Permeabilization and morphological changes in phosphatidylglycerol bilayers induced by an antimicrobial peptide, tachyplesin I

Cyclic Analogues of Horseshoe Crab Peptide Tachyplesin I with Anticancer and Cell Penetrating Properties

Tachyplesin-I (TI) is a host defense peptide from the horseshoe crab Tachypleus tridentatus that has outstanding potential as an anticancer therapeutic lead. Backbone cyclized TI (cTI) has similar anticancer properties to TI but has higher stability and lower hemolytic activity. We designed and synthesized cTI analogues to further improve anticancer potential and investigated structure-activity relationships based on peptide-membrane interactions, cellular uptake, and anticancer activity. The membrane-binding affinity and cytotoxic activity of cTI were found to be highly dependent on peptide hydrophobicity and charge. We describe two analogues with increased selectivity toward melanoma cells and one analogue with the ability to enter cells with high efficacy and low toxicity. Overall, the structure-activity relationship study shows that cTI can be developed as a membrane-active antimelanoma lead, or be employed as a cell penetrating peptide scaffold that can target and enter cells without damaging their integrity.

Reversible liposome association induced by LAH4: a peptide with potent antimicrobial and nucleic acid transfection activities

We report on the reversible association of anionic liposomes induced by an antimicrobial peptide (LAH4). The process has been characterized for mixed membranes of POPC and POPS at molar ratios of 1:1, 3:1, and 9:1. Although the vesicles remain in suspension in the presence of excess amounts of peptide, the addition of more lipids results in surface charge neutralization, aggregation of the liposomes, and formation of micrometer-sized structures that coexist in equilibrium with vesicles in suspension. At low ratios of anionic lipids, vesicle aggregation is a reversible process, and vesicle disassembly is observed upon inversion of the surface charge by further supplementation with anionic vesicles. In contrast, a different process, membrane fusion, occurs in the presence of high phosphatidylserine concentrations. Upon binding to membranes containing low POPS concentrations, the peptide adopts an in-plane alpha-helical structure, a secondary structure that is conserved during vesicle association and dissociation. Our finding that peptides are essential for vesicle aggregation contributes to a better understanding of the activity of antimicrobial peptides, and suggests an additional layer of complexity in membrane-protein lipid interactions.

Divorcing folding from function: how acylation affects the membrane-perturbing properties of an antimicrobial peptide

Many small cationic peptides, which are unstructured in aqueous solution, have antimicrobial properties. These properties are assumed to be linked to their ability to permeabilize bacterial membranes, accompanied by the transition to an alpha-helical folding state. Here we show that there is no direct link between folding of the antimicrobial peptide Novicidin (Nc) and its membrane permeabilization. N-terminal acylation with C8-C16 alkyl chains and the inclusion of anionic lipids both increase Nc's ability to form alpha-helical structure in the presence of vesicles. Nevertheless, both acylation and anionic lipids reduce the extent of permeabilization of these vesicles and lead to slower permeabilization kinetics. Furthermore, acylation significantly decreases antimicrobial activity. Although acyl chains of increasing length also increase the tendency of the peptides to aggregate in solution, this cannot rationalize our results since permeabilization and antimicrobial activities are observed well below concentrations where aggregation occurs. This suggests that significant induction of alpha-helical structure is not a prerequisite for membrane perturbation in this class of antimicrobial peptides. Our data suggests that for Nc, induction of alpha-helical structure may inhibit rather than facilitate membrane disruption, and that a more peripheral interaction may be the most efficient permeabilization mechanism. Furthermore, acylation leads to a deeper embedding in the membrane, which could lead to an anti-permeabilizing "plugging" effect.

Antimicrobial peptides: linking partition, activity and high membrane-bound concentrations

An increasing amount of information on the action of antimicrobial peptides (AMPs) at the molecular level has not yet been translated into a comprehensive understanding of effects in bacteria. Although some biophysical attributes of AMPs have been correlated with macroscopic features, the physiological relevance of other properties has not yet been addressed. Pertinent and surprising conclusions have therefore been left unstated. Strong membrane-binding and micromolar therapeutic concentrations of AMPs indicate that membrane-bound concentrations may be reached that are higher than intuitively expected, triggering disruptive effects on bacteria.

Orientation and helical conformation of a tissue-specific hunter-killer peptide in micelles

Hunter-killer peptides are chimeric synthetic peptides that selectively target specific cell types for an apoptotic death. These peptides, which are models for potential therapeutics, contain a homing sequence for receptor-mediated interactions and a pro-apoptotic sequence. Homing domains have been designed to target angiogenic tumor cells, prostate cells, arthritic tissue and, most recently, adipose tissue. After a receptor-mediated internalization, the apoptotic sequence, which contains D-enantiomer amino acids, initiates apoptosis through mitochondrial membrane disruption. We have begun structure and functional studies on a peptide (HKP1) that specifically targets angiogenic tumor cells for apoptosis. As a model for mitochondrial membrane disruption, we have examined peptide-induced leakage of a calcein fluorophore from large unilamellar vesicles. These experiments demonstrate more potent leakage activity by HKP1 than the peptide lacking the homing domain. Circular dichroism and 2D homonuclear NMR experiments demonstrate that this tumor-specific HKP adopts a left-handed amphipathic helix in association with dodecylphosphorylcholine micelles in a parallel orientation to the lipid-water interface with the homing domain remaining exposed to solvent. The amphipathic helix of the apoptotic domain orients with nonpolar leucine and alanine residues inserting most deeply into the lipid environment.

Transient vesicle leakage initiated by a synthetic apoptotic peptide derived from the death domain of neurotrophin receptor, p75NTR

Peptides that induce apoptosis have potential as anticancer therapeutics. The design of safe, effective cancer therapeutic peptides requires characterization of the physical and chemical properties that influence activation of cell death in neoplastic cells. NTR365 is a synthetic pro-apoptotic peptide with an amino acid sequence derived from the death domain of p75(NTR). These studies were initiated to identify a potential mechanism for the apoptotic activity of NTR365 identified by Rabizadeh et al. We examined the interactions of this synthetic pro-apoptotic peptide with phospholipid vesicles. Fluorescence experiments demonstrate that the peptide induces leakage from large unilamellar vesicles. Leakage activity is transient and dependent on the presence of anionic lipid in the vesicles. Circular dichroism studies show that the NTR365 adopts a different conformation and may have altered vesicle affinity under conditions conducive to leakage. The active conformation of NTR365 differs from that of the NMR derived conformation. A related peptide with a single substitution is not apoptotically active, does not form a helical structure in the presence of vesicles and does not induce appreciable vesicle leakage under the same conditions as NTR365. These studies suggest that the demonstrated apoptotic activity of a closely related NTR364 peptide is linked to disruption of a membrane barrier and to the ability of the peptide to form a helical structure.

Structural aspects of the interaction of peptidyl-glycylleucine-carboxyamide, a highly potent antimicrobial peptide from frog skin, with lipids

The interaction of PGLa (peptidyl-glycylleucine-carboxyamide), a 21-amino-acid residue cationic peptide, isolated from the skin of the South African clawed frog, Xenopus laevis, with model membrane systems was investigated. Our studies focussed on the importance of the difference in the phospholipid composition of bacterial and erythrocyte membranes. This is of particular interest to gain information on the specificity of membranolysis exhibited by this peptide against bacteria but not against erythrocytes. In phosphate buffer at physiological pH, as well as in the presence of the zwitterionic phosphatidylcholine and sphingomyelin. the peptide had a random structure but it adopted an alpha-helical conformation in the presence of negatively charged lipids. Furthermore, calorimetric experiments showed that PGLa had no effects on the thermotropic phase behavior of liposomes composed of the choline phosphatides, while separation of a distinct peptide-rich domain was observed for phosphatidylglycerol liposomes. In addition to the main transition of pure 1,2-dipalmitoylglycerophosphoglycerol at 40 degrees C a second transition owing to the peptide-perturbed lipid domains was found at 41 degrees C. This conclusion is supported by X-ray diffraction experiments which indicated that PGLa penetrates into the hydrophobic core of the bilayer inducing an untilting of the hydrocarbon chains as observed in the gel phase of the pure lipid. These results demonstrate that this antibacterial peptide specifically interacts with negatively charged lipid membranes, which are characteristic of bacterial membranes. This can be explained based on the structural features of PGLa.