[Insect antimicrobial peptides: structures, properties and gene regulation]

Anticancer Mechanisms and Potential Anticancer Applications of Antimicrobial Peptides and Their Nano Agents

Traditional chemotherapy is one of the main methods of cancer treatment, which is largely limited by severe side effects and frequent development of multi-drug resistance by cancer cells. Antimicrobial peptides (AMPs) with high efficiency and low toxicity, as one of the most promising new drugs to replace chemoradiotherapy, have become a current research hotspot, attracting the attention of worldwide researchers. AMPs are natural-source small peptides from the innate immune system, and certain AMPs can selectively kill a broad spectrum of cancer cells while exhibiting less damage to normal cells. Although it involves intracellular mechanisms, AMPs exert their anti-cancer effects mainly through membrane destruction effect; thus, AMPs also hold unique advantages in fighting drug-resistant cancer cells. However, the poor stability and hemolytic toxicity of peptides limit their clinical application. Fortunately, functionalized nanoparticles have many possibilities in overcoming the shortcomings of AMPs, which provides a huge prospect for better application of AMPs. In this paper, we briefly introduce the characteristics and different sources of AMPs, review and summarize the mechanisms of action and the research status of AMPs used as an anticancer therapy, and finally focus on the further use of AMPs nano agents in the anti-cancer direction.

Herbicide exposure alters the effect of the enthomopathogen Beauveria bassiana on immune gene expression in mealworm beetles

Concerns have grown worldwide about the potentially far-reaching effects of herbicides on functional biodiversity in agroecosystems. Repeated applications over time can lead to accumulation of residues in soil, water, and food and may have negative impacts on non-target organisms. However, the effects of herbicide residues on interspecific relationships, such as host-pathogen interactions, are poorly studied. In this study, we evaluated the effects of two different concentrations of a commercial pendimethalin-based formulation (PND), the residual contamination (S, 13 ppm) in treated soils and the maximum residue level allowed by the European Commission in cereals (EU, 0.05 ppm). We tested the effect of PND on the biological interaction between the mealworm beetle Tenebrio molitor Linnaeus, 1758 and the entomopathogenic fungus Beauveria bassiana Vuillemin, 1912 (Bb, strain KVL 03-144) at two concentrations (LC50 5 × 105 conidia mL-1 and LC100 1 × 107 conidia mL-1). We checked the survival of beetles exposed to PND or/and inoculated with B. bassiana, the expression of four antimicrobial peptides (AMPs), and finally how PND affects in vitro germination of fungus. The exposure to PND had no significant effects on the survival of either control or Bb-exposed beetles. In the mealworm beetle, upregulation of gene expression of the inducible AMPs Tenecin 1, 2, and 4 was observed in PND-treated beetles after inoculation with Bb, while the levels of the non-inducible AMP Tenecin 3 were similar between treatments. In conclusion, our findings demonstrate that admitted residual doses of currently used herbicides modify an important component of the inducible immune response of an insect. This did not translate into an effect on the survival to B. bassiana in our system. However, residual doses of the herbicide at 13 ppm may temporarily affect fungal germination. These results raise questions about the compatibility of bioinsecticides with synthetic pesticides and the effects of herbicide residues on host-pathogen interactions.

Identification and Functional Analysis of a Defensin CcDef2 from Coridius chinensis

Coridius chinensis belongs to Dinidoridae, Hemiptera. Previous studies have indicated that C. chinensis contains abundant polypeptides with antibacterial and anticancer activities. Antimicrobial peptides (AMPs), as endogenous peptides with immune function, play an indispensable role in the process of biological development and immunity. AMPs have become one of the most potential substitutes for antibiotics due to their small molecular weight and broad-spectrum antimicrobial activity. In this study, a defensin CcDef2 from C. chinensis was characterized based on bioinformatics and functional analyses. The mature peptide of CcDef2 is a typical cationic peptide composed of 43 amino acid residues with five cations, and contains three intramolecular disulfide bonds and a typical cysteine-stabilized αβ motif in defensins. Phylogenetic analysis showed that CcDef2 belongs to the insect defensin family. Analysis of gene expression patterns showed that CcDef2 was expressed throughout developmental stages of C. chinensis with high levels at the nymphal stage and in adult tissues tested with the highest level in the fat body. In addition, the CcDef2 expression was significantly upregulated in adults infected by bacteria. After expressed in Escherichia coli BL21(DE3) and renatured, the recombinant CcDef2 showed a significant antibacterial effect on three kinds of Gram-positive bacteria. These results indicate that CcDef2 is an excellent antibacterial peptide and a highly effective immune effector in the innate immunity of C. chinensis. This study provides a foundation for further understanding the function of CcDef2 and developing new antimicrobial drugs.

Novel antimicrobial cecropins derived from O. curvicornis and D. satanas dung beetles

Antibiotic resistance is an increasing global problem and therapeutic alternatives to traditional antibiotics are needed. Antimicrobial and host defense peptides represent an attractive source for new therapeutic strategies, given their wide range of activities including antimicrobial, antitumoral and immunomodulatory. Insects produce several families of these peptides, including cecropins. Herein, we characterized the sequence, structure, and biological activity of three cecropins called satanin 1, 2, and curvicin, found in the transcriptome of two dung beetle species Dichotomius satanas and Onthophagus curvicornis. Sequence and circular dichroism analyses show that they have typical features of the cecropin family: short length (38-39 amino acids), positive charge, and amphipathic α-helical structure. They are active mainly against Gram-negative bacteria (3.12-12.5 μg/mL), with low toxicity on eukaryotic cells resulting in high therapeutic indexes (TI > 30). Peptides also showed effects on TNFα production in LPS-stimulated PBMCs. The biological activity of Satanin 1, 2 and Curvicin makes them interesting leads for antimicrobial strategies.

Time- and tissue-specific antimicrobial activity of the common bed bug in response to blood feeding and immune activation by bacterial injection

Unlike almost all hematophagous insects, common bed bugs, Cimex lectularius, are not known to transmit pathogens to humans. To help unravel the reasons for their lack of vector competence, we studied the time- and tissue-dependent expression of innate immune factors after blood feeding or immune activation through the intrathoracic injection of bacteria. We used minimum inhibitory concentration (MIC¹) bioassays and the Kirby-Bauer protocol to evaluate antimicrobial peptide (AMP²) activity in tissue extracts from the midguts or 'rest of body' (RoB³) tissues (containing hemolymph and fat body AMPs) against Gram-positive and Gram-negative bacteria. We compared AMP activity between blood-fed female bed bugs and yellow fever mosquitoes, Aedes aegypti and determined how female and male bed bugs respond to immune challenges, and how long AMP gene expression remains elevated in bed bugs following a blood meal. Blood meal-induced AMP activity is 4-fold stronger in female bed bugs than in female mosquitoes. Male bed bugs have elevated AMP activity within 8 h of a blood meal or an intrathoracic injection with bacteria, with the strongest activity expressed in RoB tissue 24 h after the immune challenge. Female bed bugs have a stronger immune response than males within 24 h of a blood meal. The effects of blood meal-induced elevated AMP activity lasts longer against the Gram-positive bacterium, Bacillus subtilis, than against the Gram-negative bacterium Escherichia coli. Unravelling the specific immune pathways that are activated in the bed bugs' immune responses and identifying the bed bug-unique AMPs might help determine why these insects are not vectors of human parasites.

Antimicrobial Peptides from Black Soldier Fly (Hermetia illucens) as Potential Antimicrobial Factors Representing an Alternative to Antibiotics in Livestock Farming

Functional antimicrobial peptides (AMPs) are an important class of effector molecules of innate host immune defense against pathogen invasion. Inability of microorganisms to develop resistance against the majority of AMPs has made them alternatives to antibiotics, contributing to the development of a new generation of antimicrobials. Due to extensive biodiversity, insects are one of the most abundant sources of novel AMPs. Notably, black soldier fly insect (BSF; Hermetia illucens (Diptera: Stratiomyidae)) feeds on decaying substrates and displays a supernormal capacity to survive under adverse conditions in the presence of abundant microorganisms, therefore, BSF is one of the most promising sources for identification of AMPs. However, discovery, functional investigation, and drug development to replace antibiotics with AMPs from Hermetia illucens remain in a preliminary stage. In this review, we provide general information on currently verified AMPs of Hermetia illucens, describe their potential medical value, discuss the mechanism of their synthesis and interactions, and consider the development of bacterial resistance to AMPs in comparison with antibiotics, aiming to provide a candidate for substitution of antibiotics in livestock farming or, to some extent, for blocking the horizontal transfer of resistance genes in the environment, which is beneficial to human and animal welfare.

Spatzle4 gene of silkworm, Bombyx mori: identification, immune response, and the effect of RNA interference on the antimicrobial peptides' expression in the integument

Insects, including silkworms, can protect themselves from exotic invasions by innate immune responses. In the immune response of Drosophila, the activation of Toll receptors is strictly dependent on the product of Spatzle. In this study, we cloned the 1567 bp BmSpz4 cDNA which contained a complete 1386 bp open reading frame, encoding 461 amino acids, out of which the forgoing 19 residues were signal peptide. The result of the cDNA sequencing showed that we found a longer transcript than the one included in large scale full-Length cDNA sequencing data by Yoshitaka Suetsugu et al. in 2013. Several spliced variants of BmSpz4 have been found, based on our preliminary results. It was shown by the RT-PCR that BmSpz4 was expressed highest in the head, lower in the integument and testis. The expression of BmSpz4 in the integument of silkworm was upregulated by formalin-inactivated Gram-positive bacteria and fungi but not by Gram-negative bacteria, when compared to the control group of PBS injection. This phenomenon was the same as the one found in the Toll signaling pathway of Drosophila. In addition, the result of double-stranded RNA interference of BmSpz4 also demonstrated that it had a corresponding interference effect on the expression of the integument antimicrobial peptides induced by bacillus and yeast. Thus, it may be concluded that BmSpz4 plays an important role in the innate immunity against microbe infection in the integument of silkworm, Bombyx mori.

An overview of antifungal peptides derived from insect

Fungi are not classified as plants or animals. They resemble plants in many ways but do not produce chlorophyll or make their own food photosynthetically like plants. Fungi are useful for the production of beer, bread, medicine, etc. More complex than viruses or bacteria; fungi can be destructive human pathogens responsible for various diseases in humans. Most people have a strong natural immunity against fungal infection. However, fungi can cause diseases when this immunity breaks down. In the last few years, fungal infection has increased strikingly and has been accompanied by a rise in the number of deaths of cancer patients, transplant recipients, and acquired immunodeficiency syndrome (AIDS) patients owing to fungal infections. The growth rate of fungi is very slow and quite difficult to identify. A series of molecules with antifungal activity against different strains of fungi have been found in insects, which can be of great importance to tackle human diseases. Insects secrete such compounds, which can be peptides, as a part of their immune defense reactions. Active antifungal peptides developed by insects to rapidly eliminate infectious pathogens are considered a component of the defense munitions. This review focuses on naturally occurring antifungal peptides from insects and their challenges to be used as armaments against human diseases.

Insect-derived cecropins display activity against Acinetobacter baumannii in a whole-animal high-throughput Caenorhabditis elegans model

The rise of multidrug-resistant Acinetobacter baumannii and a concomitant decrease in antibiotic treatment options warrants a search for new classes of antibacterial agents. We have found that A. baumannii is pathogenic and lethal to the model host organism Caenorhabditis elegans and have exploited this phenomenon to develop an automated, high-throughput, high-content screening assay in liquid culture that can be used to identify novel antibiotics effective against A. baumannii. The screening assay involves coincubating C. elegans with A. baumannii in 384-well plates containing potential antibacterial compounds. At the end of the incubation period, worms are stained with a dye that stains only dead animals, and images are acquired using automated microscopy and then analyzed using an automated image analysis program. This robust assay yields a Z' factor consistently greater than 0.7. In a pilot experiment to test the efficacy of the assay, we screened a small custom library of synthetic antimicrobial peptides (AMPs) that were synthesized using publicly available sequence data and/or transcriptomic data from immune-challenged insects. We identified cecropin A and 14 other cecropin or cecropin-like peptides that were able to enhance C. elegans survival in the presence of A. baumannii. Interestingly, one particular hit, BR003-cecropin A, a cationic peptide synthesized by the mosquito Aedes aegypti, showed antibiotic activity against a panel of Gram-negative bacteria and exhibited a low MIC (5 μg/ml) against A. baumannii. BR003-cecropin A causes membrane permeability in A. baumannii, which could be the underlying mechanism of its lethality.