A Voltage-Responding Ion Channel Derived by C-Terminal Modification of Gramicidin A

C-Terminal modification of polytheonamide B uncouples its dual functions in MCF-7 cancer cells

Polytheonamide B (1) is an exceptionally large peptide that forms a transmembrane ion channel. The potent cytotoxicity of 1 against MCF-7 cancer cells originates from its two ion transport functions. Compound 1 depolarizes the plasma membrane and neutralizes acidic lysosomes. Here, we describe how we uncoupled these functions by designing and synthesizing new analogues of 1.

Regulation of Lipid Bilayer Ion Permeability by Antibacterial Polymethyloxazoline-Polyethyleneimine Copolymers

Amphiphilic antimicrobial polymers display activity against the outer bacterial cell membrane, triggering various physiological effects. We investigated the regulation of ion transport across the lipid bilayer to understand differences in biological activity for a series of amphiphilic polymethyloxazoline - polyethyleneimine copolymers. The results confirmed that the tested structures were able to increase the permeability of the lipid bilayer (LB) membrane or its rupture. Black lipid membrane (BLM) experiments show that the triggered conductance profile and its character is strongly correlated with the polymer structure and zeta potential. The polymer exhibiting the highest antimicrobial activity promotes ion transport by using a unique mechanism and step-like characteristics with well-defined discreet openings and closings. The molecule was incorporated into the membrane in a reproducible way, and the observed channel-like activity could be responsible for the antibacterial activity of this molecule.

Phospholipid-Dependent Functions of a Macrocyclic Analogue of the Ion-Channel-Forming Antibiotic Gramicidin A

An ion-channel-forming natural peptide, gramicidin A (1), exhibits potent antimicrobial activity against Gram-positive bacteria, although medical applications are limited to topical use due to its mammalian cytotoxicity. We recently reported that the artificial macrocyclic analogue 2 provides a promising starting point for developing new ion-channel-based systemic antibacterial agents because of its low mammalian cytotoxicity compared to that of the parent 1. To dissect the molecular factors involved in the species selectivity of 2, we evaluated the ion transport activities, phospholipid affinities, and conformational properties of 1 and 2 using various compositions of phospholipids. A combination of lipid dot blot assays and circular dichroism (CD) analysis with H⁺/Na⁺ exchange assays revealed that the higher H⁺/Na⁺ exchange activity of 2 than that of 1 in liposomes containing 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (POPC) or 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (POPG) is attributable to its higher affinity towards the phospholipids than that of 1. Notably, we also discovered that 2 showed weaker H⁺/Na⁺ exchange activity in liposomes containing 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylethanolamine (POPE). CD analysis of 2 in liposomes indicated that the weak H⁺/Na⁺ exchange activity is induced by disturbance of the ion-conducting β^6.3-helical conformation in the POPE-containing lipid bilayer. These results suggest that the POPE-induced attenuation of the ion-conducting activity of 2 contributes to the alleviation of undesirable mammalian cytotoxicity of 2 compared to that of 1.

Effect of charge status on the ion transport and antimicrobial activity of synthetic channels

The charge status of channels formed by pillararene–gramicidin hybrid molecules has a significant impact on their trans-membrane transport properties, membrane-association abilities and antimicrobial activities.

Gramicidin A Mutants with Antibiotic Activity against Both Gram-Positive and Gram-Negative Bacteria

Antimicrobial peptides (AMPs) have shown potential as alternatives to traditional antibiotics for fighting infections caused by antibiotic-resistant bacteria. One promising example of this is gramicidin A (gA). In its wild-type sequence, gA is active by permeating the plasma membrane of Gram-positive bacteria. However, gA is toxic to human red blood cells at similar concentrations to those required for it to exert its antimicrobial effects. Installing cationic side chains into gA has been shown to lower its hemolytic activity while maintaining the antimicrobial potency. In this study, we present the synthesis and the antibiotic activity of a new series of gA mutants that display cationic side chains. Specifically, by synthesizing alkylated lysine derivatives through reductive amination, we were able to create a broad selection of structures with varied activities towards Staphylococcus aureus and methicillin-resistant S. aureus (MRSA). Importantly, some of the new mutants were observed to have an unprecedented activity towards important Gram-negative pathogens, including Escherichia coli, Klebsiella pneumoniae and Psuedomonas aeruginosa.

Cyclodextrin ion channels

Seventeen derivatives of α- and β-cyclodextrins were prepared from the cyclodextrin per-6-azide by "click" cyclization with terminal alkynes. Sixteen of these "half-channel" compounds showed significant activity as ion channels in planar bilayer members as assessed by the voltage-clamp technique. Activity ranged from persistent square-top openings to highly erratic conductance; mixed behaviours were evident in virtually all data recorded. Some of the erratic behaviours were shown to follow an apparent power-law distribution of open duration times. The activities observed for the suite were summarized using a model-free activity grid method which displays conductance, duration, and opening behaviour. The overall activity shows the clustering of conductance-duration indicating that activity arises from system properties rather that solely as a property of the compound. The activity grids also support an analysis of structure-activity relationships as they apply to the global behaviour of the compounds and reveal the complexity of a single structure change in controlling the distribution of concurrent conductance behaviours. Transient blockage of channel activity by the hydrophobic guest of the cyclodextrin (1-adamantyl carboxylate) is consistent with the formation of an end-to-end dimer channel among several other competing and interconverting structures.

Ion-channels: goals for function-oriented synthesis

Ion channels provide a conductance pathway for the passive transport of ions across membranes. These functional molecules perform key tasks in biological systems such as neuronal signaling, muscular control, and sensing. Recently, function-oriented synthesis researchers began to focus on ion channels with the goal of modifying the function of existing ion channels (ion selectivity, gating) or creating new channels with novel functions. Both approaches, ion channel engineering and de novo design, have involved synthetic chemists, biochemists, structural biologists, and neurochemists. Researchers characterize the function of ion channels by measuring their conductance in samples of biological membranes (patch clamp) or artificial membranes (planar lipid bilayers). At the single molecule level, these measurements require special attention to the purity of the sample, a challenge that synthetic chemists should be aware of. Ideally, researchers study the function of channels while also acquiring structural data (X-ray, NMR) to understand and predict how synthetic modifications alter channel function. Long-term oriented researchers would like to apply synthetic ion channels to single molecule sensing and to implantat these synthetic systems in living organisms as tools or for the treatment of channelopathies. In this Account, we discuss our own work on synthetic ion channels and explain the shift of our research focus from a de novo design of oligo-THFs and oligo-THF-amino acids to ion channel engineering. We introduce details about two biological lead structures for ion channel engineering: the gramicidin β(6,3) helix as an example of a channel with a narrow ion conductance pathway and the outer membrane porins (OmpF, OmpG) with their open β-barrel structure. The increase and the reversal of ion selectivity of these systems and the hydrophobic match/mismatch of the channel with the phospholipid bilayer are of particular interest. For engineering ion channels, we need to supplement the single-point attachment of a synthetic modulator with the synthesis of a more challenging two-point attachment. The successful function-oriented synthesis of ion channels will require interdisciplinary efforts that include new electrophysiology techniques, efficient synthesis (peptide/protein/organic), and good structural analysis.

Chemical construction and structural permutation of potent cytotoxin polytheonamide B: discovery of artificial peptides with distinct functions

Polytheonamide B (1), isolated from the marine sponge Theonella swinhoei, is a posttranslationally modified ribosomal peptide (MW 5030 Da) that displays extraordinary cytotoxicity. Among its 48 amino acid residues, this peptide includes a variety D- and L-amino acids that do not occur in proteins, and the chiralities of these amino acids alternate in sequence. These structural features induce the formation of a stable β6.3-helix, giving rise to a tubular structure of over 4 nm in length. In the biological setting, this fold is believed to transport cations across the lipid bilayer through a pore, thereby acting as an ion channel. In this Account, we discuss the construction and structural permutations of this potent cytotoxin. First we describe the 161-step chemical construction of this unusual peptide 1. By developing a synthetic route to 1, we established the chemical basis for subsequent SAR studies to pinpoint the proteinogenic and nonproteinogenic building blocks within the molecule that confer its toxicity and channel function. Using fully synthetic 1, we generated seven analogues with point mutations, and studies of their activity revealed the importance of the N-terminal moiety. Next, we simplified the structure of 1 by substituting six amino acid residues of 1 to design a more synthetically accessible analogue 9. This dansylated polytheonamide mimic 9 was synthesized in 127 total steps, and we evaluated its function to show that it can emulate the toxic and ion channel activities of 1 despite its multiple structural modifications. Finally, we applied a highly automated synthetic route to 48-mer 9 to generate 13 substructures of 27-39-mers. The 37-mer 12 exhibited nanomolar level toxicity through a potentially distinct mode of action from 1 and 9. The SAR studies of polytheonamide B and the 21 artificial analogues have deepened our understanding of the precise structural requirements for the biological functions of 1. They have also led to the discovery of artificial molecules with various toxicities and channel functions. These achievements demonstrate the benefits of total synthesis and the importance of efficient construction of complex molecules. The knowledge accumulated through these studies will be useful for the rational design of new tailor-made channel peptides and cytotoxic molecules with desired functions.

Selective modification of the N-terminal structure of polytheonamide B significantly changes its cytotoxicity and activity as an ion channel

Chemical point mutation: Polytheonamide B is a naturally occurring polypeptide containing 48 amino acids. It both displays potent cytotoxicity and acts as a monovalent cation channel in vitro. Chemoselective methods to modify the 44th, N-, and C-terminal residues of the natural product have been developed, and evaluation of the resultant derivatives suggests that the intrinsic activities of the peptide can only be altered by switching its N-terminal substitution.

A semi-synthetic ion channel platform for detection of phosphatase and protease activity

Sensitive methods to probe the activity of enzymes are important for clinical assays and for elucidating the role of these proteins in complex biochemical networks. This paper describes a semi-synthetic ion channel platform for detecting the activity of two different classes of enzymes with high sensitivity. In the first case, this method uses single ion channel conductance measurements to follow the enzyme-catalyzed hydrolysis of a phosphate group attached to the C-terminus of gramicidin A (gA, an ion channel-forming peptide) in the presence of alkaline phosphatase (AP). Enzymatic hydrolysis of this phosphate group removes negative charges from the entrance of the gA pore, resulting in a product with measurably reduced single ion channel conductance compared to the original gA-phosphate substrate. This technique employs a standard, commercial bilayer setup and takes advantage of the catalytic turnover of enzymes and the amplification characteristics of ion flux through individual gA pores to detect picomolar concentrations of active AP in solution. Furthermore, this technique makes it possible to study the kinetics of an enzyme and provides an estimate for the observed rate constant (k(cat)) and the Michaelis constant (K(M)) by following the conversion of the gA-phosphate substrate to product over time in the presence of different concentrations of AP. In the second case, modification of gA with a substrate for proteolytic cleavage by anthrax lethal factor (LF) afforded a sensitive method for detection of LF activity, illustrating the utility of ion channel-based sensing for detection of a potential biowarfare agent. This ion channel-based platform represents a powerful, novel approach to monitor the activity of femtomoles to picomoles of two different classes of enzymes in solution. Furthermore, this platform has the potential for realizing miniaturized, cost-effective bioanalytical assays that complement currently established assays.

Local antibiotics in dermatology

Although the vast majority of skin infection must be treated with systemic antibiotics, topical antibiotics are used overwhelmingly in the world, often as self-prescribed medications without taking into account the sensitivity of the presumed bacteria. Dermatologists are aware that different types of topical antibiotics kill different species of bacteria and tend to be more specific in their prescriptions. At present local antibiotics are advised to treat minor superficial uncomplicated skin infections (e.g., impetigo) and to prevent bacterial infections caused into minor cuts, scrapes, and burns. The role of topical antibiotics in the management of acne and atopic dermatitis is controversial. Retapamulin, a novel topical antibacterial agent, will probably replace the use of the old mupirocin and fusidic acid.