An intron enhancer activates the immunoglobulin-related Hemolin gene in Hyalophora cecropia

CmHem, a hemolin-like gene identified from Cnaphalocrocis medinalis, involved in metamorphosis and baculovirus infection

Background

As a member of the immunoglobulin superfamily, hemolins play a vital role in insect development and defense against pathogens. However, the innate immune response of hemolin to baculovirus infection varies among different insects.

Methods and results

In this study, the hemolin-like gene from a Crambidae insect, Cnaphalocrocis medinalis, CmHem was cloned, and its role in insect development and baculovirus infection was analyzed. A 1,528 bp contig as potential hemolin-like gene of C. medinalis was reassembled from the transcriptome. Further, the complete hemolin sequence of C. medinalis (CmHem) was cloned and sequenced. The cDNA of CmHem was 1,515 bp in length and encoded 408 amino acids. The deduced amino acid of CmHem has relatively low identities (41.9-62.3%) to various insect hemolins. However, it contains four Ig domains similarity to other insect hemolins. The expression level of CmHem was the highest in eggs, followed by pupae and adults, and maintained a low expression level at larval stage. The synthesized siRNAs were injected into mature larvae, and the CmHem transcription decreased by 51.7%. Moreover, the abdominal somites of larvae became straightened, could not pupate normally, and then died. Infection with a baculovirus, C. medinalis granulovirus (CnmeGV), the expression levels of CmHem in the midgut and fat body of C. medinalis significantly increased at 12 and 24 h, respectively, and then soon returned to normal levels.

Conclusions

Our results suggested that hemolin may be related to the metamorphosis of C. medinalis. Exposure to baculovirus induced the phased expression of hemolin gene in the midgut and fat body of C. medinalis, indicated that hemolin involved in the immune recognition of Crambidae insects to baculovirus.

Identification and functional analysis of a Hemolin like protein from Litopenaeus vannamei

Hemolin is a specific immune protein belonging to immunoglobulin superfamily and firstly identified in insects. Growing evidences suggest that Hemolin can be activated by bacterial and viral infections and may play an important role in antimicrobial immunity. In this paper, we firstly identified a Hemolin-like protein from Litopenaeus vannamei (LvHemolin). Sequence analysis showed that LvHemolin shares high similarity with insect Hemolins and is mainly composed of seven immunoglobulin (Ig) domains which form a 'horseshoe' tertiary structure. Tissue distribution analysis demonstrated that LvHemolin mainly expressed in stomach, gill, epithelium and pyloric cecum of L. vannamei. After challenge with pathogens or stimulants, expression of LvHemolin was significantly up-regulated in both gill and stomach. Agglutination analysis demonstrated that recombinant LvHemolin protein purified from Escherichia coli could accelerate the agglutination of Vibrio parahaemolyticus, E. coli, Staphylococcus aureus, and Bacillus subtilis in the presence of Ca(2+). To verify the immune function of LvHemolin in vivo, shrimps were injected with gene-specific dsRNA, followed by challenge with white spot syndrome virus (WSSV) or V. parahaemolyticus. The results revealed that silence of LvHemolin could increase the cumulative mortalities of shrimps challenged by pathogens and increase the WSSV copies in shrimp tissues. These suggested that Hemolin could play an important role in shrimp innate immune defense against bacterial and viral infections.

Immunity in lepidopteran insects

Lepidopteran insects provide important model systems for innate immunity of insects, particularly for cell biology of hemocytes and biochemical analyses of plasma proteins. Caterpillars are also among the most serious agricultural pests, and understanding of their immune systems has potential practical significance. An early response to infection in lepidopteran larvae is the activation of hemocyte adhesion, leading to phagocytosis, nodule formation, or encapsulation. Plasmatocytes and granular cells are the hemocyte types involved in these responses. Infectious microorganisms are recognized by binding of hemolymph plasma proteins to microbial surface components. This "pattern recognition" triggers phagocytosis and nodule formation, activation of prophenoloxidase and melanization and the synthesis of antimicrobial proteins that are secreted into the hemolymph. Many hemolymph proteins that function in such innate immune responses of insects were first discovered in lepidopterans. Microbial proteinases and nucleic acids released from lysed host cells may also activate lepidopteran immune responses. Hemolymph antimicrobial peptides and proteins can reach high concentrations and may have activity against a broad spectrum of microorganisms, contributing significantly to clearing of infections. Serine proteinase cascade pathways triggered by microbial components interacting with pattern recognition proteins stimulate activation of the cytokine Spätzle, which initiates the Toll pathway for expression of antimicrobial peptides. A proteinase cascade also results inproteolytic activation of phenoloxidase and production of melanin coatings that trap and kill parasites and pathogens. The proteinases in hemolymph are regulated by specific inhibitors, including members of the serpin superfamily. New developments in lepidopteran functional genomics should lead to much more complete understanding of the immune systems of this insect group.

A novel association between clustered NF-kappaB and C/EBP binding sites is required for immune regulation of mosquito Defensin genes

A comparative analysis identified key cis-acting regulatory elements responsible for the temporal control of mosquito Defensin gene expression. The promoters of Anopheles gambiae Defensin 1 and two isoforms of Aedes aegypti Defensin A are up-regulated by immune challenge. This stimulated activity depends upon a cluster of three NF-kappaB binding sites and closely associated C/EBP-like motifs, which function as a unit for optimal promoter activity. Binding of NF-kappaB and C/EBP like transcription factors is confirmed by electrophoretic mobility shift assay, including supershifts with antibodies to C/EBP. KappaB-like motifs are abundant within antimicrobial peptide gene promoters and most are very closely associated with putative C/EBP binding sites. This novel association between NF-kappaB and C/EBP binding sites may, therefore, be of widespread significance.

20-Hydroxyecdysone indirectly regulates Hemolin gene expression in Hyalophora cecropia

Development and innate immune defence are two vital processes that have been demonstrated to use the same or similar molecules and signalling pathways in insects. Hemolin is a moth haemolymph protein belonging to the immunoglobulin superfamily. It is strongly induced upon bacterial infection. However, recent studies indicate a developmental regulation of hemolin. We show that the steroid hormone 20-hydroxyecdysone (20E) can activate the expression of Hyalophora cecropia Hemolin (HcHemolin) in the fat body of diapausing pupae. Using the protein synthesis inhibitor cycloheximide we demonstrate that Hemolin up-regulation by 20E requires ongoing protein synthesis. Moreover, 20E enhances transcription of the Hemolin gene in response to bacteria. Comparing the upstream regions of Manduca sexta Hemolin (MsHemolin) and HcHemolin, we identified four putative regulatory sites. Two are putative hormone response elements (HREs), one with an imperfect inverted repeat (HRE-IR) and one with a monomeric site (HRE-M). An additional monomeric hormone receptor site (MRE) is present only in HcHemolin. The third conserved motif is similar to the interferon (IFN) regulatory factor binding element (IRF-E) and IFN-stimulated response element (ISRE). The fourth conserved element is a kappaB motif situated between the Cap-site and the TATA-box. Finally, by electrophoresis mobility shift assay we demonstrate that the HRE-IR forms specific complexes with nuclear extract proteins of normal pupae that increase after 20E stimulation.

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.

Baculovirus and dsRNA induce Hemolin, but no antibacterial activity, in Antheraea pernyi

Hemolin is one of the haemolymph proteins most strongly induced upon bacterial infection in Lepidoptera. When we applied RNA interference (RNAi) to suppress Hemolin expression in the Chinese oak silk moth Antheraea pernyi, we discovered that Hemolin is induced by double-stranded RNA (dsRNA) per se. As dsRNA is recognized as a virus pattern molecule, we then investigated the effect of a baculovirus (ApNPV) infection. We found that Hemolin is induced and expressed with similar kinetics as upon dsRNA injection. Notably, no Attacin gene expression or antibacterial activity was recorded. When baculovirus and high amounts of dsRNA were coinjected, the viral symptoms appeared earlier with Hemolin dsRNA than with GFP dsRNA. This indicates that silencing of hemolin affected the progress of the viral infection.