The relationship between the binding to and permeabilization of phospholipid bilayer membranes by GS14dK4, a designed analog of the antimicrobial peptide gramicidin S

Submission for Special Issue: The Role of Platelet Activation in the Pathophysiology of HIV, Tuberculosis, and Pneumococcal Disease. Bedaquiline Suppresses ADP-Mediated Activation of Human Platelets In Vitro via Interference With Phosphatidylinositol 3-Kinase

Although bedaquiline has advanced the treatment of multidrug-resistant tuberculosis (TB), concerns remain about the cardiotoxic potential of this agent, albeit by unexplored mechanisms. Accordingly, we have investigated augmentation of the reactivity of human platelets in vitro as a potential mechanism of bedaquiline-mediated cardiotoxicity. Platelet-rich plasma (PRP) or isolated cells prepared from the blood of healthy, adult humans were treated with bedaquiline (0.625–10 µg/ml), followed by activation with adenosine 5’-diphosphate (ADP), thrombin or the thromboxane A₂ receptor agonist (U46619). Expression of platelet CD62P (P-selectin), platelet aggregation, Ca²⁺ fluxes and phosphorylation of Akt1 were measured using flow cytometry, spectrophotometry, fluorescence spectrometry, and by ELISA procedures, respectively. Exposure to bedaquiline caused dose-related inhibition of ADP-activated, but not thrombin- or U46619-activated, expression of CD62P by platelets, achieving statistical significance at a threshold concentration of 5 µg/ml and was paralleled by inhibition of aggregation and Ca²⁺ mobilization. These ADP-selective inhibitory effects of bedaquiline on platelet activation were mimicked by wortmannin, an inhibitor of phosphatidylinositol 3-kinase (PI3-K), implicating PI3-K as being a common target of both agents, a contention that was confirmed by the observed inhibitory effects of bedaquiline on the phosphorylation of Akt1 following activation of platelets with ADP. These apparent inhibitory effects of bedaquiline on the activity of PI3-K may result from the secondary cationic amphiphilic properties of this agent. If operative in vivo, these anti-platelet effects of bedaquiline may contribute to ameliorating the risk of TB-associated cardiovascular disease, but this remains to be explored in the clinical setting.

Clofazimine, but Not Isoniazid or Rifampicin, Augments Platelet Activation in vitro

Although the inclusion of the cationic amphiphilic, anti-mycobacterial agent, clofazimine, in the chemotherapeutic regimens of patients with multidrug-resistant tuberculosis (TB) has contributed to improved outcomes, concerns remain about the cardiotoxic potential of this agent. Accordingly, the current study was undertaken with the primary objective of investigating the effects of clofazimine, on the reactivity of human platelets in vitro, a seemingly unexplored, mechanism of cardiotoxicity. Platelet-rich plasma (PRP) prepared from the blood of healthy, adult humans was treated with clofazimine (0.625–10 mg/L), or the primary anti-TB agents, isoniazid and rifampicin (at final concentrations of 5 and 10 mg/L), followed by addition of either adenosine 5′-diphosphate (ADP) or thrombin and measurement of platelet activation according to the magnitude of expression of CD62P (P-selectin), as well as the CD62P-mediated formation of heterotypic neutrophil:platelet (NP) aggregates, using flow cytometry. Clofazimine, but neither isoniazid nor rifampicin, caused dose-related potentiation of both ADP- and thrombin-activated expression of CD62P by platelets, achieving statistical significance at threshold concentrations of 0.625 and 2.5 mg/L, respectively, as well as significant formation of N:P aggregates. These stimulatory effects of clofazimine on platelet activation were partly attenuated by pre-treatment of PRP with the membrane-stabilizing agent, α-tocopherol, possibly consistent with a membrane-disruptive mechanism. In conclusion, clofazimine, at concentrations within the therapeutic range, augments platelet activation in vitro, probably by a mechanism linked to membrane destabilization. If operative in vivo, these pro-thrombotic activities of clofazimine may predispose for development of microvascular occlusion, exacerbating an already existing high risk for development of TB-associated cardiovascular disease.

Antimicrobial activity and mechanism of action of a novel cationic α-helical octadecapeptide derived from heat shock protein 70 of rice

Hsp70(241-258), an octadecapeptide derived from the heat shock protein 70 (Hsp70) of rice (Oryza sativa L. japonica), is a novel cationic α-helical antimicrobial peptide (AMP) that contains four lysine, two arginine, and two histidine residues. The antimicrobial activity of Hsp70(241-258) against Porphyromonas gingivalis, a periodontal pathogen, and Candida albicans, an opportunistic fungal pathogen, was quantitatively evaluated using a chemiluminescence method that measures ATP derived from viable cells. The 50% growth-inhibitory concentrations of Hsp70(241-258) against P. gingivalis and C. albicans cells were 63 μM and 70 μM, respectively. Hsp70(241-258) had little or no hemolytic activity even at 1mM, and showed negligible cytotoxicity up to 300 μM. The degrees of calcein leakage from large unilamellar vesicles, which mimic the membranes of Gram-negative bacteria, and 3,3'-dipropylthiadicarbocyanine iodide release from P. gingivalis cells induced by the addition of Hsp70(241-258) increased in a concentration-dependent manner. When Hsp70(241-258) was added to calcein-acetoxymethyl ester-loaded C. albicans cells, calcein release from the cells increased in a concentration-dependent manner. Flow cytometric analysis also showed that the percentages of C. albicans cells stained with propidium iodide, a DNA-intercalating dye, increased as the concentration of Hsp70(241-258) added was increased. Therefore, Hsp70(241-258) appears to exhibit antimicrobial activity against P. gingivalis and C. albicans through membrane disruption. These results suggest that Hsp70(241-258) could be useful as a safe and potent AMP against P. gingivalis and C. albicans in many fields of health care, especially in the control of oral infections.

Lipid composition-dependent membrane fragmentation and pore-forming mechanisms of membrane disruption by pexiganan (MSI-78)

The potency and selectivity of many antimicrobial peptides (AMPs) are correlated with their ability to interact with and disrupt the bacterial cell membrane. In vitro experiments using model membranes have been used to determine the mechanism of membrane disruption of AMPs. Because the mechanism of action of an AMP depends on the ability of the model membrane to accurately mimic the cell membrane, it is important to understand the effect of membrane composition. Anionic lipids that are present in the outer membrane of prokaryotes but are less common in eukaryotic membranes are usually thought to be key for the bacterial selectivity of AMPs. We show by fluorescence measurements of peptide-induced membrane permeabilization that the presence of anionic lipids at high concentrations can actually inhibit membrane disruption by the AMP MSI-78 (pexiganan), a representative of a large class of highly cationic AMPs. Paramagnetic quenching studies suggest MSI-78 is in a surface-associated inactive mode in anionic sodium dodecyl sulfate micelles but is in a deeply buried and presumably more active mode in zwitterionic dodecylphosphocholine micelles. Furthermore, a switch in mechanism occurs with lipid composition. Membrane fragmentation with MSI-78 can be observed in mixed vesicles containing both anionic and zwitterionic lipids but not in vesicles composed of a single lipid of either type. These findings suggest membrane affinity and membrane permeabilization are not always correlated, and additional effects that may be more reflective of the actual cellular environment can be seen as the complexity of the model membranes is increased.

Antimicrobial activity and mechanism of action of a novel cationic α-helical dodecapeptide, a partial sequence of cyanate lyase from rice

CL(14-25), a dodecapeptide, that is a partial region near N-terminus of cyanate lyase (CL, EC 4.3.99.1) from rice (Oryza sativa L. japonica), contains three arginine and two lysine residues. It was a novel cationic α-helical antimicrobial peptide. The antimicrobial activity of CL(14-25) against Porphyromonas gingivalis, a periodontal pathogen, was quantitatively evaluated by a chemiluminescence method that measures ATP derived from viable cells. The 50% growth-inhibitory concentration of CL(14-25) against P. gingivalis cells was 145 μM. CL(14-25), even at a concentration of 800 μM, had no hemolytic activity. When giant unilamellar vesicles (GUVs) that mimic the membrane composition of Gram-negative bacteria were used, microscopy image analysis suggested that CL(14-25) disrupted GUVs in a detergent-like manner. Therefore, CL(14-25) appears to exhibit antimicrobial activity through membrane disruption. To investigate the contribution of cationic amino acid residues in CL(14-25) to its antimicrobial activity, we synthesized four truncated CL analogs, in which one or two cationic amino acid residues were deleted from the N- and C- termini of CL(14-25). The degrees of calcein leakage from large unilamellar vesicles (LUVs) and 3,3'-dipropylthiadicarbocyanine iodide (diSC3-5) release from P. gingivalis cells induced by truncated CL analogs were closely related to their antimicrobial activities. CL analogs, which were truncated by removing an arginine residue from the N-terminus and a lysine residue from the C-terminus maintained their antimicrobial activity. However, CL analogs, which were further truncated by removing two arginine residues from the N-terminus, and an arginine and a lysine residue from the C-terminus, rarely exhibited antimicrobial activity.

Membrane interactions of the amphipathic amino terminus of huntingtin

The amino-terminal domain of huntingtin (Htt17), located immediately upstream of the decisive polyglutamine tract, strongly influences important properties of this large protein and thereby the development of Huntington's disease. Htt17 markedly increases polyglutamine aggregation rates and the level of huntingtin's interactions with biological membranes. Htt17 adopts a largely helical conformation in the presence of membranes, and this structural transition was used to quantitatively analyze membrane association as a function of lipid composition. The apparent membrane partitioning constants increased in the presence of anionic lipids but decreased with increasing amounts of cholesterol. When membrane permeabilization was tested, a pronounced dye release was observed from 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) vesicles and 75:25 (molar ratio) POPC/1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-l-serine vesicles but not across bilayers that better mimic cellular membranes. Solid-state nuclear magnetic resonance structural investigations indicated that the Htt17 α-helix adopts an alignment parallel to the membrane surface, and that the tilt angle (∼75°) was nearly constant in all of the membranes that were investigated. Furthermore, the addition of Htt17 resulted in a decrease in the lipid order parameter in all of the membranes that were investigated. The lipid interactions of Htt17 have pivotal implications for membrane anchoring and functional properties of huntingtin and concomitantly the development of the disease.

Secondary Structural Preferences of Some Antibacterial Cyclooctapeptides in the Presence of Calcium(II)

The purpose of this study is to understand the interactions of some antibacterial cationic amphipathic cyclooctapeptides with calcium(II) and their secondary structural preferences. The thermodynamic parameters associated with calcium(II) interactions, between the antibacterial active cyclooctapeptides (COP 1-6) and those that did not exhibit significant activities (COP 7-9), were studied by isothermal titration calorimetry. Calcium(II) binding in the absence and presence of micellar dodecylphosphocholine (DPC), a membrane mimicking detergent, was conducted by circular dichroism (CD). Both groups of cyclopeptides showed weak binding affinities for calcium(II) (Kb ca. 10(-3) M(-1)). However, CD data showed that the antimicrobial peptides COP 1-6 adopted a twisted beta-sheet structure (positive CD absorption band at ca. 203 nm) in the presence of calcium(II) in micellar DPC. In contrast, COP 7-9, which lacked antibacterial activity, adopted a different conformational structure (negative CD absorption band at ca. 203 nm). These results indicate that these cyclopeptides could adopt secondary structural preferences in the presence of calcium(II) amidst a hydrophobic environment to elicit their antibacterial activity. These findings could be useful in facilitating the design of cyclopeptide derivatives that can adopt this beta-sheet-like secondary structure and, thereby, provide a useful molecular template for crafting antibacterial compounds.

Will new generations of modified antimicrobial peptides improve their potential as pharmaceuticals?

The concept of antimicrobial peptides (AMPs) as potent pharmaceuticals is firmly established in the literature, and most research articles on this topic conclude by stating that AMPs represent promising therapeutic agents against bacterial and fungal pathogens. Indeed, early research in this field showed that AMPs were diverse in nature, had high activities with low minimal inhibitory concentrations, had broad spectrums of activity against bacterial, fungal and viral pathogens, and could easily be manipulated to alter their specificities, reduce their cytotoxicities and increase their antimicrobial activities. Unfortunately, commercial development of these peptides, for even the simplest of applications, has been very limited. With some peptides there are obstacles with their manufacture, in vivo efficacy and in vivo retention. More recently, the focus has shifted. Contemporary research now uses a more sophisticated approach to develop AMPs that surmount many of these prior obstacles. AMP mimetics, hybrid AMPs, AMP congeners, cyclotides and stabilised AMPs, AMP conjugates and immobilised AMPs have all emerged with selective or 'targeted' antimicrobial activities, improved retention, or unique abilities that allow them to bind to medical or industrial surfaces. These groups of new peptides have creative medical and industrial application potentials to treat antibiotic-resistant bacterial infections and septic shock, to preserve food or to sanitise surfaces both in vitro and in vivo.

Metal-binding dependent disruption of membranes by designed helices

The de novo design of molecular switching peptides is of increasing interest because it tests and extends our fundamental understanding of this process while laying the groundwork for the creation of new chemical and biological sensors. Here, an alpha-helical amphiphilic cell-lytic peptide, mastoparan X, was engineered to bind divalent cations. Binding of Zn(II) or Ni(II) to the designed peptide Mst-HH stabilizes the lytic amphiphilic structure and increases the activity of the peptide. Although both Zn(II) and Ni(II) activate Mst-HH for membrane lysis, they appear to do so via different mechanisms. Additionally, a series of metal binding-site mutants were synthesized to assess the relationship of charge and helical propensity to the toxicity and switchability. Additionally, by changing the characteristics of the metal-binding ligands, we can vary the selectivity of the site.

Sequence inversion and phenylalanine surrogates at the beta-turn enhance the antibiotic activity of gramicidin S

A series of gramicidin S (GS) analogues have been synthesized where the Phe (i + 1) and Pro (i + 2) residues of the beta-turn have been swapped while the respective chiralities (D-, L-) at each position are preserved, and Phe is replaced by surrogates with aromatic side chains of diverse size, orientation, and flexibility. Although most analogues preserve the beta-sheet structure, as assessed by NMR, their antibiotic activities turn out to be highly dependent on the bulkiness and spatial arrangement of the aromatic side chain. Significant increases in microbicidal potency against both Gram-positive and Gram-negative pathogens are observed for several analogues, resulting in improved therapeutic profiles. Data indicate that seemingly minor replacements at the GS beta-turn can have significant impact on antibiotic activity, highlighting this region as a hot spot for modulating GS plasticity and activity.

Therapeutic index of gramicidin S is strongly modulated by D-phenylalanine analogues at the beta-turn

Analogues of the cationic antimicrobial peptide gramicidin S (GS), cyclo(Val-Orn-Leu-D-Phe-Pro)2, with d-Phe residues replaced by different (restricted mobility, mostly) surrogates have been synthesized and used in SAR studies against several pathogenic bacteria. While all D-Phe substitutions are shown by NMR to preserve the overall beta-sheet conformation, they entail subtle structural alterations that lead to significant modifications in biological activity. In particular, the analogue incorporating D-Tic (1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid) shows a modest but significant increase in therapeutic index, mostly due to a sharp decrease in hemolytic effect. The fact that NMR data show a shortened distance between the D-Tic aromatic ring and the Orn delta-amino group may help explain the improved antibiotic profile of this analogue.

Mechanism of antibacterial action of dermaseptin B2: interplay between helix-hinge-helix structure and membrane curvature strain

Dermaseptin B2 (Drs B2) is a 33-residue-long cationic, alpha-helical antimicrobial peptide endowed with membrane-damaging activity against a broad spectrum of microorganisms, including bacteria, yeasts, fungi, and protozoa, but its precise mechanism of action remained ill-defined. A detailed characterization of peptide-membrane interactions of Drs B2 was undertaken in comparison with a C-terminal truncated analogue, [1-23]-Drs B2, that was virtually inactive on bacteria despite retaining the cationic charge of the full-length peptide. Both peptides were tested on living cells using membrane permeabilization assays and on large unilamellar and multilamellar phospholipid vesicles composed of binary lipid mixtures by dye leakage assay, fluorescence spectroscopy, circular dichroism, and differential scanning calorimetry and also on SDS micelles using NMR spectroscopy. The results indicate that Drs B2 induces a strong perturbation of anionic lipid bilayers, resides at the hydrocarbon core-water interface, parallel to the plane of the membrane, and interacts preferentially with the polar head groups and glycerol backbone region of the anionic phospholipids, as well as the region of the lipid acyl chain near the bilayer surface. The interfacial location of Drs B2 induces a positive curvature of the bilayer and clustering of anionic lipids, consistent with a carpet mechanism, that may lead to the formation of mixed peptide-phospholipid toroidal, transient pores and membrane permeation/disruption once a threshold peptide accumulation is reached. In constrast, the truncated [1-23]-Drs B2 analogue interacts at the head group level without penetrating and perturbing the hydrophobic core of the bilayer. NMR study in SDS micelles showed that [1-23]-Drs B2 adopts a well-defined helix encompassing residues 2-20, whereas Drs B2 was previously found to adopt helical structures interrupted around the Val(9)-Gly(10) segment. Thus the antibacterial activity of Drs B2 depends markedly on a threshold number of hydrophobic residues to be present on both extremities of the helix. In a membrane environment with a strong positive curvature strain, Drs B2 can adopt a flexible helix-hinge-helix structure that facilitates the concomitant insertion of the strongly hydrophobic N- and C-termini of the peptide into the acyl core of the membrane.

A survey of the year 2007 literature on applications of isothermal titration calorimetry

Elucidation of the energetic principles of binding affinity and specificity is a central task in many branches of current sciences: biology, medicine, pharmacology, chemistry, material sciences, etc. In biomedical research, integral approaches combining structural information with in-solution biophysical data have proved to be a powerful way toward understanding the physical basis of vital cellular phenomena. Isothermal titration calorimetry (ITC) is a valuable experimental tool facilitating quantification of the thermodynamic parameters that characterize recognition processes involving biomacromolecules. The method provides access to all relevant thermodynamic information by performing a few experiments. In particular, ITC experiments allow to by-pass tedious and (rarely precise) procedures aimed at determining the changes in enthalpy and entropy upon binding by van't Hoff analysis. Notwithstanding limitations, ITC has now the reputation of being the "gold standard" and ITC data are widely used to validate theoretical predictions of thermodynamic parameters, as well as to benchmark the results of novel binding assays. In this paper, we discuss several publications from 2007 reporting ITC results. The focus is on applications in biologically oriented fields. We do not intend a comprehensive coverage of all newly accumulated information. Rather, we emphasize work which has captured our attention with originality and far-reaching analysis, or else has provided ideas for expanding the potential of the method.