The homeodomain derived peptide Penetratin induces curvature of fluid membrane domains

Effect of the Lipid Landscape on the Efficacy of Cell-Penetrating Peptides

Every cell biological textbook teaches us that the main role of the plasma membrane is to separate cells from their neighborhood to allow for a controlled composition of the intracellular space. The mostly hydrophobic nature of the cell membrane presents an impenetrable barrier for most hydrophilic molecules larger than 1 kDa. On the other hand, cell-penetrating peptides (CPPs) are capable of traversing this barrier without compromising membrane integrity, and they can do so on their own or coupled to cargos. Coupling biologically and medically relevant cargos to CPPs holds great promise of delivering membrane-impermeable drugs into cells. If the cargo is able to interact with certain cell types, uptake of the CPP-drug complex can be tailored to be cell-type-specific. Besides outlining the major membrane penetration pathways of CPPs, this review is aimed at deciphering how properties of the membrane influence the uptake mechanisms of CPPs. By summarizing an extensive body of experimental evidence, we argue that a more ordered, less flexible membrane structure, often present in the very diseases planned to be treated with CPPs, decreases their cellular uptake. These correlations are not only relevant for understanding the cellular biology of CPPs, but also for rationally improving their value in translational or clinical applications.

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.

Amphiphilic Gold Nanoparticles: A Biomimetic Tool to Gain Mechanistic Insights into Peptide-Lipid Interactions

Functional peptides are now widely used in a myriad of biomedical and clinical contexts, from cancer therapy and tumor targeting to the treatment of bacterial and viral infections. Underlying this diverse range of applications are the non-specific interactions that can occur between peptides and cell membranes, which, in many contexts, result in spontaneous internalization of the peptide within cells by avoiding energy-driven endocytosis. For this to occur, the amphipathicity and surface structural flexibility of the peptides play a crucial role and can be regulated by the presence of specific molecular residues that give rise to precise molecular events. Nevertheless, most of the mechanistic details regulating the encounter between peptides and the membranes of bacterial or animal cells are still poorly understood, thus greatly limiting the biomimetic potential of these therapeutic molecules. In this arena, finely engineered nanomaterials—such as small amphiphilic gold nanoparticles (AuNPs) protected by a mixed thiol monolayer—can provide a powerful tool for mimicking and investigating the physicochemical processes underlying peptide-lipid interactions. Within this perspective, we present here a critical review of membrane effects induced by both amphiphilic AuNPs and well-known amphiphilic peptide families, such as cell-penetrating peptides and antimicrobial peptides. Our discussion is focused particularly on the effects provoked on widely studied model cell membranes, such as supported lipid bilayers and lipid vesicles. Remarkable similarities in the peptide or nanoparticle membrane behavior are critically analyzed. Overall, our work provides an overview of the use of amphiphilic AuNPs as a highly promising tailor-made model to decipher the molecular events behind non-specific peptide-lipid interactions and highlights the main affinities observed both theoretically and experimentally. The knowledge resulting from this biomimetic approach could pave the way for the design of synthetic peptides with tailored functionalities for next-generation biomedical applications, such as highly efficient intracellular delivery systems.

Unconventional Secretion, Gate to Homeoprotein Intercellular Transfer

Unconventional secretion allows for the secretion of fully mature and biologically active proteins mostly present in the cytoplasm or nucleus. Besides extra vesicle-driven secretion, non-extravesicular pathways also exist that specifically rely on the ability of the secreted proteins to translocate directly across the plasma membrane. This is the case for several homeoproteins, a family of over 300 transcription factors characterized by the structure of their DNA-binding homeodomain. The latter highly conserved homeodomain is necessary and sufficient for secretion, a process that requires PI(4,5)P2 binding, as is the case for FGF2 and HIV Tat unconventional secretion. An important feature of homeoproteins is their ability to cross membranes in both directions and thus to transfer between cells. This confers to homeoproteins their paracrine activity, an essential facet of their physiological functions.

Studies of cell-penetrating peptides by biophysical methods

Biophysical studies have a very high impact on the understanding of internalization, molecular mechanisms, interactions, and localization of CPPs and CPP/cargo conjugates in live cells or in vivo. Biophysical studies are often first carried out in test-tube set-ups or in vitro, leading to the complicated in vivo systems. This review describes recent studies of CPP internalization, mechanisms, and localization. The multiple methods in these studies reveal different novel and important aspects and define the rules for CPP mechanisms, hopefully leading to their improved applicability to novel and safe therapies.

Physicochemical Features and Peculiarities of Interaction of AMP with the Membrane

Antimicrobial peptides (AMPs) are anti-infectives that have the potential to be used as a novel and untapped class of biotherapeutics. Modes of action of antimicrobial peptides include interaction with the cell envelope (cell wall, outer- and inner-membrane). A comprehensive understanding of the peculiarities of interaction of antimicrobial peptides with the cell envelope is necessary to perform a rational design of new biotherapeutics, against which working out resistance is hard for microbes. In order to enable de novo design with low cost and high throughput, in silico predictive models have to be invoked. To develop an efficient predictive model, a comprehensive understanding of the sequence-to-function relationship is required. This knowledge will allow us to encode amino acid sequences expressively and to adequately choose the accurate AMP classifier. A shared protective layer of microbial cells is the inner, plasmatic membrane. The interaction of AMP with a biological membrane (native and/or artificial) has been comprehensively studied. We provide a review of mechanisms and results of interactions of AMP with the cell membrane, relying on the survey of physicochemical, aggregative, and structural features of AMPs. The potency and mechanism of AMP action are presented in terms of amino acid compositions and distributions of the polar and apolar residues along the chain, that is, in terms of the physicochemical features of peptides such as hydrophobicity, hydrophilicity, and amphiphilicity. The survey of current data highlights topics that should be taken into account to come up with a comprehensive explanation of the mechanisms of action of AMP and to uncover the physicochemical faces of peptides, essential to perform their function. Many different approaches have been used to classify AMPs, including machine learning. The survey of knowledge on sequences, structures, and modes of actions of AMP allows concluding that only possessing comprehensive information on physicochemical features of AMPs enables us to develop accurate classifiers and create effective methods of prediction. Consequently, this knowledge is necessary for the development of design tools for peptide-based antibiotics.

Nanoscale Visualization of Morphological Alteration of Live-Cell Membranes by the Interaction with Oligoarginine Cell-Penetrating Peptides

The interactions between the cell membrane and biomolecules remain poorly understood. For example, arginine-rich cell-penetrating peptides (CPPs), including octaarginines (R8), are internalized by interactions with cell membranes. However, during the internalization process, the exact membrane dynamics introduced by these CPPs are still unknown. Here, we visualize arginine-rich CPPs and cell-membrane interaction-induced morphological changes using a system that combines scanning ion-conductance microscopy and spinning-disk confocal microscopy, using fluorescently labeled R8. This system allows time-dependent, nanoscale visualization of structural dynamics in live-cell membranes. Various types of membrane remodeling caused by arginine-rich CPPs are thus observed. The induction of membrane ruffling and the cup closure are observed as a process of endocytic uptake of the peptide. Alternatively suggested is the concave structural formation accompanied by direct peptide translocation through cell membranes. Studies using R8 without fluorescent labeling also demonstrate a non-negligible effect of the fluorescent moiety on membrane structural alteration.

In Vitro Assays: Friends or Foes of Cell-Penetrating Peptides

The cell membrane is a complex and highly regulated system that is composed of lipid bilayer and proteins. One of the main functions of the cell membrane is the regulation of cell entry. Cell-penetrating peptides (CPPs) are defined as peptides that can cross the plasma membrane and deliver their cargo inside the cell. The uptake of a peptide is determined by its sequence and biophysicochemical properties. At the same time, the uptake mechanism and efficiency are shown to be dependent on local peptide concentration, cell membrane lipid composition, characteristics of the cargo, and experimental methodology, suggesting that a highly efficient CPP in one system might not be as productive in another. To better understand the dependence of CPPs on the experimental system, we present a review of the in vitro assays that have been employed in the literature to evaluate CPPs and CPP-cargos. Our comprehensive review suggests that utilization of orthogonal assays will be more effective for deciphering the true ability of CPPs to translocate through the membrane and enter the cell cytoplasm.

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.

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.

Cell-Surface Interactions on Arginine-Rich Cell-Penetrating Peptides Allow for Multiplex Modes of Internalization

One of the recent hot topics in peptide-related chemical biology research is the potential of cell-penetrating peptides (CPPs). Owing to their ability to deliver exogenous molecules into cells easily and effectively, their flexible design that allows transporters to comprise various chemical structures and functions, and their potential in chemical and cell biology studies and clinical applications, CPPs have been attracting enormous interest among researchers in related fields. Consequently, publications on CPPs have increased significantly. Although there are many types of CPPs with different physicochemical properties and applications, arginine-rich CPPs, which include the human immunodeficiency virus type 1 (HIV-1) TAT peptide and oligoarginines, are among the most extensively employed and studied. Previous studies demonstrated the importance of the guanidino group in arginine, which confers flexibility in transporter design. Therefore, in addition to peptides, various transporters rich in guanidino groups, which do not necessarily share specific chemical and three-dimensional structures, have been developed. Typically, cell-penetrating transporters have 6-12 guanidino groups. Since the pK_a of the guanidino group in arginine is approximately 12.5, these molecules are highly basic and hydrophilic. Our group is interested in why these cationic molecules can penetrate cells. Understanding their mechanism of action should lead to the rational design of intracellular delivery systems that have high efficacy. Additionally, novel cellular uptake mechanisms may be elucidated during the course of these studies. Therefore, our group is trying to understand the basic aspects underlying the ability of these peptides to penetrate cells. Regarding the delivery of biopharmaceuticals including proteins and nucleic acids, achieving efficient and effective delivery to target organs and cells is one of the biggest challenges. Furthermore, when the target sites of these drug molecules are within cells, effective cell penetration becomes another obstacle. Cells are surrounded by a membrane that separates the inside of the cell from its outside. This barrier function is critical for keeping cellular contents inside cells, and without this, cells cannot function. Therefore, understanding the mechanism of action of CPPs is necessary to overcome these obstacles and will allow us not only to improve CPP-mediated delivery but also to create other types of intracellular delivery systems. In this Account, we summarize the current knowledge on the mechanisms of internalization of arginine-rich CPPs, from the viewpoints of both direct cell-membrane penetration (i.e., physicochemical aspects) and endocytic uptake (i.e., physiological aspects), and discuss the implications of this knowledge. We also discussed loosening of lipid packing as a factor to promote direct cell-membrane penetration.

Dimerization in tailoring uptake efficacy of the HSV-1 derived membranotropic peptide gH625

gH625 constitutes a promising delivery vehicle for the transport of therapeutic biomacromolecules across membrane barriers. We report an application of multivalency to create a complex nanosystem for delivery and to elucidate the mechanism of peptide-lipid bilayer interactions. Multivalency may offer a route to enhance gH625 cellular uptake as demonstrated by results obtained on dimers of gH625 by fluorescence spectroscopy, circular dichroism, and surface plasmon resonance. Moreover, using both phase contrast and light sheet fluorescence microscopy we were able to characterize and visualize for the first time the fusion of giant unilamellar vesicles caused by a membranotropic peptide.

Current Understanding of Direct Translocation of Arginine-Rich Cell-Penetrating Peptides and Its Internalization Mechanisms

Arginine-rich cell-penetrating peptides (CPPs) including Tat, Penetratin and oligoarginine peptides are a series of short peptides that can be efficiently internalized into cells and have been widely used as carriers for intracellular delivery of bioactive molecules. In the early phase of the study, CPPs, as well as their conjugates, were thought to rapidly enter cells by direct penetration through membranes, which was later found to be an experimental artifact that was concluded from observations in fixed cells. Although re-evaluation using living unfixed cells revealed that endocytosis has a major role in internalization of these peptides, there are a number of studies reporting that, even if fixation is avoided, direct translocation across plasma membranes and cytosolic distribution of arginine-rich CPPs are still observed in cells without membrane perturbation. In addition, amphiphilic counteranions such as pyrenebutyrate dramatically accelerate direct translocation of these peptides into cells. These results suggest that there are at least two pathways, i.e., endocytosis and direct translocation, both of which would contribute to cellular internalization of arginine-rich CPPs. In this review, we first introduce the story of fixation artifact, which indeed led to the critical progress in CPP study, and then summarize the current understanding for direct translocation of arginine-rich CPPs. Comprehensive understanding of direct translocation of these peptides and its mechanistic elucidation would provide useful knowledge for developing methodologies that would enable efficient intracellular delivery.

Intelligent substance delivery into cells using cell-penetrating peptides

Cell-penetrating peptides (CPPs) are oligopeptides that can permeate the cell membrane. The use of a CPP-mediated transport system could be an excellent method for delivering cell-impermeable substances such as proteins, antibodies, antisense oligonucleotides, siRNAs, plasmids, drugs, fluorescent compounds, and nanoparticles as covalently or noncovalently conjugated cargo into cells. Nonetheless, the mechanisms through which CPPs are internalized remain unclear. Endocytosis and direct translocation through the membrane are the generally accepted routes. Internalization via both pathways can occur simultaneously, depending on cellular conditions. However, the peculiar property of CPPs has attracted many researchers, especially in drug discovery or development, who intend to deliver impermeable substances into cells through the cell membrane. The delivery of drugs using CPPs may non-invasively solve the problem of drug penetration into cells with the added benefit of low cytotoxicity. Moreover, macromolecules can also be delivered by this transport system. In this review, I discuss the possibilities and advantages of substance delivery into cells using CPPs.

Massive glycosaminoglycan-dependent entry of Trp-containing cell-penetrating peptides induced by exogenous sphingomyelinase or cholesterol depletion

Among non-invasive cell delivery strategies, cell-penetrating peptide (CPP) vectors represent interesting new tools. To get fundamental knowledge about the still debated internalisation mechanisms of these peptides, we modified the membrane content of cells, typically by hydrolysis of sphingomyelin or depletion of cholesterol from the membrane outer leaflet. We quantified and visualised the effect of these viable cell surface treatments on the internalisation efficiency of different CPPs, among which the most studied Tat, R9, penetratin and analogues, that all carry the N-terminal biotin-Gly4 tag cargo. Under these cell membrane treatments, only penetratin and R6W3 underwent a massive glycosaminoglycan (GAG)-dependent entry in cells. Internalisation of the other peptides was only slightly increased, similarly in the absence or the presence of GAGs for R9, and only in the presence of GAGs for Tat and R6L3. Ceramide formation (or cholesterol depletion) is known to lead to the reorganisation of membrane lipid domains into larger platforms, which can serve as a trap and cluster receptors. These results show that GAG clustering, enhanced by formation of ceramide, is efficiently exploited by penetratin and R6W3, which contains Trp residues in their sequence but not Tat, R9 and R6L3. Hence, these data shed new lights on the differences in the internalisation mechanism and pathway of these peptides that are widely used in delivery of cargo molecules.

The uptake of HIV Tat peptide proceeds via two pathways which differ from macropinocytosis

Cell penetrating peptides (CPPs) have been extensively studied as vectors for cellular delivery of therapeutic molecules, yet the identity of their uptake routes remained unclear and is still under debate. In this study we provide new insights into CPP entry routes by quantitatively measuring the intracellular uptake of FAM-labeled Tat-peptide under rigorous kinetic and thermal conditions. The uptake of Tat-peptide between 4 and 15°C corresponds to Q10=1.1, proceeding through a prompt (<5 min), temperature-independent process, suggesting direct membrane translocation. At longer durations, Tat rate of uptake shows linear dependence on temperature with Q10=1.44, accompanied by activation energy Ea=4.45 Kcal/mole. These values are significantly lower than those we found for the macropinocytosis probe dextran (Q10=2.2 and Ea=7.2 Kcal/mole) which possesses an exponential dependence on temperature, characteristic of endocytosis processes. Tat-peptide and dextran do not interfere with each other's uptake rate and the ratio of Tat-peptide uptake to its extracellular concentration is ~15 times higher than that for dextran. In addition, Phloretin, a modulator of cell membrane dipole potential, is shown to increase dextran uptake but to reduce that of Tat. We conclude that the uptake of Tat differs from that of dextran in all parameters. Tat uptake proceeds by dual entry routes which differ by their energy dependence.

Curvature engineering: positive membrane curvature induced by epsin N-terminal peptide boosts internalization of octaarginine

Epsin-1 is a representative protein for inducing the positive curvature necessary for the formation of clathrin-coated pits. Here we demonstrate that the N-terminus 18-residue peptide of epsin-1 (EpN18) has this ability per se, as proved by differential scanning calorimetry (DSC) and solid-state NMR. Moreover, it is shown how this positive curvature promotion can be exploited for promoting the direct penetration of a representative cell-penetrating peptide (CPP), octaarginine (R8), through artificial and plasma membranes. This synergistic effect has been used for the efficient delivery of a proapoptotic domain peptide (PAD), which induced high level of apoptosis only when coadministered with R8 and EpN18, thus emphasizing the importance of positive curvature induction for achieving the desired ultimate cargo bioavailability.

Cell-penetrating peptides: 20 years later, where do we stand?

Twenty years ago, the discovery of peptides able to cross cellular membranes launched a novel field in molecular delivery based on these non-invasive vectors, most commonly called cell-penetrating peptides (CPPs) or protein transduction domains (PTDs). These peptides were shown to efficiently transport various biologically active molecules inside living cells, and thus are considered promising devices for medical and biotechnological developments. Moreover, CPPs emerged as potential tools to study the prime mechanisms of cellular entry across the plasma membrane. This review is dedicated to CPP fundamentals, with an emphasis on the molecular requirements and mechanism of their entry into eukaryotic cells.

Observing the translocation of a mitochondria-penetrating peptide with solid-state NMR

A new class of penetrating peptides that can target the mitochondria with high specificity was recently discovered. In this work, we developed a model inner mitochondrial membrane, equipped with a transmembrane gradient, suitable for solid-state NMR experiments. Using solid-state NMR, we observed a mitochondria-penetrating peptide interacting with the model inner mitochondrial membrane to gain insight into the mechanism of translocation. The paramagnetic relaxation effect due to Mn(2+) ions on (13)C magic angle spinning NMR was used to measure the insertion depth of the peptide and its distribution in each monolayer of the membrane. We found that at low peptide concentration the peptide binds to the outer leaflet and at high concentration, it crosses the hydrophobic bilayer core and is distributed in both leaflets. In both concentration regimes, the peptide binds at the C2 position on the lipid acyl chain. The mitochondria-penetrating peptide crossed to the inner leaflet of the model membranes without disrupting the lamellarity. These results provide evidence that supports the electroporation model of translocation. We estimated the energy associated with crossing the inner mitochondrial membrane. We found that the transmembrane potential provides sufficient energy for the peptide to cross the hydrophobic core, which is the most unfavorable step in translocation.

siRNA delivery: from lipids to cell-penetrating peptides and their mimics

To deliver siRNA for therapeutic use, several hurdles must be addressed. Metabolic degradation must be blocked, and the RNAi cellular machinery is located in the cytoplasm, while double-stranded siRNA is large, highly charged and impermeable to cell membranes. To date, the solutions to the delivery issues have mostly involved different forms of lipid particle encapsulation. Cell-penetrating peptides and their mimics or analogues offer a different approach and this is an emerging field with the first in vivo examples now reported. Recent reports point to lipid receptors being involved in the cellular uptake of both types of transporter. This review examines the delivery of siRNA with a focus on cell-penetrating peptides and their small molecule and oligomeric mimics. The current status of siRNA delivery methods in clinical trials is examined. It now seems that the goal of delivering siRNA therapeutically is achievable but will they form part of a sustainable healthcare portfolio for the future.

The enhanced membrane interaction and perturbation of a cell penetrating peptide in the presence of anionic lipids: toward an understanding of its selectivity for cancer cells

Cell penetrating peptides (CPPs) are usually short, highly cationic peptides that are capable of crossing the cell membrane and transport cargos of varied size and nature in cells by energy- and receptor-independent mechanisms. An additional potential is the newly discovered anti-tumor activity of certain CPPs, including RW16 (RRWRRWWRRWWRRWRR) which is derived from penetratin and is investigated here. The use of CPPs in therapeutics, diagnosis and potential application as anti-tumor agents increases the necessity of understanding their mode of action, a subject yet not totally understood. With this in mind, the membrane interaction and perturbation mechanisms of RW16 with both zwitterionic and anionic lipid model systems (used as representative models of healthy vs tumor cells) were investigated using a large panoply of biophysical techniques. It was shown that RW16 autoassociates and that its oligomerization state highly influences its membrane interaction. Overall a stronger association and perturbation of anionic membranes was observed, especially in the presence of oligomeric peptide, when compared to zwitterionic ones. This might explain, at least in part, the anti-tumor activity and so the selective interaction with cancer cells whose membranes have been shown to be especially anionic. Hydrophobic contacts between the peptide and lipids were also shown to play an important role in the interaction. That probably results from the tryptophan insertion into the fatty acid lipid area following a peptide flip after the first electrostatic recognition. A model is presented that reflects the ensemble of results.

Interaction of α-synuclein and a cell penetrating fusion peptide with higher eukaryotic cell membranes assessed by ¹⁹F NMR

We show that fluorine NMR can be used to monitor the insertion and change in conformation of a ¹⁹F-labeled cell-penetrating peptide upon interacting with the cellular plasma membrane. α-Synuclein and a construct comprising a cell-penetrating peptide covalently attached to its N-terminus were studied. Important information about the interaction of the proteins with CHO-K1 cells was obtained by monitoring the diminution of ¹⁹F resonances of 3-fluoro-l-tyrosine labeled proteins. For α-synuclein, a decrease in the resonance from position 39 was observed indicating that only the N-terminal third region of the protein interacts with plasma membrane. However, when the fusion construct was incubated with the cells, a decrease in the resonance from the fusion peptide region was noted with no change in the resonances from α-synuclein region. Longer incubation, studied by using confocal fluorescence microscopy, revealed that the fusion construct translocates into the cells, but α-synuclein alone did not cross the membrane in significant amounts.

Metabolic energy-independent mechanism of internalization for the cell penetrating peptide penetratin

Cellular uptake of vector peptides used for internalization of hydrophilic molecules into cells is known to follow two different pathways: direct translocation of the plasma membrane and internalization by endocytosis followed by release into the cytosol. These pathways differ in their energy dependence. The first does not need metabolic energy while the second requires metabolic energy. Herein we used erythrocytes and plasma membrane vesicles to study membrane perturbations induced by the cell penetrating peptide penetratin. The results show that cell penetrating peptides are able to be internalized by two metabolic energy-independent pathways: direct crossing of the plasma membrane and endocytosis-like mechanisms. The last mechanism involves the induction of membrane negative curvature resulting in invaginations that mimic the endosomal uptake in the absence of ATP. This new mechanism called "physical endocytosis" or "self-induced endocytosis" might explain different data concerning the independence or dependence on metabolic energy during cellular uptake and reveals the autonomous capacity of peptides to induce their internalization.

Transient focal membrane deformation induced by arginine-rich peptides leads to their direct penetration into cells

Endocytosis has been implicated in the cellular uptake of arginine-rich, cell-penetrating peptides (CPPs). However, accumulating evidence suggests that certain conditions allow the direct, non-endocytic penetration of arginine-rich peptides through the plasma membrane. We previously showed that Alexa Fluor 488-labeled dodeca-arginine (R12-Alexa488) directly enters cells at specific sites on the plasma membrane and subsequently diffuses throughout cells. In this study, we found that the peptide influx was accompanied by the formation of unique, "particle-like" multivesicular structures on the plasma membrane, together with topical inversion of the plasma membrane. Importantly, the conjugation of dodeca-arginine (R12) to Alexa Fluor 488 or a peptide tag derived from hemagglutinin (HAtag) significantly accelerated particle formation, suggesting that the chemical properties of the attached molecules (cargo molecules) may contribute to translocation of the R12 peptide. Coincubation with R12-HAtag allowed the membrane-impermeable R4-Alexa488 to permeate cells. These results suggest that R12 peptides attached to hydrophobic cargo molecules stimulate dynamic morphological alterations in the plasma membrane, and that these structural changes allow the peptides to permeate the plasma membrane. These findings may provide a novel mode of cell permeabilization by arginine-rich peptides as a means of drug delivery.

Tubular structures in heterogeneous membranes induced by the cell penetrating peptide penetratin

The delivery of active molecules into cells requires the efficient translocation of the plasma membrane barrier. Penetratin is a promising cell penetrating peptide is which crosses the cell membrane by a receptor and metabolic energy-independent mechanism. In previous work, we have shown that basic peptides induce membrane invaginations (i.e., tubes formation by induction of negative curvature of membranes) suggesting a new mechanism for cellular uptake of cell penetrating peptides: "physical endocytosis". These effects on membrane curvature are favored in pure liquid disordered but not in pure liquid ordered (raft-like) membrane domains. Herein, we present experiments in heterogeneous membranes composed of mixed domains. The results show that Penetratin is able to induce invaginations in membranes in which liquid ordered and liquid disordered membranes coexist. We suggest that Penetratin is able to recruit specific lipids locally forming fluid membrane patches dispersed inside a liquid ordered membrane zone resulting in the invagination of tubes composed of heterogeneous membrane domains.

Distinct behaviour of the homeodomain derived cell penetrating peptide penetratin in interaction with different phospholipids

Background

Penetratin is a protein transduction domain derived from the homeoprotein Antennapedia. Thereby it is currently used as a cell penetrating peptide to introduce diverse molecules into eukaryotic cells, and it could also be involved in the cellular export of transcription factors. Moreover, it has been shown that it is able to act as an antimicrobial agent. The mechanisms involved in all these processes are quite controversial.

Methodology/Principal Findings

In this article, we report spectroscopic, calorimetric and biochemical data on the penetratin interaction with three different phospholipids: phosphatidylcholine (PC) and phosphatidylethanolamine (PE) to mimic respectively the outer and the inner leaflets of the eukaryotic plasma membrane and phosphatidylglycerol (PG) to mimic the bacterial membrane. We demonstrate that with PC, penetratin is able to form vesicle aggregates with no major change in membrane fluidity and presents no well defined secondary structure organization. With PE, penetratin aggregates vesicles, increases membrane rigidity and acquires an α-helical structure. With PG membranes, penetratin does not aggregate vesicles but decreases membrane fluidity and acquires a structure with both α-helical and β–sheet contributions.

Conclusions/Significance

These data from membrane models suggest that the different penetratin actions in eukaryotic cells (membrane translocation during export and import) and on prokaryotes may result from different peptide and lipid structural arrangements. The data suggest that, for eukaryotic cell penetration, penetratin does not acquire classical secondary structure but requires a different conformation compared to that in solution.

Different molecular arrangements of the tetrameric annexin 2 modulate the size and dynamics of membrane aggregation

Annexin 2, a member of the annexin family of Ca2+-dependent membrane binding proteins is found in monomeric and heterotetrameric forms and has been involved in different membrane related functions. The heterotetrameric annexin 2 is composed of a dimer of S100A10, a member of the S100 family of Ca2+ binding proteins and two annexin 2 molecules ((Anx2-S100A10)2). Different molecular models including tetramers and octamers in which S100A10 is localized in the centre of the complex with the annexin 2 molecules positioned around S100A10 had been proposed. Herein, the organization of the (Anx2-S100A10)2 complex in conditions in which membranes are able to bridge was studied. We performed Cryo-electron microscopy observations of the tetrameric annexin 2 on the membrane surface, and study the S100A10 accessibility to antibodies by flow "cytometry". We also studied the kinetics and size evolution of vesicle aggregates by dynamic light scattering. The results show that the protein is able to organize in three different arrangements depending on the presence of Ca2+ and pH and that the aggregation is faster in the presence of Ca2+ compared with the aggregation in its absence. In one arrangement the S100A10 molecule is exposed to the solvent allowing its interaction with other proteins. The presented results will serve as a molecular basis to explain some of the functions of the tetrameric annexin 2.