Improved transfer efficiency of supercharged 36 + GFP protein mediate nucleic acid delivery

The potential of nucleic acid therapeutics to treat diseases by targeting specific cells has resulted in its increasing number of uses in clinical settings. However, the major challenge is to deliver bio-macromolecules into target cells and/or subcellular locations of interest ahead in the development of delivery systems. Although, supercharged residues replaced protein 36 + GFP can facilitate itself and cargoes delivery, its efficiency is still limited. Therefore, we combined our recent progress to further improve 36 + GFP based delivery efficiency. We found that the penetration efficacy of 36 + GFP protein was significantly improved by fusion with CPP-Dot1l or treatment with penetration enhancer dimethyl sulfoxide (DMSO) in vitro. After safely packaged with plasmid DNA, we found that the efficacy of in vitro and in vivo transfection mediated by 36 + GFP-Dot1l fusion protein is also significantly improved than 36 + GFP itself. Our findings illustrated that fusion with CPP-Dot1l or incubation with DMSO is an alternative way to synergically promote 36 + GFP mediated plasmid DNA delivery in vitro and in vivo.

Transportation of Single-Domain Antibodies through the Blood-Brain Barrier

Single-domain antibodies derive from the heavy-chain-only antibodies of Camelidae (camel, dromedary, llama, alpaca, vicuñas, and guananos; i.e., nanobodies) and cartilaginous fishes (i.e., VNARs). Their small size, antigen specificity, plasticity, and potential to recognize unique conformational epitopes represent a diagnostic and therapeutic opportunity for many central nervous system (CNS) pathologies. However, the blood–brain barrier (BBB) poses a challenge for their delivery into the brain parenchyma. Nevertheless, numerous neurological diseases and brain pathologies, including cancer, result in BBB leakiness favoring single-domain antibodies uptake into the CNS. Some single-domain antibodies have been reported to naturally cross the BBB. In addition, different strategies and methods to deliver both nanobodies and VNARs into the brain parenchyma can be exploited when the BBB is intact. These include device-based and physicochemical disruption of the BBB, receptor and adsorptive-mediated transcytosis, somatic gene transfer, and the use of carriers/shuttles such as cell-penetrating peptides, liposomes, extracellular vesicles, and nanoparticles. Approaches based on single-domain antibodies are reaching the clinic for other diseases. Several tailoring methods can be followed to favor the transport of nanobodies and VNARs to the CNS, avoiding the limitations imposed by the BBB to fulfill their therapeutic, diagnostic, and theragnostic promises for the benefit of patients suffering from CNS pathologies.

Applications of amphipathic and cationic cyclic cell-penetrating peptides: Significant therapeutic delivery tool

A new class of peptides, cyclic cell-penetrating peptides (CPPs), has great potential for delivering a vast variety of therapeutics intracellularly for treating diverse ailments. CPPs have been used previously; however, their further use is limited due to instability, toxicity, endosomal degradation, and insufficient cellular penetration. Cyclic CPPs are being investigated in delivering therapeutics to treat various ailments, including multi-drug resistant microbial infections, HIV, and cancer. They can act as a carrier for a variety of cargos and target intracellularly. Approximately 40 cyclic peptides-based therapeutics are available in the market, and annually one cyclic peptide-based drug enters the market. Numerous research and review articles have been published in the last decade about linear and cyclic peptides separately. This review is the first to provide a comprehensive deliberation about cationic and amphipathic cyclic CPPs. Herein, we highlights their structures, significant advantages, translocation mechanisms, and delivery application in the area of biomedical sciences.

Discovery of Membrane-Permeating Cyclic Peptides via mRNA Display

Small synthetic peptides capable of crossing biological membranes represent valuable tools in cell biology and drug delivery. While several cell-penetrating peptides (CPPs) of natural or synthetic origin have been reported, no peptide is currently known to cross both cytoplasmic and outer embryonic membranes. Here, we describe a method to engineer membrane-permeating cyclic peptides (MPPs) with broad permeation activity by screening mRNA display libraries of cyclic peptides against embryos at different developmental stages. The proposed method was demonstrated by identifying peptides capable of permeating Drosophila melanogaster (fruit fly) embryos and mammalian cells. The selected peptide cyclo[Glut-MRKRHASRRE-K*] showed a strong permeation activity of embryos exposed to minimal permeabilization pretreatment, as well as human embryonic stem cells and a murine fibroblast cell line. Notably, in both embryos and mammalian cells, the cyclic peptide outperformed its linear counterpart and the control MPPs. Confocal microscopy and single cell flow cytometry analysis were utilized to assess the degree of permeation both qualitatively and quantitatively. These MPPs have potential application in studying and nondisruptively controlling intracellular or intraembryonic processes.

Cell-penetrating peptide enhanced insulin buccal absorption

Non-injectable delivery of peptides and proteins is not feasible due to the limitations of large molecular mass, high hydrophilic properties, and gastrointestinal degradation. Therefore, proposing a new method to solve this problem is a burning issue. The objective of this study was to propose a novel protein delivery strategy to overcome the poor efficacy and irritation of buccal insulin delivery. In this study, we applied a conjugate of cell-penetrating peptides (LMWP) and insulin (INS-PEG-LMWP) for buccal delivery. INS-PEG-LMWP was prepared using insulin solution and mixture as references. The transport behaviour, in vivo bioactivity, hypoglycaemic effect, and safety of INS-PEG-LMWP were systematically characterised. An in vitro study demonstrated that the uptake and transportation of INS-PEG-LMWP across buccal mucosal multilayers significantly increased. By comparing the effects of different endocytic inhibitors on INS-PEG-LMWP uptake, the conjugate might be delivered via an energy independent, electrostatically adsorbed pathway. INS-PEG-LMWP's relative pharmacological bioavailability was high and its relative bioavailability was up to 26.86%, demonstrating no visible mucosal irritation. Cell-penetrating peptides are likely to become a reliable and safe tool for overcoming insulin's low permeability through the epithelial multilayers, the major barrier to buccal delivery.

Dual-Blockade Immune Checkpoint for Breast Cancer Treatment Based on a Tumor-Penetrating Peptide Assembling Nanoparticle

Cancer immunotherapy can enhance the antitumor effect of drugs through a combinatorial approach in a synergistic manner. However, the effective targeted delivery of various drugs remains a challenge. We generated a peptide assembling tumor-targeted nanodelivery system based on a breast cancer homing and penetrating peptide for the codelivery of a programmed cell death ligand 1 (PD-L1) small interfering RNA (siRNA) (siPD-L1) and an indoleamine 2,3-dioxygenase inhibitor as a dual blockade of an immune checkpoint. The vector is capable of specifically accumulating in the breast cancer tumor site in a way that allows the siRNA to escape from endosomal vesicles after being endocytosed by tumor cells. The drug within these cells then acts to block tryptophan metabolism. The results showed that locally released siPD-L1 and 1-methyl-dl-tryptophan favor the survival and activation of cytotoxic T lymphocytes, resulting in apoptosis of breast cancer cells. Therefore, this study provides a potential approach for treating breast cancer by blocking immunological checkpoints through the assembly of micelles with functional peptides.

Octopus-like Flexible Vector for Noninvasive Intraocular Delivery of Short Interfering Nucleic Acids

Gene therapy is promising for chronic posterior ocular diseases, which are causal factors for severe vision impairment and even blindness worldwide. However, the inherent absorption barriers of the eye restrict intraocular delivery of therapeutic nucleic acids via topical instillation. Safe and efficient nonviral vectors for ocular gene therapy are still unmet clinical desires. Herein, an octopus-like flexible multivalent penetratin (MVP) was designed to facilitate condensation and delivery of therapeutic nucleic acids using multiarm polyethylene glycol (PEG) as a core and conjugating penetratin at each end of the PEG arms as outspread tentacles. Among the MVPs, 8-valent penetratin (8VP) stably compacted nucleic acids into positively charged polyplexes smaller than 100 nm, promoting cellular uptake efficiency (approaching 100%) and transfection rate (over 75%). After being instilled into the conjunctival sac, 8VP enabled rapid (<10 min) and prolonged (>6 h) distribution of nucleic acids in the retina via a noncorneal pathway. In a retinoblastoma-bearing mice model, topical instillation of 8VP/siRNA efficiently inhibited the protein expression of intraocular tumor without toxicity. MVP is advantageous over the commercial transfection reagent in safety and efficiency, and therefore provides a promising vector for noninvasive intraocular gene delivery.

Dual functionalized liposomes for efficient co-delivery of anti-cancer chemotherapeutics for the treatment of glioblastoma

Glioblastoma is a hostile brain tumor associated with high infiltration leading to poor prognosis. Anti-cancer chemotherapeutic agents have limited access into the brain due to the presence of the blood brain barrier (BBB). In this study, we designed a dual functionalized liposomal delivery system, surface modified with transferrin (Tf) for receptor mediated transcytosis and a cell penetrating peptide-penetratin (Pen) for enhanced cell penetration. We loaded doxorubicin and erlotinib into liposomes to enhance their translocation across the BBB to glioblastoma tumor. In vitro cytotoxicity and hemocompatibility studies demonstrated excellent biocompatibility for in vivo administration. Co-delivery of doxorubicin and erlotinib loaded Tf-Pen liposomes revealed significantly (p < 0.05) higher translocation (~15%) across the co-culture endothelial barrier resulting in regression of tumor in the in vitro brain tumor model. The biodistribution of Tf-Pen liposomes demonstrated ~12 and 3.3 fold increase in doxorubicin and erlotinib accumulation in mice brain, respectively compared to free drugs. In addition, Tf-Pen liposomes showed excellent antitumor efficacy by regressing ~90% of tumor in mice brain with significant increase in the median survival time (36 days) along with no toxicity. Thus, we believe that this study would have high impact for treating patients with glioblastoma.

Cancer targeting peptides

Despite continuing advances in the development of biomacromolecules for therapeutic purposes, successful application of these often large and hydrophilic molecules has been hindered by their inability to efficiently traverse the cellular plasma membrane. In recent years, cell-penetrating peptides (CPPs) have received considerable attention as a promising class of delivery vectors due to their ability to mediate the efficient import of a large number of cargoes in vitro and in vivo. However, the lack of target specificity of CPPs remains a major obstacle to their clinical development. To address this issue, researchers have developed strategies in which chemotherapeutic drugs are conjugated to cancer targeting peptides (CTPs) that exploit the unique characteristics of the tumor microenvironment or cancer cells, thereby improving cancer cell specificity. This review highlights several of these strategies that are currently in use, and discusses how multi-component nanoparticles conjugated to CTPs can be designed to provide a more efficient cancer therapeutic delivery strategy.

Tumor targeting and microenvironment-responsive multifunctional fusion protein for pro-apoptotic peptide delivery

The great therapeutic potential of peptides has not yet been achieved, mainly due to their remarkably short in vivo half-life. Although conjugation to macromolecules has been an effective way of improving protein in vivo half-life, the steric hindrance of macromolecules usually reduces the in vivo efficacy of peptides. Here we report a complex delivery system made from PsTag polypeptide, polyglutamic acid chain, matrix metalloproteinase 2 (MMP2)-degradable domain and cationic cell penetrating peptide for anticancer peptide delivery. Clear evidence was shown in vitro and in vivo to demonstrate that this multifunctional protein fusing a pro-apoptotic KLAKLAKKLAKLAK (KLA), named PAK, can increase circulation time in blood, enhance accumulation at tumor sites, eliminate the PsTag domain and the polyanionic sequence when triggered by tumor overexpressing MMP2, and then expose the cell penetrating peptide to realize the potent cellular uptake of KLA. Treatment of tumor-bearing mice with PAK could markedly induce tumor cells apoptosis and inhibit tumor growth, with no significant adverse effects. These results suggest our fusion protein can be a potential delivery system for peptide delivery in cancer treatments.

Engineered Histidine-Enriched Facial Lipopeptides for Enhanced Intracellular Delivery of Functional siRNA to Triple Negative Breast Cancer Cells

Cytosolic delivery of functional siRNA remains the major challenge to develop siRNA-based therapeutics. Designing clinically safe and effective siRNA transporter to deliver functional siRNA across the plasma and endosomal membrane remains a key hurdle. With the aim of improving endosomal release, we have designed cyclic and linear peptide-based transporters having an Arg-^DHis-Arg template. Computational studies show that the Arg-^DHis-Arg template is also stabilized by the Arg-His side-chain hydrogen bonding interaction at physiological pH, which dissociates at lower pH. The overall atomistic interactions were examined by molecular dynamics simulations, which indicate that the extent of peptide_siRNA assembly formation depends greatly on physicochemical properties of the peptides. Our designed peptides having the Arg-^DHis-Arg template and two lipidic moieties facilitate high yield of intracellular delivery of siRNA. Additionally, unsaturated lipid, linoleic acid moieties were introduced to promote fusogenicity and facilitate endosomal release and cytosolic delivery. Interestingly, such protease-resistant peptides provide serum stability to siRNA and exhibit high efficacy of erk1 and erk2 gene silencing in the triple negative breast cancer (TNBC) cell line. The peptide having two linoleyl moieties demonstrated comparable efficacy with commercial transfection reagent HiPerFect, as evidenced by the erk1 and erk2 gene knockdown experiment. Additionally, our study shows that ERK1/2 silencing siRNA and doxorubicin-loaded gramicidin-mediated combination therapy is more effective than siRNA-mediated gene silencing-based monotherapy for TNBC treatment.

Targeted Interleukin-22 Gene Delivery in the Liver by Polymetformin and Penetratin-Based Hybrid Nanoparticles to Treat Nonalcoholic Fatty Liver Disease

Nonalcoholic fatty liver disease (NAFLD) is now a leading cause of chronic liver disease, and there is currently no available treatment strategy. Interleukin-22 (IL-22) has been recognized as a promising agent for alleviating NAFLD, but the efficacy of IL-22 is far from satisfactory because safe dose of IL-22 elicited limited improvement, whereas higher concentration might induce serious side effects and off-target toxicities. Thus, targeted and sustained expression of IL-22 in the liver is necessary. To meet the challenge, we elaborately developed a novel polymetformin carrier by conjugating biguanide to chitosan, termed chitosan-metformin (CM), which could exert advanced gene delivery efficiency and possess intrinsic therapeutic efficacy from metformin for NAFLD. CM accompanied with penetratin and DSPE-PEG2000 could self-assemble to form stable nanocomplexes with IL-22 gene via electrostatic interaction. This nanoparticle (CDPIA) exerted desirable particle size at ∼100 nm, fine morphology, and efficient cellular internalization. Furthermore, CDPIA also demonstrated a unique superiority in endosomal escape capacity and satisfactory biocompatibility as well as predominant liver accumulation. Most importantly, CDPIA distinctly alleviated hepatic steatosis, restored insulin sensitivity, and improved metabolic syndrome in high-fat-diet-fed mice model. This liver-targeted delivery of IL-22 activated STAT3/Erk1/2 and Nrf2/SOD1 signaling transductions as well as modulated lipid-metabolism-related gene expression. These findings altogether demonstrated that the polymetformin and penetratin-based hybrid nanoparticles could be exploited as a novel safe and efficient strategy for the improvement of NAFLD.

New Cell-Penetrating Peptide (KRP) with Multiple Physicochemical Properties Endows Doxorubicin with Tumor Targeting and Improves Its Therapeutic Index

Cell-penetrating peptides (CPPs) are considered as promising drug carriers by virtue of their potent cell-penetrating capacity. However, lack of targetability still represents a bottleneck for their systemic administration. Here, we synthesized a lysine-rich CPP named KRP and developed a tumor-targeted drug delivery system (DDS) by linking KRP and doxorubicin (DOX) with stable covalent bonds (thioether bond and amide bond). Through in vitro and in vivo tests, we confirmed that the multiple physicochemical properties of KRP endow KRP-DOX with multiple synergistic functions, including good biocompatibility and biodistribution, selective accumulation in tumor tissues, inclination to remain in tumor tissues and be internalized by tumor cells; stable covalent bonds prevent free DOX release from KRP-DOX in blood stream, shield normal tissues from the toxic effect of DOX, and lead to the majority of DOX delivery into tumor cells by KRP; lysosome escape of KRP-DOX ensures its tumor-killing effect. In addition, the simple chemical composition and modification reduce the risk of immunogenicity and metabolite toxicity. Our study provides a simple, safe, and efficient platform for tumor-targeted DDS.

HOPS-dependent endosomal fusion required for efficient cytosolic delivery of therapeutic peptides and small proteins

Protein therapeutics represent a significant and growing component of the modern pharmacopeia, but their potential to treat human disease is limited because most proteins fail to traffic across biological membranes. Recently, we discovered a class of cell-permeant miniature proteins (CPMPs) containing a precisely defined, penta-arginine (penta-Arg) motif that traffics readily to the cytosol and nucleus of mammalian cells with efficiencies that rival those of hydrocarbon-stapled peptides active in animals and man. Like many cell-penetrating peptides (CPPs), CPMPs enter the endocytic pathway; the difference is that CPMPs containing a penta-Arg motif are released efficiently from endosomes, while other CPPs are not. Here, we seek to understand how CPMPs traffic from endosomes into the cytosol and what factors contribute to the efficiency of endosomal release. First, using two complementary cell-based assays, we exclude endosomal rupture as the primary means of endosomal escape. Next, using an RNA interference screen, fluorescence correlation spectroscopy, and confocal imaging, we identify VPS39-a gene encoding a subunit of the homotypic fusion and protein-sorting (HOPS) complex-as a critical determinant in the trafficking of CPMPs and hydrocarbon-stapled peptides to the cytosol. Although CPMPs neither inhibit nor activate HOPS function, HOPS activity is essential to efficiently deliver CPMPs to the cytosol. CPMPs localize within the lumen of Rab7+ and Lamp1+ endosomes and their transport requires HOPS activity. Overall, our results identify Lamp1+ late endosomes and lysosomes as portals for passing proteins into the cytosol and suggest that this environment is prerequisite for endosomal escape.

Acid-Induced Activated Cell-Penetrating Peptide-Modified Cholesterol-Conjugated Polyoxyethylene Sorbitol Oleate Mixed Micelles for pH-Triggered Drug Release and Efficient Brain Tumor Targeting Based on a Charge Reversal Mechanism

Glioblastoma multiforme is the most devastating malignant brain tumor in adults. Even with the standard care of therapy, the prognosis remains dismal due to tumor heterogeneity, tumor infiltration, and, more importantly, the restrictive nature of the blood-brain barrier (BBB). To overcome the challenge of effectively delivering therapeutic cargo into the brain, herein a "smart", multifunctional polymeric micelle was developed using a cholesterol-conjugated polyoxyethylene sorbitol oleate. A cell-penetrating peptide, arginine-glycine repeats (RG)₅, was incorporated into the micelles to improve cellular uptake, while a pH-sensitive masking sequence, histidine-glutamic acid repeats (HE)₅, was introduced for charge shielding to minimize nonspecific binding and uptake at physiological pH. Results demonstrated that (RG)₅- and (HE)₅-modified mixed micelles were optimized using this strategy to effectively mask the cationic charges of the activated cell-penetrating peptide (RG)₅ at physiological pH, i.e., limiting internalization, and were selectively triggered in response to a mildly acidic microenvironment in vitro based on a charge reversal mechanism. In vivo results further confirmed that such micelles preferentially accumulated in both brain and tumor tissues in both xenograft and orthotropic glioma mouse models. Furthermore, micelles significantly inhibited tumor growth with limited toxicity to peripheral tissues. The combination of BBB penetration, tumor targeting, potent efficacy, and high tolerance of these micelles strongly suggests that they could be a promising candidate for safe and effective drug delivery to the brain.

Enhanced intranasal insulin delivery by formulations and tumor protein-derived protein transduction domain as an absorption enhancer

One of the key factors for successful development of an intranasal insulin formulation is an absorption enhancer that would deliver insulin efficiently across nasal membranes without causing damage to mucosa or inducing protein aggregation under physiological conditions. In the present study, a protein transduction domain (PTD1) and its L-form with the double substitution A6L and I8A (PTD4), derived from human translationally controlled tumor protein, were used as absorption enhancers. PTD4 exhibited higher compatibility with insulin in terms of biophysical properties analyzed using μDSC, DLS, and CD. In addition, thermodynamic properties indicated stable complex formation but higher propensity of protein aggregation. Arginine hydrochloride (ArgHCl) was used to suppress protein aggregation and carbohydrates (i.e., mannitol, sucrose, and glycerin) were used as osmolytes in the formulation. The relative bioavailability of insulin co-administered intranasally using PTD4, 16 mg/mL glycerin and 100 mM ArgHCl was 58% and that using PTD4, 1 w/v% sucrose, and 25 mM ArgHCl was 53% of the bioavailability obtained via the subcutaneous route. These values represented a remarkable increase in bioavailability of intranasal insulin, causing a significant decrease in blood glucose levels within one hour. The pharmacokinetic properties of intranasal absorption were dependent on the concentration of carbohydrates used. These results suggest that the newly designed formulations with PTD represent a useful platform for intranasal delivery of insulin and other biomolecules.

Lipid-modified cell-penetrating peptide-based self-assembly micelles for co-delivery of narciclasine and siULK1 in hepatocellular carcinoma therapy

Hepatocellular carcinoma (HCC) is the most frequent type of primary liver cancer, and one therapeutic approach is to target both the AMPK and autophagy pathways in order to synergistically promote programmed cell death. Here, a series of amphiphilic, lipid-modified cell-penetrating peptides were synthesized and allowed to self-assemble into micelles loaded with the AMPK activator narciclasine (Narc) and short interfering RNA targeting the unc-51-like kinase 1 (siULK1). The size of these micelles, their efficiency of transfection into cells, and their ability to release drug or siRNA cargo in vitro were pH-sensitive, such that drug release was facilitated in the acidic microenvironment of the tumor. Transfecting the micelles into HCC cells significantly inhibited protective autophagy within tumor cells, and delivering the micelles into mice carrying HCC xenografts induced apoptosis, slowed tumor growth, and inhibited autophagy. Our results indicate that co-delivering Narc and siULK1 in biocompatible micelles can safely inhibit tumor growth and protective autophagy, justifying further studies into this promising therapeutic approach against HCC.

STATEMENT OF SIGNIFICANCE

We have focused on the targeted therapy of HCC via synergistically inhibiting the autophagy and inducing apoptosis. The lipid-modified cell-penetrating peptide can not only aggregate into micelles to load natural product narciclasine and ULK1 siRNA simultaneously, but also facilitate uptake and endosome escape with a pH-sensitive manner in HepG2 cells. HepG2 cell treated with siULK1-M-Narc has increased apoptotic levels and declined autophagy via the targeted regulation of AMPK-ULK1 signaling axis. The in vivo studies have confirmed that siULK1-M-Narc efficiently reduce the growth of tumor on HCC xenograft models with good safety. Thus, we suppose the lipid-modified cell-penetrating peptide has good application prospects in the targeted combinational therapy of HCC.

Cell Penetrating Capacity and Internalization Mechanisms Used by the Synthetic Peptide CIGB-552 and Its Relationship with Tumor Cell Line Sensitivity

CIGB-552 is a twenty-amino-acid novel synthetic peptide that has proven to be effective in reducing tumor size and increasing lifespan in tumor-bearing mice. Such capability is conferred by its cell-penetrating peptide character, which allows it to enter cells and elicit a pro-apoptotic effect through its major mediator, COMMD1 protein. Cell-penetrating peptides are able to use different internalization mechanisms, such as endocytosis or direct transduction through the plasma membrane. Although CIGB-552 cytotoxicity has been evaluated in several non-tumor- and tumor-derived cell lines, no data regarding the relationship between cell line sensitivity, cell penetrating capacity, the internalization mechanisms involved, COMMD1 expression levels, or its subcellular localization has yet been produced. Here, we present the results obtained from a comparative analysis of CIGB-552 sensitivity, internalization capacity and the mechanisms involved in three human tumor-derived cell lines from different origins: mammary gland, colon and lung (MCF-7, HT-29 and H460, respectively). Furthermore, cell surface markers relevant for internalization processes such as phosphatidylserine, as well as CIGB-552 target COMMD1 expression/localization, were also evaluated. We found that both endocytosis and transduction are involved in CIGB-552 internalization in the three cell lines evaluated. However, CIGB-552 incorporation efficiency and contribution of each mechanism is cell-line dependent. Finally, sensitivity was directly correlated with high internalization capacity in those cell lines where endocytosis had a major contribution on CIGB-552 internalization.

Cell Penetrating Peptides as Molecular Carriers for Anti-Cancer Agents

Cell membranes with their selective permeability play important functions in the tight control of molecular exchanges between the cytosol and the extracellular environment as the intracellular membranes do within the internal compartments. For this reason the plasma membranes often represent a challenging obstacle to the intracellular delivery of many anti-cancer molecules. The active transport of drugs through such barrier often requires specific carriers able to cross the lipid bilayer. Cell penetrating peptides (CPPs) are generally 5–30 amino acids long which, for their ability to cross cell membranes, are widely used to deliver proteins, plasmid DNA, RNA, oligonucleotides, liposomes and anti-cancer drugs inside the cells. In this review, we describe the several types of CPPs, the chemical modifications to improve their cellular uptake, the different mechanisms to cross cell membranes and their biological properties upon conjugation with specific molecules. Special emphasis has been given to those with promising application in cancer therapy.

Perfluoroarene-Based Peptide Macrocycles to Enhance Penetration Across the Blood-Brain Barrier

Here we describe the utility of peptide macrocyclization through perfluoroaryl-cysteine SNAr chemistry to improve the ability of peptides to cross the blood-brain barrier. Multiple macrocyclic analogues of the peptide transportan-10 were investigated that displayed increased uptake in two different cell lines and improved proteolytic stability. One of these analogues (M13) exhibited substantially increased delivery across a cellular spheroid model of the blood-brain barrier. Through ex vivo imaging of mouse brains, we demonstrated that this perfluoroarene-based macrocycle of TP10 exhibits increased penetration of the brain parenchyma following intravenous administration in mice. Finally, we evaluated macrocyclic analogues of the BH3 domain of the BIM protein to assess if our approach would be applicable to a peptide of therapeutic interest. We identified a BIM BH3 analogue that showed increased penetration of the brain tissue in mice.

Manipulating the membrane penetration mechanism of helical polypeptides via aromatic modification for efficient gene delivery

The delivery performance of non-viral gene vectors is greatly related to their intracellular kinetics. Cationic helical polypeptides with potent membrane penetration properties and gene transfection efficiencies have been recently developed by us. However, they suffer from severe drawbacks in terms of their membrane penetration mechanisms that mainly include endocytosis and pore formation. The endocytosis mechanism leads to endosomal entrapment of gene cargos, while the charge- and helicity-induced pore formation causes appreciable cytotoxicity at high concentrations. With the attempt to overcome such critical challenges, we incorporated aromatic motifs into the design of helical polypeptides to enhance their membrane activities and more importantly, to manipulate their membrane penetration mechanisms. The aromatically modified polypeptides exhibited higher cellular internalization level than the unmodified analogue by up to 2.5 folds. Such improvement is possibly because aromatic domains promoted the polypeptides to penetrate cell membranes via direct transduction, a non-endocytosis and non-pore formation mechanism. As such, gene cargos were more efficiently delivered into cells by bypassing endocytosis and subsequently avoiding endosomal entrapment, and the material toxicity associated with excessive pore formation was also reduced. The top-performing aromatic polypeptide containing naphthyl side chains at the incorporated content of 20mol% revealed notably higher transfection efficiencies than commercial reagents in melanoma cells in vitro (by 11.7 folds) and in vivo (by 9.1 folds), and thus found potential utilities toward topical gene delivery for cancer therapy.

STATEMENT OF SIGNIFICANCE

Cationic helical polypeptides, as efficient gene delivery materials, suffer from severe drawbacks in terms of their membrane penetration mechanisms. The main cell penetration mechanisms involved are endocytosis and pore formation. However, the endocytosis mechanism has the limitation of endosomal entrapment of gene cargos, while the charge- and helicity-induced pore formation causes cytotoxicity at high concentrations. To address such critical issues toward the maximization of gene delivery efficiency, we incorporated aromatic domains into helical polypeptides to promote the cell membrane penetrations via direct transduction, which is a non-endocytosis and non-pore formation mechanism. The manipulation of their membrane penetration mechanisms allows gene cargos to be more efficiently delivered by bypassing endocytosis and subsequently avoiding endosomal entrapment.

Targeting neuronal nitric oxide synthase by a cell penetrating peptide Tat-LK15/siRNA bioconjugate

We developed a cell penetrating peptide (CPP) Tat-LK15, as a siRNA carrier to target nNOS. The feasibility, stability, efficiency and selectivity of this peptide-siRNA complex were evaluated in rat neuronal cells. We also compared the new method with conventional siRNA carrier Lipofectamine™. It was found that the CPP Tat-LK15 effectively and specifically delivered nNOS-siRNA into Rat retinal ganglia (RGC-5) cells and silenced the expression of nNOS. The CPP Tat-LK15 can conjugate with siRNA to form stable complex at a ratio of 2:1 (peptide/siRNA, w/w), which maintained stable in serum for as long as 4h. The CPP Tat-LK15 was low-toxicity to cells, as the apoptosis rate of treat cells was not increased significantly when the used peptide lower than 10μg/mL. Moreover, the cellular uptake of nNOS siRNA by Rat Neurons-dorsal spinal cord (RNdsc) cells was also significantly more than naked siRNA by RNdsc cells. The CPP Tat-LK15 was an efficient and stable, and non-cytotoxic siRNA delivery to neurons and effectively silenced the nNOS expression. The CPP Tat-LK15 mediated siRNA delivery was a potential tool to treat neuropathic diseases involving NO or nNOS neurotoxic cascades.

Tumor penetrating peptides for improved drug delivery

In vivo screening of phage libraries in tumor-bearing mice has been used to identify peptides that direct phage homing to a tumor. The power of in vivo phage screening is illustrated by the recent discovery of peptides with unique tumor-penetrating properties. These peptides activate an endocytic transport pathway related to but distinct from macropinocytosis. They do so through a complex process that involves binding to a primary, tumor-specific receptor, followed by a proteolytic cleavage, and binding to a second receptor. The second receptor, neuropilin-1 (or neuropilin-2) activates the transport pathway. This trans-tissue pathway, dubbed the C-end Rule (CendR) pathway, mediates the extravasation transport through extravascular tumor tissue of payloads ranging from small molecule drugs to nanoparticles. The CendR technology provides a solution to a major problem in tumor therapy, poor penetration of drugs into tumors. Targeted delivery with tumor-penetrating peptides has been shown to specifically increase the accumulation of drugs, antibodies and nanotherapeutics in experimental tumors in vivo, and in human tumors ex vivo. Remarkably the payload does not have to be coupled to the peptide; the peptide activates a bulk transport system that sweeps along a drug present in the blood. Treatment studies in mice have shown improved anti-tumor efficacy and less damage to normal tissues with drugs ranging from traditional chemotherapeutics to antibodies, and to nanoparticle drugs.

The Role of Cell-Penetrating Peptide and Transferrin on Enhanced Delivery of Drug to Brain

The challenge of effectively delivering therapeutic agents to brain has led to an entire field of active research devoted to overcome the blood brain barrier (BBB) and efficiently deliver drugs to brain. This review focusses on exploring the facets of a novel platform designed for the delivery of drugs to brain. The platform was constructed based on the hypothesis that a combination of receptor-targeting agent, like transferrin protein, and a cell-penetrating peptide (CPP) will enhance the delivery of associated therapeutic cargo across the BBB. The combination of these two agents in a delivery vehicle has shown significantly improved (p < 0.05) translocation of small molecules and genes into brain as compared to the vehicle with only receptor-targeting agents. The comprehensive details of the uptake mechanisms and properties of various CPPs are illustrated here. The application of this technology, in conjunction with nanotechnology, can potentially open new horizons for the treatment of central nervous system disorders.

Low cost delivery of proteins bioencapsulated in plant cells to human non-immune or immune modulatory cells

Targeted oral delivery of GFP fused with a GM1 receptor binding protein (CTB) or human cell penetrating peptide (PTD) or dendritic cell peptide (DCpep) was investigated. Presence of GFP(+) intact plant cells between villi of ileum confirm their protection in the digestive system from acids/enzymes. Efficient delivery of GFP to gut-epithelial cells by PTD or CTB and to M cells by all these fusion tags confirm uptake of GFP in the small intestine. PTD fusion delivered GFP more efficiently to most tissues or organs than the other two tags. GFP was efficiently delivered to the liver by all fusion tags, likely through the gut-liver axis. In confocal imaging studies of human cell lines using purified GFP fused with different tags, GFP signal of DCpep-GFP was only detected within dendritic cells. PTD-GFP was only detected within kidney or pancreatic cells but not in immune modulatory cells (macrophages, dendritic, T, B, or mast cells). In contrast, CTB-GFP was detected in all tested cell types, confirming ubiquitous presence of GM1 receptors. Such low-cost oral delivery of protein drugs to sera, immune system or non-immune cells should dramatically lower their cost by elimination of prohibitively expensive fermentation, protein purification cold storage/transportation and increase patient compliance.

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

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

Dual Receptor Recognizing Cell Penetrating Peptide for Selective Targeting, Efficient Intratumoral Diffusion and Synthesized Anti-Glioma Therapy

Cell penetrating peptides (CPPs) were widely used for drug delivery to tumor. However, the nonselective in vivo penetration greatly limited the application of CPPs-mediated drug delivery systems. And the treatment of malignant tumors is usually followed by poor prognosis and relapse due to the existence of extravascular core regions of tumor. Thus it is important to endue selective targeting and stronger intratumoral diffusion abilities to CPPs. In this study, an RGD reverse sequence dGR was conjugated to a CPP octa-arginine to form a CendR (R/KXXR/K) motif contained tandem peptide R8-dGR (RRRRRRRRdGR) which could bind to both integrin αvβ3 and neuropilin-1 receptors. The dual receptor recognizing peptide R8-dGR displayed increased cellular uptake and efficient penetration ability into glioma spheroids in vitro. The following in vivo studies indicated the active targeting and intratumoral diffusion capabilities of R8-dGR modified liposomes. When paclitaxel was loaded in the liposomes, PTX-R8-dGR-Lip induced the strongest anti-proliferation effect on both tumor cells and cancer stem cells, and inhibited the formation of vasculogenic mimicry channels in vitro. Finally, the R8-dGR liposomal drug delivery system prolonged the medium survival time of intracranial C6 bearing mice by 2.1-fold compared to the untreated group, and achieved an exhaustive anti-glioma therapy including anti-tumor cells, anti-vasculogenic mimicry and anti-brain cancer stem cells. To sum up, all the results demonstrated that R8-dGR was an ideal dual receptor recognizing CPP with selective glioma targeting and efficient intratumoral diffusion, which could be further used to equip drug delivery system for effective glioma therapy.

Biodistribution, Stability, and Blood Distribution of the Cell Penetrating Peptide Maurocalcine in Mice

Maurocalcine (MCa) is the first natural cell penetrating peptide to be discovered in animal venom. In addition to the fact that it represents a potent vector for the cell penetration of structurally diverse therapeutic compounds, MCa also displays several distinguishing features that make it a potential peptide of choice for clinical and biotechnological applications. The aim of the present study was to gain new information about the properties of MCa in vivo in order to delineate the future potential applications of this vector. For this purpose, two analogues of this peptide with (Tyr-MCa) and without (Lin-Tyr-MCa) disulfide bridges were synthesized, radiolabeled with ¹²⁵I, and their in vitro stabilities were first evaluated in mouse blood. The results indicated that ¹²⁵I-Tyr-MCa was stable in vitro and that the disulfide bridges conferred a competitive advantage for the stability of peptide. Following in vivo injection in mice, ¹²⁵I-Tyr-MCa targeted peripheral organs with interesting quantitative differences and the main route of peptide elimination was renal.

Taurine Boosts Cellular Uptake of Small D-Peptides for Enzyme-Instructed Intracellular Molecular Self-Assembly

Due to their biostability, D-peptides are emerging as an important molecular platform for biomedical applications. Being proteolytically resistant, D-peptides lack interactions with endogenous transporters and hardly enter cells. Here we show that taurine, a natural amino acid, drastically boosts the cellular uptake of small D-peptides in mammalian cells by >10-fold, from 118 μM (without conjugating taurine) to >1.6 mM (after conjugating taurine). The uptake of a large amount of the ester conjugate of taurine and D-peptide allows intracellular esterase to trigger intracellular self-assembly of the D-peptide derivative, further enhancing their cellular accumulation. The study on the mechanism of the uptake reveals that the conjugates enter cells via both dynamin-dependent endocytosis and macropinocytosis, but likely not relying on taurine transporters. Differing fundamentally from the positively charged cell-penetrating peptides, the biocompatibility, stability, and simplicity of the enzyme-cleavable taurine motif promise new ways to promote the uptake of bioactive molecules for countering the action of efflux pump and contributing to intracellular molecular self-assembly.

Activable Cell-Penetrating Peptide Conjugated Prodrug for Tumor Targeted Drug Delivery

In this paper, an activable cell-penetrating peptide (CR8G3PK6, ACPP) with a shielding group of 2,3-dimethylmaleic anhydride (DMA) was conjugated with antitumor drug doxorubicin (DOX) to construct a novel prodrug (DOX-ACPP-DMA) for tumor targeted drug delivery. The shielding group of DMA linked to the primary amines of K6 through the amide bond was used to block the cell-penetrating function of the polycationic CPP (R8) through intramolecular electrostatic attraction at physiological pH 7.4. At tumor extracellular pH 6.8, the hydrolysis of DMA led to charge reversal, activating the pristine function of CPP for improved cellular uptake by tumor cells. Confocal laser scanning microscopy (CLSM) and flow cytometry studies revealed that the cellular uptake of DOX-ACPP-DMA was significantly enhanced after acid-triggered activation in both HeLa and COS7 cells. After cell internalization, the overexpressed intracellular proteases would further trigger drug release in cells. Both in vitro and in vivo investigations showed that the peptidic prodrug exhibited significant tumor growth inhibition and demonstrated great potential for tumor therapy.

Cell penetrating peptide-based polyplexes shelled with polysaccharide to improve stability and gene transfection

Cell-penetrating peptides (CPP) have been widely developed as a strategy to enhance cell penetrating ability and transfection. In this work, octa-arginine modified dextran gene vector with pH-sensitivity was developed via host-guest interactions. α-Cyclodextrin was modified with octa-arginine (CDR), which had excellent cell penetrating ability. Dextran was selected as a backbone and modified with azobenzene as guest units by acid-labile imine bonds (Az-I-Dex). The supramolecular polymer CDR/Az-I-Dex with high a C/A molar ratio (molar ratio of CD on CDR to Az on Az-I-Dex) was unfavorable for DNA condensation. The dextran shell of CDR/Az-I-Dex/DNA polyplexes improved the stability under physiological conditions. However, once treated with acetate buffer (pH 5.4) for 3 h, large aggregates formed rapidly due to the cleavage of the dextran shell. As expected, the vector had cell viability of 80% even when the CDR concentration increased to 100 μg mL(-1). Moreover, due to the effective cellular uptake efficiency, CDR/Az-I-Dex/DNA polyplexes had 6-300 times higher transfection efficiency than CDR/DNA polyplexes. It was even higher than high molecular weight PLL-based polyplexes of HEK293 T cells. Importantly, chloroquine as an endosomal escape agent could not improve the transfection of CDR/Az-I-Dex/DNA polyplexes, which indicated that the CDR/Az-I-Dex supramolecular polymer had its own ability for endosomal escape. These results suggested that the CPP-based polyplexes shelled with polysaccharide can be promising non-viral gene delivery carriers.

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.

Gadolinium-functionalized aggregation-induced emission dots as dual-modality probes for cancer metastasis study

Understanding the localization and engraftment of tumor cells at postintravasation stage of metastasis is of high importance in cancer diagnosis and treatment. Advanced fluorescent probes and facile methodologies for cell tracing play a key role in metastasis studies. In this work, we design and synthesize a dual-modality imaging dots with both optical and magnetic contrast through integration of a magnetic resonance imaging reagent, gadolinium(III), into a novel long-term cell tracing probe with aggregation-induced emission (AIE) in far-red/near-infrared region. The obtained fluorescent-magnetic AIE dots have both high fluorescence quantum yield (25%) and T1 relaxivity (7.91 mM(-1) s(-1) ) in aqueous suspension. After further conjugation with a cell membrane penetrating peptide, the dual-modality dots can be efficiently internalized into living cells. The gadolinium(III) allows accurate quantification of biodistribution of cancer cells via intraveneous injection, while the high fluorescence provides engraftment information of cells at single cellular level. The dual-modality AIE dots show obvious synergistic advantages over either single imaging modality and hold great promises in advanced biomedical studies.

Lipid envelope-type nanoparticle incorporating a multifunctional peptide for systemic siRNA delivery to the pulmonary endothelium

A system that permits the delivery of cargoes to the lung endothelium would be extraordinarily useful in terms of curing a wide variety of lung-related diseases. This study describes the development of a multifunctional envelope-type nanodevice (MEND) that targets the lung endothelium, delivers its encapsulated siRNA to the cytoplasm, and eradicates lung metastasis. The key to the success can be attributed to the presence of a surface-modified GALA peptide that has dual functions: targeting the sialic acid-terminated sugar chains on the pulmonary endothelium and subsequently delivering the encapsulated cargoes to the cytosol via endosomal membrane fusion, analogous to the influenza virus. The active targeting of MENDs without the formation of large aggregates was verified by intravital real-time confocal laser scanning microscopy in living lung tissue. The GALA-modified MEND is a promising carrier that opens a new generation of therapeutic approaches for satisfying unmet medical needs in curing lung diseases.

Cell-penetrating peptides: from cell cultures to in vivo applications

The field of gene therapy is starting to move towards clinical applications but is currently limited by the lack of efficient delivery systems. Cell-penetrating peptides provide a means of cellular delivery for gene therapy applications as well as delivery of traditional drugs. Using cell-penetrating peptides a range of different cargoes have been successfully delivered into a number of cell types, in vitro as well as in vivo. In this review we discuss uptake mechanisms of different cell-penetrating peptides, with or without cargo. The transition from in vitro to in vivo applications and strategies to increase the bioavailability of cell-penetrating peptides are also discussed.

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.

Bioportide: an emergent concept of bioactive cell-penetrating peptides

Cell-penetrating peptides (CPPs) have proven utility for the highly efficient intracellular delivery of bioactive cargoes that include peptides, proteins, and oligonucleotides. The many strategies developed to utilize CPPs solely as pharmacokinetic modifiers necessarily requires them to be relatively inert. Moreover, it is feasible to combine one or multiple CPPs with bioactive cargoes either by direct chemical conjugation or, more rarely, as non-covalent complexes. In terms of the message-address hypothesis, this combination of cargo (message) linked to a CPP (address) as a tandem construct conforms to the sychnological organization. More recently, we have introduced the term bioportide to describe monomeric CPPs that are intrinsically bioactive. Herein, we describe the design and biochemical properties of two rhegnylogically organized monometic CPPs that collectively modulate a variety of biological and pathophysiological phenomena. Thus, camptide, a cell-penetrant sequence located within the first intracellular loop of a human calcitonin receptor, regulates cAMP-dependent processes to modulate insulin secretion and viral infectivity. Nosangiotide, a bioportide derived from endothelial nitric oxide synthase, potently inhibits many aspects of the endothelial cell morphology and movement and displays potent anti-angiogenic activity in vivo. We conclude that, due to their capacity to translocate and target intracellular signaling events, bioportides represent an innovative generic class of bioactive agents.

Direct cytosolic delivery of cargoes in vivo by a chimera consisting of D- and L-arginine residues

The ability of cell-penetrating peptides (CPPs) to deliver a range of membrane-impermeable molecules into living cells makes them attractive potential vehicles for therapeutics. However, in vivo, the efficiency of CPP delivery to the cytosol remains unsatisfactory owing to endosomal entrapment and/or systemic toxicity, which severely restrict their bioavailability and efficacy in in vivo applications. In this study, we developed a series of novel chimeras consisting of various numbers of d- and l-arginine residues and investigated their cellular uptake behaviors and systemic toxicities. We demonstrated that the intracellular distribution, uptake efficiency, and systemic toxicity of these oligoarginines were all significantly affected by the number of d-arginine residues in the peptide sequence. We also found that a hybrid peptide, (rR)(3)R(2), possessed low systemic toxicity, high uptake efficiency, and, remarkably, achieved efficient cytosolic delivery not only in cultured cells but also in living tissue cells in mice after intravenous injection, implying that this heterogeneous motif might have promising applications in the delivery of cargoes of small sizes directed to cytosolic targets in vivo. Our studies into the uptake mechanism of (rR)(3)R(2) indicate that its cellular uptake was not affected by pharmacological or physical inhibitors of endocytosis but by the elimination of the membrane potential, suggesting that (rR)(3)R(2) does not enter the cells via endocytosis but rather through direct membrane translocation driven by the membrane potential. The results here might provide useful guidelines for the design and application of CPPs in drug delivery.

Cell-penetrating peptide-functionalized quantum dots for intracellular delivery

Quantum dots (QDs) are luminescent semiconductor nanocrystals that are widely used as fluorescent probes in biomedical applications, including cellular imaging and tumor tracking. Cell-penetrating peptides (CPPs), also called protein transduction domains (PTDs), are short basic peptides that permeate cell membranes and are able to deliver a variety of macromolecule cargoes, such as DNAs, RNAs, proteins, and nanomaterials. Here we review strategies to couple QDs to CPPs, by either covalent linkages or noncovalent interactions, to provide a tool to study intracellular delivery. This facilitated transport of QDs by CPPs into cells is both simple and efficient. Accordingly, CPP-QD nanoparticles are likely to be of broad utility in biological research and advance the development of medical and pharmaceutical therapeutics.

Cellular internalization of quantum dots noncovalently conjugated with arginine-rich cell-penetrating peptides

Protein transduction domains comprised of basic amino acid-rich peptides, can efficiently deliver covalently fused macromolecules into cells. Quantum dots (QDs) are luminescent semiconductor nanocrystals that are finding increasing application in biological imaging. Previous studies showed that protein transduction domains mediate the internalization of covalently attached QDs. In this study, we demonstrate that arginine-rich intracellular delivery peptides (cell-penetrating peptides; CPPs), analogs of naturally-occuring protein transduction domains, deliver noncovalently associated QDs into living cells; CPPs dramatically increase the rate and efficiency of cellular uptake of QD probes. The optimal molecular ratio between arginine-rich CPPs and QD cargoes for cellular internalization is approximately 60:1. Upon entry into cells, the QDs are concentrated in the perinuclear region. There is no cytotoxicity following transport of QDs present at concentrations up to 200 nM. The mechanism for arginine-rich CPP/QD complexes to traverse cell membrane appears to involve a combination of internalization pathways. These results provide insight into the mechanism of arginine-rich CPP delivery of noncovalently attached cargoes, and may provide a powerful tool for imaging in vivo.

[Comparison of mechanisms and cellular uptake of cell-penetrating peptide on different cell lines]

Cell-penetrating peptide (CPP) can be used in pharmaceutics as a highly efficient drug delivery transporter. In this study, four tumor cell lines (MCF-7, MDA-MB-231, C6, and B16F10) were used to observe the uptake of fluorescein isothiocyanate (FITC) labeled CPP and the effects of time and concentration of CPP on cell penetration was studied. The CPP exocytosis on C6 cell line was observed, and its exocytosis kinetics was described by zero order equation. In addition, low-temperature condition (4 degrees C) and endocytosis inhibitors were utilized to investigate the mechanism of CPP uptake by cells. Low-temperature condition did not show significantly inhibition on CPP uptake. Heparin, a membrane glycoprotein receptor inhibitor, showed strong inhibition effect (only 3%-10% of the control) on CPP uptake. Chlorpromazine, chloroquine and 5-(N-ethyl-N-isopropyl)-amiloride (EIPA) showed little effect on CPP uptake. This study indicated that CPP penetration had little selectivity on cell type, but the amount and rate of CPP penetration into cells were related to the type of cell lines. The adsorption of CPP on cell membrane induced by sulfate proteoglycan plays an important role on CPP penetration.

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.