Characterization and functional recovery of a novel antimicrobial peptide (CECdir-CECret) from inclusion bodies after expression in Escherichia coli

Rapid and efficient production of cecropin A antibacterial peptide in Escherichia coli by fusion with a self-aggregating protein

BACKGROUND

Cecropin A (CeA), a natural cationic antimicrobial peptide, exerts potent antimicrobial activity against a broad spectrum of Gram-positive and Gram-negative bacteria, making it an attractive candidate substitute for antimicrobials. However, the low production rate and cumbersome, expensive processes required for both its recombinant and chemical synthesis have seriously hindered the exploitation and application of CeA. Here, we utilized a short β-structured self-aggregating protein, ELK16, as a fusion partner of CeA, which allowed the efficient production of high-purity CeA antibacterial peptide with a simple inexpensive process.

RESULTS

In this study, three different approaches to the production of CeA peptide were investigated: an affinity tag (His-tag)-fused protein expression system (AT-HIS system), a cell-free protein expression system (CF system), and a self-assembling peptide (ELK16)-fused protein expression system (SA-ELK16 system). In the AT-HIS and CF systems, the CeA peptide was obtained with purities of 92.1% and 90.4%, respectively, using one or more affinity-chromatographic purification steps. The procedures were tedious and costly, with CeA yields of only 0.41 and 0.93 μg/mg wet cell weight, respectively. Surprisingly, in the SA-ELK16 system, about 6.2 μg/mg wet cell weight of high-purity (approximately 99.8%) CeA peptide was obtained with a simple low-cost process including steps such as centrifugation and acetic acid treatment. An antimicrobial test showed that the high-purity CeA produced in this study had the same antimicrobial activity as synthetic CeA peptide.

CONCLUSIONS

In this study, we designed a suitable expression system (SA-ELK16 system) for the production of the antibacterial peptide CeA and compared it with two other protein expression systems. A high yield of high-purity CeA peptide was obtained with the SA-ELK16 system, which greatly reduced the cost and time required for downstream processing. This system may provide a platform for the laboratory scale production of the CeA antibacterial peptide.

Identification of Peptides in Flowers of Sambucus nigra with Antimicrobial Activity against Aquaculture Pathogens

The elder (Sambucus spp.) tree has a number of uses in traditional medicine. Previous studies have demonstrated the antimicrobial properties of elderberry liquid extract against human pathogenic bacteria and also influenza viruses. These properties have been mainly attributed to phenolic compounds. However, other plant defense molecules, such as antimicrobial peptides (AMPs), may be present. Here, we studied peptide extracts from flowers of Sambucus nigra L. The mass spectrometry analyses determined peptides of 3 to 3.6 kDa, among them, cysteine-rich peptides were identified with antimicrobial activity against various Gram-negative bacteria, including recurrent pathogens of Chilean aquaculture. In addition, membrane blebbing on the bacterial surface after exposure to the cyclotide was visualized by SEM microscopy and SYTOX Green permeabilization assay showed the ability to disrupt the bacterial membrane. We postulate that these peptides exert their action by destroying the bacterial membrane.

Chemical Synthesis and Functional Analysis of VarvA Cyclotide

Cyclotides are circular peptides found in various plant families. A cyclized backbone, together with multiple disulfide bonds, confers the peptides’ exceptional stability against protease digestion and thermal denaturation. In addition, the features of these antimicrobial molecules make them suitable for use in animal farming, such as aquaculture. Fmoc solid phase peptide synthesis on 2-chlorotrityl chlorine (CTC) resin using the “tea-bag” approach was conducted to generate the VarvA cyclotide identified previously from Viola arvensis. MALDI-TOF mass spectrometry determined the correct peptide amino acid sequence and the cyclization sites-critical in this multicyclic compound. The cyclotide showed antimicrobial activity against various Gram-negative bacteria, including recurrent pathogens present in Chilean aquaculture. The highest antimicrobial activity was found to be against Flavobacterium psychrophilum. In addition, membrane blebbing on the bacterial surface after exposure to the cyclotide was visualized by SEM microscopy and the Sytox Green permeabilization assay showed the ability to disrupt the bacterial membrane. We postulate that this compound can be proposed for the control of fish farming infections.

Synthetic hepcidin from fish: Uptake and protection against Vibrio anguillarum in sea bass (Dicentrarchus labrax)

The generation of a variety of new therapeutic agents to control and reduce the effects of pathogen in aquaculture is urgently needed. The antimicrobial peptides (AMPs) are one of the major components of the innate defenses and typically have broad-spectrum antimicrobial activity. However, absorption and distributions of exogenous AMPs for therapeutics application on farmed fish species need to be studied. Previous studies in our laboratory have shown the properties of hepcidin as an effective antimicrobial peptide produced in fish in response to LPS and iron. Therefore, we decided to investigate the antimicrobial activity of four synthetic variants of hepcidin against Vibrio anguillarum in vitro, and using the more effective peptide we demonstrated the pathogen's ability to protect against the infection in European Sea bass. Additionally the uptake of this peptide after ip injection was demonstrated, reaching its distribution organs such as intestine, head kidney, spleen and liver. The synthetic peptide did not show cytotoxic effects and significantly reduced the accumulated mortalities percentage (23.5%) compared to the European Sea bass control (72.5%) at day 21. In conclusion, synthetic hepcidin shows antimicrobial activity against V. anguillarum and the in vivo experiments suggest that synthetic hepcidin was distributed trough the different organs in the fish. Thus, synthetic hepcidin antimicrobial peptide could have high potential for therapeutic application in farmed fish species.

Antimicrobial activity of trout hepcidin

Hepcidin is an antimicrobial peptide and a hormone produced mostly the liver. It is a cysteine-rich peptide with a highly conserved β-sheet structure. Recently, we described the hepcidin expression in liver of rainbow trout and its inducibility by iron overloading and lipopolysaccharide (LPS). Thus, in this work, we focused in analyzing the importance of the peptide conformation associated to its oxidative state in the antimicrobial activity. This peptide showed a α-helix conformation in reduced state and the characteristic β-sheet conformation in the oxidized state. Antimicrobial activity assays showed that the oxidized peptide is more effective than the reduced peptide against Escherichia coli and the important salmon fish pathogen Piscirickettsia salmonis. In addition, confocal analysis of P. salmonis culture exposed to trout hepcidin coupled with rhodamine revealed the intracellular location of this peptide and Sytox permeation assay showed that membrane disruption is not the mechanism of its antimicrobial action. Moreover, a conserved ATCUN motif was detected in the N-terminus of this peptide. This sequence has been described as a small metal-binding site that has been implicated in DNA cleavage. In this work we proved that this peptide is able to induce DNA hydrolysis in the presence of ascorbate and CuCl2. When the same experiments were carried out using a variant with truncated N-terminus no DNA hydrolysis was observed. Our results suggest that correct folding of hepcidin is required for its antimicrobial activity and most likely the metal-binding site (ATCUN motif) present in its N-terminus is involved in the oxidative damage to macromolecules.

Transmission electron microscopic morphological study and flow cytometric viability assessment of Acinetobacter baumannii susceptible to Musca domestica cecropin

Multidrug-resistant (MDR) Acinetobacter baumannii infections are difficult to treat owing to the extremely limited armamentarium. Expectations about antimicrobial peptides' use as new powerful antibacterial agents have been raised on the basis of their unique mechanism of action. Musca domestica cecropin (Mdc), a novel antimicrobial peptide from the larvae of Housefly (Musca domestica), has potently active against Gram-positive and Gram-negative bacteria standard strain. Here we evaluated the antibacterial activity of Mdc against clinical isolates of MDR-A. baumannii and elucidate the related antibacterial mechanisms. The minimal inhibitory concentration (MIC) of Mdc was 4 μg/mL. Bactericidal kinetics of Mdc revealed rapid killing of A. baumannii (30 min). Flow cytometry using viability stain demonstrated that Mdc causes A. baumannii membrane permeabilization in a concentration- and time-dependent process, which correlates with the bactericidal action. Moreover, transmission electron microscopic (TEM) examination showed that Mdc is capable of disrupting the membrane of bacterial cells, resulting in efflux of essential cytoplasmic components. Overall, Mdc could be a promising antibacterial agent for MDR-A. baumannii infections.

Inhibitory effect of short cationic homopeptides against gram-positive bacteria

In the selection or design of antimicrobial peptides, the key role played by cationic amino acids and chain length on the inhibitory potency and specificity is not clear. A fundamental study was conducted using chemically synthesized homopeptides of L-Lys and L-Arg ranging from 7 to 14 residues. Their effect on growth inhibition was evaluated over a wide range of Gram-positive bacteria at different levels of concentration. Interestingly, at lower concentrations (10 μM), Lys homopeptides with odd number of residues, especially with 11 residues, showed a broader inhibitory activity than those with even number of residues. At higher peptide concentrations (>20 μM), the inhibitory activity of Lys homopeptides was directly related to the number of residues in the chain. In contrast, Arg homopeptides, at lower concentrations, did not exhibit a defined pattern of bacterial inhibition related to the number of residues; however, at higher concentrations (>20 μM), the inhibitory effects were more pronounced. Lys homopeptides at concentrations up to 300 μM showed a remarkably lower toxicity against CHSE-214 cells. Arg homopeptides exhibited negligible cytotoxicity up to chain length of 11 residues at concentrations lower than 100 μM, but an abrupt increase in toxicity resulted when the peptide chain length reached 12 amino acid residues and higher concentrations. All synthesized homopeptides displayed characteristic polyproline II helix conformation in both buffer and liposomes, as shown by CD spectroscopy. This result suggests that short Lys homopeptides with an odd number of residues (9 and 11) have a broad spectrum of activity against Gram-positive bacterial cells compared with Arg homopeptides, which in turn showed a considerably higher selectivity toward those cells. By investigating the differences between Lys and Arg homopeptides, this study contributes to the understanding of their mechanism of growth inhibition and selectivity. Thus, it provides further guidelines for a rational design of short antimicrobial peptides.

A natural peptide and its variants derived from the processing of infectious pancreatic necrosis virus (IPNV) displaying enhanced antimicrobial activity: a novel alternative for the control of bacterial diseases

The larger segment of the infectious pancreatic necrosis virus (IPNV) codifies most of the structural and non-structural proteins of the virus in two overlapping open reading frames (ORFs). The longer of the two ORF is expressed as a polyprotein which generates a number of variable length peptides of unknown function during processing. Since an appealing hypothesis would be that these peptides are generated by the virus to act as antimicrobial agents that favor viral infectivity in their fish host, we decided to test this possibility by selecting a master peptide and using it to generate substitution variants that may enhance their antimicrobial potential. A 20-residue master peptide (p20) was selected from the well-described maturation process of the structural viral protein VP2; several variants were then designed and chemically synthesized, ranging in size from 16 to 20 residues. The synthesized peptides were tested for in vitro activity against several prototype bacterial pathogens using standardized laboratory procedures. Chemically synthesized p20 and all its variants displayed broad activity against the tested bacteria and none of them were toxic to eukaryotic cells at least 10× the concentration used against the bacteria. Interestingly, when p20 was tested against the very aggressive bacterial pathogen Piscirickettsia salmonis, a common co-infectant of IPNV in salmonid fish, the specific activity of the novel peptide was significantly higher than that displayed for bactericidal fish farm antibiotics such as oxolinic acid, flumequine and florfenicol, which are commonly used to control Piscirickettsiosis in the field. It is potentially significant that the approach presented in this report provides a novel alternative for generating new and ideally more efficient and friendly safeguards for bacterial prophylaxis.

Choosing anti-Plasmodium molecules for genetically modifying mosquitoes: focus on peptides

In the wake of the development of insecticide resistance in mosquitoes, novel strategies for halting malaria transmission are being developed. These include the genetic modification (GM) of mosquitoes to become incompetent vectors. Although mosquito GM technologies are progressing rapidly, the rationale behind choosing anti-parasite molecules to be expressed by mosquitoes has received less attention. Here, questions are explored that that should be addressed during the strategic selection of these anti-Plasmodium molecules, focusing on antimicrobial peptides. Properties that will enhance the likelihood of success are discussed, and the need to plan an initial strategy to eliminate molecules that cause fitness costs to the mosquito is considered. Effector molecules with proven anti-sporogonic stage activity are reviewed, and the activity of a selection of these molecules is detailed.

Expression of the antimicrobial peptide cecropin fused with human lysozyme in Escherichia coli

Lysozyme is an abundant, cationic antimicrobial protein that plays an important role in host defense. It targets the beta (1-4) glycosidic bond between N-acetylglucosamine and N-acetylmuramic residues that make up peptidoglycan, making lysozyme highly active against Gram-positive bacteria. However, lysozyme alone is inactive against Gram-negative bacteria because it cannot reach the peptidoglycan layer. Cecropins are cationic molecules with a wide range of antimicrobial activities. The main target for these peptides is the cytoplasmic membrane. We resume that cecopin may disrupt the outer membrane, giving the enzyme access to the peptidoglycan in cell wall. So in the present study, novel hybrid protein combining Musca domestica cecropin (Mdc) with human lysozyme (Hly) was designed. The DNA sequence encoding recombination fusion protein Mdc-hly was cloned into the pET-32a vector for protein expression in Escherichia coli strain BL21 (DE3). The protein was expressed as a His-tagged fusion protein, and the Mdc-hly was released from the fusion by enterokinase cleavage and separated from the carrier thioredoxin. Antimicrobial activity assays showed that the recombinant fusion protein Mdc-hly has improved in vitro antimicrobial activity and action spectrum compared to Mdc and hly. Mdc-hly may have important potential application as a future safely administered human drug and food additive.

Generation and characterization of the antibacterial activity of a novel hybrid antimicrobial peptide comprising functional domains from different insect cecropins

The search for new antimicrobial compounds involves finding novel sources of chemotherapeutic compounds or manipulating and combining structures from existing molecules. Small antimicrobial peptides (AMPs) are components of innate immune defenses characterized in greatest detail in insect-derived AMPs. We have generated hybrid AMPs (hAMPs) by combining functional motifs from different insect AMPs as a proof of principle that we can generate molecules with lower minimum inhibitory concentrations, and with different activity and target specificity than either parent molecule. A two-helix, cecropin-like hAMP was created by linking the N-terminal alpha helix of cecropin A from Aedes aegypti to the C-terminal alpha helix of cecropin A1 from Drosophila melanogaster. This molecule exhibits antibacterial activity at sub-micromolar concentrations with a target specificity that differs from either parent molecule. Antibacterial activity of the hybrid molecule was found to be greater against Gram-negative than Gram-positive bacteria. No hemolysis was observed in sheep red blood cells exposed to concentrations up to 50 micromol/L, suggesting the peptide is not detrimental to eukaryotic cells.

Novel expression vector for secretion of cecropin AD in Bacillus subtilis with enhanced antimicrobial activity

Cecropin AD, a chimeric antimicrobial peptide obtained from cecropins, is effective at killing specific microorganisms. However, a highly efficient expression system is still needed to allow for commercial application of cecropin AD. For the exogenous expression of cecropin AD, we fused the cecropin AD gene with a small ubiquitin-like modifier (SUMO) gene and a signal peptide of SacB, while a Bacillus subtilis expression system was constructed based on Bacillus subtilis cells genetically modified by the introduction of an operon including an isopropyl-beta-D-thiogalactopyranoside (IPTG)-inducible Spac promoter, a signal peptide of amyQ, and the SUMO protease gene. The recombinant cecropin AD was expressed, and 30.6 mg of pure recombinant protein was purified from 1 liter of culture supernatant. The purified cecropin AD displayed antimicrobial activity against some pathogens, such as Staphylococcus aureus and Escherichia coli, and was especially effective toward Staphylococcus aureus, with MICs of <0.05 microM (0.2 microg/ml). Stability analysis results showed that the activity of cecropin AD was not influenced by temperatures as high as 55 degrees C for 20 min; however, temperatures above 85 degrees C (for 20 min) decreased the antimicrobial activity of cecropin AD. Varying the pH from 4.0 to 9.0 did not appear to affect the activity of cecropin AD, but some loss of potency was observed at pH values lower than pH 4.0. Under the challenge of several proteases (proteinase K, trypsin, and pepsin), cecropin AD maintained functional activity. The results indicated that the recombinant product expressed by the designed Bacillus subtilis expression system was a potent antimicrobial agent and could be applied to control infectious diseases of farm animals or even humans.

A novel antifungal peptide designed from the primary structure of a natural antimicrobial peptide purified from Argopecten purpuratus hemocytes

We have isolated and purified a natural antimicrobial peptide from Argopecten purpuratus hemocytes. 47 residues were determined from its primary structure representing the N-terminal of the complete sequence. This peptide of 5100.78Da was chemically synthesized and named Ap. The peptide has 25% of hydrophobic amino acids with a net charge of +1, and partial homology with known active antimicrobial peptides. Based on that sequence, a new peptide was designed and modeled to increase hydrophobicity and cationicity. The designed 30-residue peptide was chemically synthesized resulting in a novel 38% hydrophobic molecule named peptide Ap-S, with a net charge of +5 and 3028Da. A secondary structure was shown by circular dichroism, thus exposing a hydrophobic epitope toward the N-terminus and a hydrophilic one toward the C-terminus, improving amphipathicity. Ap-S was much more active than the parental Ap. Ap-S up to 100microM has no cytotoxic effect against fish cell line CHSE-214. We demonstrated that the chemical modification of a natural peptide and the chemical synthesis of derived molecules may be a powerful tool for obtaining substitutes to conventional antibiotics, displaying the many advantages of antimicrobial peptides and overcoming the limitations of natural peptides for large-scale production and application, such as the low specific activity and the minute amounts recovered in vivo. This peptide may have a relevant application in aquaculture by controlling Saprolegna sp., a parasitic pathogen fungus that attacks the culture of fish in different stages of their growth, from egg to adult.

Properties and applications of antimicrobial peptides in biodefense against biological warfare threat agents

Recent advances in knowledge of the properties of antimicrobial peptides (AMPs) are reviewed. AMPs are typically small, positively charged, amphipathic peptides that interact electrostatically and non-stereospecifically with the bacterial cell membrane, resulting in its permeabilization and cell death. Classes of AMPs, their mechanisms of action, hemolytic activity, and cytotoxicity towards host cells are discussed. A particular focus is AMPs with potential for use in defense against biological warfare agents. Some AMPs cytotoxic to Bacillus anthracis have been described. Synthesis of these peptides in multivalent form leads to a synergistic increase in antibacterial activity. Strategies to enhance the potency, stability, and selectivity of AMPs are discussed.