The Efficacies of Cell-Penetrating Peptides in Accumulating in Large Unilamellar Vesicles Depend on their Ability To Form Inverted Micelles

Cell Membrane Perforation: Patterns, Mechanisms and Functions

Cell membrane is crucial for the cellular activities, and any disruption to it may affect the cells. It is demonstrated that cell membrane perforation is associated with some biological processes like programmed cell death (PCD) and infection of pathogens. Specific developments make it a promising technique to perforate the cell membrane controllably and precisely. The pores on the cell membrane provide direct pathways for the entry and exit of substances, and can also cause cell death, which means reasonable utilization of cell membrane perforation is able to assist intracellular delivery, eliminate diseased or cancerous cells, and bring about other benefits. This review classifies the patterns of cell membrane perforation based on the mechanisms into 1) physical patterns, 2) biological patterns, and 3) chemical patterns, introduces the characterization methods and then summarizes the functions according to the characteristics of reversible and irreversible pores, with the aim of providing a comprehensive summary of the knowledge related to cell membrane perforation and enlightening broad applications in biomedical science.

Leaflet-Dependent Effect of Anionic Lipids on Membrane Insertion by Cationic Cell-Penetrating Peptides

Cationic membrane-permeating peptides can cross membranes unassisted by transmembrane protein machinery, and there is consensus that anionic lipids facilitate this process. Although membranes are asymmetric in lipid composition, investigations of the impact of anionic lipids on peptide-membrane insertion in model vesicles primarily use symmetric anionic lipid distributions between bilayer leaflets. Here, we investigate the leaflet-specific influence of three anionic lipid headgroups [phosphatidic acid (PA), phosphatidylserine (PS), and phosphatidylglycerol (PG)] on insertion into model membranes by three cationic membrane-permeating peptides (NAF-144-67, R6W3, and WWWK). We report that outer leaflet anionic lipids enhanced peptide-membrane insertion for all peptides while inner leaflet anionic lipids did not have a significant effect except in the case of NAF-144-67 incubated with PA-containing vesicles. The insertion enhancement was headgroup-dependent for arginine-containing peptides but not WWWK. These results provide significant new insight into the potential role of membrane asymmetry in insertion of peptides into model membranes.

A Second Life for MAP, a Model Amphipathic Peptide

Cell-penetrating peptides (CPP) have been shown to be efficient in the transport of cargoes into the cells, namely siRNA and DNA, proteins and peptides, and in some cases, small therapeutics. These peptides have emerged as a solution to increase drug concentrations in different tissues and various cell types, therefore having a relevant therapeutic relevance which led to clinical trials. One of them, MAP, is a model amphipathic peptide with an α-helical conformation and both hydrophilic and hydrophobic residues in opposite sides of the helix. It is composed of a mixture of alanines, leucines, and lysines (KLALKLALKALKAALKLA). The CPP MAP has the ability to translocate oligonucleotides, peptides and small proteins. However, taking advantage of its unique properties, in recent years innovative concepts were developed, such as in silico studies of modelling with receptors, coupling and repurposing drugs in the central nervous system and oncology, or involving the construction of dual-drug delivery systems using nanoparticles. In addition to designs of MAP-linked vehicles and strategies to achieve highly effective yet less toxic chemotherapy, this review will be focused on unique molecular structure and how it determines its cellular activity, and also intends to address the most recent and frankly motivating issues for the future.

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.

Impact of Cyclization and Methylation on Peptide Penetration through Droplet Interface Bilayers

Cell-penetrating peptides enter cells via diverse mechanisms, such as endocytosis, active transport, or direct translocation. For the design of orally delivered cell-penetrating peptides, it is crucial to know the contribution of these different mechanisms. In particular, the ability of a peptide to translocate through a lipid bilayer remains a key parameter for the delivery of cargos. However, existing approaches used to assess translocation often provide discrepant results probably because they have different sensitivities to the distinct translocation mechanisms. Here, we focus on the passive permeation of a range of hydrophobic cyclic peptides inspired by somatostatin, a somatotropin release-inhibiting factor. Using droplet interface bilayers (DIB), we assess the passive membrane permeability of these peptides and study the impact of the peptide cyclization and backbone methylation on translocation rates. Cyclization systematically improved the permeability of the tested peptides while methylation did not. By studying the interaction of the peptides with the DIB interfaces, we found membrane insertion and peptide intrinsic diffusion to be two independent factors of permeability. Compared to the industrial gold standard Caco-2 and parallel artificial membrane permeability assay (PAMPA) models, DIBs provide intermediate membrane permeability values, closer to Caco-2. Even for conditions where Caco-2 and PAMPA are discrepant, the DIB approach also gives results closer to Caco-2. Thereupon, DIBs represent a robust alternative to the PAMPA approach for predicting the permeability of peptides, even if the latter present extremely small structural differences.

Genetic and Covalent Protein Modification Strategies to Facilitate Intracellular Delivery

Protein-based therapeutics represent a rapidly growing segment of approved disease treatments. Successful intracellular delivery of proteins is an important precondition for expanded in vivo and in vitro applications of protein therapeutics. Direct modification of proteins and peptides for improved cytosolic translocation are a promising method of increasing delivery efficiency and expanding the viability of intracellular protein therapeutics. In this Review, we present recent advances in both synthetic and genetic protein modifications for intracellular delivery. Active endocytosis-based and passive internalization pathways are discussed, followed by a review of modification methods for improved cytosolic delivery. After establishing how proteins can be modified, general strategies for facilitating intracellular delivery, such as chemical supercharging or inclusion of cell-penetrating motifs, are covered. We then outline protein modifications that promote endosomal escape. We finally examine the delivery of two potential classes of therapeutic proteins, antibodies and associated antibody fragments, and gene editing proteins, such as cas9.

Challenges and Methods for the Study of CPP Translocation Mechanisms

Fluorescence-based methods are widely used to detect crossing of peptides across model or biological membranes. For membrane-active peptides, i.e., peptides that have strong membrane tropism, fluorescence experiments must be accompanied by relevant controls, otherwise they can lead to inconsistent interpretation and underestimation of their limitations. Here we describe how to prepare samples to study fluorescent peptide crossing droplet interface bilayer (model membrane) or cell membrane (biological membrane) and the pitfalls that can affect observational qualitative and quantitative data.

A Single GUV Method for Revealing the Action of Cell-Penetrating Peptides in Biomembranes

The mechanism of entry of cell-penetrating peptides (CPPs) into the cytosol of various cells has been studied by examining the interaction of CPPs with lipid bilayers and their entry into lipid vesicle lumens using various methods. Here we describe a single giant unilamellar vesicle (GUV) method to study CPPs. In this new method, we use GUVs containing small GUVs in the mother GUV lumen or GUVs containing large unilamellar vesicles (LUVs) in the GUV lumen and investigate the interaction of fluorescent probe-labeled CPPs with single GUVs in real time using confocal laser scanning microscopy. This method can detect CPPs in the GUV lumen with high sensitivity, allowing immediate measurement of the time course of entry of CPPs into the vesicle lumen. This method allows simultaneous measurement of the entry of CPPs and of CPP-induced pore formation, allowing the relationship between the two events to be determined. One can also simultaneously measure the entry of CPPs and the CPP concentration in the GUV membrane. The rate of entry of CPPs into a single GUV lumen can be estimated by obtaining the fraction of GUVs into which CPPs entered before a specific time t without pore formation among all examined GUVs (i.e., the fraction of entry) and the lumen intensity due to LUVs with bound CPPs. This method is therefore useful for elucidating the mechanism of entry of CPPs into lipid vesicles.

Identification and Characteristics of Fusion Peptides Derived From Enveloped Viruses

Membrane fusion events allow enveloped viruses to enter and infect cells. The study of these processes has led to the identification of a number of proteins that mediate this process. These proteins are classified according to their structure, which vary according to the viral genealogy. To date, three classes of fusion proteins have been defined, but current evidence points to the existence of additional classes. Despite their structural differences, viral fusion processes follow a common mechanism through which they exert their actions. Additional studies of the viral fusion proteins have demonstrated the key role of specific proteinogenic subsequences within these proteins, termed fusion peptides. Such peptides are able to interact and insert into membranes for which they hold interest from a pharmacological or therapeutic viewpoint. Here, the different characteristics of fusion peptides derived from viral fusion proteins are described. These criteria are useful to identify new fusion peptides. Moreover, this review describes the requirements of synthetic fusion peptides derived from fusion proteins to induce fusion by themselves. Several sequences of the viral glycoproteins E1 and E2 of HCV were, for example, identified to be able to induce fusion, which are reviewed here.

Translocation of the nonlabeled antimicrobial peptide PGLa across lipid bilayers and its entry into vesicle lumens without pore formation

Fluorescent-probe-labeled peptides are used to study the interactions of peptides with cells and lipid vesicles but labeling peptides with fluorescent probes can significantly change these interactions. We recently developed a new method to detect the entry of nonlabeled peptides into the lumen of single giant unilamellar vesicles (GUVs). Here we applied this method to examine the interaction of the antimicrobial peptide PGLa with single GUVs to elucidate whether PGLa can enter the GUV lumen without pore formation. First, we examined the interaction of nonlabeled PGLa with single GUVs comprising dioleoylphosphatidylglycerol (DOPG) and dioleoylphosphatidylcholine (DOPC) (4/6) whose lumens contain the fluorescent probe AF647 and DOPG/DOPC (8/2)-large unilamellar vesicles encapsulating a high concentration of calcein. After a large lag period from starting the interaction with PGLa, the fluorescence intensity of the GUV lumen due to calcein (I_calcein) increased gradually without leakage of AF647, indicating that PGLa enters the GUV lumen without pore formation in the GUV membrane. The fraction of entry of PGLa increased with increasing PGLa concentration. Simultaneous measurement of the fractional area change of the GUV membrane (δ) and PGLa-induced increase in I_calcein showed that the entry of PGLa occurs only during the second increase in δ, indicating that PGLa enters the lumen during its translocation from the outer leaflet to the inner leaflet. The fraction of entry of PGLa without pore formation increased with increasing membrane tension. Based on these results, we discuss the elementary processes and the mechanism of the entry of PGLa into the GUV lumen.

Effect of membrane potential on entry of lactoferricin B-derived 6-residue antimicrobial peptide into single Escherichia coli cells and lipid vesicles

The antimicrobial peptide (AMP) derived from lactoferricin B, LfcinB (4-9) (RRWQWR), and lissamine rhodamine B red-labeled peptide (Rh-LfcinB (4-9)) exhibit strong antimicrobial activities, and they can enter Escherichia coli cells without damaging the cell membranes. Thus, these peptides are cell-penetrating peptide (CPP) -type AMPs. In this study, to elucidate the effect of the membrane potential (Δφ) on the action of the CPP-type AMP, Rh-LfcinB (4-9), we investigated the interactions of Rh-LfcinB (4-9) with single E. coli cells and spheroplasts containing calcein in the cytosol using confocal laser scanning microscopy. At low peptide concentrations, Rh-LfcinB (4-9) entered the cytosol of single E. coli cells and spheroplasts without damaging the cell membranes, and the H⁺-ionophore carbonyl cyanide m-chlorophenyl-hydrazone (CCCP) suppressed its entry. The studies using the time-kill method indicate that these low concentrations of peptide exhibit antimicrobial activity but CCCP inhibits this activity. Next, we investigated the effect of Δφ on the interaction of Rh-LfcinB (4-9) with single giant unilamellar vesicles (GUVs) comprising E. coli polar lipid extracts and containing a fluorescent probe, Alexa Fluor 647 hydrazide. At low concentrations (0.2-0.5 μM), Rh-LfcinB (4-9) showed significant entry to the single GUV lumen without pore formation in the presence of Δφ. The fraction of entry of peptide increased with increasing negative membrane potential, indicating that the rate of peptide entry into the GUV lumen increased with increasing negative membrane potential. These results indicate that Δφ enhances the entry of Rh-LfcinB (4-9) into single E. coli cells, spheroplasts, and GUVs and its antimicrobial activity.IMPORTANCE: Bacterial cells have a membrane potential (Δφ), but the effect of Δφ on action of cell-penetrating peptide-type antimicrobial peptides (AMPs) is not clear. Here, we investigated the effect of Δφ on the action of fluorescent probe-labeled AMP derived from lactoferricin B, Rh-LfcinB (4-9). At low peptide concentrations, Rh-LfcinB (4-9) enters the cytosol of Escherichia coli cells and spheroplasts without damaging their cell membrane, but a protonophore suppresses this entry and its antimicrobial activity. The rate of entry of Rh-LfcinB (4-9) into the giant unilamellar vesicles (GUVs) comprising E. coli lipids without pore formation increases with increasing Δφ. These results indicate that Δφ enhances the antimicrobial activity of Rh-LfcinB (4-9) and hence LfcinB (4-9) by increasing the rate of their entry into the cytosol.

NanoClick: A High Throughput, Target-Agnostic Peptide Cell Permeability Assay

Macrocyclic peptides open new opportunities to target intracellular protein-protein interactions (PPIs) that are often considered nondruggable by traditional small molecules. However, engineering sufficient membrane permeability into these molecules is a central challenge for identifying clinical candidates. Currently, there is a lack of high-throughput assays to assess peptide permeability, which limits our capacity to engineer this property into macrocyclic peptides for advancement through drug discovery pipelines. Accordingly, we developed a high throughput and target-agnostic cell permeability assay that measures the relative cumulative cytosolic exposure of a peptide in a concentration-dependent manner. The assay was named NanoClick as it combines in-cell Click chemistry with an intracellular NanoBRET signal. We validated the approach using known cell penetrating peptides and further demonstrated a correlation to cellular activity using a p53/MDM2 model system. With minimal change to the peptide sequence, NanoClick enables the ability to measure uptake of molecules that enter the cell via different mechanisms such as endocytosis, membrane translocation, or passive permeability. Overall, the NanoClick assay can serve as a screening tool to uncover predictive design rules to guide structure-activity-permeability relationships in the optimization of functionally active molecules.

Binding and crossing: Methods for the characterization of membrane-active peptides interactions with membranes at the molecular level

Antimicrobial and cell-penetrating peptides have been the object of extensive studies for more than 60 years. Initially these two families were studied separately, and more recently parallels have been drawn. These studies have given rise to numerous methodological developments both in terms of observation techniques and membrane models. This review presents some of the most recent original and innovative developments in this field, namely droplet interface bilayers (DIBs), new fluorescence approaches, force measurements, and photolabelling.

Surface charge density and fatty acids enhance the membrane permeation rate of CPP-cargo complexes

The CPP-effect makes reference to the process by which the membrane translocation rate of a cargo is enhanced by chemical functionalization with cell-penetrating peptides (CPPs). In this work we combine a simple kinetic model with free-energy calculations to explore the energetic basis of the CPP-effect. Two polyglicines are selected as model hydrophilic cargoes, and nona-arginine as a prototypical CPP. We assess the cargo carrying efficiency of nona-arginine by comparing the adsorption and insertion energies of the cargoes, the cargo-free CPPs, and the CPP-cargo complexes, into lipid membranes of varying composition. We also analyze the effect of modifying the type and concentration of anionic lipids, and the implication of these factors on the translocation rate of the CPP-cargo complex. Of particular interest is the evaluation of the catalytic role of palmitic acid (palmitate) as a promoter of the CPP-effect. We also analyse the influence of the size of the cargo on the membrane adsorption and insertion energies. Our results show that the efficiency of nona-arginine as a transmembrane carrier of simple hydrophilic molecules is modulated by the size of the cargo, and is strongly enhanced by increasing the concentration of anionic lipids and of ionized fatty acids in the membrane.

Role of Membrane Potential on Entry of Cell-Penetrating Peptide Transportan 10 into Single Vesicles

Cell-penetrating peptides (CPPs) can translocate across plasma membranes to enter the cytosol of eukaryotic cells without decreasing cell viability. We revealed the mechanism underlying this translocation by examining the effect of membrane potential, φ_m, on the entry of a CPP, transportan 10 (TP10), into the lumen of single giant unilamellar vesicles (GUVs). For this purpose, we used the single GUV method to detect the entry of carboxyfluorescein (CF)-labeled TP10 (CF-TP10) into the lumen of single GUVs. First, we used various K⁺ concentration differences to apply different negative membrane potentials on single GUVs containing gramicidin A in their membrane and confirmed these potentials using the φ_m-sensitive fluorescent probe 3,3'-dihexyloxacarbocyanine iodine. The fluorescence intensity of the GUV membranes (i.e., the rim intensity) due to 3,3'-dihexyloxacarbocyanine iodine increased with |φ_m| up to 118 mV, and its dependence on |φ_m| less than 28 mV agreed with a theoretical estimation (i.e., the dye concentration in the inner leaflet of a GUV is larger than that in the outer leaflet according to the Boltzmann distribution). We then examined the effect of φ_m on the entry of CF-TP10 into GUVs using single GUVs containing small GUVs or large unilamellar vesicles inside the mother GUV lumen. We found that CF-TP10 entered the GUV lumen without pore formation and the rate of entry of CF-TP10 into the GUV lumen, V_entry, increased with an increase in |φ_m|. The rim intensity due to CF-TP10 increased with an increase in |φ_m|, indicating that the CF-TP10 concentration in the inner leaflet of the GUV increased with |φ_m|. These results indicate that the φ_m-induced elevation in V_entry can be explained by the increase in CF-TP10 concentration in the inner leaflet with |φ_m|. We discuss the mechanism underlying this effect of membrane potential based on the pre-pore model of the translocation of CF-TP10 across a GUV membrane.

Interaction of a Polyarginine Peptide with Membranes of Different Mechanical Properties

The membrane translocation efficiency of cell penetrating peptides (CPPs) has been largely studied, and poly-arginines have been highlighted as particularly active CPPs, especially upon negatively charged membranes. Here we inquire about the influence of membrane mechanical properties in poly-arginine adsorption, penetration and translocation, as well as the subsequent effect on the host membrane. For this, we selected anionic membranes exhibiting different rigidity and fluidity, and exposed them to the nona-arginine KR₉C. Three different membrane compositions were investigated, all of them having 50% of the anionic lipid 1,2-dioleoyl-sn-glycero-3-phospho-(1’-rac-glycerol) (DOPG), thus, ensuring a high affinity of the peptide for membrane surfaces. The remaining 50% was a saturated PC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine, DPPC), an unsaturated PC (1,2-dioleoyl-sn-glycero-3-phosphocholine, DOPC) or a mixture of DOPC with cholesterol. Peptide-membrane interactions were studied using four complementary models for membranes: Langmuir monolayers, Large Unilamellar Vesicles, Black Lipid Membranes and Giant Unilamellar Vesicles. The patterns of interaction of KR₉C varied within the different membrane compositions. The peptide strongly adsorbed on membranes with cholesterol, but did not incorporate or translocate them. KR₉C stabilized phase segregation in DPPC/DOPG films and promoted vesicle rupture. DOPC/DOPG appeared like the better host for peptide translocation: KR₉C adsorbed, inserted and translocated these membranes without breaking them, despite softening was observed.

Understanding Cell Penetration of Cyclic Peptides

Approximately 75% of all disease-relevant human proteins, including those involved in intracellular protein-protein interactions (PPIs), are undruggable with the current drug modalities (i.e., small molecules and biologics). Macrocyclic peptides provide a potential solution to these undruggable targets because their larger sizes (relative to conventional small molecules) endow them the capability of binding to flat PPI interfaces with antibody-like affinity and specificity. Powerful combinatorial library technologies have been developed to routinely identify cyclic peptides as potent, specific inhibitors against proteins including PPI targets. However, with the exception of a very small set of sequences, the vast majority of cyclic peptides are impermeable to the cell membrane, preventing their application against intracellular targets. This Review examines common structural features that render most cyclic peptides membrane impermeable, as well as the unique features that allow the minority of sequences to enter the cell interior by passive diffusion, endocytosis/endosomal escape, or other mechanisms. We also present the current state of knowledge about the molecular mechanisms of cell penetration, the various strategies for designing cell-permeable, biologically active cyclic peptides against intracellular targets, and the assay methods available to quantify their cell-permeability.

The role of membrane tension in the action of antimicrobial peptides and cell-penetrating peptides in biomembranes

For antimicrobial peptides (AMPs) with antimicrobial and bactericidal activities and cell-penetrating peptides (CPPs) with activity to permeate through plasma membrane, their interactions with lipid bilayer region in plasma membrane play important roles in these functions. However, the elementary processes and mechanisms of their functions have not been clear. The single giant unilamellar vesicle (GUV) method has revealed the details of elementary processes of interaction of some AMPs and CPPs with lipid vesicles. In this review, we summarize the mode of action of AMPs such as magainin 2 (Mag) and CPPs such as transportan 10 (TP10), revealed by the single GUV methods, and especially we focus on the role of membrane tension in actions of Mag and TP10 and the mechanisms of their actions. First, we explain the characteristics of the single GUV method briefly. Next, we summarize the recent view on the effect of tension on physical properties of lipid bilayers and describe the role of tension in actions of Mag and TP10. Some experimental results indicate that Mag-induced pore is a stretch-activated pore. The effect of packing of transbilayer asymmetric lipid on Mag-induced pore formation is described. On the other hand, entry of fluorescent dye, carboxyfluorescein (CF)-labeled TP10 (i.e., CF-TP10), into single GUVs without pore formation is affected by tension and high concentration of cholesterol. Pre-pore model for translocation of CF-TP10 across lipid bilayer is described. The experimental methods and their analysis described here are useful for investigation of functions of the other types of AMPs, CPPs, and proteins.

Cholesterol re-organisation and lipid de-packing by arginine-rich cell penetrating peptides: Role in membrane translocation

Cell penetrating peptides (CPPs) are able to transport hydrophilic molecules inside cells. To reach the cytosol, the peptide associated with a cargo must cross the plasma or the endosomal membrane. Different molecular mechanisms for peptide internalisation into cells have been proposed and it is becoming clear that the cellular internalisation mechanisms are different depending on the peptide sequence and structure and the target membrane. Herein, the penetration of three peptides into large unilamellar vesicles were studied: the homeodomain derived 16-residues penetratin, nona-arginine (R9), and a small peptide containing 6 arginine and 3 tryptophan residues (RW9). The membrane models were composed of phospholipids from natural sources containing different molecular species. We observed that among the three peptides, only the amphipathic peptide RW9 was able to cross the membrane vesicles in the liquid disordered state. The changes in the distribution of the previously characterized cholesterol-pyrene probe show that cholesterol-pyrene molecules dissociate from clusters upon membrane interaction with the three peptides and that the cholesterol environment becomes more disordered in the presence of RW9. Finally, we studied the effect of the peptides on lipid ordering on giant plasma membrane vesicles. The amphipathic peptides RW9 and its longer homologue RW16 induced lipid de-packing in plasma membrane vesicles. Overall, the data suggest that a disordered membrane favours the translocation of RW9, that the membrane cholesterol is redistributed during peptide interaction, and that the peptide amphipathic character is important to increase membrane fluidity and peptide membrane translocation.

Negative Dipole Potentials and Carboxylic Polar Head Groups Foster the Insertion of Cell-Penetrating Peptides into Lipid Monolayers

Cell-penetrating peptides (CPPs) are polycationic sequences of amino acids recognized as some of the most effective vehicles for delivering membrane-impermeable cargos into cells. CPPs can traverse cell membranes by direct translocation, and assessing the role of lipids on the membrane permeation process is important to convene a complete model of the CPP translocation. In this work, we focus on the biophysical basis of peptide-fatty acid interactions, analyzing how the acid-base and electrostatic properties of the lipids determine the CPP adsorption and incorporation into a Langmuir monolayer, focusing thus on the first two stages of the direct translocation mechanism. We sense the binding and insertion of the peptide into the lipid structure by measuring the changes in the surface pressure, the surface potential, and the reflectivity of the interface. We show that, beyond the presence of anionic moieties, negative dipole potentials and carboxylic polar head groups significantly promote the insertion of the peptide into the monolayer. On the basis of our results, we propose the appearance of stable CPP-lipid complexes whose kinetics of formation depends on the length of the lipids' hydrocarbon chains.

Entry of a Six-Residue Antimicrobial Peptide Derived from Lactoferricin B into Single Vesicles and Escherichia coli Cells without Damaging their Membranes

Lactoferricin B (LfcinB) and shorter versions of this peptide have antimicrobial activity. However, the elementary processes of interactions of these peptides with lipid membranes and bacteria are still not well understood. To elucidate the mechanism of their antimicrobial activity, we investigated the interactions of LfcinB (4-9) (its sequence of RRWQWR) with Escherichia coli cells and giant unilamellar vesicles (GUVs). LfcinB (4-9) and lissamine rhodamine B red-labeled LfcinB (4-9) (Rh-LfcinB (4-9)) did not induce an influx of a membrane-impermeant fluorescent probe, SYTOX green, from the outside of E. coli cells into their cytoplasm, indicating that no damage occurred in their plasma membrane. To examine the activity of LfcinB (4-9) to enter E. coli cytoplasm, we investigated the interaction of Rh-LfcinB (4-9) with single cells of E. coli containing calcein using confocal microscopy. We found that Rh-LfcinB (4-9) entered the cytoplasm without leakage of calcein. Next, we investigated the interactions of Rh-LfcinB (4-9) with single GUVs of dioleoylphosphatidylglycerol (DOPG) and dioleoylphosphatidylcholine (DOPC) mixtures containing a fluorescent probe, Alexa Fluor 647 hydrazide (AF647), using the single GUV method. The results indicate that Rh-LfcinB (4-9) outside the GUV translocated through the GUV membrane and entered its lumen without leakage of AF647. Interaction of Rh-LfcinB (4-9) with DNA increased its fluorescence intensity greatly. Therefore, we can conclude that Rh-LfcinB (4-9) can translocate across lipid membrane regions of the plasma membrane of E. coli cells to enter their cytoplasm without leakage of calcein and its antimicrobial activity is not due to damage of their plasma membranes.

Effects of Mechanical Properties of Lipid Bilayers on the Entry of Cell-Penetrating Peptides into Single Vesicles

The translocation of cell-penetrating peptides (CPPs) through plasma membranes of living cells is an important physiological phenomenon in biomembranes. To reveal the mechanism underlying the translocation of a CPP, transportan 10 (TP10), through lipid bilayers, we examined the effects of the mechanical properties of lipid bilayers on the entry of carboxyfluorescein (CF)-labeled TP10 (CF-TP10) into a giant unilamellar vesicle (GUV) using the single GUV method. First, we examined the effect of lateral tension in membranes on the entry of CF-TP10 into single GUVs comprising a mixture of dioleoylphosphatidylglycerol (DOPG) and dioleoylphosphatidylcholine (DOPC) (2/8). CF-TP10 entered the GUV lumen before the membrane permeation of Alexa Fluor 647 hydrazide (AF647) from the GUV and thus before pore formation in the membrane. The fraction of entry of CF-TP10 before pore formation and the rate of membrane rupture increased with tension. The CF-TP10-induced fractional change in the membrane area increased continuously with time until membrane rupture, but it increased more slowly than did the CF-TP10 concentration in the GUV membrane. A high mole fraction of cholesterol inhibited the entry of CF-TP10 into single GUVs by suppressing the translocation of CF-TP10 from the external to the internal monolayer, although higher concentrations of CF-TP10 induced the formation of pores through which CF-TP10 rapidly translocated. Suppression of the translocation of CF-TP10 by cholesterol can be reasonably explained by the large line tension of a prepore. We discussed the role of mechanical properties in membranes on the entry of CF-TP10 into single GUVs and proposed a hypothesis of the mechanism that CF-TP10 translocates across a bilayer through transient hydrophilic prepores in the membrane.

Peptide-based strategies for enhanced cell uptake, transcellular transport, and circulation: Mechanisms and challenges

Peptides are emerging as a new tool in drug and gene delivery. Peptide-drug conjugates and peptide-modified drug delivery systems provide new opportunities to avoid macrophage recognition and subsequent phagocytosis, cross endothelial and epithelial barriers, and enter the cytoplasm of target cells. Peptides are relatively small, low-cost, and are stable in a wide range of biological conditions. In this review, we summarize recent work in designing peptides to enhance penetration of biological barriers, increase cell uptake, and avoid the immune system. We highlight recent successes and contradictory results, and outline common emerging concepts and design rules. The development of sequence-structure-function relationships and standard protocols for benchmarking will be a key to progress in the field.

Self-aggregation propensity of the Tat peptide revealed by UV-Vis, NMR and MD analyses

The self-aggregation propensity of unlabeled and TAMRA-labeled Tat₁₁ peptides has been revealed for the first time by UV-Vis, NMR and MD analyses.

The role of Tat peptide self-aggregation in membrane pore stabilization: insights from a computational study

Role of Tat peptide self-aggregation to direct transduction in cells is highlighted in a computational study of dimer versus monomer.

A One-Pot Chemically Cleavable Bis-Linker Tether Strategy for the Synthesis of Heterodimeric Peptides

Heterodimeric peptides linked by disulfide bonds are attractive drug targets. However, their chemical assembly can be tedious, time-consuming, and low yielding. Inspired by the cellular synthesis of pro-insulin in which the two constituent peptide chains are expressed as a single-chain precursor separated by a connecting C-peptide, we have developed a novel chemically cleavable bis-linker tether which allows the convenient assembly of two peptide chains as a single "pro"-peptide on the same solid support. Following the peptide cleavage and post-synthetic modifications, this bis-linker tether can be removed in one-step by chemical means. This method was used to synthesize a drug delivery-cargo conjugate, TAT-PKCi peptide, and a two-disulfide bridged heterodimeric peptide, thionin (7-19)-(24-32R), a thionin analogue. To our knowledge, this is the first report of a one-pot chemically cleavable bis-linker strategy for the facile synthesis of cross-bridged two-chain peptides.

Unsaturated acyl chains dramatically enhanced cellular uptake by direct translocation of a minimalist oligo-arginine lipopeptide

The recurring issue with cell penetrating peptides is how to increase direct translocation vs. endocytosis, to avoid premature degradation. Acylation by a cis unsaturated chain (C22:6) of a short cationic peptide provides a new rational design to favour diffuse cytosolic and dense Golgi localisations.

Membrane Oxidation Enables the Cytosolic Entry of Polyarginine Cell-penetrating Peptides

Arginine-rich peptides can penetrate cells and consequently be used as delivery agents in various cellular applications. The activity of these reagents is often context-dependent, and the parameters that impact cell entry are not fully understood, giving rise to variability and limiting progress toward their usage. Herein, we report that the cytosolic penetration of linear polyarginine peptides is dependent on the oxidation state of the cell. In particular, we find that hypoxia and cellular antioxidants inhibit cell penetration. In contrast, oxidants promote cytosolic cell entry with an efficiency proportional to the level of reactive oxygen species generated within membranes. Moreover, an antibody that binds to oxidized lipids inhibits cell penetration, whereas extracellularly administered pure oxidized lipids enhance peptide transport into cells. Overall, these data indicate that oxidized lipids are capable of mediating the transport of polyarginine peptides across membranes. These data may also explain variability in cell-penetrating peptide performance in different experimental conditions. These new findings therefore provide new opportunities for the rational design of future cell-permeable compounds and for the optimization of delivery protocols.

Tuning the properties of a novel short cell-penetrating peptide by intramolecular cyclization with a triazole bridge

Cyclic versus linear: cyclic triazole-bridged cell-penetrating peptides are optimally arranged within the membrane, thus at the same time inducing suitable DNA complexation and successful peptide membrane insertion.

Spontaneous membrane-translocating peptides: influence of peptide self-aggregation and cargo polarity

Peptides that translocate spontaneously across cell membranes could transform the field of drug delivery by enabling the transport of otherwise membrane-impermeant molecules into cells. In this regard, a 9-aminoacid-long motif (representative sequence: PLIYLRLLR, hereafter Translocating Motif 9, TM9) that spontaneously translocates across membranes while carrying a polar dye was recently identified by high-throughput screening. Here we investigate its transport properties by a combination of in cuvette physico-chemical assays, rational mutagenesis, live-cell confocal imaging and fluorescence correlation spectroscopy measurements. We unveil TM9 ability to self-aggregate in a concentration-dependent manner and demonstrate that peptide self-aggregation is a necessary –yet not sufficient– step for effective membrane translocation. Furthermore we show that membrane crossing can occur with apolar payloads while it is completely inhibited by polar ones. These findings are discussed and compared to previous reports. The present results impose a careful rethinking of this class of sequences as direct-translocation vectors suitable for delivery purposes.

Mechanism Matters: A Taxonomy of Cell Penetrating Peptides

The permeability barrier imposed by cellular membranes limits the access of exogenous compounds to the interior of cells. Researchers and patients alike would benefit from efficient methods for intracellular delivery of a wide range of membrane-impermeant molecules, including biochemically active small molecules, imaging agents, peptides, peptide nucleic acids, proteins, RNA, DNA, and nanoparticles. There has been a sustained effort to exploit cell penetrating peptides (CPPs) for the delivery of such useful cargoes in vitro and in vivo because of their biocompatibility, ease of synthesis, and controllable physical chemistry. Here, we discuss the many mechanisms by which CPPs can function, and describe a taxonomy of mechanisms that could be help organize future efforts in the field.

Sensing pH via p-cyanophenylalanine fluorescence: Application to determine peptide pKa and membrane penetration kinetics

We expand the spectroscopic utility of a well-known infrared and fluorescence probe, p-cyanophenylalanine, by showing that it can also serve as a pH sensor. This new application is based on the notion that the fluorescence quantum yield of this unnatural amino acid, when placed at or near the N-terminal end of a polypeptide, depends on the protonation status of the N-terminal amino group of the peptide. Using this pH sensor, we are able to determine the N-terminal pKa values of nine tripeptides and also the membrane penetration kinetics of a cell-penetrating peptide. Taken together, these examples demonstrate the applicability of using this unnatural amino acid fluorophore to study pH-dependent biological processes or events that accompany a pH change.

When cationic cell-penetrating peptides meet hydrocarbons to enhance in-cell cargo delivery

Cell-penetrating peptides (CPPs) are short sequences often rich in cationic residues with the remarkable ability to cross cell membranes. In the past 20 years, CPPs have gained wide interest and have found numerous applications in the delivery of bioactive cargoes to the cytosol and even the nucleus of living cells. The covalent or non-covalent addition of hydrocarbon moieties to cationic CPPs alters the hydrophobicity/hydrophilicity balance in their sequence. Such perturbation dramatically influences their interaction with the cell membrane, might induce self-assembling properties and modifies their intracellular trafficking. In particular, the introduction of lipophilic moieties changes the subcellular distribution of CPPs and might result in a dramatically increase of the internalization yield of the co-transported cargoes. Herein, we offer an overview of different aspects of the recent findings concerning the properties of CPPs covalently or non-covalently associated to hydrocarbons. We will focus on the impact of the hydrocarbon moieties on the delivery of various cargoes, either covalently or non-covalently bound to the modified CPPs. We will also provide some key elements to rationalize the influence of the hydrocarbons moieties on the cellular uptake. Furthermore, the recent in vitro and in vivo successful applications of acylated CPPs will be summarized to provide a broad view of the versatility of these modified CPPs as small-molecules and oligonucleotides vectors.