Disulphide bonds of the peptide protegrin-1 are not essential for antimicrobial activity and haemolytic activity

Single Disulfide Bond in Host Defense Thanatin Analog Peptides: Antimicrobial Activity, Atomic-Resolution Structures and Target Interactions

Host defense antimicrobial peptides (AMPs) are promising lead molecules with which to develop antibiotics against drug-resistant bacterial pathogens. Thanatin, an inducible antimicrobial peptide involved in the host defense of Podisus maculiventris insects, is gaining considerable attention in the generation of novel classes of antibiotics. Thanatin or thanatin-based analog peptides are extremely potent in killing bacterial pathogens in the Enterobacteriaceae family, including drug-resistant strains of Escherichia coli and Klebsiella pneumoniae. A single disulfide bond that covalently links two anti-parallel β-strands in thanatin could be pivotal to its selective antibacterial activity and mode of action. However, potential correlations of the disulfide covalent bond with structure, activity and target binding in thanatin peptides are currently unclear to. Here, we examined a 16-residue designed thanatin peptide, namely disulfide-bonded VF16QK, and its Cys to Ser substituted variant, VF16QKSer, to delineate their structure-activity relationships. Bacterial growth inhibitory activity was only detected for the disulfide-bonded VF16QK peptide. Mechanistically, both peptides vastly differ in their bacterial cell permeabilizations, atomic-resolution structures, interactions with the LPS-outer membrane and target periplasmic protein LptAm binding. In particular, analysis of the 3-D structures of the two peptides revealed an altered folded conformation for the VF16QKSer peptide that was correlated with diminished LPS-outer membrane permeabilization and target interactions. Analysis of docked complexes of LPS-thanatin peptides indicated potential structural requirements and conformational adaptation for antimicrobial activity. Collectively, these observations contrast with those for the disulfide-bonded β-hairpin antimicrobial protegrin and tachyplesin peptides, where disulfide bonds are dispensable for activity. We surmise that the atomistic structures and associated molecular interactions presented in this work can be utilized to design novel thanatin-based antibiotics.

Single Deletion Unmasks Hidden Anti-Gram-Negative Bacterial Activity of an Insect Defensin-Derived Peptide

Insect defensins are a large family of antimicrobial peptides primarily active against Gram-positive bacteria. Here, we explore their hidden anti-Gram-negative bacterial potential via a nature-guided strategy inspired by natural deletion variants of Drosophila defensins. Referring to these variants, we deleted the equivalent region of an insect defensin with the first cysteine-containing N-terminus, and the last three cysteine-containing C-terminal regions remained. This 15-mer peptide exhibits low solubility and specifically targets Gram-positive bacteria. Further deletion of alanine-9 remarkably improves its solubility, unmasks its hidden anti-Gram-negative bacterial activity, and alters its states in different environments. Intriguingly, compared with the oxidized form, the 14-mer reduced peptide shows increased activity on Gram-positive and Gram-negative bacteria through a membrane-disruptive mechanism. The broad-spectrum activity and tolerance to high-salt environments and human serum, together with no toxicity to mammalian or human cells, make it a promising candidate for the design of new peptide antibiotics against Gram-negative bacterial infections.

Matching amino acids membrane preference profile to improve activity of antimicrobial peptides

Antimicrobial peptides (AMPs) are cationic antibiotics that can kill multidrug-resistant bacteria via membrane insertion. However, their weak activity limits their clinical use. Ironically, the cationic charge of AMPs is essential for membrane binding, but it obstructs membrane insertion. In this study, we postulate that this problem can be overcome by locating cationic amino acids at the energetically preferred membrane surface. All amino acids have an energetically preferred or less preferred membrane position profile, and this profile is strongly related to membrane insertion. However, most AMPs do not follow this profile. One exception is protegrin-1, a powerful but neglected AMP. In the present study, we found that a potent AMP, WCopW5, strongly resembles protegrin-1 and that the match between its sequence and the preferred position profile closely correlates with its antimicrobial activity. One of its derivatives, WCopW43, has antimicrobial activity comparable to that of the most effective AMPs in clinical use.

Adaptively evolved human oral actinomyces-sourced defensins show therapeutic potential

The development of eukaryote‐derived antimicrobial peptides as systemically administered drugs has proven a challenging task. Here, we report the first human oral actinomyces‐sourced defensin—actinomycesin—that shows promise for systemic therapy. Actinomycesin and its homologs are only present in actinobacteria and myxobacteria, and share similarity with a group of ancient invertebrate‐type defensins reported in fungi and invertebrates. Signatures of natural selection were detected in defensins from the actinomyces colonized in human oral cavity and ruminant rumen and dental plaque, highlighting their role in adaptation to complex multispecies bacterial communities. Consistently, actinomycesin exhibited potent antibacterial activity against oral bacteria and clinical isolates of Staphylococcus and synergized with two classes of human salivary antibacterial factors. Actinomycesin specifically inhibited bacterial peptidoglycan synthesis and displayed weak immunomodulatory activity and low toxicity on human and mammalian cells and ion channels in the heart and central nervous system. Actinomycesin was highly efficient in mice infected with Streptococcus pneumoniae and mice with MRSA‐induced experimental peritoneal infection. This work identifies human oral bacteria as a new source of systemic anti‐infective drugs.

Murepavadin, a Small Molecule Host Defense Peptide Mimetic, Activates Mast Cells via MRGPRX2 and MrgprB2

Pseudomonas aeruginosa is a frequent cause of hospital-acquired wound infection and is difficult to treat because it forms biofilms and displays antibiotic resistance. Previous studies in mice demonstrated that mast cells (MCs) not only contribute to P. aeruginosa eradication but also promote wound healing via an unknown mechanism. We recently reported that host defense peptides (HDPs) induce human MC degranulation via Mas-related G protein-coupled receptor-X2 (MRGPRX2). Small molecule HDP mimetics have distinct advantages over HDPs because they are inexpensive to synthesize and display high stability, bioavailability, and low toxicity. Murepavadin is a lipidated HDP mimetic, (also known as POL7080), which displays antibacterial activity against a broad panel of multi-drug-resistant P. aeruginosa. We found that murepavadin induces Ca2+ mobilization, degranulation, chemokine IL-8 and CCL3 production in a human MC line (LAD2 cells) endogenously expressing MRGPRX2. Murepavadin also caused degranulation in RBL-2H3 cells expressing MRGPRX2 but this response was significantly reduced in cells expressing missense variants within the receptor's ligand binding (G165E) or G protein coupling (V282M) domains. Compound 48/80 induced β-arrestin recruitment and promoted receptor internalization, which resulted in substantial decrease in the subsequent responsiveness to the MRGPRX2 agonist. By contrast, murepavadin did not cause β-arrestin-mediated MRGPRX2 regulation. Murepavadin induced degranulation in mouse peritoneal MCs via MrgprB2 (ortholog of human MRGPRX2) and caused increased vascular permeability in wild-type mice but not in MrgprB2-/- mice. The data presented herein demonstrate that murepavadin activates human MCs via MRGPRX2 and murine MCs via MrgprB2 and that MRGPRX2 is resistant to β-arrestin-mediated receptor regulation. Thus, besides its direct activity against P. aeruginosa, murepavadin may contribute to bacterial clearance and promote wound healing by harnessing MC's immunomodulatory property via the activation of MRGPRX2.

Experimental and Computational Characterization of Oxidized and Reduced Protegrin Pores in Lipid Bilayers

Protegrin-1 (PG-1), an 18-residue β-hairpin stabilized by two disulfide bonds, is a member of a family of powerful antimicrobial peptides which are believed to act through membrane permeabilization. Here we used a combination of experimental and computational approaches to characterize possible structural arrangements of PG-1 in lipid bilayers mimicking bacterial membranes. We have measured the dose-response function of the PG-1-induced leakage of markers of various sizes from vesicles and found it to be consistent with the formation of pores of two different sizes. The first one allows the release of small dyes and occurs at peptide:lipid ratios < 0.006. Above this ratio, larger pores are observed through which the smallest of dextrans FD4 can be released. In parallel with pore formation, we observe a general large-scale destabilization of vesicles which is probably related to complete rupture of some vesicles. The population of vesicles that are completely ruptured depends linearly on PG-1:lipid ratio. Neither pore size, nor vesicle rupture are influenced by the formation of disulfide bonds. Previous computational work on oxidized protegrin is complemented here by all-atom MD simulations of PG-1 with reduced disulfide bonds both in solution (monomer) and in a bilayer (dimer and octamer). The simulations provide molecular insights into the influence of disulfide bonds on peptide conformation, aggregation, and oligomeric structure.

Protegrin-1 cytotoxicity towards mammalian cells positively correlates with the magnitude of conformational changes of the unfolded form upon cell interaction

Porcine protegrin-1 (PG-1) is a broad-spectrum antimicrobial peptide (AMP) with potent antimicrobial activities. We produced recombinant PG-1 and evaluated its cytotoxicity toward various types of mammalian cell lines, including embryonic fibroblasts, retinal cells, embryonic kidney cells, neuroblastoma cells, alveolar macrophage cells, and neutrophils. The sensitivity of the different mammalian cells to cytotoxic damage induced by PG-1 differed significantly among the cell types, with retinal neuron cells and neutrophils being the most significantly affected. A circular dichroism analysis showed there was a precise correlation between conformational changes in PG-1 and the magnitude of cytotoxicity among the various cell type. Subsequently, a green fluorescent protein (GFP) penetration assay using positively charged GFPs indicated there was a close correlation between the degree of penetration of charged GFP into cells and the magnitude of PG-1 cytotoxicity. Furthermore, we also showed that inhibition of the synthesis of anionic sulphated proteoglycans on the cell surface decreases the cytotoxic damage induced by PG-1 treatment. Taken together, the observed cytotoxicity of PG-1 towards different membrane surfaces is highly driven by the membrane’s anionic properties. Our results reveal a possible mechanism underlying cell-type dependent differences in cytotoxicity of AMPs, such as PG-1, toward mammalian cells.

The drosomycin multigene family: three-disulfide variants from Drosophila takahashii possess antibacterial activity

Drosomycin (DRS) is a strictly antifungal peptide in Drosophila melanogaster, which contains four disulfide bridges (DBs) with three buried in molecular interior and one exposed on molecular surface to tie the amino- and carboxyl-termini of the molecule together (called wrapper disulfide bridge, WDB). Based on computational analysis of genomes of Drosophila species belonging to the Oriental lineage, we identified a new multigene family of DRS in Drosphila takahashii that includes a total of 11 DRS-encoding genes (termed DtDRS-1 to DtDRS-11) and a pseudogene. Phylogenetic tree and synteny analyses reveal orthologous relationship between DtDRSs and DRSs, indicating that orthologous genes of DRS-1, DRS-2, DRS-3 and DRS-6 have undergone duplication in D. takahashii and three amplifications (DtDRS-9 to DtDRS-11) of DRS-3 have lost WDB. Among the 11 genes, five are transcriptionally active in adult fruitflies. The ortholog of DRS (DtDRS-1) shows high structural and functional similarity to DRS while two WDB-deficient members display antibacterial activity accompanying complete loss or remarkable reduction of antifungal activity. To the best of our knowledge, this is the first report on the presence of three-disulfide antibacterial DRSs in a specific Drosophila species, suggesting a potential role of DB loss in neofunctionalization of a protein via structural adjustment.

The Disulfide Bond of the Peptide Thanatin Is Dispensible for Its Antimicrobial Activity In Vivo and In Vitro

Thanatin (THA) displays potent antibiotic activity, especially against extended-spectrum-β-lactamase (ESBL)-producing Escherichia coli both in vitro and in vivo, with minimal hemolytic toxicity and satisfactory stability in plasma. However, the high cost of thanatin significantly limits its development and clinical application. To reduce the cost of peptide synthesis, a formulation of cyclic thanatin (C-thanatin) called linear thanatin (L-thanatin) was synthesized and its activity was evaluated in vivo and in vitro Results showed that C-thanatin and L-thanatin MICs did not differ against eight Gram-negative and two Gram-positive bacterial strains. Furthermore, the survival rates of ESBL-producing-E. coli-infected mice were consistent after C-thanatin or L-thanatin treatment at 5 or 10 mg/kg of body weight. Neither C-thanatin nor L-thanatin showed toxicity for human red blood cells (hRBCs) and human umbilical vein endothelial cells (HUVECs) at a concentration as high as 256 μg/ml. Results of circular dichroism spectroscopy indicated that the secondary structure of L-thanatin is extremely similar to that of C-thanatin. Membrane permeabilization and depolarization assays showed that C-thanatin and L-thanatin have similar abilities to permeabilize the outer and inner membranes and to induce membrane depolarization in ESBL-producing E. coli However, neither of them caused significant HUVEC membrane permeability. These findings indicate that the two peptides have similar effects on bacterial cell membranes and that the disulfide bond in thanatin is not essential for its antimicrobial activities in vivo and in vitro L-thanatin is thus a promising low-cost peptide candidate for treating ESBL-producing E. coli infections.

An insect defensin-derived β-hairpin peptide with enhanced antibacterial activity

Insect defensins are a class of small, cysteine-rich antimicrobial peptides primarily active on Gram-positive bacteria. Their roles in maggot therapy for treating chronic wound infection have been reported recently. However, a relatively narrow antibacterial spectrum together with the lack of a cost-effective means of commercial-scale production has limited their application. To further exploit the therapeutic potential of these molecules, we engineered the carboxyl-terminal β-sheet of navidefensin2-2, an insect defensin from Nasonia vitripennis, based on its structural similarity to naturally occurring microbicidal β-hairpin peptides. The designed peptide of 14 residues, referred to as NvBH, spans the β-sheet region of the defensin with two amino acids substituted for assembly of a disulfide-bonded amphipathic β-hairpin structure. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) combined with circular dichroism (CD) analysis shows that the oxidized NvBH (oNvBH), produced from the synthetic peptide by air oxidization in an alkaline environment, folds into a typical β-hairpin structure linked by two disulfide bridges (Cys1-Cys4; Cys2-Cys3). However, such a structure appears not to be functionally necessary as synthetic NvBH with a spontaneously oxidized disulfide bridge (Cys2-Cys3) (termed poNvBH) displayed similar antibacterial potency to oNvBH. In comparison with oNvBH, poNvBH exhibited higher serum stability and more resistance on tryptic digestion. These two forms of peptides are capable of killing an array of Gram-positive (including antibiotic-resistant strains of Staphylococcus) and Gram-negative bacterial pathogens at low micromolar concentrations through a membrane disruptive mode of action. Our work indicates that the β-sheet region of insect defensins is a promising subdomain of proteins in anti-infective drug discovery.

Membrane interactions and pore formation by the antimicrobial peptide protegrin

Protegrin is an antimicrobial peptide with a β-hairpin structure stabilized by a pair of disulfide bonds. It has been extensively studied by solid-state NMR and computational methods. Here we use implicit membrane models to examine the binding of monomers on the surface and in the interior of the membrane, the energetics of dimerization, the binding to membrane pores, and the stability of different membrane barrel structures in pores. Our results challenge a number of conclusions based on previous experimental and theoretical work. The burial of monomers into the membrane interior is found to be unfavorable for any membrane thickness. Because of its imperfect amphipathicity, protegrin binds weakly, at most, on the surface of zwitterionic membranes. However, it binds more favorably onto toroidal pores. Anionic charge on the membrane facilitates the binding due to electrostatic interactions. Solid-state NMR results have suggested a parallel NCCN association of monomers in dimers and association of dimers to form octameric or decameric β-barrels. We find that this structure is not energetically plausible for binding to bilayers, because in this configuration the hydrophobic sides of two monomers point in opposite directions. In contrast, the antiparallel NCCN and especially the parallel NCNC octamers are stable and exhibit a favorable binding energy to the pore. The results of 100-ns simulations in explicit bilayers corroborate the higher stability of the parallel NCNC barrel compared with the parallel NCCN barrel. The ability to form pores in zwitterionic membranes provides a rationalization for the peptide's cytotoxicity. The discrepancies between our results and experiment are discussed, and new experiments are proposed to resolve them and to test the validity of the models.

Structural perspectives on antimicrobial chemokines

Chemokines are best known as signaling proteins in the immune system. Recently however, a large number of human chemokines have been shown to exert direct antimicrobial activity. This moonlighting activity appears to be related to the net high positive charge of these immune signaling proteins. Chemokines can be divided into distinct structural elements and some of these have been studied as isolated peptide fragments that can have their own antimicrobial activity. Such peptides often encompass the α-helical region found at the C-terminal end of the parent chemokines, which, similar to other antimicrobial peptides, adopt a well-defined membrane-bound amphipathic structure. Because of their relatively small size, intact chemokines can be studied effectively by NMR spectroscopy to examine their structures in solution. In addition, NMR relaxation experiments of intact chemokines can provide detailed information about the intrinsic dynamic behavior; such analyses have helped for example to understand the activity of TC-1, an antimicrobial variant of CXCL7/NAP-2. With chemokine dimerization and oligomerization influencing their functional properties, the use of NMR diffusion experiments can provide information about monomer-dimer equilibria in solution. Furthermore, NMR chemical shift perturbation experiments can be used to map out the interface between self-associating subunits. Moreover, the unusual case of XCL1/lymphotactin presents a chemokine that can interconvert between two distinct folds in solution, both of which have been elucidated. Finally, recent advances have allowed for the determination of the structures of chemokines in complex with glycosaminoglycans, a process that could interfere with their antimicrobial activity. Taken together, these studies highlight several different structural facets that contribute to the way in which chemokines exert their direct microbicidal actions.

Chemical synthesis and biological evaluation of an antimicrobial peptide gonococcal growth inhibitor

Gonococcal growth inhibitor 1 (GGI-1) is a 44-residue peptide with potent anti-Legionella activity. It has been isolated from Staphylococcus haemolyticus but, to date, its chemical synthesis has not been reported. Acquisition of this peptide via this means would enable a more detailed examination of its antimicrobial properties. However, its synthesis represents a significant challenge because of two predicted "difficult sequences" within the peptide. Its successful solid-phase assembly is reported in this paper, and was accomplished by use of simple palliative measures including the introduction of a single pseudo-proline isostere in order to counteract on-resin aggregation. The peptide had moderate antimicrobial activity against Escherichia coli but was inactive against another Gram-negative bacterium and two Gram-positive bacteria (Bacillus species). It had significant haemolytic activity, with a H(50) (concentration of peptide that causes 50 % haemolysis) of 20 and 125 μM for two blood samples from different donors. An alternative therapeutic index to that proposed for GGI-1 in a recent publication is proposed.

Potent antimicrobial peptides with selectivity for Bacillus anthracis over human erythrocytes

In this study, 39 antimicrobial peptides, most with documented low haemolytic activity and potent efficacy against Gram-negative and Gram-positive bacteria, were evaluated for their haemolytic activity against human red blood cells as well as their antimicrobial activity against Escherichia coli, Burkholderia thailandensis, Bacillus globigii and Bacillus anthracis. The majority of the peptides had a minimum inhibitory concentration (MIC) of <10 μM against B. globigii. However, only eight of these (CaLL, Ci-MAM-A24, LLaMA, Ltc2a, OV-5, papillosin, smapspin and smapspin-G) had a MIC<10 μM against B. anthracis. All except one (papillosin) were ineffective at 100 μM against B. thailandensis and none had potent activity against E. coli. Potent activity against B. anthracis was associated with significant haemolytic activity, but the ratio of the concentration of peptide that caused 50% haemolysis to the concentration that inhibited growth of B. anthracis by 50% (the therapeutic index) varied from 0.8 to 34.2. Two peptides (papillosin and Ltc2a) had a therapeutic index >30 and could be considered as candidates for further development for potential medical countermeasures against anthrax. Although B. globigii has often been used as a non-pathogenic simulant for B. anthracis, in this study it was found that the sensitivity of B. globigii to peptides was not a reliable predictor of the sensitivity of B. anthracis to the same peptides.