Methodological and cellular aspects that govern the internalization mechanisms of arginine-rich cell-penetrating peptides

Molecular partners for interaction and cell internalization of cell-penetrating peptides: how identical are they?

Cell-penetrating peptides are short basic peptide sequences that might display amphipathic properties. These positively charged peptides internalize into all cell types, albeit with different efficiency. Cell-penetrating peptides use all routes of pinocytosis to internalize, in addition to direct membrane translocation that requires interaction with lipid membrane domains. These differences in internalization efficiency according to the peptide sequence and cell type suggest that the cell-penetrating peptides interact with different molecular partners at the cell surface. This review will first report on data that describe the molecular interaction of the most popular cell-penetrating peptides (penetratin, Tat and oligoarginine) with carbohydrates and lipids. The second part of the review will be dedicated to cell studies that have reported how cell surface composition influences cell internalization. Discussion will focus on the gap between in vitro and in cellulo studies, and more specifically to which extent the interaction with molecules found in membranes reflect the internalization efficiency of the peptides.

A class of human proteins that deliver functional proteins into mammalian cells in vitro and in vivo

We discovered a class of naturally occurring human proteins with unusually high net positive charge that can potently deliver proteins in functional form into mammalian cells both in vitro and also in murine retina, pancreas, and white adipose tissues in vivo. These findings represent diverse macromolecule delivery agents for in vivo applications, and also raise the possibility that some of these human proteins may penetrate cells as part of their native biological functions.

Cell-penetrating peptides-based strategies for the delivery of splice redirecting antisense oligonucleotides

Progress in our understanding of the molecular pathogenesis of human malignancies has provided therapeutic targets amenable to oligonucleotide (ON)-based strategies. Antisense ON-mediated splicing regulation in particular offers promising prospects since the majority of human genes undergo alternative splicing and since splicing defects have been found in many diseases. However, their implementation has been hampered so far by the poor bioavailability of nucleic acids-based drugs. Cell-penetrating peptides (CPPs) now appear as promising non-viral delivery vector for non-permeant biomolecules. We describe here new CPPs allowing the delivery of splice redirecting steric-block ON using either chemical conjugation or non-covalent complexation. We also describe a convenient and robust splice redirecting assay which allows the quantitative assessment of ON nuclear delivery.

Evaluation of [125I]IPOS as a molecular imaging probe for hypoxia-inducible factor-1-active regions in a tumor: comparison among single-photon emission computed tomography/X-ray computed tomography imaging, autoradiography, and immunohistochemistry

To image hypoxia-inducible factor-1 (HIF-1)-active tumors, we previously developed a chimeric protein probe ([(123/125) I]IPOS) that is degraded in the same manner as HIF-1α under normoxic conditions. In the present study, we aim to show that the accumulation of radioiodinated POS reflects the expression of HIF-1. In vivo single-photon emission computed tomography (SPECT)/X-ray CT (CT) imaging, autoradiography, and double-fluorescent immunostaining for HIF-1α and pimonidazole (PIMO) were carried out 24 h after the injection of [(125) I]IPOS. Tumor metabolite analysis was also carried out. A tumor was clearly visualized by multi-pinhole, high-resolution SPECT/CT imaging with [(125) I]IPOS. The obtained images were in accordance with the corresponding autoradiograms and with the results of ex vivo biodistribution. A metabolite analysis revealed that 77% of the radioactivity was eluted in the macromolecular fraction, suggesting that the radioactivity mainly existed as [(125) I]IPOS in the tumors. Immunohistochemistry revealed that the HIF-1α-positive areas and PIMO-positive areas were not always identical, only some of the regions were positive for both markers. The areas showing [(125) I]IPOS accumulation were positively and significantly correlated with the HIF-1α-positive areas (R = 0.75, P < 0.0001). The correlation coefficient between [(125) I]IPOS-accumulated areas and HIF-1α-positive areas was significantly greater than that between the [(125) I]IPOS-accumulated areas and the PIMO-positive areas (P < 0.01). These findings indicate that [(125) I]IPOS accumulation reflects HIF-1 expression. Thus, [(123/125) I]IPOS can serve as a useful probe for the molecular imaging of HIF-1-active tumors.

Distinct transduction modes of arginine-rich cell-penetrating peptides for cargo delivery into tumor cells

The application of cell-penetrating peptides (CPPs) for delivering various cargo molecules with biological functions into cells has gained much attention in recent years. However, the internalization mechanisms and delivery properties of CPP-cargo remains controversial. In this study, low- and high-molecular-weight cargoes attached to arginine-rich CPPs were employed: the former was the fluorescein isothiocyanate-labeled nona-arginine (CPP-FITC), and the latter was the fluorescently labeled nona-arginine-avidin complex (CPP-avidin). We measured the intracellular trafficking of CPP-FITC and CPP-avidin in four cancer cell lines in a series of microenvironments altered by the presence or absence of serum, different temperatures and different incubation times. The results revealed that CPP-cargo delivery exhibited no specificity toward any cell line, but the levels were found to be related to cell type and cargo. Furthermore, their endocytic mechanisms were investigated via incubation with related endocytic inhibitors. Two different types of CPP-cargo were required to cross the plasma membrane to bind to cell surface-associated heparan sulfate proteoglycans in a time-dependent manner. CPPs and small cargoes attached to CPP may enter cells rapidly via direct translocation in addition to the endocytic route. Translocation of large components linked to CPP tended to be mediated by macropinocytosis in an energy-dependent manner with slower rates for larger compounds. In contrast, the clathrin-dependent pathway is not essential to the translocation of either type of CPP-cargo.

A gene delivery system for human cells mediated by both a cell-penetrating peptide and a piggyBac transposase

The piggyBac (PB) transposable element has recently accumulated enormous attention as a tool for the transgenesis in various eukaryotic organisms. Arginine-rich cell-penetrating peptides (CPPs) are protein transduction domains containing a large amount of basic amino acids that were found to be capable of delivering biologically active macromolecules into living cells. In this study, we demonstrate a strategy, which we called "transposoduction", which is a one-plasmid gene delivery system mediated by the nontoxic CPP-piggyBac transposase (CPP-PBase) fusion protein to accomplish both protein transduction and transposition. CPPs were proven to be able to synchronously deliver covalently linked PBase and noncovalently linked a cis plasmid into human cells. The expression of promoterless reporter genes coding for red (dTomato) and yellow (mOrange) fluorescent proteins (RFP and YFP) with PB elements could be detected in cells treated with the PBase-expressing plasmid after 3 days indicating transposition of coding regions to downstream of endogenous promoter sequences. An enhanced green fluorescent protein (EGFP) plasmid-based excision assay further confirmed the efficiency of the bifunctional CPP-PBase fusion protein. In conclusion, this strategy representing a combinational concept of both protein transduction and mobile transposition may provide tremendous potential for safe and efficient cell line transformation, gene therapy and functional genomics.

Exogenous and cell surface glycosaminoglycans alter DNA delivery efficiency of arginine and lysine homopeptides in distinctly different ways

Glycosaminoglycans (GAGs) expressed ubiquitously on the cell surface are known to interact with a variety of ligands to mediate different cellular processes. However, their role in the internalization of cationic gene delivery vectors such as liposomes, polymers, and peptides is still ambiguous and seems to be controlled by multiple factors. In this report, taking peptides as model systems, we show that peptide chemistry is one of the key factors that determine the dependence on cell surface glycosaminoglycans for cellular internalization and gene delivery. Arginine peptides and their complexes with plasmid DNA show efficient uptake and functional gene transfer independent of the cell surface GAGs. On the other hand, lysine peptides and complexes primarily enter through a GAG-dependent pathway. The peptide-DNA complexes also show differential interaction with soluble GAGs. In the presence of exogenous GAGs under certain conditions, arginine peptide-DNA complexes show increased transfection efficiency that is not observed with lysine. This is attributed to a change in the complex nature that ensures better protection of the compacted DNA in the case of arginine complexes, whereas the lysine complexes get destabilized under these conditions. The presence of a GAG coating also ensures better cell association of arginine complexes, resulting in increased uptake. Our results indicate that the role of both the cell surface and exogenous glycosaminoglycans in gene delivery is controlled by the nature of the peptide and its complex with DNA.

A single replacement of histidine to arginine in EGFR-lytic hybrid peptide demonstrates the improved anticancer activity

We previously reported that novel targeted "hybrid peptide" in which epidermal growth factor receptor (EGFR) binding peptide was conjugated with lytic-type peptide had selective cytotoxic activity to EGFR expressing cancer cell lines, and in vivo analysis revealed that this EGFR-lytic peptide displayed significant antitumor activity in a xenograft model of human breast cancer which was resistant to tyrosine kinase inhibitor drugs. As an attempt to improve the selective anticancer activity of EGFR-lytic peptide, we modified the EGFR-binding peptide through introducing the mutation of amino acid according to biophysical analysis by biomolecular interaction and circular dichroism (CD) spectra. When cytotoxic activity of EGFR-lytic or EGFR(2R)-lytic hybrid peptides was investigated in various human cancer and normal cell lines, it was demonstrated that EGFR(2R)-lytic, in which second histidine (H) of EGFR-binding peptide was replaced to arginine (R) had 1.2-1.9-fold higher cytotoxic activity than that of original EGFR-lytic peptide. In vivo analysis also revealed that this modified peptide displayed significant antitumor activity at as low as 1 mg/kg dosage. These results suggest that mutated arginine on EGFR-lytic peptide produces higher binding ability to EGFR on cancer cells, and thereby the improved anticancer activity.

Cooperative, heparan sulfate-dependent cellular uptake of dimeric guanidinoglycosides

Oligoarginine and guanidinium-rich molecular transporters have been shown to facilitate the intracellular delivery of a diverse range of biologically relevant cargos. Several such transporters have been suggested to interact with cell-surface heparan sulfate proteoglycans as part of their cell-entry pathway. Unlike for other guanidinium-rich transporters, the cellular uptake of guanidinoglycosides at nanomolar concentrations is exclusively heparan sulfate dependent. As distinct cells differ in their expression levels and/or the composition of cell-surface heparan sulfate proteoglycans, one might be able to exploit such differences to selectively target certain cell types. To systematically investigate the nature of their cell-surface interactions, monomeric and dimeric guanidinoglycosides were synthesized by using neomycin, paromomycin, and tobramycin as scaffolds. These transporters differ in the number and 3D arrangement of their guanidinium groups. Their cellular uptake was measured by flow cytometry in wild-type and mutant Chinese hamster ovary cells after the corresponding fluorescent streptavidin-phycoerythrin-Cy5 conjugates had been generated. All derivatives showed negligible uptake in mutant cells lacking heparan sulfate. Decreasing the number of guanidinium groups diminished uptake, but the three dimensional arrangement of these groups was less important for cellular delivery. Whereas conjugates prepared with the monomeric carriers showed significantly reduced uptake in mutant cells expressing heparan sulfate chains with altered patterns of sulfation, conjugates prepared with the dimeric guanidinoglycosides could overcome this deficiency and maintain high levels of uptake in such deficient cells. This finding suggests that cellular uptake depends on the valency of the transporter and both the content and arrangement of the sulfate groups on the cell-surface receptors. Competition studies with chemically desulfated or carboxy-reduced heparin derivatives corroborated these observations. Taken together, these findings show that increasing the valency of the transporters retains heparan sulfate specificity and provides reagents that could distinguish different cell types based on the specific composition of their cell-surface heparan sulfate proteoglycans.

Peptide nanoparticles for oligonucleotide delivery

In the past two decades, different methods have emerged for intervention with gene expression, which can be generally referred to as gene therapy. Oligonucleotides (ONs) and their analogs form the basis of the molecules that can be used to modulate gene expression. Unfortunately, due to their physicochemical properties, these molecules require assistance in their intracellular delivery. Cell-penetrating peptides (CPPs) are one class of nonviral delivery vectors that, because of their remarkable translocation properties, have been intensely utilized for the delivery of ON-based molecules, both in vitro and in vivo. This chapter concentrates on the applications of CPPs that directly form nanoparticles with different ONs and facilitate their intracellular delivery.

Targeting antibodies to the cytoplasm

A growing number of research consortia are now focused on generating antibodies and recombinant antibody fragments that target the human proteome. A particularly valuable application for these binding molecules would be their use inside a living cell, e.g., for imaging or functional intervention. Animal-derived antibodies must be brought into the cell through the membrane, whereas the availability of the antibody genes from phage display systems allows intracellular expression. Here, the various technologies to target intracellular proteins with antibodies are reviewed.

Construction of cell penetrating peptide vectors with N-terminal stearylated nuclear localization signal for targeted delivery of DNA into the cell nuclei

Cellular uptake and nuclear localization are two barriers to gene delivery. Here, we designed new gene delivery carriers with an N-terminal stearylated (STR) nuclear localization signal (NLS), PKKKRKV, present in the Simian Virus 40 large T antigen with the aim to overcome limitations, such as cell membrane and nuclear pores, offering attractive possibilities to enhance gene delivery. Four vectors with different structures of N-stearylated nuclear localization signal-octaarginine peptide (STR-PKKKRKV-R(8) or STR-NLS-R(8), STR-VKRKKKP-R(8) or STR-reverse NLS-R(8), PKKKRKV-R(8) or NLS-R(8), and VKRKKKP-R(8) or reverse NLS-R(8)) were compared. The gene expression mediated by these vectors in dividing and non-dividing cells (both in 293T and HeLa cell lines) was investigated. The most efficient N/P ratio was 4 for STR-PKKKRKV-R(8,) STR-VKRKKKP-R(8,) and 0.25 for PKKKRKV-R(8), VKRKKKP-R(8.) The maximum transfection activity of these vehicles (VKRKKKP-R(8)) was up to 80% as effective as jetPEI™ and the vehicles did not exhibit cytotoxicity. Interestingly, N-stearylated peptides presented lower transfection activity compared to peptides without N-stearylation at lower N/P ratios (0.25 to 1). Confocal study showed that the vectors could effectively promote the nuclear translocation.

Synthesis of a pH-sensitive nitrilotriacetic linker to peptide transduction domains to enable intracellular delivery of histidine imidazole ring-containing macromolecules

Intracellular delivery of functional macromolecules using peptide transduction domains (PTDs) is an exciting technology with both experimental and therapeutic applications. Recent data indicate that PTD-mediated transduction occurs via fluid-phase macropinocytosis involving an intracellular pH drop to approximately 5. Nitrilotriacetic acid (NTA)-coordinated metals avidly bind hexahistidine-tagged macromolecules, including peptides and proteins. Histidine's imidazole ring has a pK(a) of 6, making this an attractive target for the biological pH drop of PTD-mediated macropinocytotic delivery. The objective of this study was to develop a pH-sensitive PTD delivery peptide (NTA(3)-PTD). We demonstrate the in vitro function of this novel peptide by delivering fluorescently labeled peptides (1.6 kDa) and functional enzymes, beta-galactosidase (119 kDa) and Cre recombinase (37 kDa). Furthermore, the NTA(3)-PTD peptide was able to deliver functional Cre recombinase in an in vivo mouse model.

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

We describe the synthesis and cellular delivery properties of multivalent and branched delivery systems consisting of cell-penetrating peptides assembled onto a peptide scaffold using native chemical ligation. A trimeric delivery system presenting three copies of the prototypical cell-penetrating peptide TAT shows an endosomolytic activity much higher than its monomeric and dimeric counterparts. This novel reagent promotes the endosomal release of macromolecules internalized into cells by endocytosis, and as a result, it can be used to achieve cytosolic delivery of bioactive but cell-impermeable macromolecules in either cis (covalent conjugation) or trans (simple coincubation).

Cyclodextrin-based gene delivery systems

Cyclodextrin (CD) history has been largely dominated by their unique ability to form inclusion complexes with guests fitting in their hydrophobic cavity. Chemical funcionalization was soon recognized as a powerful mean for improving CD applications in a wide range of fields, including drug delivery, sensing or enzyme mimicking. However, 100 years after their discovery, CDs are still perceived as novel nanoobjects of undeveloped potential. This critical review provides an overview of different strategies to promote interactions between CD conjugates and genetic material by fully exploiting the inside-outside/upper-lower face anisotropy of the CD nanometric platform. Covalent modification, self-assembling and supramolecular ligation can be put forward with the ultimate goal to build artificial viruses for programmed and efficient gene therapy (222 references).

Basic tetrapeptides as potent intracellular inhibitors of type A botulinum neurotoxin protease activity

Botulinum neurotoxins (BoNT) are the most potent of all toxins that cause flaccid muscle paralysis leading to death. They are also potential biothreat agents. A systematic investigation of various short peptide inhibitors of the BoNT protease domain with a 17-residue peptide substrate led to arginine-arginine-glycine-cysteine having a basic tetrapeptide structure as the most potent inhibitor. When assayed in the presence of dithiothreitol (DTT), the inhibitory effect was drastically reduced. Replacing the terminal cysteine with one hydrophobic residue eliminated the DTT effect but with two hydrophobic residues made the pentapeptide a poor inhibitor. Replacing the first arginine with cysteine or adding an additional cysteine at the N terminus did not improve inhibition. When assessed using mouse brain lysates, the tetrapeptides also inhibited BoNT/A cleavage of the endogenous SNAP-25. The peptides penetrated the neuronal cell lines, N2A and BE(2)-M17, without adversely affecting metabolic functions as measured by ATP production and P-38 phosphorylation. Biological activity of the peptides persisted within cultured chick motor neurons and rat and mouse cerebellar neurons for more than 40 h and inhibited BoNT/A protease action inside the neurons in a dose- and time-dependent fashion. Our results define a tetrapeptide as the smallest peptide inhibitor in the backdrop of a large substrate protein of 200+ amino acids having multiple interaction regions with its cognate enzyme. The inhibitors should also be valuable candidates for drug development.

A specific binding site for the prorenin propart peptide Arg10-Arg20 does not occur on human endothelial cells

INTRODUCTION

We looked for novel binding sites for the human prorenin 'decoy peptide' sometimes called 'handle region peptide' on human endothelial cells.

METHOD

The biotinylated peptide biotin-Acp-RIFLKRMPSIR (B-PR), an unlabelled peptide PR1 (RIFLKRMPSIR) and a scrambled peptide scPR1 (SRRMIFPIKLR) were synthesized. B-PR was added to human umbilical cord endothelial cells (HUVECs) maintained in serum-free medium, with or without excess unlabelled peptide or 'scrambled' peptide as blocker. Biotin-labelled HUVEC proteins were extracted, the amount of bound tracer was measured, and the identity of the binding proteins was analysed by sodium-dodecyl-sulphate polyacrylamide gel electrophoresis (SDS-PAGE) and by liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS).

RESULTS

Biotinylated peptide bound to the HUVEC proteins with a major labelled band at 68,600 ± 1503 kDa (mean ± SEM, n = 5 runs). Unlabelled peptide and scrambled peptide equally displaced the labelled peptide, indicating that the binding was non-specific for amino acid sequence. LC-MS/MS showed that binding was mainly to cytoskeletal proteins.

CONCLUSION

The binding of the human prorenin peptide R¹⁰IFLKRMPSIR²⁰ to HUVEC proteins is not specific for amino acid sequence and probably involves a general peptide/protein uptake mechanism. We could not detect a specific prorenin propart binding site in these cells.

Adenosine analogue-oligo-arginine conjugates (ARCs) serve as high-affinity inhibitors and fluorescence probes of type I cGMP-dependent protein kinase (PKGIalpha)

INTRODUCTION

Type I cGMP-dependent protein kinase (PKGIalpha) belongs to the family of cyclic nucleotide-dependent protein kinases and is one of the main effectors of cGMP. PKGIalpha is involved in regulation of cardiac contractility, vasorelaxation, and blood pressure; hence, the development of potent modulators of PKGIalpha would lead to advances in the treatment of a variety of cardiovascular diseases.

AIM

Representatives of ARC-type compounds previously characterized as potent inhibitors and high-affinity fluorescent probes of PKA catalytic subunit (PKAc) were tested towards PKGIalpha to determine that ARCs could serve as activity regulators and sensors for the latter protein kinase both in vitro and in complex biological systems.

RESULTS

Structure-activity profiling of ARCs with PKGIalpha in vitro demonstrated both similarities as well as differences to corresponding profiling with PKAc, whereas ARC-903 and ARC-668 revealed low nanomolar displacement constants and inhibition IC(50) values with both cyclic nucleotide-dependent kinases. The ability of ARC-based fluorescent probes to penetrate cell plasma membrane was demonstrated in the smooth muscle tissue of rat cerebellum isolated arteries, and the compound with the highest affinity in vitro (ARC-903) showed also potential for in vivo applications, fully abolishing the PKG1alpha-induced vasodilation.

Oligoarginine-linked polymers as a new class of penetration enhancers

Oligoarginines, which are known as cell-penetrating peptides, enhance the cellular uptake of poorly membrane-permeable bioactive molecules that are chemically conjugated to them. We designed a novel polymer: oligoarginine-linked poly(N-vinylacetamide-co-acrylic acid), with the expectation that the polymers will enhance the cellular uptake of the bioactive molecules that are physically mixed with them. Oligoarginines were grafted onto the polymer backbone through the chemical reaction with acrylic acid functional groups. The changes in the blood glucose concentration after nasal administration of insulin with and without the polymer were monitored in mice. The blood glucose concentration was slightly reduced when insulin was given solely at a dose of 10IU/kg. A D-octaarginine-linked poly(N-vinylacetamide-co-acrylic acid) with a grafting degree of 2% significantly enhanced the insulin-induced hypoglycemic effect. A similar enhancement was not observed when the polymer was substituted with intact D-octaarginine. The penetration-enhancing function of D-octaarginine-linked poly(N-vinylacetamide-co-acrylic acid) increased dramatically with an increase in the grafting degree of D-octaarginine. Substitution of D-octaarginine with the corresponding optical isomer and an increase in the number of arginine residues rather reduced the penetration-enhancing function. In vitro cell studies also indicated that a D-octaarginine-linked poly(N-vinylacetamide-co-acrylic acid) with a grafting degree of 17% enabled fluorescein isothiocyanate-dextran to effectively penetrate the cell membrane. Results demonstrated that our oligoarginine-linked polymer has a potential to provide a new class of penetration enhancers.

Small-molecule FRET probes for protein kinase activity monitoring in living cells

In this study, the applicability of fluorescently labeled adenosine analogue-oligoarginine conjugates (ARC-Photo probes) for monitoring of protein kinase A (PKA) activity in living cells was demonstrated. ARC-Photo probes possessing subnanomolar affinity towards the catalytic subunit of PKA (PKAc) and competitive with the regulatory subunit (PKAr), penetrate cell plasma membrane and associate with PKAc fused with yellow fluorescent protein (PKAc-YFP). Detection of inter-molecular Förster resonance energy transfer (FRET) efficiency between the fluorophores of the fusion protein and ARC-Photo probe can be used for both the evaluation of non-labeled inhibitors of PKAc and for monitoring of cAMP signaling via detection of changes in the activity of PKA as a cAMP downstream effector.

Guanidinylated neomycin mediates heparan sulfate-dependent transport of active enzymes to lysosomes

Guanidinylated neomycin (GNeo) can transport bioactive, high molecular weight cargo into the interior of cells in a process that depends on cell surface heparan sulfate proteoglycans. In this report, we show that GNeo-modified quantum dots bind to cell surface heparan sulfate, undergo endocytosis and eventually reach the lysosomal compartment. An N-hydroxysuccinimide activated ester of GNeo (GNeo-NHS) was prepared and conjugated to two lysosomal enzymes, beta-D-glucuronidase (GUS) and alpha-L-iduronidase. Conjugation did not interfere with enzyme activity and enabled binding of the enzymes to heparin-Sepharose and heparan sulfate on primary human fibroblasts. Cells lacking the corresponding lysosomal enzyme took up sufficient amounts of the conjugated enzymes to restore normal turnover of glycosaminoglycans. The high capacity of proteoglycan-mediated uptake suggests that this method of delivery might be used for enzyme replacement or introduction of foreign enzymes into cells.

Targeting a ruthenium complex to the nucleus with short peptides

In an effort to develop octahedral metal complexes as chemotherapeutic and diagnostic agents targeted to DNA, it is critical to optimize the properties of their cellular uptake. Appending d-octaarginine has been found to improve both the uptake and nuclear localization efficiency of these complexes, but the increased positive charge interferes with selective DNA binding and hence activity. Herein, we evaluate the nuclear entry of a series of luminescent ruthenium peptide conjugates of shorter sequence and lower charge. As is the case for the d-octaarginine conjugate (Ru-D-R8), the tetrapeptide RrRK (where r=d-arginine) facilitates nuclear localization of the ruthenium complex above a threshold concentration, though the threshold is higher for this conjugate (Ru-RrRK) than for Ru-D-R8. Furthermore, appended fluorescein, which lowers the threshold concentration for Ru-D-R8, does not improve nuclear entry of Ru-RrRK, indicating that fluorescein conjugation is not a general strategy for modulating the distribution of cell-penetrating peptides. Similarly, the concentration required for nuclear entry of Ru-RrRK is much higher than has been reported for a thiazole orange RrRK conjugate, demonstrating the influence of payload on the efficiency of uptake and localization of cell-penetrating peptides.

Dendritic Guanidines as Efficient Analogues of Cell Penetrating Peptides

The widespread application of cell penetrating agents to clinical therapeutics and imaging agents relies on the ability to prepare them on a large scale and to readily conjugate them to their cargos. Dendritic analogues of cell penetrating peptides, with multiple guanidine groups on their peripheries offer advantages as their high symmetry allows them to be efficiently synthesized, while orthogonal functionalities at their focal points allow them to be conjugated to cargo using simple synthetic methods. Their chemical structures and properties are also highly tunable as their flexibility and the number of guanidine groups can be tuned by altering the dendritic backbone or the linkages to the guanidine groups. This review describes the development of cell-penetrating dendrimers based on several different backbones, their structure-property relationships, and comparisons of their efficacies with those of known cell penetrating peptides. The toxicities of these dendritic guanidines are also reported as well as their application towards the intracellular delivery of biologically significant cargos including proteins and nanoparticles.

Arginine-rich intracellular delivery peptides synchronously deliver covalently and noncovalently linked proteins into plant cells

Protein transduction domains (PTDs) are small peptides with a high content of basic amino acids, and they are responsible for cellular uptake. Many PTDs, including arginine-rich intracellular delivery (AID) peptides, have been shown to transport macromolecules across membranes and into cells. In this study, we demonstrated for the first time that AID peptides could rapidly and efficiently deliver proteins into plant cells in both covalent and noncovalent protein transductions (CNPT) simultaneously. The optimal molecular ratio between an AID peptide carrier and cargo in CNPT was about 3:1. Fluorescence resonance energy transfer (FRET) analysis revealed protein-protein interactions between AID peptide carriers and cargos after CNPT in cells. The possible mechanisms of AID peptides-mediated cellular entry might involve a combination of multiple internalization pathways. Therefore, applications by AID peptide-mediated CNPT may provide a simple and direct transport strategy for delivering two proteins in agricultural systems.

New daunomycin-oligoarginine conjugates: synthesis, characterization, and effect on human leukemia and human hepatoma cells

In this article, the synthesis, a novel chromatographic procedure and characteristics of a new class of daunomycin (Dau)-oligoarginine conjugates are described. In these compounds oligoarginine with 6 or 8 residues (Arg(n), n = 6, 8) is attached to Dau by different covalent bond: squaric amide (Dau- square-Arg(n)), oxime (Dau=N-O-CH2-CO-Arg(n)), or hydrazone (H-Glu(Arg(n))-NH-N=Dau). Conjugates were characterized by RP-HPLC and mass spectrometry. We report also on our findings concerning chemical and biological properties of Dau-conjugates as a function of covalent linkage, site of conjugation and length of the oligoarginine moiety. Stability, fluorescent properties as well as cytostatic effect and cellular uptake of these compounds were studied. Dau-conjugates with squaric amide or oxime linkage were stable, but continuous release of free Dau was observed from the hydrazone conjugate in solution. We found that some spectral characteristics (e.g., the amplitude of the emission spectrum) of conjugates could be sensitive for the site of coupling (amino vs. oxo function). Cytostasis and cellular uptake of conjugates were investigated both on human leukemia (HL-60) and human hepatoma (HepG2) cell lines by MTT assay and flow cytometry We found that cytostatic effect and uptake properties of Dau-conjugates were dependent on the acid stability of the linkage (hydrazone vs. oxime/amide) applied and more markedly on the cell line studied.

Cell-penetrating peptide exploited syndecans

Cell-penetrating peptides (CPPs) are short peptides capable of translocating across the plasma membrane of live cells and transporting conjugated compounds intracellularly. Fifteen years after discovering the first model cationic CPPs, penetratin and TAT, CPP internalization is still challenging many questions. Particularly it has been unknown whether CPPs enter the cells with or without mediation of a specific surface receptor. Here we report that syndecan-4, the universally expressed isoform of the syndecan family of transmembrane proteoglycans, binds and mediates transport of the three most frequently utilized cationic CPPs (penetratin, octaarginine and TAT) into the cells. Quantitative uptake studies and mutational analyses demonstrate that attachment of the cationic CPPs is mediated by specific interactions between the heparan sulfate chains of syndecan-4 and the CPPs. Protein kinase C alpha is also heavily involved in the uptake mechanism. The collected data give the first direct evidence on the receptor-mediated uptake of cationic CPPs and may replace the long-thought, but already contradicted membrane penetration hypothesis. Thus our study might give an answer for a decade long debate and foster the development of rationalized, syndecan-4 targeted novel delivery technologies.

The glycosaminoglycan-binding domain of PRELP acts as a cell type-specific NF-kappaB inhibitor that impairs osteoclastogenesis

The PRELP heparin sulfate–binding protein translocates to the nucleus, where it impairs NF-κB transcriptional activity, which in turn regulates bone homeostasis.

Proline/arginine-rich end leucine-rich repeat protein (PRELP) is a glycosaminoglycan (GAG)- and collagen-binding anchor protein highly expressed in cartilage, basement membranes, and developing bone. We observed that PRELP inhibited in vitro and in vivo mouse osteoclastogenesis through its GAG-binding domain (^hbdPRELP), involving (a) cell internalization through a chondroitin sulfate– and annexin II–dependent mechanism, (b) nuclear translocation, (c) interaction with p65 nuclear factor κB (NF-κB) and inhibition of its DNA binding, and (d) impairment of NF-κB transcriptional activity and reduction of osteoclast-specific gene expression. ^hbdPRELP does not disrupt the mitogen-activated protein kinase signaling nor does it impair cell survival. ^hbdPRELP activity is cell type specific, given that it is internalized by the RAW264.7 osteoclast-like cell line but fails to affect calvarial osteoblasts, bone marrow macrophages, and epithelial cell lines. In vivo, ^hbdPRELP reduces osteoclast number and activity in ovariectomized mice, underlying its physiological and/or pathological importance in skeletal remodeling.

The role of intermolecular interactions with penetratin and its analogue on the enhancement of absorption of nasal therapeutic peptides

We investigated the relationship between intermolecular binding and the ability of novel cell-penetrating peptides (CPPs) to enhance the nasal absorption of therapeutic peptides and proteins. The absorption-enhancing effect of a novel l-penetratin analogue, 'shuffle (R,K fix) 2' coadministered with different biotherapeutic peptides was evaluated after nasal administration in rats. Shuffle (R,K fix) 2 significantly increased the nasal absorption of insulin, glucagon-like-peptide-1 (GLP-1) and exendin-4, compared with the absorption seen with l-penetratin. Intermolecular binding was analyzed by surface plasmon resonance (SPR)-based binding assay. The binding characteristics implied that the higher the amount of CPP bound, the greater the nasal drug absorption. In addition, the calculated binding ratio between CPP and drug proved a critical aspect in enhancing the absorption of insulin and GLP-1. This difference in the enhancing effect of CPPs on nasal drug absorption is attributed to the degree of binding with the therapeutic macromolecule.