Orientation and Conformation of Cell-Penetrating Peptide Penetratin in Phospholipid Vesicle Membranes Determined by Polarized-Light Spectroscopy

Systemic and brain delivery of antidiabetic peptides through nasal administration using cell-penetrating peptides

The intranasal route has emerged as a promising strategy that can direct delivery of drugs into the systemic circulation because the high-vascularized nasal cavity, among other advantages, avoids the hepatic first-pass metabolism. The nose-to-brain pathway provides a non-invasive alternative to other routes for the delivery of macromolecular therapeutics. A great variety of methodologies has been developed to enhance the efficiency of transepithelial translocation of macromolecules. Among these, the use of cell-penetrating peptides (CPPs), short protein transduction domains (PTDs) that facilitate the intracellular transport of various bioactive molecules, has become an area of extensive research in the intranasal delivery of peptides and proteins either to systemic or to brain compartments. Some CPPs have been applied for the delivery of peptide antidiabetics, including insulin and exendin-4, for treating diabetes and Alzheimer's disease. This review highlights the current status of CPP-driven intranasal delivery of peptide drugs and its potential applicability as a universal vehicle in the nasal drug delivery.

Membrane Association Modes of Natural Anticancer Peptides: Mechanistic Details on Helicity, Orientation, and Surface Coverage

Anticancer peptides (ACPs) could potentially offer many advantages over other cancer therapies. ACPs often target cell membranes, where their surface mechanism is coupled to a conformational change into helical structures. However, details on their binding are still unclear, which would be crucial to reach progress in connecting structural aspects to ACP action and to therapeutic developments. Here we investigated natural helical ACPs, Lasioglossin LL-III, Macropin 1, Temporin-La, FK-16, and LL-37, on model liposomes, and also on extracellular vesicles (EVs), with an outer leaflet composition similar to cancer cells. The combined simulations and experiments identified three distinct binding modes to the membranes. Firstly, a highly helical structure, lying mainly on the membrane surface; secondly, a similar, yet only partially helical structure with disordered regions; and thirdly, a helical monomeric form with a non-inserted perpendicular orientation relative to the membrane surface. The latter allows large swings of the helix while the N-terminal is anchored to the headgroup region. These results indicate that subtle differences in sequence and charge can result in altered binding modes. The first two modes could be part of the well-known carpet model mechanism, whereas the newly identified third mode could be an intermediate state, existing prior to membrane insertion.

New short cationic antibacterial peptides. Synthesis, biological activity and mechanism of action

We report a theoretical and experimental study on a new series of small-sized antibacterial peptides. Synthesis and bioassays for these peptides are reported here. In addition, we evaluated different physicochemical parameters that modulate antimicrobial activity (charge, secondary structure, amphipathicity, hydrophobicity and polarity). We also performed molecular dynamic simulations to assess the interaction between these peptides and their molecular target (the membrane). Biophysical characterization of the peptides was carried out with different techniques, such as circular dichroism (CD), linear dichroism (LD), infrared spectroscopy (IR), dynamic light scattering (DLS), fluorescence spectroscopy and TEM studies using model systems (liposomes) for mammalian and bacterial membranes. The results of this study allow us to draw important conclusions on three different aspects. Theoretical and experimental results indicate that small-sized peptides have a particular mechanism of action that is different to that of large peptides. These results provide additional support for a previously proposed four-step mechanism of action. The possible pharmacophoric requirement for these small-sized peptides is discussed. Furthermore, our results indicate that a net +4 charge is the adequate for 9 amino acid long peptides to produce antibacterial activity. The information reported here is very important for designing new antibacterial peptides with these structural characteristics.

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.

Probing Microscopic Orientation in Membranes by Linear Dichroism

The cell membrane is an ordered environment, which anisotropically affects the structure and interactions of all of its molecules. Monitoring membrane orientation at a local level is rather challenging but could reward crucial information on protein conformation and interactions in the lipid bilayer. We monitored local lipid ordering changes upon varying the cholesterol concentration using polarized light spectroscopy and pyrene as a membrane probe. Pyrene, with a shape intermediate between a disc and a rod, can detect microscopic orientation variations at the level of its size. The global membrane orientation was determined using curcumin, a probe with nonoverlapping absorption relative to that of pyrene. While the macroscopic orientation of a liquid-phase bilayer decreases with increasing cholesterol concentration, the local orientation is improved. Pyrene is found to be sensitive to the local effects induced by cholesterol and temperature on the bilayer. Disentangling local and global orientation effects in membranes could provide new insights into functionally significant interactions of membrane proteins.

Amphiphilic macromolecules on cell membranes: from protective layers to controlled permeabilization

Antimicrobial and cell-penetrating peptides have inspired developments of abiotic membrane-active polymers that can coat, penetrate, or break lipid bilayers in model systems. Application to cell cultures is more recent, but remarkable bioactivities are already reported. Synthetic polymer chains were tailored to achieve (i) high biocide efficiencies, and selectivity for bacteria (Gram-positive/Gram-negative or bacterial/mammalian membranes), (ii) stable and mild encapsulation of viable isolated cells to escape immune systems, (iii) pH-, temperature-, or light-triggered interaction with cells. This review illustrates these recent achievements highlighting the use of abiotic polymers, and compares the major structural determinants that control efficiency of polymers and peptides. Charge density, sp. of cationic and guanidinium side groups, and hydrophobicity (including polarity of stimuli-responsive moieties) guide the design of new copolymers for the handling of cell membranes. While polycationic chains are generally used as biocidal or hemolytic agents, anionic amphiphilic polymers, including Amphipols, are particularly prone to mild permeabilization and/or intracell delivery.

Investigation of homeodomain membrane translocation properties: insights from the structure determination of engrailed-2 homeodomain in aqueous and membrane-mimetic environments

In addition to their well-known DNA-binding properties, homeodomains have the ability to efficiently translocate across biological membranes through still poorly-characterized mechanisms. To date, most biophysical studies addressing the mechanisms of internalization have focused on small synthetic peptides rather than full-length globular homeodomains. In this work, we characterized the conformational properties of chicken Engrailed 2 homeodomain (En2HD) in aqueous solution and in membrane mimetic environments using circular dichroism, Trp fluorescence, and NMR spectroscopy. En2HD adopts a well-defined three-helical bundle fold in aqueous solution. The Trp-48 residue, which is critical for internalization, is fully buried in the hydrophobic core. Circular dichroism and fluorescence reveal that a conformational transition occurs in anionic lipid vesicles and in micelles. En2HD loses its native three-dimensional structure in micellar environments but, remarkably, near-native helical secondary structures are maintained. Long-range interactions could be detected using site-directed spin labels, indicating that the three helices do not adopt extended orientations. Noncovalent paramagnetic probes yielded information about helix positioning and unveiled the burial of critical aromatic and basic residues within the micelles. Our results suggest that electrostatic interactions with membranes may be determinant in inducing a conformational change enabling Trp-48 to insert into membranes.

Probing the orientational distribution of dyes in membranes through multiphoton microscopy

Numerous dyes are available or under development for probing the structural and functional properties of biological membranes. Exogenous chromophores adopt a range of orientations when bound to membranes, which have a drastic effect on their biophysical behavior. Here, we present a method that employs optical anisotropy data from three polarization-imaging techniques to establish the distribution of orientations adopted by molecules in monolayers and bilayers. The resulting probability density functions, which contain the preferred molecular tilt μ and distribution breadth γ, are more informative than an average tilt angle [φ]. We describe a methodology for the extraction of anisotropy data through an image-processing technology that decreases the error in polarization measurements by about a factor of four. We use this technique to compare di-4-ANEPPS and di-8-ANEPPS, both dipolar dyes, using data from polarized 1-photon, 2-photon fluorescence and second-harmonic generation imaging. We find that di-8-ANEPPS has a lower tilt but the same distributional width. We find the distribution of tilts taken by di-4-ANEPPS in two phospholipid membrane models: giant unilamellar vesicles and water-in-oil droplet monolayers. Both models result in similar distribution functions with average tilts of 52° and 47°, respectively.

Oral biodrug delivery using cell-penetrating peptide

During the past few decades, the novel biotherapeutic agents such as peptides and proteins have been contributed to the treatment of several diseases. However, their oral absorption is significantly limited due to their poor delivery through the intestinal mucosa. Therefore, the feasible approaches are needed for improving the oral bioavailability of biodrugs. Recently, cell-penetrating peptides (CPPs) such as HIV-1 Tat, penetratin and oligoarginine are considered as a useful tool for the intracellular delivery of therapeutic macromolecules. Hence, it was expected that the ability of CPPs may be applicable to enhance the absorption of biodrugs through intestinal epithelial membrane. CPPs are likely to become powerful tools for overcoming the low permeability of therapeutic peptides and proteins through the intestinal membrane, the major barrier to their oral delivery. Further advantage of this promising strategy is that this successful intestinal absorption could be achieved by more convenient methodology, coadministration of CPP with drugs via intermolecular interaction among them. Hereafter, the further establishment of delivery system based on CPPs is required to realize the development of the oral forms of therapeutic peptides and proteins. The aim here is to introduce our vision focusing on oral biodrug delivery by the use of CPPs as potential peptide carrier in order to provide new information in the design and development of new oral delivery systems for novel biotherapeutics.

Dissociation of protonated peptides containing adjacent arginines

The dissociation of protonated peptides containing adjacent arginines has been studied by electrospray ionization-tandem mass spectrometry (ESI MS/MS) and theoretical calculations. The experimental results show that singly protonated peptides cleave at the Arg-Arg amide bond and generate the y(1) ion when adjacent arginines are the C-terminal residues. The major cleavage occurs at the C-terminal amide bond and produces the b(n-1) ion when adjacent arginines are not the C-terminal residues. The diketopiperazine and oxazolone fragmentation pathways of protonated NRR (Asn-Arg-Arg) have been investigated at the B3LYP/6-31G(d) and B3LYP/6-31++G(d,p) levels of theory. The geometries and energies of transition state species and hydrogen-bonding interaction are also discussed.

Assigning membrane binding geometry of cytochrome C by polarized light spectroscopy

In this work we demonstrate how polarized light absorption spectroscopy (linear dichroism (LD)) analysis of the peptide ultraviolet-visible spectrum of a membrane-associated protein (cytochrome (cyt) c) allows orientation and structure to be assessed with quite high accuracy in a native membrane environment that can be systematically varied with respect to lipid composition. Cyt c binds strongly to negatively charged lipid bilayers with a distinct orientation in which its alpha-helical segments are on average parallel to the membrane surface. Further information is provided by the LD of the pi-pi( *) transitions of the heme porphyrin and transitions of aromatic residues, mainly a single tryptophan. A good correlation with NMR data was found, and combining NMR structural data with LD angular data allowed the whole protein to be docked to the lipid membrane. When the redox state of cyt c was changed, distinct variations in the LD spectrum of the heme Soret band were seen corresponding to changes in electronic transition energies; however, no significant change in the overall protein orientation or structure was observed. Cyt c is known to interact in a specific manner with the doubly negatively charged lipid cardiolipin, and incorporation of this lipid into the membrane at physiologically relevant levels was indeed found to affect the protein orientation and its alpha-helical content. The detail in which cyt c binding is described in this study shows the potential of LD spectroscopy using shear-deformed lipid vesicles as a new methodology for exploring membrane protein structure and orientation.

Self-assembled nanogaps via seed-mediated growth of end-to-end linked gold nanorods

Gold nanorods (AuNRs) are of interest for a wide range of applications, ranging from imaging to molecular electronics, and they have been studied extensively for the past decade. An important issue in AuNR applications is the ability to self-assemble the rods in predictable structures on the nanoscale. We here present a new way to end-to-end link AuNRs with a single or few linker molecules. Whereas methods reported in the literature so far rely on modification of the AuNRs after the synthesis, we here dimerize gold nanoparticle seeds with a water-soluble dithiol-functionalized polyethylene glycol linker and expose the linked seeds to growth conditions identical to the synthesis of unlinked AuNRs. Doing so, we obtain a large fraction of end-to-end linked rods, and transmission electron microscopy provides evidence of a 1-2 nm wide gap between the AuNRs. Flow linear dichroism demonstrates that a large fraction of the rods are flexible around the hinging molecule in solution, as expected for a molecularly linked nanogap. By using excess of gold nanoparticles relative to the linking dithiol molecule, this method can provide a high probability that a single molecule is connecting the two rods. In essence, our methods hence demonstrate the fabrication of a nanostructure with a molecule connected to two nanoelectrodes by bottom-up chemical assembly.

Effect of cell-penetrating peptides on the nasal absorption of insulin

The goal of this study was to evaluate whether cell-penetrating peptides (CPPs) affect the nasal absorption of insulin. L- or D-forms of penetratin, or the L- or D-forms of octaarginine (L- or D-R8), was used as first time for nasal insulin delivery. Furthermore, the concentration of lactate dehydrogenase (LDH) in nasal lavage fluid was determined and a histopathological study of nasal respiratory epithelium was conducted. CPPs dramatically increased nasal insulin absorption, and it was more pronounced for L- and D-penetratin than L- or D-R8. L-penetratin was the most effective promoter of insulin absorption compared with others CPPs. A dose-dependent relationship of L-penetratin and insulin bioavailability was statically significant. The pharmacological availability and bioavailability of nasally administered insulin was up to 76.7% and 50.7% relative to the subcutaneous route, respectively. In contrast, increasing the D-penetratin concentration decreased the efficiency of nasal insulin absorption. There was no significant difference in the release of LDH in nasal lavage fluid and the integrity of nasal respiratory epithelium when L-penetratin was present. In conclusion, these data demonstrate that L-penetratin markedly increased the permeability of insulin across the nasal membrane without causing detectable damage to the integrity of cells in the nasal respiratory mucosa.

Phospholipid membranes decorated by cholesterol-based oligonucleotides as soft hybrid nanostructures

DNA monomers and oligomers are currently showing great promise as building blocks for supramolecular arrays that can self-assemble in a fashion preprogrammed by the base pairing code. The design and build-up of hybrid DNA/amphiphilic self-assemblies can expand the range of possible architectures and enhance the selectivity toward a well-specified geometry. We report on the self-assembly properties in aqueous solution of a cholesteryl-tetraethylenglycol single stranded 18-mer oligonucleotide (ON 1TEG-Chol) and on its spontaneous insertion in fluid phospholipid membranes. Up to 500 units of these lipophilic ss-oligonucleotides can be incorporated in the outer leaflet of 350 A radius POPC vesicle. The insertion and hybridization with the complementary oligonucleotide are monitored through light scattering as an increase of hydrodynamic thickness, which is interpreted in terms of average distance between anchoring sites. The conformation of the ss-oligonucleotidic portion is strongly dependent on surface coverage, passing from a quasi-random coil to a more rigid configuration, as concentration increases. Interestingly, conformational details affect in a straightforward fashion the hybridization kinetics. Liposomes with single- and double-strand decorations remain stable within the experimental time window (about one week). The structure represents an example of successful and stable amphiphile/DNA supramolecular hybrid, where a DNA guest is held in a membrane by hydrophobic interactions. The lipophilic oligonucleotide under investigation is therefore a suitable building block that can effectively serve as a hydrophobic anchor in the fluid bilayer to assemble supramolecular constructs based on the DNA digital code.

Membrane-associated proteins and peptides

This chapter discusses the practical aspects of setting up molecular dynamics simulations for membrane-associated proteins and peptides. Special emphasis lies on the analysis of such systems. The main focus is the association between a cationic peptide and an anionic lipid bilayer--a peptide/lipid-bilayer system--but the extension onto more complicated systems is discussed. Topology files for selected lipids and several new analysis tools relevant for protein-membrane simulations are presented, the most important ones of which are: g_helixaxis, to calculate the axis of a helix and its angle with the bilayer; g_arom, to calculate aromatic order parameters; and g_under, to calculate which lipids interact with the protein. A procedure is explained to calculate properties involving peptide-interacting lipids only, as opposed to all lipids.

Membrane binding of pH-sensitive influenza fusion peptides. positioning, configuration, and induced leakage in a lipid vesicle model

pH-sensitive HA2 fusion peptides from influenza virus hemagglutinin have potential as endosomal escape-inducing components in peptide-based drug delivery. Polarized light spectroscopy and tryptophan fluorescence were used to assess the conformation, orientation, effect on lipid order, and binding kinetics of wild-type peptide HA2(1-23) and a glutamic acid-enriched analogue (INF7) in large unilamellar POPC or POPC/POPG (4:1) lipid vesicles (LUVs). pH-sensitive membrane leakage was established for INF7 but not HA2(1-23) using an entrapped-dye assay. A correlation is indicated between leakage and a low degree of lipid chain order (assessed by linear dichroism, LD, of the membrane orientation probe retinoic acid). Both peptides display poor alignment in zwitterionic POPC LUVs compared to POPC/POPG (4:1) LUVs, and it was found that peptide-lipid interactions display slow kinetics (hours), resulting in reduced lipid order and increased tryptophan shielding. At pH 7.4, INF7 displays tryptophan emission and LD features indicative of a surface-orientated peptide, suggesting that its N-terminal glutamic acid residues prevent deep penetration into the hydrocarbon core. At pH 5.0, INF7 displays weaker LD signals, indicating poor orientation, possibly due to aggregation. By contrast, the orientation of the HA2(1-23) peptide backbone supports previously reported oblique insertion ( approximately 60-65 degrees relative to the membrane normal), and aromatic side-chain orientations are consistent with an interfacial (pH-independent) location of the C-terminus. We propose that a conformational change upon reduction of pH is limited to minor rearrangements of the peptide "hinge region" around Trp14 and repositioning of this residue.

Tryptophan orientation in model lipid membranes

Tryptophans in membrane proteins display strong preference for the lipid membrane interface and are important for anchoring proteins at the proper longitudinal level. Linear dichroism spectroscopy on indoles in shear-deformed liposomes has been used to show that this positioning is accompanied by an intrinsically adopted orientation, also observed for tryptophans in membrane-bound peptides. Similarities in orientation of different indoles suggest that tryptophan will adopt this orientation independent of the protein it is part of. From the orientation of indole electronic transition moments L(a), L(b) and B(b), a binding model is proposed where the indole long axis is approximately 60-65 degrees from the membrane normal and the indole plane is at an oblique angle. We propose that dipole-dipole interactions and steric constraints in the membrane hydrocarbon region determine positioning and orientation of tryptophans whereas hydrogen bonding and cation-pi interactions with lipid head-groups, though contributing to the membrane affinity of indoles, are less important.

Retinoid Chromophores as Probes of Membrane Lipid Order

There is a great need for development of independent methods to study the structure and function of membrane-associated proteins and peptides. Polarized light spectroscopy (linear dichroism, LD) using shear-aligned lipid vesicles as model membranes has emerged as a promising tool for the characterization of the binding geometry of membrane-bound biomolecules. Here we explore the potential of retinoic acid, retinol, and retinal to function as probes of the macroscopic alignment of shear-deformed 100 nm liposomes. The retinoids display negative LD, proving their preferred alignment perpendicular to the membrane surface. The magnitude of the LD indicates the order retinoic acid > retinol > retinal regarding the degree of orientation in all tested lipid vesicle types. It is concluded that mainly nonspecific electrostatic interactions govern the apparent orientation of the retinoids within the bilayer. We propose a simple model for how the effective orientation may be related to the polarity of the end groups of the retinoid probes, their insertion depths, and their angular distribution of configurations around the membrane normal. Further, we provide evidence that the retinoids can sense subtle structural differences due to variations in membrane composition and we explore the pH sensitivity of retinoic acid, which manifests in variations in absorption maximum wavelength in membranes of varying surface charge. Based on LD measurements on cholesterol-containing liposomes, the influence of membrane constituents on bending rigidity and vesicle deformation is considered in relation to the macroscopic alignment, as well as to lipid chain order on the microscopic scale.

Non-metabolic membrane tubulation and permeability induced by bioactive peptides

Background

Basic cell-penetrating peptides are potential vectors for therapeutic molecules and display antimicrobial activity. The peptide-membrane contact is the first step of the sequential processes leading to peptide internalization and cell activity. However, the molecular mechanisms involved in peptide-membrane interaction are not well understood and are frequently controversial. Herein, we compared the membrane activities of six basic peptides with different size, charge density and amphipaticity: Two cell-penetrating peptides (penetratin and R9), three amphipathic peptides and the neuromodulator substance P.

Methodology/Principal Findings

Experiments of X ray diffraction, video-microscopy of giant vesicles, fluorescence spectroscopy, turbidimetry and calcein leakage from large vesicles are reported. Permeability and toxicity experiments were performed on cultured cells. The peptides showed differences in bilayer thickness perturbations, vesicles aggregation and local bending properties which form lipidic tubular structures. These structures invade the vesicle lumen in the absence of exogenous energy.

Conclusions/Significance

We showed that the degree of membrane permeabilization with amphipathic peptides is dependent on both peptide size and hydrophobic nature of the residues. We propose a model for peptide-induced membrane perturbations that explains the differences in peptide membrane activities and suggests the existence of a facilitated “physical endocytosis,” which represents a new pathway for peptide cellular internalization.

Membrane Interactions of Cell-Penetrating Peptides Probed by Tryptophan Fluorescence and Dichroism Techniques: Correlations of Structure to Cellular Uptake

This work reports on the binding and conformation of a series of CPPs in the bilayer membranes of large unilamellar vesicles and the effect of the presence of cholesterol. We show a negative correlation between alpha-helical structure and uptake efficiency for penetratin peptides where the two central arginine residues of penetratin are thought to be important for breaking the secondary structure. Penetratin alpha-helicity is also reduced upon incorporation of cholesterol into the membrane. Flow linear dichroism in the far-UV region shows that the penetratin peptides adopt a preferential orientation of the alpha-helix parallel to the bilayer, and the linear dichroism (LD) spectrum in the aromatic region indicates that the tryptophan residues are preferentially oriented parallel to the membrane. The Tat analogue TatP59W and the oligoarginine R7W, which are more efficient CPPs than penetratin, bind to membranes as random coils and do not show any orientation in LD, again indicating that alpha-helicity reduces uptake efficiency. Further, we observe large variations in tryptophan quantum yields for the five CPPs in this study and discuss this in terms of the ability to cause lipid rearrangement. Binding isotherms show that cholesterol increases the affinity of the peptide for the membrane, but tryptophan fluorescence lifetimes are essentially unaltered by incorporation of as much as 40 mol % cholesterol into the membrane, suggesting the absence of specific peptide-cholesterol interactions. Fluorescence emission maxima are insensitive to cholesterol and indicate that the peptide is positioned in the headgroup region. The results on peptide-membrane interactions are discussed in terms of possible uptake mechanisms.

Implicit solvent simulations of peptide interactions with anionic lipid membranes

A recently developed implicit membrane model (IMM1) is supplemented with a Gouy-Chapman term describing counterion-screened electrostatic interactions of a solute with negatively charged membrane lipids. The new model is tested on peptides that bind to anionic membranes. Pentalysine binds just outside the plane of negative charge, whereas Lys-Phe peptides insert their aromatic rings into the hydrophobic core. Melittin and magainin 2 bind more strongly to anionic than to neutral membranes and in both cases insert their hydrophobic residues into the hydrocarbon core. The third domain of Antennapedia homeodomain (penetratin) binds as an alpha-helix in the headgroup region. Cardiotoxin II binds strongly to anionic membranes but marginally to neutral ones. In all cases, the location and configuration of the peptides are consistent with experimental data, and the effective energy changes upon binding compare favorably with experimental binding free energies. The model opens the way to exploring the effect of membrane charge on the location, conformation, and dynamics of a large variety of biologically active peptides on membranes.

Break on through to the Other Side-Biophysics and Cell Biology Shed Light on Cell-Penetrating Peptides

Cell-penetrating peptides (CPPs) have become widely used vectors for the cellular import of molecules in basic and applied biomedical research. Despite the broad acceptance of these molecules as molecular carriers, the details of the mode of cellular internalization and membrane permeation remain elusive. Within the last two years endocytosis has been demonstrated to be a route of uptake shared by several CPPs. These findings had a significant impact on CPP research. State-of-the-art cell biology is now required to advance the understanding of the intracellular fate of the CPP and cargo molecules. Owing to their presumed ability to cross lipid bilayers, CPPs also represent highly interesting objects of biophysical research. Numerous studies have investigated structure-activity relationships of CPPs with respect to their ability to bind to a lipid bilayer or to cross this barrier. Endocytosis route only relocates the membrane permeation from the cell surface to endocytic compartments. Therefore, biophysical experiments are key to a mechanistic molecular understanding of the cellular uptake of CPPs. However, biophysical investigations have to consider the molecular environment encountered by a peptide inside and outside a cell. In this contribution we will review biophysical and cell-biology data obtained for several prominent CPPs. Furthermore, we will summarize recent findings on the cell-penetrating characteristics of antimicrobial peptides and the antimicrobial properties of CPPs. Peptides of both groups have overlapping characteristics. Therefore, both fields may greatly benefit from each other. The review will conclude with a perspective of how biophysics and cell biology may synergize even more efficiently in the future.

Lipid membrane-induced optimization for ligand-receptor docking: recent tools and insights for the "membrane catalysis" model

Cells in living organisms are regulated by chemical and physical stimuli from their environment. Often, ligands interact with membrane receptors to trigger responses and Sargent and Schwyzer conceived a model to describe this process, "membrane catalysis". There is a notion that the physical organization of membranes can control the response of cells by speeding up reactions. We revisit the "membrane catalysis" model in the light of recent technical, methodological and theoretical advances and how they can be exploited to highlight the details of membrane mediated ligand-receptor interactions. We examine the possible effects that ligand concentration causes in the membrane catalysis and focus our attention in techniques used to determine the partition constant. The hypothetical diffusional advantage associated with membrane catalysis is discussed and the applicability of existing models is assessed. The role of in-depth location and orientation of ligands is explored emphasizing the contribution of new analysis methods and spectroscopic techniques. Results suggest that membranes can optimize the interaction between ligands and receptors through several different effects but the relative contribution of each must be carefully investigated. We certainly hope that the conjugation of the methodological and technical advances here reported will revive the interest in the membrane catalysis model.

Translocation of histone proteins across lipid bilayers and Mycoplasma membranes

We show that the three core histones H2A, H3 and H4 can transverse lipid bilayers of large unilamellar vesicles (LUVs) and multilamellar vesicles (MLVs). In contrast, the histone H2B, although able to bind to the liposomes, fails to penetrate the unilamellar and the multilamellar vesicles. Translocation across the lipid bilayer was determined using biotin-labeled histones and an ELISA-based system. Following incubation with the liposomes, external membrane-bound biotin molecules were neutralized by the addition of avidin. Penetrating biotin-histone conjugates were exposed by Triton treatment of the neutralized liposomes. The intraliposomal biotin-histone conjugates, in contrast to those attached only to the external surface, were attached to the detergent lysed lipid molecules. Thus, biotinylated histone molecules that were exposed only following detergent treatment of the liposomes were considered to be located at the inner leaflet of the lipid bilayers. The penetrating histone molecules failed to mediate translocation of BSA molecules covalently attached to them. Translocation of the core histones, including H2B, was also observed across mycoplasma cell membranes. The extent of this translocation was inversely related to the degree of membrane cholesterol. The addition of cholesterol also reduced the extent of histone penetration into the MLVs. Although able to bind biotinylated histones, human erythrocytes, erythrocyte ghosts and Escherichia coli cells were impermeable to them. Based on the present and previous data histones appear to be characterized by the same features that characterize cell penetrating peptides and proteins (CPPs).

Vesicle size-dependent translocation of penetratin analogs across lipid membranes

The recent discoveries of serious artifacts associated with the use of cell fixation in studies of the cellular uptake of cell-penetrating peptides (CPPs) have prompted a reevaluation of the current understanding of peptide-mediated cellular delivery. Following a report on the differential cellular uptake of a number of penetratin analogs in unfixed cells, we here investigate their membrane translocation abilities in large and giant unilamellar vesicles (LUVs and GUVs, respectively). Surprisingly, in contrast to the behavior in living cells, all peptides readily entered the giant vesicles (>1 microm) as proved by confocal microscopy, while none of them could cross the membranes of LUVs (100 nm). For determination of the location of the peptides in the LUVs, a new concept was introduced, based on sensitive resonance energy transfer (RET) measurements of the enhanced fluorescence of acceptor fluorophores present solely in the inner leaflet. An easily adopted method to prepare such asymmetrically labeled liposomes is described. The membrane insertion depths of the tryptophan moieties of the peptides were determined by use of brominated lipids and found to be very similar for all of the peptides studied. We also demonstrate that infrared spectroscopy on the lipid carbonyl stretch vibration peak is a convenient technique to determine phospholipid concentration.

Penetratin-membrane association: W48/R52/W56 shield the peptide from the aqueous phase

Using molecular dynamics simulations, we studied the mode of association of the cell-penetrating peptide penetratin with both a neutral and a charged bilayer. The results show that the initial peptide-lipid association is a fast process driven by electrostatic interactions. The homogeneous distribution of positively charged residues along the axis of the helical peptide, and especially residues K46, R53, and K57, contribute to the association of the peptide with lipids. The bilayer enhances the stability of the penetratin helix. Oriented parallel to the lipid-water interface, the subsequent insertion of the peptide through the bilayer headgroups is significantly slower. The presence of negatively charged lipids considerably enhances peptide binding. Lateral side-chain motion creates an opening for the helix into the hydrophobic core of the membrane. The peptide aromatic residues form a pi-stacking cluster through W48/R52/W56 and F49/R53, protecting the peptide from the water phase. Interaction with the penetratin peptide has only limited effect on the overall membrane structure, as it affects mainly the conformation of the lipids which interact directly with the peptide. Charge matching locally increases the concentration of negatively charged lipids, lateral lipid diffusion locally decreases. Lipid disorder increases, through decreased order parameters of the lipids interacting with the penetratin side chains. Penetratin molecules at the membrane surface do not seem to aggregate.

Fluorescence imaging of two-photon linear dichroism: cholesterol depletion disrupts molecular orientation in cell membranes

The plasma membrane of cells is an ordered environment, giving rise to anisotropic orientation and restricted motion of molecules and proteins residing in the membrane. At the same time as being an organized matrix of defined structure, the cell membrane is heterogeneous and dynamic. Here we present a method where we use fluorescence imaging of linear dichroism to measure the orientation of molecules relative to the cell membrane. By detecting linear dichroism as well as fluorescence anisotropy, the orientation parameters are separated from dynamic properties such as rotational diffusion and homo energy transfer (energy migration). The sensitivity of the technique is enhanced by using two-photon excitation for higher photo-selection compared to single photon excitation. We show here that we can accurately image lipid organization in whole cell membranes and in delicate structures such as membrane nanotubes connecting two cells. The speed of our wide-field imaging system makes it possible to image changes in orientation and anisotropy occurring on a subsecond timescale. This is demonstrated by time-lapse studies showing that cholesterol depletion rapidly disrupts the orientation of a fluorophore located within the hydrophobic region of the cell membrane but not of a surface bound probe. This is consistent with cholesterol having an important role in stabilizing and ordering the lipid tails within the plasma membrane.

Linear dichroism of biomolecules: which way is up?

Understanding the organization of molecules in naturally occurring ordered arrays (e.g. membranes, protein fibres and DNA strands) is of great importance to understanding biological function. Unfortunately, few biophysical techniques provide detailed structural information on these non-crystalline systems. UV, visible and IR linear dichroism have the potential to provide such information. Recent advances in technology and simulations allow this potential to be fulfilled, and can now provide a detailed understanding of the molecular mechanisms of such fundamental biological processes as amyloid fibre formation and membrane protein folding.