Thermodynamics of the alpha-helix-coil transition of amphipathic peptides in a membrane environment: implications for the peptide-membrane binding equilibrium

Characterization of Copper(II) and Zinc(II) Complexes of Peptides Mimicking the CuZnSOD Enzyme

Antimicrobial peptides are short cationic peptides that are present on biological surfaces susceptible to infection, and they play an important role in innate immunity. These peptides, like other compounds with antimicrobial activity, often have significant superoxide dismutase (SOD) activity. One direction of our research is the characterization of peptides modeling the CuZnSOD enzyme and the determination of their biological activity, and these results may contribute to the development of novel antimicrobial peptides. In the framework of this research, we have synthesized 10, 15, and 16-membered model peptides containing the amino acid sequence corresponding to the Cu(II) and Zn(II) binding sites of the CuZnSOD enzyme, namely the Zn(II)-binding HVGD sequence (80-83. fragments), the Cu(II)-binding sequence HVH (fragments 46-48), and the histidine (His63), which links the two metal ions as an imidazolate bridge: Ac-FHVHEGPHFN-NH2 (L1(10)), Ac-FHVHAGPHFNGGHVG-NH2 (L2(15)), and Ac-FHVHEGPHFNGGHVGD-NH2 (L3(16)). pH-potentiometric, UV-Vis-, and CD-spectroscopy studies of the Cu(II), Zn(II), and Cu(II)-Zn(II) mixed complexes of these peptides were performed, and the SOD activity of the complexes was determined. The binding sites preferred by Cu(II) and Zn(II) were identified by means of CD-spectroscopy. From the results obtained for these systems, it can be concluded that in equimolar solution, the -(NGG)HVGD- sequence of the peptides is the preferred binding site for copper(II) ion. However, in the presence of both metal ions, according to the native enzyme, the -HVGD- sequence offers the main binding site for Zn(II), while the majority of Cu(II) binds to the -FHVH- sequence. Based on the SOD activity assays, complexes of the 15- and 16-membered peptide have a significant SOD activity. Although this activity is smaller than that of the native CuZnSOD enzyme, the complexes showed better performance in the degradation of superoxide anion than other SOD mimics. Thus, the incorporation of specific amino acid sequences mimicking the CuZnSOD enzyme increases the efficiency of model systems in the catalytic decomposition of superoxide anion.

Molecular understanding of calorimetric protein unfolding experiments

Testing and predicting protein stability gained importance because proteins, including antibodies, became pharmacologically relevant in viral and cancer therapies. Isothermal scanning calorimetry is the principle method to study protein stability. Here, we use the excellent experimental heat capacity C_p(T) data from the literature for a critical inspection of protein unfolding as well as for the test of a new cooperative model. In the relevant literature, experimental temperature profiles of enthalpy, H_cal(T), entropy, S_cal(T), and free energy, G_cal(T) are missing. First, we therefore calculate the experimental H_cal(T), S_cal(T), and G_cal(T) from published C_p(T) thermograms. Considering only the unfolding transition proper, the heat capacity and all thermodynamic functions are zero in the region of the native protein. In particular, the free energy of the folded proteins is also zero and G_cal(T) displays a trapezoidal temperature profile when cold denaturation is included. Second, we simulate the DSC-measured thermodynamic properties with a new molecular model based on statistical-mechanical thermodynamics. The model quantifies the protein cooperativity and predicts the aggregate thermodynamic variables of the system with molecular parameters only. The new model provides a perfect simulation of all thermodynamic properties, including the observed trapezoidal G_cal(T) temperature profile. Importantly, the new cooperative model can be applied to a broad range of protein sizes, including antibodies. It predicts not only heat and cold denaturation but also provides estimates of the unfolding kinetics and allows a comparison with molecular dynamics calculations and quasielastic neutron scattering experiments.

Untangling the complexity of membrane protein folding

Delineating the folding steps of helical-bundle membrane proteins has been a challenging task. Many questions remain unanswered, including the conformation and stability of the states populated during folding, the shape of the energy barriers between the states, and the role of lipids as a solvent in mediating the folding. Recently, theoretical frames have matured to a point that permits detailed dissection of the folding steps, and advances in experimental techniques at both single-molecule and ensemble levels enable selective modulation of specific steps for quantitative determination of the folding energy landscapes. We also discuss how lipid molecules would play an active role in shaping the folding energy landscape of membrane proteins, and how folding of multi-domain membrane proteins can be understood based on our current knowledge. We conclude this review by offering an outlook for emerging questions in the study of membrane protein folding.

Antimicrobial peptides: mechanism of action and lipid-mediated synergistic interactions within membranes

Biophysical and structural studies of peptide-lipid interactions, peptide topology and dynamics have changed our view of how antimicrobial peptides insert and interact with membranes. Clearly, both peptides and lipids are highly dynamic, and change and mutually adapt their conformation, membrane penetration and detailed morphology on a local and a global level. As a consequence, peptides and lipids can form a wide variety of supramolecular assemblies in which the more hydrophobic sequences preferentially, but not exclusively, adopt transmembrane alignments and have the potential to form oligomeric structures similar to those suggested by the transmembrane helical bundle model. In contrast, charged amphipathic sequences tend to stay intercalated at the membrane interface. Although the membranes are soft and can adapt, at increasing peptide density they cause pronounced disruptions of the phospholipid fatty acyl packing. At even higher local or global concentrations the peptides cause transient membrane openings, rupture and ultimately lysis. Interestingly, mixtures of peptides such as magainin 2 and PGLa, which are stored and secreted naturally as a cocktail, exhibit considerably enhanced antimicrobial activities when investigated together in antimicrobial assays and also in pore forming experiments applied to biophysical model systems. Our most recent investigations reveal that these peptides do not form stable complexes but act by specific lipid-mediated interactions and the nanoscale properties of phospholipid bilayers.

Revealing the Mechanisms of Synergistic Action of Two Magainin Antimicrobial Peptides

The study of peptide-lipid and peptide-peptide interactions as well as their topology and dynamics using biophysical and structural approaches have changed our view how antimicrobial peptides work and function. It has become obvious that both the peptides and the lipids arrange in soft supramolecular arrangements which are highly dynamic and able to change and mutually adapt their conformation, membrane penetration, and detailed morphology. This can occur on a local and a global level. This review focuses on cationic amphipathic peptides of the magainin family which were studied extensively by biophysical approaches. They are found intercalated at the membrane interface where they cause membrane thinning and ultimately lysis. Interestingly, mixtures of two of those peptides namely magainin 2 and PGLa which occur naturally as a cocktail in the frog skin exhibit synergistic enhancement of antimicrobial activities when investigated together in antimicrobial assays but also in biophysical experiments with model membranes. Detailed dose-response curves, presented here for the first time, show a cooperative behavior for the individual peptides which is much increased when PGLa and magainin are added as equimolar mixture. This has important consequences for their bacterial killing activities and resistance development. In membranes that carry unsaturations both peptides align parallel to the membrane surface where they have been shown to arrange into mesophases involving the peptides and the lipids. This supramolecular structuration comes along with much-increased membrane affinities for the peptide mixture. Because this synergism is most pronounced in membranes representing the bacterial lipid composition it can potentially be used to increase the therapeutic window of pharmaceutical formulations.

Conformational Changes of Anoplin, W-MreB1-9, and (KFF)3K Peptides near the Membranes

Many peptides interact with biological membranes, but elucidating these interactions is challenging because cellular membranes are complex and peptides are structurally flexible. To contribute to understanding how the membrane-active peptides behave near the membranes, we investigated peptide structural changes in different lipid surroundings. We focused on two antimicrobial peptides, anoplin and W-MreB_1–9, and one cell-penetrating peptide, (KFF)₃K. Firstly, by using circular dichroism spectroscopy, we determined the secondary structures of these peptides when interacting with micelles, liposomes, E. coli lipopolysaccharides, and live E. coli bacteria. The peptides were disordered in the buffer, but anoplin and W-MreB_1–9 displayed lipid-induced helicity. Yet, structural changes of the peptide depended on the composition and concentration of the membranes. Secondly, we quantified the destructive activity of peptides against liposomes by monitoring the release of a fluorescent dye (calcein) from the liposomes treated with peptides. We observed that only for anoplin and W-MreB_1–9 calcein leakage from liposomes depended on the peptide concentration. Thirdly, bacterial growth inhibition assays showed that peptide conformational changes, evoked by the lipid environments, do not directly correlate with the antimicrobial activity of the peptides. However, understanding the relation between peptide structural properties, mechanisms of membrane disruption, and their biological activities can guide the design of membrane-active peptides.

Synergistic Biophysical Techniques Reveal Structural Mechanisms of Engineered Cationic Antimicrobial Peptides in Lipid Model Membranes

In the quest for new antibiotics, two novel engineered cationic antimicrobial peptides (eCAPs) have been rationally designed. WLBU2 and D8 (all 8 valines are the d-enantiomer) efficiently kill both Gram-negative and -positive bacteria, but WLBU2 is toxic and D8 nontoxic to eukaryotic cells. We explore protein secondary structure, location of peptides in six lipid model membranes, changes in membrane structure and pore evidence. We suggest that protein secondary structure is not a critical determinant of bactericidal activity, but that membrane thinning and dual location of WLBU2 and D8 in the membrane headgroup and hydrocarbon region may be important. While neither peptide thins the Gram-negative lipopolysaccharide outer membrane model, both locate deep into its hydrocarbon region where they are primed for self-promoted uptake into the periplasm. The partially α-helical secondary structure of WLBU2 in a red blood cell (RBC) membrane model containing 50 % cholesterol, could play a role in destabilizing this RBC membrane model causing pore formation that is not observed with the D8 random coil, which correlates with RBC hemolysis caused by WLBU2 but not by D8.

EcDBS1R6: A novel cationic antimicrobial peptide derived from a signal peptide sequence

BACKGROUND

Bacterial infections represent a major worldwide health problem the antimicrobial peptides (AMPs) have been considered as potential alternative agents for treating these infections. Here we demonstrated the antimicrobial activity of EcDBS1R6, a peptide derived from a signal peptide sequence of Escherichia coli that we previously turned into an AMP by making changes through the Joker algorithm.

METHODS

Antimicrobial activity was measured by broth microdilution method. Membrane integrity was measured using fluorescent probes and through scanning electron microscopy imaging. A sliding window of truncated peptides was used to determine the EcDBS1R6 active core. Molecular dynamics in TFE/water environment was used to assess the EcDBS1R6 structure.

RESULTS

Signal peptides are known to naturally interact with membranes; however, the modifications introduced by Joker transformed this peptide into a membrane-active agent capable of killing bacteria. The C-terminus was unable to fold into an α-helix whereas its fragments showed poor or no antimicrobial activity, suggesting that the EcDBS1R6 antibacterial core was located at the helical N-terminus, corresponding to the signal peptide portion of the parent peptide.

CONCLUSION

The strategy of transforming signal peptides into AMPs appears to be promising and could be used to produce novel antimicrobial agents.

GENERAL SIGNIFICANCE

The process of transforming an inactive signal peptide into an antimicrobial peptide could open a new venue for creating new AMPs derived from signal peptides.

Thermal and Chemical Unfolding of a Monoclonal IgG1 Antibody: Application of the Multistate Zimm-Bragg Theory

The thermal unfolding of a recombinant monoclonal antibody IgG1 (mAb) was measured with differential scanning calorimetry (DSC). The DSC thermograms reveal a pretransition at 72°C with an unfolding enthalpy of ΔH_cal ∼200-300 kcal/mol and a main transition at 85°C with an enthalpy of ∼900-1000 kcal/mol. In contrast to small single-domain proteins, mAb unfolding is a complex reaction that is analyzed with the multistate Zimm-Bragg theory. For the investigated mAb, unfolding is characterized by a cooperativity parameter σ ∼6 × 10^-5 and a Gibbs free energy of unfolding of g_nu ∼100 cal/mol per amino acid. The enthalpy of unfolding provides the number of amino acid residues ν participating in the unfolding reaction. On average, ν∼220 ± 50 amino acids are involved in the pretransition and ν∼850 ± 30 in the main transition, accounting for ∼90% of all amino acids. Thermal unfolding was further studied in the presence of guanidineHCl. The chemical denaturant reduces the unfolding enthalpy ΔH_cal and lowers the midpoint temperature T_m. Both parameters depend linearly on the concentration of denaturant. The guanidineHCl concentrations needed to unfold mAb at 25°C are predicted to be 2-3 M for the pretransition and 5-7 M for the main transition, varying with pH. GuanidineHCl binds to mAb with an exothermic binding enthalpy, which partially compensates the endothermic mAb unfolding enthalpy. The number of guanidineHCl molecules bound upon unfolding is deduced from the DSC thermograms. The bound guanidineHCl-to-unfolded amino acid ratio is 0.79 for the pretransition and 0.55 for the main transition. The pretransition binds more denaturant molecules and is more sensitive to unfolding than the main transition. The current study shows the strength of the Zimm-Bragg theory for the quantitative description of unfolding events of large, therapeutic proteins, such as a monoclonal antibody.

Thermal and Chemical Unfolding of Lysozyme. Multistate Zimm-Bragg Theory Versus Two-State Model

Thermal and chemical unfolding of lysozyme in the presence of the guanidine HCl denaturant is a model system to compare the conventional two-state model of protein unfolding with the multistate Zimm-Bragg theory. The two-state model is shown to be the noncooperative limit of the Zimm-Bragg theory. In particular, the Zimm-Bragg theory provides a molecular interpretation of the empirical linear extrapolation method (LEM) of the two-state model. Differential scanning calorimetry (DSC) experiments reported in the literature are analyzed with both methods. Lysozyme unfolding is associated with a large endothermic enthalpy that decreases significantly upon addition of guanidine HCl. In contrast, the Gibbs free energy of unfolding is small, negative, and independent of the guanidine HCl concentration, contradicting, in part, the conclusions of the LEM. The unfolding enthalpy is compensated by an even larger entropy term. The multistate Zimm-Bragg theory predicts a larger conformational enthalpy and a smaller Gibbs free energy than the two-state model. The Zimm-Bragg theory provides the protein cooperativity parameter, the average length of independently folding protein domains, and the Gibbs free energy of unfolding of individual amino acid residues. Guanidine HCl binding to lysozyme is exothermic and counteracts the endothermic unfolding enthalpy. The number of bound denaturant molecules is determined from the decrease in enthalpy and is extrapolated to the guanidine HCl-to-amino acid stoichiometry at complete lysozyme unfolding. Chemical unfolding isotherms measured with circular dichroism (CD) spectroscopy are analyzed with both models. The chemical Zimm-Bragg theory is a cooperative molecular model, yielding the guanidine HCl binding constant and the protein cooperativity parameter. It allows a quantitative comparison between thermal and chemical protein unfolding. The two reactions have almost identical changes in Gibbs free energy. However, thermal unfolding is significantly more cooperative than chemical unfolding. Finally, distinct differences are observed in thermal unfolding between DSC and CD spectroscopy.

De novo Design of Selective Membrane-Active Peptides by Enzymatic Control of Their Conformational Bias on the Cell Surface

Selectively targeting the membrane-perturbing potential of peptides towards a distinct cellular phenotype allows one to target distinct populations of cells. We report the de novo design of a new class of peptide whose ability to perturb cellular membranes is coupled to an enzyme-mediated shift in the folding potential of the peptide into its bioactive conformation. Cells rich in negatively charged surface components that also highly express alkaline phosphatase, for example many cancers, are susceptible to the action of the peptide. The unfolded, inactive peptide is dephosphorylated, shifting its conformational bias towards cell-surface-induced folding to form a facially amphiphilic membrane-active conformer. The fate of the peptide can be further tuned by peptide concentration to affect either lytic or cell-penetrating properties, which are useful for selective drug delivery. This is a new design strategy to afford peptides that are selective in their membrane-perturbing activity.

Comparing the Membrane-Interaction Profiles of Two Antiviral Peptides: Insights into Structure-Function Relationship

In recent years, certain amphipathic, α-helical peptides have been discovered that inhibit medically important enveloped viruses by disrupting the lipid membrane surrounding individual virus particles. Interestingly, only a small subset of amphipathic, α-helical peptides demonstrate inhibitory activity, and there is broad interest in understanding how the structures of these peptides contribute to functional activity against lipid membranes. To address this question, herein, we employed multiple surface-sensitive measurement techniques along with computational simulations in order to investigate how AH and C5A peptides, two of the most biologically active peptides in this class, interact with model lipid membranes while gaining insight into membrane-induced peptide conformational changes. Circular dichroism spectroscopy experiments revealed that both AH and C5A peptides undergo pronounced coil-to-helix transitions in the presence of lipid membrane environments, and the C5A conformational change was the largest. Time-lapsed fluorescence microscopy measurements were conducted to monitor the interaction of peptides with arrays of tethered, individual lipid vesicles and showed that C5A potently lyses lipid vesicles indiscriminate of vesicle size at peptide concentrations as low as 10 nM whereas AH peptide preferentially lyses lipid vesicles with high membrane curvature and is less potent than C5A. These findings were complemented by electrochemical impedance spectroscopy measurements on a tethered lipid bilayer membrane platform, which indicated that C5A solubilizes lipid membranes in a manner that is distinct from how AH disrupts lipid membranes via pore formation. Computational simulations supported that the distinct membrane-interaction profiles arise from different helical folding patterns, whereby AH monomers predominantly exist as two shorter helices with a hinge in-between and C5A monomers form a single helix. Taken together, our findings demonstrate that membrane-active antiviral peptides can exhibit distinct membrane-interaction profiles that confer different degrees of targeting selectivity, and the corresponding structural insights will be useful for peptide engineering applications.

Unveiling the binding and orientation of the antimicrobial peptide Plantaricin 149 in zwitterionic and negatively charged membranes

Antimicrobial peptides are a large group of natural compounds which present promising properties for the pharmaceutical and food industries, such as broad-spectrum activity, potential for use as natural preservatives, and reduced propensity for development of bacterial resistance. Plantaricin 149 (Pln149), isolated from Lactobacillus plantarum NRIC 149, is an intrinsically disordered peptide with the ability to inhibit bacteria from the Listeria and Staphylococcus genera, and which is capable of promoting inhibition and disruption of yeast cells. In this study, the interactions of Pln149 with model membranes composed of zwitterionic and/or anionic phospholipids were investigated using a range of biophysical techniques, including isothermal titration calorimetry, surface tension measurements, synchrotron radiation circular dichroism spectroscopy, oriented circular dichroism spectroscopy, and optical microscopy, to elucidate these peptides' mode of interactions and provide insight into their functional roles. In anionic model membranes, the binding of Pln149 to lipid bilayers is an endothermic process and induces a helical secondary structure in the peptide. The helices bind parallel to the surfaces of lipid bilayers and can promote vesicle disruption, depending on peptide concentration. Although Pln149 has relatively low affinity for zwitterionic liposomes, it is able to adsorb at their lipid interfaces, disturbing the lipid packing, assuming a similar parallel helix structure with a surface-bound orientation, and promoting an increase in the membrane surface area. Such findings can explain the intriguing inhibitory action of Pln149 in yeast cells whose cell membranes have a significant zwitterionic lipid composition.

The Mechanisms of Action of Cationic Antimicrobial Peptides Refined by Novel Concepts from Biophysical Investigations

Even 30 years after the discovery of magainins, biophysical and structural investigations on how these peptides interact with membranes can still bear surprises and add new interesting detail to how these peptides exert their antimicrobial action. Early on, using oriented solid-state NMR spectroscopy, it was found that the amphipathic helices formed by magainins are active when being oriented parallel to the membrane surface. More recent investigations indicate that this in-planar alignment is also found when PGLa and magainin in combination exert synergistic pore-forming activities, where studies on the mechanism of synergistic interaction are ongoing. In a related manner, the investigation of dimeric antimicrobial peptide sequences has become an interesting topic of research which bears promise to refine our views how antimicrobial action occurs. The molecular shape concept has been introduced to explain the effects of lipids and peptides on membrane morphology, locally and globally, and in particular of cationic amphipathic helices that partition into the membrane interface. This concept has been extended in this review to include more recent ideas on soft membranes that can adapt to external stimuli including membrane-disruptive molecules. In this manner, the lipids can change their shape in the presence of low peptide concentrations, thereby maintaining the bilayer properties. At higher peptide concentrations, phase transitions occur which lead to the formation of pores and membrane lytic processes. In the context of the molecular shape concept, the properties of lipopeptides, including surfactins, are shortly presented, and comparisons with the hydrophobic alamethicin sequence are made.

Calcineurin B homologous protein 3 binds with high affinity to the CHP binding domain of the human sodium/proton exchanger NHE1

The Na+/H+ exchanger NHE1 is critical for cell vitality as it controls intracellular pH and cell volume. Its functionality is influenced by calcineurin B homologous proteins (CHPs). The human isoform CHP3 is important for transport of NHE1 to the plasma membrane and for its activity. Here, we characterized the binding interaction of human CHP3 with the regulatory domain of NHE1. The exact binding site of CHP3 was previously debated. CHP3 as well as both regions of NHE1 in question were produced and purified. CHP3 specifically formed stable complexes with the CHP-binding region (CBD) of NHE1 (residues 503-545) in size-exclusion chromatography (SEC), but not with the C-terminal region (CTD, residues 633-815). CTD was functional as shown by Ca2+-dependent binding of calmodulin in SEC analysis. CHP3 bound with high affinity to CBD with an equilibrium dissociation constant (KD) of 56 nM determined by microscale thermophoresis. The high affinity was substantiated by isothermal calorimetry analysis (KD = 3 nM), which also revealed that the interaction with CBD is strongly exothermic (ΔG° = -48.6 kJ/mol, ΔH = -75.3 kJ/mol, -TΔS° = 26.7 kJ/mol). The data provide insights in the molecular mechanisms that underlie the regulatory interaction of CHP3 and NHE1 and more general of calcineurin homologous proteins with their target proteins.

Lipid-Mediated Interactions between the Antimicrobial Peptides Magainin 2 and PGLa in Bilayers

A synergistic enhancement of activities has been described for the amphipathic cationic antimicrobial peptides magainin 2 and PGLa when tested in antimicrobial assays or in biophysical experiments using model membranes. In the presence of magainin 2, PGLa changes from an in-planar alignment parallel to the membrane surface to a more transmembrane orientation when investigated in membranes made from fully saturated PC or PC/PG, but not when one of the fatty acyl chains is unsaturated. Such lipid-mediated changes in the membrane topology of polypeptide domains could provide an interesting mechanism for the regulation of membrane proteins. Here we investigated the PGLa topology in a wide variety of membranes made of saturated or unsaturated PE, PC, and/or PG using ¹⁵N solid-state NMR spectroscopy. In contrast to predictions made by previous models the data show that membrane curvature has only a minor effect on PGLa realignment. Furthermore, using ²H solid-state NMR spectroscopy of deuterated phospholipid fatty acyl chains the order parameters of the lipids were investigated in the presence of PGLa, magainin, or equimolar peptide mixtures. Both peptides cause a pronounced decrease in the order parameters when oriented parallel to the membrane surface, an effect that reverts when PGLa flips into transmembrane alignments. Taken together, these data are suggestive that the magainin-induced disordering of fatty acyl chains provides an important driving force for PGLa realignment.

Fluorophore labeling of a cell-penetrating peptide significantly alters the mode and degree of biomembrane interaction

The demand for highly efficient macromolecular drugs, used in the treatment of many severe diseases, is continuously increasing. However, the hydrophilic character and large molecular size of these drugs significantly limit their ability to permeate across cellular membranes and thus impede the drugs in reaching their target sites in the body. Cell-penetrating peptides (CPP) have gained attention as promising drug excipients, since they can facilitate drug permeation across cell membranes constituting a major biological barrier. Fluorophores are frequently covalently conjugated to CPPs to improve detection, however, the ensuing change in physico-chemical properties of the CPPs may alter their biological properties. With complementary biophysical techniques, we show that the mode of biomembrane interaction may change considerably upon labeling of the CPP penetratin (PEN) with a fluorophore. Fluorophore-PEN conjugates display altered modes of membrane interaction with increased insertion into the core of model cell membranes thereby exerting membrane-thinning effects. This is in contrast to PEN, which localizes along the head groups of the lipid bilayer, without affecting the thickness of the lipid tails. Particularly high membrane disturbance is observed for the two most hydrophobic PEN conjugates; rhodamine B or 1-pyrene butyric acid, as compared to the four other tested fluorophore-PEN conjugates.

Biophysical Investigations Elucidating the Mechanisms of Action of Antimicrobial Peptides and Their Synergism

Biophysical and structural investigations are presented with a focus on the membrane lipid interactions of cationic linear antibiotic peptides such as magainin, PGLa, LL37, and melittin. Observations made with these peptides are distinct as seen from data obtained with the hydrophobic peptide alamethicin. The cationic amphipathic peptides predominantly adopt membrane alignments parallel to the bilayer surface; thus the distribution of polar and non-polar side chains of the amphipathic helices mirror the environmental changes at the membrane interface. Such a membrane partitioning of an amphipathic helix has been shown to cause considerable disruptions in the lipid packing arrangements, transient openings at low peptide concentration, and membrane disintegration at higher peptide-to-lipid ratios. The manifold supramolecular arrangements adopted by lipids and peptides are represented by the 'soft membranes adapt and respond, also transiently' (SMART) model. Whereas molecular dynamics simulations provide atomistic views on lipid membranes in the presence of antimicrobial peptides, the biophysical investigations reveal interesting details on a molecular and supramolecular level, and recent microscopic imaging experiments delineate interesting sequences of events when bacterial cells are exposed to such peptides. Finally, biophysical studies that aim to reveal the mechanisms of synergistic interactions of magainin 2 and PGLa are presented, including unpublished isothermal titration calorimetry (ITC), circular dichroism (CD) and dynamic light scattering (DLS) measurements that suggest that the peptides are involved in liposome agglutination by mediating intermembrane interactions. A number of structural events are presented in schematic models that relate to the antimicrobial and synergistic mechanism of amphipathic peptides when they are aligned parallel to the membrane surface.

Antimicrobial peptides: biochemical determinants of activity and biophysical techniques of elucidating their functionality

Antimicrobial peptides (AMPs) have been established over millennia as powerful components of the innate immune system of many organisms. Due to their broad spectrum of activity and the development of host resistance against them being unlikely, AMPs are strong candidates for controlling drug-resistant pathogenic microbial pathogens. AMPs cause cell death through several independent or cooperative mechanisms involving membrane lysis, non-lytic activity, and/or intracellular mechanisms. Biochemical determinants such as peptide length, primary sequence, charge, secondary structure, hydrophobicity, amphipathicity and host cell membrane composition together influence the biological activities of peptides. A number of biophysical techniques have been used in recent years to study the mechanisms of action of AMPs. This work appraises the molecular parameters that determine the biocidal activity of AMPs and overviews their mechanisms of actions and the diverse biochemical, biophysical and microscopy techniques utilised to elucidate these.

Calorimetry Methods to Study Membrane Interactions and Perturbations Induced by Antimicrobial Host Defense Peptides

Biological membranes play an important role in determining the activity and selectivity of antimicrobial host defense peptides (AMPs). Several biophysical methods have been developed to study the interactions of AMPs with biological membranes. Isothermal titration calorimetry and differential scanning calorimetry (ITC and DSC, respectively) are powerful techniques as they provide a unique label-free approach. ITC allows for a complete thermodynamic characterization of the interactions between AMPs and membranes. DSC allows one to study the effects of peptide binding on the packing of the phospholipids in the membrane. Used in combination with mimetic models of biological membranes, such as phospholipid vesicles, the role of different phospholipid headgroups and distinct acyl chains can be characterized. In these protocols the use of ITC and DSC methods for the study of peptide-membrane interactions will be presented, highlighting the importance of membrane model systems selected to represent bacterial and mammalian cells. These studies provide valuable insights into the mechanisms involved in the membrane binding and perturbation properties of AMPs.

Cooperative protein unfolding. A statistical-mechanical model for the action of denaturants

Knowledge of protein stability is of utmost importance in various fields of biotechnology. Protein stability can be assessed in solution by increasing the concentration of denaturant and recording the structural changes with spectroscopic or thermodynamic methods. The standard interpretation of the experimental data is to assume a 2-state equilibrium between completely folded and completely unfolded protein molecules. Here we propose a cooperative model based on the statistical-mechanical Zimm-Bragg theory. In this model protein unfolding is driven by the weak binding of a rather small number of denaturant molecules, inducing the cooperative unfolding with multiple dynamic intermediates. The modified Zimm-Bragg theory is applied to published thermodynamic and spectroscopic data leading to the following conclusions. (i) The binding constant K_D is correlated with the midpoint concentration, c₀, of the unfolding reaction according to c₀≅1/K_D. The better the binding of denaturant the lower is the concentration to achieve unfolding. (ii) The binding constant K_D agrees with direct thermodynamic measurements. A rather small number of bound denaturants suffices to induce the cooperative unfolding of the whole protein. (iii) Chemical unfolding occurs in the concentration range Δc_D=c_end-c_ini. The theory predicts the unfolding energy per amino acid residue as g_nu=RTK_D(c_end-c_ini). The Gibbs free energy of an osmotic gradient of the same size is ΔG_Diff=-RTln(c_end/c_ini). In all examples investigated ΔG_Diff exactly balances the unfolding energy g_nu. The total unfolding energy is thus close to zero. (iv) Protein cooperativity in chemical unfolding is rather low with cooperativity parameters σ≥3x10^-3. As a consequence, the theory predicts a dynamic mixture of conformations during the unfolding reaction. The probabilities of individual conformations are easily accessible via the partition function Z(c_D,σ).

Disruption of drug-resistant biofilms using de novo designed short α-helical antimicrobial peptides with idealized facial amphiphilicity

The escalating threat of antimicrobial resistance has increased pressure to develop novel therapeutic strategies to tackle drug-resistant infections. Antimicrobial peptides have emerged as a promising class of therapeutics for various systemic and topical clinical applications. In this study, the de novo design of α-helical peptides with idealized facial amphiphilicities, based on an understanding of the pertinent features of protein secondary structures, is presented. Synthetic amphiphiles composed of the backbone sequence (X₁Y₁Y₂X₂)_n, where X₁ and X₂ are hydrophobic residues (Leu or Ile or Trp), Y₁ and Y₂ are cationic residues (Lys), and n is the number repeat units (2 or 2.5 or 3), demonstrated potent broad-spectrum antimicrobial activities against clinical isolates of drug-susceptible and multi-drug resistant bacteria. Live-cell imaging revealed that the most selective peptide, (LKKL)₃, promoted rapid permeabilization of bacterial membranes. Importantly, (LKKL)₃ not only suppressed biofilm growth, but effectively disrupted mature biofilms after only 2h of treatment. The peptides (LKKL)₃ and (WKKW)₃ suppressed the production of LPS-induced pro-inflammatory mediators to levels of unstimulated controls at low micromolar concentrations. Thus, the rational design strategies proposed herein can be implemented to develop potent, selective and multifunctional α-helical peptides to eradicate drug-resistant biofilm-associated infections.

STATEMENT OF SIGNIFICANCE

Antimicrobial peptides (AMPs) are increasingly explored as therapeutics for drug-resistant and biofilm-related infections to help expand the size and quality of the current antibiotic pipeline in the face of mounting antimicrobial resistance. Here, synthetic peptides rationally designed based upon principles governing the folding of natural α-helical AMPs, comprising the backbone sequence (X₁Y₁Y₂X₂)_n, and which assemble into α-helical structures with idealized facial amphiphilicity, is presented. These multifunctional peptide amphiphiles demonstrate high bacterial selectivity, promote the disruption of pre-formed drug-resistant biofilms, and effectively neutralize endotoxins at low micromolar concentrations. Overall, the design strategies presented here could provide a useful tool for developing therapeutic peptides with broad-ranging clinical applications from the treatment and prevention of drug-resistant biofilms to the neutralization of bacterial endotoxins.

Synthetic Random Copolymers as a Molecular Platform To Mimic Host-Defense Antimicrobial Peptides

Synthetic polymers have been used as a molecular platform to develop host-defense antimicrobial peptide (AMP) mimetics which are effective in killing drug-resistant bacteria. In this topical review, we will discuss the AMP-mimetic design and chemical optimization strategies as well as the biological and biophysical implications of AMP mimicry by synthetic polymers. Traditionally, synthetic polymers have been used as a chemical means to replicate the chemical functionalities and physicochemical properties of AMPs (e.g., cationic charge, hydrophobicity) to recapitulate their mode of action. However, we propose a new perception that AMP-mimetic polymers are an inherently bioactive platform as whole molecules, which mimic more than the side chain functionalities of AMPs. The tunable nature and chemical simplicity of synthetic random polymers facilitate the development of potent, cost-effective, broad-spectrum antimicrobials. The polymer-based approach offers the potential for many antimicrobial applications to be used directly in solution or attached to surfaces to fight against drug-resistant bacteria.

Characterizing and Controlling the Loading and Release of Cationic Amphiphilic Peptides onto and from PEG-Stabilized Lipodisks

Recent studies have identified PEG-stabilized lipid nanodisks (lipodisks) as promising carriers for cationic amphiphilic peptides with antimicrobial and anticancer activity. Using fluorimetric and nanogravimetric methods, we have in this work characterized the parameters describing and controlling the binding of three selected peptides (melittin, LL37, and magainin 2) onto lipodisks. It was found that the affinity of melittin for lipodisks is independent of the disk size and rim charge. On the other hand, the number of binding sites is strongly dependent on both parameters, with the highest loading being obtained for small disks with a negatively charged rim. An optimized composition of the lipodisks was utilized to study the loading of antimicrobial peptides magainin 2 and human LL37. It was observed that although magainin 2 can be loaded in large amounts, it is released very fast upon dilution, which limits future therapeutic applications. In contrast, LL37 can be loaded at relevant concentrations and the formulation is stable. This opens up for applications of LL37-loaded lipodisks as antibiotics and in anticancer treatments.

Thermal protein unfolding by differential scanning calorimetry and circular dichroism spectroscopy Two-state model versus sequential unfolding

Thermally-induced protein unfolding is commonly described with the two-state model. This model assumes only two types of protein molecules in solution, the native (N) and the denatured, unfolded (U) protein. In reality, protein unfolding is a multistep process, even if intermediate states are only sparsely populated. As an alternative approach we explore the Zimm-Bragg theory, originally developed for the α-helix-to-random coil transition of synthetic polypeptides. The theory includes intermediate structures with concentrations determined by the cooperativity of the unfolding reaction. We illustrate the differences between the two-state model and the Zimm-Bragg theory with measurements of apolipoprotein A-1 and lysozyme by differential scanning calorimetry (DSC) and CD spectroscopy. Nine further protein examples are taken from the literature. The Zimm-Bragg theory provides a perfect fit of the calorimetric unfolding transitions for all proteins investigated. In contrast, the transition curves and enthalpies predicted by the two-state model differ considerably from the experimental results. Apolipoprotein A-1 is ~50% α-helical at ambient temperature and its unfolding follows the classical α-helix-to-random coil equilibrium. The unfolding of proteins with little α-helix content, such as lysozyme, can also be analyzed with the Zimm-Bragg theory by introducing the concept of 'folded' and 'unfolded' peptide units assuming an average unfolding enthalpy per peptide unit. DSC is the method of choice to measure the unfolding enthalpy, , but CD spectroscopy in combination with the two-state model is often used to deduce the unfolding enthalpy. This can lead to erroneous result. Not only are different enthalpies required to describe the CD and DSC transition curves but these values deviate distinctly from the experimental result. In contrast, the Zimm-Bragg theory predicts the DSC and CD unfolding transitions with the same set of parameters.

Destabilization of α-Helical Structure in Solution Improves Bactericidal Activity of Antimicrobial Peptides: Opposite Effects on Bacterial and Viral Targets

We have previously examined the mechanism of antimicrobial peptides on the outer membrane of vaccinia virus. We show here that the formulation of peptides LL37 and magainin-2B amide in polysorbate 20 (Tween 20) results in greater reductions in virus titer than formulation without detergent, and the effect is replicated by substitution of polysorbate 20 with high-ionic-strength buffer. In contrast, formulation with polysorbate 20 or high-ionic-strength buffer has the opposite effect on bactericidal activity of both peptides, resulting in lesser reductions in titer for both Gram-positive and Gram-negative bacteria. Circular dichroism spectroscopy shows that the differential action of polysorbate 20 and salt on the virucidal and bactericidal activities correlates with the α-helical content of peptide secondary structure in solution, suggesting that the virucidal and bactericidal activities are mediated through distinct mechanisms. The correlation of a defined structural feature with differential activity against a host-derived viral membrane and the membranes of both Gram-positive and Gram-negative bacteria suggests that the overall helical content in solution under physiological conditions is an important feature for consideration in the design and development of candidate peptide-based antimicrobial compounds.

Pore Structure and Synergy in Antimicrobial Peptides of the Magainin Family

Magainin 2 and PGLa are among the best-studied cationic antimicrobial peptides. They bind preferentially to negatively charged membranes and apparently cause their disruption by the formation of transmembrane pores, whose detailed structure is still unclear. Here we report the results of 5–9 μs all-atom molecular dynamics simulations starting from tetrameric transmembrane helical bundles of these two peptides, as well as their stoichiometric mixture, and the analog MG-H2 in DMPC or 3:1 DMPC/DMPG membranes. The simulations produce pore structures that appear converged, although some effect of the starting peptide arrangement (parallel vs. antiparallel) is still observed on this timescale. The peptides remain mostly helical and adopt tilted orientations. The calculated tilt angles for PGLa are in excellent agreement with recent solid state NMR experiments. The antiparallel dimer structure in the magainin 2 simulations resembles previously determined NMR and crystal structures. More transmembrane orientations and a larger and more ordered pore are seen in the 1:1 heterotetramer with an antiparallel helix arrangement. Insights into the mechanism of synergy between these two peptides are obtained via implicit solvent modeling of homo- and heterodimers and analysis of interactions in the atomistic simulations. This analysis suggests stronger pairwise interactions in the heterodimer than in the two homodimers.

The emergence of antibiotic resistance has created a compelling need for new potent antibiotics. Antimicrobial peptides, naturally produced by many organisms, have long been pursued as a means to fill this gap. However, they have not yet realized their practical potential, partly due to a lack of full understanding of their mechanism of action. The formation of pores by these peptides in bacterial membranes has been demonstrated by multiple biophysical approaches, but the detailed structure of these pores is still unclear. The magainin family of antimicrobial peptides have been thoroughly studied over the last three decades. We have generated atomic-resolution models of membrane pores generated by tetrameric assemblies of different magainin peptides by means of molecular dynamics simulations on the multimicrosecond timescale. The results complement previous experimental findings and set the stage for further development of theoretical and experimental approaches that may ultimately allow the rational design of improved antibiotics.

Thermodynamic and Biophysical Analysis of the Membrane-Association of a Histidine-Rich Peptide with Efficient Antimicrobial and Transfection Activities

LAH4-L1 is a synthetic amphipathic peptide with antimicrobial activity. The sequence of the 23 amino acid peptide was inspired by naturally occurring frog peptides such as PGLa and magainin. LAH4-L1 also facilitates the transport of nucleic acids through the cell membrane. We have investigated the membrane binding properties and energetics of LAH4-L1 at pH 5.5 with physical-chemical methods. CD spectroscopy was employed to quantitate the membrane-induced random coil-to-helix transition of LAH4-L1. Binding isotherms were obtained with CD spectroscopy as a function of the lipid-to-protein ratio for neutral and negatively charged membranes and were analyzed with both the Langmuir multisite adsorption model and the surface partition/Gouy-Chapman model. According to the Langmuir adsorption model each molecule LAH4-L1 binds 4 POPS molecules, independent of the POPS concentration in the membrane. This is supported by the surface partition/Gouy-Chapman model which predicts an electric charge of LAH4-L1 of z = 4. Binding affinity is dominated by electrostatic attraction. The thermodynamics of the binding process was elucidated with isothermal titration calorimetry. The ITC data revealed that the binding process is composed of at least three different reactions, that is, a coil-to-helix transition with an exothermic enthalpy of about -11 kcal/mol and two endothermic processes with enthalpies of ∼4 and ∼8 kcal/mol, respectively, which partly compensate the exothermic enthalpy of the conformational change. The major endothermic reaction is interpreted as a deprotonation reaction following the insertion of a highly charged cationic peptide into a nonpolar environment.

The SMART model: Soft Membranes Adapt and Respond, also Transiently, in the presence of antimicrobial peptides

Biophysical and structural studies of peptide-lipid interactions, peptide topology and dynamics have changed our view on how antimicrobial peptides insert and interact with membranes. Clearly, both the peptides and the lipids are highly dynamic, change and mutually adapt their conformation, membrane penetration and detailed morphology on a local and a global level. As a consequence, the peptides and lipids can form a wide variety of supramolecular assemblies in which the more hydrophobic sequences preferentially, but not exclusively, adopt transmembrane alignments and have the potential to form oligomeric structures similar to those suggested by the transmembrane helical bundle model. In contrast, charged amphipathic sequences tend to stay intercalated at the membrane interface where they cause pronounced disruptions of the phospholipid fatty acyl packing. At increasing local or global concentrations, the peptides result in transient membrane openings, rupture and ultimately lysis. Depending on peptide-to-lipid ratio, lipid composition and environmental factors (temperature, buffer composition, ionic strength, etc.), the same peptide sequence can result in a variety of those responses. Therefore, the SMART model has been introduced to cover the full range of possibilities. With such a view in mind, novel antimicrobial compounds have been designed from amphipathic polymers, peptide mimetics, combinations of ultra-short polypeptides with hydrophobic anchors or small designer molecules.

Membrane partitioning of the pore-forming domain of colicin A. Role of the hydrophobic helical hairpin

The colicins are bacteriocins that target Escherichia coli and kill bacterial cells through different mechanisms. Colicin A forms ion channels in the inner membranes of nonimmune bacteria. This activity resides exclusively in its C-terminal fragment (residues 387-592). The soluble free form of this domain is a 10 α-helix bundle. The hydrophobic helical hairpin, H8-H9, is buried inside the structure and shielded by eight amphipathic surface helices. The interaction of the C-terminal colicin A domain and several chimeric variants with lipidic vesicles was examined here by isothermal titration calorimetry. In the mutant constructions, natural sequences of the hydrophobic helices H8 and H9 were either removed or substituted by polyalanine or polyleucine. All the constructions fully associated with DOPG liposomes including the mutant that lacked helices H8 and H9, indicating that amphipathic rather than hydrophobic helices were the major determinants of the exothermic binding reactions. Alanine is not specially favored in the lipid-bound form; the chimeric construct with polyalanine produced lower enthalpy gain. On the other hand, the large negative heat capacities associated with partitioning, a characteristic feature of the hydrophobic effect, were found to be dependent on the sequence hydrophobicity of helices H8 and H9.

Mechanical activation of vinculin binding to talin locks talin in an unfolded conformation

The force-dependent interaction between talin and vinculin plays a crucial role in the initiation and growth of focal adhesions. Here we use magnetic tweezers to characterise the mechano-sensitive compact N-terminal region of the talin rod, and show that the three helical bundles R1–R3 in this region unfold in three distinct steps consistent with the domains unfolding independently. Mechanical stretching of talin R1–R3 enhances its binding to vinculin and vinculin binding inhibits talin refolding after force is released. Mutations that stabilize R3 identify it as the initial mechano-sensing domain in talin, unfolding at ∼5 pN, suggesting that 5 pN is the force threshold for vinculin binding and adhesion progression.

Chemistry and biology of cyclotides: circular plant peptides outside the box

Cyclotides stand out as the largest family of circular proteins of plant origin hitherto known, with more than 280 sequences isolated at peptide level and many more predicted from gene sequences. Their unusual stability resulting from the signature cyclic cystine knot (CCK) motif has triggered a broad interest in these molecules for potential therapeutic and agricultural applications. Since the time of the first cyclotide discovery, our laboratory in Uppsala has been engaged in cyclotide discovery as well as the development of protocols to isolate and characterize these seamless peptides. We have also developed methods to chemically synthesize cyclotides by Fmoc-SPPS, which are useful in protein grafting applications. In this review, experience in cyclotide research over two decades and the recent literature related to their structures, synthesis, and folding as well the recent proof-of-concept findings on their use as "epitope" stabilizing scaffolds are summarized.

Nuclear pore targeting of the yeast Pom33 nucleoporin depends on karyopherin- and lipid-binding

Pom33 is an integral membrane protein of the yeast nuclear pore complex (NPC), required for proper NPC distribution and assembly. To characterize Pom33 NPC-targeting determinants, we performed immunoprecipitation experiments followed by mass spectrometry analyses. This identified a novel Pom33 partner, the nuclear import factor Kap123. In vitro experiments revealed a direct interaction between Pom33 C-terminal domain (CTD) and Kap123. In silico analysis predicted the presence of two amphipathic α-helices within Pom33-CTD. Circular dichroism and liposome co-flotation assays showed that this domain is able to fold into α-helices in the presence of liposomes and preferentially binds to highly curved lipid membranes. When expressed in yeast, under conditions abolishing Pom33-CTD membrane association, this domain behaves as a Kap123-dependent nuclear localization signal (NLS). While deletion of Pom33 C-terminal domain (Pom33ΔCTD-GFP) impairs Pom33 stability and NPC targeting, mutants affecting either Kap123 binding or the amphipathic properties of the α-helices do not display any detectable defect. However, combined impairment of lipid and Kap123 binding affects Pom33 targeting to NPCs. These data highlight the requirement of multiple determinants and mechanisms for proper NPC localization of Pom33.

riDOM, a cell penetrating peptide. Interaction with phospholipid bilayers

Melittin is an amphipathic peptide which has received much attention as a model peptide for peptide-membrane interactions. It is however not suited as a transfection agent due to its cytolytic and toxicological effects. Retro-inverso-melittin, when covalently linked to the lipid 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (riDOM), eliminates these shortcomings. The interaction of riDOM with phospholipid membranes was investigated with circular dichroism (CD) spectroscopy, dynamic light scattering, ζ-potential measurements, and high-sensitivity isothermal titration calorimetry. riDOM forms cationic nanoparticles with a diameter of ~13nm which are well soluble in water and bind with high affinity to DNA and lipid membranes. When dissolved in bilayer membranes, riDOM nanoparticles dissociate and form transient pores. riDOM-induced membrane leakiness is however much reduced compared to that of authentic melittin. The secondary structure of the ri-melittin is not changed when riDOM is transferred from water to the membrane and displays a large fraction of β-structure. The (31)P NMR spectrum of the nanoparticle is however transformed into a typical bilayer spectrum. The Gibbs free energy of riDOM binding to bilayer membranes is -8.0 to -10.0kcal/mol which corresponds to the partition energy of just one fatty acyl chain. Half of the hydrophobic surface of the riDOM lipid extension with its 2 oleic acyl chains is therefore involved in a lipid-peptide interaction. This packing arrangement guarantees a good solubility of riDOM both in the aqueous and in the membrane phase. The membrane binding enthalpy is small and riDOM binding is thus entropy-driven.

Structure, antimicrobial activities and mode of interaction with membranes of novel [corrected] phylloseptins from the painted-belly leaf frog, Phyllomedusa sauvagii

Transcriptomic and peptidomic analysis of skin secretions from the Painted-belly leaf frog Phyllomedusa sauvagii led to the identification of 5 novel phylloseptins (PLS-S2 to -S6) and also of phylloseptin-1 (PSN-1, here renamed PLS-S1), the only member of this family previously isolated in this frog. Synthesis and characterization of these phylloseptins revealed differences in their antimicrobial activities. PLS-S1, -S2, and -S4 (79–95% amino acid sequence identity; net charge  = +2) were highly potent and cidal against Gram-positive bacteria, including multidrug resistant S. aureus strains, and killed the promastigote stage of Leishmania infantum, L. braziliensis and L. major. By contrast, PLS-S3 (95% amino acid identity with PLS-S2; net charge  = +1) and -S5 (net charge  = +2) were found to be almost inactive against bacteria and protozoa. PLS-S6 was not studied as this peptide was closely related to PLS-S1. Differential scanning calorimetry on anionic and zwitterionic multilamellar vesicles combined with circular dichroism spectroscopy and membrane permeabilization assays on bacterial cells indicated that PLS-S1, -S2, and -S4 are structured in an amphipathic α-helix that disrupts the acyl chain packing of anionic lipid bilayers. As a result, regions of two coexisting phases could be formed, one phase rich in peptide and the other lipid-rich. After reaching a threshold peptide concentration, the disruption of lipid packing within the bilayer may lead to local cracks and disintegration of the microbial membrane. Differences in the net charge, α-helical folding propensity, and/or degree of amphipathicity between PLS-S1, -S2 and -S4, and between PLS-S3 and -S5 appear to be responsible for their marked differences in their antimicrobial activities. In addition to the detailed characterization of novel phylloseptins from P. sauvagii, our study provides additional data on the previously isolated PLS-S1 and on the mechanism of action of phylloseptins.

Effect of amino acid distribution of amphipathic helical peptide derived from human apolipoprotein A-I on membrane curvature sensing

Amphipathic helix, which senses membrane curvature, is of growing interest. Here we explore the effect of amino acid distribution of amphipathic helical peptide derived from the C-terminal region (residues 220-241) of human apolipoprotein (apo) A-I on membrane curvature sensing. This peptide preferred a curved membrane in a manner similar to full-length apoA-I, although its model peptide did not sense membrane curvature. Substitution of several residues both on the polar and non-polar faces of the amphipathic helix had no significant effect on sensing, suggestive of the elaborate molecular architecture in the C-terminal helical region of apoA-I to exert lipid efflux function.

Melittin-grafted HPMA-oligolysine based copolymers for gene delivery

Non-viral gene delivery systems capable of transfecting cells in the brain are critical in realizing the potential impact of nucleic acid therapeutics for diseases of the central nervous system. In this study, the membrane-lytic peptide melittin was incorporated into block copolymers synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization. The first block, designed for melittin conjugation, was composed of N-(2-hydroxypropyl)methacrylamide (HPMA) and pyridyl disulfide methacrylamide (PDSMA) and the second block, designed for DNA binding, was composed of oligo-l-lysine (K10) and HPMA. Melittin modified with cysteine at the C-terminus was conjugated to the polymers through the pyridyl disulfide pendent groups via disulfide exchange. The resulting pHgMelbHK10 copolymers are more membrane-lytic than melittin-free control polymers, and efficiently condensed plasmid DNA into salt-stable particles (~100-200 nm). The melittin-modified polymers transfected both HeLa and neuron-like PC-12 cells more efficiently than melittin-free polymers although toxicity associated with the melittin peptide was observed. Optimized formulations containing the luciferase reporter gene were delivered to mouse brain by intraventricular brain injections. Melittin-containing polyplexes produced about 35-fold higher luciferase activity in the brain compared to polyplexes without melittin. Thus, the melittin-containing block copolymers described in this work are promising materials for gene delivery to the brain.