Mechanisms of antimicrobial, cytolytic, and cell-penetrating peptides: from kinetics to thermodynamics

All-or-none versus graded: single-vesicle analysis reveals lipid composition effects on membrane permeabilization

We report a single-vesicle approach to compare the all-or-none and graded mechanisms of lipid bilayer permeabilization by CpreTM and NpreTM, two peptides derived from the membrane-proximal external region of the HIV fusion glycoprotein gp41 subunit. According to bulk requenching assays, these peptides permeabilize large unilamellar vesicles via all-or-none and graded mechanisms, respectively. Visualization of the process using giant unilamellar vesicles shows that the permeabilization of individual liposomes by these two peptides differs in kinetics, degree of dye filling, and stability of the permeabilized state. All-or-none permeabilization by CpreTM is characterized by fast and total filling of the individual vesicles. This process is usually accompanied by the formation of stably open pores, as judged from the capacity of the vesicles to incorporate a second dye added after several hours. In contrast, graded permeabilization by NpreTM is transient and exhibits slower kinetics, which leads to partial filling of the individual liposomes. Of importance, quantitative analysis of vesicle population distribution allowed the identification of mixed mechanisms of membrane permeabilization and the assessment of cholesterol effects. Specifically, the presence of this viral envelope lipid increased the stability of the permeating structures, which may have implications for the fusogenic activity of gp41.

Spotlight on Human LL-37, an Immunomodulatory Peptide with Promising Cell-Penetrating Properties

Cationic antimicrobial peptides are major components of innate immunity and help control the initial steps of the infectious process. They are expressed not only by immunocytes, but also by epithelial cells. They share an amphipathic secondary structure with a polar cationic site, which explains their tropism for prokaryote membranes and their hydrophobic site contributing to the destructuration of these membranes. LL-37 is the only cationic antimicrobial peptide derived from human cathelicidin. LL-37 can also cross the plasma membrane of eukaryotic cells, probably through special domains of this membrane called lipid rafts. This transfer could be beneficial in the context of vaccination: the activation of intracellular toll-like receptors by a complex formed between CpG oligonucleotides and LL-37 could conceivably play a major role in the building of a cellular immunity involving NK cells.

Synthesis, characterization, and thermodynamic study of a polyvalent lytic peptide-polymer conjugate as novel anticancer agent

We designed and synthesized a new polyvalent lytic peptide-polymer conjugate as a novel chemotherapeutic agent capable of overcoming multidrug resistance. A hexapeptide (KWKWKW or (KW)₃) was designed and conjugated to dextran in multiple copies to afford a polyvalent conjugate. A robust synthesis procedure involving click chemistry and the detailed characterization of the conjugate were reported here. The conjugate Dex-(KW)₃ exhibited significantly enhanced anticancer potency in vitro by up to 500-fold compared to monomeric (KW)₃. The LC₅₀ value was comparable to that of conventional lytic peptides which have more than 20 residues. No hemolytic activity was shown by the conjugates up to 300 μM. Thermodynamic study indicated that the binding of conjugates was predominantly entropy-driven while the binding of free peptides was mainly enthalpy-driven, implying a deeper penetration of conjugate into the core of lipid bilayer. The binding affinity of conjugate to neutral membrane is much higher than that to free peptide (K(conj) ≈ 8822.9 M⁻¹, K(pep) ≈ 1884.7 M⁻¹). In binding to negatively charged membrane, the conjugate surpassed free peptides at high concentrations when the binding of free peptides became saturated. The higher binding capability, attributed to the high local concentration of peptides mounted on a polymer backbone, explains the superior anticancer activity of polyvalent Dex-(KW)₃.

Describing the mechanism of antimicrobial peptide action with the interfacial activity model

Antimicrobial peptides (AMPs) have been studied for three decades, and yet a molecular understanding of their mechanism of action is still lacking. Here we summarize current knowledge for both synthetic vesicle experiments and microbe experiments, with a focus on comparisons between the two. Microbial experiments are done at peptide to lipid ratios that are at least 4 orders of magnitude higher than vesicle-based experiments. To close the gap between the two concentration regimes, we propose an "interfacial activity model", which is based on an experimentally testable molecular image of AMP-membrane interactions. The interfacial activity model may be useful in driving engineering and design of novel AMPs.

Probing membrane topology of the antimicrobial peptide distinctin by solid-state NMR spectroscopy in zwitterionic and charged lipid bilayers

Distinctin is a 47-residue antimicrobial peptide, which interacts with negatively charged membranes and is active against Gram-positive and Gram-negative bacteria. Its primary sequence comprises two linear chains of 22 (chain 1) and 25 (chain 2) residues, linked by a disulfide bridge between Cys19 of chain 1 and Cys23 of chain 2. Unlike other antimicrobial peptides, distinctin in the absence of the lipid membrane has a well-defined three-dimensional structure, which protects it from protease degradation. Here, we used static solid-state NMR spectroscopy in mechanically aligned lipid bilayers (charged or zwitterionic) to study the topology of distinctin in lipid bilayers. We found that this heterodimeric peptide adopts an ordered conformation absorbed on the surface of the membrane, with the long helix (chain 2), approximately parallel to the lipid bilayer (~5° from the membrane plane) and the short helix (chain 1) forming a ~24° angle with respect to the bilayer plane. Since the peptide does not disrupt the macroscopic alignment of charged or zwitterionic lipid bilayers at lipid-to-protein molar ratio of 50:1, it is possible that higher peptide concentrations might be needed for pore formation, or alternatively, distinctin elicits its cell disruption action by another mechanism.

Diffusion as a probe of the heterogeneity of antimicrobial peptide-membrane interactions

Many antimicrobial peptides (AMPs) function by forming various oligomeric structures and/or pores upon binding to bacterial membranes. Because such peptide aggregates are capable of inducing membrane thinning and membrane permeabilization, we expected that AMP binding would also affect the diffusivity or mobility of the lipid molecules in the membrane. Herein, we show that measurements of the diffusion times of individual lipids through a confocal volume via fluorescence correlation spectroscopy (FCS) provide a sensitive means of probing the underlying AMP-membrane interactions. In particular, results obtained with two well-studied AMPs, magainin 2 and mastoparan X, and two model membranes indicate that this method is capable of revealing structural information, especially the heterogeneity of the peptide-membrane system, that is otherwise difficult to obtain using common ensemble methods. Moreover, because of the high sensitivity of FCS, this method allows examination of the effect of AMPs on the membrane structure at very low peptide/lipid ratios.

Structural contributions to the intracellular targeting strategies of antimicrobial peptides

The interactions of cationic amphipathic antimicrobial peptides (AMPs) with anionic biological membranes have been the focus of much research aimed at improving the activity of such compounds in the search for therapeutic leads. However, many of these peptides are thought to have other polyanions, such as DNA or RNA, as their ultimate target. Here a combination of fluorescence and circular dichroism (CD) spectroscopies has been used to assess the structural properties of amidated versions of buforin II, pleurocidin and magainin 2 that support their varying abilities to translocate through bacterial membranes and bind to double stranded DNA. Unlike magainin 2 amide, a prototypical membrane disruptive AMP, buforin II amide adopts a poorly helical structure in membranes closely mimicking the composition of Gram negative bacteria, such as Escherichia coli, and binds to a short duplex DNA sequence with high affinity, ultimately forming peptide-DNA condensates. The binding affinities of the peptides to duplex DNA are shown to be related to the structural changes that they induce. Furthermore, CD also reveals the conformation of the bound peptide buforin II amide. In contrast with a synthetic peptide, designed to adopt a perfect amphipathic alpha-helix, buforin II amide adopts an extended or polyproline II conformation when bound to DNA. These results show that an alpha-helix structure is not required for the DNA binding and condensation activity of buforin II amide.

Novel triblock oligopeptides as efficient nonviral vectors: characterisation and further insights

The delivery of immunomodulatory genes into the cornea prior to transplantation is one promising strategy to improve graft survival rates. We recently reported a class of novel triblock oligopeptides that could mediate efficient gene transfer into corneal endothelial cells. Now these peptides are characterised further and it is show that they lack distinct secondary structures. Peptide complexes are also demonstrated to be weakly haemolytic and transfection efficiency is shown to be sensitive to several experimental conditions. SEM and FRET confocal images are used to study the particle morphology and to show that they condense their DNA cargo well.

Structural polymorphism of non-covalent peptide-based delivery systems: highway to cellular uptake

During the last two decades, delivery has become a major challenge for the development of new therapeutic molecules for the clinic. Although, several strategies either viral or non viral have been proposed to favor cellular uptake and targeting of therapeutics, only few of them have reach preclinical evaluation. Amongst them, cell-penetrating peptide (CPP) constitutes one of the most promising strategy and has applied for systemic in vivo delivery of a variety of therapeutic molecules. Two CPP-strategies have been described; using peptide carriers either covalently-linked to the cargo or forming non-covalent stable complexes with cargo. Peptide-based nanoparticle delivery system corresponds to small amphipathic peptides able to form stable nanoparticles with either proteins/peptides or nucleic acids and to enter the cell independently of the endosomal pathway. Three families of peptide-based nanoparticle systems; MPG, PEP and CADY have been successfully used for the delivery of various biologically active cargoes both ex vivo and in vivo in several animal models. This review will focus on the mechanism of the peptide-based nanoparticles; PEP, MPG and CADY in a structural and biophysical context. It will also highlight the major parameters associated to particle formation/stabilization and the impact of the carrier structural polymorphism in triggering cellular uptake.

Orthologs of magainin, PGLa, procaerulein-derived, and proxenopsin-derived peptides from skin secretions of the octoploid frog Xenopus amieti (Pipidae)

The Volcano clawed frog Xenopus amieti Kobel, du Pasquier, Fischberg, and Gloor, 1980, with a chromosome number of 2n=72, is believed to have undergone two rounds of genome duplication since evolving from a diploid ancestor. Nine peptides with differential antimicrobial activity against Escherichia coli and Staphylococcus aureus were isolated from norepinephrine-stimulated skin secretions of X. amieti that showed structural similarity to peptides previously isolated from the tetraploid frog X. laevis (2n=36) and the diploid frog Silurana (formerly Xenopus) tropicalis (2n=20). Two peptides (magainin-AM1 and -AM2) are othologous to the magainins, two peptides (PGLa-AM1 and -AM2) orthologous to peptide glycine-leucine-amide, four peptides (CPF-AM1, -AM2, -AM3, -AM4) orthologous to caerulein-precursor fragments, and one peptide (XPF-AM1) structurally similar to xenopsin-precursor fragments were characterized. CFP-AM1 (GLGSVLGKALKIGANLL.NH(2)) was the most potent peptide present in the secretions and magainin-AM2 (GVSKILHSAGKFGKAFLGEIMKS) was the most abundant. The data indicate that nonfunctionalization has been the most common fate of duplicated antimicrobial peptide genes following the polyploidization events in the X. amieti lineage. However, the very low antimicrobial activity of the magainin-AM1 and PGLa-AM2 paralogs suggests the possibility that certain peptides may have evolved toward a new, as yet undetermined, function (neofunctionalization).

Secondary structure in de novo designed peptides induced by electrostatic interaction with a lipid bilayer membrane

We show that it is possible to induce a defined secondary structure in de novo designed peptides upon electrostatic attachment to negatively charged lipid bilayer vesicles without partitioning of the peptides into the membrane, and that the secondary structure can be varied via small changes in the primary amino acid sequence of the peptides. The peptides have a random-coil conformation in solution, and results from far-UV circular dichroism spectroscopy demonstrate that the structure induced by the interaction with silica nanoparticles is solely alpha-helical and also strongly pH-dependent. The present study shows that negatively charged vesicles, to which the peptides are electrostatically adsorbed via cationic amino acid residues, induce either alpha-helices or beta-sheets and that the conformation is dependent on both lipid composition and variations in peptide primary structure. The pH-dependence of the vesicle-induced peptide secondary structure is weak, which correlates well with small differences in the vesicles' electrophoretic mobility, and thus the surface charge, as the pH is varied.

High-throughput discovery of broad-spectrum peptide antibiotics

Membrane-permeabilizing peptide antibiotics are an underutilized weapon in the battle against drug-resistant microorganisms. This is true, in part, because of the bottleneck caused by the lack of explicit design principles and the paucity of simple high-throughput methods for selection. In this work, we characterize the requirements for broad-spectrum antimicrobial activity by membrane permeabilization and find that different microbial membranes have very different susceptibilities to permeabilization by individual antimicrobial peptides. Broad-spectrum activity requires only that an AMP have at least a small amount of membrane-permeabilizing activity against multiple classes of microbes, a feature that we show to be rare in a peptide library containing many members with species-specific activity. We compare biological and vesicle-based high-throughput strategies for selecting such broad-spectrum AMPs from combinatorial peptide libraries and demonstrate that a simple in vitro, lipid vesicle-based high-throughput screen is the most effective strategy for rapid discovery of novel, broad-spectrum antimicrobial peptides.

Lipid clustering by three homologous arginine-rich antimicrobial peptides is insensitive to amino acid arrangement and induced secondary structure

Three Arg-rich nonapeptides, containing the same amino acid composition but different sequences, PFWRIRIRR-amide (PR-9), RRPFWIIRR-amide (RR-9) and PRFRWRIRI-amide (PI-9), are able to induce segregation of anionic lipids from zwitterionic lipids, as shown by changes in the phase transition properties of lipid mixtures detected by differential scanning calorimetry and freeze fracture electron microscopy. The relative Minimal Inhibitory Concentration (MIC) of these three peptides against several strains of Gram positive bacteria correlated well with the extent to which the lipid composition of the bacterial membrane facilitated peptide-induced clustering of anionic lipids. The lower activity of these three peptides against Gram negative bacteria could be explained by the retention of these peptides in the LPS layer. The membrane morphologies produced by PR-9 as well as by a cathelicidin fragment, KR-12 that had previously been shown to induce anionic lipid clustering, was directly visualized using freeze fracture electron microscopy. This work shows the insensitivity of phase segregation to the specific arrangement of the cationic charges in the peptide sequence as well as to their tendency to form different secondary structures. It also establishes the role of anionic lipid clustering in the presence of zwitterionic lipids in determining antimicrobial selectivity.

Helical membrane peptides to modulate cell function

In recent years there has been an abundance of research into the potential of helical peptides to influence cell function. These peptides have been used to achieve a variety of different outcomes from cell repair to cell death, depending upon the peptide sequence and the nature of its interactions with cell membranes and membrane proteins. In this critical review, we summarise several mechanisms by which helical peptides, acting as either transporters, inhibitors, agonists or antibiotics, can have significant effects on cell membranes and can radically affect the internal mechanisms of the cell. The various approaches to peptide design are discussed, including the role of naturally-occurring proteins in the design of these helical peptides and current breakthroughs in the use of non-natural (and therefore more stable) peptide scaffolds. Most importantly, the current successful applications of these peptides, and their potential uses in the field of medicine, are reviewed (131 references).

The effect of acidic residues and amphipathicity on the lytic activities of mastoparan peptides studied by fluorescence and CD spectroscopy

Some mastoparan peptides extracted from social wasps display antimicrobial activity and some are hemolytic and cytotoxic. Although the cell specificity of these peptides is complex and poorly understood, it is believed that their net charges and their hydrophobicity contribute to modulate their biological activities. We report a study, using fluorescence and circular dichroism spectroscopies, evaluating the influence of these two parameters on the lytic activities of five mastoparans in zwitterionic and anionic phospholipid vesicles. Four of these peptides, extracted from the venom of the social wasp Polybia paulista, present both acidic and basic residues with net charges ranging from +1 to +3 which were compared to Mastoparan-X with three basic residues and net charge +4. Previous studies revealed that these peptides have moderate-to-strong antibacterial activity against Gram-positive and Gram-negative microorganisms and some of them are hemolytic. Their affinity and lytic activity in zwitterionic vesicles decrease with the net electrical charges and the dose response curves are more cooperative for the less charged peptides. Higher charged peptides display higher affinity and lytic activity in anionic vesicles. The present study shows that the acidic residues play an important role in modulating the peptides' lytic and biological activities and influence differently when the peptide is hydrophobic or when the acidic residue is in a hydrophilic peptide.

HAMLET interacts with lipid membranes and perturbs their structure and integrity

BACKGROUND

Cell membrane interactions rely on lipid bilayer constituents and molecules inserted within the membrane, including specific receptors. HAMLET (human alpha-lactalbumin made lethal to tumor cells) is a tumoricidal complex of partially unfolded alpha-lactalbumin (HLA) and oleic acid that is internalized by tumor cells, suggesting that interactions with the phospholipid bilayer and/or specific receptors may be essential for the tumoricidal effect. This study examined whether HAMLET interacts with artificial membranes and alters membrane structure.

METHODOLOGY/PRINCIPAL FINDINGS

We show by surface plasmon resonance that HAMLET binds with high affinity to surface adherent, unilamellar vesicles of lipids with varying acyl chain composition and net charge. Fluorescence imaging revealed that HAMLET accumulates in membranes of vesicles and perturbs their structure, resulting in increased membrane fluidity. Furthermore, HAMLET disrupted membrane integrity at neutral pH and physiological conditions, as shown by fluorophore leakage experiments. These effects did not occur with either native HLA or a constitutively unfolded Cys-Ala HLA mutant (rHLA(all-Ala)). HAMLET also bound to plasma membrane vesicles formed from intact tumor cells, with accumulation in certain membrane areas, but the complex was not internalized by these vesicles or by the synthetic membrane vesicles.

CONCLUSIONS/SIGNIFICANCE

The results illustrate the difference in membrane affinity between the fatty acid bound and fatty acid free forms of partially unfolded HLA and suggest that HAMLET engages membranes by a mechanism requiring both the protein and the fatty acid. Furthermore, HAMLET binding alters the morphology of the membrane and compromises its integrity, suggesting that membrane perturbation could be an initial step in inducing cell death.

Inhibition of Escherichia coli ATP synthase by amphibian antimicrobial peptides

Previously melittin, the alpha-helical basic honey bee venom peptide, was shown to inhibit F(1)-ATPase by binding at the beta-subunit DELSEED motif of F(1)F(o)-ATP synthase. Herein, we present the inhibitory effects of the basic alpha-helical amphibian antimicrobial peptides, ascaphin-8, aurein 2.2, aurein 2.3, carein 1.8, carein 1.9, citropin 1.1, dermaseptin, maculatin 1.1, maganin II, MRP, or XT-7, on purified F(1) and membrane bound F(1)F(0)Escherichia coli ATP synthase. We found that the extent of inhibition by amphibian peptides is variable. Whereas MRP-amide inhibited ATPase essentially completely (approximately 96% inhibition), carein 1.8 did not inhibit at all (0% inhibition). Inhibition by other peptides was partial with a range of approximately 13-70%. MRP-amide was also the most potent inhibitor on molar scale (IC(50) approximately 3.25 microM). Presence of an amide group at the c-terminal of peptides was found to be critical in exerting potent inhibition of ATP synthase ( approximately 20-40% additional inhibition). Inhibition was fully reversible and found to be identical in both F(1)F(0) membrane preparations as well as in isolated purified F(1). Interestingly, growth of E. coli was abrogated in the presence of ascaphin-8, aurein 2.2, aurein 2.3, citropin 1.1, dermaseptin, magainin II-amide, MRP, MRP-amide, melittin, or melittin-amide but was unaffected in the presence of carein 1.8, carein 1.9, maculatin 1.1, magainin II, or XT-7. Hence inhibition of F(1)-ATPase and E. coli cell growth by amphibian antimicrobial peptides suggests that their antimicrobial/anticancer properties are in part linked to their actions on ATP synthase.

Dual functions of the human antimicrobial peptide LL-37-target membrane perturbation and host cell cargo delivery

The mechanisms behind target vs. host cell recognition of the human antimicrobial peptide LL-37 remain ill-defined. Here, we have investigated the membrane disruption capacity of LL-37 using large unilamellar vesicles (LUVs) composed of varying mixtures of POPC, POPG and cholesterol to mimic target and host membranes respectively. We show that LL-37 is unable to induce leakage of entrapped calcein from zwitterionic POPC LUVs, whereas leakage from LUVs partially composed of POPG is fast and efficient. In accordance with typical antimicrobial peptide behavior, cholesterol diminished LL-37 induced leakage. By using linear dichroism and flow oriented LUVs, we found that LL-37 orients with the axis of its induced α-helix parallel to the membrane surface in POPC:POPG (7:3) LUVs. In the same system, we also observed a time-dependent increase of the parallel α-helix LD signal on timescales corresponding to the leakage kinetics. The increased LD may be connected to a peptide translocation step, giving rise to mass balance across the membrane. This could end the leakage process before it is complete, similar to what we have observed. Confocal microscopy studies of eukaryotic cells show that LL-37 is able to mediate the cell delivery of non-covalently linked fluorescent oligonucleotides, in agreement with earlier studies on delivery of plasmid DNA (Sandgren et al., J. Biol. Chem. 279 (2004) 17951). These observations highlight the potential dual functions of LL-37 as an antimicrobial agent against bacterial target cells and a cell-penetrating peptide that can deliver nucleic acids into the host cells.

Probing excited states and activation energy for the integral membrane protein phospholamban by NMR CPMG relaxation dispersion experiments

Phospholamban (PLN) is a dynamic single-pass membrane protein that inhibits the flow of Ca(2+) ions into the sarcoplasmic reticulum (SR) of heart muscle by directly binding to and inhibiting the SR Ca(2+)ATPase (SERCA). The PLN monomer is the functionally active form that exists in equilibrium between ordered (T state) and disordered (R state) states. While the T state has been fully characterized using a hybrid solution/solid-state NMR approach, the R state structure has not been fully portrayed. It has, however, been detected by both NMR and EPR experiments in detergent micelles and lipid bilayers. In this work, we quantitatively probed the mus to ms dynamics of the PLN excited states by observing the T state in DPC micelles using CPMG relaxation dispersion NMR spectroscopy under functional conditions for SERCA. The (15)N backbone and (13)C(delta1) Ile-methyl dispersion curves were fit using a two-state equilibrium model, and indicate that residues within domain Ia (residues 1-16), the loop (17-22), and domain Ib (23-30) of PLN undergo mus-ms dynamics (k(ex)=6100+/-800 s(-1) at 17 degrees C). We measured k(ex) at additional temperatures, which allowed for a calculation of activation energy equal to approximately 5 kcal/mol. This energy barrier probably does not correspond to the detachment of the amphipathic domain Ia, but rather the energy needed to unwind domain Ib on the membrane surface, likely an important mechanism by which PLN converts between high and low affinity states for its binding partners.