Generation of endosomolytic reagents by branching of cell-penetrating peptides: tools for the delivery of bioactive compounds to live cells in cis or trans

Cell-Penetrating Peptide-Mediated Biomolecule Transportation in Artificial Lipid Vesicles and Living Cells

Signal transduction and homeostasis are regulated by complex protein interactions in the intracellular environment. Therefore, the transportation of impermeable macromolecules (nucleic acids, proteins, and drugs) that control protein interactions is essential for modulating cell functions and therapeutic applications. However, macromolecule transportation across the cell membrane is not easy because the cell membrane separates the intra/extracellular environments, and the types of molecular transportation are regulated by membrane proteins. Cell-penetrating peptides (CPPs) are expected to be carriers for molecular transport. CPPs can transport macromolecules into cells through endocytosis and direct translocation. The transport mechanism remains largely unclear owing to several possibilities. In this review, we describe the methods for investigating CPP conformation, translocation, and cargo transportation using artificial membranes. We also investigated biomolecular transport across living cell membranes via CPPs. Subsequently, we show not only the biochemical applications but also the synthetic biological applications of CPPs. Finally, recent progress in biomolecule and nanoparticle transportation via CPPs into specific tissues is described from the viewpoint of drug delivery. This review provides the opportunity to discuss the mechanism of biomolecule transportation through these two platforms.

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.

Cell-Penetrating Peptides (CPPs) as Therapeutic and Diagnostic Agents for Cancer

Cell-Penetrating Peptides (CPPs) are short peptides consisting of <30 amino acids. Their ability to translocate through the cell membrane while carrying large cargo biomolecules has been the topic of pre-clinical and clinical trials. The ability to deliver cargo complexes through membranes yields potential for therapeutics and diagnostics for diseases such as cancer. Upon cellular entry, some CPPs have the ability to target specific organelles. CPP-based intracellular targeting strategies hold tremendous potential as they can improve efficacy and reduce toxicities and side effects. Further, recent clinical trials show a significant potential for future CPP-based cancer treatment. In this review, we summarize recent advances in CPPs based on systematic searches in PubMed, Embase, Web of Science, and Scopus databases until 30 September 2022. We highlight targeted delivery and explore the potential uses for CPPs as diagnostics, drug delivery, and intrinsic anti-cancer agents.

The role of cell-penetrating peptides in potential anti-cancer therapy

Due to the complex physiological structure, microenvironment and multiple physiological barriers, traditional anti-cancer drugs are severely restricted from reaching the tumour site. Cell-penetrating peptides (CPPs) are typically made up of 5-30 amino acids, and can be utilised as molecular transporters to facilitate the passage of therapeutic drugs across physiological barriers. Up to now, CPPs have widely been used in many anti-cancer treatment strategies, serving as an excellent potential choice for oncology treatment. However, their drawbacks, such as the lack of cell specificity, short duration of action, poor stability in vivo, compatibility problems (i.e. immunogenicity), poor therapeutic efficacy and formation of unwanted metabolites, have limited their further application in cancer treatment. The cellular uptake mechanisms of CPPs involve mainly endocytosis and direct penetration, but still remain highly controversial in academia. The CPPs-based drug delivery strategy could be improved by clever design or chemical modifications to develop the next-generation CPPs with enhanced cell penetration capability, stability and selectivity. In addition, some recent advances in targeted cell penetration that involve CPPs provide some new ideas to optimise CPPs.

Redesigning of Cell-Penetrating Peptides to Improve Their Efficacy as a Drug Delivery System

Cell-penetrating peptides (CPP) are promising tools for the transport of a broad range of compounds into cells. Since the discovery of the first members of this peptide family, many other peptides have been identified; nowadays, dozens of these peptides are known. These peptides sometimes have very different chemical–physical properties, but they have similar drawbacks; e.g., non-specific internalization, fast elimination from the body, intracellular/vesicular entrapment. Although our knowledge regarding the mechanism and structure–activity relationship of internalization is growing, the prediction and design of the cell-penetrating properties are challenging. In this review, we focus on the different modifications of well-known CPPs to avoid their drawbacks, as well as how these modifications may increase their internalization and/or change the mechanism of penetration.

Strategies for improving the safety and RNAi efficacy of noncovalent peptide/siRNA nanocomplexes

In the past decades, the striking development of cationic polypeptides and cell-penetrating peptides (CPPs) tailored for small interfering RNA (siRNA) delivery has been fuelled by the conception of nuclear acid therapy and precision medicine. Owing to their amino acid compositions, inherent secondary structures as well as diverse geometrical shapes, peptides or peptide-containing polymers exhibit good biodegradability, high flexibility, and bio-functional diversity as nonviral siRNA vectors. Also, a variety of noncovalent nanocomplexes could be built via self-assembling and electrostatic interactions between cationic peptides and siRNAs. Although the peptide/siRNA nanocomplex-based RNAi therapies, STP705 and MIR-19, are under clinical trials, a guideline addressing the current bottlenecks of peptide/siRNA nanocomplex delivery is in high demand for future research and development. In this review, we present strategies for improving the safety and RNAi efficacy of noncovalent peptide/siRNA nanocomplexes in the treatment of genetic disorders. Through thorough analysis of those RNAi formulations using different delivery strategies, we seek to shed light on the rationale of peptide design and modification in constructing robust siRNA delivery systems, including targeted and co-delivery systems. Based on this, we provide a timely and comprehensive understanding of how to engineer biocompatible and efficient peptide-based siRNA vectors.

Tricyclic cell-penetrating peptides for efficient delivery of functional antibodies into cancer cells

The intracellular environment hosts a large number of cancer- and other disease-relevant human proteins. Targeting these with internalized antibodies would allow therapeutic modulation of hitherto undruggable pathways, such as those mediated by protein-protein interactions. However, one of the major obstacles in intracellular targeting is the entrapment of biomacromolecules in the endosome. Here we report an approach to delivering antibodies and antibody fragments into the cytosol and nucleus of cells using trimeric cell-penetrating peptides (CPPs). Four trimers, based on linear and cyclic sequences of the archetypal CPP Tat, are significantly more potent than monomers and can be tuned to function by direct interaction with the plasma membrane or escape from vesicle-like bodies. These studies identify a tricyclic Tat construct that enables intracellular delivery of functional immunoglobulin-G antibodies and Fab fragments that bind intracellular targets in the cytosol and nuclei of live cells at effective concentrations as low as 1 μM.

Macropinocytosis-Inducible Extracellular Vesicles Modified with Antimicrobial Protein CAP18-Derived Cell-Penetrating Peptides for Efficient Intracellular Delivery

The antimicrobial protein CAP18 (approximate molecular weight: 18 000), which was first isolated from rabbit granulocytes, comprises a C-terminal fragment that has negatively charged lipopolysaccharide binding activity. In this study, we found that CAP18 (106-121)-derived (sC18)₂ peptides have macropinocytosis-inducible biological functions. In addition, we found that these peptides are highly applicable for use as extracellular vesicle (exosomes, EV)-based intracellular delivery, which is expected to be a next-generation drug delivery carrier. Here, we demonstrate that dimerized (sC18)₂ peptides can be easily introduced on EV membranes when modified with a hydrophobic moiety, and that they show high potential for enhanced cellular uptake of EVs. By glycosaminoglycan-dependent induction of macropinocytosis, cellular EV uptake in targeted cells was strongly increased by the peptide modification made to EVs, and intriguingly, our herein presented technique is efficiently applicable for the cytosolic delivery of the biologically cell-killing functional toxin protein, saporin, which was artificially encapsulated in the EVs by electroporation, suggesting a useful technique for EV-based intracellular delivery of biofunctional molecules.

Cytosolic Delivery of Macromolecules in Live Human Cells Using the Combined Endosomal Escape Activities of a Small Molecule and Cell Penetrating Peptides

Ineffective cellular delivery is a common problem in numerous biological applications. Developing delivery reagents that work robustly in a variety of experimental settings remains a challenge. Herein, we report how peptides derived from the prototypical cell penetrating peptide TAT can be used in combination with a small molecule, UNC7938, to deliver macromolecules into the cytosol of cells by a simple co-incubation protocol. We establish successful delivery of peptides, DNA plasmids, and a single-chain variable fragment antibody. We also demonstrate that delivery works in hard-to-transfect mammalian cells and under conditions typically inhibitory to cell-penetrating peptides. Mechanistically, UNC7938 destabilizes the membrane of endosomes. This, in turn, enhances the endosome-leakage activity of cell-penetrating peptides and facilitates the endosomal escape of macromolecules initially internalized by mammalian cells via endocytosis. This combined selective membrane-destabilization represents a new chemical space for delivery tools and provides a novel solution to the problem of endosomal entrapment that often limits the effectiveness of reagent-based delivery approaches.

Cell-penetrating peptide: a means of breaking through the physiological barriers of different tissues and organs

The selective infiltration of cell membranes and tissue barriers often blocks the entry of most active molecules. This natural defense mechanism prevents the invasion of exogenous substances and limits the therapeutic value of most available molecules. Therefore, it is particularly important to find appropriate ways of membrane translocation and therapeutic agent delivery to its target site. Cell penetrating peptides (CPPs) are a group of short peptides harnessed in this condition, possessing a significant capacity for membrane transduction and could be exploited to transfer various biologically active cargoes into the cells. Since their discovery, CPPs have been employed for delivery of a wide variety of therapeutic molecules to treat various disorders including cranial nerve involvement, ocular inflammation, myocardial ischemia, dermatosis and cancer. The promising results of CPPs-derived therapeutics in various tumor models demonstrated a potential and worthwhile scope of CPPs in chemotherapy. This review describes the detailed description of CPPs and CPPs-assisted molecular delivery against various tissues and organs disorders. An emphasis is focused on summarizing the novel insights and achievements of CPPs in surmounting the natural membrane barriers during the last 5 years.

gH625-liposomes as tool for pituitary adenylate cyclase-activating polypeptide brain delivery

The blood-brain barrier (BBB) regulates the traffic of molecules into the central nervous system (CNS) and also limits the drug delivery. Due to their flexible properties, liposomes are an attractive tool to deliver drugs across the BBB. We previously characterized gH625, a peptide derived from Herpes simplex virus 1. The present study investigates the efficiency of liposomes functionalized on their surface with gH625 to promote the brain uptake of neuroprotective peptide PACAP (pituitary adenylate cyclase-activating polypeptide). Using a rat in vitro BBB model, we showed that the liposomes preparations were non-toxic for the endothelial cells, as assessed by analysis of tight junction protein ZO1 organization and barrier integrity. Next, we found that gH625 improves the transfer of liposomes across endothelial cell monolayers, resulting in both low cellular uptake and increased transport of PACAP. Finally, in vivo results demonstrated that gH625 ameliorates the efficiency of liposomes to deliver PACAP to the mouse brain after intravenous administration. gH625-liposomes improve both PACAP reaching and crossing the BBB, as showed by the higher number of brain cells labelled with PACAP. gH625-liposomes represent a promising strategy to deliver therapeutic agents to CNS and to provide an effective imaging and diagnostic tool for the brain.

Novel pH Selective, Highly Lytic Peptides Based on a Chimeric Influenza Hemagglutinin Peptide/Cell Penetrating Peptide Motif

Delivery of macromolecular cargos such as siRNA to the cytosol after endocytosis remains a critical challenge. Numerous approaches including viruses, lipid nanoparticles, polymeric constructs, and various peptide-based approaches have yet to yield a general solution to this delivery issue. In this manuscript, we describe our efforts to design novel endosomolytic peptides that could be used to facilitate the release of cargos from a late endosomal compartment. These amphiphilic peptides, based on a chimeric influenza hemagglutinin peptide/cell-penetrating peptide (CPP) template, utilize a pH-triggering mechanism in which the peptides are protonated after acidification of the endosome, and thereby adopt an alpha-helical conformation. The helical forms of the peptides are lytically active, while the non-protonated forms are much less or non-lytically active at physiological pH. Starting from an initial lead peptide (INF7-Tat), we systematically modified the sequence of the chimeric peptides to obtain peptides with greatly enhanced lytic activity that maintain good pH selectivity in a red blood cell hemolysis assay.

Simple and efficient delivery of cell-impermeable organic fluorescent probes into live cells for live-cell superresolution imaging

Numerous commercial organic fluorophores with excellent optical properties are precluded from live-cell superresolution imaging due to poor cell permeability. Here, we develop a simple but effective strategy that renders cells permeable to cell-impermeable, organic fluorescent probes by using a novel peptide vehicle, PV-1. By simple coincubation with PV-1, 22 different cell-impermeable, organic fluorescent probes were efficiently delivered into live cells and specifically labeled a variety of organelles. Moreover, PV-1 can simultaneously transfer up to three different probes into live cells. By using PV-1 and these cell-impermeable fluorescent probes, we obtained multicolor, long-term, live-cell superresolution images of various organelles, which allowed us to study the dynamic interactions between them. PV-1, together with these organic fluorescent probes, will greatly broaden the applications of superresolution imaging technology in diverse live-cell studies and opens up a new avenue in the design and application of peptide vehicles.

Endosomal Escape and Cytosolic Penetration of Macromolecules Mediated by Synthetic Delivery Agents

Cell delivery reagents often exploit the endocytic pathway as a route of cell entry. Once endocytosed, these reagents must overcome endosomal entrapment to ensure the release of their macromolecular cargo into the cytosol of cells. In this review, we describe several examples of prototypical synthetic reagents that are capable of endosomal escape and examine their mechanisms of action, their efficiencies, and their effects on cells. Although these delivery systems are chemically distinct, some commonalities in how they interact with cellular membranes can be inferred. This, in turn, sheds some light on the process of endosomal escape, and may help guide the development and optimization of next-generation delivery tools.

Machine learning antimicrobial peptide sequences: Some surprising variations on the theme of amphiphilic assembly

Antimicrobial peptides (AMPs) collectively constitute a key component of the host innate immune system. They span a diverse space of sequences and can be α-helical, β-sheet, or unfolded in structure. Despite a wealth of knowledge about them from decades of experiments, it remains difficult to articulate general principles governing such peptides. How are they different from other molecules that are also cationic and amphiphilic? What other functions, in immunity and otherwise, are enabled by these simple sequences? In this short review, we present some recent work that engages these questions using methods not usually applied to AMP studies, such as machine learning. We find that not only do AMP-like sequences confer membrane remodeling activity to an unexpectedly broad range of protein classes, their cationic and amphiphilic signature also allows them to act as meta-antigens and self-assemble with immune ligands into nanocrystalline complexes for multivalent presentation to Toll-like receptors.

Efficient Delivery of Macromolecules into Human Cells by Improving the Endosomal Escape Activity of Cell-Penetrating Peptides: Lessons Learned from dfTAT and its Analogs

Cell-penetrating peptides (CPPs) are typically prone to endocytic uptake into human cells. However, they are often inefficient at escaping from endosomes, which limits their ability to deliver cargos into cells. This review highlights the efforts that our laboratory has devoted toward developing CPPs that can mediate the leakage of endosomal membranes, and consequently gain better access to the intracellular milieu. In particular, we have identified a CPP named dimeric fluorescent TAT (dfTAT) with high endosomolytic activity. We describe how we have used this reagent and its analogs to develop efficient cytosolic delivery protocols and learn about molecular and cellular parameters that control the cell permeation process. Specifically, we discuss how late endosomes represent exploitable gateways for intracellular entry. We also describe how certain features in CPPs, including guanidinium content, charge density, multimerization, chirality, and susceptibility to degradation modulate the activity that these peptidic agents take toward endosomal membranes and cytosolic egress.

Fluorescent and Photosensitizing Conjugates of Cell-Penetrating Peptide TAT(47-57): Design, Microwave-Assisted Synthesis at 60 °C, and Properties

Conjugates based on cell-penetrating peptides (CPPs) are scientifically relevant owing to their structural complexity; their ability to enter cells and deliver drugs, labels, antioxidants, bioactive compounds, or DNA fragments; and, consequently, their potential for application in research and biomedicine. In this study, carboxyamidated fluorescently labeled conjugates FAM-GG-TAT(47-57)-NH2 and FAM-PEG6-TAT(47-57)-NH2 and photosensitizer-labeled conjugate Chk-PEG6-TAT(47-57)-NH2 [where TAT(47-57) is the CPP, 5(6)-carboxyfluorescein is the (FAM) fluorophore, chlorin k (Chk) is the photosensitizer, and the dipeptide glycyl-glycine (GG) or hexaethylene glycol (PEG6) is the spacer] were originally designed, prepared, and fully characterized. Practically, all chemical reactions of the synthetic steps (peptide synthesis, spacer incorporation, and conjugation) were microwave-assisted at 60 °C using optimized protocols to give satisfying yields and high-quality products. Detailed analyses of the conjugates using spectrofluorimetry and singlet oxygen detection showed that they display photophysical properties typical of FAM or Chk. Anticandidal activity assays showed that not only this basic property of TAT(47-57) was preserved in the conjugates but also that the minimal inhibitory concentration was slightly reduced for cells incubated with PS-bearing conjugate Chk-PEG6-TAT(47-57)-NH2. Overall, these results indicated that the synthetic approach on-resin assisted by microwaves at 60 °C is simple, straightforward, selective, metal-free, sufficiently fast, cleaner, and more cost-effective than those previously used for preparing this type of macromolecule. Furthermore, such new data show that microwaves at 60 °C and/or conjugation did not harm the integrity of the conjugates' constituents. Therefore, FAM-GG-TAT(47-57)-NH2, FAM-PEG6-TAT(47-57)-NH2, and Chk-PEG6-TAT(47-57)-NH2 have high potential for practical applications in biochemistry, biophysics, and therapeutics.

Unlocking Endosomal Entrapment with Supercharged Arginine-Rich Peptides

Endosomal entrapment is a common bottleneck in various macromolecular delivery approaches. Recently, the polycationic peptide dfTAT was identified as a reagent that induces the efficient leakage of late endosomes and, thereby, enhances the penetration of macromolecules into the cytosol of live human cells. To gain further insights into the features that lead to this activity, the role of peptide sequence was investigated. We establish that the leakage activity of dfTAT can be recapitulated by polyarginine analogs but not by polylysine counterparts. Efficiencies of peptide endocytic uptake increase linearly with the number of arginine residues present. In contrast, peptide cytosolic penetration displays a threshold behavior, indicating that a minimum number of arginines is required to induce endosomal escape. Increasing arginine content above this threshold further augments delivery efficiencies. Yet, it also leads to increasing the toxicity of the delivery agents. Together, these data reveal a relatively narrow arginine-content window for the design of optimally active endosomolytic agents.

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

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

Nanoparticle cellular uptake by dendritic wedge peptides: achieving single peptide facilitated delivery

Significant efforts are being undertaken to optimize the cargo carrying capacity and especially the cellular delivery efficiency of functionalized nanoparticles for applications in biological research and pharmacological delivery. One approach to increasing nanoparticle surface cargo display capacity is to decrease the number of moieties required for mediating cellular delivery by improving their efficiency. We describe a series of multivalent cell penetrating peptide (CPP) dendrimers that facilitate rapid cellular delivery of prototypical nanoparticle-semiconductor quantum dots (QDs). The modular CPP dendrimers were assembled through an innovative convergent oxime ligation strategy between (Arg₉)_n motifs and a dendritic QD-coordination scaffold. Dendrimeric peptides sequentially incorporate a terminal (His)₆ motif for metal-affinity QD coordination, a Pro₉ spacer, a branching poly-lysine scaffold, and wedged display of (Arg₉)_n binding motifs with n = 1×, 2×, 4×, 8×, 16× multivalency. QD dendrimer display capacity was estimated using structural simulations and QD-(Arg₉)_1-16 conjugates characterized by dynamic light scattering along with surface plasmon resonance-based binding assays to heparan sulfate proteoglycan surfaces. Cellular uptake via endocytosis was confirmed and peptide delivery kinetics investigated as a function of QD-(Arg₉)_1-16 conjugate exposure time and QD assembly ratio where cellular viability assays reflected no overt cytotoxicity. The ability of single dendrimer conjugates to facilitate cellular uptake was confirmed for QD-(Arg₉)_2-16 repeats along with the ability to deliver >850 kDa of protein cargo per QD. Minimizing the number of CPPs required for cellular uptake is critical for expanding nanoparticle cargo carrying capacity and can allow for inclusion of additional sensors, therapeutics and contrast agents on their surface.

Interactions between Membranes and "Metaphilic" Polypeptide Architectures with Diverse Side-Chain Populations

At physiological conditions, most proteins or peptides can fold into relatively stable structures that present on their molecular surfaces specific chemical patterns partially smeared out by thermal fluctuations. These nanoscopically defined patterns of charge, hydrogen bonding, and/or hydrophobicity, along with their elasticity and shape stability (folded proteins have Young's moduli of ∼1 × 10⁸ Pa), largely determine and limit the interactions of these molecules, such as molecular recognition and allosteric regulation. In this work, we show that the membrane-permeating activity of antimicrobial peptides (AMPs) and cell-penetrating peptides (CPPs) can be significantly enhanced using prototypical peptides with "molten" surfaces: metaphilic peptides with quasi-liquid surfaces and adaptable shapes. These metaphilic peptides have a bottlebrush-like architecture consisting of a rigid helical core decorated with mobile side chains that are terminated by cationic or hydrophobic groups. Computer simulations show that these flexible side chains can undergo significant rearrangement in response to different environments, giving rise to adaptable surface chemistry of the peptide. This quality makes it possible to control their hydrophobicity over a broad range while maintaining water solubility, unlike many AMPs and CPPs. Thus, we are able to show how the activity of these peptides is amplified by hydrophobicity and cationic charge, and rationalize these results using a quantitative mean-field theory. Computer simulations show that the shape-changing properties of the peptides and the resultant adaptive presentation of chemistry play a key enabling role in their interactions with membranes.

Virus-inspired nucleic acid delivery system: Linking virus and viral mimicry

Targeted delivery of nucleic acids into disease sites of human body has been attempted for decades, but both viral and non-viral vectors are yet to meet our expectations. Safety concerns and low delivery efficiency are the main limitations of viral and non-viral vectors, respectively. The structure of viruses is both ordered and dynamic, and is believed to be the key for effective transfection. Detailed understanding of the physical properties of viruses, their interaction with cellular components, and responses towards cellular environments leading to transfection would inspire the development of safe and effective non-viral vectors. To this goal, this review systematically summarizes distinctive features of viruses that are implied for efficient nucleic acid delivery but not yet fully explored in current non-viral vectors. The assembly and disassembly of viral structures, presentation of viral ligands, and the subcellular targeting of viruses are emphasized. Moreover, we describe the current development of cationic material-based viral mimicry (CVM) and structural viral mimicry (SVM) in these aspects. In light of the discrepancy, we identify future opportunities for rational design of viral mimics for the efficient delivery of DNA and RNA.

The Late Endosome and Its Lipid BMP Act as Gateways for Efficient Cytosolic Access of the Delivery Agent dfTAT and Its Macromolecular Cargos

Endosomal entrapment is a severely limiting bottleneck in the delivery of biologics into cells. The compound dfTAT was recently found to circumvent this problem by mediating endosomal leakage efficiently and without toxicity. Herein, we report on the mechanism of endosomal escape of this cell-penetrating peptide. By modulating the trafficking of the peptide within the endocytic pathway, we identify late endosomes as the organelles rendered leaky by dfTAT. We establish that dfTAT binds bis(monoacylglycero)phosphate (BMP), a lipid found in late endosomes, and that the peptide causes the fusion and leakage of bilayers containing BMP. Together, these data identify late endosomes as desirable gateways for cell penetration and BMP as a cellular factor that can be exploited for the development of future delivery agents.

Applications and Challenges for Use of Cell-Penetrating Peptides as Delivery Vectors for Peptide and Protein Cargos

The hydrophilic nature of peptides and proteins renders them impermeable to cell membranes. Thus, in order to successfully deliver peptide and protein-based therapeutics across the plasma membrane or epithelial and endothelial barriers, a permeation enhancing strategy must be employed. Cell-penetrating peptides (CPPs) constitute a promising tool and have shown applications for peptide and protein delivery into cells as well as across various epithelia and the blood-brain barrier (BBB). CPP-mediated delivery of peptides and proteins may be pursued via covalent conjugation of the CPP to the cargo peptide or protein or via physical complexation obtained by simple bulk-mixing of the CPP with its cargo. Both approaches have their pros and cons, and which is the better choice likely relates to the physicochemical properties of the CPP and its cargo as well as the route of administration, the specific barrier and the target cell. Besides the physical barrier, a metabolic barrier must be taken into consideration when applying peptide-based delivery vectors, such as the CPPs, and stability-enhancing strategies are commonly employed to prolong the CPP half-life. The mechanisms by which CPPs translocate cell membranes are believed to involve both endocytosis and direct translocation, but are still widely investigated and discussed. The fact that multiple factors influence the mechanisms responsible for cellular CPP internalization and the lack of sensitive methods for detection of the CPP, and in some cases the cargo, further complicates the design and conduction of conclusive mechanistic studies.

Endosomal escape efficiency of fusogenic B18 and B55 peptides fused with anti-EGFR single chain Fv as estimated by nuclear translocation

Although monoclonal antibodies have been used not only as analytical tools but also as biologic therapeutics, they cannot target intracellular proteins due to their large molecular size and low membrane permeability, which limit their applications. During previous attempts to delivery antibodies intracellularly, the low efficiency of escape from endosomes to the cytosol reduced the bioavailability of antibodies or antibody-conjugated effectors. Recently, we found that the fusogenic peptides (FPs) B18 and B55 from bindin, a sea urchin gamete recognition protein, facilitated the endosomal escape of FP-fused enhanced green fluorescent protein (eGFP) and/or of co-administered cargos such as dextrans [Niikura et al. A fusogenic peptide from a sea urchin fertilization protein promotes intracellular delivery of biomacromolecules by facilitating endosomal escape. J.

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Release 2015;212:85-93]. In this study, we constructed FP-fused anti-epidermal growth factor receptor (EGFR) single-chain Fv (αEGFR[scFv]) proteins and evaluated their endosomal escape efficiency by utilizing a nuclear localization signal). When the FP-fused αEGFR[scFv] proteins were incubated with A431 cells, the estimated endosomal escape efficiency of αEGFR[scFv]-B18 was significantly higher than that of αEGFR[scFv] alone, suggesting that the B18 peptide facilitates endosomal escape of the conjugated scFv in cis. Moreover, αEGFR[scFv]-B55 promoted the intracellular uptake of co-administered eGFP and dextrans in trans. These results imply that B18- and B55-fused antibodies may be useful for the cell-specific intracellular delivery of biomacromolecules.

Delivery of Proteins, Peptides or Cell-impermeable Small Molecules into Live Cells by Incubation with the Endosomolytic Reagent dfTAT

Macromolecular delivery strategies typically utilize the endocytic pathway as a route of cellular entry. However, endosomal entrapment severely limits the efficiency with which macromolecules penetrate the cytosolic space of cells. Recently, we have circumvented this problem by identifying the reagent dfTAT, a disulfide bond dimer of the peptide TAT labeled with the fluorophore tetramethylrhodamine. We have generated a fluorescently labeled dimer of the prototypical cell-penetrating peptide (CPP) TAT, dfTAT, which penetrates live cells and reaches the cytosolic space of cells with a particularly high efficiency. Cytosolic delivery of dfTAT is achieved in multiple cell lines, including primary cells. Moreover, delivery does not noticeably impact cell viability, proliferation or gene expression. dfTAT can deliver small molecules, peptides, antibodies, biologically active enzymes and a transcription factor. In this report, we describe the protocols involved in dfTAT synthesis and cellular delivery. The manuscript describes how to control the amount of protein delivered to the cytosolic space of cells by varying the amount of protein administered extracellularly. Finally, the current limitations of this new technology and steps involved in validating delivery are discussed. The described protocols should be extremely useful for cell-based assays as well as for the ex vivo manipulation and reprogramming of cells.

Designed cell penetrating peptide dendrimers efficiently internalize cargo into cells

Redesigning linear cell penetrating peptides (CPPs) into a multi-branched topology with short dipeptide branches gave cell penetrating peptide dendrimers (CPPDs) with higher cell penetration, lower toxicity and hemolysis and higher serum stability than linear CPPs. Their use is demonstrated by delivering a cytotoxic peptide and paclitaxel into cells.

Branched dimerization of Tat peptide improves permeability to HeLa and hippocampal neuronal cells

A designed, dimeric analogue of TAT-peptide translocates through HeLa and primary neuronal cell membrane in a non-linear dependence on concentration.

gH625: a milestone in understanding the many roles of membranotropic peptides

Here, we review the current knowledge about viral derived membranotropic peptides, and we discuss how they may be used for many therapeutic applications. While they have been initially discovered in viral fusion proteins and have been involved in the mechanism of viral entry, it is now clear that their features and their mode of interaction with membrane bilayers can be exploited to design viral inhibitors as well as to favor delivery of cargos across the cell membrane and across the blood–brain barrier. The peptide gH625 has been extensively used for all these purposes and provides a significant contribution to the field. We describe the roles of this sequence in order to close the gap between the many functions that are now emerging for membranotropic peptides.

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Membranotropic peptides and their therapeutic applications

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Membrane fusion, viral inhibition, drug delivery

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gH625, a peptide derived from Herpes simplex virus type I: a case study

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gH625 in vitro and in vivo delivery across the blood–brain barrier

The tetrapeptide core of the carrier peptide Xentry is cell-penetrating: novel activatable forms of Xentry

Here we describe a structure-function analysis of the cell-penetrating peptide Xentry derived from the X-protein of the hepatitis B virus. Remarkably, the tetrapeptide core LCLR retains the cell-penetrating ability of the parental peptide LCLRPVG, as either an L- or D-enantiomer. Substitution of the cysteine with leucine revealed that the cysteine is essential for activity. In contrast, the C-terminal arginine could be substituted in the L-isomer with lysine, histidine, glutamic acid, glutamine, and asparagine, though the resulting peptides displayed distinct cell-type-specific uptake. Substitution of the leucines in the D-isomer with other hydrophobic residues revealed that leucines are optimal for activity. Surprisingly, linear di- and tetra-peptide forms of Xentry are not cell-permeable. Protease-activatable forms of Xentry were created by fusing Xentry to itself via a protease-cleavable peptide, or by attaching a heparin mimic peptide to the N-terminus. These novel activatable forms of Xentry were only taken up by MCF-7 cells after cleavage by matrix metalloproteinase 9, and could be used to deliver drugs specifically to tumours.

Improved bioactivity of antimicrobial peptides by addition of amino-terminal copper and nickel (ATCUN) binding motifs

Antimicrobial peptides (AMPs) are promising candidates to help circumvent antibiotic resistance, which is an increasing clinical problem. Amino-terminal copper and nickel (ATCUN) binding motifs are known to actively form reactive oxygen species (ROS) upon metal binding. The combination of these two peptidic constructs could lead to a novel class of dual-acting antimicrobial agents. To test this hypothesis, a set of ATCUN binding motifs were screened for their ability to induce ROS formation, and the most potent were then used to modify AMPs with different modes of action. ATCUN binding motif-containing derivatives of anoplin (GLLKRIKTLL-NH2), pro-apoptotic peptide (PAP; KLAKLAKKLAKLAK-NH2), and sh-buforin (RAGLQFPVGRVHRLLRK-NH2) were synthesized and found to be more active than the parent AMPs against a panel of clinically relevant bacteria. The lower minimum inhibitory concentration (MIC) values for the ATCUN-anoplin peptides are attributed to the higher pore-forming activity along with their ability to cause ROS-induced membrane damage. The addition of the ATCUN motifs to PAP also increases its ability to disrupt membranes. DNA damage is the major contributor to the activity of the ATCUN-sh-buforin peptides. Our findings indicate that the addition of ATCUN motifs to AMPs is a simple strategy that leads to AMPs with higher antibacterial activity and possibly to more potent, usable antibacterial agents.

Cell-penetrating peptides: design, synthesis, and applications

The intrinsic property of cell-penetrating peptides (CPPs) to deliver therapeutic molecules (nucleic acids, drugs, imaging agents) to cells and tissues in a nontoxic manner has indicated that they may be potential components of future drugs and disease diagnostic agents. These versatile peptides are simple to synthesize, functionalize, and characterize yet are able to deliver covalently or noncovalently conjugated bioactive cargos (from small chemical drugs to large plasmid DNA) inside cells, primarily via endocytosis, in order to obtain high levels of gene expression, gene silencing, or tumor targeting. Typically, CPPs are often passive and nonselective yet must be functionalized or chemically modified to create effective delivery vectors that succeed in targeting specific cells or tissues. Furthermore, the design of clinically effective systemic delivery systems requires the same amount of attention to detail in both design of the delivered cargo and the cell-penetrating peptide used to deliver it.

Mimicry of high-density lipoprotein: functional peptide-lipid nanoparticles based on multivalent peptide constructs

We describe an approach for engineering peptide-lipid nanoparticles that function similarly to high-density lipoprotein (HDL). Branched, multivalent constructs, bearing multiple 23- or 16-amino-acid peptides, were designed, synthesized, and combined with phospholipids to produce nanometer-scale discoidal HDL-like particles. A variety of biophysical techniques were employed to characterize the constructs, including size exclusion chromatography, analytical ultracentrifuge sedimentation, circular dichroism, transmission electron microscopy, and fluorescence spectroscopy. The nanoparticles functioned in vitro (human and mouse plasma) and in vivo (mice) to rapidly remodel large native HDLs into small lipid-poor HDL particles, which are key acceptors of cholesterol in reverse cholesterol transport. Fluorescent labeling studies showed that the constituents of the nanoparticles readily distributed into native HDLs, such that the peptide constructs coexisted with apolipoprotein A-I (apoA-I), the main structural protein in HDLs. Importantly, nanolipid particles containing multivalent peptides promoted efficient cellular cholesterol efflux and were functionally superior to those derived from monomeric apoA-I mimetic peptides. The multivalent peptide-lipid nanoparticles were also remarkably stable toward enzymatic digestion in vitro and displayed long half-lives and desirable pharmacokinetic profiles in mice, providing a real practical advantage over previously studied linear or tandem helical peptides. Encouragingly, a two-week exploratory efficacy study in a widely used animal model for atherosclerosis research (LDLr-null mice) using nanoparticles constructed from a trimeric peptide demonstrated an exceptional 50% reduction in the plasma total cholesterol levels compared to the control group. Altogether, the studies reported here point to an attractive avenue for designing synthetic, HDL-like nanoparticles, with potential for treating atherosclerosis.

Mechanistic and functional aspects of the interaction of AR-23 with mammalian cell membrane and improvement of branched polyethylenimine-mediated gene transfection

BACKGROUND

Previous studies have suggested that reducing the positive charge of melittin could increase endosomal release activity and improve branched polyethylenimine (BPEI)-mediated transfection. AR-23 is a melittin-related peptide from Rana tagoi, which shows 81% sequence identity with melittin but has less positively-charged residues than melittin. The present study aimed to investigate the mechanistic and functional aspects of the interaction of AR-23 with mammalian cells and thus improve BPEI-mediated gene transfection.

METHODS

AR23 and two AR-23 analogs (AR-20 without positively-charged residues and AR-26 with the same positively-charged residues as melittin) were analyzed. Circular dichroism (CD) spectrometry was used to analyze the secondary structures of the peptides. Peptide-induced depolarization of cell membrane, the membrane-lytic activity of the peptides, and their potency with respect to enhancing the cellular uptake of calcein were evaluated. The physicochemical characters of complexes were measured and the effect of the peptides on BPEI-mediated transfection was determined.

RESULTS

The CD spectra results indicated that a positive charge in AR-23 played a crucial role in maintaining the α-helical conformation, whereas an extra positive charge could not increase α-helical formation. AR-23 displayed a similar depolarization ability to melittin. However, AR-23 showed a lower membrane lytic activity under physiological conditions and a higher lytic activity at endosomal pH than melittin and AR-26, which possess more positive charges. Compared to melittin and AR-26, AR-23, with a higher endosomal escaping activity, resulted in a higher enhancement of BPEI-mediated gene transfection, as well as the maintainance of a lower cytotoxicity.

CONCLUSIONS

We suggest that AR-23 may be considered as a potential enhancer for improving the transfection efficiency of cationic polymers.

Photoinactivation of Gram positive and Gram negative bacteria with the antimicrobial peptide (KLAKLAK)(2) conjugated to the hydrophilic photosensitizer eosin Y

We test the hypothesis that the antimicrobial peptide (KLAKLAK)(2) enhances the photodynamic activity of the photosensitizer eosin Y upon conjugation. The conjugate eosin-(KLAKLAK)(2) was obtained by solid-phase peptide synthesis. Photoinactivation assays were performed against the Gram-negative bacteria Escherichia coli , Pseudomonas aeruginosa , and multidrug resistant Acinetobacter baumannii AYE, as well as the Gram-positive bacteria Staphylococcus aureus , and Staphylococcus epidermidis . Partitioning assays were performed with E. coli and S. aureus . Photohemolysis and photokilling assays were also performed to assess the photodynamic activity of the conjugate toward mammalian cells. Eosin-(KLAKLAK)(2) photoinactivates 99.999% of 10(8) CFU/mL of most bacteria tested at a concentration of 1 μM or below. In contrast, neither eosin Y nor (KLAKLAK)(2) cause any significant photoinactivation under similar conditions. The increase in photodynamic activity of the photosensitizer conferred by the antimicrobial peptide is in part due to the fact that (KLAKLAK)(2) promotes the association of eosin Y to bacteria. Eosin-(KLAKLAK)(2) does not significantly associate with red blood cells or the cultured mammalian cell lines HaCaT, COS-7, and COLO 316. Consequently, little photodamage or photokilling is observed with these cells under conditions for which bacterial photoinactivation is achieved. The peptide (KLAKLAK)(2) therefore significantly enhances the photodynamic activity of eosin Y toward both Gram-positive and Gram-negative bacteria while interacting minimally with human cells. Overall, our results suggest that antimicrobial peptides such as (KLAKLAK)(2) might serve as attractive agents that can target photosensitizers to bacteria specifically.

Improving the endosomal escape of cell-penetrating peptides and their cargos: strategies and challenges

Cell penetrating peptides (CPPs) can deliver cell-impermeable therapeutic cargos into cells. In particular, CPP-cargo conjugates tend to accumulate inside cells by endocytosis. However, they often remain trapped inside endocytic organelles and fail to reach the cytosolic space of cells efficiently. In this review, the evidence for CPP-mediated endosomal escape is discussed. In addition, several strategies that have been utilized to enhance the endosomal escape of CPP-cargos are described. The recent development of branched systems that display multiple copies of a CPP is presented. The use of viral or synthetic peptides that can disrupt the endosomal membrane upon activation by the low pH of endosomes is also discussed. Finally, we survey how CPPs labeled with chromophores can be used in combination with light to stimulate endosomal lysis. The mechanisms and challenges associated with these intracellular delivery methodologies are discussed.

Dimerization of a cell-penetrating peptide leads to enhanced cellular uptake and drug delivery

Over the past 20 years, cell-penetrating peptides (CPPs) have gained tremendous interest due to their ability to deliver a variety of therapeutically active molecules that would otherwise be unable to cross the cellular membrane due to their size or hydrophilicity. Recently, we reported on the identification of a novel CPP, sC18, which is derived from the C-terminus of the 18 kDa cationic antimicrobial protein. Furthermore, we demonstrated successful application of sC18 for the delivery of functionalized cyclopentadienyl manganese tricarbonyl (cymantrene) complexes to tumor cell lines, inducing high cellular toxicity. In order to increase the potential of the organometallic complexes to kill tumor cells, we were looking for a way to enhance cellular uptake. Therefore, we designed a branched dimeric variant of sC18, (sC18)(2), which was shown to have a dramatically improved capacity to internalize into various cell lines, even primary cells, using flow cytometry and fluorescence microscopy. Cell viability assays indicated increased cytotoxicity of the dimer presumably caused by membrane leakage; however, this effect turned out to be dependent on the specific cell type. Finally, we could show that conjugation of a functionalized cymantrene with (sC18)(2) leads to significant reduction of its IC(50) value in tumor cells compared to the respective sC18 conjugate, proving that dimerization is a useful method to increase the drug-delivery potential of a cell-penetrating peptide.

A cell penetrating peptide-integrated and enediyne-energized fusion protein shows potent antitumor activity

Arginine-rich peptides belong to a subclass of cell penetrating peptides that are taken up by living cells and can be detected freely diffusing inside the cytoplasm and nucleoplasm. This phenomenon has been attributed to either an endocytotic mode of uptake and a subsequent release from vesicles or a direct membrane penetration. Lidamycin is an antitumor antibiotic, which consists of an active enediyne chromophore (AE) and a noncovalently bound apoprotein (LDP). In the present study, a fusion protein (Arg)(9)-LDP composed of cell penetrating peptide (Arg)(9) and LDP was prepared by DNA recombination, and the enediyne-energized fusion protein (Arg)(9)-LDP-AE was prepared by molecular reconstitution. The data in fixed cells demonstrated that (Arg)(9)-LDP could rapidly enter cells, and the results based on fluorescence activated cell sorting indicated that the major route for (Arg)(9)-mediated cellular uptake of protein molecules was endocytosis. (Arg)(9)-LDP-AE demonstrated more potent cytotoxicity against different carcinoma cell lines than lidamycin in vitro. In the mouse hepatoma 22 model, (Arg)(9)-LDP-AE (0.3mg/kg) suppressed the tumor growth by 89.2%, whereas lidamycin (0.05 mg/kg) by 74.6%. Furthermore, in the glioma U87 xenograft model in nude mice, (Arg)(9)-LDP-AE at 0.2mg/kg suppressed tumor growth by 88.8%, compared with that of lidamycin by 62.9% at 0.05 mg/kg. No obvious toxic effects were observed in all groups during treatments. The results showed that energized fusion protein (Arg)(9)-LDP-AE was more effective than lidamycin and would be a promising candidate for glioma therapy. In addition, this approach to manufacturing fusion proteins might serve as a technology platform for the development of new cell penetrating peptides-based drugs.

Dendrimer functionalization with a membrane-interacting domain of herpes simplex virus type 1: towards intracellular delivery

A poly(amide)-based dendrimer was synthesized and functionalized with the membrane-interacting peptide gH(625-644) (gH625) derived from the herpes simplex virus type 1 (HSV-1) envelope glycoprotein H, which has previously been shown to assist in delivering large cargoes across the cellular membrane. We demonstrate that the attachment of the gH625 peptide sequence to the termini of a dendrimer allows the conjugate to penetrate into the cellular matrix, whereas the unfunctionalized dendrimer is excluded from translocation. The peptide-functionalized dendrimer is rapidly taken into the cells mainly through a non-active translocation mechanism. Our results suggest that the presented peptidodendrimeric scaffold may be a promising material for efficient drug delivery.