Non-metabolic membrane tubulation and permeability induced by bioactive peptides

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.

Critical contribution of the intracellular C-terminal region to TRESK channel activity is revealed by the epithelial Na+ current ratio (ENaR) method

TRESK (K_2P18.1) possesses unique structural proportions within the K_2P background potassium channel family. The previously described TRESK regulatory mechanisms are based on the long intracellular loop between the second and third transmembrane segments (TMS). However, the functional significance of the exceptionally short intracellular C-terminal region (iCtr) following the fourth TMS has not yet been examined. In the present study, we investigated TRESK constructs modified at the iCtr by two-electrode voltage clamp and the newly developed epithelial sodium current ratio (ENaR) method in Xenopus oocytes. The ENaR method allowed the evaluation of channel activity by exclusively using electrophysiology, and provided data that are otherwise not readily available under whole-cell conditions. TRESK homodimer was connected with two ENaC (epithelial Na⁺ channel) heterotrimers and the Na⁺ current was measured as an internal reference, proportional to the number of channels in the plasma membrane. Modifications of TRESK iCtr resulted in diverse functional effects, indicating a complex contribution of this region to K⁺ channel activity. Mutations of positive residues in proximal iCtr locked TRESK in a low activity, calcineurin-insensitive state, although this phosphatase binds to distant motifs in the loop region. Accordingly, mutations in proximal iCtr may prevent the transmission of modulation to the gating machinery. Replacing distal iCtr with a sequence designed to interact with the inner surface of the plasma membrane increased the activity of the channel to unprecedented levels, as indicated by ENaR and single channel measurements. In conclusion, the distal iCtr is a major positive determinant of TRESK function.

Recent Advances of Studies on Cell-Penetrating Peptides Based on Molecular Dynamics Simulations

With the ability to transport cargo molecules across cell membranes with low toxicity, cell-penetrating peptides (CPPs) have become promising candidates for next generation peptide-based drug delivery vectors. Over the past three decades since the first CPP was discovered, a great deal of work has been done on the cellular uptake mechanisms and the applications for the delivery of therapeutic molecules, and significant advances have been made. But so far, we still do not have a precise and unified understanding of the structure-activity relationship of the CPPs. Molecular dynamics (MD) simulations provide a method to reveal peptide-membrane interactions at the atomistic level and have become an effective complement to experiments. In this paper, we review the progress of the MD simulations on CPP-membrane interactions, including the computational methods and technical improvements in the MD simulations, the research achievements in the CPP internalization mechanism, CPP decoration and coupling, and the peptide-induced membrane reactions during the penetration process, as well as the comparison of simulated and experimental results.

Tubulation and dispersion of oil by bacterial growth on droplets

Bacteria on surfaces exhibit collective behaviors, such as active turbulence and active stresses, which result from their motility, growth, and interactions with their local surroundings. However, interfacial deformations on soft surfaces and liquid interfaces caused by active growth, particularly over long time scales, are not well understood. Here, we describe experimental observations on the emergence of tubular structures arising from the growth of rod-shaped bacteria at the interface of oil droplets in water. Using microfluidics and timelapse microscopy, the dimensions and extension rates of individual tubular structures as well as bulk bio-aggregate formation are quantified for hundreds of droplets over 72 hours. Tubular structures are comparable in length to the initial droplet radius and are composed of an outer shell of bacteria that stabilize an inner filament of oil. The oil filament breaks up into smaller microdroplets dispersed within the bacterial shell. This work provides insight into active stresses at deformable interfaces and improves our understanding of microbial oil biodegradation and its potential influence on the transport of droplets in the ocean water column.

The glycine arginine-rich domain of the RNA-binding protein nucleolin regulates its subcellular localization

Nucleolin is a multifunctional RNA Binding Protein (RBP) with diverse subcellular localizations, including the nucleolus in all eukaryotic cells, the plasma membrane in tumor cells, and the axon in neurons. Here we show that the glycine arginine rich (GAR) domain of nucleolin drives subcellular localization via protein-protein interactions with a kinesin light chain. In addition, GAR sequences mediate plasma membrane interactions of nucleolin. Both these modalities are in addition to the already reported involvement of the GAR domain in liquid-liquid phase separation in the nucleolus. Nucleolin transport to axons requires the GAR domain, and heterozygous GAR deletion mice reveal reduced axonal localization of nucleolin cargo mRNAs and enhanced sensory neuron growth. Thus, the GAR domain governs axonal transport of a growth controlling RNA-RBP complex in neurons, and is a versatile localization determinant for different subcellular compartments. Localization determination by GAR domains may explain why GAR mutants in diverse RBPs are associated with neurodegenerative disease.

Inactivation of HIV-1 in Polarized Infant Tonsil Epithelial Cells by Human Beta-Defensins 2 and 3 Tagged with the Protein Transduction Domain of HIV-1 Tat

Mother-to-child transmission (MTCT) of HIV-1 may occur during pregnancy, labor, and breastfeeding; however, the molecular mechanism of MTCT of virus remains poorly understood. Infant tonsil mucosal epithelium may sequester HIV-1, serving as a transient reservoir, and may play a critical role in MTCT. Innate immune proteins human beta-defensins 2 (hBD-2) and -3 may inactivate intravesicular virions. To establish delivery of hBD-2 and -3 into vesicles containing HIV-1, we tagged hBDs with the protein transduction domain (PTD) of HIV-1 Tat, which facilitates an efficient translocation of proteins across cell membranes. Our new findings showed that hBD-2 and -3 proteins tagged with PTD efficiently penetrated polarized tonsil epithelial cells by endocytosis and direct penetration. PTD-initiated internalization of hBD-2 and -3 proteins into epithelial cells led to their subsequent penetration of multivesicular bodies (MVB) and vacuoles containing HIV-1. Furthermore, PTD played a role in the fusion of vesicles containing HIV-1 with lysosomes, where virus was inactivated. PTD-initiated internalization of hBD-2 and -3 proteins into ex vivo tonsil tissue explants reduced the spread of virus from epithelial cells to CD4+ T lymphocytes, CD68+ macrophages, and CD1c+ dendritic cells, suggesting that this approach may serve as an antiviral strategy for inactivating intraepithelial HIV-1 and reducing viral MTCT.

Tubulation of Supported Lipid Bilayer Membranes Induced by Photosensitized Lipid Oxidation

We show that photosensitized phospholipid oxidation, initiated by the lipid-conjugated fluorophore TopFluor-PC, causes defects, namely, membrane tubes and vesicle-like structures, in supported lipid bilayers (SLBs). Lipid oxidation is detrimental to the integrity of the lipid molecules; when oxidized, they undergo a conformational expansion, which causes membrane tubes to protrude from the SLB. Lipid oxidation is verified by FT-IR spectroscopy, and area expansion is observed in Langmuir trough experiments. Upon growing to a critical length, the membrane tubes arising from SLBs rapidly undergo transition to vesicle-like structures. We find a correlation between the maximum tube length and the diameter of the resulting vesicle, suggesting the conservation of the surface area between these features. We use geometric modeling and the measured tube length and vesicle radius to calculate the tube radius; our calculated mean tube diameter of 243 nm is comparable to other groups' experimental findings. In the presence of fluid flow, membrane tubes can be extended to tens to hundreds of microns in length. SLBs composed of saturated lipids resist light-induced tubulation, and the inclusion of the lipophilic antioxidant α-tocopherol attenuates the tubulation process and increases the light intensity threshold for tubulation.

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.

Amphipathic Helices of Cellular Proteins Can Replace the Helix in M2 of Influenza A Virus with Only Small Effects on Virus Replication

M2 of influenza virus functions as a proton channel during virus entry. In addition, an amphipathic helix in its cytoplasmic tail plays a role during budding. It targets M2 to the assembly site where it inserts into the inner membrane leaflet to induce curvature that causes virus scission. Since vesicularization of membranes can be performed by a variety of amphiphilic peptides, we used reverse genetics to investigate whether the peptides can substitute for M2's helix. Virus could not be generated if M2's helix was deleted or replaced by a peptide predicted not to form an amphiphilic helix. In contrast, viruses could be rescued if the M2 helix was exchanged by helices known to induce membrane curvature. Infectious virus titers were marginally reduced if M2 contains the helix of the amphipathic lipid packing sensor from the Epsin N-terminal homology domain or the nonnatural membrane inducer RW16. Transmission electron microscopy of infected cells did not reveal unequivocal evidence that virus budding or membrane scission was disturbed in any of the mutants. Instead, individual virus mutants exhibit other defects in M2, such as reduced surface expression, incorporation into virus particles, and ion channel activity. The protein composition and specific infectivity were also altered for mutant virions. We conclude that the presence of an amphiphilic helix in M2 is essential for virus replication but that other helices can replace its basic (curvature-inducing) function.IMPORTANCE Influenza virus is unique among enveloped viruses since it does not rely on the cellular ESCRT machinery for budding. Instead, viruses encode their own scission machine, the M2 protein. M2 is targeted to the edge of the viral assembly site, where it inserts an amphiphilic helix into the membrane to induce curvature. Cellular proteins utilize a similar mechanism for scission of vesicles. We show that the helix of M2 can be replaced by helices from cellular proteins with only small effects on virus replication. No evidence was obtained that budding is disturbed, but individual mutants exhibit other defects in M2 that explain the reduced virus titers. In contrast, no virus could be generated if the helix of M2 is deleted or replaced by irrelevant sequences. These experiments support the concept that M2 requires an amphiphilic helix to induce membrane curvature, but its biophysical properties are more important than the amino acid sequence.

Fluctuations and conformational stability of a membrane patch with curvature inducing inclusions

Membranes with curvature inducing inclusions display a range of cooperative phenomena, which can be linked to biomembrane function, e.g. membrane tubulation, vesiculation, softening and spontaneous tension. We investigate how these phenomena are related for a fluctuating, framed membrane through analysis of a descretized membrane model by Monte Carlo simulation techniques. The membrane model is based on a dynamically triangulated surface equipped with non-interacting, up-down symmetry breaking inclusions where only terms coupled linearly to mean-curvature are maintained. We show that the lateral configurational entropy plays a key role for the mechanical properties of the semi-flexible membrane, e.g. a pronounced softening at intermediate inclusion coverages of the membrane and generation of membrane tension. Tensionless framed membranes will remain quasi-flat up to some threshold coverage, where a shape instability occurs with formation of pearling or tubular membranes, which below full coverage is associated with segregation of inclusions between the curved and flat membrane geometries. For inclusions with preference for highly curved membranes the instability appears at dilute inclusion coverages and is accompanied by strong configurational fluctuations.

Electrostatically Driven Encapsulation of Hydrophilic, Non-Conformational Peptide Epitopes into Liposomes

Since the first use of liposomes as carriers for antigens, much work has been done to elucidate the mechanisms involved in the encapsulation of vaccine-relevant biomolecules. However, only a few studies have specifically investigated the encapsulation of hydrophilic, non-conformational peptide epitopes. We performed comprehensive and systematic screening studies, in order to identify conditions that favor the electrostatic interaction of such peptides with lipid membranes. Moreover, we have explored bi-terminal sequence extension as an approach to modify the isoelectric point of peptides, in order to modulate their membrane binding behavior and eventually shift/expand the working range under which they can be efficiently encapsulated in an electrostatically driven manner. The findings of our membrane interaction studies were then applied to preparing peptide-loaded liposomes. Our results show that the magnitude of membrane binding observed in our exploratory in situ setup translates to corresponding levels of encapsulation efficiency in both of the two most commonly employed methods for the preparation of liposomes, i.e., thin-film hydration and microfluidic mixing. We believe that the methods and findings described in the present studies will be of use to a wide audience and can be applied to address the ongoing relevant issue of the efficient encapsulation of hydrophilic biomolecules.

Cell penetrating peptides: the potent multi-cargo intracellular carriers

Introduction: Cell penetrating peptides (CPPs) known as protein translocation domains (PTD), membrane translocating sequences (MTS), or Trojan peptides (TP) are able to cross biological membranes without clear toxicity using different mechanisms, and facilitate the intracellular delivery of a variety of bioactive cargos. CPPs could overcome some limitations of drug delivery and combat resistant strains against a broad range of diseases. Despite delivery of different therapeutic molecules by CPPs, they lack cell specificity and have a short duration of action. These limitations led to design of combined cargo delivery systems and subsequently improvement of their clinical applications. Areas covered: This review covers all our studies and other researchers in different aspects of CPPs such as classification, uptake mechanisms, and biomedical applications. Expert opinion: Due to low cytotoxicity of CPPs as compared to other carriers and final degradation to amino acids, they are suitable for preclinical and clinical studies. Generally, the efficiency of CPPs was suitable to penetrate the cell membrane and deliver different cargos to specific intracellular sites. However, no CPP-based therapeutic approach has approved by FDA, yet; because there are some disadvantages for CPPs including short half-life in blood, and nonspecific CPP-mediated delivery to normal tissue. Thus, some methods were used to develop the functions of CPPs in vitro and in vivo including the augmentation of cell specificity by activatable CPPs, specific transport into cell organelles by insertion of corresponding localization sequences, incorporation of CPPs into multifunctional dendrimeric or liposomal nanocarriers to improve selectivity and efficiency especially in tumor cells.

Annexin A2 expression and partners during epithelial cell differentiation

The members of the annexin family of calcium- and phospholipid-binding proteins participate in different cellular processes. Annexin A2 binds to S100A10, forming a functional heterotetrameric protein that has been involved in many cellular functions, such as exocytosis, endocytosis, cell junction formation, and actin cytoskeleton dynamics. Herein, we studied annexin A2 cellular movements and looked for its partners during epithelial cell differentiation. By using immunofluorescence, mass spectrometry (MS), and western blot analyses after S100A10 affinity column separation, we identified several annexin A2–S100A10 partner candidates. The association of putative annexin A2–S100A10 partner candidates obtained by MS after column affinity was validated by immunofluorescence and sucrose density gradient separation. The results show that three proteins are clearly associated with annexin A2: E-cadherin, actin, and caveolin 1. Overall, the data show that annexin A2 can associate with molecular complexes containing actin, caveolin 1, and flotillin 2 before epithelial differentiation and with complexes containing E-cadherin, actin, and caveolin 1, but not flotillin 2 after cell differentiation. The results indicate that actin, caveolin 1, and E-cadherin are the principal protein partners of annexin A2 in epithelial cells and that the serine phosphorylation of the N-terminal domain does not play an essential role during epithelial cell differentiation.

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.

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.

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.

Arginine-rich cell-penetrating peptides induce membrane multilamellarity and subsequently enter via formation of a fusion pore

Arginine-rich cell-penetrating peptides do not enter cells by directly passing through a lipid membrane; they instead passively enter vesicles and live cells by inducing membrane multilamellarity and fusion. The molecular picture of this penetration mode, which differs qualitatively from the previously proposed direct mechanism, is provided by molecular dynamics simulations. The kinetics of vesicle agglomeration and fusion by an iconic cell-penetrating peptide-nonaarginine-are documented via real-time fluorescence techniques, while the induction of multilamellar phases in vesicles and live cells is demonstrated by a combination of electron and fluorescence microscopies. This concert of experiments and simulations reveals that the identified passive cell penetration mechanism bears analogy to vesicle fusion induced by calcium ions, indicating that the two processes may share a common mechanistic origin.

Arginine-rich cell-penetrating peptides induce membrane multilamellarity and subsequently enter via formation of a fusion pore

Arginine-rich cell-penetrating peptides do not enter cells by directly passing through a lipid membrane; they instead passively enter vesicles and live cells by inducing membrane multilamellarity and fusion. The molecular picture of this penetration mode, which differs qualitatively from the previously proposed direct mechanism, is provided by molecular dynamics simulations. The kinetics of vesicle agglomeration and fusion by an iconic cell-penetrating peptide-nonaarginine-are documented via real-time fluorescence techniques, while the induction of multilamellar phases in vesicles and live cells is demonstrated by a combination of electron and fluorescence microscopies. This concert of experiments and simulations reveals that the identified passive cell penetration mechanism bears analogy to vesicle fusion induced by calcium ions, indicating that the two processes may share a common mechanistic origin.

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.

A retro-inverso cell-penetrating peptide for siRNA delivery

Background

Small interfering RNAs (siRNAs) are powerful tools to control gene expression. However, due to their poor cellular permeability and stability, their therapeutic development requires a specific delivery system. Among them, cell-penetrating peptides (CPP) have been shown to transfer efficiently siRNA inside the cells. Recently we developed amphipathic peptides able to self-assemble with siRNAs as peptide-based nanoparticles and to transfect them into cells. However, despite the great potential of these drug delivery systems, most of them display a low resistance to proteases.

Results

Here, we report the development and characterization of a new CPP named RICK corresponding to the retro-inverso form of the CADY-K peptide. We show that RICK conserves the main biophysical features of its L-parental homologue and keeps the ability to associate with siRNA in stable peptide-based nanoparticles. Moreover the RICK:siRNA self-assembly prevents siRNA degradation and induces inhibition of gene expression.

Conclusions

This new approach consists in a promising strategy for future in vivo application, especially for targeted anticancer treatment (e.g. knock-down of cell cycle proteins).Graphical abstract

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.

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.

DNA-histone complexes as ligands amplify cell penetration and nuclear targeting of anti-DNA antibodies via energy-independent mechanisms

We have generated three monoclonal cell-penetrating antibodies (CPAbs) from a non-immunized lupus-prone (NZB × NZW)F1 mouse that exhibited high anti-DNA serum titres. These CPAbs are polyreactive because they bind to DNA and other cellular components, and localize mainly in the nucleus of HeLa cells, albeit with a distinct nuclear labelling profile. Herein, we have examined whether DNA-histone complexes (DHC) binding to CPAbs, before cell entry, could modify the cell penetration of CPAbs or their nuclear staining properties. By applying confocal microscopy and image analysis, we found that extracellular binding of purified CPAbs to DHC significantly enhanced their subsequent cell-entry, both in terms of percentages of positively labelled cells and fluorescence intensity (internalized CPAb amount), whereas there was a variable effect on their nuclear staining profile. Internalization of CPAbs, either alone or bound to DHC, remained unaltered after the addition of endocytosis-specific inhibitors at 37° or assay performance at 4°, suggesting the involvement of energy-independent mechanisms in the internalization process. These findings assign to CPAbs a more complex pathogenetic role in systemic lupus erythematosus where both CPAbs and nuclear components are abundant.

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.

Paracentrin 1, a synthetic antimicrobial peptide from the sea-urchin Paracentrotus lividus, interferes with staphylococcal and Pseudomonas aeruginosa biofilm formation

The rise of antibiotic-resistance as well as the reduction of investments by pharmaceutical companies in the development of new antibiotics have stimulated the investigation for alternative strategies to conventional antibiotics. Many antimicrobial peptides show a high specificity for prokaryotes and a low toxicity for eukaryotic cells and, due to their mode of action the development of resistance is considered unlikely. We recently characterized an antimicrobial peptide that was called Paracentrin 1 from the 5-kDa peptide fraction from the coelomocyte cytosol of the Paracentrotus lividus. In this study, the chemically synthesized Paracentrin 1, was tested for its antimicrobial and antibiofilm properties against reference strains of Gram positive and Gram negative. The Paracentrin 1 was active against planktonic form of staphylococcal strains (reference and isolates) and Pseudomonas aeruginosa ATCC 15442 at concentrations ranging from 12.5 to 6.2 mg/ml. The Paracentrin 1 was able to inhibit biofilm formation of staphylococcal and Pseudomonas aeruginosa strains at concentrations ranging from 3.1 to 0.75 mg/ml. We consider the tested peptide as a good starting molecule for novel synthetic derivatives with improved pharmaceutical potential.

Membrane changes under oxidative stress: the impact of oxidized lipids

Studying photosensitized oxidation of unsaturated phospholipids is of importance for understanding the basic processes underlying photodynamic therapy, photoaging and many other biological dysfunctions. In this review we show that the giant unilamellar vesicle, when used as a simplified model of biological membranes, is a powerful tool to investigate how in situ photogenerated oxidative species impact the phospholipid bilayer. The extent of membrane damage can be modulated by choosing a specific photosensitizer (PS) which is activated by light irradiation and can react by either type I and or type II mechanism. We will show that type II PS generates only singlet oxygen which reacts to the phospholipid acyl double bond. The byproduct thus formed is a lipid hydroperoxide which accumulates in the membrane as a function of singlet oxygen production and induces an increase in its area without significantly affecting membrane permeability. The presence of a lipid hydroperoxide can also play an important role in the formation of the lipid domain for mimetic plasma membranes. Lipid hydroperoxides can be also transformed in shortened chain compounds, such as aldehydes and carboxylic acids, in the presence of a PS that reacts via the type I mechanism. The presence of such byproducts may form hydrophilic pores in the membrane for moderate oxidative stress or promote membrane disruption for massive oxidation. Our results provide a new tool to explore membrane response to an oxidative stress and may have implications in biological signaling of redox misbalance.

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.

Therapeutic potential of cell-penetrating peptides

The ability of cell-penetrating peptides to cross plasma membranes has been used for various applications, including the delivery of bioactive molecules to inhibit disease-producing cellular mechanisms. Selective drug delivery into target cells improves drug distribution and decreases dosing and toxicity. In this review, the authors outline the main challenges in the field, namely clarification of mechanisms of entry into cells, as well as current and future perspectives regarding cell-penetrating peptides application for human therapeutics. Here, the authors discuss some of the factors that influence efficacy of delivery and review the current status of preclinical studies and clinical trials involving the use of cell-penetrating peptide-mediated delivery of therapeutics.

The interaction of polyglutamine peptides with lipid membranes is regulated by flanking sequences associated with huntingtin

Huntington disease (HD) is caused by an expanded polyglutamine (poly(Q)) repeat near the N terminus of the huntingtin (htt) protein. Expanded poly(Q) facilitates formation of htt aggregates, eventually leading to deposition of cytoplasmic and intranuclear inclusion bodies containing htt. Flanking sequences directly adjacent to the poly(Q) domain, such as the first 17 amino acids on the N terminus (Nt17) and the polyproline (poly(P)) domain on the C-terminal side of the poly(Q) domain, heavily influence aggregation. Additionally, htt interacts with a variety of membraneous structures within the cell, and Nt17 is implicated in lipid binding. To investigate the interaction between htt exon1 and lipid membranes, a combination of in situ atomic force microscopy, Langmuir trough techniques, and vesicle permeability assays were used to directly monitor the interaction of a variety of synthetic poly(Q) peptides with different combinations of flanking sequences (KK-Q35-KK, KK-Q35-P10-KK, Nt17-Q35-KK, and Nt17-Q35-P10-KK) on model membranes and surfaces. Each peptide aggregated on mica, predominately forming extended, fibrillar aggregates. In contrast, poly(Q) peptides that lacked the Nt17 domain did not appreciably aggregate on or insert into lipid membranes. Nt17 facilitated the interaction of peptides with lipid surfaces, whereas the poly(P) region enhanced this interaction. The aggregation of Nt17-Q35-P10-KK on the lipid bilayer closely resembled that of a htt exon1 construct containing 35 repeat glutamines. Collectively, this data suggests that the Nt17 domain plays a critical role in htt binding and aggregation on lipid membranes, and this lipid/htt interaction can be further modulated by the presence of the poly(P) domain.

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.

Degradation of an old human protein: age-dependent cleavage of γS-crystallin generates a peptide that binds to cell membranes

Long-lived proteins exist in a number of tissues in the human body; however, little is known about the reactions involved in their degradation over time. Lens proteins, which do not turn over, provide a useful system to examine such processes. Using a combination of Western blotting and proteomic methodology, age-related changes to a major protein, γS-crystallin, were studied. By teenage years, insoluble intact γS-crystallin was detected, indicative of protein denaturation. This was not the only change, however, because blots revealed evidence of significant cross-linking as well as cleavage of γS-crystallin in all adult lenses. Cleavage at a serine residue near the C terminus was a major reaction that caused the release of a 12-residue peptide, SPAVQSFRRIVE, which bound tightly to lens cell membranes. Several other crystallin-derived peptides with double basic residues also lodged in the cell membrane fraction. Model studies showed that once cleaved from γS-crystallin, SPAVQSFRRIVE adopts a markedly different shape from that in the intact protein. Further, the acquired helical conformation may explain why the peptide seems to affect water permeability. This observation may help explain the changes to cell membranes known to be associated with aging in human lenses. Age-related cleavage of long-lived proteins may therefore yield peptides with untoward biological activity.

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.

Perforin activity at membranes leads to invaginations and vesicle formation

The cytotoxic cell granule secretory pathway is essential for immune defence. How the pore-forming protein perforin (PFN) facilitates the cytosolic delivery of granule-associated proteases (granzymes) remains enigmatic. Here we show that PFN is able to induce invaginations and formation of complete internal vesicles in giant unilamellar vesicles. Formation of internal vesicles depends on native PFN and calcium and antibody labeling shows the localization of PFN at the invaginations. This vesiculation is recapitulated in large unilamellar vesicles and in this case PFN oligomers can be seen associated with the necks of the invaginations. Capacitance measurements show PFN is able to increase a planar lipid membrane surface area in the absence of pore formation, in agreement with the ability to induce invaginations. Finally, addition of PFN to Jurkat cells causes the formation of internal vesicles prior to pore formation. PFN is capable of triggering an endocytosis-like event in addition to pore formation, suggesting a new paradigm for its role in delivering apoptosis-inducing granzymes into target cells.

Revisiting peptide amphiphilicity for membrane pore formation

It has previously been shown that an amphipathic de novo designed peptide made of 10 leucines and four phenylalanines substituted with crown ethers induces vesicle leakage without selectivity. To gain selectivity against negatively charged dimyristoylphosphatidylglycerol (DMPG) bilayers, one or two leucines of the peptide were substituted with positively charged residues at each position. All peptides induce significant calcein leakage of DMPG vesicles. However, some peptides do not induce significant leakage of zwitterionic dimyristoylphosphatidylcholine vesicles and are thus active against only bacterial model membranes. The intravesicular leakage is induced by pore formation instead of membrane micellization. Nonselective peptides are mostly helical, while selective peptides mainly adopt an intermolecular β-sheet structure. This study therefore demonstrates that the position of the lysine residues significantly influences the secondary structure and bilayer selectivity of an amphipathic 14-mer peptide, with β-sheet peptides being more selective than helical peptides.

Identification and characterization of a new family of cell-penetrating peptides: cyclic cell-penetrating peptides

Cell-penetrating peptides can translocate across the plasma membrane of living cells and thus are potentially useful agents in drug delivery applications. Disulfide-rich cyclic peptides also have promise in drug design because of their exceptional stability, but to date only one cyclic peptide has been reported to penetrate cells, the Momordica cochinchinensis trypsin inhibitor II (MCoTI-II). MCoTI-II belongs to the cyclotide family of plant-derived cyclic peptides that are characterized by a cyclic cystine knot motif. Previous studies in fixed cells showed that MCoTI-II could penetrate cells but kalata B1, a prototypic cyclotide from a separate subfamily of cyclotides, was bound to the plasma membrane and did not translocate into cells. Here, we show by live cell imaging that both MCoTI-II and kalata B1 can enter cells. Kalata B1 has the same cyclic cystine knot structural motif as MCoTI-II but differs significantly in sequence, and the mechanism by which these two peptides enter cells also differs. MCoTI-II appears to enter via macropinocytosis, presumably mediated by interaction of positively charged residues with phosphoinositides in the cell membrane, whereas kalata B1 interacts directly with the membrane by targeting phosphatidylethanolamine phospholipids, probably leading to membrane bending and vesicle formation. We also show that another plant-derived cyclic peptide, SFTI-1, can penetrate cells. SFTI-1 includes just 14 amino acids and, with the exception of its cyclic backbone, is structurally very different from the cyclotides, which are twice the size. Intriguingly, SFTI-1 does not interact with any of the phospholipids tested, and its mechanism of penetration appears to be distinct from MCoTI-II and kalata B1. The ability of diverse disulfide-rich cyclic peptides to penetrate cells enhances their potential in drug design, and we propose a new classification for them, i.e. cyclic cell-penetrating peptides.

Octa-arginine mediated delivery of wild-type Lnk protein inhibits TPO-induced M-MOK megakaryoblastic leukemic cell growth by promoting apoptosis

Background

Lnk plays a non-redundant role by negatively regulating cytokine signaling of TPO, SCF or EPO. Retroviral expression of Lnk has been shown to suppress hematopoietic leukemic cell proliferation indicating its therapeutic value in cancer therapy. However, retroviral gene delivery carries risks of insertional mutagenesis. To circumvent this undesired consequence, we fused a cell permeable peptide octa-arginine to Lnk and evaluated the efficacy of inhibition of leukemic cell proliferation in vitro.

Methodology/Principal Findings

In this study, proliferation assays, flow cytometry, Western Blot analyses were performed on wild-type (WT), mutant Lnk R8 or BSA treated M-MOK cells. We found that delivered WT, but not mutant Lnk R8 blocked TPO-induced M-MOK megakaryoblastic leukemic cell proliferation. In contrast, WT Lnk R8 showed no growth inhibitive effect on non-hematopoietic HELA or COS-7 cell. Moreover, we demonstrated that TPO-induced M-MOK cell growth inhibition by WT Lnk R8 was dose-dependent. Penetrated WT Lnk R8 induced cell cycle arrest and apoptosis. Immunoprecipitation and Western blots data indicated WT Lnk R8 interacted with endogeneous Jak2 and downregulated Jak-Stat and MAPK phosphorylation level in M-MOK cells after TPO stimulation. Treatment with specific inhibitors (TG101348 and PD98059) indicated Jak-Stat and MAPK pathways were crucial for TPO-induced proliferation of M-MOK cells. Further analyses using TF-1 and HEL leukemic cell-lines showed that WT Lnk R8 inhibited Jak2-dependent cell proliferation. Using cord blood-derived CD34+ stem cells, we found that delivered WT Lnk R8 blocked TPO-induced megakaryopoiesis in vitro.

Conclusions/Significance

Intracellular delivery of WT Lnk R8 fusion protein efficiently inhibited TPO-induced M-MOK leukemic cell growth by promoting apoptosis. WT Lnk R8 protein delivery may provide a safer and more practical approach to inhibit leukemic cell growth worthy of further development.