Probing the impact of valency on the routing of arginine-rich peptides into eukaryotic cells

Arginine Surface Density of Nanoparticles Controls Nonendocytic Cell Uptake and Autophagy Induction

Nonendocytic cell uptake of nanomaterials is challenging, which requires specific surface chemistry and smaller particle size. Earlier works have shown that an arginine-terminated nanoparticle of <10-20 nm size shows nonendocytic uptake via direct membrane penetration. However, the roles of surface arginine density and the arginine-arginine distance at the nanoparticle surface in controlling such nonendocytic uptake mechanism is not yet explored. Here we show that a higher arginine density at the nanoparticle surface with an arginine-arginine distance of <3 nm is the most critical aspect for such nonendocytic uptake. We have used quantum dot (QD)-based nanoparticles as a model for fluorescent tracking inside cells and for quantitative estimation of cellular uptake. We found that arginine-terminated nanoparticles of 10 nm size can opt for the energy-dependent endocytosis pathway if the arginine-arginine distance is >3 nm. In contrast, nanoparticles with <3 nm arginine-arginine distance rapidly enter into the cell via the nonendocytic approach, are freely available in the cytosol in large amounts to capture the cellular adenosine triphosphate (ATP), generate oxidative stress, and induce ATP-deficient cellular autophagy. This work shows that arginine-arginine distance at the nanoparticle surface is another fundamental parameter, along with the particle size, for the nonendocytic cell uptake of foreign materials and to control intracellular activity. This approach may be utilized in designing nanoprobes and nanocarriers with more efficient biomedical performances.

Second-Generation Polyamidoamine Dendrimer Conjugated with Oligopeptides Can Enhance Plasmid DNA Delivery In Vitro

Poly(amidoamine) (PAMAM) dendrimers have attracted considerable attention in the field of gene therapy due to their flexibility in introducing different functional moieties and reduced toxicity at low generations. However, their transfection efficiency remains a limitation. Therefore, an essential approach for improving their transfection efficiency as gene carriers involves modifying the structure of PAMAM by conjugating functional groups around their surface. In this study, we successfully conjugated an RRHRH oligopeptide to the surface of PAMAM generation 2 (PAMAM G2) to create RRHRH-PAMAM G2. This construction aims to condense plasmid DNA (pDNA) and facilitate its penetration into cell membranes, leading to its promising potential for gene therapy. RRHRH-PAMAM G2/pDNA complexes were smaller than 100 nm and positively charged. Nano-polyplexes can enter the cell and show a high transfection efficiency after 24 h of transfection. The RRHRH-PAMAM G2 was non-toxic to HeLa, NIH3T3, A549, and MDA-MB-231 cell lines. These results strongly suggest that RRHRH-PAMAM G2 holds promise as a gene carrier for gene therapy owing to its biocompatibility and ability to deliver genes to the cell.

Elucidating the Impact of Payload Conjugation on the Cell-Penetrating Efficiency of the Endosomal Escape Peptide dfTAT: Implications for Future Designs for CPP-Based Delivery Systems

Cell-penetrating peptides (CPPs) are promising tools for the intracellular delivery of various biological payloads. However, the impact of payload conjugation on the cell-penetrating activity of CPPs is poorly understood. This study focused on dfTAT, a modified version of the HIV-TAT peptide with enhanced endosomal escape activity, to explore how different payloads affect its cell-penetrating activity. We systematically examined dfTAT conjugated with the SnoopTag/SnoopCatcher pair and found that while smaller payloads such as short peptides do not significantly impair dfTAT's cell delivery activity, larger payloads markedly reduce both its endocytic uptake and endosomal escape efficiency. Our results highlight the role of the payload size and bulk in limiting CPP-mediated delivery. While further research is needed to understand the molecular underpinnings of these effects, our findings pave the way for developing more effective CPP-based delivery systems.

A Novel Immunotoxin Targeting Epithelial Cell Adhesion Molecule Using Single Domain Antibody Fused to Diphtheria Toxin

Epithelial Cell Adhesion Molecule (EpCAM) is overexpressed in a variety of cancers such as colon, stomach, pancreas, and prostate adenocarcinomas. Inhibition of EpCAM is considered as a potential target for cancer therapy. In current study, anti-EpCAM immunotoxin (α-EpCAM IT) was developed using genetic fusion of α-EpCAM single domain antibody (nanobody) (α-EpCAM Nb) to truncated form of diphtheria toxin. The expression of recombinant α-EpCAM IT was induced by Isopropyl β-d-1-thiogalactopyranoside (IPTG) and confirmed by SDS-PAGE and western blot. Recombinant α-EpCAM IT was purified from the inclusion bodies and refolded using urea gradient procedure. The cytotoxicity and apoptosis activity of α-EpCAM IT on EpCAM over-expressing (MCF7), low-expressing (HEK293), and no-expressing (HUVEC) cells were evaluated by 3-4,5-Dimethylthiazol-2-yl (MTT) assay and annexin V-FITC-PI assay as well. In addition, anti-tumor activity of α-EpCAM IT was evaluated on nude mice bearing MCF7 tumor cells. Results showed success expression and purification of α-EpCAM IT. The α-EpCAM IT showed time and dose-dependent anti-proliferative activity on MCF-7 cells. However, α-EpCAM IT did not show any anti-proliferative activity on HEK293 and HUVEC cells as well. In addition, the annexin V-FITC-PI assay results showed that α-EpCAM IT significantly increased apoptotic rate in MCF-7 cells with no effect on HEK293 and HUVEC as well. Moreover, α-EpCAM IT significantly reduced tumor size in vivo study. The achieved results indicate the potential of designing α-EpCAM IT as a novel therapeutic for cancer therapy.

Cell-penetrating peptide: A powerful delivery tool for DNA-free crop genome editing

Genome editing system based on the CRISPR/Cas (clustered regularly interspaced short palindromic repeats) technology is a milestone for biology. However, public concerns regarding genetically modified organisms (GMOs) and recalcitrance in the crop of choice for regeneration have limited its application. Cell-penetrating peptides (CPPs) are derived from protein transduction domains (PTDs) that can take on various cargoes across the plant wall, and membrane of target cells. Selected CPPs show mild cytotoxicity and are a suitable delivery tool for DNA-free genome editing. Moreover, CPPs may also be applied for the transient delivery of morphogenic transcription factors, also known as developmental regulators (DRs), to overcome the bottleneck of the crop of choice regeneration. In this review, we introduce a brief history of cell-penetrating peptides and discuss the practice of CPP-mediated DNA-free transfection and the prospects of this potential delivery tool for improving crop genome editing.

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.

Glycopolymer-Cell-Penetrating Peptide (CPP) Conjugates for Efficient Epidermal Growth Factor Receptor (EGFR) Silencing

Overexpression of epidermal growth factor receptor (EGFR) is observed in multiple cancers such as colorectal, lung, and cervical solid tumors. Regulating the EGFR expression is an efficient strategy to manage these malignancies, and it can be achieved by using short interfering RNA (siRNA). Cell-penetrating peptides (CPPs) demonstrated an excellent capability to enhance the cellular uptake of siRNA, but high knockdown efficiencies have not been achieved due to endosomal entrapment. In this work, Schiff's base reaction was used to modify a block {P[LAEMA(2-lactobionamidoethyl methacrylamide)₃₇]-b-P[FPMA(4-formyl phenyl methacrylate)₂-st-DMA(N,N-dimethylacrylamide)₂], P2} and two statistical [P(LAEMA₂₃-st-FPMA₃) (P3) and P(LAEMA₂₅-st-FPMA₂-st-DMA₂) (P4)] aldehyde-based and galactose-based polymers, prepared via reversible addition-fragmentation chain-transfer (RAFT) polymerization. An arginine-rich peptide (ARP, KRRKRRRRRK) was used as a cell-penetrating peptide (CPP) and conjugated to the polymers via a Schiff base reaction. The resulting glycopolymer-peptide conjugates were utilized to condense the siRNA to prepare polyplexes with multivalent CPPs (MCPPs, a nanoparticle with multiple copies of the CPP) to enhance the endosomal escape. The polyplexes have different surface properties as determined by the architecture of polymers and the insertion of dimethyl amide moieties. The enhancement of cellular internalization of ARP was observed by labeling the polyplexes with fluorescein isothiocyanate (FITC)-siRNA showing a localization of polyplexes in the cytoplasm of a HeLa (cervical cancer) cell line. In the in vitro EFGR silencing study, the statistical glycopolymer-peptide (P3-P) polyplexes had superior EGFR silencing efficiency in comparison with the other polymers that were studied. Furthermore, P3-P polyplexes led to less off-targeting silencing than lipofectamine 3000. These encouraging results confirmed the potency of decorating galactose-based polymers with CPP, like ARP for their application in siRNA delivery and management of cervical carcinomas.

Efficient intracellular delivery of proteins by a multifunctional chimaeric peptide in vitro and in vivo

Protein delivery with cell-penetrating peptide is opening up the possibility of using targets inside cells for therapeutic or biological applications; however, cell-penetrating peptide-mediated protein delivery commonly suffers from ineffective endosomal escape and low tolerance in serum, thereby limiting in vivo efficacy. Here, we present an intracellular protein delivery system consisting of four modules in series: cell-penetrating peptide, pH-dependent membrane active peptide, endosome-specific protease sites and a leucine zipper. This system exhibits enhanced delivery efficiency and serum tolerance, depending on proteolytic cleavage-facilitated endosomal escape and leucine zipper-based dimerisation. Intravenous injection of protein phosphatase 1B fused with this system successfully suppresses the tumour necrosis factor-α-induced systemic inflammatory response and acetaminophen-induced acute liver failure in a mouse model. We believe that the strategy of using multifunctional chimaeric peptides is valuable for the development of cell-penetrating peptide-based protein delivery systems, and facilitate the development of biological macromolecular drugs for use against intracellular targets.

Fine-tuning bispecific therapeutics

Bispecific therapeutics target two distinct antigens simultaneously and provide novel functionalities that are not attainable with single monospecific molecules or combinations of them. The unique potential of bispecific therapeutics is driving extensive efforts to discover synergistic dual targets, design molecular formats to integrate bispecific elements, and accelerate successful clinical translation. In particular, the past decade has witnessed a boom in the design and development of bispecific antibody formats with more than 100 collections to date. Despite the remarkable progress that has been made to expand the number of formats, qualitative fine-tuning of bispecific formats is needed to achieve optimal dual-target engagement based on understanding of the spatiotemporal interdependence of the two physically linked binding specificities and the complex target biology associated with bispecific approaches. This review provides insights into the design parameters - including affinity, valency, and geometry - that need to be considered at an early stage of development in order to take the best advantage of bispecific therapeutics.

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.

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.

Tetramer formation of tumor suppressor protein p53: Structure, function, and applications

Tetramer formation of p53 is essential for its tumor suppressor function. p53 not only acts as a tumor suppressor protein by inducing cell cycle arrest and apoptosis in response to genotoxic stress, but it also regulates other cellular processes, including autophagy, stem cell self-renewal, and reprogramming of differentiated cells into stem cells, immune system, and metastasis. More than 50% of human tumors have TP53 gene mutations, and most of them are missense mutations that presumably reduce tumor suppressor activity of p53. This review focuses on the role of the tetramerization (oligomerization), which is modulated by the protein concentration of p53, posttranslational modifications, and/or interactions with its binding proteins, in regulating the tumor suppressor function of p53. Functional control of p53 by stabilizing or inhibiting oligomer formation and its bio-applications are also discussed. © 2015 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 106: 598-612, 2016.

Syndecan-4 Is a Receptor for Clathrin-Mediated Endocytosis of Arginine-Rich Cell-Penetrating Peptides

Arginine-rich cell-penetrating peptides (CPPs) such as Tat and oligoarginine peptides have been widely used as carriers for intracellular delivery of bioactive molecules. Despite accumulating evidence for involvement of endocytosis in the cellular uptake of arginine-rich CPPs, the primary cell-surface receptors for these peptide carriers that would initiate endocytic processes leading to intracellular delivery of bioactive cargoes have remained poorly understood. Our previous attempt to identify membrane receptors for octa-arginine (R8) peptide, one of the representative arginine-rich CPPs, using the photo-cross-linking probe bearing a photoreactive diazirine was not successful due to considerable amounts of cellular proteins nonspecifically bound to the affinity beads. To address this issue, here we developed a photo-cross-linking probe in which a cleavable linker of a diazobenzene moiety was employed to allow selective elution of cross-linked proteins by reducing agent-mediated cleavage. We demonstrated that introduction of the diazobenzene moiety into the photoaffinity probe enables efficient purification of cross-linked proteins with significant reduction of nonspecific binding proteins, leading to successful identification of 17 membrane-associated proteins that would interact with R8 peptide. RNAi-mediated knockdown experiments in combination with the pharmacological inhibitors revealed that, among the proteins identified, syndecan-4, one of the heparan sulfate proteoglycans, is an endogenous membrane-associated receptor for the cellular uptake of R8 peptide via clathrin-mediated endocytosis. This syndecan-4-dependent pathway was also involved in the intracellular delivery of bioactive proteins mediated by R8 peptide. These results reveal that syndecan-4 is a primary cell-surface target for R8 peptide that allows intracellular delivery of bioactive cargo molecules via clathrin-mediated endocytosis.

Cell penetrating peptide-modified poly(lactic-co-glycolic acid) nanoparticles with enhanced cell internalization

The surface modification of nanoparticles (NPs) can enhance the intracellular delivery of drugs, proteins, and genetic agents. Here we studied the effect of different surface ligands, including cell penetrating peptides (CPPs), on the cell binding and internalization of poly(lactic-co-glycolic) (PLGA) NPs. Relative to unmodified NPs, we observed that surface-modified NPs greatly enhanced cell internalization. Using one CPP, MPG (unabbreviated notation), that achieved the highest degree of internalization at both low and high surface modification densities, we evaluated the effect of two different NP surface chemistries on cell internalization. After 2h, avidin-MPG NPs enhanced cellular internalization by 5 to 26-fold relative to DSPE-MPG NP formulations. Yet, despite a 5-fold increase in MPG density on DSPE compared to Avidin NPs, both formulations resulted in similar internalization levels (48 and 64-fold, respectively) after 24h. Regardless of surface modification, all NPs were internalized through an energy-dependent, clathrin-mediated process, and became dispersed throughout the cell. Overall both Avidin- and DSPE-CPP modified NPs significantly increased internalization and offer promising delivery options for applications in which internalization presents challenges to efficacious delivery.

Small molecule antagonists of cell-surface heparan sulfate and heparin-protein interactions

Surfen, bis-2-methyl-4-amino-quinolyl-6-carbamide, was previously reported as a small molecule antagonist of heparan sulfate (HS), a key cell-surface glycosaminoglycan found on all mammalian cells. To generate structure-activity relationships, a series of rationally designed surfen analogs was synthesized, where its dimeric structure, exocyclic amines, and urea linker region were modified to probe the role of each moiety in recognizing HS. An in vitro assay monitoring inhibition of fibroblast growth factor 2 binding to wild-type CHO cells was utilized to quantify interactions with cell surface HS. The dimeric molecular structure of surfen and its aminoquinoline ring systems was essential for its interaction with HS, and certain dimeric analogs displayed higher inhibitory potency than surfen and were also shown to block downstream FGF signaling in mouse embryonic fibroblast cells. These molecules were also able to antagonize other HS-protein interactions including the binding of soluble RAGE to HS. Importantly, selected molecules were shown to neutralize heparin and other heparinoids, including the synthetic pentasaccharide fondaparinux, in a factor Xa chromogenic assay and in vivo in mice. These results suggest that small molecule antagonists of heparan sulfate and heparin can be of therapeutic potential for the treatment of disorders involving glycosaminoglycan-protein interactions.

HIV-1 drug discovery: targeting folded RNA structures with branched peptides

Human immunodeficiency virus type 1 (HIV-1) is an RNA virus that is prone to high rates of mutation. While the disease is managed with current antiretroviral therapies, drugs with a new mode of action are needed. A strategy towards this goal is aimed at targeting the native three-dimensional fold of conserved RNA structures. This perspective highlights medium-sized peptides and peptidomimetics used to target two conserved RNA structures of HIV-1. In particular, branched peptides have the capacity to bind in a multivalent fashion, utilizing a large surface area to achieve the necessary affinity and selectivity toward the target RNA.

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.

Enhanced cellular uptake of engineered spider silk particles

Drug delivery systems allow tissue/cell specific targeting of drugs in order to reduce total drug amounts administered to an organism and potential side effects upon systemic drug delivery.

A survey on "Trojan Horse" peptides: opportunities, issues and controlled entry to "Troy"

Cell-penetrating peptides (CPPs), often vividly termed as the "Trojan Horse" peptides, have attracted considerable interest for the intracellular delivery of a wide range of cargoes, such as small molecules, peptides, proteins, nucleic acids, contrast agents, nanocarriers and so on. Some preclinical and clinical developments of CPP conjugates demonstrate their promise as therapeutic agents for drug discovery. There is increasing evidence to suggest that CPPs have the potential to cross several bio-barriers (e.g., blood-brain barriers, intestinal mucosa, nasal mucosa and skin barriers). Despite revolutionary process in many aspects, there are a lot of basic issues unclear for these entities, such as internalization mechanisms, translocation efficiency, translocation kinetics, metabolic degradation, toxicity, side effect, distribution and non-specificity. Among them, non-specificity remains a major drawback for the in vivo application of CPPs in the targeted delivery of cargoes. So far, diverse organelle-specific CPPs or controlled delivery strategies have emerged and improved their specificity. In this review, we will look at the opportunities of CPPs in clinical development, bio-barriers penetration and nanocarriers delivery. Then, a series of basic problems of CPPs will be discussed. Finally, this paper will highlight the use of various controlled strategies in the organelle-specific delivery and targeted delivery of CPPs. The purpose of this review will be to emphasize most influential advance in this field and present a fundamental understanding for challenges and utilizations of CPPs. This will accelerate their translation as efficient vectors from the in vitro setting into the clinic arena, and retrieve the entry art to "Troy".

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.

Oligopeptide-terminated poly(β-amino ester)s for highly efficient gene delivery and intracellular localization

The main limitation of gene therapy towards clinics is the lack of robust, safe and efficient gene delivery vectors. This paper describes new polycations for gene delivery based on poly(β-amino ester)s (pBAE) containing terminal oligopeptides. The authors developed oligopeptide-modified pBAE-pDNA nanoparticles that achieve better cellular viability and higher transfection efficacy than other end-modified pBAE and commercial transfection agents. Gene expression in highly permissive cell lines was remarkably high, but transfection efficiency in less-permissive cell lines was highly dependent on oligopeptide composition and nanoparticle formulation. Moreover, the use of selected oligopeptides in the pBAE formulation led to preferential intracellular localization of the particles. Particle analysis of highly efficient pBAE formulations revealed different particle sizes and charge features, which indicates chemical pseudotyping of the particle surface, related to the oligopeptide chemical nature. In conclusion, chemical modification at the termini of pBAE with amine-rich oligopeptides is a powerful strategy for developing delivery systems for future gene therapy applications.

Cell-penetrating peptides: 20 years later, where do we stand?

Twenty years ago, the discovery of peptides able to cross cellular membranes launched a novel field in molecular delivery based on these non-invasive vectors, most commonly called cell-penetrating peptides (CPPs) or protein transduction domains (PTDs). These peptides were shown to efficiently transport various biologically active molecules inside living cells, and thus are considered promising devices for medical and biotechnological developments. Moreover, CPPs emerged as potential tools to study the prime mechanisms of cellular entry across the plasma membrane. This review is dedicated to CPP fundamentals, with an emphasis on the molecular requirements and mechanism of their entry into eukaryotic cells.

The Pseudomonas aeruginosa exotoxin A translocation domain facilitates the routing of CPP-protein cargos to the cytosol of eukaryotic cells

The use of cell-penetrating peptides (CPPs), such as polyarginine, has been shown to facilitate the import of drugs and other cargos into cells. However, a major obstacle limiting their use as delivery agents is their entrapment following internalization into endocytic vesicles, leading to either their recycling out of cells or their degradation in lysosomes. To address this challenge, we fused a CPP sequence to the translocation domain of Pseudomonas aeruginosa exotoxin A (ETA) to facilitate the endosomal escape of imported CPP-containing protein constructs. Specifically, a fusion protein incorporating ten arginines linked to residues 253 to 412 of ETA (ETA(253-412)) was tested for its ability to effectively route a protein cargo (enhanced green fluorescent protein, eGFP) to the cytosol of cells. Using flow cytometry and fluorescence live-cell imaging, we observed a 5-fold improvement of cellular uptake as well as a 40-fold increase in cytosolic delivery of the CPP-ETA(253-412)-eGFP construct in relation to CPP-eGFP. Furthermore, analysis of intracellular routing events indicated that the incorporation of ETA(253-412) within the CPP-containing protein fusion construct avoided lysosomal degradation by re-directing the construct from early endosomes to the ER lumen and finally to the cytosol. Studies using inhibitors of vesicular transport confirmed that the ER lumen is a key compartment reached by the CPP-ETA(253-412)-eGFP construct before accessing the cytosol. Together, these findings suggest that incorporating a CPP motif and the ETA translocation domain into protein constructs can facilitate their cytosolic delivery.

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.

Quantitative analysis of affinity enhancement by noncovalently oligomeric ligands

Designed ligands that self-assemble noncovalently via an independent oligomerization domain have demonstrated enhancement in affinity for a variety of chemical and biological targets. To better understand the thermodynamic linkage between enhanced receptor binding and self-assembly, we have developed linkage models for the three commonly encountered types of noncovalently oligomeric ligands: homofunctional oligomeric ligands, heterodimeric ligands that target a single receptor, and bispecific ligands that crosslink noninteracting receptors. Expressions and numerical approaches for exact analysis as a function of total ligand concentrations are provided. We apply the linkage models to the binding data for two published noncovalently oligomeric ligands: one targeting a small molecule (phosphocholine) and the other targeting a soluble protein (tumor necrosis factor α). The linkage models provide a quantitative measure of the potential and realized enhancement in affinity that could inform and guide design optimization efforts, and they reveal physical insight that would elude model-free analysis. Incorporation of the linkage models, therefore, is expected to be valuable in the rational engineering of noncovalently oligomeric ligands.

Inhibition of tumor suppressor protein p53-dependent transcription by a tetramerization domain peptide via hetero-oligomerization

Tumor suppressor protein p53 induces cell cycle arrest, apoptosis, and senescence in response to cellular stresses. The p53 tetramer formation is essential for its functions. Despite of these crucial functions of p53 for integrity of genome, activation of the p53 signal pathway causes low induced pluripotent stem (iPS) cell generation efficiency. In this study, we report transient inhibition of p53-dependent transcription using a p53 tetramerization domain peptide that contains cell penetrating and nuclear localization signals. The peptide was efficiently introduced into cells and inhibited p21 expression via hetero-tetramerization with endogenous p53 protein. This method can be applied towards safe and efficient iPS cell generation.

Modulation of the biological activity of microRNA-210 with peptide nucleic acids (PNAs)

Herein we describe the activity of a peptide nucleic acid (PNA) that targets microRNA-210 (miR-210), which is associated with hypoxia and is modulated during erythroid differentiation. PNAs directed against miR-210 were designed to bind with high affinity to the target RNA strand and to undergo efficient uptake in target cells. A polyarginine-PNA conjugate directed against miR-210 (Rpep-PNA-a210) showed both very high affinity for RNA and efficient uptake into target cells without the need for transfection reagents. An unmodified PNA of the same sequence displayed the ability to bind RNA, but cellular uptake was very poor. Consistent with this, only Rpep-PNA-a210 strongly inhibited miR-210 activity, as evaluated by assays on undifferentiated K562 cells and on cells treated with mithramycin, which was found to induce erythroid differentiation and miR-210 overexpression. Targeting miR-210 by Rpep-PNA-a210 resulted in: 1) a decrease in miR-210 levels as measured by RT-PCR, 2) up-regulation of raptor mRNA, 3) a decrease in γ-globin mRNA, and 4) decreased expression of differentiated functions (i.e., proportion of benzidine-positive cells, content of embryo-fetal hemoglobins). The efficient delivery of anti-miR PNAs through a suitable peptide carrier (Rpep-PNA-a210) leads to the inhibition of miR-210 activity, altering the expression of miR-210-regulated erythroid functions.

TAT fusion protein transduction into isolated mitochondria is accelerated by sodium channel inhibitors

Stringent control of ion and protein transport across the mitochondrial membranes is required to maintain mitochondrial function and biogenesis. In particular, the inner mitochondrial membrane is generally impermeable to proteins entering the matrix except via tightly regulated protein import mechanisms. Recently, cell penetrant peptides have been shown to move across the inner mitochondrial membrane in a manner suggesting an independent mechanism. HIV-1 transactivator of transcription (TAT) is an arginine-rich cell penetrant peptide, 47YGRKKRRQRRR57, which can transduce full-length proteins not only across the cell membrane but also into intracellular organelles. In this study, we investigated the ability of a TAT-containing protein to move into the mitochondrial matrix. Using a novel FACS assay for isolated, purified mitochondria, we show that TAT can deliver a modified fluorescent protein, mMDH-GFP, to the matrix of mitochondria and it is subsequently processed by the matrix peptidases. In addition, transduction of TAT-mMDH-GFP into mitochondria is independent of canonical protein import pathways as well as mitochondrial membrane potential. In direct contrast to published reports regarding the cell membrane where the sodium channel inhibitor, amiloride, blocks endocytosis and inhibits TAT transduction, TAT transduction into mitochondria is markedly increased by this same sodium channel inhibitor. These results confirm that the cell penetrant peptide, TAT, can readily transduce a protein cargo into the mitochondrial matrix. These results also demonstrate a novel role for mitochondrial sodium channels in mediating TAT transduction into mitochondria that is independent of endocytotic mechanisms. The mechanism of TAT transduction into mitochondria therefore is distinctly different from transduction across the cell membrane.

Design and evaluation of variants of the protein transduction domain originated from translationally controlled tumor protein

Protein transduction domains (PTDs) have been successfully employed to deliver therapeutic cargos both in vitro and in vivo because of their cellular penetrating ability. We previously reported that a 10-amino acid peptide (MIIYRDLISH) derived from the NH(2)-terminus of human translationally controlled tumor protein (TCTP) functions as a PTD. TCTP-PTD is quite different from other well-known PTDs in its hydrophobic composition and structural character, and the sequence requirements for transduction remain unknown. To identify the role of each residue, we compared the cellular uptake of various deletion mutants and Ala substituents of TCTP-PTD. The results showed that the amino terminal residues and the hydrophobic nature of the peptide, with a minimal length of nine residues, were necessary for transduction. Based on the elucidated sequence requirements, we designed and evaluated variants to improve the efficiency and solubility through sequential modification of TCTP-PTD. During the optimization process, we also delineated the contribution of residues and the advantageous composition of sequences for cellular uptake.

Intracellular delivery of quantum dots mediated by a histidine- and arginine-rich HR9 cell-penetrating peptide through the direct membrane translocation mechanism

Functional peptides that transfer biomaterials, such as semiconductor quantum dots (QDs), into cells in biomaterial research have been developed in recent years. Delivery of QDs conjugated with cell-penetrating peptides (CPPs) into cells by the endocytic pathway was problematic in biomedical applications because of lysosomal trapping. Here, we demonstrate that histidine- and arginine-rich CPPs (HR9 peptides) stably and noncovalently combined with QDs are able to enter into cells in an extremely short period (4 min). Interrupting both F-actin polymerization and active transport did not inhibit the entry of HR9/QD complexes into cells, indicating that HR9 penetrates cell membrane directly. Subcellular colocalization studies indicated that QDs delivered by HR9 stay in cytosol without any organelle capture. Dimethyl sulphoxide, ethanol and oleic acid, but not pyrenebutyrate, enhanced HR9-mediated intracellular delivery of QDs by promoting the direct membrane translocation pathway. HR9 and HR9/QDs were not cytotoxic. These findings suggest that HR9 could be an efficient carrier to deliver drugs without interfering with their therapeutic 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.

Synthesis and splice-redirecting activity of branched, arginine-rich peptide dendrimer conjugates of peptide nucleic acid oligonucleotides

Arginine-rich cell-penetrating peptides have found excellent utility in cell and in vivo models for enhancement of delivery of attached charge-neutral PNA or PMO oligonucleotides. We report the synthesis of dendrimeric peptides containing 2- or 4-branched arms each having one or more R-Ahx-R motifs and their disulfide conjugation to a PNA705 splice-redirecting oligonucleotide. Conjugates were assayed in a HeLa pLuc705 cell assay for luciferase up-regulation and splicing redirection. Whereas 8-Arg branched peptide−PNA conjugates showed poor activity compared to a linear (R-Ahx-R)₄−PNA conjugate, 2-branched and some 4-branched 12 and 16 Arg peptide−PNA conjugates showed activity similar to that of the corresponding linear peptide−PNA conjugates. Many of the 12- and 16-Arg conjugates retained significant activity in the presence of serum. Evidence showed that biological activity in HeLa pLuc705 cells of the PNA conjugates of branched and linear (R-Ahx-R) peptides is associated with an energy-dependent uptake pathway, predominantly clathrin-dependent, but also with some caveolae dependence.

Engineered biological entities for drug delivery and gene therapy protein nanoparticles

The development of genetic engineering techniques has speeded up the growth of the biotechnological industry, resulting in a significant increase in the number of recombinant protein products on the market. The deep knowledge of protein function, structure, biological interactions, and the possibility to design new polypeptides with desired biological activities have been the main factors involved in the increase of intensive research and preclinical and clinical approaches. Consequently, new biological entities with added value for innovative medicines such as increased stability, improved targeting, and reduced toxicity, among others have been obtained. Proteins are complex nanoparticles with sizes ranging from a few nanometers to a few hundred nanometers when complex supramolecular interactions occur, as for example, in viral capsids. However, even though protein production is a delicate process that imposes the use of sophisticated analytical methods and negative secondary effects have been detected in some cases as immune and inflammatory reactions, the great potential of biodegradable and tunable protein nanoparticles indicates that protein-based biotechnological products are expected to increase in the years to come.

Cell line-dependent internalization pathways determine DNA transfection efficiency of decaarginine-PEG-lipid

Previously, we have reported that decaarginine-conjugated PEG-lipids (R10B) efficiently delivered plasmid DNA (pDNA) into human cervical carcinoma HeLa cells via macropinocytosis; however, the mechanism of cellular uptake by R10B was not evaluated in other cell lines. In this study, we investigated the internalization mechanism by R10B/pDNA complex (R10B-lipoplex) in human prostate tumor PC-3 and human nasopharyngeal tumor KB cells, and compared with that in HeLa cells. Although it was necessary for R10B-lipoplex to associate with heparan sulfate (HS) on the cell surface in all cell lines, the R10B-lipoplex was internalized primarily through clathrin-mediated endocytosis in PC-3 and KB cells, and macropinosytosis in HeLa cells. In HeLa cells, treatment with the R10B-lipoplex induced the formation of lamellipodia for macropinocytosis, but did not in KB and PC-3 cells. Furthermore, the highest transfection efficiency by R10B-lipoplex was observed in HeLa cells. These findings indicated that the R10B-lipoplex induced the formation of lamellipodia in HeLa cells after binding to HS on the cells and was then internalized by macropinocytosis, which could induce high gene expression because of escaping degradation in lysosomes. Cell physiology might be a critical factor in cellular internalization and efficient transfection by cell penetration peptide.

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).

Branched oligomerization of cell-permeable peptides markedly enhances the transduction efficiency of adenovirus into mesenchymal stem cells

Cell-permeable peptides (CPPs) promote the transduction of nonpermissive cells by recombinant adenovirus (rAd) to improve the therapeutic efficacy of rAd. In this study, branched oligomerization of CPPs significantly enhanced the transduction of human mesenchymal stem cells (MSCs) by rAd in a CPP type-independent manner. In particular, tetrameric CPPs increased transduction efficiency at 3000-5000-fold lower concentrations than did monomeric CPPs. Although branched oligomerization of CPPs also increases cytotoxicity, optimal concentrations of tetrameric CPPs required for maximum transduction are at least 300-1000-fold lower than those causing 50% cytotoxicity. Furthermore, although only approximately 60% of MSCs were maximally transduced at 500 muM of monomeric CPPs, >95% of MSCs were transduced with 0.1 muM of tetrameric CPPs. Tetrameric CPPs also significantly increased the formation and net surface charge of CPP/rAd complexes, as well as the binding of rAd to cell membranes at a greater degree than did monomeric CPPs, followed by rapid internalization into MSCs. In a critical-size calvarial defect model, the inclusion of tetrameric CPPs in ex vivo transduction of rAd expressing bone morphogenetic protein 2 into MSCs promoted highly mineralized bone formation. In addition, MSCs that were transduced with rAd expressing brain-derived neurotrophic factor in the presence of tetrameric CPPs improved functional recovery in a spinal cord injury model. These results demonstrated the potential for tetrameric CPPs to provide an innovative tool for MSC-based gene therapy and for in vitro gene delivery to MSCs.

Preparation of an artificial cell cycle progressor with a novel mechanism

Control of cell cycle progression in somatic cells or terminally differentiated cells is a key technology for cell-based therapies such as regenerative therapy. We have prepared an artificial cell cycle progression peptide composed of a human immunodeficiency virus-derived Tat protein transduction domain (PTD) and a p53 genetic suppressor element (GSE123). The peptide significantly promoted hepatocyte growth factor-induced DNA synthesis and the proliferation of primary mouse hepatocytes, which are highly differentiated somatic cells. The addition of a nuclear localization signal (NLS) sequence to the peptide increased the internalization of the peptide to the nuclear fraction. Distribution analysis using a fluoresein isothiocyanate-labeled peptide indicated that the NLS enabled the peptide to escape from the lysosomes to the cytosol. As a result, the NLS-Tat-GSE123 peptide induced potent cell proliferation of primary mouse hepatocytes in vitro. The use of this peptide as an artificial cell growth enhancer, bypassing a specific receptor, is a useful tool for the study of regenerative therapy and cell signaling.

A peptide-based dendrimer that enhances the splice-redirecting activity of PNA conjugates in cells

The full therapeutic potential of oligonucleotide (ON)-based agents has been hampered by cellular delivery challenges. Cell-penetrating peptides (CPP) represent promising delivery vectors for nucleic acids, and their potential has recently been evaluated using a functional splicing redirection assay, which capitalizes on the nuclear delivery of splice-correcting steric-block ON analogues such as peptide nucleic acids (PNA). Despite encouraging in vitro and in vivo data with arginine-rich CPP-steric block conjugates, mechanistic studies have shown that entrapment within the endosome/lysosome compartment after endocytosis remains a limiting factor. Previous work from our group has shown that CPP oligomerization greatly improves cellular delivery and increases transfection of plasmid DNA. We now report the chemical synthesis and the evaluation of multivalent CPP-PNA constructs incorporating monomeric (p53(mono)) and dendrimer-like tetrameric (p53(tet)) forms of the p53 tetramerization domain containing peptide, a 10 arginine CPP domain (R10), and a splice redirecting PNA (PNA705). These CPP-PNA conjugates were termed R10p53(tet)-PNA705 and R10p53(mono)-PNA705, referring to their oligomerization state. The present study demonstrates that the splicing redirection efficiency of PNA705 is much greater in the context of the tetrameric R10p53(tet)-PNA705 construct than for the monomeric and occurs at nanomolar concentrations, demonstrating that multivalency is an important factor in delivering PNA into cells.

An improved cell-penetrating, caspase-activatable, near-infrared fluorescent peptide for apoptosis imaging

Apoptosis is required for normal cellular homeostasis, and deregulation of the apoptotic process is implicated in various diseases. Previously, we developed a cell-penetrating near-infrared fluorescence (NIRF) probe based on an activatable strategy to detect apoptosis-associated caspase activity in vivo. This probe consisted of a cell-penetrating Tat peptide conjugated to an effector recognition sequence (DEVD) that was flanked by a fluorophore-quencher pair (Alexa Fluor 647 and QSY 21). Once exposed to effector caspases, the recognition sequence was cleaved, resulting in separation of the fluorophore-quencher pair and signal generation. Herein, we present biochemical analysis of a second generation probe, KcapQ, with a modified cell-penetrating peptide sequence (KKKRKV). This modification resulted in a probe that was more sensitive to effector caspase enzymes, displayed an unexpectedly higher quenching efficiency between the fluorophore-quencher pair, and was potentially less toxic to cells. Assays using recombinant caspase enzymes revealed that the probe was specific for effector caspases (caspase 3 > 7 > 6). Analysis of apoptosis in HeLa cells treated with doxorubicin showed probe activation specific to apoptotic cells. In a rat model of retinal neuronal excitotoxicity, intravitreal injection of N-methyl-d-aspartate (NMDA) induced apoptosis of retinal ganglion cells (RGCs). Eyecup and retinal flat-mount images of NMDA-pretreated animals injected intravitreally with KcapQ using a clinically applicable protocol showed specific and widely distributed cell-associated fluorescence signals compared to untreated control animals. Fluorescence microscopy images of vertical retinal sections from NMDA-pretreated animals confirmed that activated probe was predominantly localized to RGCs and colocalized with TUNEL labeling. Thus, KcapQ represents an improved effector caspase-activatable NIRF probe for enhanced noninvasive analysis of apoptosis in whole cells and live animals.

Pyrenebutyrate-mediated delivery of quantum dots across the plasma membrane of living cells

Quantum dots have been delivered directly across the plasma membrane to the cytosol of living cells using a combination of a cationic peptide, polyarginine, and a hydrophobic counterion, pyrenebutyrate. Quantum dot delivery did not disrupt the plasma membrane and bypassed the barrier of endocytic vesicles. Cellular uptake was independent of temperature but highly dependent on the surface charge of the quantum dot and the membrane potential of the cell, suggesting a direct translocation across the membrane. This method of delivery can find immediate application for quantum dots and may be broadly applicable to other nanoparticles.

Heparin antagonism by polyvalent display of cationic motifs on virus-like particles

Particles to the rescue! The construction of cationic amino acid motifs on the surface of bacteriophage Qbeta by genetic engineering or chemical conjugation gives particles that are potent inhibitors of the anticoagulant action of heparin, which is a common anticlotting agent subject to clinical overdose.Polyvalent interactions allow biological structures to exploit low-affinity ligand-receptor binding events to affect physiological responses. We describe here the use of bacteriophage Qbeta as a multivalent platform for the display of polycationic motifs that act as heparin antagonists. Point mutations to the coat protein allowed us to generate capsids bearing the K16M, T18R, N10R, or D14R mutations; because 180 coat proteins form the capsid, the mutants provide a spectrum of particles differing in surface charge by as much as +540 units (K16M vs. D14R). Whereas larger poly-Arg insertions (for example, C-terminal Arg(8)) did not yield intact virions, it was possible to append chemically synthesized oligo-Arg peptides to stable wild-type (WT) and K16M platforms. Heparin antagonism by the particles was evaluated by using the activated partial thrombin time (aPTT) clotting assay; this revealed that T18R, D14R, and WT-(R(8)G(2))(95) were the most effective at disrupting heparin-mediated anticoagulation (>95 % inhibition). This activity agreed with measurements of zeta potential (ZP) and retention time on cation exchange chromatography for the genetic constructs, which distribute their added positive charge over the capsid surface (+180 and +360 for T18R and D14R relative to WT). The potent activity of WT-(R(8)G(2))(95), despite its relatively diminished overall surface charge is likely a consequence of the particle's presentation of locally concentrated regions with high positive charge density that interact with heparin's extensively sulfated domains. The engineered cationic capsids retained their ability to inhibit heparin at high concentrations and showed no anticlotting activity of the kind that limits the utility of antiheparin polycationic agents that are currently in clinical use.

Cell entry of arginine-rich peptides is independent of endocytosis

Arginine-rich peptides are 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 endocytic mode of uptake and a subsequent release from vesicles or to direct membrane penetration (transduction). To distinguish between both possibilities, we have blocked endocytic pathways suggested to be involved in uptake of cell-penetrating peptides. We have then monitored by confocal microscopy the uptake and distribution of the cell-penetrating transactivator of transcription (TAT) peptide into living mammalian cells over time. To prevent side effects of chemical inhibitors, we used genetically engineered cells as well as different temperature. We found that a knockdown of clathrin-mediated endocytosis and a knock-out of caveolin-mediated endocytosis did not affect the ability of TAT to enter cells. In addition, the TAT peptide showed the same intracellular distribution throughout the cytoplasm and nucleus as in control cells. Even incubation of cells at 4 °C did not abrogate TAT uptake nor change its intracellular distribution. We therefore conclude that this distribution results from TAT peptide that directly penetrated (transduced) the plasma membrane. The formation of nonselective pores is unlikely, because simultaneously added fluorophores were not taken up together with the TAT peptide. In summary, although the frequency and kinetics of TAT transduction varied between cell types, it was independent of endocytosis.

Dendritic cell acquisition of epitope cargo mediated by simple cationic peptide structures

In this study we evaluate the uptake by murine dendritic cells (DCs) of different synthetic, branched cationic peptide structures with a view to facilitating peptide epitope delivery. The level of cell uptake by fluorescenated peptides was measured by flow cytometry following quenching of extracellular fluorescence with trypan blue. Branched peptides containing either N-terminal arginine or N-terminal lysine residues were able to mediate cell entry but the peptide containing four arginine residues in a branching configuration (R4) was found to be superior not only to other branched peptides in translocating to the cell interior and also to a peptide containing four arginine residues arranged linearly. Fluorescenated R4 was found to be localized within intracellular vesicle-like compartments as well as being distributed throughout the cell cytoplasm. Uptake of R4 utilized an energy-dependent process that appeared to involve phosphatidylinositol-3-kinase and could induce intermediate levels of DC maturation. R4 when conjugated to a T-helper cell and CTL epitope construct was able to induce antigen-specific CD8+ T-cell mediated immune responses in mice when administered in adjuvant as were DCs that were pulsed with this construct and then matured with LPS. Fluorescenated R4 was also found to translocate into the interior of other cell types indicating that it may be useful for the delivery of peptide cargo into other specialized cells.