The Plasticins: Membrane Adsorption, Lipid Disorders, and Biological Activity

Peptide Dimerization as a Strategy for the Development of Antileishmanial Compounds

Leishmaniasis is recognized as a serious public health problem in Brazil and around the world. The limited availability of drugs for treatment, added to the diversity of side effects and the emergence of resistant strains, shows the importance of research focused on the development of new molecules, thus contributing to treatments. Therefore, this work aimed to identify leishmanicidal compounds using a peptide dimerization strategy, as well as to understand their mechanisms of action. Herein, it was demonstrated that the dimerization of the peptide TSHa, (TSHa)2K, presented higher potency and selectivity than its monomeric form when evaluated against Leishmania mexicana and Leishmania amazonensis. Furthermore, these compounds are capable of inhibiting the parasite cysteine protease, an important target explored for the development of antileishmanial compounds, as well as to selectively interact with the parasite membranes, as demonstrated by flow cytometry, permeabilization, and fluorescence microscopy experiments. Based on this, the identified molecules are candidates for use in in vivo studies with animal models to combat leishmaniasis.

A redox-responsive macrocycle based on the crown ether C7Te for enhanced bacterial inhibition

Due to increasing bacterial resistance to disinfectants, there is an urgent need for new therapeutic agents and strategies to effectively inhibit bacteria. Accordingly, we have designed and synthesized a novel crown ether known as C7Te, and its oxidized form C7TeO. These compounds have demonstrated antibacterial effectiveness against Gram-negative E. coli (BL21). Notably, C7Te has the capability to enhance the inhibition of E. coli and the prevention of biofilm formation by H2O2 through a redox response. It can also effectively disrupt preformed E. coli biofilms by penetrating biofilm barriers effectively. Additionally, computer modeling of the bacterial cell membrane using nanodiscs composed of phospholipids and encircled amphipathic proteins with helical belts has revealed that C7Te can insert into and interact with phospholipids and proteins. This interaction results in the disruption of the bacterial cell membrane leading to bacterial cell death. The utilization of redox-responsive crown ethers to augment the antibacterial capabilities of H2O2-based disinfectants represents a novel approach to supramolecular bacterial inhibition.

Characterization of a New Immunosuppressive and Antimicrobial Peptide, DRS-DA2, Isolated from the Mexican Frog, Pachymedusa dacnicolor

Inflammatory and antimicrobial diseases constitute a major burden for society, and fighting them is a WHO strategic priority. Most of the treatments available to fight inflammatory diseases are anti-inflammatory drugs, such as corticosteroids or immunomodulators that lack cellular specificity and lead to numerous side effects. In addition to suppressing undesired inflammation and reducing disease progression, these drugs lessen the immune system protective functions. Furthermore, treating infectious diseases is more and more challenging due to the rise of microbial resistance to antimicrobial drugs. Thus, controlling the inflammatory process locally without compromising the ability to combat infections is an essential feature in the treatment of inflammatory diseases. We isolated three forms (DRS-DA2N, DRS-DA2NE, and DRS-DA2NEQ) of the same peptide, DRS-DA2, which belongs to the dermaseptin family, from the Mexican tree frog Pachymedusa dacnicolor. Interestingly, DRS-DA2N and DRS-DA2NEQ exhibit a dual activity by inducing the death of leukocytes as well as that of Gram-negative and Gram-positive bacteria, including multiresistant strains, without affecting other cells such as epithelial cells or erythrocytes. We showed that the death of both immune cells and bacteria is induced rapidly by DRS-DA2 and that the membrane is permeabilized, leading to the loss of membrane integrity. We also validated the capacity of DRS-DA2 to regulate the pool of inflammatory cells in vivo in a mouse model of noninfectious peritonitis. After the induction of peritonitis, a local injection of DRS-DA2N could decrease the number of inflammatory cells locally in the peritoneal cavity without inducing a systemic effect, as no changes in the number of inflammatory cells could be detected in blood or in the bone marrow. Collectively, these data suggest that this peptide could be a promising tool in the treatment of inflammatory diseases, such as inflammatory skin diseases, as it could reduce the number of inflammatory cells locally without suppressing the ability to combat infections.

Development of New Leishmanicidal Compounds via Bioconjugation of Antimicrobial Peptides and Antileishmanial Guanidines

Leishmaniasis refers to a collection of diseases caused by protozoa from the Leishmania genus. These diseases, along with other parasitic afflictions, pose a significant public health issue, particularly given the escalating number of at-risk patients. This group includes immunocompromised individuals and those residing in impoverished conditions. The treatment of leishmaniasis is crucial, particularly in light of the mortality rate associated with nontreatment, which stands at 20-30,000 deaths per year globally. However, the therapeutic options currently available are limited, often ineffective, and potentially toxic. Consequently, the pursuit of new therapeutic alternatives is warranted. This study aims to design, synthesize, and evaluate the leishmanicidal activity of antimicrobial peptides functionalized with guanidine compounds and identify those with enhanced potency and selectivity against the parasite. Accordingly, three bioconjugates were obtained by using the solid-phase peptide synthesis protocol. Each proved to be more potent against intracellular amastigotes than their respective peptide or guanidine compounds alone and demonstrated higher selectivity to the parasites than to the host cells. Thus, the conjugation strategy employed with these compounds effectively contributes to the development of new molecules with leishmanicidal activity.

Antimicrobial Peptides: An Update on Classifications and Databases

Antimicrobial peptides (AMPs) are distributed across all kingdoms of life and are an indispensable component of host defenses. They consist of predominantly short cationic peptides with a wide variety of structures and targets. Given the ever-emerging resistance of various pathogens to existing antimicrobial therapies, AMPs have recently attracted extensive interest as potential therapeutic agents. As the discovery of new AMPs has increased, many databases specializing in AMPs have been developed to collect both fundamental and pharmacological information. In this review, we summarize the sources, structures, modes of action, and classifications of AMPs. Additionally, we examine current AMP databases, compare valuable computational tools used to predict antimicrobial activity and mechanisms of action, and highlight new machine learning approaches that can be employed to improve AMP activity to combat global antimicrobial resistance.

Phylloseptin-1 is Leishmanicidal for Amastigotes of Leishmania amazonensis Inside Infected Macrophages

Leishmania protozoans are the causal agents of neglected diseases that represent an important public health issue worldwide. The growing occurrence of drug-resistant strains of Leishmania and severe side effects of available treatments represent an important challenge for the leishmaniases treatment. We have previously reported the leishmanicidal activity of phylloseptin-1 (PSN-1), a peptide found in the skin secretion of Phyllomedusa azurea (=Pithecopus azureus), against Leishmania amazonensis promastigotes. However, its impact on the amastigote form of L. amazonensis and its impact on infected macrophages are unknown. In this work, we evaluated the effects of PSN-1 on amastigotes of L. amazonensis inside macrophages infected in vitro. We assessed the production of hydrogen peroxide and nitric oxide, as well as the levels of inflammatory and immunomodulatory markers (TGF-β, TNF-α and IL-12), in infected and non-infected macrophages treated with PSN-1. Treatment with PSN-1 decreased the number of infected cells and the number of ingested amastigotes per cell when compared with the untreated cells. At 32 µM (64 µg/mL), PSN-1 reduced hydrogen peroxide levels in both infected and uninfected macrophages, whereas it had little effect on NO production or TGF-β release. The effect of PSN-1 on IL-12 and TNF-α secretion depended on its concentration, but, in general, their levels tended to increase as PSN-1 concentration increased. Further in vitro and in vivo studies are needed to clarify the mechanisms of action of PSN-1 and its interaction with the immune system aiming to develop pharmacological applications.

Peptidoglycan potentiates the membrane disrupting effect of the carboxyamidated form of DMS-DA6, a Gram-positive selective antimicrobial peptide isolated from Pachymedusa dacnicolor skin

The occurrence of nosocomial infections has been on the rise for the past twenty years. Notably, infections caused by the Gram-positive bacteria Staphylococcus aureus represent a major clinical problem, as an increase in antibiotic multi-resistant strains has accompanied this rise. There is thus a crucial need to find and characterize new antibiotics against Gram-positive bacteria, and against antibiotic-resistant strains in general. We identified a new dermaseptin, DMS-DA6, produced by the skin of the Mexican frog Pachymedusa dacnicolor, with specific antibacterial activity against Gram-positive bacteria. This peptide is particularly effective against two multiple drug-resistant strains Enterococcus faecium BM4147 and Staphylococcus aureus DAR5829, and has no hemolytic activity. DMS-DA6 is naturally produced with the C-terminal carboxyl group in either the free or amide forms. By using Gram-positive model membranes and different experimental approaches, we showed that both forms of the peptide adopt an α-helical fold and have the same ability to insert into, and to disorganize a membrane composed of anionic lipids. However, the bactericidal capacity of DMS-DA6-NH2 was consistently more potent than that of DMS-DA6-OH. Remarkably, rather than resulting from the interaction with the negatively charged lipids of the membrane, or from a more stable conformation towards proteolysis, the increased capacity to permeabilize the membrane of Gram-positive bacteria of the carboxyamidated form of DMS-DA6 was found to result from its enhanced ability to interact with peptidoglycan.

Effect of Different Broad Waveband Lights on Membrane Lipids of a Cyanobacterium, Synechococcus sp., as Determined by UPLC-QToF-MS and Vibrational Spectroscopy

Differential profile of membrane lipids and pigments of a Synechococcus sp. cyanobacterial strain cells exposed to blue, green, red and white light are determined by means of liquid chromatography and mass spectrometry or diode array detection. Raman and ATR-IR spectra of intact cells under the diverse light wavebands are also reported. Blue light cells exhibited an increased content of photosynthetic pigments as well as specific species of membrane glycerolipids as compared to cells exposed to other wavebands. The A630/A680 ratio indicated an increased content of phycobilisomes (PBS) in the blue light-exposed cells. Some differences in the protein conformation between the four light waveband-exposed cells were deduced from the variable absorbance at specific wavenumbers in the FT-Raman and ATR-FTIR spectra, in particular bands assigned to amide I and amide II. Bands from 1180 to 950 cm(-1) in the ATR-FTIR spectrum suggest degraded outer membrane polysaccharide in the blue light-exposed cells.

The diversity and evolution of anuran skin peptides

Amphibians exhibit various, characteristic adaptations related to their "incomplete" shift from the aquatic to the terrestrial habitat. In particular, the integument was subject to a number of specialized modifications during the evolution of these animals. In this review, we place special emphasis on endogenous host-defence skin peptides from the cuteanous granular glands anuran amphibians (frogs and toads). The overview on the two broad groups of neuroactive and antimicrobial peptides (AMPs) goes beyond a simple itemization in that we provide a new perspective into the evolution and function of anuran AMPs. Briefly, these cationic, amphipathic and α-helical peptides are traditionally viewed as being part of the innate immune system, protecting the moist skin against invading microorganisms through their cytolytic action. However, the complete record of anuran species investigated to date suggests that AMPs are distributed sporadically (i.e., non-universally) across Anura. Together with the intriguing observation that virtually all anurans known to produce neuropeptides in their granular glands also co-secrete cytolytic peptides, we call the traditional role for AMPs as being purely antimicrobial into question and present an alternative scenario. We hypothesize AMPs to assist neuroactive peptides in their antipredator role through their cytolytic action increasing the delivery of the latter to the endocrine and nervous system of the predator. Thus, AMPs are more accurately viewed as cytolysins and their contribution to the immune system is better regarded as an accessory benefit.

Molecular understanding of a potential functional link between antimicrobial and amyloid peptides

Antimicrobial and amyloid peptides do not share common sequences, typical secondary structures, or normal biological activity but both the classes of peptides exhibit membrane-disruption ability to induce cell toxicity. Different membrane-disruption mechanisms have been proposed for antimicrobial and amyloid peptides, individually, some of which are not exclusive to either peptide type, implying that certain common principles may govern the folding and functions of different cytolytic peptides and associated membrane disruption mechanisms. Particularly, some antimicrobial and amyloid peptides have been identified to have dual complementary amyloid and antimicrobial properties, suggesting a potential functional link between amyloid and antimicrobial peptides. Given that some similar structural and membrane-disruption characteristics exist between the two classes of peptides, this review summarizes major findings, recent advances, and future challenges related to antimicrobial and amyloid peptides and strives to illustrate the similarities, differences, and relationships in the sequences, structures, and membrane interaction modes between amyloid and antimicrobial peptides, with a special focus on direct interactions of the peptides with the membranes. We hope that this review will stimulate further research at the interface of antimicrobial and amyloid peptides - which has been studied less intensively than either type of peptides - to decipher a possible link between both amyloid pathology and antimicrobial activity, which can guide drug design and peptide engineering to influence peptide-membrane interactions important in human health and diseases.

Influence of surface chemistry on particle internalization into giant unilamellar vesicles

Cellular uptake of materials plays a key role in their biomedical applications. In this work, based on the cell-mimic giant unilamellar vesicles (GUVs) and a novel type of microscale materials consisting of stimuli-responsive poly(N-isopropylacrylamide) microgel particles and the incorporated lipids, the influence of particle surface chemistry, including hydrophobic/hydrophilic property and lipid decorations, on the adsorption and consequent internalization of particles into GUVs was investigated. It is found that the decoration of particle surface with lipids facilitates the adsorption of particles on GUV membrane. After that, the hydrophobic property of particle surface further triggers the internalization of particles into GUVs. These results demonstrate the importance of surface properties of particles on their interactions with lipid membranes and are helpful to the understanding of cellular uptake mechanism.

A glycine zipper motif mediates the formation of toxic β-amyloid oligomers in vitro and in vivo

Background

The β-amyloid peptide (Aβ) contains a Gly-XXX-Gly-XXX-Gly motif in its C-terminal region that has been proposed to form a "glycine zipper" that drives the formation of toxic Aβ oligomers. We have tested this hypothesis by examining the toxicity of Aβ variants containing substitutions in this motif using a neuronal cell line, primary neurons, and a transgenic C. elegans model.

Results

We found that a Gly37Leu substitution dramatically reduced Aβ toxicity in all models tested, as measured by cell dysfunction, cell death, synaptic alteration, or tau phosphorylation. We also demonstrated in multiple models that Aβ Gly37Leu is actually anti-toxic, thereby supporting the hypothesis that interference with glycine zipper formation blocks assembly of toxic Aβ oligomers. To test this model rigorously, we engineered second site substitutions in Aβ predicted by the glycine zipper model to compensate for the Gly37Leu substitution and expressed these in C. elegans. We show that these second site substitutions restore in vivo Aβtoxicity, further supporting the glycine zipper model.

Conclusions

Our structure/function studies support the view that the glycine zipper motif present in the C-terminal portion of Aβ plays an important role in the formation of toxic Aβ oligomers. Compounds designed to interfere specifically with formation of the glycine zipper could have therapeutic potential.

Vibrational spectroscopy of biomembranes

Vibrational spectroscopy, commonly associated with IR absorption and Raman scattering, has provided a powerful approach for investigating interactions between biomolecules that make up cellular membranes. Because the IR and Raman signals arise from the intrinsic properties of these molecules, vibrational spectroscopy probes the delicate interactions that regulate biomembranes with minimal perturbation. Numerous innovative measurements, including nonlinear optical processes and confined bilayer assemblies, have provided new insights into membrane behavior. In this review, we highlight the use of vibrational spectroscopy to study lipid-lipid interactions. We also examine recent work in which vibrational measurements have been used to investigate the incorporation of peptides and proteins into lipid bilayers, and we discuss the interactions of small molecules and drugs with membrane structures. Emerging techniques and measurements on intact cellular membranes provide a prospective on the future of vibrational spectroscopic studies of biomembranes.

Ctenidins: antimicrobial glycine-rich peptides from the hemocytes of the spider Cupiennius salei

Three novel glycine-rich peptides, named ctenidin 1-3, with activity against the Gram-negative bacterium E. coli, were isolated and characterized from hemocytes of the spider Cupiennius salei. Ctenidins have a high glycine content (>70%), similarly to other glycine-rich peptides, the acanthoscurrins, from another spider, Acanthoscurria gomesiana. A combination of mass spectrometry, Edman degradation, and cDNA cloning revealed the presence of three isoforms of ctenidin, at least two of them originating from simple, intronless genes. The full-length sequences of the ctenidins consist of a 19 amino acid residues signal peptide followed by the mature peptides of 109, 119, or 120 amino acid residues. The mature peptides are post-translationally modified by the cleavage of one or two C-terminal cationic amino acid residue(s) and amidation of the newly created mature C-terminus. Tissue expression analysis revealed that ctenidins are constitutively expressed in hemocytes and to a small extent also in the subesophageal nerve mass.

Antimicrobial peptides from Phyllomedusa frogs: from biomolecular diversity to potential nanotechnologic medical applications

Screening for new bioactive peptides in South American anurans has been pioneered in frogs of the genus Phyllomedusa. All frogs of this genus have venomous skin secretions, i.e., a complex mixture of bioactive peptides against potential predators and pathogens that presumably evolved in a scenario of predator-prey interaction and defense against microbial invasion. For every new anuran species studied new peptides are found, with homologies to hormones, neurotransmitters, antimicrobials, and several other peptides with unknown biological activity. From Vittorio Erspamer findings, this genus has been reported as a "treasure store" of bioactive peptides, and several groups focus their research on these species. From 1966 to 2009, more than 200 peptide sequences from different Phyllomedusa species were deposited in UniProt and other databases. During the last decade, the emergence of high-throughput molecular technologies involving de novo peptide sequencing via tandem mass spectrometry, cDNA cloning, pharmacological screening, and surface plasmon resonance applied to peptide discovery, led to fast structural data acquisition and the generation of peptide molecular libraries. Research groups on bioactive peptides in Brazil using these new technologies, accounted for the exponential increase of new molecules described in the last decade, much higher than in any previous decades. Recently, these secretions were also reported as a rich source of multiple antimicrobial peptides effective against multidrug resistant strains of bacteria, fungi, protozoa, and virus, providing instructive lessons for the development of new and more efficient nanotechnological-based therapies for infectious diseases treatment. Therefore, novel drugs arising from the identification and analysis of bioactive peptides from South American anuran biodiversity have a promising future role on nanobiotechnology.

Comparative study of two plasticins: specificity, interfacial behavior, and bactericidal activity

A comparative study was designed to evaluate the staphylococcidal efficiency of two sequence-related plasticins from the dermaseptin superfamily we screened previously. Their bactericidal activities against Staphylococcus aureus as well as their chemotactic potential were investigated. The impact of the GraS/GraR two-component system involved in regulating resistance to cationic antimicrobial peptides (CAMPs) was evaluated. Membrane disturbing activity was quantified by membrane depolarization assays using the diS-C3 probe and by membrane integrity assays measuring beta-galactosidase activity with recombinant strain ST1065 reflecting compromised membranes and cytoplasmic leakage. Interactions of plasticins with membrane models composed of either zwitterionic lipids mimicking the S. aureus membrane of CAMP-resistant strains or anionic lipids mimicking the negative charge-depleted membrane of CAMP-sensitive strains were analyzed by jointed Brewster angle microscopy (BAM), polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), and differential scanning calorimetry (DSC) to yield detailed information about the macroscopic interfacial organization, in situ conformation, orientation of the peptides at the lipid-solvent interface, and lipid-phase disturbance. We clearly found evidence of distinct interfacial behaviors of plasticins we linked to the distribution of charges along the peptides and structural interconversion properties at the membrane interface. Our results also suggest that amidation might play a key role in GraS/GraR-mediated CAMP sensing at the bacterial surface.

A glycine-leucine-rich peptide structurally related to the plasticins from skin secretions of the frog Leptodactylus laticeps (Leptodactylidae)

A glycine-leucine-rich peptide was isolated from norepinephrine-stimulated skin secretions of the Sante Fe frog Leptodactylus laticeps (Leptodactylidae) whose primary structure (Gly-Leu-Val-Asn-Gly-Leu-Leu-Ser-Ser-Val-Leu-Gly-Gly-Gly-Gln-Gly-Gly-Gly-Gly-Leu-Leu-Gly-Gly-Ile-Leu) contains the (GXXXG)(3) motif found in the plasticins, previously identified only in phyllomedusid frogs (Hylidae). Circular dichroism studies showed that the secondary structure of the peptide, termed plasticin-L1, was markedly solvent-dependent displaying a random coil conformation in water, a beta-sheet structure in methanol, and an alpha-helical conformation in 50% trifluoroethanol-water. A synthetic replicate of the peptide did not inhibit the growth of Escherichia coli or Staphylococcus aureus or lyse human erythrocytes at concentrations up to 500 microM. At relatively high concentrations (>or=1 microM), the peptide produced a significant (P<0.05), although modest (139% of basal rate at 3 microM), increase in the rate of glucose-induced release of insulin from rat clonal BRIN-BD11 beta cells without increasing the rate of release of lactate dehydrogenase. A peptide, termed ocellatin-L2 was also identified in the skin secretion that was identical to the previously described ocellatin-L1 except for the substitution Asn(23)-->Asp. Ocellatin-L2 was devoid of antimicrobial and hemolytic activity but also showed significant activity in stimulating insulin release from BRIN-BD11 cells (181% of basal rate at 3 microM).

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 consistent nomenclature of antimicrobial peptides isolated from frogs of the subfamily Phyllomedusinae

A growing number of cationic antimicrobial peptides have been isolated from the skin of hylid frogs belonging to the Phyllomedusinae subfamily. The amino acid sequences of these peptides are currently located in several databases under identifiers with no consistent system of nomenclature to describe them. In order to provide a workable terminology for antimicrobial peptides from Phyllomedusid frogs, we have made a systematic effort to collect, analyze, and classify all the Phyllomedusid peptide sequences available in databases. We propose that frogs belonging to the Phyllomedusinae subfamily should be described by the species names set out in Amphibian Species of the World: http://research.amnh.org/herpetology/amphibia/index.php, American Museum of Natural History, New York, USA. Multiple alignments analysis of at least 80 antimicrobial peptides isolated from 12 Phyllomedusinae species were distributed in seven distinct peptide families including dermaseptin, phylloseptin, plasticin, dermatoxin, phylloxin, hyposin and orphan peptides, and will be considered as the name of the headgroup of each family. The parent peptide's name should be followed by the first upper letter of the species for orthologous peptides and publication date determines priority. For example, the abbreviation B for bicolor and H for hypochondrialis. When two species begin with the same letter, two letters in upper case should be used (the first letter followed by the second or the third letter and so on). For example, the abbreviation DI for distincta, DU for duellmani, VA for vaillanti and VN for vanzolinii. Paralogous peptides should bear letter(s) in upper case followed by numbers.

Intrinsic flexibility and structural adaptability of Plasticins membrane-damaging peptides as a strategy for functional versatility

The Plasticins are a family of antimicrobial, 23-29-residue Gly-Leu-rich ortholog peptides from the frog skin that have very similar amino acid sequences, hydrophobicities, and amphipathicities but differ markedly in their conformational plasticity and spectrum of activity. The intrinsic flexibility and structural malleability of Plasticins modulate their ability to bind to and disrupt the bilayer membranes of prokaryotic and eukaryotic cells, and/or to reach intracellular targets, therefore, triggering functional versatility. The discussion is opened herein on several examples of other membrane-active peptides, like viral fusion peptides, cell-penetrating peptides, that are able to display antimicrobial activity. Hence, Plasticins could be regarded as models of multipotent membrane-active peptides guided by structural plasticity.

Structural malleability of plasticins: Preorganized conformations in solution and relevance for antimicrobial activity

Plasticins (23 long-residue glycine-leucine-rich dermaseptin-related peptides produced by the skin of South American hylids) have very similar amino acid sequences, hydrophobicities, and amphipathicities, but differ in their membrane-damaging properties and structurations (i.e. destabilized helix states, beta-hairpin, beta-sheet, and disordered states) at anionic and zwitterionic membrane interfaces. Structural malleability of plasticins in aqueous solutions together with parameters that may govern their ability to fold within beta-hairpin like structures were analyzed through circular dichroism and FTIR spectroscopic studies completed by molecular dynamics simulations in polar mimetic media. The goal of this study was to probe to which extent pre-existent peptide conformations, i.e. intrinsic "conformational landscape", may be responsible for variability in bioactive conformation and antimicrobial/hemolytic mechanisms of action of these peptides in relation with their various membrane disturbing properties. All plasticins present a turn region that does not always result in folding into a beta-hairpin shaped conformation. Residue at position 8 plays a major role in initiating the folding, while position 12 is not critical. Conformational stability has no major impact on antimicrobial efficacy. However, preformed beta-hairpin in solution may act as a conformational lock that prevents switch to alpha-helical structure. This lock lowers the antimicrobial efficiency and explains subtle differences in potencies of the most active antimicrobial plasticins.