role in host defense against pathogen invasion in S. paramamosain.

Positional information, positional error, and readout precision in morphogenesis: a mathematical framework

The concept of positional information is central to our understanding of how cells determine their location in a multicellular structure and thereby their developmental fates. Nevertheless, positional information has neither been defined mathematically nor quantified in a principled way. Here we provide an information-theoretic definition in the context of developmental gene expression patterns and examine the features of expression patterns that affect positional information quantitatively. We connect positional information with the concept of positional error and develop tools to directly measure information and error from experimental data. We illustrate our framework for the case of gap gene expression patterns in the early Drosophila embryo and show how information that is distributed among only four genes is sufficient to determine developmental fates with nearly single-cell resolution. Our approach can be generalized to a variety of different model systems; procedures and examples are discussed in detail.

New gene evolution in the bonus-TIF1-γ/TRIM33 family impacted the architecture of the vertebrate dorsal-ventral patterning network

Uncovering how a new gene acquires its function and understanding how the function of a new gene influences existing genetic networks are important topics in evolutionary biology. Here, we demonstrate nonconservation for the embryonic functions of Drosophila Bonus and its newest vertebrate relative TIF1-γ/TRIM33. We showed previously that TIF1-γ/TRIM33 functions as an ubiquitin ligase for the Smad4 signal transducer and antagonizes the Bone Morphogenetic Protein (BMP) signaling network underlying vertebrate dorsal-ventral axis formation. Here, we show that Bonus functions as an agonist of the Decapentaplegic (Dpp) signaling network underlying dorsal-ventral axis formation in flies. The absence of conservation for the roles of Bonus and TIF1-γ/TRIM33 reveals a shift in the dorsal-ventral patterning networks of flies and mice, systems that were previously considered wholly conserved. The shift occurred when the new gene TIF1-γ/TRIM33 replaced the function of the ubiquitin ligase Nedd4L in the lineage leading to vertebrates. Evidence of this replacement is our demonstration that Nedd4 performs the function of TIF1-γ/TRIM33 in flies during dorsal-ventral axis formation. The replacement allowed vertebrate Nedd4L to acquire novel functions as a ubiquitin ligase of vertebrate-specific Smad proteins. Overall our data reveal that the architecture of the Dpp/BMP dorsal-ventral patterning network continued to evolve in the vertebrate lineage, after separation from flies, via the incorporation of new genes.

Cloning and characterization of a clip domain serine protease and its homolog (masquerade) from Eriocheir sinensis

Serine proteinases (SPs) or SP homologs (SPHs) including clip domain SPs (cSPs) or SPHs (cSPHs) play critical roles in digestion, embryonic development, hemolymph coagulation, and melanization. In this study, one cSP (EscSP) and one SPH, similar to Drosophila masquerade (EsMas), were identified from hepatopancreas of the Chinese mittern crab Eriocheir sinensis. They both possess the clip domains at the N-terminal, EscSP has only one clip domain, but EsMas has seven clip domains. One SP or SP-like domain was at the C-terminal of EscSP and EsMas respectively. In contrast to EscSP, absence of a catalytic residue of Ser resulted in the loss of SP activity of EsMas. Tissue distribution analysis showed that EscSP mRNA was mainly expressed in hepatopancreas, nerve and eyestalk tissue; whereas the EsMas transcript was mainly distributed in eyestalk, muscle, nerve and hemocytes. EscSP in hemocytes showed significant increase after a lipopolysaccharide (LPS) or peptidoglycan (PGN) challenge. However, down-regulation of EsMas was observed in hemocytes challenged by LPS from 2 to 24 h, by contrast EsMas could be induced by PGN challenge at 2 and 24 h. All these findings indicated that EscSP and EsMas might be involved in the innate immune defenses in E. sinensis.

Self-organized shuttling: generating sharp dorsoventral polarity in the early Drosophila embryo

Morphogen gradients pattern tissues and organs during development. When morphogen production is spatially restricted, diffusion and degradation are sufficient to generate sharp concentration gradients. It is less clear how sharp gradients can arise within the source of a broadly expressed morphogen. A recent solution relies on localized production of an inhibitor outside the domain of morphogen production, which effectively redistributes (shuttles) and concentrates the morphogen within its expression domain. Here, we study how a sharp gradient is established without a localized inhibitor, focusing on early dorsoventral patterning of the Drosophila embryo, where an active ligand and its inhibitor are concomitantly generated in a broad ventral domain. Using theory and experiments, we show that a sharp Toll activation gradient is produced through "self-organized shuttling," which dynamically relocalizes inhibitor production to lateral regions, followed by inhibitor-dependent ventral shuttling of the activating ligand Spätzle. Shuttling may represent a general paradigm for patterning early embryos.

Graded dorsal and differential gene regulation in the Drosophila embryo

A gradient of Dorsal activity patterns the dorsoventral (DV) axis of the early Drosophila melanogaster embryo by controlling the expression of genes that delineate presumptive mesoderm, neuroectoderm, and dorsal ectoderm. The availability of the Drosophila melanogaster genome sequence has accelerated the study of embryonic DV patterning, enabling the use of systems-level approaches. As a result, our understanding of Dorsal-dependent gene regulation has expanded to encompass a collection of more than 50 genes and 30 cis-regulatory sequences. This information, which has been integrated into a spatiotemporal atlas of gene regulatory interactions, comprises one of the best-understood networks controlling any developmental process to date. In this article, we focus on how Dorsal controls differential gene expression and how recent studies have expanded our understanding of Drosophila embryonic development from the cis-regulatory level to that controlling morphogenesis of the embryo.

The Drosophila serine protease homologue Scarface regulates JNK signalling in a negative-feedback loop during epithelial morphogenesis

In Drosophila melanogaster, dorsal closure is a model of tissue morphogenesis leading to the dorsal migration and sealing of the embryonic ectoderm. The activation of the JNK signal transduction pathway, specifically in the leading edge cells, is essential to this process. In a genome-wide microarray screen, we identified new JNK target genes during dorsal closure. One of them is the gene scarface (scaf), which belongs to the large family of trypsin-like serine proteases. Some proteins of this family, like Scaf, bear an inactive catalytic site, representing a subgroup of serine protease homologues (SPH) whose functions are poorly understood. Here, we show that scaf is a general transcriptional target of the JNK pathway coding for a secreted SPH. scaf loss-of-function induces defects in JNK-controlled morphogenetic events such as embryonic dorsal closure and adult male terminalia rotation. Live imaging of the latter process reveals that, like for dorsal closure, JNK directs the dorsal fusion of two epithelial layers in the pupal genital disc. Genetic data show that scaf loss-of-function mimics JNK over-activity. Moreover, scaf ectopic expression aggravates the effect of the JNK negative regulator puc on male genitalia rotation. We finally demonstrate that scaf acts as an antagonist by negatively regulating JNK activity. Overall, our results identify the SPH-encoding gene scaf as a new transcriptional target of JNK signalling and reveal the first secreted regulator of the JNK pathway acting in a negative-feedback loop during epithelial morphogenesis.

Distinct functional specificities are associated with protein isoforms encoded by the Drosophila dorsal-ventral patterning gene pipe

Spatially regulated transcription of the pipe gene in ventral cells of the Drosophila ovary follicle cell epithelium is a key event that specifies progeny embryo dorsal-ventral (DV) polarity. pipe encodes ten putative protein isoforms, all of which exhibit similarity to vertebrate glycosaminoglycan-modifying enzymes. Expression of one of the isoforms, Pipe-ST2, in follicle cells has previously been shown to be essential for DV patterning. pipe is also expressed in the embryonic salivary gland and its expression there is required for normal viability. Here, we show that in addition to Pipe-ST2, seven of the other Pipe isoforms are expressed in the ovary, whereas all Pipe isoforms are abundantly expressed in the embryo. Of the ten isoforms, only Pipe-ST2 can restore ventral and lateral pattern elements to the progeny of otherwise pipe-null mutant females. By contrast, three Pipe isoforms, but not Pipe-ST2, support the production of a novel pipe-dependent epitope present in the embryonic salivary gland. These data indicate that differences in functional specificity, and presumably enzymatic specificity, are associated with several of the Pipe isoforms. In addition, we show that uniform expression of the Pipe-ST2 isoform in the follicle cell layer of females otherwise lacking pipe expression leads to the formation of embryos with a DV axis that is appropriately oriented with respect to the intrinsic polarity of the eggshell. This suggests the existence of a second mechanism that polarizes the Drosophila embryo, in addition to the ventrally restricted transcription of the pipe gene.

Clip domain serine protease and its homolog respond to Vibrio challenge in Chinese white shrimp, Fenneropenaeus chinensis

Clip domain serine proteases and their homologs are involved in invertebrate innate immunity, including hemolymph coagulation, antimicrobial peptide synthesis, cell adhesion, and melanization. Recognition of pathogens by pattern recognition receptors can trigger activation of a serine protease cascade. We report here the cDNA cloning of a serine protease (FcSP) and a serine protease homolog (FcSPH) from Chinese white shrimp, Fenneropenaeus chinensis. Both FcSP and FcSPH possess a clip domain at the N-terminal and an SP or SP-like domain at the C-terminal. In contrast to FcSP, FcSPH lacks a catalytic residue and is catalytically inactive. Tissue distribution and time course qRT-PCR analysis indicates that FcSP and FcSPH can respond to Vibrio anguillarum challenge in hemocytes, hepatopancreas and intestine. In situ hybridization analysis shows that FcSP is distributed in hemocytes and gills, and originated mainly from the hemocytes. FcSPH protein is expressed in gills and stomach of non-challenged shrimp. Its expression in gill mainly originates from the hemocytes in it. Two immunoreactive bands of FcSP can be detected in gills and stomach of non-challenged shrimp. FcSP protein is partially cleaved in non-challenged shrimp, while FcSPH protein is unprocessed in unchallenged shrimp and is partially cleaved after V. anguillarum challenge. Our results suggest that this Clip domain serine protease and its homolog may be involved in the serine protease cascade and play an important role in innate immunity of the shrimp.

Molecular cloning, characterization and expression analysis of a clip-domain serine protease from pearl oyster Pinctada fucata

The clip-domain serine proteases (SPs) are the essential components of extracellular signaling cascade in various biological processes, especially in embryonic development and the innate immune responses of invertebrate. Herein, we described the isolation and characterization of pearl oyster Pinctada fucata clip-domain SP gene (designated as poSP). The poSP cDNA was 1080 bp long and consisted of a 5'-untranslated region (UTR) of 13 bp, a 3'-UTR of 68 bp with a polyadenylation signal (AATAAA) at 22 nucleotides upstream of the poly(A) tail, and an open reading frame (ORF) of 999 bp encoding a polypeptide of 332 amino acids with an estimated molecular mass of 36.5 kDa and a theoretical isoelectric point of 7.3. A clip-domain and a trypsin-like serine protease domain were identified in the poSP using SMART analysis. Homology analysis of the deduced amino acid sequence of the poSP with other known SP sequences by MatGAT software revealed that the poSP shared 47.0-68.4% similarity to the other known SP sequences. The poSP mRNA was expressed in haemocytes, gonad, digestive gland and mantle, but not expressed in adductor muscle and gill. The poSP mRNA was up-regulated and increased nearly double-fold after LPS or Vibrio alginolyticus stimulation, respectively. These results suggested that the poSP was an inducible acute-phase protein that perhaps involved in the innate immune response of pearl oyster.

Preliminary X-ray crystallographic analysis of the catalytic domain of prophenoloxidase activating factor-I

Clip-domain serine proteases (SPs) have been identified in invertebrates as crucial enzymes that are involved in diverse extracellular signalling pathways. Prophenoloxidase (proPO) activating factor-I (PPAF-I), a catalytically active clip-domain SP, cleaves proPO. To date, no crystal structures of a catalytically active clip-domain SP have been determined. Here, the results of crystallization and preliminary X-ray analysis of the SP domain of PPAF-I are reported. The crystal of the PPAF-I SP domain was obtained using the hanging-drop vapour-diffusion method in a precipitant solution containing 0.15 M lithium sulfate, 30% polyethylene glycol 4000 and 0.1 M Tris-HCl pH 8.0. The crystal diffracts X-rays to 1.7 angstroms resolution using a synchrotron-radiation source. The crystal belongs to space group P2(1)2(1)2(1), with one molecule in the asymmetric unit and unit-cell parameters a = 38.3, b = 53.3, c = 116.6 angstroms, alpha = beta = gamma = 90 degrees. A molecular-replacement solution has been found using kallikrein as a starting model, resulting in an interpretable electron-density map.

Prophenoloxidase activation is not required for survival to microbial infections in Drosophila

The antimicrobial defence of Drosophila relies on cellular and humoral processes, of which the inducible synthesis of antimicrobial peptides has attracted interest in recent years. Another potential line of defence is the activation, by a proteolytic cascade, of phenoloxidase, which leads to the production of quinones and melanin. However, in spite of several publications on this subject, the contribution of phenoloxidase activation to resistance to infections has not been established under appropriate in vivo conditions. Here, we have isolated the first Drosophila mutant for a prophenoloxidase-activating enzyme (PAE1). In contrast to wild-type flies, PAE1 mutants fail to activate phenoloxidase in the haemolymph following microbial challenge. Surprisingly, we find that these mutants are as resistant to infections as wild-type flies, in the total absence of circulating phenoloxidase activity. This raises the question with regard to the precise function of phenoloxidase activation in defence, if any.

Expression of18-wheeler in the follicle cell epithelium affects cell migration and egg morphology inDrosophila

The Drosophila ovary is a model system for examining the genetic control of epithelial morphogenesis. The somatic follicle cells form a polarized epithelium surrounding the 16-cell germ line cyst. The integrity of this epithelium is essential for the successful completion of oogenesis. Reciprocal signaling between germ line and somatic cells establishes embryonic and eggshell polarity. The follicle cells are responsible for shaping the egg and secreting the eggshell. Follicle cells at the boundary between the nurse cells and the oocyte migrate centripetally to cover the anterior end of the oocyte and secrete the operculum. Dorsal anterior main body follicle cells undergo elaborate patterning to produce the dorsal appendages. We have examined the expression of the Toll-like receptor, 18-wheeler (18w), in the ovary and find it to be restricted to subpopulations of follicle cells. Females carrying loss-of-function 18w mutant clones in their ovaries show delayed follicle cell migrations. The eggs laid by such females also show morphological defects in egg shape and dorsal appendage morphology. We propose that the 18W protein plays an adhesive or signaling role in regions of the epithelium engaged in cell migration.

Crystal structure of a clip-domain serine protease and functional roles of the clip domains

Clip-domain serine proteases (SPs) are the essential components of extracellular signaling cascades in various biological processes, especially in embryonic development and the innate immune responses of invertebrates. They consist of a chymotrypsin-like SP domain and one or two clip domains at the N-terminus. Prophenoloxidase-activating factor (PPAF)-II, which belongs to the noncatalytic clip-domain SP family, is indispensable for the generation of the active phenoloxidase leading to melanization, a major defense mechanism of insects. Here, the crystal structure of PPAF-II reveals that the clip domain adopts a novel fold containing a central cleft, which is distinct from the structures of defensins with a similar arrangement of cysteine residues. Ensuing studies demonstrated that PPAF-II forms a homo-oligomer upon cleavage by the upstream protease and that the clip domain of PPAF-II functions as a module for binding phenoloxidase through the central cleft, while the clip domain of a catalytically active easter-type SP plays an essential role in the rapid activation of its protease domain.

Genomic Regulatory Networks and Animal Development

The synthesis of gene expression data and cis-regulatory analysis permits the elucidation of genomic regulatory networks. These networks provide a direct visualization of the functional interconnections among the regulatory genes and signaling components leading to cell-specific patterns of gene activity. Complex developmental processes are thereby illuminated in ways not revealed by the conventional analysis of individual genes. In this review, we describe emerging networks in several different model systems, and compare them with the gene regulatory network that controls dorsoventral patterning of the Drosophila embryo.

Overexpression and preliminary X-ray crystallographic analysis of prophenoloxidase activating factor II, a clip domain family of serine proteases

A clip domain family of serine proteases has been identified in invertebrates as a crucial enzyme involved in diverse biological processes including immune responses and embryonic development. Although these proteins contain at least one clip domain at the N-terminal of the serine protease domain, the roles and three-dimensional structure of the clip domain are unknown. Prophenoloxidase activating factor-II (PPAF-II), a clip domain family of serine proteases, derived from the beetle Holotrichia diomphalia larvae, was overexpressed in the baculovirus system, and crystallized using the hanging-drop vapor-diffusion method. High-quality single crystals of PPAF-II were obtained in a precipitant solution containing 0.15 M ammonium sulfate, 1.25 M lithium sulfate monohydrate, and 0.1 M sodium citrate dehydrate (pH 5.5). These crystals belong to space group C2 with unit-cell parameters a=107.84, b=76.78, c=70.49 A and beta=113.93 degrees , and contain one or two molecules in the asymmetric unit. Determination of the three-dimensional structure of PPAF-II would clarify the functions of the clip domains.

Drosophila WntD is a target and an inhibitor of the Dorsal/Twist/Snail network in the gastrulating embryo

The maternal Toll signaling pathway sets up a nuclear gradient of the transcription factor Dorsal in the early Drosophila embryo. Dorsal activates twist and snail, and the Dorsal/Twist/Snail network activates and represses other zygotic genes to form the correct expression patterns along the dorsoventral axis. An essential function of this patterning is to promote ventral cell invagination during mesoderm formation, but how the downstream genes regulate ventral invagination is not known. We show here that wntD is a novel member of the Wnt family. The expression of wntD is activated by Dorsal and Twist, but the expression is much reduced in the ventral cells through repression by Snail. Overexpression of WntD in the early embryo inhibits ventral invagination, suggesting that the de-repressed WntD in snail mutant embryos may contribute to inhibiting ventral invagination. The overexpressed WntD inhibits invagination by antagonizing Dorsal nuclear localization, as well as twist and snail expression. Consistent with the early expression of WntD at the poles in wild-type embryos, loss of WntD leads to posterior expansion of nuclear Dorsal and snail expression, demonstrating that physiological levels of WntD can also attenuate Dorsal nuclear localization. We also show that the de-repressed WntD in snail mutant embryos contributes to the premature loss of snail expression, probably by inhibiting Dorsal. Thus, these results together demonstrate that WntD is regulated by the Dorsal/Twist/Snail network, and is an inhibitor of Dorsal nuclear localization and function.

Profiling early infection responses: Pseudomonas aeruginosa eludes host defenses by suppressing antimicrobial peptide gene expression

Insights into the host factors and mechanisms mediating the primary host responses after pathogen presentation remain limited, due in part to the complexity and genetic intractability of host systems. Here, we employ the model Drosophila melanogaster to dissect and identify early host responses that function in the initiation and progression of Pseudomonas aeruginosa pathogenesis. First, we use immune potentiation and genetic studies to demonstrate that flies mount a heightened defense against the highly virulent P. aeruginosa strain PA14 when first inoculated with strain CF5, which is avirulent in flies; this effect is mediated via the Imd and Toll signaling pathways. Second, we use whole-genome expression profiling to assess and compare the Drosophila early defense responses triggered by the PA14 vs. CF5 strains to identify genes whose expression patterns are different in susceptible vs. resistant host-pathogen interactions, respectively. Our results identify pathogenesis- and defense-specific genes and uncover a previously undescribed mechanism used by P. aeruginosa in the initial stages of its host interaction: suppression of Drosophila defense responses by limiting antimicrobial peptide gene expression. These results provide insights into the genetic factors that mediate or restrict pathogenesis during the early stages of the bacterial-host interaction to advance our understanding of P. aeruginosa-human infections.

Comparative and functional genomics of the innate immune system in the malaria vector Anopheles gambiae

In much of Africa, the mosquito Anopheles gambiae is the major vector of human malaria, a devastating infectious disease caused by Plasmodium parasites. Vector and parasite interact at multiple stages and locations, and the nature and effectiveness of this reciprocal interaction determines the success of transmission. Many of the interactions engage the mosquito's innate immunity, a primitive but very effective defense system. In some cases, the mosquito kills the parasite, thus blocking the transmission cycle. However, not all interactions are antagonistic; some represent immune evasion. The sequence of the A. gambiae genome revealed numerous potential components of the innate immune system, and it established that they evolve rapidly, as summarized in the present review. Their rapid evolution by gene family expansion diversification as well as the prevalence of haplotype alleles in the best-studied families may reflect selective adaptation of the immune system to the exigencies of multiple immune challenges in a variety of ecologic niches. As a follow-up to the comparative genomic analysis, the development of functional genomic methodologies has provided novel opportunities for understanding the immune system and the nature of its interactions with the parasite. In this context, identification of both Plasmodium antagonists and protectors in the mosquito represents a significant conceptual advance. In addition to providing fundamental understanding of primitive immune systems, studies of mosquito interactions with the parasite open unprecedented opportunities for novel interventions against malaria transmission. The generation of transgenic mosquitoes that resist malaria infection in the wild and the development of antimalarial 'smart sprays' capable of disrupting interactions that are protective of the parasite, or reinforcing others that are antagonistic, represent technical challenges but also immense opportunities for improvement of global health.

Whole-genome analysis of Drosophila gastrulation

Microarray assays and bioinformatics methods have identified many of the genes and associated regulatory DNAs that control the early phases of gastrulation in Drosophila. The localized activities of these genes are coordinated by a nuclear gradient of the maternal regulatory factor, Dorsal, that is established shortly after fertilization. At least half of the Dorsal target genes encode transcription factors or signaling components that lead to the restricted activation of FGF, EGF, and TGF-beta signaling pathways in the mesoderm, neurogenic ectoderm and dorsal ectoderm, respectively. Recent work has yielded insights into how these signaling pathways control gastrulation, particularly in the context of the Dorsal-mediated gene regulation network

[Establishment and expression of embryonic axes: comparisons between different model organisms]

In an accompanying article (C. Sardet et al. m/s 2004; 20 : 414-423) we reviewed determinants of polarity in early development and the mechanisms which regulate their localization and expression. Such determinants have for the moment been identified in only a few species: the insect Drosophila melanogaster, the worm Caenorhabditis elegans, the frog Xenopus laevis and the ascidians Ciona intestinalis and Holocynthia roretzi. Although oogenesis, fertilization, and cell divisions in these embryos differ considerably, with respect to early polarities certain common themes emerge, such as the importance of cortical mRNAs, the PAR polarity proteins, and reorganizations mediated by the cytoskeleton. Here we highlight similarities and differences in axis establishment between these species, describing them in a chronological order from oocyte to gastrula, and add two more classical model organisms, sea urchin and mouse, to complete the comparisons depicted in the form of a Poster which can be downloaded from the site http://biodev.obs-vlfr.fr/biomarcell.

The repressor function of snail is required for Drosophila gastrulation and is not replaceable by Escargot or Worniu

Mesoderm formation in the Drosophila embryo depends on the maternal Toll signaling pathway. The Toll pathway establishes the Dorsal nuclear gradient, which regulates many zygotic genes to establish the mesodermal fate and promote the invagination of ventral cells. An important target gene of Dorsal is snail, which is required for proper mesoderm invagination. The Snail protein contains five zinc fingers and is a transcriptional repressor. However, it is not clear whether repressing target genes is a requirement for Snail to control ventral invagination. To examine such requirement, we conducted a series of genetic rescue experiments in snail mutant embryos. Snail, Worniu, and Escargot are closely related zinc-finger proteins and have equal functions during neuroblast development. However, among these three proteins, only Snail can rescue the mesoderm invagination phenotype. Moreover, the ability of various Snail mutant constructs to repress gene expression correlates with their ability to control invagination. This unique property of Snail in mesoderm formation can be attributed mostly to the CtBP co-repressor interaction motifs in the N-terminus, not to the C-terminal DNA-binding zinc fingers. Ectopic expression of Snail outside the ventral domain is not sufficient to induce cell movement even though repression of target genes still occurs. Together, the results show that the repressor function of Snail is essential for gastrulation. The repression of target genes by Snail may permit other factors in the ventral cells to positively promote mesoderm invagination.

Towards an understanding of the role of NOD2/CARD15 in the pathogenesis of Crohn's disease

With the recent discovery of NOD2 as the first susceptibility gene linked with Crohn's disease, research is now focused on attempting to explain the biological role of NOD2 and how mutations can contribute to the development of this inflammatory disease. Biochemical studies have revealed that NOD2 is in fact a protein involved in the innate immune detection of bacterial products. More specifically, NOD2 recognizes a fragment of peptidoglycan, called muramyl dipeptide, that is found in the cell walls of both Gram-negative and Gram-positive bacteria. This recognition event triggers a pro-inflammatory signalling cascade regulated by the transcription factor NF-kappa B. The complex cellular responses emanating from the interaction of NOD2 and its ligand are thought to touch on many aspects of immune function, including bacterial killing, cytokine release, stimulation and maturation of antigen-presenting cells, and the regulation of the adaptive immune response. Defining these NOD2-regulated responses, and how mutations in the gene encoding this protein disrupt these responses, will be key to understanding the pathogenesis of Crohn's disease.

Activation tagging in plants: a tool for gene discovery

A significant limitation of classical loss-of-function screens designed to dissect genetic pathways is that they rarely uncover genes that function redundantly, are compensated by alternative metabolic or regulatory circuits, or which have an additional role in early embryo or gametophyte development. Activation T-DNA tagging is one approach that has emerged in plants to help circumvent these potential problems. This technique utilises a T-DNA sequence that contains four tandem copies of the cauliflower mosaic virus (CaMV) 35S enhancer sequence. This element enhances the expression of neighbouring genes either side of the randomly integrated T-DNA tag, resulting in gain-of-function phenotypes. Activation tagging has identified a number of genes fundamental to plant development, metabolism and disease resistance in Arabidopsis. This review provides selected examples of these discoveries to highlight the utility of this technology. The recent development of activation tagging strategies for other model plant systems and the construction of new more sophisticated vectors for the generation of conditional alleles are also discussed. These recent advances have significantly expanded the horizons for gain-of-function genetics in plants.

Polarisation des oeufs et des embryons : principes communs

Embryonic development depends on the establishment of polarities which define the axial characteristics of the body. In a small number of cases such as the embryo of the fly drosophila, developmental axes are established well before fertilization while in other organisms such as the nematode worm C. elegans these axes are set up only after fertilization. In most organisms the egg posesses a primary (A-V, Animal-Vegetal) axis acquired during oogenesis which participates in the establishment of the embryonic axes. Such is the case for the eggs of ascidians or the frog Xenopus whose AV axes are remodelled by sperm entry to yield the embryonic axes. Embryos of different species thus acquire an anterior end and a posterior end (Antero-Posterior, A-P axis), dorsal and ventral sides (D-V axis) and then a left and a right side.

Myoblast determination in the somatic and visceral mesoderm depends on Notch signalling as well as on milliways(mili(Alk)) as receptor for Jeb signalling

The visceral muscles of the Drosophila midgut consist of syncytia and arise by fusion of founder and fusion-competent myoblasts, as described for the somatic muscles. A single-step fusion results in the formation of binucleate circular midgut muscles, whereas a multiple-step fusion process produces the longitudinal muscles. A prerequisite for muscle fusion is the establishment of myoblast diversity in the mesoderm prior to the fusion process itself. We provide evidence for a role of Notch signalling during establishment of the different cell types in the visceral mesoderm, demonstrating that the basic mechanism underlying the segregation of somatic muscle founder cells is also conserved during visceral founder cell determination. Searching for genes involved in the determination and differentiation of the different visceral cell types, we identified two independent mutations causing loss of visceral midgut muscles. In both of these mutants visceral muscle founder cells are missing and the visceral mesoderm consists of fusion-competent myoblasts only. Thus, no fusion occurs resulting in a complete disruption of visceral myogenesis. Subsequent characterisation of the mutations revealed that they are novel alleles of jelly belly (jeb) and the Drosophila Alk homologue named milliways (mili(Alk)). We show that the process of founder cell determination in the visceral mesoderm depends on Jeb signalling via the Milliways/Alk receptor. Moreover, we demonstrate that in the somatic mesoderm determination of the opposite cell type, the fusion-competent myoblasts, also depends on Jeb and Alk, revealing different roles for Jeb signalling in specifying myoblast diversity. This novel mechanism uncovers a crosstalk between somatic and visceral mesoderm leading not only to the determination of different cell types but also maintains the separation of mesodermal tissues, the somatic and splanchnic mesoderm.

Mosaic analyses reveal the function ofDrosophila Rasin embryonic dorsoventral patterning and dorsal follicle cell morphogenesis

In Drosophila melanogaster, the Ras signal transduction pathway is the primary effector of receptor tyrosine kinases, which govern diverse developmental programs. During oogenesis, epidermal growth factor receptor signaling through the Ras pathway patterns the somatic follicular epithelium, establishing the dorsoventral asymmetry of eggshell and embryo. Analysis of follicle cell clones homozygous for a null allele of Ras demonstrates that Ras is required cell-autonomously to repress pipe transcription, the critical first step in embryonic dorsoventral patterning. The effects of aberrant pipe expression in Ras mosaic egg chambers can be ameliorated, however, by post-pipe patterning events, which salvage normal dorsoventral polarity in most embryos derived from egg chambers with dorsal Ras clones. The patterned follicular epithelium also determines the final shape of the eggshell, including the dorsal respiratory appendages, which are formed by the migration of two dorsolateral follicle cell populations. Confocal analyses of mosaic egg chambers demonstrate that Ras is required both cell- and non cell-autonomously for morphogenetic behaviors characteristic of dorsal follicle cell migration, and reveal a novel, Ras-dependent pattern of basal E-cadherin localization in dorsal midline follicle cells.

Genetic and molecular analysis of region 88E9;88F2 in Drosophila melanogaster, including the ear gene related to human factors involved in lineage-specific leukemias

We identified and characterized the Drosophila gene ear (ENL/AF9-related), which is closely related to mammalian genes that have been implicated in the onset of acute lymphoblastic and myelogenous leukemias when their products are fused as chimeras with those of human HRX, a homolog of Drosophila trithorax. The ear gene product is present in all early embryonic cells, but becomes restricted to specific tissues in late embryogenesis. We mapped the ear gene to cytological region 88E11-13, near easter, and showed that it is deleted by Df(3R)ea(5022rx1), a small, cytologically invisible deletion. Annotation of the completed Drosophila genome sequence suggests that this region might contain as many as 26 genes, most of which, including ear, are not represented by mutant alleles. We carried out a large-scale noncomplementation screen using Df(3R)ea(5022rx1) and chemical (EMS) mutagenesis from which we identified seven novel multi-allele recessive lethal complementation groups in this region. An overlapping deficiency, Df(3R)Po(4), allowed us to map several of these groups to either the proximal or the distal regions of Df(3R)ea(5022rx1). One of these complementation groups likely corresponds to the ear gene as judged by map location, terminal phenotype, and reduction of EAR protein levels.

The functions ofpannierduringDrosophilaembryogenesis

The pannier (pnr) gene of Drosophila encodes a zinc-finger transcription factor of the GATA family and is involved in several developmental processes during embryonic and imaginal development. We report some novel aspects of the regulation and function of pnr during embryogenesis. Previous work has shown that pnr is activated by decapentaplegic (dpp) in early development, but we find that after stage 10, the roles are reversed and pnr becomes an upstream regulator of dpp. This function of pnr is necessary for the activation of the Dpp pathway in the epidermal cells implicated in dorsal closure and is not mediated by the JNK pathway, which is also necessary for Dpp activity in these cells. In addition, we show that pnr behaves as a selector-like gene in generating morphological diversity in the dorsoventral body axis. It is responsible for maintaining a subdivision of the dorsal half of the embryo into two distinct, dorsomedial and dorsolateral, regions, and also specifies the identity of the dorsomedial region. These results, together with prior work on its function in adults, suggest that pnr is a major factor in the genetic subdivision of the body of Drosophila.

Drosophila-raf acts to elaborate dorsoventral pattern in the ectoderm of developing embryos

In the early Drosophila embryo the activity of the EGF-receptor (Egfr) is required to instruct cells to adopt a ventral neuroectodermal fate. Using a gain-of-function mutation we showed that D-raf acts to transmit this and other late-acting embryonic Egfr signals. A novel role for D-raf was also identified in lateral cell development using partial loss-of-function D-raf mutations. Thus, we provide evidence that zygotic D-raf acts to specify cell fates in two distinct pathways that generate dorsoventral pattern within the ectoderm. These functional requirements for D-raf activity occur subsequent to its maternal role in organizing the anterioposterior axis. The consequences of eliminating key D-raf regulatory domains and specific serine residues in the transmission of Egfr and lateral epidermal signals were also addressed here.

Activation of a protease cascade involved in patterning the Drosophila embryo

Dorsoventral patterning of the Drosophila embryo is initiated by a ventralizing signal. Production of this signal requires the serine proteases Gastrulation Defective (GD), Snake, and Easter, which genetic studies suggest act sequentially in a cascade that is activated locally in response to a ventral cue provided by the pipe gene. Here, we demonstrate biochemically that GD activates Snake, which in turn activates Easter. We also provide evidence that GD zymogen cleavage is important for triggering this cascade but is not spatially localized by pipe. Our results suggest that a broadly, rather than locally, activated protease cascade produces the ventralizing signal, so a distinct downstream step in this cascade must be spatially regulated to restrict signaling to the ventral side of the embryo.

Merlin, the Drosophila homologue of neurofibromatosis-2, is specifically required in posterior follicle cells for axis formation in the oocyte

In Drosophila, the formation of the embryonic axes is initiated by Gurken, a transforming growth factor alpha signal from the oocyte to the posterior follicle cells, and an unknown polarising signal back to the oocyte. We report that Drosophila Merlin is specifically required only within the posterior follicle cells to initiate axis formation. Merlin mutants show defects in nuclear migration and mRNA localisation in the oocyte. Merlin is not required to specify posterior follicle cell identity in response to the Gurken signal from the oocyte, but is required for the unknown polarising signal back to the oocyte. Merlin is also required non-autonomously, only in follicle cells that have received the Gurken signal, to maintain cell polarity and limit proliferation, but is not required in embryos and larvae. These results are consistent with the fact that human Merlin is encoded by the gene for the tumour suppressor neurofibromatosis-2 and is a member of the Ezrin-Radixin-Moesin family of proteins that link actin to transmembrane proteins. We propose that Merlin acts in response to the Gurken signal by apically targeting the signal that initiates axis specification in the oocyte.

The Drosophila dorsoventral determinant PIPE contains ten copies of a variable domain homologous to mammalian heparan sulfate 2-sulfotransferase

In Drosophila, the gene PIPE is expressed in follicle cells, the somatic cells that surround the forming egg during maturation, specifically on one side of the egg chamber. This asymmetry establishes the dorsoventral axis of the future embryo. Through the action of PIPE, the ligand SPATZLE, that is located in the perivitelline fluid of the embryo, is activated ventrally. This signal activates TOLL, a membrane-bound receptor. According to present knowledge, PIPE encodes two different transcripts, one of which restored ventral pattern elements to embryos when introduced into mutant pipe females. Here we show that PIPE is far more complex than previously reported. It encodes not two, but at least ten different transcripts, two of which are localized to ventral follicle cells. The transcripts contain one of ten copies of a variable domain, all homologous to heparan sulfate 2-sulfotransferase, an enzyme known to modify heparan sulfate proteoglycans, which are molecules that can bind ligands. The complex gene structure of PIPE thus evolved by duplications of one exon, a strategy used by genes of the immunoglobulin superfamily to generate molecular diversity. We show that PIPE transcripts can be eliminated by RNAi, although in this method double-stranded RNA is injected in embryos, while PIPE transcripts appear in the adult ovary. Our data suggest that at least two different PIPE transcripts redundantly provide the ventralizing PIPE function. 3' of PIPE we identified an enhancer element that drives a lacZ reporter gene specifically in ventral follicle cells. Since PIPE transcripts are found in salivary glands, and since expression of salivary gland genes is dependent on signaling molecules, we speculate that PIPE became localized to ventral follicle cells by a preexisting control system after acquiring a follicle cell enhancer.