Interaction of S413-PV cell penetrating peptide with model membranes: relevance to peptide translocation across biological membranes

Vogel H, Park Y. Antifungal and Antibiofilm Activities and the Mechanism of Action of Repeating Lysine-Tryptophan Peptides against Candida albicans

The rapid increase in the emergence of antifungal-resistant Candida albicans strains is becoming a serious health concern. Because antimicrobial peptides (AMPs) may provide a potential alternative to conventional antifungal agents, we have synthesized a series of peptides with a varying number of lysine and tryptophan repeats (KWn-NH2). The antifungal activity of these peptides increased with peptide length, but only the longest KW5 peptide displayed cytotoxicity towards a human keratinocyte cell line. The KW4 and KW5 peptides exhibited strong antifungal activity against C. albicans, even under conditions of high-salt and acidic pH, or the addition of fungal cell wall components. Moreover, KW4 inhibited biofilm formation by a fluconazole-resistant C. albicans strain. Circular dichroism and fluorescence spectroscopy indicated that fungal liposomes could interact with the longer peptides but that they did not release the fluorescent dye calcein. Subsequently, fluorescence assays with different dyes revealed that KW4 did not disrupt the membrane integrity of intact fungal cells. Scanning electron microscopy showed no changes in fungal morphology, while laser-scanning confocal microscopy indicated that KW4 can localize into the cytosol of C. albicans. Gel retardation assays revealed that KW4 can bind to fungal RNA as a potential intracellular target. Taken together, our data indicate that KW4 can inhibit cellular functions by binding to RNA and DNA after it has been translocated into the cell, resulting in the eradication of C. albicans.

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.

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.

Acylation of the S413-PV cell-penetrating peptide as a means of enhancing its capacity to mediate nucleic acid delivery: Relevance of peptide/lipid interactions

BACKGROUND

Cell-penetrating peptides (CPPs) have been extensively exploited in gene therapy approaches as vectors for intracellular delivery of bioactive molecules. The ability of CPPs to be internalized into cells and their capacity to complex nucleic acids depend on their molecular structure, both primary and secondary, namely regarding hydrophobicity/hydrophilicity. CPP acylation has been used as a strategy to improve this structural feature.

METHODS

Acyl groups (from 6 to 18 carbon atoms) were attached to the S4₁₃-PV peptide and their effects on the peptide competence to complex siRNAs and to mediate gene silencing in glioblastoma (GBM) cells were studied. A systematic characterization of membrane interactions with S4₁₃-PV acyl-derivatives was also conducted, using different biophysical techniques (surface pressure-area isotherms in Langmuir monolayers, DSC and ³¹P NMR) to unravel a relationship between CPP biological activity and CPP effects on membrane stability and lipid organization.

RESULTS

A remarkable concordance was noticed between acylated-S4₁₃-PV peptide competence to promote gene silencing in GBM cells and disturbance induced in membrane models, the lauroyl- and myristoyl-S4₁₃-PV peptides being the most effective. A cut-off effect was described for the first time regarding the influence of acyl-chain length on CPP bioactivity.

CONCLUSIONS

C12-S4₁₃-PV showed high capacity to destabilize lipid bilayers, to escape from lysosomal degradation and to mediate gene silencing without promoting cytotoxicity.

GENERAL SIGNIFICANCE

Besides unraveling a new CPP with high potential to be employed as a gene delivery vector, this work emphasizes the benefit from allying biophysical and biological studies towards a proper CPP structural refinement for successful pre-clinical/clinical application.

Helical Poly(arginine) Mimics with Superior Cell-Penetrating and Molecular Transporting Properties

Poly(arginine) mimics bearing long hydrophobic side chains adopt stable helical conformation and exhibit helix-related cell-penetrating properties. Elongating polypeptide backbone length and increasing side chain hydrophobicity further increase the helicities of poly(arginine) mimics. They show superior cell membrane permeability up to two orders of magnitude higher than that of HIV-TAT peptide and excellent DNA and siRNA delivery efficiencies in various mammalian cells.

Structural polymorphism of non-covalent peptide-based delivery systems: highway to cellular uptake

During the last two decades, delivery has become a major challenge for the development of new therapeutic molecules for the clinic. Although, several strategies either viral or non viral have been proposed to favor cellular uptake and targeting of therapeutics, only few of them have reach preclinical evaluation. Amongst them, cell-penetrating peptide (CPP) constitutes one of the most promising strategy and has applied for systemic in vivo delivery of a variety of therapeutic molecules. Two CPP-strategies have been described; using peptide carriers either covalently-linked to the cargo or forming non-covalent stable complexes with cargo. Peptide-based nanoparticle delivery system corresponds to small amphipathic peptides able to form stable nanoparticles with either proteins/peptides or nucleic acids and to enter the cell independently of the endosomal pathway. Three families of peptide-based nanoparticle systems; MPG, PEP and CADY have been successfully used for the delivery of various biologically active cargoes both ex vivo and in vivo in several animal models. This review will focus on the mechanism of the peptide-based nanoparticles; PEP, MPG and CADY in a structural and biophysical context. It will also highlight the major parameters associated to particle formation/stabilization and the impact of the carrier structural polymorphism in triggering cellular uptake.

Cell-Penetrating Peptides-Mechanisms of Cellular Uptake and Generation of Delivery Systems

The successful clinical application of nucleic acid-based therapeutic strategies has been limited by the poor delivery efficiency achieved by existing vectors. The development of alternative delivery systems for improved biological activity is, therefore, mandatory. Since the seminal observations two decades ago that the Tat protein, and derived peptides, can translocate across biological membranes, cell-penetrating peptides (CPPs) have been considered one of the most promising tools to improve non-invasive cellular delivery of therapeutic molecules. Despite extensive research on the use of CPPs for this purpose, the exact mechanisms underlying their cellular uptake and that of peptide conjugates remain controversial. Over the last years, our research group has been focused on the S4₁₃-PV cell-penetrating peptide, a prototype of this class of peptides that results from the combination of 13-amino-acid cell penetrating sequence derived from the Dermaseptin S4 peptide with the SV40 large T antigen nuclear localization signal. By performing an extensive biophysical and biochemical characterization of this peptide and its analogs, we have gained important insights into the mechanisms governing the interaction of CPPs with cells and their translocation across biological membranes. More recently, we have started to explore this peptide for the intracellular delivery of nucleic acids (plasmid DNA, siRNA and oligonucleotides). In this review we discuss the current knowledge of the mechanisms responsible for the cellular uptake of cell-penetrating peptides, including the S4₁₃-PV peptide, and the potential of peptide-based formulations to mediate nucleic acid delivery.

A non-covalent strategy combining cationic lipids and CPPs to enhance the delivery of splice correcting oligonucleotides

Modulation of pre-mRNA splicing by steric-block oligonucleotides constitutes a promising strategy for the treatment of many diseases, but requires efficient delivery to cell nuclei. In the present study, we evaluated the efficacy of a non-covalent strategy that combines a cell penetrating peptide with a lipoplex-based formulation to mediate the delivery of splice-switching oligonucleotides. The splice correcting ability of these new formulations was assessed using splice-switching oligonucleotides targeted towards the mutated splicing site of human beta-globin pre-mRNA in the HeLa pLuc/705 splice correction model. Importantly, the optimal splice correcting activity was exhibited by the formulation containing both lipid and peptide components, the order of component addition in these formulations being crucial for their efficacy. Our results demonstrate that the inclusion of cationic liposomes in the formulation provides the ability to improve release from endocytic vesicles, a barrier that severely limits the efficiency of oligonucleotide delivery by cell penetrating peptides. On the other hand, cell penetrating peptides potentiate the cellular uptake and delivery of the oligonucleotides by the lipoplexes. Moreover, when combining cell penetrating peptides with the lipoplex formulations, a significant reduction in the amount of required cationic lipid could be achieved, while maintaining or even enhancing biological activity.

Biophysical interactions with model lipid membranes: applications in drug discovery and drug delivery

The transport of drugs or drug delivery systems across the cell membrane is a complex biological process, often difficult to understand because of its dynamic nature. In this regard, model lipid membranes, which mimic many aspects of cell-membrane lipids, have been very useful in helping investigators to discern the roles of lipids in cellular interactions. One can use drug-lipid interactions to predict pharmacokinetic properties of drugs, such as their transport, biodistribution, accumulation, and hence efficacy. These interactions can also be used to study the mechanisms of transport, based on the structure and hydrophilicity/hydrophobicity of drug molecules. In recent years, model lipid membranes have also been explored to understand their mechanisms of interactions with peptides, polymers, and nanocarriers. These interaction studies can be used to design and develop efficient drug delivery systems. Changes in the lipid composition of cells and tissue in certain disease conditions may alter biophysical interactions, which could be explored to develop target-specific drugs and drug delivery systems. In this review, we discuss different model membranes, drug-lipid interactions and their significance, studies of model membrane interactions with nanocarriers, and how biophysical interaction studies with lipid model membranes could play an important role in drug discovery and drug delivery.

Recent developments in peptide-based nucleic acid delivery

Despite the fact that non-viral nucleic acid delivery systems are generally considered to be less efficient than viral vectors, they have gained much interest in recent years due to their superior safety profile compared to their viral counterpart. Among these synthetic vectors are cationic polymers, branched dendrimers, cationic liposomes and cell-penetrating peptides (CPPs). The latter represent an assortment of fairly unrelated sequences essentially characterised by a high content of basic amino acids and a length of 10–30 residues. CPPs are capable of mediating the cellular uptake of hydrophilic macromolecules like peptides and nucleic acids (e.g. siRNAs, aptamers and antisense-oligonucleotides), which are internalised by cells at a very low rate when applied alone. Up to now, numerous sequences have been reported to show cell-penetrating properties and many of them have been used to successfully transport a variety of different cargos into mammalian cells. In recent years, it has become apparent that endocytosis is a major route of internalisation even though the mechanisms underlying the cellular translocation of CPPs are poorly understood and still subject to controversial discussions. In this review, we will summarise the latest developments in peptide-based cellular delivery of nucleic acid cargos. We will discuss different mechanisms of entry, the intracellular fate of the cargo, correlation studies of uptake versus biological activity of the cargo as well as technical problems and pitfalls.

Thermodynamic studies and binding mechanisms of cell-penetrating peptides with lipids and glycosaminoglycans

Cell-penetrating peptides (CPPs) traverse the membrane of biological cells at low micromolar concentrations and are able to take various cargo molecules along with. Despite large differences in their chemical structure, CPPs share the structural similarity of a high cationic charge density. This property confers to them the ability to bind electrostatically membrane constituents such as anionic lipids and glycosaminoglycans (GAGs). Controversies exist, however, about the biological response after the interaction of CPPs with such membrane constituents. Present review compares thermodynamic binding studies with conditions of the biological CPP uptake. It becomes evident that CPPs enter biological cells by different and probably competing mechanisms. For example, some amphipathic CPPs traverse pure lipid model membranes at low micromolar concentrations--at least in the absence of cargos. In contrast, no direct translocation at these conditions is observed for non-amphipathic CPPs. Finally, CPPs bind GAGs at low micromolar concentrations with potential consequences for endocytotic pathways.