Mutations in masquerade, a novel serine-protease-like molecule, affect axonal guidance and taste behavior in Drosophila

Cleavage activation and functional comparison of Manduca sexta serine protease homologs SPH1a, SPH1b, SPH4, and SPH101 in conjunction with SPH2

Phenoloxidase (PO) is a crucial component of the insect immune response against microbial infection. In the tobacco hornworm Manduca sexta, PO is generated from its precursor proPO by prophenoloxidase activating proteases (PAPs) in the presence of two noncatalytic serine protease homologs (SPHs). cDNA cloning and genome analysis indicate that SPH1a (formerly known as SPH1), SPH1b, SPH4, SPH101, and SPH2 contain a clip domain, a linker, and a protease-like domain (PLD). The first 22 residues of the SPH1b, SPH4, and SPH101 PLDs are identical, and differ from SPH1a only at position 4, Thr¹⁵⁴ substituted with Asn¹⁵⁴ in SPH1a. While the sequence from Edman degradation was used to establish PAP cofactor as a high M_r complex of SPH1a and SPH2, this assignment needed further validation, especially because SPH1b mRNA levels are much higher than SPH1a's and better correlate with SPH2 transcription. Thus, here we determined expression profiles of these SPH genes in different tissues from various developmental stages using highly specific primers. High levels of SPH1b and SPH2 proteins, low SPH4, and no SPH1a or SPH101 were detected in hemolymph from larvae in the feeding, wandering and bar stages, pupae, and adults by targeted LC-MS/MS analysis, based on unique peptides from the trypsin-treated SPHs. We expressed the five proSPHs in baculovirus-infected Sf9 cells for use as standards to identify and quantify their counterparts in plasma samples. Moreover, we tested their cleavage by PAP3 and efficacy of the SPH1a, 1b, 4, and 101 as SPH2 partners in PAP3-mediated proPO activation. PAP3 processed proSPH1b and 101 more readily than proSPH1a and 4; PAP3 activated proPO more efficiently in the presence of SPH2 with SPH101 or SPH1b than with SPH1a or SPH4. These results generally agree with their order of appearance or sequence similarity: SPH101 > SPH1b (98%) > SPH1a (90%) > SPH4 (83%). In other words, likely due to positive selection, products of the newly duplicated genes (SPH1b and SPH101) are more favorable substrates of PAP3 and better SPH2 partners in forming a high M_r cofactor than SPH1a or SPH4 is. Electrophoresis on native gel and immunoblot analysis further indicated that SPH101 or 1b form high M_r complexes more readily than SPH1a or 4 does. In comparison, SPH2 showed a small mobility decrease and then increase on native gel after PAP3 cleavage at the first site. Since the natural cofactor in bar-stage hemolymph is complexes of SPH1 and 2 with an average M_r of 790 kDa, PAP3-activated SPH2 may associate with the higher M_r SPH1b scaffolds to form super-complexes. Their structures and formation in relation to cleavage of SPH1b at different sites await further exploration.

Serine pseudoproteases in physiology and disease

Serine proteases (SPs) constitute a very important family of enzymes, both physiologically and pathologically. The effects produced by these proteins have been explained by their proteolytic activity. However, the discovery of pharmacologically active SP molecules that show no enzymatic activity, as the so-called pseudo SPs or SP homologs (SPHs), has exposed a profoundly neglected possibility of nonenzymatic functions of these SP molecules. In this review, the most thoroughly described SPHs are presented. The main physiological domains in which SPHs operate appear to be in reproduction, embryonic development, immune response, host defense, and hemostasis. Hitherto unexplained actions of SPs should therefore be considered also as the result of the ligand-like attributes of SPs. The gain of a novel function by an SPH is a consequence of specific amino acid replacements that have resulted in a novel interaction interface or a 'catalytic trap'. Unraveling the SP/SPH interactome will provide a description of previously unknown physiological functions of SPs/SPHs, aiding the creation of innovative medical approaches.

Building a platform for predicting functions of serine protease-related proteins in Drosophila melanogaster and other insects

Serine proteases (SPs) and serine protease homologs (SPHs) play essential roles in insect physiological processes including digestion, defense and development. Studies of insect genomes, transcriptomes and proteomes have generated a vast amount of information on these proteins, dwarfing the biological data acquired from a few model species. The large number and high diversity of homologous sequences makes it a challenge to use the limited functional information for making predictions across a broad taxonomic group of insects. In this work, we have extensively updated the framework of knowledge on the SP-related proteins in Drosophila melanogaster by identifying 52 new SPs/SPHs, classifying the 257 proteins into four groups (CLIP, gut, single- and multi-domain SPs/SPHs), and detecting inherent connections among phylogenetic relationships, genomic locations and expression profiles for 99 of the genes. Information on the existence of specific proteins in eggs, larvae, pupae and adults is presented to facilitate future research. More importantly, we have developed an approach to reveal close homologous or orthologous relationships among SPs/SPHs from D. melanogaster, Anopheles gambiae, Apis mellifera, Manduca sexta, and Tribolium castaneum thus inspiring functional studies in these and other holometabolous insects. This approach is useful for tackling similar problems on large and diverse protein families in other groups of organisms.

A serine protease homologue Bombyx mori scarface induces a short and fat body shape in silkworm

Body shape is one of the most prominent and basic characteristics of any organism. In insects, abundant variations in body shape can be observed both within and amongst species. However, the molecular mechanism underlying body shape fine-tuning is very complex and has been largely unknown until now. In the silkworm Bombyx mori, the tubby (tub) mutant has an abnormal short fat body shape and the abdomen of tub larvae expands to form a fusiform body shape. Morphological investigation revealed that the body length was shorter and the body width was wider than that of the Dazao strain. Thus, this mutant is a good model for studying the molecular mechanisms of body shape fine-tuning. Using positional cloning, we identified a gene encoding the serine protease homologue, B. mori scarface (Bmscarface), which is associated with the tub phenotype. Sequence analysis revealed a specific 312-bp deletion from an exon of Bmscarface in the tub strain. In addition, recombination was not observed between the tub and Bmscarface loci. Moreover, RNA interference of Bmscarface resulted in the tub-like phenotype. These results indicate that Bmscarface is responsible for the tub mutant phenotype. This is the first study to report that mutation of a serine protease homologue can induce an abnormal body shape in insects.

The genomic and transcriptomic analyses of serine proteases and their homologs in an endoparasitoid, Pteromalus puparum

In insects, serine proteases (SPs) and serine protease homologs (SPHs) constitute a large family of proteins involved in multiple physiological processes such as digestion, development, and immunity. Here we identified 145 SPs and 38 SPHs in the genome of an endoparasitoid, Pteromalus puparum. Gene duplication and tandem repeats were observed in this large SPs/SPHs family. We then analyzed the expression profiles of SP/SPH genes in response to different microbial infections (Gram-positive bacterium Micrococcus luteus, Gram-negative bacterium Escherichia coli, and entomopathogenic fungus Beauveria bassiana), as well as in different developmental stages and tissues. Some SPs/SPHs also displayed distinct expression patterns in venom gland, suggesting their specific physiological functions as venom proteins. Our finding lays groundwork for further research of SPs and SPHs expressed in the venom glands.

Sequence conservation, phylogenetic relationships, and expression profiles of nondigestive serine proteases and serine protease homologs in Manduca sexta

Serine protease (SP) and serine protease homolog (SPH) genes in insects encode a large family of proteins involved in digestion, development, immunity, and other processes. While 68 digestive SPs and their close homologs are reported in a companion paper (Kuwar et al., in preparation), we have identified 125 other SPs/SPHs in Manduca sexta and studied their structure, evolution, and expression. Fifty-two of them contain cystine-stabilized structures for molecular recognition, including clip, LDLa, Sushi, Wonton, TSP, CUB, Frizzle, and SR domains. There are nineteen groups of genes evolved from relatively recent gene duplication and sequence divergence. Thirty-five SPs and seven SPHs contain 1, 2 or 5 clip domains. Multiple sequence alignment and molecular modeling of the 54 clip domains have revealed structural diversity of these regulatory modules. Sequence comparison with their homologs in Drosophila melanogaster, Anopheles gambiae and Tribolium castaneum allows us to classify them into five subfamilies: A are SPHs with 1 or 5 group-3 clip domains, B are SPs with 1 or 2 group-2 clip domains, C, D1 and D2 are SPs with a single clip domain in group-1a, 1b and 1c, respectively. We have classified into six categories the 125 expression profiles of SP-related proteins in fat body, brain, midgut, Malpighian tubule, testis, and ovary at different stages, suggesting that they participate in various physiological processes. Through RNA-Seq-based gene annotation and expression profiling, as well as intragenomic sequence comparisons, we have established a framework of information for future biochemical research of nondigestive SPs and SPHs in this model species.

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.

High-throughput functional screen of mouse gastrula cDNA libraries reveals new components of endoderm and mesoderm specification

This study describes a cross-species functional screen of mouse gastrula cDNA libraries for components of endoderm and mesoderm specification. Pools of 96 cDNAs from arrayed mouse gastrula cDNA libraries were transcribed into mRNA and injected into either the presumptive mesoderm or the ectoderm of one-cell Xenopus laevis embryos. Injected embryos were examined at gastrula stage by in situ hybridization with endoderm or mesoderm markers. Using this approach, we screened over 700 pools or approximately 60,000 cDNAs. We identified 17 unique cDNAs that function during mesoderm and/or endoderm specification and 16 that cause general morphology changes. Identified molecules fall into eight general functional groups as follows: cell cycle components (seven), transcription factors (four), extracellular secreted molecules (seven), transmembrane receptors (one), intracellular signaling components (five), microtubule components (two), metabolism molecules (three), and unknown (four). Several of the genes we identified would not have been predicted to be involved in endoderm or mesoderm specification, highlighting the usefulness of nonbiased screening approaches. This includes Otx2, which we show is a downstream target of Xsox17beta. The speed, low cost, and high efficiency of this cross-species screen makes it an ideal method for examining cDNAs from difficult-to-obtain sources. Therefore, this approach complements the current mouse molecular genetics systems and provides a powerful means for the genome-wide examination of mammalian gene function.

Drosophila NAB (dNAB) is an orphan transcriptional co-repressor required for correct CNS and eye development

The mammalian NAB proteins have been identified previously as potent co-repressors of the EGR family of zinc finger transcription factors. Drosophila NAB (dNAB), like its mammalian counterparts, binds EGR1 and represses EGR1-mediated transcriptional activation from a synthetic promoter. In contrast, dNAB does not bind the Drosophila EGR-related protein klumpfuss. dnab RNA is expressed exclusively in a subset of neuroblasts in the embryonic and larval central nervous system (CNS), as well as in several larval imaginal disc tissues. Here, we describe the creation of targeted deletion mutations in the dnab gene and the identification of additional, EMS-induced dnab mutations by genetic complementation analysis. Null alleles in dnab cause larval locomotion defects and early larval lethality (L1-L2). A putative hypomorphic allele in dnab instead causes early adult lethality due to severe locomotion defects. In the dnab -/- CNS, axon outgrowth/guidance and glial development appear normal; however, a subset of eve+ neurons forms in reduced numbers. In addition, mosaic analysis in the eye reveals that dnab -/- clones are either very small or absent. Similarly, dNAB overexpression in the eye causes eyes to be very small with few ommatidia. These dramatic eye-specific phenotypes will prove useful for enhancer/suppressor screens to identify dnab-interacting genes.

A CUB-serine protease in the olfactory organ of the spiny lobster Panulirus argus

csp, a gene encoding a protein with high sequence identity to trypsinlike serine protease and CUB domains, was identified from a cDNA library from the olfactory organ (antennular lateral flagellum) of the spiny lobster Panulirus argus. The full-length cDNA sequence of csp is 1801 bp, encoding a protein of 50.25 kD, with three domains: signal peptide, trypsinlike serine protease, and CUB (named for a class of compounds including Complement subcomponents Clr/Cls, Uegf, and Bone morphogenic protein-1). RT-PCR, Northern blots, and immunoblots showed that csp is predominantly expressed in the lateral flagellum and eyestalk. Immunocytochemistry showed that Csp is present in olfactory (aesthetasc) sensilla around auxiliary cells (glia that surround the inner dendrites of olfactory receptor neurons, ORNs) and ORN outer dendrites. We propose that Csp is expressed and secreted by auxiliary cells, associates with ORN cell membranes or extracellular matrix via the CUB domain, and has trypsinlike activity. In the eyestalk, Csp is associated with cells surrounding axons between neuropils of the eyestalk ganglia. Possible functions in the olfactory organ and eyestalk are discussed. To our knowledge, this is the first report from any olfactory system of a gene encoding a protein with serine protease and CUB domains.

The midline glia of Drosophila: a molecular genetic model for the developmental functions of glia

The Midline Glia of Drosophila are required for nervous system morphogenesis and midline axon guidance during embryogenesis. In origin, gene expression and function, this lineage is analogous to the floorplate of the vertebrate neural tube. The expression or function of over 50 genes, summarised here, has been linked to the Midline Glia. Like the floorplate, the cells which generate the Midline Glia lineage, the mesectoderm, are determined by the interaction of ectoderm and mesoderm during gastrulation. Determination and differentiation of the Midline Glia involves the Drosophila EGF, Notch and segment polarity signaling pathways, as well as twelve identified transcription factors. The Midline Glia lineage has two phases of cell proliferation and of programmed cell death. During embryogenesis, the EGF receptor pathway signaling and Wrapper protein both function to suppress apoptosis only in those MG which are appropriately positioned to separate and ensheath midline axonal commissures. Apoptosis during metamorphosis is regulated by the insect steroid, Ecdysone. The Midline Glia participate in both the attraction of axonal growth cones towards the midline, as well as repulsion of growth cones from the midline. Midline axon guidance requires the Drosophila orthologs of vertebrate genes expressed in the floorplate, which perform the same function. Genetic and molecular evidence of the interaction of attractive (Netrin) and repellent (Slit) signaling is reviewed and summarised in a model. The Midline Glia participate also in the generation of extracellular matrix and in trophic interactions with axons. Genetic evidence for these functions is reviewed.

A masquerade-like serine proteinase homologue is necessary for phenoloxidase activity in the coleopteran insect, Holotrichia diomphalia larvae

Previously, we reported the molecular cloning of cDNA for the prophenoloxidase activating factor-I (PPAF-I) that encoded a member of the serine proteinase group with a disulfide-knotted motif at the N-terminus and a trypsin-like catalytic domain at the C-terminus [Lee, S.Y., Cho, M.Y., Hyun, J.H., Lee, K.M., Homma, K.I., Natori, S. , Kawabata, S.I., Iwanaga, S. & Lee, B.L. (1998) Eur. J. Biochem. 257, 615-621]. PPAF-I is directly involved in the activation of pro-phenoloxidase (pro-PO) by limited proteolysis and the overall structure is highly similar to that of Drosophila easter serine protease, an essential serine protease zymogen for pattern formation in normal embryonic development. Here, we report purification and molecular cloning of cDNA for another 45-kDa novel PPAF from the hemocyte lysate of Holotrichia diomphalia larvae. The gene encodes a serine proteinase homologue consisting of 415 amino-acid residues with a molecular mass of 45 256 Da. The overall structure of the 45-kDa protein is similar to that of masquerade, a serine proteinase homologue expressed during embryogenesis, larval, and pupal development in Drosophila melanogaster. The 45-kDa protein contained a trypsin-like serine proteinase domain at the C-terminus, except for the substitution of Ser of the active site triad to Gly and had a disulfide-knotted domain at the N-terminus. A highly similar 45-kDa serine proteinase homologue was also cloned from the larval cDNA library of another coleopteran, Tenebrio molitor. By in vitro reconstitution experiments, we found that the purified 45-kDa serine proteinase homologue, the purified active PPAF-I and the purified pro-PO were necessary for expressing phenoloxidase activity in the Holotrichia pro-PO system. However, incubation of pro-PO with either PPAF-I or 45-kDa protein, no phenoloxidase activity was observed. Interestingly, when the 45-kDa protein was incubated with PPAF-I and pro-PO in the absence, but not in the presence of Ca2+, the 45-kDa protein was cleaved to a 35-kDa protein. RNA blot hybridization revealed that expression of the 45-kDa protein was increased in the Holotrichia hemolymph after Escherichia coli challenge.

Functional complementation of the malvolio mutation in the taste pathway of Drosophila melanogaster by the human natural resistance-associated macrophage protein 1 (Nramp-1)

The malvolio (mvl) gene of Drosophila melanogaster encodes a protein with a high degree of homology to natural resistance-associated macrophage proteins (Nramps). This family of integral membrane proteins, many of which appear to function as cation transporters, is remarkably conserved in several phylogenetically distinct species. In Drosophila melanogaster, the protein Mvl is expressed in macrophages and in differentiated neurons; loss-of-function mutations lead to defects in gustatory behaviour. The human Nramp-1 protein was expressed in Drosophila melanogaster using the hsp70 promoter. Overexpression in normal animals does not lead to any alterations in their behaviour or physiology. In mutants, however, ubiquitous expression of human Nramp-1 can totally rescue the taste defect. This finding that Nramp-1 can complement the taste defect in mvl mutants provides a potent means of exploiting behavioural genetics to dissect the function of Nramp-1 and to identify other molecules involved with this transport system.

Genetic lesions in Drosophila behavioural mutants

Over the last 30 years, several hundred behavioural mutants have been isolated in Drosophila. Only a fraction of these are well characterized genetically, behaviourally, and structurally. From six areas of behaviour a set of 24 well-studied mutants was chosen, in which the behavioural defect is probably caused by a central dysfunction and not by an impairment of sensory input or motor output. In all cases, the affected genes can be mutated to more than just a behavioural phenotype. Most genes in the sample are essential. Thus, phenotypic specificity is caused by the specificity of the mutation and not by the gene being a 'behavioural gene'. This study investigates how partial functional inactivation in these loci is brought about genetically. In particular, an attempt is made to discern whether behavioural mutations affect part of a protein's functional repertoire, a subset of protein isoforms, or the spatio-temporal expression of a gene. Not unexpectedly, in view of the predominant use of ethyl methanesulfonate (EMS) as mutagen, the majority of sampled mutations fall into the first two categories. The potentially richest source of genetic versatility, the spatio-temporal modulation of promoter activity by enhancers and silencers, has thus been insufficiently exploited for obtaining behavioural mutants. Various mutagens are reviewed as to their suitability in inducing selective regulatory mutations.

The chemorepulsive activity of secreted semaphorins is regulated by furin-dependent proteolytic processing

The semaphorins are a large group of cell surface and secreted proteins implicated in axonal pathfinding. Here we show that the secreted mouse semaphorin D (SemD) is synthesized as an inactive precursor (proSemD) and becomes repulsive for sensory and sympathetic neurites upon proteolytic cleavage. ProSemD processing can be blocked completely by an inhibitor selective for furin-like endoproteases or mutagenesis of three conserved dibasic cleavage sites. Its C-terminal pro-peptide contains a processing signal that is essential for SemD to acquire its full repulsive activity. SemD processing is regulated during the embryonic development of the mouse and determines the magnitude of its repulsive activity. Similarly to SemD, the secreted semaphorins SemA and SemE display repulsive properties that are regulated by processing. Our data suggest that differential proteolytic processing determines the repulsive potency of secreted semaphorins and implicate proteolysis as an important regulatory mechanism in axonal pathfinding.