Molecular parameters of siRNA--cell penetrating peptide nanocomplexes for efficient cellular delivery

Stereoselective uptake of cell-penetrating peptides is conserved in antisense oligonucleotide polyplexes

Stereochemistry matters. A significant conceptual advancement is presented toward the understanding of how functional characteristics of delivery peptides can translate into functional characteristics of peptide-based oligoplexes.

A convergent uptake route for peptide- and polymer-based nucleotide delivery systems

Cell-penetrating peptides (CPPs) have been used as vehicles to deliver various cargos into cells and are promising as tools to deliver therapeutic biomolecules such as oligonucleotides both in vitro and in vivo. CPPs are positively charged and it is believed that CPPs deliver their cargo in a receptor-independent manner by interacting with the negatively charged plasma membrane and thereby inducing endocytosis. In this study we examine the mechanism of uptake of several different, well known, CPPs that form complexes with oligonucleotides. We show that these CPP:oligonucleotide complexes are negatively charged in transfection-media and their uptake is mediated by class A scavenger receptors (SCARA). These receptors are known to promiscuously bind to, and mediate uptake of poly-anionic macromolecules. Uptake of CPP:oligonucleotide complexes was abolished using pharmacological SCARA inhibitors as well as siRNA-mediated knockdown of SCARA. Additionally, uptake of CPP:oligonucleotide was significantly increased by transiently overexpressing SCARA. Furthermore, SCARA inhibitors also blocked internalization of cationic polymer:oligonucleotide complexes. Our results demonstrate that the previous held belief that CPPs act receptor independently does not hold true for CPP:oligonucleotide complexes, as scavenger receptor class A (SCARA) mediates the uptake of all the examined CPP:oligonucleotide complexes in this study.

Design of a multicomponent peptide-woven nanocomplex for delivery of siRNA

We developed and tested a multicomponent peptide-woven siRNA nanocomplex (PwSN) comprising different peptides designed for efficient cellular targeting, endosomal escape, and release of siRNA. To enhance tumor-specific cellular uptake, we connected an interleukin-4 receptor-targeting peptide (I4R) to a nine-arginine peptide (9r), yielding I4R-9r. To facilitate endosomal escape, we blended endosomolytic peptides into the I4R-9r to form a multicomponent nanocomplex. Lastly, we modified 9r peptides by varying the number and positions of positive charges to obtain efficient release of siRNA from the nanocomplex in the cytosol. Using this step-wise approach for overcoming the biological challenges of siRNA delivery, we obtained an optimized PwSN with significant biological activity in vitro and in vivo. Interestingly, surface plasmon resonance analyses and three-dimensional peptide models demonstrated that our designed peptide adopted a unique structure that was correlated with faster complex disassembly and a better gene-silencing effect. These studies further elucidate the siRNA nanocomplex delivery pathway and demonstrate the applicability of our stepwise strategy to the design of siRNA carriers capable of overcoming multiple challenges and achieving efficient delivery.

High-throughput screening identifies small molecules that enhance the pharmacological effects of oligonucleotides

The therapeutic use of antisense and siRNA oligonucleotides has been constrained by the limited ability of these membrane-impermeable molecules to reach their intracellular sites of action. We sought to address this problem using small organic molecules to enhance the effects of oligonucleotides by modulating their intracellular trafficking and release from endosomes. A high-throughput screen of multiple small molecule libraries yielded several hits that markedly potentiated the actions of splice switching oligonucleotides in cell culture. These compounds also enhanced the effects of antisense and siRNA oligonucleotides. The hit compounds preferentially caused release of fluorescent oligonucleotides from late endosomes rather than other intracellular compartments. Studies in a transgenic mouse model indicated that these compounds could enhance the in vivo effects of a splice-switching oligonucleotide without causing significant toxicity. These observations suggest that selected small molecule enhancers may eventually be of value in oligonucleotide-based therapeutics.

Perspectives and Challenges of Cell-Penetrating Peptides in Effective siRNA Delivery

Over the last two decades, hundreds of cell penetrating peptides (CPPs) have been intensively developed as drug and nucleic acid delivery vectors. In many cases, however, the efficient delivery of exogenous bioactive molecules through the plasma membrane to their targets remains a tremendous challenging issue. CPPs have attracted tremendous research interest as efficient cellular delivery vehicles due to their intrinsic ability to enter cells and mediate uptake of a wide range of macromolecular cargos, such as proteins, peptides, nucleic acids, drugs and nanoparticle carriers. This review presents and discusses the current perspectives of CPP-mediated siRNA delivery system. We focus on the CPP-mediated siRNA delivery approaches, and particular emphasis is placed on the strategies for the advantages and disadvantages for each delivery approach. Lastly, the cellular uptake mechanisms of CPPs and the specific challenges associated with each delivery system of siRNAs are discussed.

Attachment of cell-binding ligands to arginine-rich cell-penetrating peptides enables cytosolic translocation of complexed siRNA

Cell-penetrating peptides (CPPs), such as nona-arginine (9R), poorly translocate siRNA into cells. Our studies demonstrate that attaching 9R to ligands that bind cell surface receptors quantitatively increases siRNA uptake and importantly, allows functional delivery of complexed siRNA. The mechanism involved accumulation of ligand-9R:siRNA microparticles on the cell membrane, which induced transient membrane inversion at the site of ligand-9R binding and rapid siRNA translocation into the cytoplasm. siRNA release also occurred late after endocytosis when the ligand was attached to the L isoform of 9R, but not the protease-resistant 9DR, prolonging mRNA knockdown. This critically depended on endosomal proteolytic activity, implying that partial CPP degradation is required for endosome-to-cytosol translocation. The data demonstrate that ligand attachment renders simple polycationic CPPs effective for siRNA delivery by restoring their intrinsic property of translocation.

The role of ligand density and size in mediating quantum dot nuclear transport

Studying the effects of the physicochemical properties of nanomaterials on cellular uptake, toxicity, and exocytosis can provide the foundation for designing safer and more effective nanoparticles for clinical applications. However, an understanding of the effects of these properties on subcellular transport, accumulation, and distribution remains limited. The present study investigates the effects of surface density and particle size of semiconductor quantum dots on cellular uptake as well as nuclear transport kinetics, retention, and accumulation. The current work illustrates that cellular uptake and nuclear accumulation of nanoparticles depend on surface density of the nuclear localization signal (NLS) peptides with nuclear transport reaching a plateau at 20% surface NLS density in as little as 30 min. These intracellular nanoparticles have no effects on cell viability up to 72 h post treatment. These findings will set a foundation for engineering more sophisticated nanoparticle systems for imaging and manipulating genetic targets in the nucleus.

Biomimetic nanoparticles for siRNA delivery in the treatment of leukaemia

Leukaemia is a bone marrow cancer occurring in acute and chronic subtypes. Acute leukaemia is a rapidly fatal cancer potentially causing death within a few weeks, if untreated. Leukaemia arises as a result of disruption to haematopoietic precursors, caused either by acquired gene fusions, gene mutations or inappropriate expression of the relevant oncogenes. Current treatment options have made significant progress, but the 5 year survival for acute leukaemia remains under 10% in elderly patients, and less than 50% for some types of acute leukaemia in younger adults. For chronic leukaemias longer survival is generally expected and for chronic myeloid leukaemia patients on tyrosine kinase inhibitors the median survival is not yet reached and is expected to exceed 10 years. Chemotherapy and haematopoietic stem cell transplantation (HSCT) for acute leukaemia provide the mainstay of therapy for patients under 65 and both carry significant morbidity and mortality. Alternative and superior therapeutic strategies for acute leukaemias are urgently required. Recent molecular-based knowledge of recurring chromosome rearrangements, in particular translocations and inversions, has resulted in significant advances in understanding the molecular pathogenesis of leukaemia. Identification of a number of unique fusion genes has facilitated the development of highly specific small interfering RNAs (siRNA). Although delivery of siRNA using multifunctional nanoparticles has been investigated to treat solid cancers, the application of this approach to blood cancers is at an early stage. This review describes current treatments for leukaemia and highlights the potential of leukaemic fusion genes as therapeutic targets for RNA interference (RNAi). In addition, the design of biomimetic nanoparticles which are capable of responding to the physiological environment of leukaemia and their potential to advance RNAi therapeutics to the clinic will be critically evaluated.

Rational modification of oligoarginine for highly efficient siRNA delivery: structure-activity relationship and mechanism of intracellular trafficking of siRNA

Recently, cell-penetrating peptides (CPPs) have received much attention for cellular delivery of therapeutic molecules. However, in the case of CPPs as carriers for siRNA delivery, their utility is often restricted by low cellular uptake and/or entrapment in endosomes. Here, in order to deliver siRNAs with high efficiency, oligoarginine, a prominent member in CPPs, is rationally modified with oligohistidine and stearyl moieties (STR-) by fully taking into account the formation of nanoparticles, uptake and intracellular trafficking. We show that when the ratio of histidine/arginine in a peptide sequence is >1.5, pronounced gene silencing is induced. Following this rule, STR-HnR8 (n=16 and 20) are developed, which show a high knockdown efficiency rarely reported before. Finally, we find that endosomal escape of siRNA induced by stearylated and oligohistidylated oligoarginine is only from "proton-sponge" effect. Taken together, our results suggest a new strategy for the improvement of CPP-based siRNA delivery systems.

FROM THE CLINICAL EDITOR

This study present a novel cell penetrating peptide-based siRNA delivery system utilizing modified oligo-arginine demonstrating a successful siRNA delivery approach.

Effects of cargo molecules on membrane perturbation caused by transportan10 based cell-penetrating peptides

Cell-penetrating peptides with the ability to escape endosomes and reach the target are of great value as delivery vectors for different bioactive cargoes and future treatment of human diseases. We have studied two such peptides, NickFect1 and NickFect51, both originated from stearylated transportan10 (PF3). To obtain more insight into the mechanism(s) of peptide delivery and the biophysical properties of an efficient vector system, we investigated the effect of different bioactive oligonucleotide cargoes on peptide-membrane perturbation and peptide structural induction. We studied the membrane interactions of the peptides with large unilamellar vesicles and compared their effects with parent peptides transportan10 and PF3. In addition, cellular uptake and peptide-mediated oligonucleotide delivery were analyzed. Calcein leakage experiments showed that similar to transportan10, NickFect51 caused a significant degree of membrane leakage, whereas NickFect1, similar to PF3, was less membrane perturbing. The results are in agreement with previously published results indicating that NickFect51 is a more efficient endosomal escaper. However, the presence of a large cargo like plasmid DNA inhibited NickFect's membrane perturbation and cellular uptake efficiency of the peptide was reduced. We conclude that the pathway for cellular uptake of peptide complexes is cargo dependent, whereas the endosomal escape efficacy depends on peptide hydrophobicity and chemical structure. For small interfering RNA delivery, NickFect51 appears to be optimal. The biophysical signature shows that the peptide alone causes membrane perturbation, but the cargo complex does not. These two biophysical characteristics of the peptide and its cargo complex may be the signature of an efficient delivery vector system.

Arginine-rich, cell penetrating peptide-anti-microRNA complexes decrease glioblastoma migration potential

MicroRNAs (miRNAs) are a class of gene regulators originating from non-coding endogenous RNAs. Altered expression, both up- and down-regulation, of miRNAs plays important roles in many human diseases. Correcting miRNA dysregulation by either inhibiting or restoring miRNA function may provide therapeutic benefit. However, efficient, nontoxic miRNA delivery systems are in need. Cell penetrating peptides (CPPs) have been widely exploited for protein, DNA, and RNA delivery. Few have examined CPP transfection efficiency with single stranded anti-miRNA. The R8 peptide condensed both siRNA and anti-miRNA. Greater than 50% of cells had anti-miRNA/R8 complexes associated and in these cells 68% of anti-miRNA escapes the endosome/lysosome. Single-stranded antisense miR-21 inhibitor (anti-miR-21) administered using the R8 peptide elicited efficient downstream gene upregulation. Glioblastoma cell migration was inhibited by 25% compared to the negative control group. To our knowledge, this is the first demonstration of miRNA modulation with anti-miR-21/R8 complexes, which has laid the groundwork for further exploring octaarginine as intracellular anti-miRNAs carrier.

Cell penetrating peptides: efficient vectors for delivery of nanoparticles, nanocarriers, therapeutic and diagnostic molecules

Efficient delivery of therapeutic and diagnostic molecules to the cells and tissues is a difficult challenge. The cellular membrane is very effective in its role as a selectively permeable barrier. While it is essential for cell survival and function, also presents a major barrier for intracellular delivery of cargo such as therapeutic and diagnostic agents. In recent years, cell-penetrating peptides (CPPs), that are relatively short cationic and/or amphipathic peptides, received great attention as efficient cellular delivery vectors due to their intrinsic ability to enter cells and mediate uptake of a wide range of macromolecular cargo such as plasmid DNA (pDNA), small interfering RNA (siRNAs), drugs, and nanoparticulate pharmaceutical carriers. This review discusses the various uptake mechanisms of these peptides. Furthermore, we discuss recent advances in the use of CPP for the efficient delivery of nanoparticles, nanocarriers, DNA, siRNA, and anticancer drugs to the cells. In addition, we have been highlighting new results for improving endosomal escape of CPP-cargo molecules. Finally, pH-responsive and activable CPPs for tumor-targeting therapy have been described.

Enhancing endosomal escape for nanoparticle mediated siRNA delivery

Gene therapy with siRNA is a promising biotechnology to treat cancer and other diseases. To realize siRNA-based gene therapy, a safe and efficient delivery method is essential. Nanoparticle mediated siRNA delivery is of great importance to overcome biological barriers for systemic delivery in vivo. Based on recent discoveries, endosomal escape is a critical biological barrier to be overcome for siRNA delivery. This feature article focuses on endosomal escape strategies used for nanoparticle mediated siRNA delivery, including cationic polymers, pH sensitive polymers, calcium phosphate, and cell penetrating peptides. Work has been done to develop different endosomal escape strategies based on nanoparticle types, administration routes, and target organ/cell types. Also, enhancement of endosomal escape has been considered along with other aspects of siRNA delivery to ensure target specific accumulation, high cell uptake, and low toxicity. By enhancing endosomal escape and overcoming other biological barriers, great progress has been achieved in nanoparticle mediated siRNA delivery.

A targeted ultrasound contrast agent carrying gene and cell-penetrating peptide: preparation and gene transfection in vitro

Targeted and high efficient gene delivery is a main issue in gene treatment. Taking advantage of ischemic memory target P-selectin and our previous study-synergistic effects of ultrasound-targeted microbubble destruction (UTMD) and TAT peptide on gene transfection, which were characterized by targeted aggregation and high efficient gene transfection, we set up a 'smart' gene delivery system-targeted ultrasound contrast agent (UCA) carrying gene and cell-permeable peptides (CPP). Such UCA had a strong binding force with DNA which was protected from being hydrolysed by nuclease. Moreover, synergistic effects of UTMD and TAT peptide increased gene transfection. Specifically, the UCA were reacted with an ischemic memory target P-selectin overexpressed by ischemic issues (including ischemic heart disease) and loaded with gene and CPP, which enabled targeted localization and gene delivery to ischemic cells overexpressing P-selectin. We demonstrated their targeting affinity for hypoxia human umbilical vein endothelial cell (HUVEC) and gene transfection in vitro. The results of confocal laser scanning microscopy (CLSM) showed that gene and CPP were distributed on the shell of UCA. Red fluorescence was observed on the surface of targeted UCA using immunofluorescent microscopy, which demonstrated that the antibody was successfully connected to the UCA. The targeted UCA was specifically and tightly binded to hypoxia HUVEC, while there were no or little non-targeted UCA binding around hypoxia HUVEC. 24h after transfection, gene transfection efficiency detected by FCM was higher in targeted group than non-targeted group. Overall, the targeted UCA carrying gene and CPP was prepared successfully. It had a strong target binding capacity to hypoxia HUVEC and high efficient gene transfection, which maybe provide a novel strategy for gene therapy.

Recent advances in nanoparticle-mediated siRNA delivery

Inhibiting specific gene expression by short interfering RNA (siRNA) offers a new therapeutic strategy to tackle many diseases, including cancer, metabolic disorders, and viral infections, at the molecular level. The macromolecular and polar nature of siRNA hinders its cellular access to exert its effect. Nanoparticulate delivery systems can promote efficient intracellular delivery. Despite showing promise in many preclinical studies and potential in some clinical trials, siRNA has poor delivery efficiency, which continues to demand innovations, from carrier design to formulation, in order to overcome transport barriers. Previous findings for optimal plasmid DNA delivery cannot be generalized to siRNA delivery owing to significant discrepancy in size and subtle differences in chain flexibility between the two types of nucleic acids. In this review, we highlight the recent advances in improving the stability of siRNA nanoparticles, understanding their intracellular trafficking and release mechanisms, and applying judiciously the promising formulations to disease models.

Nanometer-scale siRNA carriers incorporating peptidomimetic oligomers: physical characterization and biological activity

Synthetic short interfering RNA (siRNA) oligonucleotides can trigger the RNA interference pathway and lead to selective gene silencing. Despite considerable enthusiasm and investment, formidable challenges remain that may deter translating this breakthrough discovery into clinical applications. In particular, the development of efficient, nontoxic, nonimmunogenic methods for delivering siRNA in vivo has proven to be exceptionally challenging. Thorough analysis of the relationship between the structure and function of siRNA carrier systems, both in isolation and in complex with RNA, will facilitate the design of efficient nonviral siRNA delivery vehicles. In this study, we explore the relationship between the physicochemical characteristics and the biological activity of “lipitoid” compounds as potent siRNA delivery vehicles. Lipitoids are cationic peptidomimetic oligomers incorporating a peptoid and a phospholipid moiety. Lipitoids can associate with siRNA oligonucleotides and self-assemble into spherical lipitoid-based nanoparticles (LNPs), with dimensions that are dependent upon the medium and the stoichiometric ratio between the cationic monomers of the lipitoid and anionic siRNA oligonucleotides. The morphology, gene silencing efficiency, and cytotoxicity of the siRNA-loaded LNPs are similarly sensitive to the stoichiometry of the complexes. The medium in which the LNPs are formed affects the assembled cargo particles’ characteristics such as particle size, transfection efficiency, and stability. Formation of the LNPs in the biological, serum-free medium OptiMEM resulted in LNPs an order of magnitude larger than LNPs formed in water, and were twice as efficient in siRNA transfection compared to LNPs formed in water. Inhibitor studies were conducted to elucidate the efficiency of lysosomal escape and the uptake mechanism of the siRNA-loaded LNPs. Our results suggest that these lipitoid-based, siRNA-loaded spherical LNPs are internalized through a lipid raft-dependent and dynamin-mediated pathway, circumventing endosomal and lysosomal encapsulation. The lipitoid-siRNA nanospheres proved to be suitable platforms for investigating the critical parameters determining the efficiency of transfection agents, revealing the necessity for conducting characterization studies in biological media. The investigation of the LNP internalization pathway points to an alternative uptake route that bypasses the lysosome, explaining the surprisingly high efficiency of LNPs and suggesting that the uptake mechanism should be probed rather than assumed for the next generation of rationally designed transfection agents.

Contrasting effects of cysteine modification on the transfection efficiency of amphipathic peptides

Delivery of DNA to cells remains a key challenge towards development of gene therapy. A better understanding of the properties involved in stability and transfection efficiency of the vector could critically contribute to the improvement of delivery vehicles. In the present work we have chosen two peptides differing only in amphipathicity and explored how presence of cysteine affects DNA uptake and transfection efficiency. We report an unusual observation that addition of cysteine selectively increases transfection efficiency of secondary amphipathic peptide (Mgpe-9) and causes a drop in the primary amphipathic peptide (Mgpe-10). Our results point the effect of cysteine is dictated by the importance of physicochemical properties of the carrier peptide. We also report a DNA delivery agent Mgpe-9 exhibiting high transfection efficiency in multiple cell lines (including hard-to-transfect cell lines) with minimal cytotoxicity which can be further explored for in vivo applications.

Silica nanoparticles and polyethyleneimine (PEI)-mediated functionalization: a new method of PEI covalent attachment for siRNA delivery applications

Small-interfering RNA (siRNA) is a synthetic double-stranded RNA that consists of approximately 21 nucleotides (nts). It induces degradation of target mRNAs in a sequence-specific manner by the RNA interference (RNAi) mechanism. Thus, siRNAs offer a potential strategy for silencing mutated or defective genes that cause a variety of human diseases. The main obstacles of harnessing siRNAs as drugs are their inefficient delivery to cells and off-target effect making clinical applications very challenging. To address these issues, researchers have studied a variety of nanocarrier systems for siRNA delivery. This study presents the design, fabrication, and full characterization of innovative polyethyleneimine (PEI)-decorated polycationic 34.2 ± 4.2 nm silica (SiO2) NPs for siRNA-mediated gene silencing. More specifically, a new means of introduction (covalent mode of attachment) of the polycationic 25 kDa PEI polymer onto the SiO2 NP surface has been developed that makes use of an effective electrophilic double Michäel acceptor, divinyl sulfone (DVS). The resulting novel SiO2-PEI nanoparticles (SPEI NPs) have been fully characterized using a wide range of analytical, spectroscopic, and microscopic methods (TEM, DLS, ζ potential, elemental analysis (EA), XPS, TGA, and FTIR). Disclosing quite low cytotoxicity due to this unique mode of PEI covalent grafting, SPEI NPs/siRNA polyplexes have been successfully tested for the induction of gene silencing using dual-reporter luciferase transfected human osteosarcoma U2OS cells. The corresponding gene silencing data showed a clear correlation between PEI/siRNA ratios, siRNA concentration(s), and the level of gene silencing. Moreover, these SPEI NPs have been demonstrated to be thermodynamically stable with an ability to efficiently bind siRNAs and induce silencing for at least a one-year-long storage.

Development of streptavidin-based nanocomplex for siRNA delivery

In our previous study, we have identified a PCBP2 siRNA that exhibits antifibrotic activity in rat hepatic stellate cells (HSCs) by inhibition of αCP2, a protein responsible for stabilization of the collagen α1 (I) mRNA in alcoholic liver fibrosis. This study aims to develop a streptavidin-based nanocomplex that can efficiently deliver the PCBP2 siRNA to HSCs. Biotin-siRNA and biotin-cholesterol were mixed with streptavidin to form the streptavidin-biotin complex, which was further condensed electrostatically with positively charged protamine to form the final multicomponent siRNA nanocomplex in the size range of 150-250 nm. The siRNA nanocomplex does not induce cytotoxicity in rat HSCs as compared to commercially available transfection agents. The cellular uptake efficiency of the siRNA nanocomplex is higher in rat HSCs than other cell lines, such as Caco-2 and PC-3, indicating that receptor-mediated endocytosis mainly contributes to the cellular uptake of the siRNA nanocomplex. The siRNA nanocomplex exhibits more than 85% silencing effect on the PCBP2 mRNA in HSCs. Stability study indicates that the nanocomplex can efficiently protect siRNA from degradation in the serum. The streptavidin-based multicomponent siRNA nanocomplex provides a promising strategy to deliver the PCBP2 siRNA to HSCs. Moreover, the nanocomplex can be used as a platform for other diseases by changing the siRNA sequence and targeting ligand.

Mechanisms of nanoparticle-mediated siRNA transfection by melittin-derived peptides

Traditional peptide-mediated siRNA transfection via peptide transduction domains exhibits limited cytoplasmic delivery of siRNA due to endosomal entrapment. This work overcomes these limitations with the use of membrane-destabilizing peptides derived from melittin for the knockdown of NFkB signaling in a model of adult T-cell leukemia/lymphoma. While the mechanism of siRNA delivery into the cytoplasmic compartment by peptide transduction domains has not been well studied, our analysis of melittin derivatives indicates that concurrent nanocomplex disassembly and peptide-mediated endosomolysis are crucial to siRNA transfection. Importantly, in the case of the most active derivative, p5RHH, this process is initiated by acidic pH, indicating that endosomal acidification after macropinocytosis can trigger siRNA release into the cytoplasm. These data provide general principles regarding nanocomplex response to endocytosis, which may guide the development of peptide/siRNA nanocomplex-based transfection.

Cell surface clustering of heparan sulfate proteoglycans by amphipathic cell-penetrating peptides does not contribute to uptake

For arginine-rich cell-penetrating peptides (CPPs), an association with heparan sulfate (HS) chains is considered the first step in the stimulation of uptake for many cells. Much less is known about the role of HS chains in the cell-association and internalization of arginine-free amphipathic CPP such as transportan-10 (TP10). Here, we report that various TP10 analogs differ in their capacity to accumulate on HS-rich plasma membranes in an HS-dependent manner. No accumulation was observed on HS-poor plasma membranes or when HS was removed by enzymatic cleavage. The TP10 analog that strongly clustered on the cell surface, also showed a pronounced capacity to form clusters with HS chains in solution. However, aggregation occurred in a thermodynamically different way compared to the interaction of arginine-rich CPP with HS. To monitor the impact of the peptide on the aggregation of the glycocalyx by time-lapse microscopy, sialic acids were visualized by metabolic labeling using copper-free click chemistry to attach fluorophores to metabolically incorporated azido sugars. Strikingly, a highly enhanced HS-mediated accumulation on the plasma membrane of a particular TP10 analog did not correlate with a better uptake. These findings illustrate that the mode of interaction between cell-penetrating peptides and HS chains has important functional consequences regarding peptide internalization and that there is no direct coupling of interaction, accumulation and uptake.