Trichoplusia ni attacin A, a differentially displayed insect gene coding for an antibacterial protein

Insect antimicrobial peptides: potential weapons to counteract the antibiotic resistance

Misuse and overuse of antibiotics have contributed in the last decades to a phenomenon known as antibiotic resistance which is currently considered one of the principal threats to global public health by the World Health Organization. The aim to find alternative drugs has been demonstrated as a real challenge. Thanks to their biodiversity, insects represent the largest class of organisms in the animal kingdom. The humoral immune response includes the production of antimicrobial peptides (AMPs) that are released into the insect hemolymph after microbial infection. In this review, we have focused on insect immune responses, particularly on AMP characteristics, their mechanism of action and applications, especially in the biomedical field. Furthermore, we discuss the Toll, Imd, and JAK-STAT pathways that activate genes encoding for the expression of AMPs. Moreover, we focused on strategies to improve insect peptides stability against proteolytic susceptibility such as D-amino acid substitutions, N-terminus modification, cyclization and dimerization.

Insect antimicrobial peptides and their applications

Insects are one of the major sources of antimicrobial peptides/proteins (AMPs). Since observation of antimicrobial activity in the hemolymph of pupae from the giant silk moths Samia Cynthia and Hyalophora cecropia in 1974 and purification of first insect AMP (cecropin) from H. cecropia pupae in 1980, over 150 insect AMPs have been purified or identified. Most insect AMPs are small and cationic, and they show activities against bacteria and/or fungi, as well as some parasites and viruses. Insect AMPs can be classified into four families based on their structures or unique sequences: the α-helical peptides (cecropin and moricin), cysteine-rich peptides (insect defensin and drosomycin), proline-rich peptides (apidaecin, drosocin, and lebocin), and glycine-rich peptides/proteins (attacin and gloverin). Among insect AMPs, defensins, cecropins, proline-rich peptides, and attacins are common, while gloverins and moricins have been identified only in Lepidoptera. Most active AMPs are small peptides of 20-50 residues, which are generated from larger inactive precursor proteins or pro-proteins, but gloverins (~14 kDa) and attacins (~20 kDa) are large antimicrobial proteins. In this mini-review, we will discuss current knowledge and recent progress in several classes of insect AMPs, including insect defensins, cecropins, attacins, lebocins and other proline-rich peptides, gloverins, and moricins, with a focus on structural-functional relationships and their potential applications.

Enhanced antibacterial activity of an attacin-coleoptericin hybrid protein fused with a helical linker

Previously, we isolated and characterized attacin from Spodoptera exigua and a coleoptericin-like protein from Protaetia brevitarsis seulensis. In this study, we fused these two genes encoding antimicrobial proteins to obtain a hybrid protein with enhanced antimicrobial activity. To fuse the two antimicrobial proteins, we employed helical and non-helical linker sequences that function as inter-domain linkers in proteins. We used the Gly-Gly-Gly-Gly-Ser peptide as a non-helical linker. The hybrid protein produced using this linker showed less antimicrobial activity against Escherichia coli, Bacillus subtilis, Burkholderia glumae, Pseudomonas corrugate, and Erwinia rhapontici than either of the two parental antimicrobial proteins. In addition, the MIC value of the hybrid protein was 23.1 μM, which indicates poor activity against E. coli. When we used three Glu-Ala-Ala-Ala-Lys (EAAAK) peptide sequences as a helical linker to fuse the two proteins, the resultant hybrid protein had much higher antimicrobial activity than the parental antimicrobial proteins. In particular, this hybrid protein had strong antimicrobial activity against P. corrugate. These results indicate that the EAAAK motif can be used to effectively separate two antimicrobial proteins and produce a hybrid protein with more antimicrobial activity than either of the parent proteins.

Characterization and expression of attacin, an antibacterial protein-encoding gene, from the beet armyworm, Spodoptera exigua (Hübner) (Insecta: Lepidoptera: Noctuidae)

To isolate antimicrobial-related genes from the beet armyworm, Spodoptera exigua, we performed GeneFishing, a polymerase chain reaction (PCR)-based differential display technique. An attacin-like complementary DNA (cDNA) including a 3'-untranslated region was identified from among 18 over-expressed genes in microbial-infected larvae. The full-length attacin cDNA from S. exigua cDNA (Seattacin) was cloned using rapid amplification of cDNA ends PCR. The attacin-like cDNA transcript was 765 nucleotides in length, and the predicted polypeptide was 254 amino acids in length with a calculated molecular mass of 27.6 kDa and an isoelectric point of 6.44. The protein sequence of the attacin-like cDNA showed high identity to that of Trichoplusia ni (61.2%). The amino acid sequence identity of Seattacin to the orthologous proteins in Bombyx mori, Manduca sexta, Heliothis virescens, Hlicoverpa armigera, Hyphantria cunea, Hyalophora cecropia, and Drosophila melanogaster was 61.2, 46.1, 44.5, 42.2, 39.5, 45.1, and 24.0%, respectively. To examine possible immune functions of the attacin-like cDNA, its expression was investigated by reverse transcriptase PCR analysis after challenging S. exigua with microorganisms. The attacin-like cDNA was expressed at high levels 12 h post-infection, and its expression was slightly induced 4-8 h post-infection compared to control larvae inoculated with sterile water. Furthermore, induced Seattacin showed biological activity against several bacteria including Escherichia coli DH5α, Pseudomonas cichorii, Bacillus subtilis, and Listeria monocytogenes. These results suggest that the attacin-like cDNA of S. exigua codes for antimicrobial peptides.

Lysozymes and lysozyme-like proteins from the fall armyworm, Spodoptera frugiperda

Lysozyme is an important component of the insect non-specific immune response against bacteria that is characterized by its ability to break down bacterial cell-walls. By searching an EST database from the fall armyworm, Spodoptera frugiperda (Negre et al., 2006), we identified five sequences encoding proteins of the lysozyme family. The deduced protein sequences corresponded to three classical c-type lysozymes Sf-Lys1, Sf-Lys2 and Sf-Lys3, and two lysozyme-like proteins, Sf-LLP1 and Sf-LLP2. Sf-Lys1 was purified from the hemolymph of Escherichia coli-challenged S. frugiperda larvae. The mature protein had a molecular mass of 13.975 Da with an isoelectric point of 8.77 and showed 98.3% and 96.7% identity with lysozymes from Spodoptera litura and Spodoptera exigua, respectively. As the other insect lysozymes, Sf-Lys1 was active against gram positive bacteria such as Micrococcus luteus but also induced a slight permeabilization of the inner membrane of E. coli. Genes encoding these five Sf-Lys or Sf-LLPs were differentially up-regulated in three immune-competent tissues (hemocytes, fat body and gut) after challenges with non-pathogenic bacteria, E. coli and M. luteus, or entomopathogenic bacterium, Photorhabdus luminescens. Sf-Lys1 and Sf-Lys2 were mainly induced in fat body in the presence of E. coli or P. luminescens. Sf-Lys3, which had an acidic isoelectric point, was found to be the most up-regulated of all five Sf-Lys or Sf-LLPs in hemocytes and gut after challenge with P. luminescens. More molecular data are now available to investigate differences in physiological functions of these different members of the lysozyme superfamily.

Molecular cloning, expression in Escherichia coli of Attacin A gene from Drosophila and detection of biological activity

Attacin, a 20 kDa antibacterial peptide, plays an important role in immunity. To understand this gene better, gene cloning, expression and biological activity detection of Attacin A was carried out in present study. The full-length open reading frame (ORF) coding for Attacin A gene was generated using RT-PCR which takes total RNA extracted from Drosophila as the template. The gene was inserted directionally into the prokaryotic expression vector pET-32a (+). The resulting recombinant plasmid was transformed into E. coli Rosetta. SDS-PAGE was carried out to detect the expression product which was induced by IPTG. The antimicrobial activity and hemolysis activity were tested in vitro after purification. Agarose gel electrophoresis indicated that the complete ORF of Attacin A gene has been cloned successfully from Drosophila stimulated by E. coli which includes 666 bp and encodes 221 AA. The gene encoding mature Attacin A protein was amplified by PCR from the recombinant plasmid containing Attacin A, which includes 570 bp in all. SDS-PAGE analysis demonstrated that the fusion protein expressed was approximately 39.2 kDa. Biological activities detection showed that this peptide exhibited certain antibacterial activity to several G- bacteria, as well as minor hemolysis activity for porcine red blood cells. In conclusion, Attacin A gene was cloned and expressed successfully. It was the basis for further study of Attacin.

Detection of genes encoding antimicrobial peptides in Mexican strains of Trichoplusia ni (Hübner) exposed to Bacillus thuringiensis

The systemic immune response of Trichoplusia ni after Bacillus thuringiensis (Bt) exposure was evaluated by comparing the expression of genes encoding antimicrobial peptides (AMPs) in Bt-susceptible and -resistant T. ni strains that were either exposed or not to XenTari (Bt-XT). AMP genes were detected by RT-PCR using primers for attacin, gloverin, lebocin, lysozyme, and peptidoglycan recognition peptide (PGRP). In general, AMP genes were detected more frequently in Mexican field strains previously exposed to Bt (SALX and GTOX) than in a Mexican laboratory strain (NL), but expression was similar to the AMP expression in USA laboratory strains (US and USX). Among the AMPs, transcripts for lebocin were the least detected (11.7%) and those for lysozyme were the most detected (84.8%) in all samples. Lebocin was detected only in 2nd instar and pupa. All untreated controls expressed attacin. Attacin and gloverin were not detected in any midgut sample, and their highest detection was in pupa. Lysozyme was rarely detected in 2nd instar larvae from any strain or treatment but was detected in almost all midgut and hemolymph samples. Overall, AMPs were found more in T. ni strains previously exposed to Bt-XT, especially lebocin and globerin (1.8-fold increase) and PGRP (3.8-fold increase). The data suggest that the expression of AMPs in T. ni correlates to previous Bt exposure.

A protein from the cabbage looper, Trichoplusia ni, regulated by a bacterial infection is homologous to 3-dehydroecdysone 3beta-reductase

During the screening of immune-regulated genes from the cabbage looper, Trichoplusia ni, a 3-dehydroecdysone 3beta-reductase homologue (DERH) was cloned. In the course of development, 3-dehydroecdysone 3beta-reductase mediates the conversion of 3-dehydroecdysone (3dE) secreted from the prothoracic glands to ecdysone (E), which is subsequently converted to 20-hydroxyecdysone (20E), the major insect molting hormone. The cloned gene is upregulated in fat body during development and is strongly induced after the larva is challenged with bacteria. The gene codes for a 308 amino acid residue protein which shows 42.5% identity to Spodoptera littoralis 3-dehydroecdysone 3beta-reductase. Using the baculovirus expression system, the recombinant DERH was expressed. The purified protein mediates the reduction of 3-dehydromakisterone A to makisterone A, and requires NADPH as a cofactor. Western blots using an antiserum to T. ni DERH revealed the presence of the protein in larval hemolymph and integument. The data indicate that the protein is regulated developmentally and is induced after a challenge with bacteria. Immunohistochemical studies localized the enzyme exclusively in the epidermis and the cuticle.

Protein purification, cDNA cloning and gene expression of attacin, an antibacterial protein, from eri-silkworm, Samia cynthia ricini

Attacin was isolated from immunized larval hemolymph of the wild silkmoth, Samia cynthia ricini. The antibacterial effect of the attacin was limited to some species of Gram-negative bacteria. Two cDNA clones encoding attacin A and B, respectively, were isolated by screening the cDNA library from immunized fat body. The two cDNAs encoded the same length of precursor protein with 233 amino acid residues. The 46-residue prepropeptides of the two attacins were identical to each other, but 4 out of 187 residues of the mature proteins were different in each other. The two attacins show 98% identity at the amino acid level, while 92% identity at the nucleotide level. Both of the mature proteins were highly homologous to cecropia basic attacin with identity of 96%. The attacin transcripts were detected at significant level in fat body, hemocytes and Malpighian tube after injection with peptidoglycan, but not in the midgut and the silkgland. The induction of attacin gene expression was elicited most effectively by peptidoglycan and UV-killed bacteria in the fat body.

Trichoplusia ni gloverin, an inducible immune gene encoding an antibacterial insect protein

By using differential display PCR, we obtained a cDNA clone encoding a gloverin homologue from the cabbage looper, Trichoplusia ni. The expression of the gene was induced by bacterial infections. The gene codes for a 174 amino acid residue protein, including a signal sequence and a prosegment. The deduced mature protein is 14 kDa and shows 58% and 49% identity to P2 from Helicoverpa armigera and to Hyalophora gloveri gloverin, respectively. The protein was detected in hemolymph and hemocytes from bacteria-immunized animals. We expressed gloverin using the baculovirus expression system. N-terminal amino acid sequence analysis showed that the purified protein contained a propart. This progloverin inhibited the growth of E. coli and the activity is comparable to that of H. gloveri mature gloverin. Processing of progloverin was possible in vitro, using human furin.

Trichoplusia ni lebocin, an inducible immune gene with a downstream insertion element

A cDNA clone encoding a lebocin-like protein was obtained from the cabbage looper Trichoplusia ni by using differential display PCR. Northern blot analysis showed that lebocin gene expression was inducible upon bacterial challenge. Transcripts were mainly found in fat body but were also observed in hemocytes. Expression reached its highest level at 20 h and continued at least until 60 h after bacterial injection. The deduced protein is proline-rich and contains 143 amino acid residues. At position 128, a possible O-glycosylation site is observed. The whole protein shows 35% identity to Bombyx mori lebocin. The mature peptide displays an N-terminus similar to that of lebocin and a C-terminus to that of Drosophila metchnikowin. A 39-bp repetitive element is located downstream of the coding region.

A peptidoglycan recognition protein in innate immunity conserved from insects to humans

Innate nonself recognition must rely on common structures of invading microbes. In a differential display screen for up-regulated immune genes in the moth Trichoplusia ni we have found mechanisms for recognition of bacterial cell wall fragments. One bacteria-induced gene encodes a protein that, after expression in the baculovirus system, was shown to be a peptidoglycan recognition protein (PGRP). It binds strongly to Gram-positive bacteria. We have also cloned the corresponding cDNA from mouse and human and shown this gene to be expressed in a variety of organs, notably organs of the immune system-i.e., bone marrow and spleen. In addition, purified recombinant murine PGRP was shown to possess peptidoglycan affinity. From our results and the sequence homology, we conclude that PGRP is a ubiquitous protein involved in innate immunity, conserved from insects to humans.

PCR differential display of immune gene expression in Trichoplusia ni

The immune state of insects is defined by a set of proteins that is absent in the naive state. To explore the immune system of Trichoplusia ni in more detail we have employed a PCR differential display technique to compare the mRNA population of untreated last instar larvae to that of immunized animals. In the primary display, more than one hundred bands seemed induced upon bacterial challenge. When they were used as probes in Northern blots, 35% of these probes detected inducible mRNA species. Such probes were used to screen a cDNA library from immunized larvae. We isolated clones for T. ni homologs of cecropin A, lysozyme and attacin. One differentially expressed band hybridized to clones for BJHSP1, a hemacy-anin-related protein which is hormonally up-regulated in last instar larvae; this induction is probably not related to the bacterial infection. Still other probes recognized inducible mRNAs of 1.6 and 1.0 kb. The corresponding cDNA clones did not show strong sequence homology to any known proteins. We have demonstrated the potential of this PCR technique to display both known and unknown genes specific for the immune state of whole insects against a background of genes involved in larval development.