An enhancer peptide for membrane-disrupting antimicrobial peptides

Insect antimicrobial peptides show potentiating functional interactions against Gram-negative bacteria

Antimicrobial peptides (AMPs) and proteins are important components of innate immunity against pathogens in insects. The production of AMPs is costly owing to resource-based trade-offs, and strategies maximizing the efficacy of AMPs at low concentrations are therefore likely to be advantageous. Here, we show the potentiating functional interaction of co-occurring insect AMPs (the bumblebee linear peptides hymenoptaecin and abaecin) resulting in more potent antimicrobial effects at low concentrations. Abaecin displayed no detectable activity against Escherichia coli when tested alone at concentrations of up to 200 μM, whereas hymenoptaecin affected bacterial cell growth and viability but only at concentrations greater than 2 μM. In combination, as little as 1.25 μM abaecin enhanced the bactericidal effects of hymenoptaecin. To understand these potentiating functional interactions, we investigated their mechanisms of action using atomic force microscopy and fluorescence resonance energy transfer-based quenching assays. Abaecin was found to reduce the minimal inhibitory concentration of hymenoptaecin and to interact with the bacterial chaperone DnaK (an evolutionarily conserved central organizer of the bacterial chaperone network) when the membrane was compromised by hymenoptaecin. These naturally occurring potentiating interactions suggest that combinations of AMPs could be used therapeutically against Gram-negative bacterial pathogens that have acquired resistance to common antibiotics.

Production, purification, and characterization of the cecropin from Plutella xylostella, pxCECA1, using an intein-induced self-cleavable system in Escherichia coli

Antimicrobial peptides (AMPs) are widely expressed and play an important role in innate immune defense against infectious agents such as bacteria, viruses, fungi, and parasites. Cecropins are a family of AMPs synthesized in the fat body of insects that have proven effective at killing specific pathogens. In order to fulfill their clinical potential as antimicrobial drugs, a simple, cost-effective method to express AMPs is sorely needed. In this study, we expressed and characterized the cecropin from Plutella xylostella (pxCECA1) using an intein-dependent expression system in Escherichia coli. We cloned the pxCECA1 gene from larva by RT-PCR and fused the encoding sequence of mature pxCECA1 with an intein gene and a chitin-binding domain gene (CBD) in pTWIN1 plasmid. The fusion protein CBD-intein-pxCECA1 was expressed in E. coli BL21 (DE3) and separated by flowing cell extracts through a chitin column. Subsequently, self-cleavage of the intein at its C-terminus was induced in a temperature- and pH-dependent manner, resulting in the release of mature pxCECA1. The optimal conditions for self-cleavage were determined to be pH 6.0 for 48 h at 4°C, under which 12.3 mg of recombinant pxCECA1 could be recovered from 1 l of E. coli culture. The purified pxCECA1 displayed antimicrobial activity against a broad variety of gram-positive and gram-negative bacteria. This preparation was especially effective against Staphylococcus aureus, including methicillin-resistant strains. Catalase release assays demonstrated that pxCECA1 acts as a microbicidal agent. These results show for the first time that the IMPACT-TWIN expression system is an efficient, cost-effective way to produce fully functional AMPs and that the AMP pxCECA1 is a novel microbicidal agent with promising therapeutic applications.

Antimicrobial peptides: key components of the innate immune system

Life-threatening infectious diseases are on their way to cause a worldwide crisis, as treating them effectively is becoming increasingly difficult due to the emergence of antibiotic resistant strains. Antimicrobial peptides (AMPs) form an ancient type of innate immunity found universally in all living organisms, providing a principal first-line of defense against the invading pathogens. The unique diverse function and architecture of AMPs has attracted considerable attention by scientists, both in terms of understanding the basic biology of the innate immune system, and as a tool in the design of molecular templates for new anti-infective drugs. AMPs are gene-encoded short (<100 amino acids), amphipathic molecules with hydrophobic and cationic amino acids arranged spatially, which exhibit broad spectrum antimicrobial activity. AMPs have been the subject of natural evolution, as have the microbes, for hundreds of millions of years. Despite this long history of co-evolution, AMPs have not lost their ability to kill or inhibit the microbes totally, nor have the microbes learnt to avoid the lethal punch of AMPs. AMPs therefore have potential to provide an important breakthrough and form the basis for a new class of antibiotics. In this review, we would like to give an overview of cationic antimicrobial peptides, origin, structure, functions, and mode of action of AMPs, which are highly expressed and found in humans, as well as a brief discussion about widely abundant, well characterized AMPs in mammals, in addition to pharmaceutical aspects and the additional functions of AMPs.

Generation of novel cationic antimicrobial peptides from natural non-antimicrobial sequences by acid-amide substitution

Background

Cationic antimicrobial peptides (CAMPs) are well recognized to be promising as novel antimicrobial and antitumor agents. To obtain novel skeletons of CAMPs, we propose a simple strategy using acid-amide substitution (i.e. Glu→Gln, Asp→Asn) to confer net positive charge to natural non-antimicrobial sequences that have structures distinct from known CAMPs. The potential of this strategy was verified by a trial study.

Methods

The pro-regions of nematode cecropin P1-P3 (P1P-P3P) were selected as parent sequences. P1P-P3P and their acid-amide-substituted mutants (NP1P-NP3P) were chemically synthesized. Bactericidal and membrane-disruptive activities of these peptides were evaluated. Conformational changes were estimated from far-ultraviolet circular dichroism (CD) spectra.

Results

NP1P-NP3P acquired potent bactericidal activities via membrane-disruption although P1P-P3P were not antimicrobial. Far-ultraviolet CD spectra of NP1P-NP3P were similar to those of their parent peptides P1P-P3P, suggesting that NP1P-NP3P acquire microbicidal activity without remarkable conformational changes. NP1P-NP3P killed bacteria in almost parallel fashion with their membrane-disruptive activities, suggesting that the mode of action of those peptides was membrane-disruption. Interestingly, membrane-disruptive activity of NP1P-NP3P were highly diversified against acidic liposomes, indicating that the acid-amide-substituted nematode cecropin pro-region was expected to be a unique and promising skeleton for novel synthetic CAMPs with diversified membrane-discriminative properties.

Conclusions

The acid-amide substitution successfully generated some novel CAMPs in our trial study. These novel CAMPs were derived from natural non-antimicrobial sequences, and their sequences were completely distinct from any categories of known CAMPs, suggesting that such mutated natural sequences could be a promising source of novel skeletons of CAMPs.