Tryptophan fluorescence study of the interaction of penetratin peptides with model membranes

Influence of Aza-Glycine Substitution on the Internalization of Penetratin

The cell-penetrating peptide (CPP) penetratin has gained much attention over many years due to its potential role as a transporter for a broad range of cargo into cells. The modification of penetratin has been extensively investigated too. Aza-peptides are peptide analogs in which one or more of the amino residues are replaced by a semicarbazide. This substitution results in conformational restrictions and modifications in hydrogen bonding properties, which affect the structure and may lead to enhanced activity and selectivity of the modified peptide. In this work, the Trp residues of penetratin were substituted by aza-glycine or glycine residues to examine the effect of these modifications on the cellular uptake and the internalization mechanism. The substitution of Trp48 or Trp48,56 dramatically reduced the internalization, showing the importance of Trp48 in cellular uptake. Interestingly, while aza-glycine in the position of Trp56 increased the cellular uptake, Gly reduced it. The two Trp-modified derivatives showed altered internalization pathways, too. Based on our knowledge, this is the first study about the effect of aza-amino acid substitution on the cell entry of CPPs. Our results suggest that aza-amino acid insertion is a useful modification to change the internalization of a CPP.

Introduction of constrained Trp analogs in RW9 modulates structure and partition in membrane models

Over the past decades, many cell-penetrating peptides (CPP) have been studied for their capacity to cross cellular membranes, mostly in order to improve cellular uptake of therapeutic agents. Even though hydrophobic and anionic CPPs have been described, many of them are polycationic, due to the presence of several arginine (Arg) residues. Noteworthy, however, the presence of aromatic amino acids such as tryptophan (Trp) within CPPs seems to play an important role to reach high membranotropic activity. RW9 (RRWWRRWRR) is a designed CPP derived from the polyarginine R9 presenting both features. In general, when interacting with membranes, CPPs adopt an optimal conformation for membrane interactions - an amphipathic helical secondary structure in the case of RW9. Herein, we assumed that the incorporation of a locally constrained amino acid in the peptide sequence could improve the membranotropic activity of RW9, by facilitating its structuration upon contact with a membrane, while leaving a certain plasticity. Therefore, two cyclized Trp derivatives (Tcc and Aia) were synthesized to be incorporated in RW9 as surrogates of Trp residues. Thus, a series of peptides containing these building blocks has been synthesized by varying the type, position, and number of modifications. The membranotropic activity of the RW9 analogs was studied by spectrofluorescence titration of the peptides in presence of liposomes (DMPG), allowing to calculate partition coefficients (Kp). Our results indicate that the partitioning of the modified peptides depends on the type, the number and the position of the modification, with the best sequence being [Aia4]RW9. Interestingly, both NMR analysis and molecular dynamic (MD) simulations indicate that this analog presents an extended conformation similar to the native RW9, but with a much-reduced structural flexibility. Finally, cell internalization properties were also confirmed by confocal microscopy.

Label-free measurement of antimicrobial peptide interactions with lipid vesicles and nanodiscs using microscale thermophoresis

One strategy to combat antimicrobial resistance is the discovery of new classes of antibiotics. Most antibiotics will at some point interact with the bacterial membrane to either interfere with its integrity or to cross it. Reliable and efficient tools for determining the dissociation constant for membrane binding (KD) and the partitioning coefficient between the aqueous- and membrane phases (KP) are therefore important tools for discovering and optimizing antimicrobial hits. Here we demonstrate that microscale thermophoresis (MST) can be used for label-free measurement of KD by utilising the intrinsic fluorescence of tryptophan and thereby removing the need for chromophore labelling. As proof of principle, we have used the method to measure the binding of a set of small cyclic AMPs to large unilamellar vesicles (LUVs) and two types of lipid nanodiscs assembled by styrene maleic acid (SMA) and quaternary ammonium SMA (SMA-QA). The measured KD values correlate well with the corresponding measurements using surface plasmon resonance (SPR), also broadly reflecting the tested AMPs' minimal inhibition concentration (MIC) towards S. aureus and E. coli. We conclude that MST is a promising method for fast and cost-efficient detection of peptide-lipid interactions or mapping of sample conditions in preparation for more advanced studies that rely on expensive sample preparation, labelling and/or instrument time.

The impact of N-glycosylation on the properties of the antimicrobial peptide LL-III

The misuse of antibiotics has led to the emergence of drug-resistant pathogens. Antimicrobial peptides (AMPs) may represent valuable alternative to antibiotics; nevertheless, the easy degradation due to environmental stress and proteolytic enzyme action, limits their use. So far, different strategies have been developed to overcome this drawback. Among them, glycosylation of AMPs represents a promising approach. In this work, we synthesized and characterized the N-glycosilated form of the antimicrobial peptide LL-III (g-LL-III). The N-acetylglucosamine (NAG) was covalently linked to the Asn residue and the interaction of g-LL-III with bacterial model membranes, together with its resistance to proteases, were investigated. Glycosylation did not affect the peptide mechanism of action and its biological activity against both bacteria and eukaryotic cells. Interestingly, a higher resistance to the activity of proteolytic enzymes was achieved. The reported results pave the way for the successful application of AMPs in medicine and biotechnological fields.

Myxinidin-Derived Peptide against Biofilms Caused by Cystic Fibrosis Emerging Pathogens

Chronic lung infections in cystic fibrosis (CF) patients are triggered by multidrug-resistant bacteria such as Pseudomonas aeruginosa, Achromobacter xylosoxidans, and Stenotrophomonas maltophilia. The CF airways are considered ideal sites for the colonization and growth of bacteria and fungi that favor the formation of mixed biofilms that are difficult to treat. The inefficacy of traditional antibiotics reinforces the need to find novel molecules able to fight these chronic infections. Antimicrobial peptides (AMPs) represent a promising alternative for their antimicrobial, anti-inflammatory, and immunomodulatory activities. We developed a more serum-stable version of the peptide WMR (WMR-4) and investigated its ability to inhibit and eradicate C. albicans, S. maltophilia, and A. xylosoxidans biofilms in both in vitro and in vivo studies. Our results suggest that the peptide is able better to inhibit than to eradicate both mono and dual-species biofilms, which is further confirmed by the downregulation of some genes involved in biofilm formation or in quorum-sensing signaling. Biophysical data help to elucidate its mode of action, showing a strong interaction of WMR-4 with lipopolysaccharide (LPS) and its insertion in liposomes mimicking Gram-negative and Candida membranes. Our results support the promising therapeutic application of AMPs in the treatment of mono- and dual-species biofilms during chronic infections in CF patients.

Role of PI(4,5)P2 and Cholesterol in Unconventional Protein Secretion

Besides its protective role in the maintenance of cell homeostasis, the plasma membrane is the site of exchanges between the cell interior and the extracellular medium. To circumvent the hydrophobic barrier formed by the acyl chains of the lipid bilayer, protein channels and transporters are key players in the exchange of small hydrophilic compounds such as ions or nutrients, but they hardly account for the transport of larger biological molecules. Exchange of proteins usually relies on membrane-fusion events between vesicles and the plasma membrane. In recent years, several alternative unconventional protein secretion (UPS) pathways across the plasma membrane have been characterised for a specific set of secreted substrates, some of them excluding any membrane-fusion events (Dimou and Nickel, Curr Biol 28:R406-R410, 2018). One of thesbe pathways, referred as type I UPS, relies on the direct translocation of the protein across the plasma membrane and not surprisingly, lipids are essential players in this process. In this chapter, we discuss the roles of phosphatidylinositol(4,5)bisphosphate (PI(4,5)P2) and cholesterol in unconventional pathways involving Engrailed-2 homeoprotein and fibroblast growth factor 2.

Fusogenic peptide delivery of bioactive siRNAs targeting CSNK2A1 for treatment of ovarian cancer

Ovarian cancer has shown little improvement in survival among advanced-stage patients over the past decade. Current treatment strategies have been largely unsuccessful in treating advanced disease, with many patients experiencing systemic toxicity and drug-resistant metastatic cancer. This study evaluates novel fusogenic peptide carriers delivering short interfering RNA (siRNA) targeting casein kinase II, CSNK2A1, for reducing the aggressiveness of ovarian cancer. The peptides were designed to address two significant barriers to siRNA delivery: insufficient cellular uptake and endosomal entrapment. The three peptide variants developed, DIVA3, DIV3H, and DIV3W, were able to form monodisperse nanoparticle complexes with siRNA and protect siRNAs from serum and RNase degradation. Furthermore, DIV3W demonstrated optimal delivery of bioactive siRNAs into ovarian cancer cells with high cellular uptake efficiency and mediated up to 94% knockdown of CSNK2A1 mRNA compared with non-targeting siRNAs, resulting in decreased cell migration and recolonization in vitro. Intratumoral delivery of DIV3W-siCSNK2A1 complexes to subcutaneous ovarian tumors resulted in reduced CSNK2A1 mRNA and CK2α protein expression after 48 h and reduced tumor growth and migration in a 2-week multi-dosing regimen. These results demonstrate the potential of the DIV3W peptide to deliver bioactive siRNAs and confirms the role of CSNK2A1 in cell-cell communication and proliferation in ovarian cancer.

Anionic lipids induce a fold-unfold transition in the membrane-translocating Engrailed homeodomain

Homeoprotein transcription factors have the property of interacting with membranes through their DNA-binding homeodomain, which is involved in unconventional internalization and secretion. Both processes depend on membrane-translocating events but their detailed molecular mechanisms are still poorly understood. We have previously characterized the conformational properties of Engrailed 2 homeodomain (EnHD) in aqueous solution and in micelles as membrane-mimetic environments. In the present study, we used small isotropic lipid bicelles as a more relevant membrane-mimetic model to characterize the membrane-bound state of EnHD. We show that lipid bicelles, in contrast to micelles, adequately reproduce the requirement of anionic lipids in the membrane binding and conformational transition of EnHD. The fold-unfold transition of EnHD induced by anionic lipids was characterized by NMR using ¹H, ¹³C, ¹⁵N chemical shifts, nuclear Overhauser effects, residual dipolar couplings, intramolecular and intermolecular paramagnetic relaxation enhancements induced by site-directed spin-label or paramagnetic lipid probe, respectively. A global unpacking of EnHD helices is observed leading to a loss of the native fold. However, near-native propensities of EnHD backbone conformation are maintained in membrane environment, including not only the three helices but also the turn connecting helices H2 and H3. NMR and coarse-grained molecular dynamics simulations reveal that the EnHD adopts a shallow insertion in the membrane, with the three helices oriented parallel to the membrane. EnHD explores extended conformations and closed U-shaped conformations, which are stabilized by anionic lipid recruitment.

Designed Multifunctional Peptides for Intracellular Targets

Nature’s way for bioactive peptides is to provide them with several related functions and the ability to cooperate in performing their job. Natural cell-penetrating peptides (CPP), such as penetratins, inspired the design of multifunctional constructs with CPP ability. This review focuses on known and novel peptides that can easily reach intracellular targets with little or no toxicity to mammalian cells. All peptide candidates were evaluated and ranked according to the predictions of low toxicity to mammalian cells and broad-spectrum activity. The final set of the 20 best peptide candidates contains the peptides optimized for cell-penetrating, antimicrobial, anticancer, antiviral, antifungal, and anti-inflammatory activity. Their predicted features are intrinsic disorder and the ability to acquire an amphipathic structure upon contact with membranes or nucleic acids. In conclusion, the review argues for exploring wide-spectrum multifunctionality for novel nontoxic hybrids with cell-penetrating peptides.

Structural Bases for the Involvement of Phosphatidylinositol-4,5-bisphosphate in the Internalization of the Cell-Penetrating Peptide Penetratin

Cell-penetrating peptides cross cell membranes through various parallel internalization pathways. Herein, we analyze the role of the negatively charged lipid phosphatidylinositol-4,5-bisphosphate (PI(4,5)P₂) in the internalization of Penetratin. Contributions of both inner leaflet and outer leaflet pools of PI(4,5)P₂ were revealed by quantifying the internalization of Penetratin in cells treated with PI(4,5)P₂ binders. Studies on model systems showed that Penetratin has a strong affinity for PI(4,5)P₂ and interacts selectively with this lipid, even in the presence of other negatively charged lipids, as demonstrated by affinity photo-crosslinking experiments. Differential scanning calorimetry experiments showed that Penetratin induces lateral segregation in PI(4,5)P₂-containing liposomes, which was confirmed by coarse-grained molecular dynamics simulations. NMR experiments indicated that Penetratin adopts a stabilized helical conformation in the presence of PI(4,5)P₂-containing membranes, with an orientation parallel to the bilayer plane, which was also confirmed by all-atom simulations. NMR and photo-crosslinking experiments also suggest a rather shallow insertion of the peptide in the membrane. Put together, our findings suggest that PI(4,5)P₂ is a privileged interaction partner for Penetratin and that it plays an important role in Penetratin internalization.

Endocytosis Involved d-Oligopeptide of Tryptophan and Arginine Displays Ordered Nanostructures and Cancer Cell Stereoselective Toxicity by Autophagy

Owing to their self-aggregation propensity and selective interaction with the anionic membranes, the peptides rich in tryptophan (Trp) and arginine (Arg) are considered for the development of novel anticancer therapeutics. However, the structural insights from the perspective of backbone chirality and spatial orientation of side chains into the selective toxicity of peptides are limited. Here, we investigated the selectivity and cellular uptake of HHC36, a Trp/Arg-rich nonapeptide, and its d-enantiomer (^allDHHC36) and a retroinverso analogue in the lung A549 and breast MDA-MB-231 cancer cells. We realized that the d-peptides can specifically induce autophagy at nontoxic concentrations only in the A549 cells supported from the LC 3-II immunostaining expression in the vicinity of the nucleus and the ultrastructural analysis revealing the autophagosome formation. The autophagic flux was also remarkable in the cells exposed to d-peptides at a far lower concentration in synergism with doxorubicin (DOX). In marked contrast, nonselective cell death was observed only if a high amount of HHC36 was applied. HHC36 tended to irregular collagen-like fibrils relative to ^allDHHC36 that distinctly formed higher-order coiled nanostructures. Interestingly, the short d-peptide fragments were generated in a harsh oxidative condition. Compared with the direct membrane transduction of HHC36, the entry of d-peptides into the lung cancer cells was controlled by endocytosis through the contribution of heparan sulfate proteoglycans (HSPGs) and cholesterol (CHO). However, both l- and d-peptides feasibly crossed the membrane and localized inside the S-phase-arrested cell nucleus. This suggested the likelihood of peptide intercalation with DNA that might differently appear in selective and/or nonselective deaths. These results unraveled the d-handedness-selective toxicity of a self-assembling Trp/Arg-rich sequence that is dependent on the cell type from the aspects of the density of anionic charges and CHO in the outer leaflet of the plasma membrane, as well as the intracellular redox imbalance that may drive the formation of toxic peptide nanostructure fragments.

Evaluating the interaction of human serum albumin (HSA) and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) liposomes in different aqueous environments using anisotropy resolved multi-dimensional emission spectroscopy (ARMES)

Studying the interaction between plasma proteins and liposomes is critical, particularly for their use as drug delivery systems. Here, the efficacy of anisotropy resolved multidimensional emission spectroscopy (ARMES) for investigating the interaction of human serum albumin (HSA) with liposomes was explored and compared to conventional spectroscopic techniques. Dynamic Light Scattering (DLS) and absorbance spectroscopy (with Multivariate Curve Resolution (MCR) modeling) indicated that the highest degree of liposome rupturing, and aggregation occurred in water, with less in ammonium bicarbonate buffer (ABC) and phosphate buffered saline (PBS). Fluorescence emission spectra of HSA-liposome mixtures revealed significant hypsochromic shifts for water and ABC, but much less in PBS, where the data suggests a non-penetrating protein layer was formed. Average fluorescence lifetimes decreased upon liposome interaction in water (6.2→5.2 ns) and ABC buffer (6.3→5.6 ns) but increased slightly for PBS (5.6→5.8 ns). ARMES using polarized Total Synchronous Fluorescence Scan measurements with parallel factor (PARAFAC) analysis resolved intrinsic HSA fluorescence into two components for interactions in water and ABC buffer, but only one component for PBS. These components, in water and ABC buffer, corresponded to two different HSA populations, one blue-shifted and penetrating the liposomes (λ_ex/em = ~ 280/320 nm) and a second, similar to free HSA in solution (λ_ex/em = ~ 282/356 nm). PARAFAC scores for water and ABC buffer suggested that a large proportion of HSA interacted in an end on configuration. ARMES provides a new way for investigating protein-liposome interactions that exploits the full intrinsic emission space of the protein and thus avoids the use of extrinsic labels. The use of multivariate data analysis provided a comprehensive and structured framework to extract a variety of useful information (resolving different fluorescent species, quantifying their signal contribution, and extracting light scatter signals) all of which can be used to discriminate between interaction mechanisms.

Biocompatibility and Comfort during Extended Wear of Mel4 Peptide-Coated Antimicrobial Contact Lenses

(1) Purpose: This study aimed to investigate the effects of Mel4 antimicrobial contact lenses (MACL) on the ocular surface and comfort during extended wear. (2) Methods: A prospective, randomised, double-masked, contralateral clinical trial was conducted with 176 subjects to evaluate the biocompatibility of contralateral wear of MACL. The wearing modality was 14-day extended lens wear for three months. The participants were assessed at lens dispensing, after one night, two weeks, one month and three months of extended wear and one month after study completion. (3) Results: There were no significant differences (p > 0.05) in ocular redness or palpebral roughness between Mel4 and control eyes at any of the study visits. There was no significant difference (p > 0.05) in corneal staining between Mel4 and control eyes. There were no significant differences in front surface wettability or deposits or back surface debris (p > 0.05). No statistically significant differences (p > 0.05) were found in comfort, dryness, CLDEQ-8 scores lens or edge awareness. There was no evidence for delayed reactions on the ocular surface after cessation of lens wear. (4) Conclusion: The novel MACLs showed similar comfort to control lenses and were biocompatible during extended wear. Thus, these lenses were compatible with the ocular surface.

Genetic, cellular, and structural characterization of the membrane potential-dependent cell-penetrating peptide translocation pore

Cell-penetrating peptides (CPPs) allow intracellular delivery of bioactive cargo molecules. The mechanisms allowing CPPs to enter cells are ill-defined. Using a CRISPR/Cas9-based screening, we discovered that KCNQ5, KCNN4, and KCNK5 potassium channels positively modulate cationic CPP direct translocation into cells by decreasing the transmembrane potential (V_m). These findings provide the first unbiased genetic validation of the role of V_m in CPP translocation in cells. In silico modeling and live cell experiments indicate that CPPs, by bringing positive charges on the outer surface of the plasma membrane, decrease the V_m to very low values (–150 mV or less), a situation we have coined megapolarization that then triggers formation of water pores used by CPPs to enter cells. Megapolarization lowers the free energy barrier associated with CPP membrane translocation. Using dyes of varying dimensions in CPP co-entry experiments, the diameter of the water pores in living cells was estimated to be 2 (–5) nm, in accordance with the structural characteristics of the pores predicted by in silico modeling. Pharmacological manipulation to lower transmembrane potential boosted CPP cellular internalization in zebrafish and mouse models. Besides identifying the first proteins that regulate CPP translocation, this work characterized key mechanistic steps used by CPPs to cross cellular membranes. This opens the ground for strategies aimed at improving the ability of cells to capture CPP-linked cargos in vitro and in vivo.

Before a drug can have its desired effect, it must reach its target tissue or organ, and enter its cells. This is not easy because cells are surrounded by the plasma membrane, a fat-based barrier that separates the cell from its external environment. The plasma membrane contains proteins that act as channels, shuttling specific molecules in and out of the cell, and it also holds charge, with its inside surface being more negatively charged than its outside surface.

Cell-penetrating peptides are short sequences of amino acids (the building blocks that form proteins) that carry positive charges. These positive charges allow them to cross the membrane easily, but it is not well understood how.

To find out how cell-penetrating peptides cross the membrane, Trofimenko et al. attached them to dyes of different sizes. This revealed that the cell-penetrating peptides enter the cell through temporary holes called water pores, which measure about two nanometres across. The water pores form when the membrane becomes ‘megapolarized’, this is, when the difference in charge between the inside and the outside of the membrane becomes greater than normal. This can happen when the negative charge on the inside surface or the positive charge on the outer surface of the membrane increase. Megapolarization depends on potassium channels, which transport positive potassium ions outside the cell, making the outside of the membrane positive. When cell-penetrating peptides arrive at the outer surface of the cell near potassium channels, they make it even more positive. This increases the charge difference between the inside and the outside of the cell, allowing water pores to form. Once the peptides pass through the pores, the charge difference between the inside and the outside of the cell membrane dissipates, and the pores collapse.

Drug developers are experimenting with attaching cell-penetrating peptides to drugs to help them get inside their target cells. Currently there are several experimental medications of this kind in clinical trials. Understanding how these peptides gain entry, and what size of molecule they could carry with them, provides solid ground for further drug development.

Role of Chiral Configuration in the Photoinduced Interaction of D- and L-Tryptophan with Optical Isomers of Ketoprofen in Linked Systems

The study of the L- and D-amino acid properties in proteins and peptides has attracted considerable attention in recent years, as the replacement of even one L-amino acid by its D-analogue due to aging of the body is resulted in a number of pathological conditions, including Alzheimer’s and Parkinson’s diseases. A recent trend is using short model systems to study the peculiarities of proteins with D-amino acids. In this report, the comparison of the excited states quenching of L- and D-tryptophan (Trp) in a model donor–acceptor dyad with (R)- and (S)-ketoprofen (KP-Trp) was carried out by photochemically induced dynamic nuclear polarization (CIDNP) and fluorescence spectroscopy. Quenching of the Trp excited states, which occurs via two mechanisms: prevailing resonance energy transfer (RET) and electron transfer (ET), indeed demonstrates some peculiarities for all three studied configurations of the dyad: (R,S)-, (S,R)-, and (S,S)-. Thus, the ET efficiency is identical for (S,R)- and (R,S)-enantiomers, while RET differs by 1.6 times. For (S,S)-, the CIDNP coefficient is almost an order of magnitude greater than for (R,S)- and (S,R)-. To understand the source of this difference, hyperpolarization of (S,S)-and (R,S)- has been calculated using theory involving the electron dipole–dipole interaction in the secular equation.

SARS-CoV-2 NSP1 C-terminal (residues 131-180) is an intrinsically disordered region in isolation

The NSP1- C terminal structure in complex with ribosome using cryo-EM is available now, and the N-terminal region structure in isolation is also deciphered in literature. However, as a reductionist approach, the conformation of NSP1- C terminal region (NSP1-CTR; amino acids 131-180) has not been studied in isolation. We found that NSP1-CTR conformation is disordered in an aqueous solution. Further, we examined the conformational propensity towards alpha-helical structure using trifluoroethanol, we observed induction of helical structure conformation using CD spectroscopy. Additionally, in SDS, NSP1-CTR shows a conformational change from disordered to ordered, possibly gaining alpha-helix in part. But in the presence of neutral lipid DOPC, a slight change in conformation is observed, which implies the possible role of hydrophobic interaction and electrostatic interaction on the conformational changes of NSP1. Fluorescence-based studies have shown a blue shift and fluorescence quenching in the presence of SDS, TFE, and lipid vesicles. In agreement with these results, fluorescence lifetime and fluorescence anisotropy decay suggest a change in conformational dynamics. The zeta potential studies further validated that the conformational dynamics are primarily because of hydrophobic interaction. These experimental studies were complemented through Molecular Dynamics (MD) simulations, which have shown a good correlation and testifies our experiments. We believe that the intrinsically disordered nature of the NSP1-CTR will have implications for enhanced molecular recognition feature properties of this IDR, which may add disorder to order transition and disorder-based binding promiscuity with its interacting proteins.

A Global Review on Short Peptides: Frontiers and Perspectives

Peptides are fragments of proteins that carry out biological functions. They act as signaling entities via all domains of life and interfere with protein-protein interactions, which are indispensable in bio-processes. Short peptides include fundamental molecular information for a prelude to the symphony of life. They have aroused considerable interest due to their unique features and great promise in innovative bio-therapies. This work focusing on the current state-of-the-art short peptide-based therapeutical developments is the first global review written by researchers from all continents, as a celebration of 100 years of peptide therapeutics since the commencement of insulin therapy in the 1920s. Peptide "drugs" initially played only the role of hormone analogs to balance disorders. Nowadays, they achieve numerous biomedical tasks, can cross membranes, or reach intracellular targets. The role of peptides in bio-processes can hardly be mimicked by other chemical substances. The article is divided into independent sections, which are related to either the progress in short peptide-based theranostics or the problems posing challenge to bio-medicine. In particular, the SWOT analysis of short peptides, their relevance in therapies of diverse diseases, improvements in (bio)synthesis platforms, advanced nano-supramolecular technologies, aptamers, altered peptide ligands and in silico methodologies to overcome peptide limitations, modern smart bio-functional materials, vaccines, and drug/gene-targeted delivery systems are discussed.

A review on recent advances in amino acid and peptide-based fluorescence and its potential applications

Fluorescence is widely used to detect functional groups and ions, and peptides are used in various fields due to their excellent biological activity.

Spectroscopic, zeta potential and molecular dynamics studies of the interaction of antimicrobial peptides with model bacterial membrane

Peptide-membrane interactions play a key role in the mechanisms of activity of antimicrobial peptides. Here, methods of fluorescence spectroscopy, zeta potential, and molecular dynamics modeling were used to study the interaction of new antimicrobial peptide megin with model bacterial membrane. The Gibbs free energy of -6 kcal/mol characterizes the interaction of the peptides with liposomes containing DOPE and POPG lipids. Fluorescence data, acrylamide quenching, and MD simulations show that megin peptides are mainly located at the lipid/water interface and are aligned parallel to the bilayer surface in a carpet like manner. Measurements of zeta potential demonstrate the decrease of the negative potential of liposomes in the presence of peptides. The influence of megin on the membrane properties is also confirmed by molecular dynamics simulations. Insertion of peptides into the membrane disturbs lipid ordering, decreases the order parameters of lipids, and facilitates penetration of water molecules through the membrane. According to our results, we proposed that the megin antimicrobial activity can be explained by the carpet model of peptide activity.

Lipidated Short Analogue of α-Melanocyte Stimulating Hormone Exerts Bactericidal Activity against the Stationary Phase of Methicillin-Resistant Staphylococcus aureus and Inhibits Biofilm Formation

Stationary phase Staphylococcus aureus, especially methicillin-resistant S. aureus (MRSA), has been widely associated with many persistent infections as well as biofilm-associated infections, which are challenging due to their increasing antibiotic resistance. α-Melanocyte stimulating hormone (α-MSH) is an antimicrobial peptide (AMP) with well-established potent activity against S. aureus , but little is known about its antimicrobial efficacy against the stationary phase of the bacteria. We investigated the in vitro activities of two palmitoylated analogues, Pal-α-MSH(6-13) and Pal-α-MSH(11-13), of the C-terminal fragments of α-MSH against biofilm-producing strains of methicillin-sensitive S. aureus (MSSA) and MRSA. While both the peptides demonstrated anti-staphylococcal efficacy, Pal-α-MSH(11-13) emerged as the most effective AMP as palmitoylation led to a remarkable enhancement in its activity against stationary phase bacteria. Similar to α-MSH, both the designed analogues were membrane-active and exhibited improved bacterial membrane depolarization and permeabilization, as further confirmed via electron microscopy studies. Of the two peptides, Pal-α-MSH(11-13) was able to retain its activity in the presence of standard microbiological media, which otherwise is a major limiting factor toward the therapeutic use of α-MSH-based peptides. More importantly, Pal-α-MSH(11-13) was also highly effective in inhibiting the formation of biofilms. Furthermore, it did not lead to resistance development in MRSA cells even upon 18 serial passages at sub-MIC concentrations. These observations support the potential use of Pal-α-MSH(11-13) in the treatment of planktonic as well as sessile S. aureus infections.

Stereoselectivity of Electron and Energy Transfer in the Quenching of (S/R)-Ketoprofen-(S)-Tryptophan Dyad Excited State

Photoinduced elementary processes in chiral linked systems, consisting of drugs and tryptophan (Trp) residues, attract considerable attention due to several aspects. First of all, these are models that allow one to trace the full and partial charge transfer underlying the binding of drugs to enzymes and receptors. On the other hand, Trp fluorescence is widely used to establish the structure and conformational mobility of proteins due to its high sensitivity to the microenvironment. Therefore, the study of mechanisms of Trp fluorescence quenching in various systems has both fundamental and practical interest. An analysis of the photo-chemically induced dynamic nuclear polarization (CIDNP) and Trp fluorescence quenching in (R/S)-ketoprofen-(S)-tryptophan ((S/R)-KP-(S)-Trp) dyad carried out in this work allowed us to trace the intramolecular reversible electron transfer (ET) and obtain evidence in favor of the resonance energy transfer (RET). The fraction of dyad’s singlet excited state, quenched via ET, was shown to be 7.5 times greater for the (S,S)-diastereomer than for the (R,S) analog. At the same time, the ratio of the fluorescence quantum yields shows that quenching effectiveness of (S,S)-diastereomer to be 5.4 times lower than for the (R,S) analog. It means that the main mechanism of Trp fluorescence quenching in (S/R)-KP-(S)-Trp dyad is RET.

Biophysical Insight on the Membrane Insertion of an Arginine-Rich Cell-Penetrating Peptide

Cell-penetrating peptides (CPPs) are short peptides that can translocate and transport cargoes into the intracellular milieu by crossing biological membranes. The mode of interaction and internalization of cell-penetrating peptides has long been controversial. While their interaction with anionic membranes is quite well understood, the insertion and behavior of CPPs in zwitterionic membranes, a major lipid component of eukaryotic cell membranes, is poorly studied. Herein, we investigated the membrane insertion of RW16 into zwitterionic membranes, a versatile CPP that also presents antibacterial and antitumor activities. Using complementary approaches, including NMR spectroscopy, fluorescence spectroscopy, circular dichroism, and molecular dynamic simulations, we determined the high-resolution structure of RW16 and measured its membrane insertion and orientation properties into zwitterionic membranes. Altogether, these results contribute to explaining the versatile properties of this peptide toward zwitterionic lipids.

The Development of an Antimicrobial Contact Lens - From the Laboratory to the Clinic

Contact lens wear is generally safe and provides excellent vision. However, contact lens wear is often associated with the risk of developing ocular surface infection and inflammation, and in severe cases, the infection can result in loss of vision. Antimicrobial peptide-coated contact lenses have been made to help reduce the incidence of infection and inflammation. This paper reviews the research progress from conception, through the laboratory and preclinical tests to the latest information on clinical testing of an antimicrobial contact lens. We provide insights into the pathways followed and pitfalls that have been encountered. The journey has not always been linear or smooth, but has resulted in some of the first published clinical testing of antimicrobial peptide-coated contact lenses in humans. We hope this may help lead to the development and commercialisation of antimicrobial contact lenses in the future.

Global Analysis of Intercellular Homeodomain Protein Transfer

The homeodomain is found in hundreds of transcription factors that play roles in fate determination via cell-autonomous regulation of gene expression. However, some homeodomain-containing proteins (HPs) are thought to be secreted and penetrate neighboring cells to affect the recipient cell fate. To determine whether this is a general characteristic of HPs, we carried out a large-scale validation for intercellular transfer of HPs. Our screening reveals that intercellular transfer is a general feature of HPs, but it occurs in a cell-context-sensitive manner. We also found the secretion is not solely a function of the homeodomain, but it is supported by external motifs containing hydrophobic residues. Thus, mutations of hydrophobic residues of HPs abrogate secretion and consequently interfere with HP function in recipient cells. Collectively, our study proposes that HP transfer is an intercellular communication method that couples the functions of interacting cells.

Lee et al. evaluate capabilities of homeodomain proteins (HPs) for transfer between cells. They find that intercellular transfer is a general but cell-context-sensitive property of HP. Intercellular HP transfer can be an unconventional way for the cells to communicate with neighboring cells that associate structurally and functionally.

Structural and mechanistic insights into EchAMP: A antimicrobial protein from the Echidna milk

BACKGROUND

Antibiotic resistance is a problem that necessitates the identification of new antimicrobial molecules. Milk is known to have molecules with antimicrobial properties (AMPs). Echidna Antimicrobial Protein (EchAMP) is one such lactation specific AMP exclusively found in the milk of Echidna, an egg-laying mammal geographically restricted to Australia and New Guinea. Previous studies established that EchAMP exhibits substantial bacteriostatic activity against multiple bacterial genera. However, the subsequent structural and functional studies were hindered due to the unavailability of pure protein.

RESULTS

In this study, we expressed EchAMP protein using a heterologous expression system and successfully purified it to >95% homogeneity. The purified recombinant protein exhibits bacteriolytic activity against both Gram-positive and Gram-negative bacteria as confirmed by live-dead staining and scanning electron microscopy. Structurally, this AMP belongs to the family of intrinsically disordered proteins (IDPs) as deciphered by the circular-dichroism, tryptophan fluorescence, and NMR spectroscopy. Nonetheless, EchAMP has the propensity to acquire structure with amphipathic molecules, or membrane mimics like SDS, lipopolysaccharides, and liposomes as again observed through multiple spectroscopic techniques.

CONCLUSIONS

Recombinant EchAMP exhibits broad-spectrum bacteriolytic activity by compromising the bacterial cell membrane integrity. Hence, we propose that this intrinsically disordered antimicrobial protein interact with the bacterial cell membrane and undergoes conformational changes to form channels in the membrane resulting in cell lysis.

GENERAL SIGNIFICANCE

EchAMP, the evolutionarily conserved, lactation specific AMP from an oviparous mammal may find application as a broad-spectrum antimicrobial against pathogens that affect mammary gland or otherwise cause routine infections in humans and livestock.

A Comparative Study of the Antimicrobial and Structural Properties of Short Peptides and Lipopeptides Containing a Repetitive Motif KLFK

BACKGROUND

In the last years, Antimicrobial Peptides (AMPs) and lipopeptides have received attention as promising candidates to treat infections caused by resistant microorganisms.

OBJECTIVE

The main objective of this study was to investigate the effect of repetitive KLFK motifs and the attachment of aliphatic acids to the N-terminus of (KLFK)n peptides on therapeutic properties.

METHODS

Minimal inhibitory concentration against Gram (+) and (-) bacteria and yeast of synthetic compounds were determined by broth microtiter dilution method, and the toxicity was evaluated by hemolysis assay. Membrane-peptide interaction studies were performed with model phospholipid membranes mimicking those of bacterial and mammalian cells by Fluorescence Spectroscopy. The secondary structure in solution and membranes was determined by Circular Dichroism.

RESULTS

Our results showed that the resulting compounds have inhibitory activity against bacteria and fungi. The (KLFK)3 peptide showed the highest therapeutic index against bacterial and yeast strains, and the (KLFK)2 peptide conjugated with octanoic acid was the most active against yeasts. All the lipopeptides containing long-chain fatty acids (C14 or longer) were highly hemolytic at low concentrations. The antimicrobial activity of (KLFK)2 and (KLFK)3 lipopeptides was mainly associated with improved stability of the amphipathic secondary structure, which showed high contributions of α-helix in dipalmitoylphosphatidylglycerol (DPPG) vesicles.

CONCLUSION

The repetition of the KLFK sequence and the conjugation with lipid tails allowed obtained compounds with high antimicrobial activity and low toxicity, becoming good candidates for treating infectious diseases.

Cu-ATCUN Derivatives of Sub5 Exhibit Enhanced Antimicrobial Activity via Multiple Modes of Action

Antimicrobial peptides (AMPs) are short, amphipathic peptides that are typically cationic in sequence and display broad-spectrum activity against bacteria, fungi, and protists. Herein, we report the effect of appending the amino terminal copper and nickel binding motif (ATCUN) to Sub5. The Cu-ATCUN derivatives show a two- to three-fold increase in antimicrobial activity for a variety of microbes, relative to Sub5, with MICs as low as 0.3 ± 0.1 μM toward Enterococcus faecium. Sub5 and the ATCUN derivatives bind both plasmid DNA and 16s A-site rRNA with low micromolar affinity. Native Sub5 and the metallopeptide derivatives were shown to promote damage against DNA to similar extents in cellular studies against both Escherichia coli and Staphylococcus epidermidis, with an almost threefold higher activity against the latter organism. Liposome experiments show that the metallopeptides have a greater affinity for model membranes of E. coli and S. aureus relative to Sub5, which correlates with their enhanced antimicrobial activity. Sub5 and the metalloderivatives also display no cytotoxicity toward adult human dermal fibroblasts. Addition of the ATCUN motif conferred the ability to promote lipid oxidation toward E. coli and S. epidermidis and enhanced membrane permeability, as evidenced by the extent of ATP leaked from cellular membranes relative to Sub5 alone. These data suggest that Cu-ATCUN derivatives inhibit microbes through multiple modes of action, resulting in an enhancement in their overall potency.

Peptide-Based Nanoparticles to Rapidly and Efficiently "Wrap 'n Roll" siRNA into Cells

Delivery of small interfering RNA (siRNA) as a therapeutic tool is limited due to critical obstacles such as the cellular barrier, the negative charges of the siRNA molecule, and its instability in serum. Several siRNA delivery systems have been constructed using cell-penetrating peptides (CPPs) since the CPPs have shown a high potential for oligonucleotide delivery into the cells, especially by forming nanoparticles. In this study, we have developed a new family of short (15mer or 16mer) tryptophan-(W) and arginine-(R) rich Amphipathic Peptides (WRAP) able to form stable nanoparticles and to enroll siRNA molecules into cells. The lead peptides, WRAP1 and WRAP5, form defined nanoparticles smaller than 100 nm as characterized by biophysical methods. Furthermore, they have several benefits as oligonucleotide delivery tools such as the rapid encapsulation of the siRNA, the efficient siRNA delivery in several cell types, and the high gene silencing activity, even in the presence of serum. In conclusion, we have designed a new family of CPPs specifically dedicated for siRNA delivery through nanoparticle formation. Our results indicate that the WRAP family has significant potential for the safe, efficient, and rapid delivery of siRNA for diverse applications.

Lysine to arginine mutagenesis of chlorotoxin enhances its cellular uptake

Chlorotoxin (CTX), a disulfide-rich peptide from the scorpion Leiurus quinquestriatus, has several promising biopharmaceutical properties, including preferential affinity for certain cancer cells, high serum stability, and cell penetration. These properties underpin its potential for use as a drug design scaffold, especially for the treatment of cancer; indeed, several analogs of CTX have reached clinical trials. Here, we focus on its ability to internalize into cells-a trait associated with a privileged subclass of peptides called cell-penetrating peptides-and whether it can be improved through conservative substitutions. Mutants of CTX were made using solid-phase peptide synthesis and internalization into human cervical carcinoma (HeLa) cells was monitored by fluorescence and confocal microscopy. CTX_M1 (ie, [K15R/K23R]CTX) and CTX_M2 (ie, [K15R/K23R/Y29W]CTX) mutants showed at least a twofold improvement in uptake compared to CTX. We further showed that these mutants internalize into HeLa cells largely via an energy-dependent mechanism. Importantly, the mutants have high stability, remaining intact in serum for over 24 h; thus, retaining the characteristic stability of their parent peptide. Overall, we have shown that simple conservative substitutions can enhance the cellular uptake of CTX, suggesting that such type of mutations might be useful for improving uptake of other peptide toxins.

Membrane affinity and fluorescent labelling: comparative study of monolayer interaction, cellular uptake and cytotoxicity profile of carboxyfluorescein-conjugated cationic peptides

Fluorescent labelling is a common approach to reveal the molecular details of cellular uptake, internalisation, transport, distribution processes in biological systems. The conjugation with a fluorescent moiety might affect relevant physico-chemical and in vitro transport properties of the bioactive component. A representative set of seven cationic peptides-including cell-penetrating peptides as well as antimicrobial peptides and synthetic derivatives-was selected for our comparative study. Membrane affinity of the peptides and their 5(6)-carboxyfluorescein (Cf) derivatives was determined quantitatively and compared applying Langmuir monolayer of zwitterionic (DPPC) and negatively charged (DPPC + DPPG) lipids as cell membrane models. The interaction with neutral lipid layer is mainly governed by the overall hydrophobicity of the molecule which is remarkably increased by Cf-conjugation for the most hydrophobic Magainin, Melittin and Transportan. A significantly enhanced membrane affinity was detected in negatively charged lipid model monolayer for all of the peptides since the combination of electrostatic and hydrophobic interaction is active in that case. The Cf-conjugation improved the penetration ability of Penetratin and Dhvar4 suggesting that both the highly charged character (Z/n) and the increased hydrophobicity by Cf-conjugation present important contribution to membrane interaction. This effect might also responsible for the observed high in vitro internalisation rate of Penetratin and Dhvar4, while according to in vitro studies they did not cause damage of cell membrane. From the experiments with the given seven cationic peptides, it can be concluded that the Cf-conjugation alters the degree of membrane interaction of such peptides which are moderately hydrophobic and highly charged.

Synthetic molecular evolution of hybrid cell penetrating peptides

Peptides and analogs such as peptide nucleic acids (PNA) are promising tools and therapeutics, but the cell membrane remains a barrier to intracellular targets. Conjugation to classical cell penetrating peptides (CPPs) such as pTat_48–60 (tat) and pAntp_43–68 (penetratin) facilitates delivery; however, efficiencies are low. Lack of explicit design principles hinders rational improvement. Here, we use synthetic molecular evolution (SME) to identify gain-of-function CPPs with dramatically improved ability to deliver cargoes to cells at low concentration. A CPP library containing 8192 tat/penetratin hybrid peptides coupled to an 18-residue PNA is screened using the HeLa pTRE-LucIVS2 splice correction reporter system. The daughter CPPs identified are one to two orders of magnitude more efficient than the parent sequences at delivery of PNA, and also deliver a dye cargo and an anionic peptide cargo. The significant increase in performance following a single iteration of SME demonstrates the power of this approach to peptide sequence optimization.

Therapeutic peptide nucleic acids can be delivered into cells by conjugation to cell penetrating peptides (CPPs), but efficiency is usually low. Here the authors use synthetic molecular evolution and a luciferase-based library screen to generate new CPPs with improved efficiency and lower toxicity.

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.

Vesicles mimicking normal and cancer cell membranes exhibit differential responses to the cell-penetrating peptide Pep-1

The cell-penetrating peptide (CPP) Pep-1 presents a great potential in drug delivery due to its intrinsic property to cross plasma membrane. However, its mechanism of entry into the cell remains unresolved. In this study, we compare the selectivity of Pep-1 towards vesicles mimicking normal and cancer cell membranes. The interaction was performed in a wide range of peptide-to-lipid molar ratios using infrared (IR), fluorescence, scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) techniques. At low peptide concentration, fluorescence experiments show that lipid-phosphatidylserine (PS) seems to enable Pep-1 translocation into cancer cell membrane as evidenced by the blue shift of its maximal emission wavelength. DSC data show that Pep-1 induces segregation of lipids. At high peptide concentration, IR data indicate that the interaction of Pep-1 is relatively stronger with normal cell membrane than with cancer cell membrane through the phosphate groups, while the interaction is weaker with normal cell membrane than with cancer cell membrane through the carbonyl groups. TGA and DSC data reveal that vesicles of normal cell membrane are thermally more stable than vesicles of cancer cell membrane. This suggests that the additional lipid PS included in cancer cell membrane has a destabilizing effect on the membrane structure. SEM images reveal that Pep-1 form superstructures including spherical particles and fibrils in the presence of both model membranes. PS seems to enhance peptide transport across cellular membranes. The biophysical techniques in this study provide valuable insights into the properties of CPPs in drug delivery systems.

Indoloazepinone-Constrained Oligomers as Cell-Penetrating and Blood-Brain-Barrier-Permeating Compounds

Non-cationic and amphipathic indoloazepinone-constrained (Aia) oligomers have been synthesized as new vectors for intracellular delivery. The conformational preferences of the [l-Aia-Xxx]_n oligomers were investigated by circular dichroism (CD) and NMR spectroscopy. Whereas Boc-[l-Aia-Gly]_2,4 -OBn oligomers 12 and 13 and Boc-[l-Aia-β³ -h-l-Ala]_2,4 -OBn oligomers 16 and 17 were totally or partially disordered, Boc-[l-Aia-l-Ala]₂ -OBn (14) induced a typical turn stabilized by C₅ - and C₇ -membered H-bond pseudo-cycles and aromatic interactions. Boc-[l-Aia-l-Ala]₄ -OBn (15) exhibited a unique structure with remarkable T-shaped π-stacking interactions involving the indole rings of the four l-Aia residues forming a dense hydrophobic cluster. All of the proposed FITC-6-Ahx-[l-Aia-Xxx]₄ -NH₂ oligomers 19-23, with the exception of FITC-6-Ahx-[l-Aia-Gly]₄ -NH₂ (18), were internalized by MDA-MB-231 cells with higher efficiency than the positive references penetratin and Arg₈ . In parallel, the compounds of this series were successfully explored in an in vitro blood-brain barrier (BBB) permeation assay. Although no passive diffusion permeability was observed for any of the tested Ac-[l-Aia-Xxx]₄ -NH₂ oligomers in the PAMPA model, Ac-[l-Aia-l-Arg]₄ -NH₂ (26) showed significant permeation in the in vitro cell-based human model of the BBB, suggesting an active mechanism of cell penetration.

Binding, folding and insertion of a β-hairpin peptide at a lipid bilayer surface: Influence of electrostatics and lipid tail packing

Antimicrobial peptides (AMPs) act as host defenses against microbial pathogens. Here we investigate the interactions of SVS-1 (KVKVKVKV^dP^lPTKVKVKVK), an engineered AMP and anti-cancer β-hairpin peptide, with lipid bilayers using spectroscopic studies and atomistic molecular dynamics simulations. In agreement with literature reports, simulation and experiment show preferential binding of SVS-1 peptides to anionic over neutral bilayers. Fluorescence and circular dichroism studies of a Trp-substituted SVS-1 analogue indicate, however, that it will bind to a zwitterionic DPPC bilayer under high-curvature conditions and folds into a hairpin. In bilayers formed from a 1:1 mixture of DPPC and anionic DPPG lipids, curvature and lipid fluidity are also observed to promote deeper insertion of the fluorescent peptide. Simulations using the CHARMM C36m force field offer complementary insight into timescales and mechanisms of folding and insertion. SVS-1 simulated at an anionic mixed POPC/POPG bilayer folded into a hairpin over a microsecond, the final stage in folding coinciding with the establishment of contact between the peptide's valine sidechains and the lipid tails through a "flip and dip" mechanism. Partial, transient folding and superficial bilayer contact are seen in simulation of the peptide at a zwitterionic POPC bilayer. Only when external surface tension is applied does the peptide establish lasting contact with the POPC bilayer. Our findings reveal the influence of disruption to lipid headgroup packing (via curvature or surface tension) on the pathway of binding and insertion, highlighting the collaborative effort of electrostatic and hydrophobic interactions on interaction of SVS-1 with lipid bilayers.

Synthesis and mechanistic evaluation of novel N'-benzylidene-carbohydrazide-1H-pyrazolo[3,4-b]pyridine derivatives as non-anionic antiplatelet agents

Cardiovascular diseases (CVDs) account for over 17 million deaths globally each year, with atherosclerosis as the underlying cause of most CVDs. Herein we describe the synthesis and in vitro mechanistic evaluation of novel N'-benzylidene-carbohydrazide-1H-pyrazolo[3,4-b]pyridines (3-22) designed as non-anionic antiplatelet agents and presenting a 30-fold increase in potency compared to aspirin. The mechanism underlying their antiplatelet activity was elucidated by eliminating potential targets through a series of in vitro assays including light transmission aggregometry, clot retraction, and quantitative ELISA, further identifying the reduction in biosynthesis of thromboxane B2 as their main mechanism of action. The intrinsic fluorescence of the compounds permits their binding to platelet membranes to be readily monitored. In silico structure-activity relationship, molecular docking and dynamics studies support the biological profile of the series revealing the molecular basis of their activity and their potential as future molecular therapeutic agents.

Short peptides interfering with signaling pathways as new therapeutic tools for cancer treatment

Short peptides have many advantages, such as low molecular weight, selectivity for a specific target, organelles or cells with minimal toxicity. We describe properties of short peptides, which interfere with communication networks in tumor cells and within microenvironment of malignant gliomas, the most common brain tumors. We focus on ligand/receptor axes and intracellular signaling pathways critical for gliomagenesis that could be targeted with interfering peptides. We review structures and efficacy of organelle-specific and cell-penetrating peptides and describe diverse chemical modifications increasing proteolytic stability and protecting synthetic peptides against degradation. We report results of application of short peptides in glioma therapy clinical trials, their rises and falls. The most advanced examples of therapeutics such as short interfering peptides combined with cell-penetrating peptides that show good effectiveness in disease models are presented. It is foreseen that identification of peptides with better clinical properties may improve their success rates in clinical trials.

The chain order of binary unsaturated lipid bilayers modulated by aromatic-residue-containing peptides: an ATR-FTIR spectroscopy study

It highlights the importance of aromatic residues in influencing peptide binding to the membrane, demonstrates that the stability of the membranes depends on the lipid composition and the sequence, structural context, and orientation of the peptides.

Quantitative fluorescence spectroscopy and flow cytometry analyses of cell-penetrating peptides internalization pathways: optimization, pitfalls, comparison with mass spectrometry quantification

The mechanism of cell-penetrating peptides entry into cells is unclear, preventing the development of more efficient vectors for biotechnological or therapeutic purposes. Here, we developed a protocol relying on fluorometry to distinguish endocytosis from direct membrane translocation, using Penetratin, TAT and R9. The quantities of internalized CPPs measured by fluorometry in cell lysates converge with those obtained by our previously reported mass spectrometry quantification method. By contrast, flow cytometry quantification faces several limitations due to fluorescence quenching processes that depend on the cell line and occur at peptide/cell ratio >6.10⁸ for CF-Penetratin. The analysis of cellular internalization of a doubly labeled fluorescent and biotinylated Penetratin analogue by the two independent techniques, fluorometry and mass spectrometry, gave consistent results at the quantitative and qualitative levels. Both techniques revealed the use of two alternative translocation and endocytosis pathways, whose relative efficacy depends on cell-surface sugars and peptide concentration. We confirmed that Penetratin translocates at low concentration and uses endocytosis at high μM concentrations. We further demonstrate that the hydrophobic/hydrophilic nature of the N-terminal extremity impacts on the internalization efficiency of CPPs. We expect these results and the associated protocols to help unraveling the translocation pathway to the cytosol of cells.

Enhanced Cationic Charge is a Key Factor in Promoting Staphylocidal Activity of α-Melanocyte Stimulating Hormone via Selective Lipid Affinity

The steady rise in antimicrobial resistance poses a severe threat to global public health by hindering treatment of an escalating spectrum of infections. We have previously established the potent activity of α-MSH, a 13 residue antimicrobial peptide, against the opportunistic pathogen Staphylococcus aureus. Here, we sought to determine whether an increase in cationic charge in α-MSH could contribute towards improving its staphylocidal potential by increasing its interaction with anionic bacterial membranes. For this we designed novel α-MSH analogues by replacing polar uncharged residues with lysine and alanine. Similar to α-MSH, the designed peptides preserved turn/random coil conformation in artificial bacterial mimic 1,2-dimyristoyl-sn-glycero-3-phosphocholine:1,2-dimyristoyl-sn-glycero-3-phospho-rac-(1-glycerol) (7:3, w/w) vesicles and showed preferential insertion in the hydrophobic core of anionic membranes. Increased cationic charge resulted in considerable augmentation of antibacterial potency against MSSA and MRSA. With ~18-fold better binding than α-MSH to bacterial mimic vesicles, the most charged peptide KKK-MSH showed enhanced membrane permeabilization and depolarization activity against intact S. aureus. Scanning electron microscopy confirmed a membrane disruptive mode of action for KKK-MSH. Overall, increasing the cationic charge improved the staphylocidal activity of α-MSH without compromising its cell selectivity. The present study would help in designing more effective α-MSH-based peptides to combat clinically relevant staphylococcal infections.

A Pathway Toward Tumor Cell-Selective CPPs?

Despite the great potential of CPPs in therapeutics and diagnosis, their application still suffers from a non-negligible drawback: a complete lack of cell-type specificity. In the innumerous routes proposed for CPP cell entry there is common agreement that electrostatic interactions between cationic CPPs and anionic components in membranes, including lipids and glycosaminoglycans, play a crucial role. Tumor cells have been shown to overexpress certain glycosaminoglycans at the cell membrane surface and to possess a higher amount of anionic lipids in their outer leaflet when compared with healthy cells. Such molecules confer tumor cell membranes an enhanced anionic character, a property that could be exploited by CPPs to preferentially target these cells. Herein, these aspects are discussed in an attempt to confer CPPs certain selectivity toward cancer cells.

Classes of Cell-Penetrating Peptides

During the three decades of cell-penetrating peptides era the superfamily of CPPs has rapidly expanded, and the quest for new sequences continues. CPPs have been well recognized by scientific community and they have been used for transduction of a wide variety of molecules and particles into cultured cells and in vivo. In parallel with application of CPPs for delivering of active payloads, the mechanisms that such peptides take advantage of for gaining access to cells' insides have been in the focus of intense studies. Although the common denominator "cell penetration" unites all CPPs, the interaction partners on the cell surface, evoked cellular responses and even the uptake mechanisms might greatly vary between different peptide types. Here we present some possibilities for classification of CPPs based on their type of origin, physical-chemical properties, and the extent of modifications and design efforts. We also briefly analyze the internalization mechanisms with regard to their classification into groups based on physical-chemical characteristics.

Rational modification of a dendrimeric peptide with antimicrobial activity: consequences on membrane-binding and biological properties

Peptide-based antibiotics might help containing the rising tide of antimicrobial resistance. We developed SB056, a semi-synthetic peptide with a dimeric dendrimer scaffold, active against both Gram-negative and Gram-positive bacteria. Being the mechanism of SB056 attributed to disruption of bacterial membranes, we enhanced the amphiphilic profile of the original, empirically derived sequence [WKKIRVRLSA-NH2] by interchanging the first two residues [KWKIRVRLSA-NH2], and explored the effects of this modification on the interaction of peptide, both in linear and dimeric forms, with model membranes and on antimicrobial activity. Results obtained against Escherichia coli and Staphylococcus aureus planktonic strains, with or without salts at physiological concentrations, confirmed the added value of dendrimeric structure over the linear one, especially at physiological ionic strength, and the impact of the higher amphipathicity obtained through sequence modification on enhancing peptide performances. SB056 peptides also displayed intriguing antibiofilm properties. Staphylococcus epidermidis was the most susceptible strain in sessile form, notably to optimized linear analog lin-SB056-1 and the wild-type dendrimer den-SB056. Membrane affinity of all peptides increased with the percentage of negatively charged lipids and was less influenced by the presence of salt in the case of dendrimeric peptides. The analog lin-SB056-1 displayed the highest overall affinity, even for zwitterionic PC bilayers. Thus, in addition to electrostatics, distribution of charged/polar and hydrophobic residues along the sequence might have a significant role in driving peptide-lipid interaction. Supporting this view, dendrimeric analog den-SB056-1 retained greater membrane affinity in the presence of salt than den-SB056, despite the fact that they bear exactly the same net positive charge.

A Spectroscopic Study of the Aggregation State of the Human Antimicrobial Peptide LL-37 in Bacterial versus Host Cell Model Membranes

The LL-37 antimicrobial peptide is the only cathelicidin peptide found in humans that has antimicrobial and immunomodulatory properties. Because it exerts also chemotactic and angiogenetic activity, LL-37 is involved in promoting wound healing, reducing inflammation, and strengthening the host immune response. The key to the effectiveness of antimicrobial peptides (AMPs) lies in the different compositions of bacterial versus host cell membranes. In this context, antimicrobial peptide LL-37 and two variants were studied in the presence of model membranes with different lipid compositions and charges. The investigation was performed using an experimental strategy that combines the site-directed spin labeling-electron paramagnetic resonance technique with circular dichroism and fluorescence emission spectroscopies. LL-37 interacts with negatively charged membranes forming a stable aggregate, which can likely produce toroidal pores until the amount of bound peptide exceeds a critical concentration. At the same time, we have clearly detected an aggregate with a higher oligomeric degree for interaction of LL-37 with neutral membranes. These data confirm the absence of cell selectivity of the peptide and a more complex role in stimulating host cells.