Rational design of alpha-helical antimicrobial peptides with enhanced activities and specificity/therapeutic index

Novel antimicrobial peptides from the venom of the eusocial bee Halictus sexcinctus (Hymenoptera: Halictidae) and their analogs

Two novel antimicrobial peptides, named halictines, were isolated from the venom of the eusocial bee Halictus sexcinctus. Their primary sequences were established by ESI-QTOF mass spectrometry, Edman degradation and enzymatic digestion as Gly-Met-Trp-Ser-Lys-Ile-Leu-Gly-His-Leu-Ile-Arg-NH2 (HAL-1), and Gly-Lys-Trp-Met-Ser-Leu-Leu-Lys-His-Ile-Leu-Lys-NH2 (HAL-2). Both peptides exhibited potent antimicrobial activity against Gram-positive and Gram-negative bacteria but also noticeable hemolytic activity. The CD spectra of HAL-1 and HAL-2 measured in the presence of trifluoroethanol or SDS showed ability to form an amphipathic alpha-helical secondary structure in an anisotropic environment such as bacterial cell membrane. NMR spectra of HAL-1 and HAL-2 measured in trifluoroethanol/water confirmed formation of helical conformation in both peptides with a slightly higher helical propensity in HAL-1. Altogether, we prepared 51 of HAL-1 and HAL-2 analogs to study the effect of such structural parameters as cationicity, hydrophobicity, alpha-helicity, amphipathicity, and truncation on antimicrobial and hemolytic activities. The potentially most promising analogs in both series are those with increased net positive charge, in which the suitable amino acid residues were replaced by Lys. This improvement basically relates to the increase of antimicrobial activity against pathogenic Pseudomonas aeruginosa and to the mitigation of hemolytic activity.

Membrane poration by antimicrobial peptides combining atomistic and coarse-grained descriptions

Antimicrobial peptides (AMPs) comprise a large family of peptides that include small cationic peptides, such as magainins, which permeabilize lipid membranes. Previous atomistic level simulations of magainin-H2 peptides show that they act by forming toroidal transmembrane pores. However, due to the atomistic level of description, these simulations were necessarily limited to small system sizes and sub-microsecond time scales. Here, we study the long-time relaxation properties of these pores by evolving the systems using a coarse-grain (CG) description. The disordered nature and the topology of the atomistic pores are maintained at the CG level. The peptides sample different orientations but at any given time, only a few peptides insert into the pore. Key states observed at the CG level are subsequently back-transformed to the atomistic level using a resolution-transformation protocol. The configurations sampled at the CG level are stable in the atomistic simulation. The effect of helicity on pore stability is investigated at the CG level and we find that partial helicity is required to form stable pores. We also show that the current CG scheme can be used to study spontaneous poration by magainin-H2 peptides. Overall, our simulations provide a multi-scale view of a fundamental biophysical membrane process involving a complex interplay between peptides and lipids.

Retro-inversion enhances the adjuvant and CpG co-adjuvant activity of host defence peptide Bac2A

Host defence peptides (HDPs) have a variety of potential therapeutic applications, including as vaccine adjuvants, energizing efforts for modification strategies to address their toxicity and instability. Here we compare l, d and RI-Bac2A as vaccine adjuvants. d and RI-Bac2A are equally resistant to proteolytic degradation with no increases in toxicity, however, only RI-Bac2A maintains adjuvant activity of the natural peptide through conserved induction of a Th2 immune response. As HDPs potentiate the adjuvant activity of CpG ODNs, the isomers were also evaluated as co-adjuvants. l-Bac2A has no significant co-adjuvant activity while CpG/RI-Bac2A induces antibody titres significantly higher than CpG (P<0.01), CpG/l-Bac2A (P<0.01) or CpG/d-Bac2A (P<0.01). None of the isomers influence ODN duration or distribution but l and RI-Bac2A promote ODN uptake into B cells and antigen presenting cells. The enhanced adjuvant and co-adjuvant of RI-Bac2A is hypothesized to result from an undefined combination of increased stability and retained biological activity supporting application of retro-inversion to this, and potentially other HDPs.

Alpha-helical cationic antimicrobial peptides: relationships of structure and function

Antimicrobial peptides (AMPs), with their extraordinary properties, such as broad-spectrum activity, rapid action and difficult development of resistance, have become promising molecules as new antibiotics. Despite their various mechanisms of action, the interaction of AMPs with the bacterial cell membrane is the key step for their mode of action. Moreover, it is generally accepted that the membrane is the primary target of most AMPs, and the interaction between AMPs and eukaryotic cell membranes (causing toxicity to host cells) limits their clinical application. Therefore, researchers are engaged in reforming or de novo designing AMPs as a 'single-edged sword' that contains high antimicrobial activity yet low cytotoxicity against eukaryotic cells. To improve the antimicrobial activity of AMPs, the relationship between the structure and function of AMPs has been rigorously pursued. In this review, we focus on the current knowledge of α-helical cationic antimicrobial peptides, one of the most common types of AMPs in nature.

Critical interactions in the stability control region of tropomyosin

Our laboratory has recently described a stability control region in the two-stranded alpha-helical coiled-coil alpha-tropomyosin that accounts for overall protein stability but is not required for folding (Hodges et al., 2009). We have used a synthetic peptide approach to investigate three stability control sites within the stability control region (residues 97-118). Two of the sites, electrostatic cluster 1 (97-104, EELDRAQE) and electrostatic cluster 2 (112-118, KLEEAEK), feature sequences with unusually high charge density and the potential to form multiple intrachain and interchain salt bridges (ionic attractions). A third site (105-111, RLATALQ) features an e position Leu residue, an arrangement known previously to enhance coiled-coil stability modestly. A native peptide and seven peptide analogs of the tropomyosin sequence 85-119 were prepared by Fmoc solid-phase peptide synthesis. Thermal stability measurements by circular dichroism (CD) spectroscopy revealed the following T(m) values for the native peptide and three key analogs: 52.9 degrees C (Native), 46.0 degrees C (R101A), 45.3 degrees C (K112A/K118A), and 27.9 degrees C (L110A). The corresponding DeltaT(m) values for the analogs, relative to the native peptide, are -6.9 degrees C, -7.6 degrees C, and -25.0 degrees C, respectively. The dramatic contribution to stability made by L110e is three times greater than the contribution of either electrostatic cluster 1 or 2, likely resulting from a novel hydrophobic interaction not previously observed. These thermal stability results were corroborated by temperature profiling analyses using reversed-phase high-performance liquid chromatography (RP-HPLC). We believe that the combined contributions of the interactions within the three stability control sites are responsible for the effect of the stability control region in tropomyosin, with the Leu110e contribution being most critical.

Synthetic mimics of antimicrobial peptides--a versatile ring-opening metathesis polymerization based platform for the synthesis of selective antibacterial and cell-penetrating polymers

Natural macromolecules exhibit an extensive arsenal of properties, many of which have proven difficult to recapitulate in simpler synthetic systems. Over the last couple of years, foldamers have emerged as one important step toward increased functionality in synthetic systems. While the great majority of work in this area has focused on folded structures, hence the name, more recent progress has centered on polymers that mimic protein function. These efforts have resulted in the design of relatively simple macromolecules; one example are the synthetic mimics of antimicrobial peptides (SMAMPs) that capture the central physicochemical features of their natural archetypes irrespective of the specific folded form. Here we present our recent efforts to create polymers which display biological activity similar to natural proteins, including antimicrobial and cell-penetrating peptides.

De novo design of antimicrobial polymers, foldamers, and small molecules: from discovery to practical applications

Antimicrobial peptides (AMPs) provide protection against a variety of pathogenic bacteria and are, therefore, an important part of the innate immune system. Over the past decade, there has been considerable interest in developing AMPs as intravenously administered antibiotics. However, despite extensive efforts in the pharmaceutical and biotechnology industry, it has proven difficult to achieve this goal. While researchers have solved some relatively simple problems such as susceptibility to proteolysis, more severe problems have included the expense of the materials, toxicity, poor efficacy, and limited tissue distribution. In this Account, we describe our efforts to design and synthesize "foldamers"-- short sequence-specific oligomers based on arylamide and beta-amino acid backbones, which fold into well-defined secondary structures-- that could act as antimicrobial agents. We reasoned that small "foldamers" would be less expensive to produce than peptides, and might have better tissue distribution. It should be easier to fine-tune the structures and activities of these molecules to minimize toxicity. Because the activities of many AMPs depends primarily on their overall physicochemical properties rather than the fine details of their precise amino acid sequences, we have designed and synthesized very small "coarse-grained" molecules, which are far simpler than naturally produced AMPs. The molecular design of these foldamers epitomizes the positively charged amphiphilic structures believed to be responsible for the activity of AMPs. The designed oligomers show greater activity than the parent peptides. They have also provided leads for novel small molecule therapeutics that show excellent potency in animal models for multidrug resistant bacterial infections. In addition, such molecules can serve as relatively simple experimental systems for investigations aimed at understanding the mechanism of action for this class of antimicrobial agents. The foldamers' specificity for bacterial membranes relative to mammalian membranes appears to arise from differences in membrane composition and physical properties between these cell types. Furthermore, because experimental coarse-graining provided such outstanding results, we developed computational coarse-grained models to enable molecular dynamic simulations of these molecules with phospholipid membranes. These simulations allow investigation of larger systems for longer times than conventional molecular dynamics simulations, allowing us to investigate how physiologically relevant surface concentrations of AMP mimics affect the bilayer structure and properties. Finally, we apply the principles discovered through this work to the design of inexpensive antimicrobial polymers and materials.

Effect of L-leucine graft content on aqueous solution behavior and membrane-lytic activity of a pH-responsive pseudopeptide

A series of pH-responsive polymers have been synthesized by grafting L-leucine onto the pendant carboxylic acid groups of the linear pseudopeptide, poly(L-lysine iso-phthalamide). The effect of the degree of grafting on aqueous solution properties, cell membrane-disruptive activity, and in vitro cytotoxicity was examined by UV-visible and fluorescence spectroscopy, hemolysis, alamar blue staining, and propidium iodide fluorescence assays. Modification of poly(L-lysine iso-phthalamide) with < or =23.6 mol % L-leucine caused a marginal effect on the pH-mediated hydrophobic association and hemolytic activity. Increasing the degree of grafting from 31.9 to 61.2 mol % resulted in polymers with progressively enhanced hydrophobic association and cell membrane disruption, thus confirming that the pH responsiveness and the extent of hydrophobic association and membrane disruption of the polymers can be modulated by varying the degree of grafting with hydrophobic amino acids. The pH responses were demonstrated to be concentration-dependent. At certain degrees of leucine grafting, the polymers were nonmembrane-lytic at physiological pH but mediated considerable membrane lysis at endosomal pH values (5.0-6.8), a feature critical for potential drug delivery applications.

Antimicrobial specificity and mechanism of action of disulfide-removed linear analogs of the plant-derived Cys-rich antimicrobial peptide Ib-AMP1

Ib-AMP1 is a 20-residue disulfide-linked beta-sheet antimicrobial peptide found in the seeds of Impatiens balsamina. In order to investigate the effects of the 2 disulfide bonds on the antimicrobial specificity, to determine the mechanism of antimicrobial action of Ib-AMP1 and to develop novel cell-selective antimicrobial peptides with improved antimicrobial specificity as compared to wild-type Ib-AMP1, we synthesized a disulfide-removed linear analog of Ib-AMP1 with L-Pro, D-Pro or peptoid residues (Nala and Nlys) at the central position of the molecule. All linear analogs displayed a 3.7-4.8-fold higher antimicrobial specificity than wild-type Ib-AMP1, indicating that the disulfide bonds of Ib-AMP1 analogs are not essential for its antimicrobial specificity. Circular dichroism spectra revealed that the peptoid residues, as well as the proline at the central position of disulfide bond-removed Ib-AMP1 analogs, induce a beta-turn structure in a negatively charged bacterial membrane-mimicking environment. Ib-AMP1 was not effective in depolarizing the cytoplasmic membranes of Staphylococcus aureus and showed almost no leakage of calcein from negatively charged bacterial membranes mimicking lipid vesicles. In contrast, all linear analogs caused very weak dye leakage from negatively charged vesicles, but they almost completely depolarized the membrane potential of S. aureus cells. Collectively, our results suggest that the target of Ib-AMP1 may not be the cytoplasmic membranes of bacteria but their intracellular components. All linear analogs exhibit lethality due to their ability to form small channels that permit the transit of ions or protons and not molecules as large as calcein, and not by disrupting membranes.

Structure-activity relationships among random nylon-3 copolymers that mimic antibacterial host-defense peptides

Host-defense peptides are natural antibiotics produced by multicellular organisms to ward off bacterial infection. Since the discovery of these molecules in the 1980s, a great deal of effort has been devoted to elucidating their mechanisms of action and to developing analogues with improved properties for possible therapeutic use. The vast majority of this effort has focused on materials composed of a single type of molecule, most commonly a peptide with a specific sequence of alpha-amino acid residues. We have recently shown that sequence-random nylon-3 copolymers can mimic favorable properties of host-defense peptides, and here we document structure-activity relationships in this polymer family. Although the polymers are heterogeneous in terms of subunit order and stereochemistry, these materials display structure-activity relationships comparable to those that have been documented among host-defense peptides and analogous synthetic peptides. Previously such relationships have been interpreted in terms of a specific and regular folding pattern (usually an alpha-helix), but our findings show that these correlations between covalent structure and biological activity do not require the adoption of a specific or regular conformation. In some cases our observations suggest alternative interpretations of results obtained with discrete peptides.

Structure-function study of cathelicidin-derived bovine antimicrobial peptide BMAP-28: design of its cell-selective analogs by amino acid substitutions in the heptad repeat sequences

Although BMAP-28 is a potent cathelicidin-derived bovine antimicrobial peptide, its cytotoxic activity against the human and other mammalian cells is of concern for converting it into a novel antimicrobial drug. We have identified a short leucine and isoleucine zipper sequences at the N- and C-terminals of BMAP-28, respectively. To understand the possible role of these structural elements in BMAP-28, a number of alanine-substituted analogs were designed, synthesized and characterized along with the wild-type peptide. The substitution of amino acids at single or multiple 'a' position(s) of these structural motifs by alanine showed significant effects on the cytotoxic activity of the molecule on the human red blood cells (hRBCs) and 3T3 cells without showing much effects on their MIC values against the selected bacteria. BMAP-28 and all its analogs depolarized the Escherichia coli cells with almost equal efficacy. In contrast, the alanine-substituted analogs of BMAP-28 depolarized hRBCs much less efficiently than the parent molecule. Results further showed that BMAP-28 assembled appreciably onto the live E. coli and hRBC. However, the selected less toxic analogs of BMAP-28 although assembled as good as the parent molecule onto the live E. coli cells, their assembly onto the live mammalian hRBCs was much weaker as compared to that of the wild-type molecule. Looking at the remarkable similarity with the data presented in our previous work on melittin, it appears that probably the heptad repeat sequence possesses a general role in maintaining the cytotoxicity of the antimicrobial peptides against the mammalian cells and assembly therein.

Imcroporin, a new cationic antimicrobial peptide from the venom of the scorpion Isometrus maculates

The pace of resistance against antibiotics almost exceeds that of the development of new drugs. As many bacteria have become resistant to conventional antibiotics, new drugs or drug resources are badly needed to combat antibiotic-resistant pathogens, like methicillin-resistant Staphylococcus aureus (MRSA). Antimicrobial peptides, rich sources existing in nature, are able to effectively kill multidrug-resistant pathogens. Here, imcroporin, a new antimicrobial peptide, was screened and isolated from the cDNA library of the venomous gland of Isometrus maculates. The MIC of imcroporin against MRSA was 50 microg/ml, 8-fold lower than that of cefotaxime and 40-fold lower than that of penicillin. Imcroporin killed bacteria rapidly in vitro, inhibited bacterial growth, and cured infected mice. These results revealed that imcroporin could be considered a potential anti-infective drug or lead compound, especially for treating antibiotic-resistant pathogens.

Design of novel indolicidin-derived antimicrobial peptides with enhanced cell specificity and potent anti-inflammatory activity

Indolicidin (IN) is a 13-residue Trp-rich antimicrobial peptide isolated from bovine neutrophils. To develop novel IN-derived antimicrobial peptides with enhanced cell specificity (therapeutic index) and potent anti-inflammatory activity, several IN analogs were synthesized by Pro-->Lys substitution. All IN analogs displayed an increase in therapeutic index by 3- to 15-fold relative to parental IN. IN and its analogs induced a significant membrane depolarization against intact Staphylococcus aureus in a dose-dependent manner and depolarized membrane potential at 5 microg/ml (MIC for S. aureus) almost completely. However, these peptides caused less than 40% calcein leakage from negatively charged EYPG/EYPE liposomes mimicking bacterial membranes at 10 microg/ml. Based on these results, we hypothesize that IN and its analogs kill microorganisms via the formation of small ion channels that permit transit of ions or protons, but not molecules as large as calcein. Furthermore, IN and its analogs induced a remarkable suppression in nitric oxide (NO) production and inducible nitric oxide synthase (iNOS) mRNA expression in LPS-stimulated mouse macrophage RAW264.7 cells. All IN analogs showed LPS-binding activity comparable to that of IN. Taken together, their potent antimicrobial, anti-inflammatory and LPS-neutralizing activities similar to those of IN, coupled with their no cytotoxicity, our designed IN analogs make excellent candidates for novel antimicrobial and anti-sepsis agents.

End-tagging of ultra-short antimicrobial peptides by W/F stretches to facilitate bacterial killing

BACKGROUND

Due to increasing resistance development among bacteria, antimicrobial peptides (AMPs), are receiving increased attention. Ideally, AMP should display high bactericidal potency, but low toxicity against (human) eukaryotic cells. Additionally, short and proteolytically stable AMPs are desired to maximize bioavailability and therapeutic versatility.

METHODOLOGY AND PRINCIPAL FINDINGS

A facile approach is demonstrated for reaching high potency of ultra-short antimicrobal peptides through end-tagging with W and F stretches. Focusing on a peptide derived from kininogen, KNKGKKNGKH (KNK10) and truncations thereof, end-tagging resulted in enhanced bactericidal effect against Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus. Through end-tagging, potency and salt resistance could be maintained down to 4-7 amino acids in the hydrophilic template peptide. Although tagging resulted in increased eukaryotic cell permeabilization at low ionic strength, the latter was insignificant at physiological ionic strength and in the presence of serum. Quantitatively, the most potent peptides investigated displayed bactericidal effects comparable to, or in excess of, that of the benchmark antimicrobial peptide LL-37. The higher bactericidal potency of the tagged peptides correlated to a higher degree of binding to bacteria, and resulting bacterial wall rupture. Analogously, tagging enhanced peptide-induced rupture of liposomes, particularly anionic ones. Additionally, end-tagging facilitated binding to bacterial lipopolysaccharide, both effects probably contributing to the selectivity displayed by these peptides between bacteria and eukaryotic cells. Importantly, W-tagging resulted in peptides with maintained stability against proteolytic degradation by human leukocyte elastase, as well as staphylococcal aureolysin and V8 proteinase. The biological relevance of these findings was demonstrated ex vivo for pig skin infected by S. aureus and E. coli.

CONCLUSIONS/SIGNIFICANCE

End-tagging by hydrophobic amino acid stretches may be employed to enhance bactericidal potency also of ultra-short AMPs at maintained limited toxicity. The approach is of general applicability, and facilitates straightforward synthesis of hydrophobically modified AMPs without the need for post-peptide synthesis modifications.

De novo design and in vivo activity of conformationally restrained antimicrobial arylamide foldamers

The emergence of drug-resistant bacteria has compromised the use of many conventional antibiotics, leading to heightened interest in a variety of antimicrobial peptides. Although these peptides have attractive potential as antibiotics, their size, stability, tissue distribution, and toxicity have hampered attempts to harness these capabilities. To address such issues, we have developed small (molecular mass <1,000 Da) arylamide foldamers that mimic antimicrobial peptides. Hydrogen-bonded restraints in the arylamide template rigidify the conformation via hydrogen bond formation and increase activity toward Staphylococcus aureus and Escherichia coli. The designed foldamers are highly active against S. aureus in an animal model. These results demonstrate the application of foldamer templates as therapeutics.

Cell specificity, anti-inflammatory activity, and plausible bactericidal mechanism of designed Trp-rich model antimicrobial peptides

To develop novel short Trp-rich antimicrobial peptides (AMPs) with potent cell specificity (targeting bacteria but not eukaryotic cells) and anti-inflammatory activity, a series of 11-meric Trp-rich model peptides with different ratios of Leu and Lys/Arg residues, XXWXXWXXWXX-NH(2) (X indicates Leu or Lys/Arg), was synthesized. K(6)L(2)W(3) displayed an approximately 40-fold increase in cell specificity, compared with the natural Trp-rich AMP indolicidin (IN). Lys-containing peptides (K(8)W(3), K(7)LW(3) and K(6)L(2)W(3)) showed approximately 2- to 4-fold higher cell specificities than did their counterparts, the Arg-containing peptides (R(8)W(3), R(7)LW(3) and R(6)L(2)W(3)), indicating that multiple Lys residues are more important than multiple Arg residues in the design of AMPs with good cell specificity. The excellent resistance of d-enantiomers (K(6)L(2)W(3)-D and R(6)L(2)W(3)-D) and Orn/Nle-containing peptides (O(6)L(2)W(3) and O(6)L(2)W(3)) to trypsin digestion compared with the rapid breakdown of the l-enantiomers (K(6)L(2)W(3) and R(6)L(2)W(3)), highlights the clinical potential of such peptides. K(6)L(2)W(3), R(6)L(2)W(3), K(6)L(2)W(3)-D and R(6)L(2)W(3)-D caused weak dye leakage from bacterial membrane-mimicking negatively charged EYPG/EYPE (7:3, v/v) liposomes. Confocal microscopy showed that these peptides penetrated the cell membrane of Escherichia coli and accumulated in the cytoplasm, as observed for buforin-2. Gel retardation studies revealed that the peptides bound more strongly to DNA than did IN. These results suggested that one possible peptide bactericidal mechanism may relate to the inhibition of intracellular functions via interference with DNA/RNA synthesis. Furthermore, some model peptides, containing K(6)L(2)W(3), K(5)L(3)W(3), R(6)L(2)W(3), O(6)L(2)W(3), O(6)L(2)W(3), and K(6)L(2)W(3)-D inhibited LPS-induced inducible nitric oxide synthase (iNOS) mRNA expression, the release of nitric oxide (NO) following LPS stimulation in RAW264.7 cells and had powerful LPS binding activities at bactericidal concentrations. Collectively, our results indicated that these peptides have potential for future development as novel antimicrobial and anti-inflammatory agents.

Hydrophilic modifications of an amphiphilic polynorbornene and the effects on its hemolytic and antibacterial activity

Here we report the modification of an amphiphilic antibacterial polynorbornene, Poly3, via incorporation of hydrophilic, biocompatible groups. The sugar, zwitterionic, and polyethylene glycol based moieties were incorporated in varying ratios by copolymerization and postpolymerization techniques. Well-defined copolymers with molecular weights of 3 kDa and narrow polydispersity indices ranging from 1.08 to 1.15 were obtained. The effects of these modifications on the biological activity of these polymers were analyzed by determining their minimum inhibitory concentrations (MIC) and their hemolytic activities (HC50).

Effects of hydrophobicity on the antifungal activity of alpha-helical antimicrobial peptides

We utilized a series of analogs of D-V13K (a 26-residue amphipathic alpha-helical antimicrobial peptide, denoted D1) to compare and contrast the role of hydrophobicity on antifungal and antibacterial activity to the results obtained previously with Pseudomonas aeruginosa strains. Antifungal activity for zygomycota fungi decreased with increasing hydrophobicity (D-V13K/A12L/A20L/A23L, denoted D4, the most hydrophobic analog was sixfold less active than D1, the least hydrophobic analog). In contrast, antifungal activity for ascomycota fungi increased with increasing hydrophobicity (D4, the most hydrophobic analog was fivefold more active than D1). Hemolytic activity is dramatically affected by increasing hydrophobicity with peptide D4 being 286-fold more hemolytic than peptide D1. The therapeutic index for peptide D1 is 1569-fold and 62-fold better for zygomycota fungi and ascomycota fungi, respectively, compared with peptide D4. To reduce the hemolytic activity of peptide D4 and improve/maintain the antifungal activity of D4, we substituted another lysine residue in the center of the non-polar face (V16K) to generate D5 (D-V13K/V16K/A12L/A20L/A23L). This analog D5 decreased hemolytic activity by 13-fold, enhanced antifungal activity to zygomycota fungi by 16-fold and improved the therapeutic index by 201-fold compared with D4 and represents a unique approach to control specificity while maintaining high hydrophobicity in the two hydrophobic segments on the non-polar face of D5.

The role of hydrophobic amino acid grafts in the enhancement of membrane-disruptive activity of pH-responsive pseudo-peptides

pH-responsive polymers have been synthesised by grafting l-valine (PV-75), l-leucine (PL-75) and l-phenylalanine (PP-75) onto the pendant carboxylic acid moieties of a pseudo-peptide, poly(l-lysine iso-phthalamide), at a stoichiometric degree of substitution of 75 mol%. The effect of such modification on the pH-, concentration- and time-dependent cell membrane-disruptive activity of the grafted polymers has been investigated using a haemolysis model. At 0.025 mg mL(-1), the grafted polymers were almost non-haemolytic at pH 7.4, but mediated considerable membrane lysis after 60 min in the pH range characteristic of early endosomes, which ranked in the order: PP-75 > PL-75 > PV-75 > poly(l-lysine iso-phthalamide). PP-75 was 35-fold more lytic on a molar basis than the membrane-lytic peptide melittin. With increasing concentration, the grafted polymers showed an increased ability to lyse cell membranes and caused noticeable membrane disruption at physiological pH. The mechanism of the polymer-mediated membrane destabilisation has been investigated. The in-vitro cytotoxicity of the grafted polymers has been assessed using a propidium iodide fluorescence assay. It has been demonstrated by confocal microscopy that the grafted polymers can induce a significant release of endocytosed materials into the cytoplasm of HeLa cells, which is a feature critical for drug delivery applications.

Intrinsic amino acid side-chain hydrophilicity/hydrophobicity coefficients determined by reversed-phase high-performance liquid chromatography of model peptides: comparison with other hydrophilicity/hydrophobicity scales

An accurate determination of the intrinsic hydrophilicity/hydrophobicity of amino acid side-chains in peptides and proteins is fundamental in understanding many area of research, including protein folding and stability, peptide and protein function, protein-protein interactions and peptide/protein oligomerization, as well as the design of protocols for purification and characterization of peptides and proteins. Our definition of intrinsic hydrophilicity/hydrophobicity of side-chains is the maximum possible hydrophilicity/hydrophobicity of side-chains in the absence of any nearest-neighbor effects and/or any conformational effects of the polypeptide chain that prevent full expression of side-chain hydrophilicity/hydrophobicity. In this review, we have compared an experimentally derived intrinsic side-chain hydrophilicity/hydrophobicity scale generated from RP-HPLC retention behavior of de novo designed synthetic model peptides at pH 2 and pH 7 with other RP-HPLC-derived scales, as well as scales generated from classic experimental and calculation-based methods of octanol/water partitioning of Nalpha-acetyl-amino-acid amides or free energy of transfer of free amino acids. Generally poor correlation was found with previous RP-HPLC-derived scales, likely due to the random nature of the peptide mixtures in terms of varying peptide size, conformation and frequency of particular amino acids. In addition, generally poor correlation with the classical approaches served to underline the importance of the presence of a polypeptide backbone when generating intrinsic values. We have shown that the intrinsic scale determined here is in full agreement with the structural characteristics of amino acid side-chains.

Mechanism of a prototypical synthetic membrane-active antimicrobial: Efficient hole-punching via interaction with negative intrinsic curvature lipids

Phenylene ethynylenes comprise a prototypical class of synthetic antimicrobial compounds that mimic antimicrobial peptides produced by eukaryotes and have broad-spectrum antimicrobial activity. We show unambiguously that bacterial membrane permeation by these antimicrobials depends on the presence of negative intrinsic curvature lipids, such as phosphatidylethanolamine (PE) lipids, found in high concentrations within bacterial membranes. Plate-killing assays indicate that a PE-knockout mutant strain of Escherichia coli drastically out-survives the wild type against the membrane-active phenylene ethynylene antimicrobials, whereas the opposite is true when challenged with traditional metabolic antibiotics. That the PE deletion is a lethal mutation in normative environments suggests that resistant bacterial strains do not evolve because a lethal mutation is required to gain immunity. PE lipids allow efficient generation of negative curvature required for the circumferential barrel of an induced membrane pore; an inverted hexagonal H(II) phase, which consists of arrays of water channels, is induced by a small number of antimicrobial molecules. The estimated antimicrobial occupation in these water channels is nonlinear and jumps from approximately 1 to 3 per 4 nm of induced water channel length as the global antimicrobial concentration is increased. By comparing to exactly solvable 1D spin models for magnetic systems, we quantify the cooperativity of these antimicrobials.

The central kink region of fowlicidin-2, an alpha-helical host defense peptide, is critically involved in bacterial killing and endotoxin neutralization

Fowlicidins are a group of newly identified chicken cathelicidin host defense peptides. We have shown that the putatively mature fowlicidin-2 of 31 amino acid residues possesses potent antibacterial and lipopolysaccharide (LPS)- neutralizing activities, but with a noticeable toxicity to mammalian cells. As a first step in exploring the structure-activity relationships of fowlicidin-2, in this study we determined its tertiary structure by nuclear magnetic resonance spectroscopy. Unlike the majority of cathelicidins, which are composed of a predominant alpha-helix with a short hinge sequence near the center, fowlicidin-2 consists of 2 well-defined alpha-helical segments (residues 6-12 and 23-27) connected by a long extensive kink (residues 13-20) induced by proline. To further investigate the functional significance of each of these structural components, several N- and C-terminal deletion analogs of fowlicidin-2 were synthesized and analyzed for their antibacterial, cytotoxic and LPS-neutralizing activities. Our results indicated that neither the N- nor C-terminal alpha-helix alone is sufficient to confer any function. Rather, fowlicidin-2(1-18) and fowlicidin-2(15-31), 2 alpha-helical segments with inclusion of the central cationic kink region, retained substantial capacities to kill bacteria and neutralize the LPS-induced proinflammatory response, relative to the parent peptide. More desirably, these 2 peptide analogs showed substantially reduced toxicity to human erythrocytes and epithelial cells, indicative of improved potential as antibacterial and antisepsis agents. To our knowledge, fowlicidin-2 is the first alpha-helical cathelicidin, with the central kink region shown to be critically important in killing bacteria and neutralizing LPS.

Structural determinants of antimicrobial and antiplasmodial activity and selectivity in histidine-rich amphipathic cationic peptides

Designed histidine-rich amphipathic cationic peptides, such as LAH4, have enhanced membrane disruption and antibiotic properties when the peptide adopts an alignment parallel to the membrane surface. Although this was previously achieved by lowering the pH, here we have designed a new generation of histidine-rich peptides that adopt a surface alignment at neutral pH. In vitro, this new generation of peptides are powerful antibiotics in terms of the concentrations required for antibiotic activity; the spectrum of target bacteria, fungi, and parasites; and the speed with which they kill. Further modifications to the peptides, including the addition of more hydrophobic residues at the N terminus, the inclusion of a helix-breaking proline residue or using D-amino acids as building blocks, modulated the biophysical properties of the peptides and led to substantial changes in toxicity to human and parasite cells but had only a minimal effect on the antibacterial and antifungal activity. Using a range of biophysical methods, in particular solid-state NMR, we show that the peptides are highly efficient at disrupting the anionic lipid component of model membranes. However, we also show that effective pore formation in such model membranes may be related to, but is not essential for, high antimicrobial activity by cationic amphipathic helical peptides. The information in this study comprises a new layer of detail in the understanding of the action of cationic helical antimicrobial peptides and shows that rational design is capable of producing potentially therapeutic membrane active peptides with properties tailored to their function.

Effects of net charge and the number of positively charged residues on the biological activity of amphipathic alpha-helical cationic antimicrobial peptides

In our previous study, we utilized a 26-residue amphipathic alpha-helical antimicrobial peptide L-V13K (Chen et al., Antimicrob Agents Chemother 2007, 51, 1398-1406) as the framework to study the effects of peptide hydrophobicity on the mechanism of its antimicrobial action. In this study, we explored the effects of net charge and the number of positively charged residues on the hydrophilic/polar face of L-V13K on its biological activity (antimicrobial and hemolytic) and biophysical properties (hydrophobicity, amphipathicity, helicity, and peptide self-association). The net charge of V13K analogs at pH 7 varied between -5 and +10 and the number of positively charged residues varied from 1 to 10. The minimal inhibitory concentrations (MIC) against six strains of Pseudomonas aeruginosa as well as other gram-negative and gram-positive bacteria were determined along with the maximal peptide concentration that produces no hemolysis of human red blood cells (MHC). Our results show that the number of positively charged residues on the polar face and net charge are both important for both antimicrobial activity and hemolytic activity. The most dramatic observation is the sharp transition of hemolytic activity on increasing one positive charge on the polar face of V13K i.e., the change from +8 to +9 resulted in greater than 32-fold increase in hemolytic activity (250 microg/ml to <7.8 microg/ml, respectively).

Antimicrobial polymers prepared by ROMP with unprecedented selectivity: a molecular construction kit approach

Synthetic Mimics of Antimicrobial Peptides (SMAMPs) imitate natural host-defense peptides, a vital component of the body's immune system. This work presents a molecular construction kit that allows the easy and versatile synthesis of a broad variety of facially amphiphilic oxanorbornene-derived monomers. Their ring-opening metathesis polymerization (ROMP) and deprotection provide several series of SMAMPs. Using amphiphilicity, monomer feed ratio, and molecular weight as parameters, polymers with 533 times higher selectivitiy (selecitviy = hemolytic concentration/minimum inhibitory concentration) for bacteria over mammalian cells were discovered. Some of these polymers were 50 times more selective for Gram-positive over Gram-negative bacteria while other polymers surprisingly showed the opposite preference. This kind of "double selectivity" (bacteria over mammalian and one bacterial type over another) is unprecedented in other polymer systems and is attributed to the monomer's facial amphiphilicity.

Membrane structure and interactions of a short Lycotoxin I analogue

Lycotoxin I and Lycotoxin II are natural anti-microbial peptides that were identified in the venom of the Wolf Spider Lycosa carolinensis. These peptides were found to be potent growth inhibitors for bacteria (Escherichia coli) and yeast (Candida glabrata) at micromolar concentrations. Recently, shortened analogues of LycoI and LycoII have been reported to have decreased haemolytic effects. A shorter Lyco-I analogue studied, LycoI 1-15 (H-IWLTALKFLGKHAAK-NH2), was active only above 10 microM, but was also the least haemolytic. On the basis of these findings, we became interested in obtaining a deeper insight into the membrane activity of LycoI 1-15, as this peptide may represent the first major step for the future development of selective, i.e. non-haemolytic, Lycotoxin-based antibiotics. The interaction of this peptide with liposomes of different composition was studied by microcalorimetry [differential scanning calorimetry (DSC) and isothermal titration calorimetry (ITC)] and CD. The results obtained from the calorimetric and spectroscopic techniques were jointly discussed in an attempt to further understand the interaction of this peptide with model membranes.

Alternative stabilities of a proline-rich antibacterial peptide in vitro and in vivo

The proline-rich designer antibacterial peptide dimer A3-APO is currently under preclinical development for the treatment of systemic infections caused by antibiotic-resistant Gram-negative bacteria. The peptide showed remarkable stability in 25% mouse serum in vitro, exhibiting a half-life of approximately 100 min as documented by reversed-phase chromatography. Indeed, after a 30-min incubation period in undiluted mouse serum ex vivo, mass spectrometry failed to identify any degradation product. The peptide was still a major peak in full blood ex vivo, however, with degradation products present corresponding to amino-terminal cleavage. When injected into mice intravenously, very little, if any unmodified peptide could be detected after 30 min. Nevertheless, the major early metabolite, a full single-chain fragment, was detectable until 90 min, and this fragment exhibited equal or slightly better activity in the broth microdilution antimicrobial assay against a panel of resistant Enterobactericeae strains. The Chex1-Arg20 metabolite, when administered three times at 20 mg/kg to mice infected with a sublethal dose (over LD(50)) of an extended spectrum beta-lactamase-producing Escherichia coli strain, completely sterilized the mouse blood, similar to imipenem added at a higher dose. The longer and presumably more immunogenic prodrug A3-APO, injected subcutaneously twice over a 3-wk period, did not induce any antibody production, indicating the suitability of this peptide or its active metabolite for clinical development.

Peptoids that mimic the structure, function, and mechanism of helical antimicrobial peptides

Antimicrobial peptides (AMPs) and their mimics are emerging as promising antibiotic agents. We present a library of "ampetoids" (antimicrobial peptoid oligomers) with helical structures and biomimetic sequences, several members of which have low-micromolar antimicrobial activities, similar to cationic AMPs like pexiganan. Broad-spectrum activity against six clinically relevant BSL2 pathogens is also shown. This comprehensive structure-activity relationship study, including circular dichroism spectroscopy, minimum inhibitory concentration assays, hemolysis and mammalian cell toxicity studies, and specular x-ray reflectivity measurements shows that the in vitro activities of ampetoids are strikingly similar to those of AMPs themselves, suggesting a strong mechanistic analogy. The ampetoids' antibacterial activity, coupled with their low cytotoxicity against mammalian cells, make them a promising class of antimicrobials for biomedical applications. Peptoids are biostable, with a protease-resistant N-substituted glycine backbone, and their sequences are highly tunable, because an extensive diversity of side chains can be incorporated via facile solid-phase synthesis. Our findings add to the growing evidence that nonnatural foldamers will emerge as an important class of therapeutics.

Purification, characterization and homology analysis of ocellatin 4, a cytolytic peptide from the skin secretion of the frog Leptodactylus ocellatus

Neobatrachia is the amphibian suborder with the largest number of species and a most important source of bioactive peptides from frog skin secretions. However, 90% of the studies on this subject have been focused on the frog families Hylidae and Ranidae, while very little is known about peptides of other families, like Leptodactylidae. Our work reports for the first time the isolation and characterization of ocellatin 4 (GLLDFVTGVGKDIFAQLIKQI-NH(2)), a cytolytic peptide from the skin secretion of the South American frog Leptodactylus ocellatus. While most cytolytic amphibian skin peptides are selective for microorganisms and harmless for mammalian cells, the HC(50) of ocellatin 4 against human erythrocytes is 14.3muM. The interaction between ocellatin 4 and zwitterionic phospholipids in mammalian plasma membranes may be favored by its neutral charge at pH 7.0. Ocellatin 4 also shows some antibacterial activity (MICs(E. coli)(and)(S. aureus)=64muM) and its sequence shares similarities with the only six leptodactylid peptides previously known and with four peptides from Australian hylid frogs of the genus Litoria.

Penetration of the Antimicrobial Peptide Dicynthaurin into Phospholipid Monolayers at the Liquid–Air Interface

This work focuses on the adsorption kinetics of dicynthaurin with lipid monolayers, the effect of peptide adsorption on the structure of the lipid condensed chain lattice, peptide orientation and secondary structure in the adsorbed state. The studies with DPPG as model system revealed strong adsorption and massive incorporation of the peptide into the monolayer. Infrared reflection absorption spectroscopy (IRRAS) experiments showed that the secondary structure of the peptide is maintained upon adsorption. Specular X-ray reflectivity showed the destabilization of the condensed phase of the pure lipid monolayer and revealed a tilted orientation of the long axis of the peptide helix of about 40 degrees from the surface normal. Incorporation of the peptide was found to be pressure dependent, and at high pressure a "squeeze-out" was observed; however, the peptide remained localized to the interface, as suggested by infrared data. These findings were supported by optical fluorescence microscopy measurements which showed the squeeze-out of the peptide on water, but not under physiological conditions. The results suggest that dicynthaurin is able to adsorb to the phosphatidylglycerol-rich inner cytoplasmic membrane of bacteria and alter membrane integrity. To identify and interact with membrane motifs that are characteristic of microbes, but which are absent in eukaryotic cells, might be an intrinsic ability of peptide antibiotics.

Substitution of the leucine zipper sequence in melittin with peptoid residues affects self-association, cell selectivity, and mode of action

Melittin (ME), a non-cell-selective antimicrobial peptide, contains the leucine zipper motif, wherein every seventh amino acid is leucine or isolucine. Here, we attempted to generate novel cell-selective peptides by substituting amino acids in the leucine zipper sequence of ME with peptoid residues. We generated a series of ME analogues by replacing Leu-6, Lue-13 and Ile-20 with Nala, Nleu, Nphe, or Nlys, and we examined their secondary structure, self-association activity, cell selectivity and mode of action. Circular dichroism spectroscopy indicated that the substitutions disrupt the alpha-helical structure of ME in micelles of sodium dodecyl sulfate and on negatively charged and zwitterionic phospholipid vesicles. Substitution by Nleu, Nphe, or Nlys but not Nala disturbed the self-association in an aqueous environment, interaction with zwitterionic membranes, and toxicity to mammalian cells of ME but did not affect the interaction with negatively charged membranes or antibacterial activity. Notably, peptides with Nphe or Nlys substitution had the highest therapeutic indices, consistent with their lipid selectivity. In addition, all of peptoid residue-containing ME analogues had little or no ability to induce membrane disruption, membrane depolarization and lipid flip-flop. Taken together, our studies indicate that substitution of the leucine zipper motif in ME with peptoid residues increases its selectivity against bacterial cells by impairing self-association activity and changes its mode of antibacterial action from membrane-targeting mechanism to possible intracellular targeting mechanism. Furthermore, our ME analogues especially those with Nleu, Nphe, or Nlys substitutions, may be therapeutically useful antimicrobial peptides.

Potential therapeutic applications of the cyclotides and related cystine knot mini-proteins

Cyclotides are naturally occurring mini-proteins that have a cyclic peptide backbone and a knotted arrangement of three disulfide bonds. They are remarkably stable and have a diverse range of therapeutically useful biological activities, including antimicrobial and anti-HIV activity, although their natural function appears to be as plant defence agents. Cyclotides are amenable to chemical synthesis and the potential exists to graft new bioactivities onto their cyclic cystine knot framework as a way of stabilising peptide drugs. Over the last few years, proof-of-concept that bioactive peptide epitopes can be grafted onto cyclotides and related cystine knot mini-proteins has been obtained. The cystine knot framework is tolerant to a wide range of residue substitutions and is showing great promise as a scaffold in drug design and protein engineering.