Penetration and intracellular routing of nucleus-directed peptide-based shuttles (loligomers) in eukaryotic cells

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.

Targeted Therapy of Cancer Using Photodynamic Therapy in Combination with Multi-faceted Anti-Tumor Modalities

Photodynamic therapy (PDT) has emerged as one of the important therapeutic options in the management of cancer and other diseases. PDT involves a tumor-localized photosensitizer (PS), which when appropriately illuminated by visible light converts oxygen into cytotoxic reactive oxygen species (ROS), that attack key structural entities within the targeted cells, ultimately resulting in necrosis or apoptosis. Though PDT is a selective modality, it can be further enhanced by combining other targeted therapeutic strategies that include the use of synthetic peptides and nanoparticles for selective delivery of photosensitizers. Another potentially promising strategy is the application of targeted therapeutics that exploit a myriad of critical pathways involved in tumorigenesis and metastasis. Vascular disrupting agents that eradicate tumor vasculature during PDT and anti-angiogenic agents that targets specific molecular pathways and prevent the formation of new blood vessels are novel therapeutic approaches that have been shown to improve treatment outcome. In addition to the well-documented mechanisms of direct cell killing and damage to the tumor vasculature, PDT can also activate the body's immune response against tumors. Numerous pre-clinical studies and clinical observations have demonstrated the immuno-stimulatory capability of PDT. Herein, we aim to integrate the most important findings with regard to the combination of PDT and other novel targeted therapy approaches, detailing its potential in cancer photomedicine.

Screening of a branched peptide library with HIV-1 TAR RNA

The recognition that RNA is more than just an intermediate in the information transfer from genetic code to fully functional protein has placed it at the forefront of chemical research. RNA is important because of its vital role in regulating transcription, translation, splicing, replication and catalysis. Consequently, molecules that can bind to RNA and control its function have potential as powerful tools in biology and medicine. Herein, we report the discovery of HIV-1 TAR RNA-selective ligands using an on-bead screening of a library of 4096 branched peptides.

Multifunctionalization of dendrimers through orthogonal transformations

A straightforward methodology for the synthesis of multifunctionalized dendrimers that is based on an orthogonal functionalization strategy has been developed. Polyamide-based dendrimers that possess both a single aldehyde and a single azide moiety on their periphery have been synthesized by using a convergent synthetic strategy. These dendrimers can be functionalized quantitatively with small organic and biological molecules that contain hydrazide and/or alkyne groups in which each functional moiety is completely specific for its complementary motif. This orthogonal functionalization strategy has the potential to be used to synthesize multifunctional dendrimers for a variety of applications, which range from targeted biological delivery vehicles to optical materials.

Probing length effects and mechanism of cell penetrating agents mounted on a polyproline helix scaffold

Cell penetrating peptides (CPP) displaying a type II polyproline helix backbone of different length and amphiphilic character were synthesized and their cellular uptake was compared. The longer CPP sequence, P14LRR, displayed a 7- to 12-fold higher uptake in MCF-7 cells as compared to its shorter counterpart, P11LRR, and a 35-fold higher uptake as compared to Tatp. These results demonstrate that an increased number of cationic and hydrophobic residues can strongly influence the extent of cellular internalization. Mechanistic investigations suggest internalization via a receptor independent endocytotic pathway with these agents.

Cell-penetrating proline-rich peptidomimetics

Cell-penetrating peptides (CPPs) offer potential as delivery agents for the cellular administration of drugs. However, the pharmacological utility of CPPs that are derived from natural amino acids is limited by their rapid metabolic degradation, low membrane permeability, and toxicity. Various peptidomimetics able to overcome these problems have been described, including peptides formed by D-amino acids and beta-peptides. This chapter summarizes the synthesis of gamma-proline-derived peptides and polyproline dendrimers for drug delivery applications, and includes descriptions of several modifications in the gamma-peptides (mimicking the side chains of the alpha-amino acids) or modulating the dendrimer surface. 5(6)-Carboxyfluorescein labeling of the aforementioned peptidomimetics for use in cell translocation studies is also described. Furthermore, different protocols for the study of the drug delivery capabilities of these compounds are reviewed, including enzymatic stability studies, cellular uptake measurements by plate fluorimetry and flow cytometry, confocal laser scanning microscopy, and cytotoxicity assays.

A cell-penetrating peptide derived from mammalian cell uptake protein of Mycobacterium tuberculosis

A Mycobacterium tuberculosis membrane protein called Mycobacterium cell entry protein (Mce1A) was previously shown to mediate the uptake of nonpathogenic Escherichia coli and latex beads by nonphagocytic mammalian cells. Here we characterize further the in vitro invasive activity of Mce1A using colloidal gold nanoparticles and fluorescent latex microspheres. Mce1A-coated colloidal gold particles induced plasma membrane invagination and entered membrane-bound compartments inside HeLa cells. Few of the protein-coated particles were also found in the cytosol compartment. Cytochalasin D and nocodazole inhibited the uptake by HeLa cells, indicating that rearrangement of both microtubules and microfilaments was necessary for the uptake. The functional domain of Mce1A for invasion was narrowed to a highly basic 22-amino acid sequence termed Inv3. A synthetic Inv3 peptide stimulated uptake of colloidal gold particles as well as latex microspheres by HeLa cells. A chimeric protein composed of Inv3 sequence at the N terminus of beta-galactosidase appeared to stain the nuclear membrane, suggesting that it entered the HeLa cell cytoplasm. These observations suggest that the cell uptake activity of Mce1A is confined to a small peptide domain located in the core region of the protein. Inv3 could be used to ferry any protein in fusion with it into mammalian cells and may serve as a potent nonviral delivery system.

Medium-sized peptides as built in carriers for biologically active compounds

A growing number of oligopeptides of natural and/or synthetic origin have been described and considered as targeting structures for delivery bioactive compounds into various cell types. This review will outline the discovery of peptide sequences and the corresponding mid-sized oligopeptides with membrane translocating properties and also summarize de novo designed structures possessing similar features. Conjugates and chimera constructs derived from these sequences with covalently attached bioactive peptide, epitope, oligonucleotide, PNA, drug, reporter molecule will be reviewed. A brief note will refer to the present understanding on the uptake mechanism at the end of each section.

The importance of valency in enhancing the import and cell routing potential of protein transduction domain-containing molecules

Protein transduction domains (PTDs) are peptides that afford the internalization of cargo macromolecules (including plasmid DNA, proteins, liposomes, and nanoparticles). In the case of polycationic peptides, the efficiency of PTDs to promote cellular uptake is directly related to their molecular mass or their polyvalent presentation. Similarly, the efficiency of routing to the nucleus increases with the number of nuclear localization signals (NLS) associated with a cargo. The quantitative enhancement, however, depends on the identity of the PTD sequence as well as the targeted cell type. Thus the choice and multivalent presentation of PTD and NLS sequences are important criteria guiding the design of macromolecules intended for specific intracellular localization. This review outlines synthetic and recombinant strategies whereby PTDs and signal sequences can be assembled into multivalent peptide dendrimers and promote the uptake and routing of their cargoes. In particular, the tetramerization domain of the tumour suppressor p53 (p53tet) is emerging as a useful scaffold to present multiple routing and targeting moieties. Short cationic peptides fused to the 31-residue long p53tet sequence resulted in tetramers displaying a significant enhancement (up to 1000 fold) in terms of their ability to be imported into cells and delivered to the cell nucleus in relation to their monomeric analogues. The design of future polycationic peptide dendrimers as effective delivering vehicles will need to incorporate selective cell targeting functions and provide solutions to the issue of endosomal entrapment.

Delivery of bioactive molecules into the cell: the Trojan horse approach

In recent years, vast amounts of data on the mechanisms of neural de- and regeneration have accumulated. However, only in disproportionally few cases has this led to efficient therapies for human patients. Part of the problem is to deliver cell death-averting genes or gene products across the blood-brain barrier (BBB) and cellular membranes. The discovery of Antennapedia (Antp)-mediated transduction of heterologous proteins into cells in 1992 and other "Trojan horse peptides" raised hopes that often-frustrating attempts to deliver proteins would now be history. The demonstration that proteins fused to the Tat protein transduction domain (PTD) are capable of crossing the BBB may revolutionize molecular research and neurobiological therapy. However, it was only recently that PTD-mediated delivery of proteins with therapeutic potential has been achieved in models of neural degeneration in nerve trauma and ischemia. Several groups have published the first positive results using protein transduction domains for the delivery of therapeutic proteins in relevant animal models of human neurological disorders. Here, we give an extensive review of peptide-mediated protein transduction from its early beginnings to new advances, discuss their application, with particular focus on a critical evaluation of the limitations of the method, as well as alternative approaches. Besides applications in neurobiology, a large number of reports using PTD in other systems are included as well. Because each protein requires an individual purification scheme that yields sufficient quantities of soluble, transducible material, the neurobiologist will benefit from the experiences of other researchers in the growing field of protein transduction.

Probing essential residues for cellular uptake with a cationic nuclear localization signal sequence

The nuclear localization signal sequence (NLS) of the transcription factor NF-kappaB is a cationic peptide with the ability to cross the cytoplasmic membrane and facilitate the delivery of attached cargo, such as DNA and proteins, to cells. Previous research had pointed to the essential role of cationic residues, therefore, the importance of residues within the NLS of NF-kappaB was evaluated for cellular uptake using an alanine replacement strategy. Although it was expected that removal of the cationic groups would have the greatest effect on membrane translocation, the most significant decreases in cellular uptake occurred with the replacement of the hydrophilic Q6 (80%) and the hydrophobic L8 (70%) residues. Replacement of the positively charged residues resulted in 30-40% decrease in cellular uptake, indicating that electrostatic interactions are not the primary driving force for membrane translocation.

beta-Homolysine oligomers: a new class of Trojan carriers

We describe the design, synthesis and cell-membrane translocation properties of a series of beta-peptides with the general sequence fluorescein-Adoa-(beta-homolysine)(n)-NH(2), n=5-8 and Adoa=8-amino-3,6-dioxaoctanoic acid. These beta-peptides are able to cross the cytoplasmic membrane and accumulate in the nucleus of mammalian cells.

Translocating proline-rich peptides from the antimicrobial peptide bactenecin 7

The intracellular delivery of most peptides, proteins, and nucleotides to the cytoplasm and nucleus is impeded by the cell membrane. To allow simplified, noninvasive delivery of attached cargo, cell-permeant peptides that are either highly cationic or hydrophobic have been utilized. Because cell-permeable peptides share half of the structural features of antimicrobial peptides containing clusters of charge and hydrophobic residues, we have explored antimicrobial peptides as templates for designing cell-permeant peptides. We prepared synthetic fragments of Bac 7, an antimicrobial peptide with four 14-residue repeats from the bactenecin family. The dual functions of cell permeability and antimicrobial activity of Bac 7 were colocalized at the N-terminal 24 residues of Bac 7. In general, long fragments of Bac(1-24) containing both regions were bactericidal and cell-permeable, whereas short fragments with only a cationic or hydrophobic region were cell-permeant without the attendant microbicidal activity when measured in a fluorescence quantitation assay and by confocal microscopy. In addition, the highly cationic fragments were capable of traversing the cell membrane and residing within the nucleus. A common characteristic shared by the cell-permeant Bac(1-24) fragments, irrespective of their number of charged cationic amino acids, is their high proline content. A 10-residue proline-rich peptide with two arginine residues was capable of delivering a noncovalently linked protein into cells. Thus, the proline-rich peptides represent a potentially new class of cell-permeant peptides for intracellular delivery of protein cargo. Furthermore, our results suggest that antimicrobial peptides may represent a rich source of templates for designing cell-permeant peptides.

Hybrid tetanus toxin C fragment-diphtheria toxin translocation domain allows specific gene transfer into PC12 cells

To study the mechanism by which genes can efficiently be transferred into specific cell types, we have constructed several novel, single-chain multicomponent proteins by recombining the nontoxic C fragment of tetanus toxin and the translocation domain of diphtheria toxin together with the DNA-binding fragment of GAL4 transcription factor, for transportation of plasmid DNA into neuronal cells. The C fragment of tetanus toxin provided neuronal selectivity, the translocation domain of diphtheria toxin permitted endosomal escape, and the GAL4 domain provided binding to DNA. To assess the cellular tasks of each component in gene transfer, different combinations of these fragments were produced by polymerase chain reaction, expressed in Escherichia coli, and purified under native conditions from the soluble proteins. We show that only fusion proteins bearing the C fragment of tetanus toxin bind to gangliosides and, followed by their specific binding to differentiated PC12 cells, are internalized within 10 min. These proteins delivered the green fluorescence protein gene to PC12 cells, with the highest transfection efficiency achieved with proteins containing both the C fragment and the translocation domain. Addition of chloroquine elevated the transfection efficiency, which was further increased by incorporation of a nuclear localization signal in the delivery system. In addition, the effect of different DNA-condensing materials (poly-L-lysine, protamine, lysine(n=8)-trytophan(n=2)-lysine(n=8)) on gene transfer was investigated.

Cellular import mediated by nuclear localization signal Peptide sequences

The cellular delivery of therapeutic agents and their localization within cells is currently a great challenge in medicinal chemistry. A few cationic peptides have shown a strong propensity to cross the cytoplasmic membrane and enter cells. Nuclear localization signal (NLS) sequences are a class of highly cationic peptides that may be exploited for cellular import of linked cargo. A series of NLS sequence peptides were investigated for entry into different cancer cell lines by flow cytometry and confocal microscopy. All NLS peptides demonstrated rapid accumulation within cells when added to the cellular media. Covalent adducts of proteins and oligonucleotides with NLS peptides were also effectively imported within cells. An understanding of the structural and mechanistic properties of these sequences will provide great potential for the rational design of efficient and selective peptidic delivery systems.

In vivo generation of cytotoxic T cells from epitopes displayed on peptide-based delivery vehicles

The development of nonviral, peptide-based constructs able to elicit protective in vivo CTL responses represents a major challenge in the design of future vaccines. We report the design of branched peptide delivery vehicles, termed loligomers, that facilitate the import, processing, and presentation of CTL epitopes onto nascent MHC class I molecules. These complexes are then effectively displayed on the surface of APCs. The intracellular delivery of CTL epitopes by loligomers prolonged the expression of Ag-MHC class I complexes on the surface of APCs in comparison with free CTL epitope alone. Furthermore, the injection of CTL epitope-containing loligomers into mice led to the generation of in vivo CTL responses and the induction of autoimmune disease in an animal model. Synthetic epitope-carrying, peptide-based delivery vehicles may represent useful components to be included in the formulation of future vaccines.

Utilization of synthetic peptides containing nuclear localization signals for nonviral gene transfer systems

The ability of nonviral gene delivery systems to overcome extracellular and intracellular barriers is a critical issue for future clinical applications. In recent years, several efforts were focused on the elucidation of the gene transfer mechanisms and on the development of multicomponent systems in order to improve both targeted gene delivery and transfection efficiency. The transport of the therapeutic DNA from the cytoplasm into the nucleus is an inefficient process and is considered as the major limiting step in nondividing cells. One of the strategies to improve nuclear uptake of DNA is taking advantage of the cellular nuclear import machinery. Synthetic peptides containing a nuclear localization signal (NLS) are bound to the DNA so that the resulting DNA-NLS complex can be recognized as a nuclear import substrate by specific intracellular receptor proteins. In this review, we critically summarize recent studies applying this approach with a particular focus on NLS-sequence specificity. Implications of the observed results are also discussed in regards to future developments of this technology.

Designing peptide-based scaffolds as drug delivery vehicles

Methods for delivering drugs into cells remain an important part of the process of designing drugs. One promising approach is the concept of loligomers, synthetic peptides composed of a branched polylysine core harboring identical arms. Loligomers are typically synthesized with eight arms, each carrying peptide signals guiding their import and localization into cells. The most important advantage of loligomers is the multivalent presentation of targeting signals resulting from a tentacular arrangement. Multivalency increases the efficiency of import and intracellular routing signals as compared to similar linear peptides. Secondly, it reduces and delays the impact of peptide degradation in terms of cellular processing and compartmentalization. The vectorial delivery of nucleus-directed loligomers into cells has recently been confirmed by microscopy and flow cytometry studies. Practical uses of loligomers as intracellular vehicles include the import of plasmid DNA into cells, the conjugation of chemical groups, such as photosensitizers for use in photodynamic therapy, and the incorporation of cytotoxic T-lymphocyte (CTL) epitopes with a view to creating synthetic vaccines. Branched peptides such as loligomers represent simple and versatile molecular vehicles with potential applications in a wide variety of drug design approaches.

Vectorial delivery of macromolecules into cells using peptide-based vehicles

The ability to direct the import of therapeutic agents into cells and target them to specific organelles would greatly enhance their functional efficacy. The available spectrum of peptide-based import signals and intracellular routing signals might provide practical solutions towards achieving a guided or vectorial delivery of molecules. Multiple cell-targeting signals and routing domains can be efficiently displayed on branched peptides. These constructs are typically nonimmunogenic in the absence of adjuvant and can be easily assembled using solid phase synthesis. The vectorial delivery of larger complexes, however, will necessitate the development of alternate templates that favor the optimal presentation of all functional signals.

A CDC6 protein-binding peptide selected using a bacterial two-hybrid-like system is a cell cycle inhibitor

Peptides or small molecules able to modulate protein-protein interactions hold promise as tools with which to probe and manipulate biological pathways. An important issue in this nascent field is to evaluate different methods with which to search libraries for molecules that modulate the function of specific target proteins. One strategy is to screen libraries for molecules that bind specifically to a protein known to be critical in the pathway of interest, with the expectation that the molecules isolated will recognize regions of the target protein important for its function and thereby exhibit biological activity. Here, a peptide library was screened using a two-hybrid-like system for molecules able to bind human CDC6 protein (CDC6p), required for the initiation of DNA replication in eukaryotic cells. From a collection of over a million peptides, a single species that exhibited good affinity and specificity for binding CDC6p was obtained. When expressed in human cells, the peptide inhibited cell cycle progression and exhibited other properties expected of a CDC6p inhibitor. This approach, which does not require detailed knowledge of the mechanism of action of a protein target, may be generally useful for isolating peptides capable of manipulating biological pathways.

Noninvasive intracellular delivery of functional peptides and proteins

In order to probe intracellular signaling based on interactions of thousands of proteins expressed in the living cell, new methods of noninvasive delivery of functional peptides and proteins to cells have been developed. These include cellular import of peptides and proteins based on the cell-membrane-permeable properties of the hydrophobic region of a signal peptide sequence. The prototypical cell-permeable SN50 peptide, which contains the nuclear localization signal sequence of NK-kappaB p50, has been applied in multiple cell types to block nuclear import of this and other transcription factors. Further developments, including site-specific ligation of bipartite import peptides and production of import-competent recombinant proteins, provide the means for easy and rapid delivery of peptides and proteins to a wide spectrum of cells in order to regulate intracellular pathways involved in adhesion, signaling and trafficking to the nucleus.