Cellular entry pathway and gene transfer capacity of TAT-modified lipoplexes

Comparison of Cell-Penetrating and Fusogenic TAT-HA2 Peptide Performance in Peptideplex, Multicomponent, and Conjugate siRNA Delivery Systems

In this study, the performance of the cell-penetrating and fusogenic peptide, TAT-HA2, which consists of a cell-permeable HIV trans-activator of transcription (TAT) protein transduction domain and a pH-responsive influenza A virus hemagglutinin protein (HA2) domain, was comparatively evaluated for the first time in peptideplex, multicomponent, and conjugate siRNA delivery systems. TAT-HA2 in all three systems protected siRNA from degradation, except in the conjugate system with a low Peptide/siRNA ratio. The synergistic effect of different peptide domains enhanced the transfection efficiency of multicomponent and conjugate systems compared to that of peptideplexes, which was attributed to the surface configuration of TAT-HA2 peptides depending on the nature of attachment. Particularly, the multicomponent system showed better cellular uptake and endosomal escape than the peptideplexes, resulting in enhanced siRNA delivery in the cytoplasm. In addition, the presence of cleavable disulfide bonds in multicomponent and conjugate systems promoted the effective siRNA delivery in the cytoplasm, resulting in improved gene silencing activity. The multicomponent system reduced the level of luciferase expression in SKOV3 cells to 45% (±4). In contrast, the conjugate system and the commercially available siRNA transfection agent, Lipofectamine RNAiMax, caused luciferase suppression down to 55% (±2) at a siRNA dose of 100 nM. For the same dose, the peptideplex system could only reduce the luciferase expression to 65% (±5). None of the developed systems showed significant toxicity at any dose. Overall, the TAT-HA2 peptide is promising as a siRNA delivery vector; however, its performance depends on the nature of attachment and, as a result, its surface configuration on the developed delivery system.

Non-Viral Carriers for Nucleic Acids Delivery: Fundamentals and Current Applications

Over the past decades, non-viral DNA and RNA delivery systems have been intensively studied as an alternative to viral vectors. Despite the most significant advantage over viruses, such as the lack of immunogenicity and cytotoxicity, the widespread use of non-viral carriers in clinical practice is still limited due to the insufficient efficacy associated with the difficulties of overcoming extracellular and intracellular barriers. Overcoming barriers by non-viral carriers is facilitated by their chemical structure, surface charge, as well as developed modifications. Currently, there are many different forms of non-viral carriers for various applications. This review aimed to summarize recent developments based on the essential requirements for non-viral carriers for gene therapy.

A cationic lipid mediated CRISPR/Cas9 technique for the production of stable genome edited citrus plants

BACKGROUND

The genetic engineering of crops has enhanced productivity in the face of climate change and a growing global population by conferring desirable genetic traits, including the enhancement of biotic and abiotic stress tolerance, to improve agriculture. The clustered regularly interspaced short palindromic repeats (CRISPR/Cas9) system has been found to be a promising technology for genomic editing. Protoplasts are often utilized for the development of genetically modified plants through in vitro integration of a recombinant DNA fragment into the plant genome. We targeted the citrus Nonexpressor of Pathogenesis-Related 3 (CsNPR3) gene, a negative regulator of systemic acquired resistance (SAR) that governs the proteasome-mediated degradation of NPR1 and developed a genome editing technique targeting citrus protoplast DNA to produce stable genome-edited citrus plants.

RESULTS

Here, we determined the best cationic lipid nanoparticles to deliver donor DNA and described a protocol using Lipofectamine™ LTX Reagent with PLUS Reagent to mediate DNA delivery into citrus protoplasts. A Cas9 construct containing a gRNA targeting the CsNPR3 gene was transfected into citrus protoplasts using the cationic lipid transfection agent Lipofectamine with or without polyethylene glycol (PEG, MW 6000). The optimal transfection efficiency for the encapsulation was 30% in Lipofectamine, 51% in Lipofectamine with PEG, and 2% with PEG only. Additionally, plasmid encapsulation in Lipofectamine resulted in the highest cell viability percentage (45%) compared with PEG. Nine edited plants were obtained and identified based on the T7EI assay and Sanger sequencing. The developed edited lines exhibited downregulation of CsNPR3 expression and upregulation of CsPR1.

CONCLUSIONS

Our results demonstrate that utilization of the cationic lipid-based transfection agent Lipofectamine is a viable option for the successful delivery of donor DNA and subsequent successful genome editing in citrus.

Calcium Phosphate Nanoparticles-Based Systems for RNAi Delivery: Applications in Bone Tissue Regeneration

Bone-related injury and disease constitute a significant global burden both socially and economically. Current treatments have many limitations and thus the development of new approaches for bone-related conditions is imperative. Gene therapy is an emerging approach for effective bone repair and regeneration, with notable interest in the use of RNA interference (RNAi) systems to regulate gene expression in the bone microenvironment. Calcium phosphate nanoparticles represent promising materials for use as non-viral vectors for gene therapy in bone tissue engineering applications due to their many favorable properties, including biocompatibility, osteoinductivity, osteoconductivity, and strong affinity for binding to nucleic acids. However, low transfection rates present a significant barrier to their clinical use. This article reviews the benefits of calcium phosphate nanoparticles for RNAi delivery and highlights the role of surface functionalization in increasing calcium phosphate nanoparticles stability, improving cellular uptake and increasing transfection efficiency. Currently, the underlying mechanistic principles relating to these systems and their interplay during in vivo bone formation is not wholly understood. Furthermore, the optimal microRNA targets for particular bone tissue regeneration applications are still unclear. Therefore, further research is required in order to achieve the optimal calcium phosphate nanoparticles-based systems for RNAi delivery for bone tissue regeneration.

Reactive Self-Assembly and Specific Cellular Delivery of NCO-sP(EO-stat-PO)-Derived Nanogels

This study presents the reactive self-assembly of isocyanate functional and amphiphilic six-arm, star-shaped polyether prepolymers in water into nanogels. Intrinsic molecular amphiphilicity, mainly driven by the isophorone moiety at the distal endings of the star-shaped molecules, allows for the preparation of spherical particles with an adjustable size of 100-200 nm by self-assembly and subsequent covalent cross-linking without the need for organic solvents or surfactants. Covalent attachment of a fluorescence dye and either the cell-penetrating TAT peptide or a random control peptide sequence shows that only TAT-labeled nanogels are internalized by HeLa cells. The nanogels thus specifically enter the cells and accumulate in the perinuclear area in a time- and concentration-dependent manner.

Cleavable PEGylation: a strategy for overcoming the "PEG dilemma" in efficient drug delivery

To prolong the circulation time of drug, PEGylation has been widely used via the enhanced permeability and retention (EPR) effect, thereby providing new hope for better treatment. However, PEGylation also brings the "PEG dilemma", which is difficult for the cellular absorption of drugs and subsequent endosomal escape. As a result, the activity of drugs is inevitably lost after PEG modification. To achieve successful drug delivery for effective treatment, the crucial issue associated with the use of PEG-lipids, that is, “PEG dilemma” must be addressed. In this paper, we introduced the development and application of nanocarriers with cleavable PEGylation, and discussed various strategies for overcoming the PEG dilemma. Compared to the traditional ones, the vehicle systems with different environmental-sensitive PEG-lipids were discussed, which cleavage can be achieved in response to the intracellular as well as the tumor microenvironment. This smart cleavable PEGylation provides us an efficient strategy to overcome “PEG dilemma”, thereby may be a good candidate for the cancer treatment in future.

Smart Cell-Penetrating Peptide-Based Techniques for Intracellular Delivery of Therapeutic Macromolecules

Many therapeutic macromolecules must enter cells to take their action. However, their treatment outcomes are often hampered by their poor transportation into target cells. Therefore, efficient intracellular delivery of these macromolecules is critical for improving their therapeutic efficacy. Cell-penetrating peptide (CPP)-based approaches are one of the most efficient methods for intracellular delivery of macromolecular therapeutics. Nevertheless, poor specificity is a significant concern for systemic administrated CPP-based delivery systems. This chapter will review recent advances in CPP-mediated macromolecule delivery with a focus on various smart strategies which not only enhance the intracellular delivery but also improve the targeting specificity.

cRGD/TAT Dual-Ligand Reversibly Cross-Linked Micelles Loaded with Docetaxel Penetrate Deeply into Tumor Tissue and Show High Antitumor Efficacy in Vivo

The application of cell-penetrating peptides like TAT for in vivo targeted delivery is limited because the penetration behavior is not cell-specific. Herein, we designed cRGD and TAT comodified cross-linkable micelles (cRGD/TAT CMs), in which the TAT peptide was shielded by relatively long poly(ethylene glycol) (PEG) chains. Docetaxel (DTX)-loaded cRGD/TAT CMs were very stable with minimal drug leakage under physiological conditions, whereas rapid DTX release took place in a reductive environment. Flow cytometry showed that the cRGD/TAT CMs with molar ratios of 20% cRGD and 10% TAT (cRGD20/TAT10 CMs) were selectively and efficiently taken up by α_νβ₃-overexpressing U87MG glioma cells, with 8.3-fold and 18.3-fold higher uptake than cRGD20 CMs and PEG CMs, respectively. DTX-loaded cRGD20/TAT10 CMs exhibited a high cytotoxicity in U87MG cells, leading to rapid apoptosis of the tumor cells. Uptake mechanism studies revealed that cRGD20/TAT10 CMs mainly employed the caveolae-mediated endocytotic pathway and efficiently escaped from the lysosomes. Notably, cRGD20/TAT10 CMs had a long circulating time of 6.25 h in vivo, due to cross-linking of the micelles and shielding of the TAT peptide. Moreover, DTX-loaded cRGD20/TAT10 CMs exhibited a significantly higher accumulation and deeper penetration in subcutaneous U87MG glioma tissue compared to cRGD20 CMs and PEG CMs, leading to superior antitumor efficacy in vivo. Therefore, this dual-ligand strategy provides an effective way to realize tumor-specific penetration and inhibition.

Betanodavirus-like particles enter host cells via clathrin-mediated endocytosis in a cholesterol-, pH- and cytoskeleton-dependent manner

Betanodavirus, also referred to nervous necrosis virus (NNV), is the causative agent of the fatal disease, viral nervous necrosis and has brought significant economic losses in marine and freshwater cultured fish, especially larvae and juveniles. Here, we used an established invasion model with virus-like particle (VLP)-cells, mimicking orange-spotted grouper nervous necrosis virus (OGNNV), to investigate the crucial events of virus entry. VLP were observed in the perinuclear regions of Asian sea bass (SB) cells within 1.5 h after attachment. VLP uptake was strongly inhibited when cells were pretreated with biochemical inhibitors (chlorpromazine and dynasore) blocking clathrin-mediated endocytosis (CME) or transfected with siRNA against clathrin heavy and light chains. Inhibitors against key regulators of caveolae/raft-dependent endocytosis and macropinocytosis had no effect on VLP uptake. In contrast, disruption of cellular cholesterol by methyl-β-cyclodextrin or reduction of cholesterol fluidity by Cholera toxin B subunit significantly decreased VLP entry. Furthermore, VLP entry is dependent on low pH and cytoskeleton, demonstrated by inhibitor (chloroquine, ammonia chloride, cytochalasin D, wiskostatin, and nocodazole) perturbation. Therefore, OGNNV VLP enter SB cells via CME depending on dynamin-2, cholesterol and its fluidity, low pH, and cytoskeleton. In addition, ten more cell lines were screened for VLP entry and VLP can only enter NNV-sensitive cells, GB and SSN-1, via CME, indicating that CME is the common endocytosis pathway for VLP. These results may provide the data for NNV entry without the influence of the viral genome, an ideal model for exploring the behaviour of betanodavirus in cells, and valuable references to vaccine development.

The Antimicrobial and Antiviral Applications of Cell-Penetrating Peptides

Over the past two decades, cell-penetrating peptides (CPPs) have become increasingly popular both in research and in application. There have been numerous studies on the physiochemical characteristics and behavior of CPPs in various environments; likewise, the mechanisms of entry and delivery capabilities of these peptides have also been extensively researched. Besides the fundamental issues, there is an enormous interest in the delivery capabilities of the peptides as the family of CPPs is a promising and mostly non-toxic delivery vector candidate for numerous medical applications such as gene silencing, transgene delivery, and splice correction. Lately, however, there has been an emerging field of study besides the high-profile gene therapy applications-the use of peptides and CPPs to combat various infections caused by harmful bacteria, fungi, and viruses.In this chapter, we aim to provide a short overview of the history and properties of CPPs which is followed by more thorough descriptions of antimicrobial and antiviral peptides. To achieve this, we analyze the origin of such peptides, give an overview of the mechanisms of action and discuss the various practical applications which are ongoing or have been suggested based on research.

Screening nylon-3 polymers, a new class of cationic amphiphiles, for siRNA delivery

Amphiphilic nucleic acid carriers have attracted strong interest. Three groups of nylon-3 copolymers (poly-β-peptides) possessing different cationic/hydrophobic content were evaluated as siRNA delivery agents in this study. Their ability to condense siRNA was determined in SYBR Gold assays. Their cytotoxicity was tested by MTT assays, their efficiency of delivering Alexa Fluor-488-labeled siRNA intracellularly in the presence and absence of uptake inhibitors was assessed by flow cytometry, and their transfection efficacies were studied by luciferase knockdown in a cell line stably expressing luciferase (H1299/Luc). Endosomal release was determined by confocal laser scanning microscopy and colocalization with lysotracker. All polymers efficiently condensed siRNA at nitrogen-to-phosphate (N/P) ratios of 5 or lower, as reflected in hydrodynamic diameters smaller than that at N/P 1. Although several formulations had negative zeta potentials at N/P 1, G2C and G2D polyplexes yielded >80% uptake in H1299/Luc cells, as determined by flow cytometry. Luciferase knockdown (20–65%) was observed after transfection with polyplexes made of the high molecular weight polymers that were the most hydrophobic. The ability of nylon-3 polymers to deliver siRNA intracellularly even at negative zeta potential implies that they mediate transport across cell membranes based on their amphiphilicity. The cellular uptake route was determined to strongly depend on the presence of cholesterol in the cell membrane. These polymers are, therefore, very promising for siRNA delivery at reduced surface charge and toxicity. Our study identified nylon-3 formulations at low N/P ratios for effective gene knockdown, indicating that nylon-3 polymers are a new, promising type of gene delivery agent.

Nanoparticles and the blood-brain barrier: advancing from in-vitro models towards therapeutic significance

The blood-brain barrier is a unique cell-based restrictive barrier that prevents the entry of many substances, including most therapeutics, into the central nervous system. A wide range of nanoparticulate delivery systems have been investigated with the aim of targeting therapeutics (drugs, nucleic acids, proteins) to the brain following administration by various routes. This review provides a comprehensive description of the design and formulation of these nanoparticles including the rationale behind individual approaches. In addition, the ability of currently available in-vitro BBB models to accurately predict the in-vivo performance of targeted nanoparticles is critically assessed.

Folate and TAT peptide co-modified liposomes exhibit receptor-dependent highly efficient intracellular transport of payload in vitro and in vivo

PURPOSE

Using different chain lengths of PEG as linkers to develop a novel folate (FA) and TAT peptide co-modified doxorubicin (DOX)-loaded liposome (FA/TAT-LP-DOX) and evaluate its potential for tumor targeted intracellular drug delivery.

METHODS

FA/TAT-LP-DOX was prepared by pH gradient method and post-insertion method and the optimal ligand density was screened by MTT assay. In vitro evaluation was systematically performed through cytotoxicity assay, cellular uptake studies, subcellular localization and cellular uptake mechanism in folate receptor (FR) over-expressing KB tumor cells. In vivo tumor targeted delivery of FA/TAT-LP-DOX was also studied by in vivo fluorescence imaging in a murine KB xenograft model.

RESULTS

The particle size and zeta potential determination indicated that FA and TAT were successfully inserted into the liposome and cationic TAT peptide was completely shielded. With the optimal ligand density (5% of FA and 2.5% TAT), the FA/TAT-LP-DOX exhibited improved cytotoxity and cellular uptake efficiency compared with its single-ligand counterparts (FA-LP-DOX and PEG/TAT-LP-DOX). Competitive inhibition and uptake mechanism experiments revealed that FA and TAT peptide played a synergistic effect in facilitating intracellular transport of the liposome, and association between FA and FA receptors activated this transport process. In vivo imaging further demonstrated the superiority of FA/TAT-LP in tumor targeting and accumulation.

CONCLUSIONS

Folate and TAT peptide co-modified liposome using different chain lengths of PEG as linkers may provide a useful strategy for specific and efficient intracellular drug delivery.

Cell-penetrating peptides: design, synthesis, and applications

The intrinsic property of cell-penetrating peptides (CPPs) to deliver therapeutic molecules (nucleic acids, drugs, imaging agents) to cells and tissues in a nontoxic manner has indicated that they may be potential components of future drugs and disease diagnostic agents. These versatile peptides are simple to synthesize, functionalize, and characterize yet are able to deliver covalently or noncovalently conjugated bioactive cargos (from small chemical drugs to large plasmid DNA) inside cells, primarily via endocytosis, in order to obtain high levels of gene expression, gene silencing, or tumor targeting. Typically, CPPs are often passive and nonselective yet must be functionalized or chemically modified to create effective delivery vectors that succeed in targeting specific cells or tissues. Furthermore, the design of clinically effective systemic delivery systems requires the same amount of attention to detail in both design of the delivered cargo and the cell-penetrating peptide used to deliver it.

Recent progress of cell-penetrating peptides as new carriers for intracellular cargo delivery

The plasma membrane as a selectively permeable barrier of living cells is essential to cell survival and function. In many cases, however, the efficient passage of exogenous bioactive molecules through the plasma membrane remains a major hurdle for intracellular delivery of cargoes. During the last two decades, the potential of peptides for drug delivery into cells has been highlighted by the discovery of numerous cell-penetrating peptides (CPPs). CPPs serving as carriers can successfully intracellular transport cargoes such as siRNA, nucleic acids, proteins, small molecule therapeutic agents, quantum dots and MRI contrast agents. This review mainly introduces recent advances of CPPs as new carriers for the development of cellular imaging, nuclear localization, pH-sensitive and thermally targeted delivery systems. In particular, we highlight the exploiting of the synergistic effects of targeting ligands and CPPs. What's more, the classification and cellular uptake mechanisms of CPPs are briefly discussed as well.

Therapeutic delivery using cell-penetrating peptides

Intracellular delivery of promising therapeutic agents as well as nanocarriers presents a unique challenge. However, with the discovery of the cell-penetrating peptides (CPPs), overcoming this obstacle seems more plausible. In many cases, CPPs conjugated with therapeutic agent or therapeutic agent loaded-nanoparticles have shown promising results via increased cellular uptake. In this review, the current status of CPPs for the intracellular delivery of not just potential therapeutic small molecules but also large molecules like peptides, nucleic acids and nanocarriers is discussed. In addition, the design of ‘smart stimuli-sensitive nanocarrier’ to overcome the non-target-specificity of CPPs is also described.

2-Hydroxypropyl-β-cyclodextrin-modified SLN of paclitaxel for overcoming p-glycoprotein function in multidrug-resistant breast cancer cells

OBJECTIVES

This study aimed to evaluate the potential of solid lipid nanoparticles (SLNs) of paclitaxel (PTX) modified with a 2-hydroxypropyl-β-cyclodextrin system to enhance cellular accumulation of PTX into p-glycoprotein (p-gp)-expressing cells.

METHODS

The PTX-loaded-SLNs consisted of lipid (stearic acid) and surfactants (lecithin and poloxamer 188) and were then modified with 2-hydroxypropyl-β-cyclodextrin by a sonication method.

KEY FINDINGS

In terms of cytotoxicity, PTX-loaded SLNs modified with 2-hydroxypropyl-β-cyclodextrin showed higher cytotoxicity than other formulations. In particular, the cellular uptake of PTX from PTX-loaded SLNs modified with 2-hydroxypropyl-β-cyclodextrin was about 5.8- and 1.5-fold higher than that from PTX solution and unmodified PTX-loaded SLNs in MCF-7/ADR cells, respectively. After a 4-h incubation, clear fluorescence images inside cells were observed over time. When PTX-loaded SLNs modified with 2-hydroxypropyl-β-cyclodextrin were incubated with MCF-7/ADR cells for 4 h, cellular uptake of PTX increased 1.7-fold versus that of PTX in the presence of verapamil.

CONCLUSIONS

These results suggest that optimized SLNs modified with 2-hydroxypropyl-β-cyclodextrin may have potential as an oral drug delivery system for PTX.

Modified Tat peptide with cationic lipids enhances gene transfection efficiency via temperature-dependent and caveolae-mediated endocytosis

The HIV-1 Tat peptide has been successfully used for intracellular gene delivery. Likewise, various lipid-based methods have shown increased endocytosis and can influence endosomal escape. This study combines the favorable properties of Tat peptide with that of lipid systems for DNA delivery. We combined the lipid FuGENE HD (FH) with the Tat peptide sequence modified with histidine and cysteine residues (mTat). mTat/FH transfection was evaluated by luciferase expression plasmid in five cell types. mTat/FH produced significant improvement in transfection efficiency of all cell lines when compared to FH or mTat. Treatment with chloroquine, associated with energy-dependent endocytosis, significantly increased transfection efficiency with mTat/FH while incubation at low temperature decreased it. The zeta potential of mTat/FH/DNA was significantly higher compared to FH, mTat, or their DNA combination in the presence of serum, and it was correlated with transfection efficiency. The particle size of the FH/DNA complex was significantly reduced by addition of mTat. Filipin III, an inhibitor of caveolae-mediated endocytosis, significantly inhibited mTat/FH transfection, but transfection was increased by chlorpromazine, an inhibitor of clathrin-mediated endocytosis. These findings demonstrated the feasibility of using a combination of mTat with lipids, utilizing temperature-dependent and caveolae-mediated endocytosis, as a potentially attractive non-viral gene vector.

Emerging links between surface nanotechnology and endocytosis: impact on nonviral gene delivery

Significant effort continues to be exerted toward the improvement of transfection mediated by nonviral vectors. These endeavors are often focused on the design of particulate carriers with properties that encourage efficient accumulation at the membrane surface, particle uptake, and endosomal escape. Despite its demonstrated importance in successful nonviral transfection, relatively little investigation has been done to understand the pressures driving internalized vectors into favorable nondegradative endocytic pathways. Improvements in transfection efficiency have been noted for complexes delivered with a substrate-mediated approach, but the reasons behind such enhancements remain unclear. The phenotypic changes exhibited by cells interacting with nano- and micro-featured substrates offer hints that may explain these effects. This review describes nanoscale particulate and substrate parameters that influence both the uptake of nonviral gene carriers and the endocytic phenotype of interacting cells, and explores the molecular links that may mediate these interactions. Substrate-mediated control of endocytosis represents an exciting new design parameter that will guide the creation of efficient transgene carriers.

Tailoring lipoplex composition to the lipid composition of plasma membrane: a Trojan horse for cell entry?

The first interaction between lipoplexes and cells is charge-mediated and not specific. Endocytosis is considered to be the main pathway for lipoplex entry. Upon interaction between lipoplexes and the plasma membrane, intermixing between lipoplex and membrane lipids is necessary for efficient endocytosis. Here we study the mechanism of the different endocytic pathways in lipid-mediated gene delivery. We show that DC-Chol-DOPE/DNA lipoplexes preferentially use a raft-mediated endocytosis, while DOTAP-DOPC/DNA systems are mainly internalized by not specific fluid phase macropinocitosys. On the other hand, most efficient multicomponent lipoplexes, incorporating different lipid species in their lipid bilayer, can use multiple endocytic pathways to enter cells. Our data demonstrate that efficiency of endocytosis is regulated by shape coupling between lipoplex and membrane lipids. We suggest that such a shape-dependent coupling regulates efficient formation of endocytic vesicles thus determining the success of internalization. Our results suggest that tailoring the lipoplex lipid composition to the patchwork-like plasma membrane profile could be a successful machinery of coordinating the endocytic pathway activities and the subsequent intracellular processing.

Multiplexed supramolecular self-assembly for non-viral gene delivery

Recently, there has been success in applying a semi-rational approach to non-viral gene delivery vector development using a combinatorial/parallel synthesis approach to construct libraries of materials with unique molecular structures. In this approach, it is hoped that the random incorporation of various hydrophobic and hydrophilic domains in the library will yield candidates with the appropriate balance of DNA binding strength and endosomolytic properties to yield efficient gene delivery. Herein we describe a library approach to gene delivery vector development that relies on the supramolecular self-assembly of individual components instead of chemical reaction. Each component in the described system is capable of performing a single and well-defined purpose--DNA binding (dioleylspermine), membrane permeation (oligoarginine) or targeting (folic acid). A combination of electrostatic attraction and the hydrophobic effect is used to bring the individual groups together to form nanoscale complexes with DNA. Because the components responsible for DNA binding, membrane permeation and targeting are separate, it is possible to alter the balance between hydrophilic and hydrophobic groups by varying the relative amounts in the final formulation. By doing so, we can readily identify cell-specific formulations that have greater transfection efficiency than the individual components and have superior transfection efficiency to lipofectamine 2000 under similar conditions.

Efficient delivery of payload into tumor cells in a controlled manner by TAT and thiolytic cleavable PEG co-modified liposomes

Recently, PEGylation has been extensively employed to increase the circulation time of liposomes and enhance their accumulation in tumor tissue via the enhanced permeability and retention (EPR) effect; however, poly(ethylene glycol) (PEG) is unfavorable for the uptake of liposomes by tumor cells because of its steric hindrance. In this study, thiolytic cleavable PEG modified liposomes were used to solve this dilemma. Before arrival at the tumor tissue, PEG presents on the surface of liposomes, which is useful for passive accumulation in tumor tissue. Upon reaching the tumor tissues, the PEG chain could be removed by a safe cleaving reagent l-cysteine (l-Cys), and thus, the steric hindrance of PEG could be overcome conveniently. To further improve the uptake of liposomes, a "functional molecule" cell-penetrating peptide TAT was attached to the distal end of a shorter PEG spacer anchored to the surface of the liposomes, which could be shielded by cleavable PEG during circulation; upon arriving at tumor tissue, PEG was removed and thus the "functional molecule" TAT was exposed, and then TAT could mediate the uptake of the liposomes with high efficiency. In this study, thiolytic cleavable PEG was synthesized via a disulfide bridge, DOPE-PEG(1600)-TAT was synthesized by sulfhydryl-maleimide reaction, and then Rh-PE labeled liposomes composed of 2% DOPE-PEG(1600)-TAT and various amounts of cleavable PEG(5000) (2%, 4%, and 8%) were prepared, with particle size around 100 nm and slightly negative charge. These liposomes showed good stability in the presence of 10% serum. Their uptake by tumor cells HepG2 in vitro was assessed qualitatively and quantitatively. Liposomes modified with 2% DOPE-PEG(1600)-TAT and 8% DOPE-S-S-mPEG(5000) were regarded as the optimal formulation. In this preparation, nearly no uptake could be observed before addition of l-Cys, which meant undesired uptake during circulation could be avoided, while the uptake upon addition of l-Cys was 4 times as high as that in the absence of l-Cys. For the uptake in vivo, calcein loaded and Rh-PE labeled 8% cleavable PEG + 2% TAT modified liposomes were injected intratumorally into H22 tumor bearing mice. Confocal laser scanning microscopy (CLSM) showed that the uptake of 8% cleavable PEG + 2% TAT modified liposomes was much higher than that of 8% noncleavable PEG + 2% TAT modified liposomes in the presence of l-Cys. Thus, tumor targeted delivery could be achieved efficiently by the liposomal drug delivery system developed here in a controlled manner.

Reduction-sensitive, robust vesicles with a non-covalently modifiable surface as a multifunctional drug-delivery platform

The design and synthesis of a novel reduction-sensitive, robust, and biocompatible vesicle (SSCB[6]VC) are reported, which is self-assembled from an amphiphilic cucurbit[6]uril (CB[6]) derivative that contains disulfide bonds between hexaethylene glycol units and a CB[6] core. The remarkable features of SSCB[6]VC include: 1) facile, non-destructive, non-covalent, and modular surface modification using exceptionally strong host-guest chemistry; 2) high structural stability; 3) facile internalization into targeted cells by receptor-mediated endocytosis, and 4) efficient triggered release of entrapped drugs in a reducing environment such as cytoplasm. Furthermore, a significantly increased cytotoxicity of the anticancer drug doxorubicin to cancer cells is demonstrated using doxorubicin-loaded SSCB[6]VC, the surface of which is decorated with functional moieties such as a folate-spermidine conjugate and fluorescein isothiocyanate-spermidine conjugate as targeting ligand and fluorescence imaging probe, respectively. SSCB[6]VC with such unique features can be used as a highly versatile multifunctional platform for targeted drug delivery, which may find useful applications in cancer therapy. This novel strategy based on supramolecular chemistry and the unique properties of CB[6] can be extended to design smart multifunctional materials for biomedical applications including gene delivery.

Cell-Penetrating Peptides-Mechanisms of Cellular Uptake and Generation of Delivery Systems

The successful clinical application of nucleic acid-based therapeutic strategies has been limited by the poor delivery efficiency achieved by existing vectors. The development of alternative delivery systems for improved biological activity is, therefore, mandatory. Since the seminal observations two decades ago that the Tat protein, and derived peptides, can translocate across biological membranes, cell-penetrating peptides (CPPs) have been considered one of the most promising tools to improve non-invasive cellular delivery of therapeutic molecules. Despite extensive research on the use of CPPs for this purpose, the exact mechanisms underlying their cellular uptake and that of peptide conjugates remain controversial. Over the last years, our research group has been focused on the S4₁₃-PV cell-penetrating peptide, a prototype of this class of peptides that results from the combination of 13-amino-acid cell penetrating sequence derived from the Dermaseptin S4 peptide with the SV40 large T antigen nuclear localization signal. By performing an extensive biophysical and biochemical characterization of this peptide and its analogs, we have gained important insights into the mechanisms governing the interaction of CPPs with cells and their translocation across biological membranes. More recently, we have started to explore this peptide for the intracellular delivery of nucleic acids (plasmid DNA, siRNA and oligonucleotides). In this review we discuss the current knowledge of the mechanisms responsible for the cellular uptake of cell-penetrating peptides, including the S4₁₃-PV peptide, and the potential of peptide-based formulations to mediate nucleic acid delivery.

A non-covalent strategy combining cationic lipids and CPPs to enhance the delivery of splice correcting oligonucleotides

Modulation of pre-mRNA splicing by steric-block oligonucleotides constitutes a promising strategy for the treatment of many diseases, but requires efficient delivery to cell nuclei. In the present study, we evaluated the efficacy of a non-covalent strategy that combines a cell penetrating peptide with a lipoplex-based formulation to mediate the delivery of splice-switching oligonucleotides. The splice correcting ability of these new formulations was assessed using splice-switching oligonucleotides targeted towards the mutated splicing site of human beta-globin pre-mRNA in the HeLa pLuc/705 splice correction model. Importantly, the optimal splice correcting activity was exhibited by the formulation containing both lipid and peptide components, the order of component addition in these formulations being crucial for their efficacy. Our results demonstrate that the inclusion of cationic liposomes in the formulation provides the ability to improve release from endocytic vesicles, a barrier that severely limits the efficiency of oligonucleotide delivery by cell penetrating peptides. On the other hand, cell penetrating peptides potentiate the cellular uptake and delivery of the oligonucleotides by the lipoplexes. Moreover, when combining cell penetrating peptides with the lipoplex formulations, a significant reduction in the amount of required cationic lipid could be achieved, while maintaining or even enhancing biological activity.

Targeting the Tumour: Cell Penetrating Peptides for Molecular Imaging and Radiotherapy

Over the last couple of years, the number of original papers and reviews discussing various applications of cell penetrating peptides (CPPs) has grown exponentially. This is not remarkable since CPPs are capable of transporting the most varying cargo across cell membranes which is one of the biggest problems in drug delivery and targeted therapy. In this review, we focus on the use of CPPs and related peptides for delivery of imaging contrast agents and radionuclides to cells and tissues with the ultimate goal of in vivo molecular imaging and molecular radiotherapy of intracellular and even intranuclear targets.

Flow cytometry and live confocal analysis for the evaluation of the uptake and intracellular distribution of FITC-ODN into HaCaT cells

In this study, the mechanism of the internalization and the cellular distribution of 59 fluorescein conjugated PS-ODN (FITC-ODN) after transfection with different mixed lipidic vesicles/oligo complexes (lipoplexes) have been investigated. Mixed lipidic vesicles were prepared with one of the most used cationic lipid (DOTAP) and different amounts of a cholic acid (UDCA) to release the oligo into HaCaT cells. Using flow cytometry, the cellular uptake of the oligo was studied with and without different inhibitors able to block selectively the different pathways involved in the internalization mechanism. The intracellular distribution of the oligo was analyzed by confocal laser scanning microscopy (CLSM), treating the cells with the lipoplexes and directly observing without any fixing procedure. To better carry out the colocalization studies, fluorescent-labeled markers, specific for the different cellular compartments, were coincubated with 59 fluorescein-conjugated 29-mer phosphorotioate oligonucleotide (FITC-ODN). The different lipidic vesicles affect the internalization mechanism of FITC-ODN. After using the inhibitors, the uptake of complexes involved a different internalization mechanism. The live CLSM analysis demonstrated that, after 1 hour from the complex incubation, the oligo was transferred into cells and localized into the endosomes; after 24 hours, the oligo was intracellularly localized close to the nuclear structure in a punctuate pattern. However, the results from fusion experiments showed also a binding of a quite low amount of oligo with the cell membranes.

Delivery of macromolecules using arginine-rich cell-penetrating peptides: ways to overcome endosomal entrapment

Arginine-rich cell-penetrating peptides (AR-CPPs) are very promising tools for the delivery of therapeutic macromolecules such as peptides, proteins, and nucleic acids. These peptides allow efficient internalization of the linked cargos intracellularly through the endocytic pathway. However, when linked to bulky cargos, entrapment in the endocytic vesicles is a major limitation to the application of these peptides in cytosolic delivery. Attachment of a compatible endosomal escape device is, therefore, necessary to allow cytosolic delivery of the peptide-attached cargo. This review presents different endosomal escape devices currently in application in combination with AR-CPPs. Applications of fusogenic lipids, membrane-disruptive peptides, membrane-disruptive polymers, lysosomotropic agents, and photochemical internalization to enhance the cytosolic delivery of AR-CPPs-attached cargos are presented. The properties of each system and its mechanism of action for the enhancement of endosomal escape are discussed, together with its applications for the delivery of different macromolecules in vitro and, if applicable, in vivo.

Improved transfection of spleen-derived antigen-presenting cells in culture using TATp-liposomes

Antigen presenting cells (APC) are among the most important cells of the immune system since they link the innate and the adaptative immune responses, directing the type of immune response to be elicited. To modulate the immune response in immune preventing or treating therapies, gene delivery into immunocompetent cells could be used. However, APC are very resistant to transfection. To increase the efficiency of APC transfection, we have used liposome-based lipoplexes additionally modified with cell-penetrating TAT peptide (TATp) for better intracellular delivery of a model plasmid encoding for the enhanced-green fluorescent protein (pEGFP). pEGFP-bearing lipoplexes made of a mixture of PC:Chol:DOTAP (60:30:10 molar ratio) with the addition of 2% mol of polyethylene glycol-phosphatidylethanolamine (PEG-PE) conjugate (plain-L) or TATp-PEG-PE (TATp-L) were shown to effectively protect the incorporated DNA from degradation. Uptake assays of rhodamine-labeled lipoplexes and transfections with the EGFP reporter gene were performed with APC derived from the mouse spleen. TATp-L-based lipoplexes allowed for significantly enhanced both, the uptake and transfection in APC. Such a tool could be used for the APC transfection as a first step in immune therapy.

The third helix of the Hoxc8 homeodomain peptide enhances the efficiency of gene transfer in combination with lipofectamine

Protein transduction domains (PTDs) have been shown to cross the biological cell membranes efficiently through a receptor and energy independent mechanism. Because of its ease in membrane transducing ability, PTDs could be used as a gene delivery vector. Since we already have shown that purified Hoxc8 homeoprotein has the ability to cross the cellular membrane, we analyzed the possibility of the third helix of the Hoxc8 homeodomain as a useful gene delivery vector. For that purpose, a 16-aa long synthetic oligopeptide Hoxc8 Protein Transduction Domain (HPTD) was chemically synthesized and then tested to see whether the HPTD could form a complex with DNA or not. Gel retardation analysis revealed that the HPTD interacts with plasmid DNA efficiently but failed to transfer the DNA into the cells. However, HPTD can enhance the efficiency of gene transfer in combination with Lipofectamine which doubled the gene transfer rate into COS-7 cells compared with the DNA/Lipofectamine control. An MTT assay indicated that the amount of HPTD used in the complex for the transfection did not show any cytotoxicty in COS-7 cells. The TEM studies showed compact particle formation in the presence of HPTD. These results indicate that the HPTD could be a good candidate adjuvant molecule to enhance the gene transfer efficiency of Lipofectamine in eukaryotic cells.

Novel gelatin-siloxane nanoparticles decorated by Tat peptide as vectors for gene therapy

In principle, the technique of gene delivery involves taking complete or parts of genes that can code specific messages and delivering them to selected cells in the body. Such a transfer of plasmid DNA into mammalian cells has posed major challenges for gene therapy. A series of gelatin-siloxane nanoparticles (GS NPs) with controlled size and surface charge were synthesized through a two-step sol-gel process. In order to increase the efficiency of cellular uptake, HIV-derived Tat peptide was further grafted to GS NPs. In vitro co-location and endocytosis inhibition experiments suggested that the as-synthesized TG NPs may enter HeLa cells via a combined pathway of lipid-raft- and receptor-dependent endocytosis, and only cause little cell damage. Moreover, this study shows the encapsulation of a plasmid DNA in TG NPs to be obtained as a non-viral gene vector. This kind of encapsulation provides complete protection to the plasmid DNA from the external DNase and serum environment, and generates the hope that the resulting formulation can be developed into a potential vector for effective gene delivery. In order to check this potential, the reporter gene pSVβ-gal was encapsulated, and in vitro transfection efficiency of this system was found to be nearly 130% compared to the commercially available transfection reagent Lipofectamine™.

Intracellular delivery of nanoparticles via the HIV-1 tat peptide

Functionalized nanoparticles are heralded as part of the future with regards to targeted cell and nuclear delivery. However, direct intracellular and intranuclear delivery has, until recently, been difficult to achieve owing to the impermeable nature of the plasma and nuclear membranes. During the past 15 years, a range of peptides, termed cell-penetrating peptides (CPPs), which have the ability to translocate into living cells, have been discovered. Thus, in more recent years, the combination of CPPs with nanoparticles, enabling CPP-mediated cell delivery, has opened up many avenues of research. This review discusses the use of various CPPs, focusing on tat peptide, to functionalize nanoparticles and the possible move from the laboratory to the clinic.

Cell-penetrating and cell-targeting peptides in drug delivery

During the last decade, the potential of peptides for drug delivery into cells has been highlighted by the discovery of several cell-penetrating peptides (CPPs). CPPs are very efficient in delivering various molecules into cells. However, except in some specific cases, their lack of cell specificity remains the major drawback for their clinical development. At the same time, various peptides with specific binding activity for a given cell line (cell-targeting peptides) have also been reported in the literature. One of the goals of the next years will be to optimize the tissue and cell delivery of therapeutic molecules by means of peptides which combine both targeting and internalization advantages. In this review, we describe the main strategies that are currently in use or likely to be employed in the near future to associate both targeting and delivery properties.

Tat peptide-mediated intracellular delivery of pharmaceutical nanocarriers

Cell-penetrating peptides (CPPs) including TAT peptide (TATp) have been successfully used for intracellular delivery of a broad variety of cargoes including various nanoparticulate pharmaceutical carriers (liposomes, micelles, nanoparticles). Here, we will consider the main results in this area, with a special emphasis on TATp-mediated delivery of liposomes and DNA. We will also address the development of "smart" stimuli-sensitive nanocarriers, where cell-penetrating function can be activated by the decreased pH only inside the biological target minimizing thus the interaction of drug-loaded nanocarriers with non-target cells.

Characterization of PLL-g-PEG-DNA nanoparticles for the delivery of therapeutic DNA

Local and controlled DNA release is a critical issue in current gene therapy. As viral gene delivery systems are associated with severe security problems, nonviral gene delivery vehicles were developed. Here, DNA-nanoparticles using grafted copolymers of PLL and PEG to increase their biocompatibility and stealth properties were systematically studied. Ten different PLL-based polymers with no, low, and high PEG grafting and PEG molecular weights as well as different PLL backbone lengths were complexed with plasmids containing 3200 to 10,100 base pairs. Stable complexes were formed and selected for cytotoxicity and transfection efficiency. Predominantly, PLL-g-PEG-DNA nanoparticles grafted with 4 or 5% PEG moieties of 5 kDa transfected 40% COS-7 cells without reduction of cell viability when formed at N/P ratios between 0.1 and 12.5. The molecular weight of PLL did not significantly affect transfection efficiency or cytotoxicity indicating that a specific cationic charge-density-to-PEG-ratio is important for efficient transfection and low cytotoxicity. The PLL-g-PEG-DNA nanoparticles were spherical with a diameter of approximately 100 nm and did not aggregate over 2 weeks. Moreover, they protected included plasmid DNA against serum components and DNase I digestion. Therefore, such storage stable and versatile PLL-g-PEG-DNA nanoparticles might be useful to deliver differently sized therapeutic DNA for in vivo applications.