Regulation of serpin gene-1 in Manduca sexta

Molecular identification, characterization, and expression analysis of a serine protease inhibitor gene from cotton bollworm, Helicoverpa armigera (Lepidoptera: Noctuidae)

Serine protease inhibitors (serpins), a superfamily of protease inhibitors, are known to be involved in several physiological processes, such as development, metamorphosis, and innate immunity. In our study, a full-length serpin cDNA, designated Haserpin1, was isolated from the cotton bollworm Helicoverpa armigera. The cDNA sequence of Haserpin1 is 1176 nt long, with an open reading frame encoding 391 amino acids; there is one exon and no intron. The predicted molecular weight of Haserpin1 is 43.53 kDa, with an isoelectric point of 4.98. InterProScan was employed for Haserpin1 functional characterization, which revealed that Haserpin1 contains highly conserved signature motifs, including a reactive center loop (RCL) with a hinge region (E341-N350), the serpin signature, (F367-F375) and a predicted P1-P1' cleavage site (L357-S358), which are useful for identifying serpins. Transcripts of Haserpin1 were constitutively expressed in the fat body, suggesting that it is the major site for serpin synthesis. During the developmental stages, a fluctuation in the expression level of Haserpin1 was observed, with low expression detected at the 5th-instar larval stage. In contrast, relatively high expression was detected at the prepupal stage, suggesting that Haserpin1 might play a critical role at the H. armigera wandering stage. Although the detailed function of this serpin (Haserpin1) needs to be elucidated, our study provides a perspective for the functional investigation of serine protease inhibitor genes.

Endocrine regulation of immunity in insects

Organisms have constant contact with potentially harmful agents that can compromise their fitness. However, most of the times these agents fail to cause serious disease by virtue of the rapid and efficient immune responses elicited in the host that can range from behavioural adaptations to immune system triggering. The immune system of insects does not comprise the adaptive arm, making it less complex than that of vertebrates, but key aspects of the activation and regulation of innate immunity are conserved across different phyla. This is the case for the hormonal regulation of immunity as a part of the broad organismal responses to external conditions under different internal states. In insects, depending on the physiological circumstances, distinct hormones either enhance or suppress the immune response integrating individual (and often collective) responses physiologically and behaviourally. In this review, we provide an overview of our current knowledge on the endocrine regulation of immunity in insects, its mechanisms and implications on metabolic adaptation and behaviour. We highlight the importance of this multilayered regulation of immunity in survival and reproduction (fitness) and its dependence on the hormonal integration with other mechanisms and life-history traits.

The Manduca sexta serpinome: Analysis of serpin genes and proteins in the tobacco hornworm

Members of the serpin superfamily of proteins occur in animals, plants, bacteria, archaea and some viruses. They adopt a variety of physiological functions, including regulation of immune system, modulation of apoptosis, hormone transport and acting as storage proteins. Most members of the serpin family are inhibitors of serine proteinases. In this study, we searched the genome of Manduca sexta and identified 32 serpin genes. We analyzed the structure of these genes and the sequences of their encoded proteins. Three M. sexta genes (serpin-1, serpin-15, and serpin-28) have mutually exclusive alternatively spliced exons encoding the carboxyl-terminal reactive center loop of the protein, which is the site of interaction with target proteases. We discovered that MsSerpin-1 has 14 splicing isoforms, including two undiscovered in previous studies. Twenty-eight of the 32 M. sexta serpins include a putative secretion signal peptide and are predicted to be extracellular proteins. Phylogenetic analysis of serpins in M. sexta and Bombyx mori indicates that 17 are orthologous pairs, perhaps carrying out essential physiological functions. Analysis of the reactive center loop and hinge regions of the protein sequences indicates that 16 of the serpin genes encode proteins that may lack proteinase inhibitor activity. Our annotation and analysis of these serpin genes and their transcript profiles should lead to future advances in experimental study of their functions in insect biochemistry.

Characterization and functional analysis of serpin-28 gene from silkworm, Bombyx mori

Serine protease inhibitors (Serpins) are a broadly distributed superfamily of proteins with a SERPIN domain and participate in several immune responses. In this study, a serpin-28 gene was identified in B. mori and its role in immune regulation was investigated. This gene has an open reading frame of 1065 bp that encodes a 354-amino acid residue polypeptide containing one SERPIN domain with a predicted molecular weight of 40.3 kDa. Recombinant Bmserpin-28 protein was expressed in Escherichia coli and used to raise rabbit anti-Bmserpin-28 polyclonal antibodies. Quantitative real-time PCR analysis revealed that Bmserpin-28 was expressed in all examined tissues, with maximum expression in the fat body and silk gland. Expression pattern of different developmental stages showed that the highest expression level was in the pupae, while the lowest expression level was recorded at the egg stage. After challenge with four different microorganisms (Escherichia coli, Beauveria bassiana, Micrococcus luteus and B. mori nuclear polyhedrosis virus), the expression pattern of Bmserpin-28 was investigated in fat body and haemocyte samples. A substantial upregulation of Bmserpin-28 expression level was recorded following pathogen challenge in both the tested tissues. Furthermore, RNA interference of Bmserpin-28 resulted in significant upregulation of antimicrobial peptide genes. In summary, our results indicated that Bmserpin-28 may be involved in the innate immunity of B. mori.

Role of Antheraea pernyi serpin 12 in prophenoloxidase activation and immune responses

Serine protease inhibitors play a key role in the immune system of invertebrates by controlling proteolytic cascades. Besides its importance, the knowledge on immune functions of serpins in most of insects is fragmentary. In the present study, we identified serpin-12 from Antheraea pernyi encoding a predicted 402 amino acid residue protein (Apserpin-12). We expressed the recombinant protein in Escherichia coli and the purified protein was used for the synthesis of rabbit anti-Apserpin-12 polyclonal antibodies and functional studies. Quantitative real-time ploymerase chain reaction (qRT-PCR) analysis revealed that the knock-down of Apserpin-12 enhanced the prophenoloxidase (PPO) cascade stimulated by Micrococcus luteus in hemolymph, whereas addition of recombinant Apserpin-12 protein along with same elicitor led to down-regulate PPO activation. Following different microbial challenge (E. coli, Beauveria bassiana, M. Luteus, and nuclear polyhedrosis virus), the expression of Apserpin-12 mRNA was induced significantly. Furthermore, the Apserpin-12 double-stranded RNA administration elicited the expression of antimicrobial peptides, while the treatment with recombinant protein suppressed their expression. Tissue profile of Apserpin-12 indicated that it is expressed in all examined tissues, that is, hemolymph, malpighian tubules, midgut, silk gland, integument, and fat body with variation in their transcript levels. We concluded that Apserpin-12 may regulate PPO activation and inhibit the production of antimicrobial peptides in A. pernyi, suggesting important role in its immune system.

Characterization and functional analysis of serpin-1 like gene from oak silkworm Antheraea pernyi

Serpins are a broadly distributed family of proteases found in various organisms that play an important role in regulating the immune response. Here, we identified a serpin-1 gene from Antheraea pernyi that encodes a 279 amino acid protein with a molecular weight of 30.8 kDa. We expressed the recombinant Ap-serpin-1 protein in Escherichia coli and used the purified protein to prepare rabbit anti-Ap-serpin-1 polyclonal antibodies. We calculated the enzyme-linked immunosorbent assay titer of the antibody as 1:128000. Quantitative real-time polymerase chain reaction analysis revealed that Ap-serpin-1 was expressed in all examined tissues, including hemolymph, malpighian tubules, midgut, silk gland, integument and the fat body; the highest Ap-serpin-1 expression levels was detected in the fat body. We next investigated the expression patterns of Ap-serpin-1 in both fat body and hemolymph samples, following treatment with E. coli, Beauveria bassiana, Micrococcus luteus and nuclear polyhedrosis virus (NPV). We reported that NPV and M. luteus significantly enhanced Ap-serpin-1 expression in the fat body. While, in the hemolymph samples, treatment with B. bassiana and M. luteus was shown to upregulate Ap-serpin-1 expression at 24 h induction. Altogether, our results suggest that Ap-serpin-1 is involved in the innate immunity of A. pernyi.

Serpin-15 from Bombyx mori inhibits prophenoloxidase activation and expression of antimicrobial peptides

Serine protease inhibitors (SPIs) play a key role in physiological responses by controlling protease activities. In this study, we studied the biochemical functions of serpin-15, an SPI, from Bombyx mori (Bmserpin-15). Recombinant Bmserpin-15 was expressed in Escherichia coli cells and used to raise rabbit anti-Bmserpin-15 polyclonal antibodies. Bmserpin-15 mRNA and protein expression was detected in all tested tissues, particularly in the fat body and silk gland. After challenge with four different microorganisms (Escherichia coli, Beauveria bassiana, Micrococcus luteus and B. mori nuclear polyhedrosis virus), the expressions of Bmserpin-15 mRNA and protein were induced significantly, particularly by B. bassiana and M. luteus. Recombinant Bmserpin-15 inhibited prophenoloxidase activation, but did not affect phenoloxidase activity, in B. mori hemolymph. Injection of recombinant Bmserpin-15 into B. mori larvae reduced significantly the transcript levels of antimicrobial peptides in fat body. Our results suggested that Bmserpin-15 plays an important role in the innate immunity of B. mori.

RNAi-mediated knockdown of serine protease inhibitor genes increases the mortality of Plutella xylostella challenged by destruxin A

Destruxin A is a mycotoxin that is secreted by entomopathogenic fungi which has a broad-spectrum insecticidal effect. Previous transcript and protein profiling analysis showed that destruxin A has significant effects on the expression of serine protease inhibitor genes (serpin-2, 4, 5) in the larvae of Plutella xylostella. In the current study, we aimed to understand the role of serpins under application of destruxin A. We obtained two full-length cDNA sequences of P. xylostella serpins, named serpin-4 and serpin-5, and cloned the serpin-2 gene whose full-length has already been published. Phylogenetic analysis indicated that these two serpin genes were highly clustered with other serpins associated with the immune response in other insects. The temporal and spatial expression of serpin-2, serpin-4 and serpin-5 were determined to be the highest in the fat body and hemolymph of 4^th larval stage using qRT-PCR and western blot detection techniques. RNA interference (RNAi) mediated knockdown of P. xylostella serpin genes was carried out by microinjection of double-stranded RNA (dsRNA). The expression levels of serpins decreased significantly after RNAi. Results showed that the depletion of serpins induced cecropins expression, increased phenoloxidase (PO) activity, body melanization and mortality in the larvae of P. xylostella under the same lethal concentration of destruxin A. The superimposed effects of serpins RNAi were similar with the destruxin A treatment upon mortality of P. xylostella larvae. We discovered for the first time that serpins play indispensable role in P. xylostella when challenged by destruxin A and deduced the possible function mechanism of destruxin A. Our findings are conducive to fully understanding the potential insecticidal mechanism of destruxin A and constitute a well-defined potential molecular target for novel insecticides.

Serpin-1 splicing isoform J inhibits the proSpätzle-activating proteinase HP8 to regulate expression of antimicrobial hemolymph proteins in Manduca sexta

The innate immune system of insects include the Toll pathway, which is mediated by an extracellular serine proteinase cascade. In the tobacco hornworm, Manduca sexta, hemolymph proteinase 8 (HP8) promotes the synthesis of antimicrobial proteins by cleaving proSpätzle, the putative ligand of M. sexta Toll. HP8 has been observed to form a complex in hemolymph with M. sexta serpin-1, which has multiple alternative splicing isoforms. To investigate the regulation of HP8 and its processing of proSpätzle, we characterized the interaction of recombinant HP8 with serpin-1 isoform J (serpin-1J). Recombinant serpin-1J formed an SDS-stable complex with HP8 in vitro. The association rate constant of serpin-1J and HP8 was 1.3×10(4)M(-1)s(-1), with a stoichiometry of inhibition of 5.4. Serpin-1J inhibited the cleavage of proSpätzle by HP8. Injection of serpin-1J into M. sexta larvae resulted in decreased bacteria-induced antimicrobial activity in hemolymph and reduced expression of cecropin, attacin and hemolin mRNA in fat body. Altogether, these results suggest that serpin-1J functions to inhibit HP8 and thereby modulates the concentration of active Spätzle to regulate the Toll pathway response in M. sexta.

Analysis of mutually exclusive alternatively spliced serpin-1 isoforms and identification of serpin-1 proteinase complexes in Manduca sexta hemolymph

Mutually exclusive alternative splicing produces transcripts for 12 serpin-1 isoforms in Manduca sexta that differ only in the region encoding the carboxyl-terminal 36-40-amino acid residues. This variable region includes the reactive center loop, which determines the inhibitory selectivity of the serpin. We investigated mRNA levels of individual serpin-1 isoforms by quantitative PCR. The 12 isoforms were expressed at similar levels in hemocytes, but in fat body isoform B mRNA was present at significantly higher levels than isoforms C, D, E, F, G, J, K, and Z. To investigate the presence of individual serpin-1 isoforms in plasma we used immunoaffinity purification of serpin-1 isoforms from M. sexta plasma, followed by two-dimensional PAGE and identification of protein spots by digestion with a series of proteinases and analysis of the resulting peptides by MALDI-TOF/TOF. We identified nine of the 12 serpin-1 isoforms and, through analysis of putative serpin-1-proteinase complexes, identified three endogenous M. sexta proteinase targets of serpin-1. Our results suggest that M. sexta serpin-1 isoforms A, E, and J can inhibit hemolymph proteinase 8, which activates the cytokine spätzle. At least one isoform of serpin-1 can inhibit hemocyte proteinase 1, another M. sexta blood proteinase. In addition, a complex of serpin-1K in a complex with M. sexta midgut chymotrypsin was identified, suggesting serpin-1 isoforms may also function to protect insect tissues from digestive proteinases that may leak into the hemocoel.

Cloning, expression and characterization of four serpin-1 cDNA variants from the spruce budworm, Choristoneura fumiferana

Four cDNAs (Cfserpin-1a, Cfserpin-1b, Cfserpin-1c and Cfserpin-1d) of the Choristoneura fumiferana serpin-1 gene were cloned from an epidermis cDNA library. Analysis of the deduced amino acid sequences indicated that the cloned cDNAs encode four different proteins displaying identical N- but distinct C-termini, the latter region containing the inhibitory loop. The entire CfSerpin-1 gene is transcribed while the variants are generated. Antibodies generated against the purified recombinant serpins cross-reacted with the other three. Each of the four Cfserpin-1 cDNA variants was transcribed throughout larval development, from the 4th to the 6th instar, but transcript levels during the intermolt phases were generally higher than during the molting phase. The epidermis and fat body had higher levels of Cfserpin-1 transcripts than the midgut. Cfserpin-1 proteins, detected with the Cfserpin-1a antibody, were found in the epidermis, midgut, fat body, plasma and molting fluid of 6th instar larvae and pre-pupae. Prepupal and pupal insects had higher levels of the proteins than the 6th instar feeding larvae, despite a drop in transcript levels. Cfserpin-1a could bind with the serine proteinase elastase and form a complex in vitro. We hypothesize that the cloned serpins could be involved in the regulation of cuticle degradation during the insect molting cycle.

Differential expansion and evolution of the exon family encoding the Serpin-1 reactive centre loop has resulted in divergent serpin repertoires among the Lepidoptera

Serpins are a unique class of serine protease inhibitors that are becoming increasingly recognized as important regulators of insect defense mechanisms and developmental processes. Previously, we identified three Mamestra configurata serpins that were similar in structure to those encoded by the Manduca sexta Serpin-1 gene. To gain insight into the evolution and function of serpins in lepidopterans, we developed a bacterial artificial chromosome library and sequenced the entire M. configurata gene. The Serpin-1 gene was 28 kbp and had the capacity to encode nine serpin isoforms via alternate splicing of exons encoding variant reactive center loops onto a common scaffold. The relative abundance of each isoform was estimated by expressed sequence tag analysis and their expression patterns examined in various developmental stages and larval tissues. The organization of the M. configurata Serpin-1 gene was very similar to that of M. sexta Serpin-1; however, only the Ms Serpin-1Z (1 of 12) and the Mc Serpin-1a isoforms exhibited a high degree of similarity. Orthologs similar to this variant were also found in other lepidopterans, namely Bombyx mori and Plutella xylostella, suggesting that they are involved in a conserved biochemical process and likely represent the ancestral serpin variant. Expansion of the exon family encoding the Serpin-1 reactive centre loop region appears to be a product of recent duplication events that has given rise to different serpin repertoires in related insect taxa.

Cell-mediated immunity in arthropods: Hematopoiesis, coagulation, melanization and opsonization

The functions of hemocytes in innate immune response are reviewed with emphasized on their roles in coagulation, melanization and opsonization. Also the ways in which hemocytes are produced in and released from hematopoietic tissue are discussed.

Manduca sexta prophenoloxidase activating proteinase-1 (PAP-1) gene: organization, expression, and regulation by immune and hormonal signals

Insect phenoloxidase (PO) participates in melanotic encapsulation, wound healing, and cuticle sclerotization. It is converted from prophenoloxidase (proPO) by a proPO-activating proteinase (PAP). Manduca sexta PAP-1, the final component of a serine proteinase cascade, cleaves proPO to generate active PO. In an effort to understand the transcriptional regulation, we isolated a genomic clone of the PAP-1 gene, determined its nucleotide sequence, and elucidated its exon-intron organization. Computer analysis revealed several immune and hormone responsive elements in the upstream region. Southern blot analysis suggested that the M. sexta genome contains a single copy of PAP-1 gene. Reverse transcription-polymerase chain reaction showed that PAP-1 was constitutively expressed in fat body, trachea, and nerve tissue of the fifth instar larvae. The mRNA levels in hemocytes and fat body markedly increased in response to a bacterial challenge. We also observed tissue-specific and developmental regulation of the gene's transcription. Treating M. sexta fat body culture with 20-hydroxyecdysone reduced the PAP-1 mRNA level. These data indicated that the expression of PAP-1 gene is under the dual control of immune and hormonal signals.

Manduca sexta Serpin-4 and Serpin-5 Inhibit the Prophenol Oxidase Activation Pathway

Infection stimulates the innate immune responses of insects, including activation of prophenol oxidase (pro-PO) in plasma as the last step of a serine protease cascade. To investigate the roles of protease inhibitors in regulating this pathway, we cloned cDNAs for two new serpins (serpin-4 and serpin-5) from the tobacco hornworm, Manduca sexta. Serpin-4 and serpin-5 mRNAs are constitutively expressed at a low level in larval hemocytes and fat body and increased dramatically upon bacterial challenge. These serpins are present in larval plasma at approximately 3 (serpin-4) and approximately 1 mug/ml (serpin-5) and increased 3-8-fold by 24 h after injection of bacteria or fungi. Recombinant serpin-4 and serpin-5 decreased pro-PO activation when added to plasma, but they did not directly inhibit the pro-PO-activating proteases. Instead, they apparently regulate the pathway by inhibiting one or more target proteases upstream of the pro-PO-activating proteases.

Innate immune responses of a lepidopteran insect, Manduca sexta

Many innate immune mechanisms are conserved throughout the animal kingdom. Manduca sexta, a widely used model for insect biochemical research, employs these mechanisms to defend against invading pathogens and parasites. We have isolated from M. sexta hemolymph a group of proteins (hemolin, peptidoglycan recognition proteins, beta-1,3-glucan recognition proteins, and C-type lectins), which serve as a surveillance mechanism by binding to microbial surface molecules (e.g. peptidoglycan, lipopolysaccharide, lipoteichoic acid, and beta-1,3-glucan). The binding triggers diverse responses such as phagocytosis, nodule formation, encapsulation, melanization, and synthesis of anti-microbial peptides/proteins. Some of these responses are mediated and coordinated by serine proteinase cascades, analogous to the complement system in mammals. Our current research is focused on the proteolytic activation of prophenoloxidase (proPO)--a reaction implicated in melanotic encapsulation, wound healing, and protein cross-linking. We have isolated three proPO-activating proteinases, each of which requires serine proteinase homologs as a cofactor for generating active phenoloxidase. The proteinases and proteinase-like molecules, containing one to two clip domains at their amino-terminus, are acute-phase proteins induced upon an immune challenge. Inhibitory regulation of the proteinases by serpins and association of the proteinase homologs with a bacteria-binding lectin are important for ensuring a localized defense response. Additional serine proteinases expressed in M. sexta hemocytes and fat body have been discovered. Future research efforts will be aimed at elucidating the proteinase cascade for proPO activation and investigating the roles of proteinases in other immune responses such as processing of plasmatocyte-spreading peptide.

Purification and characterization of Manduca sexta serpin-6: a serine proteinase inhibitor that selectively inhibits prophenoloxidase-activating proteinase-3

The proteolytic activation of prophenoloxidase (proPO) is a critical defense mechanism in insects and crustaceans. We have isolated three prophenoloxidase-activating proteinases (PAPs) from cuticular extracts or hemolymph of Manduca sexta pharate pupae, which are negatively regulated by serpin-1J and serpin-3. To test if other serpins may also inhibit the PAPs, we fractionated the induced hemolymph by ammonium sulfate precipitation, gel filtration, and lectin affinity chromatography. A 47 kDa protein, designated M. sexta serpin-6, was identified in concanavalin A-bound fractions, which formed an SDS-stable complex with PAP-3. This inhibitor, not recognized by the serpin-1 or serpin-3 antibodies, was further purified on HPLC anion exchange and hydroxylapatite columns. The molecular mass and isoelectric point of serpin-6 were found to be 46,710 +/- 10 Da and 5.4. While its amino terminus was blocked, we obtained five internal peptide sequences, one of which is highly similar to M. sexta serpins-1, -2, and -3. Serpin-6 strongly inhibited PAP-3 but not PAP-1 or PAP-2, suggesting that the proPO activation by PAPs is differentially regulated by multiple serpins. When included in the reaction mixture containing proPO, PAP-3, and its cofactor, serpin-6 efficiently blocked the cleavage activation of proPO.

Manduca sexta serpin-3 regulates prophenoloxidase activation in response to infection by inhibiting prophenoloxidase-activating proteinases

Many serine proteinase inhibitors of the serpin superfamily have evolved in vertebrates and invertebrates to regulate serine proteinase cascades that mediate the host defense responses. We have isolated an immune-responsive serpin from the tobacco hornworm, Manduca sexta. This inhibitor, M. sexta serpin-3, contains a reactive site loop strikingly similar to the proteolytic activation site in prophenoloxidase (pro-PO). Molecular cloning and sequence comparison indicate that serpin-3 is orthologous to Drosophila melanogaster serpin 27A, a regulator of melanization. M. sexta serpin-3 is constitutively present in hemolymph at a low concentration of 5-12 microg/ml and increases to 30-75 microg/ml after a microbial challenge. Recombinant serpin-3 efficiently blocks pro-PO activation in the hemolymph, and it forms SDS-stable acyl-enzyme complexes with purified pro-PO-activating proteinases (PAPs) from M. sexta. PAP-serpin-3 complexes were isolated by immunoaffinity chromatography from hemolymph activated by treatment with Micrococcus luteus. Kinetic analysis of PAP-serpin-3 association strongly suggests that serpin-3 is a physiological regulator of the pro-PO activation reaction.

Identification by subtractive suppression hybridization of bacteria-induced genes expressed in Manduca sexta fat body

Insect immune processes are mediated by programs of differential gene expression. To understand the molecular regulation of the immune response in the tobacco hornworm, Manduca sexta, the relevant subset of differentially expressed genes of interest must be identified, cloned and studied in detail. In this study, suppression subtractive hybridization, a PCR-based method for cDNA subtraction was performed to identify mRNAs from fat body of immunized larvae that are not present (or present at a low level) in control larvae. A subtracted cDNA library enriched in immune-inducible genes was constructed. Northern blot analysis of a sample of clones from our subtracted library indicated that >90% of the clones randomly selected from the subtracted library are immune inducible. Sequence analysis of 238 expressed sequence tags (ESTs) revealed that 120 ESTs, representing 54 distinct genes or gene families, had sequences identical or similar to previously characterized genes, some of which have been confirmed to be involved in innate immunity. These ESTs were categorized into seven groups, including pattern recognition proteins, serine proteinases and their inhibitors, and antimicrobial proteins. 112 ESTs, about 47.5% of the library, showed no significant similarity to any known genes. The sequences identified in this M. sexta library reflect our knowledge of insect immune strategies and may facilitate better understanding of insect immune responses.

Mamestra configurata serpin-1 homologues: cloning, localization and developmental regulation

A screen of a Mamestra configurata (bertha armyworm) midgut cDNA library identified three types of cDNA clones that resemble the Manduca sexta serpin-1 gene family. Two serpins, 1b and 1c, possess a common conserved serpin amino terminal scaffold domain but bear no similarity to any members of the M. sexta gene family within the reactive centre loop. These serpins differ from one another by only two amino acids in the reactive centre loop (S(363)-->P) and serpin signature (M(369)-->T) regions. The other member, denoted serpin-1a, is closely related to the M. sexta serpin-1Z. M. configurata serpins as a group were expressed in all insect developmental stages including eggs, larvae and adult moths. Within larvae, serpin gene expression was restricted to the early to middle instar developmental phase and mainly in the fat body and hemocytes. Stress imposed by starvation strongly induced expression in fat body and to a lesser degree in alimentary organs, nervous system and Malphigian tubules. Conversely, starvation decreased expression in hemocytes. Wounding or inoculation with bacteria did not induce serpin gene transcription but did lead to the formation of higher and lower molecular weight forms, presumably serpin-protease complexes and resultant truncated serpin, respectively. Two dimensional PAGE and western blotting analysis revealed at least 12 distinct serpins consisting primarily of neutral, but also highly acidic and basic isoforms, as well as additional high and low molecular weight immuno-reactive species. Serpins-1b/1c are the more prominent serpin isoforms and are expressed predominantly in the fat body and subsequently exported to the hemolymph as revealed by western blotting and immunolocalization. The serpin-1b/1c isoform was found only as the fully glycosylated species within the hemolymph. Hemolymph protease activity was comprised mostly of serine proteases whose overall activity increased dramatically at the onset of the molt concomitant with a sharp decline in serpin gene expression.

A bacteria-induced, intracellular serpin in granular hemocytes of Manduca sexta

Serine proteinase inhibitors from the serpin superfamily have been identified as hemolymph proteins from several groups of arthropods, including horseshoe crabs, crayfish, and insects. In the tobacco hornworm, Manduca sexta, one group of serpins present in plasma is generated by alternate exon splicing from serpin gene-1. We have identified a second serpin gene from this insect, M. sexta serpin-2. A serpin-2 DNA clone was isolated from a fifth instar larval cDNA library. The full-length cDNA is 1.5 kb long and encodes a protein of 381 amino acid residues. Amino acid sequence comparisons with other invertebrate serpins reveal approximately 25-40% identity with serpin-2. An expressed sequence tag from Bombyx mori, which is very similar to M. sexta serpin-2, was identified, and the corresponding full-length cDNA sequence was determined. This silkworm homolog of serpin-2 is 57% identical to M. sexta serpin-2. Recombinant M. sexta serpin-2 was used as an antigen to generate a rabbit polyclonal antiserum. This antiserum recognized a 43 kDa protein present in hemocytes but absent from plasma. Western and Northern blot results revealed that serpin-2 gene expression increased dramatically after larvae were injected with bacteria. In situ hybridization showed that the serpin-2 mRNA is present in granular hemocytes of immune-stimulated larvae. Serpin-2 purified from hemocytes obtained 24 h after injection of larvae with bacteria lacked inhibitory activity for all proteinases tested except for human cathepsin G. The intracellular location of serpin-2 suggests a function for serpin-2 different from the plasma serpin-1 proteins.

Distribution of serpins in the tissues of the tobacco hornworm (Manduca sexta) larvae. Existence of new serpins possibly encoded by a gene distinct from the serpin-1 gene

Polyclonal antibodies were raised against the isolated hemolymph serine proteinase inhibitors (serpins) of Manduca sexta larvae. Two of these antibodies, MsH49a and MsH49b, displayed characteristic differences in labelling patterns of hemocytes, fat body, integumental epidermis and cuticle on immunoblots, and in light- and electronmicroscopic sections. The serpin composition of the latter three tissue homogenates was determined by native immunoblots and inhibitor binding assays. The results were compared to the hemolymph samples containing all the known inhibitors encoded by the well-characterized serpin-1 gene. The enzyme specificity of the MsH49b-labelled cuticular serpin was similar to serpin-1J, although its electrophoretic mobility on native PAGE was not identical with any of the known proteinase inhibitors encoded by the serpin-1 gene. Based on these data, we suggest that the cuticle and hemolymph may contain novel serpin(s) encoded by a gene other than the serpin-1 gene. Since the serpin-1J proved to be involved in the activation pathway of the prophenoloxidase system in the hemolymph, the in vivo function of cuticular MsH49b serpin was investigated by prophenoloxidase tests in native cuticular homogenates. Our results demonstrated that the cuticular serpin(s) that are labelled by the MsH49b antibody may play a determinant role in the regulation of the prophenoloxidase system of the integumental cuticle.

Serine proteinase inhibitors in arthropod immunity

Arthropod hemolymph contains proteins with serine proteinase inhibitory activity. These inhibitors may exist in plasma or in hemocyte granules. Serine proteinase inhibitors from the Kazal, Kunitz, alpha-macroglobulin, and serpin families have been identified in arthropod hemolymph and have been characterized biochemically. Two new families of low molecular weight serine proteinase inhibitors have recently been discovered: one in silkworms (the Bombyx family) and another in locusts and a crayfish. The serine proteinase inhibitors in arthropod hemolymph are likely to function in protecting their hosts from infection by pathogens or parasites. Some may inhibit fungal or bacterial proteinases. Others probably have roles in regulating endogenous proteinases involved in coagulation, prophenol oxidase activation, or cytokine activation.

Characterization and functional analysis of 12 naturally occurring reactive site variants of serpin-1 from Manduca sexta

Serpin gene-1 from the tobacco hornworm, Manduca sexta, encodes, through alternative exon usage, 12 reactive site variants (Jiang, H., Wang, Y. and Kanost, M. R., (1994) J. Biol. Chem. 269, 55-58; Jiang, H., Wang, Y., Huang, Y., Mulnix, A. B., Kadel, J., Cole, K., and Kanost, M. R. (1996) J. Biol. Chem. 271, 28017-28023). These 43-kDa proteins differ from each other only in their COOH-terminal 39-46 residues, which include the reactive site. To test the hypothesis that these proteins are proteinase inhibitors of diverse selectivities and to begin to elucidate their physiological functions, we expressed the 12 serpin-1 variants in Escherichia coli. Seven of the variants inhibited mammalian serine proteinases, with association rate constants comparable with those of human serpins. Serpin-1A, with a P1 Arg residue, inhibited both trypsin and plasmin. Serpin-1B (P1 Ala) and serpin-1F (P1 Val) inhibited porcine pancreatic elastase and human neutrophil elastase. Serpin-1H, -1K, and -1Z, all with a Tyr residue at the P1 position, inhibited chymotrypsin and cathepsin G. Serpin-1I (P1 Leu) inhibited both elastase and chymotrypsin. Nine of the serpin variants were active as inhibitors of microbial serine proteinases, including subtilisin Carlsberg, proteinase K, and two proteinases secreted by an entomopathogenic fungus, Metarhizium anisopliae. In addition, one of the serpin variants, serpin-1J, strongly inhibited activation of M. sexta hemolymph phenoloxidase, a pathway involving a serine proteinase cascade. This pathway is a component of the defensive response of insects to microbial infection. These results suggest that the products of M. sexta serpin gene-1 may be important in regulating both exogenous and endogenous serine proteinases in hemolymph.

Organization of serpin gene-1 from Manduca sexta. Evolution of a family of alternate exons encoding the reactive site loop

Manduca sexta serpin gene-1 encodes a family of serpins whose amino acid sequences are identical in their amino-terminal 336 residues but variable in their carboxyl-terminal 39-46 residues, which includes the reactive site loop (Jiang, H., Wang, Y., and Kanost, M. R. (1994) J. Biol. Chem. 269, 55-58). Here, we report the gene's complete nucleotide sequence and exon-intron structure. A unique characteristic of this gene is its exon 9, which is present in 12 alternate forms between exons 8 and 10. Isolation and characterization of cDNA clones containing exons 9C, 9H, and 9I, which were not found previously, indicate that all 12 alternate forms of exon 9 can be utilized to generate 12 different serpins. The splicing pathway apparently allows inclusion of only one exon 9 per molecule of mature serpin-1 mRNA. Analysis of exon-intron border sequences reveals unique features that may be involved in regulation of RNA splicing. The exon 9 region has apparently evolved through rounds of exon duplication and sequence divergence. The exons near the center of the region may have evolved recently, whereas the outermost exons are the most ancient. Exons 9G and 9H were duplicated as a pair from exons 9E and 9F, an event that may have occurred more than once in the history of this gene.