Cell penetrating peptide conjugates of steric block oligonucleotides

Charge neutral steric block oligonucleotide analogues, such as peptide nucleic acids (PNA) or phosphorodiamidate morpholino oligomers (PMO), have promising biological and pharmacological properties for antisense applications, such as for example in mRNA splicing redirection. However, cellular uptake of free oligomers is poor and the utility of conjugates of PNA or PMO to cell penetrating peptides (CPP), such as Tat or Penetratin, is limited by endosomal sequestration. Two new families of arginine-rich CPPs named (R-Ahx-R)(4) AhxB and R(6)Pen allow efficient nuclear delivery of splice correcting PNA and PMO at micromolar concentrations in the absence of endosomolytic agents. The in vivo efficacy of (R-Ahx-R)(4) AhxB PMO conjugates has been demonstrated in mouse models of Duchenne muscular dystrophy and in various viral infections.

Cell-penetrating-peptide-based delivery of oligonucleotides: an overview

Cationic CPPs (cell-penetrating peptides) have been used largely for intracellular delivery of low-molecular-mass drugs, biomolecules and particles. Most cationic CPPs bind to cell-associated glycosaminoglycans and are internalized by endocytosis, although the detailed mechanisms involved remain controversial. Sequestration and degradation in endocytic vesicles severely limits the efficiency of cytoplasmic and/or nuclear delivery of CPP-conjugated material. Re-routing the splicing machinery by using steric-block ON (oligonucleotide) analogues, such as PNAs (peptide nucleic acids) or PMOs (phosphorodiamidate morpholino oligomers), has consequently been inefficient when ONs are conjugated with standard CPPs such as Tat (transactivator of transcription), R(9) (nona-arginine), K(8) (octalysine) or penetratin in the absence of endosomolytic agents. New arginine-rich CPPs such as (R-Ahx-R)(4) (6-aminohexanoic acid-spaced oligo-arginine) or R(6) (hexa-arginine)-penetratin conjugated to PMO or PNA resulted in efficient splicing correction at non-cytotoxic doses in the absence of chloroquine. SAR (structure-activity relationship) analyses are underway to optimize these peptide delivery vectors and to understand their mechanisms of cellular internalization.

Cell-penetrating peptide-morpholino conjugates alter pre-mRNA splicing of DMD (Duchenne muscular dystrophy) and inhibit murine coronavirus replication in vivo

The cellular uptake of PMOs (phosphorodiamidate morpholino oligomers) can be enhanced by their conjugation to arginine-rich CPPs (cell-penetrating peptides). Here, we discuss our recent findings regarding (R-Ahx-R)(4)AhxB (Ahx is 6-aminohexanoic acid and B is beta-alanine) CPP-PMO conjugates in DMD (Duchenne muscular dystrophy) and murine coronavirus research. An (R-Ahx-R)(4)AhxB-PMO conjugate was the most effective compound in inducing the correction of mutant dystrophin transcripts in myoblasts derived from a canine model of DMD. Similarly, normal levels of dystrophin expression were restored in the diaphragms of mdx mice, with treatment starting at the neonatal stage, and protein was still detecTable 22 weeks after the last dose of an (R-Ahx-R)(4)AhxB-PMO conjugate. Effects of length, linkage and carbohydrate modification of this CPP on the delivery of a PMO were investigated in a coronavirus mouse model. An (R-Ahx-R)(4)AhxB-PMO conjugate effectively inhibited viral replication, in comparison with other peptides conjugated to the same PMO. Shortening the CPP length, modifying it with a mannosylated serine moiety or replacing it with the R(9)F(2) CPP significantly decreased the efficacy of the resulting PPMO (CPP-PMO conjugate). We attribute the success of this CPP to its stability in serum and its capacity to transport PMO to RNA targets in a manner superior to that of poly-arginine CPPs.

Switching on transgene expression by correcting aberrant splicing using multi-targeting steric-blocking oligonucleotides

BACKGROUND

Mutations leading to aberrant splicing are found as a cause of numerous pathologies. Splice-switching oligonucleotides (SSOs), which modify aberrant expression patterns of alternatively spliced mRNAs, are a novel means of potentially controlling such diseases.

METHODS

We used an experimental model in which a mutated beta-globin intron, carrying an aberrant splice site at nucleotide 705, interrupts the coding region of the luciferase reporter gene inserted in HeLa pLuc/705 cells. We have optimized delivery of splice correcting, steric-blocking 2'-O-methyl SSOs targeting the 705 mutated region (2'-O-Me SSO(705)) with DLS (DLS: delivery liposomal system) lipoplexes.

RESULTS

Optimal luciferase activity for DLS/2'-O-Me SSO(705) was achieved at 100 nM and was detectable at concentrations as low as 10 nM in serum-containing culture medium, confirming the potential of DLS lipoplex-mediated nuclear SSO delivery as observed in cellular uptake studies. We confirmed by cytofluorometry and epifluorescence microscopy the high potential of the DLS lipoplex for cellular and nuclear oligonucleotide uptake. The DLS lipoplex was then used to directly compare the intracellular efficacy of various SSO chemistries and sequences in correction of aberrant splicing. 2'-O-Methoxyethyl-oligodeoxyribonucleoside phosphorothioates had a greater activity than 2'-O-methyl phosphodiester or 2'-O-methyl-phosphorothioate oligoribonucleotides. Targeting the splicing enhancer 623 region upstream was as efficient as targeting the 705 splice site, and, remarkably, simultaneous targeting of both sites was more efficient than treatment of the cells either with 2'-O-Me SSO(705) or 2'-O-Me SSO(623) alone.

CONCLUSIONS

We demonstrated that SSOs can switch on luciferase activity in HeLa cells previously transfected with the pLuc/705 plasmid via the same DLS vector and provides a novel approach to modulate the expression of a transgene.

Peptide-based delivery of nucleic acids: design, mechanism of uptake and applications to splice-correcting oligonucleotides

CPPs (cell-penetrating peptides) have given rise to much interest for the delivery of biomolecules such as peptides, proteins or ONs (oligonucleotides). CPPs and their conjugates were initially thought to translocate through the cell membrane by a non-endocytotic mechanism which has recently been re-evaluated. Basic-amino-acid-rich CPPs first interact with cell-surface proteoglycans before being internalized by endocytosis. Sequestration and degradation in endocytotic vesicles severely limits the cytoplasmic and nuclear delivery of the conjugated biomolecules. Accordingly, splicing correction by CPP-conjugated steric-block ON analogues is inefficient in the absence of endosomolytic agents. New arginine-rich CPPs allowing efficient splicing correction by conjugated PNAs (peptide nucleic acids) or PMO (phosphorodiamidate morpholino oligomer) steric blockers in the absence of endosomolytic agents have recently been defined in our group and are currently being characterized. They offer promising leads for the development of efficient cellular delivery vectors for therapeutic steric-block ON analogues.

Efficient splicing correction by PNA conjugation to an R6-Penetratin delivery peptide

Sequence-specific interference with the nuclear pre-mRNA splicing machinery has received increased attention as an analytical tool and for development of therapeutics. It requires sequence-specific and high affinity binding of RNaseH-incompetent DNA mimics to pre-mRNA. Peptide nucleic acids (PNA) or phosphoramidate morpholino oligonucleotides (PMO) are particularly suited as steric block oligonucleotides in this respect. However, splicing correction by PNA or PMO conjugated to cell penetrating peptides (CPP), such as Tat or Penetratin, has required high concentrations (5-10 microM) of such conjugates, unless an endosomolytic agent was added to increase escape from endocytic vesicles. We have focused on the modification of existing CPPs to search for peptides able to deliver more efficiently splice correcting PNA or PMO to the nucleus in the absence of endosomolytic agents. We describe here R6-Penetratin (in which arginine-residues were added to the N-terminus of Penetratin) as the most active of all CPPs tested so far in a splicing correction assay in which masking of a cryptic splice site allows expression of a luciferase reporter gene. Efficient and sequence-specific correction occurs at 1 muM concentration of the R6Pen-PNA705 conjugate as monitored by luciferase luminescence and by RT-PCR. Some aspects of the R6Pen-PNA705 structure-function relationship have also been evaluated.

Chemical modifications to improve the cellular uptake of oligonucleotides

Specific control of gene expression by synthetic oligonucleotides (ON) is now widely used for target validation but clinical applications are limited by ON bioavailability. Moreover, most currently used strategies for physical and chemical delivery cannot be easily implemented in vivo. This article reviews new strategies which appear promising for ON delivery. The first part deals with ON chemical modifications aiming at improving cellular uptake as for instance the grafting of cationic groups on the ON backbone. The second part concerns ON conjugation to cell penetrating peptides.

Lipoplex and peptide-based strategies for the delivery of steric-block oligonucleotides

Synthetic oligonucleotides offer interesting prospects for the control of gene expression but clinical applications have been severely limited by their poor bioavailability. Cationic lipids have been widely used for the delivery of charged oligonucleotide (ON) analogues but most of the commercial formulations are toxic and poorly stable in the presence of serum proteins. We have developed a DOGS/DOPE liposome formulation named DLS (for delivery liposomal system), that allows for the efficient nuclear delivery of negatively charged antisense ON analogues as monitored by fluorescence microscopy and by their ability to correct deficient pre-mRNA splicing, even in serum-supplemented cell culture. Uncharged DNA mimics such as peptide nucleic acids (PNA), or phosphorodiamidate morpholino (PMO) ON are particularly interesting for their high metabolic stability and affinity for complementary RNA targets but they cannot be delivered with cationic lipids. Cell penetrating peptides (CPP), such as Tat or penetratin, have been used widely as conjugates for the delivery of various biomolecules and might be appropriate for neutral ON analogues. However, entrapment within endocytic vesicles severely limits the efficiency of PNA delivery by CPPs in the absence of endosomolytic drugs, such as chloroquine. The conjugation of new arginine-rich CPPs to PNA allows efficient nuclear delivery in the absence of chloroquine as monitored in a splicing correction assay. Both strategies have their advantages but DLS-mediated delivery remains more efficient than CPP delivery for the nuclear targeting of splice correcting ON analogues in vitro.

Inhibition of PAI-1 expression in breast cancer carcinoma cells by siRNA at nanomolar range

Plasminogen activator inhibitor type I (PAI-1) plays a central role in metastatic behavior by increasing cells' migratory capacities as shown in several tumoral cell lines. Moreover, in vivo high expression of this factor helps tumoral growth, both by its role in extracellular matrix remodeling and by favoring angiogenesis. High levels of PAI-1 are correlated with bad prognosis in several cancers, particularly in breast cancer. The effect of PAI-1 upon angiogenesis is also involved in atherosclerosis, in which high levels of PAI-1 expression are observed. Breast carcinoma MDA MB 231 cells are known for both having important metastatic capacities and expressing high levels of PAI-1. We have demonstrated in these cells that the transfection of PAI-1 specific small interfering RNAs (siRNA) specifically inhibited the expression of this factor by 91%. We evaluated siRNA activity by determining PAI-1 mRNA level, as well as intracellular and extracellular PAI-1 protein by using RT Q-PCR, Western blot and ELISA analyses, respectively. Data confirmed inhibition at mRNA levels (primary aim of interference), intracellular protein, and secreted PAI-1, the latter being operative successfully in the cell microenvironment. The lipidic vector Delivery Liposomes System (DLS) used was adapted to siRNA delivery as observed by particle size distribution analysis, confocal microscopy and transfection into MDA MB 231, in the presence of serum. SiRNA activity was clearly detected at concentrations as low as 10 nM. Moreover, the low cytotoxicity of this vector makes it a good candidate for future in vivo siRNA delivery.

Structural requirements for cellular uptake and antisense activity of peptide nucleic acids conjugated with various peptides

Peptide nucleic acids (PNAs) have shown great promise as potential antisense drugs; however, poor cellular delivery limits their applications. Improved delivery into mammalian cells and enhanced biological activity of PNAs have been achieved by coupling to cell-penetrating peptides (CPPs). Structural requirements for the shuttling ability of these peptides as well as structural properties of the conjugates such as the linker type and peptide position remained controversial, so far. In the present study an 18mer PNA targeted to the cryptic splice site of a mutated beta-globin intron 2, which had been inserted into a luciferase reporter gene coding sequence, was coupled to various peptides. As the peptide lead we used the cell-penetrating alpha-helical amphipathic peptide KLAL KLAL KAL KAAL KLA-NH2 [model amphipathic peptide (MAP)] which was varied with respect to charge and structure-forming properties. Furthermore, the linkage and the localization of the attached peptide (C- vs N-terminal) were modified. Positive charge as well as helicity and amphipathicity of the KLA peptide was all required for efficient dose-dependent correction of aberrant splicing. The highest antisense effect was reached within 4 h without any transfection agent. Stably linked conjugates were also efficient in correction of aberrant splicing, suggesting that a cleavable disulfide bond between CPP and PNA is clearly not essential. Moreover, the placement of the attached peptide turned out to be crucial for attaining antisense activity. Coadministration of endosome disrupting agents such as chloroquine or Ca2+ significantly increased the splicing correction efficiency of some conjugates, indicating the predominant portion to be sequestered in vesicular compartments.

Vectorization of morpholino oligomers by the (R-Ahx-R)4 peptide allows efficient splicing correction in the absence of endosomolytic agents

The efficient and non-toxic nuclear delivery of steric-block oligonucleotides (ON) is a prerequisite for therapeutic strategies involving splice correction or exon skipping. Cationic cell penetrating peptides (CPPs) have given rise to much interest for the intracellular delivery of biomolecules, but their efficiency in promoting cytoplasmic or nuclear delivery of oligonucleotides has been hampered by endocytic sequestration and subsequent degradation of most internalized material in endocytic compartments. In the present study, we compared the splice correction activity of three different CPPs conjugated to PMO(705), a steric-block ON targeted against the mutated splicing site of human beta-globin pre-mRNA in the HeLa pLuc705 splice correction model. In contrast to Tat48-60 (Tat) and oligoarginine (R(9)F(2)) PMO(705) conjugates, the 6-aminohexanoic-spaced oligoarginine (R-Ahx-R)(4)-PMO(705) conjugate was able to promote an efficient splice correction in the absence of endosomolytic agents. Our mechanistic investigations about its uptake mechanisms lead to the conclusion that these three vectors are internalized using the same endocytic route involving proteoglycans, but that the (R-Ahx-R)(4)-PMO(705) conjugate has the unique ability to escape from lysosomial fate and to access to the nuclear compartment. This vector, which has displays an extremely low cytotoxicity, the ability to function without chloroquine adjunction and in the presence of serum proteins. It thus offers a promising lead for the development of vectors able to enhance the delivery of therapeutic steric-block ON in clinically relevant models.

Impact of the guanidinium group on hybridization and cellular uptake of cationic oligonucleotides

The grafting of cationic groups to synthetic oligonucleotides (ONs) in order to reduce the charge repulsion between the negatively charged strands of a duplex or triplex, and consequently to increase a complex's stability, has been extensively studied. Guanidinium groups, which are highly basic and positively charged over a wide pH range, could be an efficient ON modification to enhance their affinity for nucleic acid targets and to improve cellular uptake. A straightforward post-synthesis method to convert amino functions attached to ONs (on sugar, nucleobase or backbone) into guanidinium tethers has been perfected. In comparison to amino groups, such cationic groups anchored to alpha-oligonucleotide phosphoramidate backbones play important roles in duplex stability, particularly with RNA targets. This high affinity could be explained by dual recognition resulting from Watson-Crick or Hoogsteen base pairing combined with cationic/anionic backbone recognition between strands involving H-bond formation and salt bridging. Molecular-dynamics simulations corroborate interactions between the cationic backbones of the alpha-ONs and the anionic backbones of the nucleic acid targets. Moreover, ONs with guanidinium modification increased cellular uptake relative to negatively charged ONs. The cellular localization of these new cationic phosphoramidate ONs is mainly cytoplasmic. The uptake of these ON analogues might occur through endocytosis.

Successful silencing of plasminogen activator inhibitor-1 in human vascular endothelial cells using small interfering RNA

Clinical as well as experimental evidence suggests that vascular overexpression of plasminogen activator inhibitor (PAI)-1, the primary physiological inhibitor of both urokinase and tissue-type plasminogen activator, may be involved in the pathophysiology of atherosclerosis and cardiovascular disease. We investigated the feasibility, efficacy and functional effects of PAI-1 gene silencing in human vascular endothelial cells using small interfering RNA. Double-stranded 21 bp-RNA molecules targeted at sequences within the human PAI-1 gene were constructed. Successful siRNA transfection of HUVEC was confirmed using fluorescence microscopy and flow cytometry. One of five candidate siRNA sequences reduced PAI-1 mRNA and protein in a concentration- and time-dependent manner. Suppression of PAI-1 mRNA was detected up to 72 hours after transfection. Moreover, siRNA treatment reduced the activity of PAI-1 released from HUVEC, and prevented the oxLDL- or LPS-induced upregulation of PAI-1 secretion. Importantly, siRNA treatment did not affect the expression of other endothelial-cell markers. Moreover, downregulation of PAI-1 significantly enhanced the ability of endothelial cells to adhere to vitronectin, and this effect could be reversed upon addition of recombinant PAI-1. SiRNA-mediated reduction of PAI-1 expression may be a promising strategy for dissecting the effects of PAI-1 on vascular homeostasis.

Endosome trapping limits the efficiency of splicing correction by PNA-oligolysine conjugates

Splicing correction by steric-blocking oligonucleotides (ON) might lead to important clinical applications but requires efficient delivery to cell nuclei. The conjugation of short oligolysine tails has been used to deliver a correcting peptide nucleic acid (PNA) sequence in a positive readout assay in which ON hybridization to the cryptic splice site is strictly required for the expression of a luciferase reporter gene. We have investigated the mechanism of cellular uptake and the efficiency of a (Lys)(8)-PNA-Lys construction in this model system. Cell uptake is temperature-dependent and leads to sequestration of the conjugate in cytoplasmic vesicles in keeping with an endocytic mechanism of internalization. Accordingly a significant and sequence-specific splicing correction is achieved only in the presence of endosome-disrupting agents as chloroquine or 0.5 M sucrose. These endosome-disrupting agents do not affect the activity of free PNA, and do not increase (Lys)(8)-PNA-Lys uptake.

Toward high yield synthesis of peptide-oligonucleotide chimera through a disulfide bridge: a simplified method for oligonucleotide activation

During the last decade, cell penetrating peptides (CPP) have been extensively used to mediate the cellular delivery of non-permeant biomolecules, including oligonucleotides (ONs). A covalent linkage between the CPP and the transported ON is required to mediate efficient cell internalization, and a disulfide bridge between the CPP and the ON has been shown to induce the most potent biological response. In this paper, we describe the activation. In a one step process of the sulfhydryl function from a synthon commercially available for ON synthesis. In addition, since the highly cationic nature of currently used CPP caused serious precipitation problems during the coupling step, we further improved the method by adsorbing the crude activated ON on an anion exchange matrix prior to specific peptide coupling.

Cell-penetrating peptide conjugates of peptide nucleic acids (PNA) as inhibitors of HIV-1 Tat-dependent trans-activation in cells

The trans-activation response (TAR) RNA stem–loop that occurs at the 5′ end of HIV RNA transcripts is an important antiviral target and is the site of interaction of the HIV-1 Tat protein together with host cellular factors. Oligonucleotides and their analogues targeted to TAR are potential antiviral candidates. We have investigated a range of cell penetrating peptide (CPP) conjugates of a 16mer peptide nucleic acid (PNA) analogue targeted to the apical stem–loop of TAR and show that disulfide-linked PNA conjugates of two types of CPP (Transportan or a novel chimeric peptide R₆-Penetratin) exhibit dose-dependent inhibition of Tat-dependent trans-activation in a HeLa cell assay when incubated for 24 h. Activity is reached within 6 h if the lysosomotropic reagent chloroquine is co-administered. Fluorescein-labelled stably-linked conjugates of Tat, Transportan or Transportan TP10 with PNA were inactive when delivered alone, but attained trans-activation inhibition in the presence of chloroquine. Confocal microscopy showed that such fluorescently labelled CPP–PNA conjugates were sequestered in endosomal or membrane-bound compartments of HeLa cells, which varied in appearance depending on the CPP type. Co-administration of chloroquine was seen in some cases to release fluorescence from such compartments into the nucleus, but with different patterns depending on the CPP. The results show that CPP–PNA conjugates of different types can inhibit Tat-dependent trans-activation in HeLa cells and have potential for development as antiviral agents. Endosomal or membrane release is a major factor limiting nuclear delivery and trans-activation inhibition.

Comparison of basic peptides- and lipid-based strategies for the delivery of splice correcting oligonucleotides

Expression of alternatively spliced mRNA variants at specific stages of development or in specific cells and tissues contributes to the functional diversity of the human genome. Aberrations in alternative splicing were found as a cause or a contributing factor to the development, progression, or maintenance of numerous diseases. The use of antisense oligonucleotides (ON) to modify aberrant expression patterns of alternatively spliced mRNAs is a novel means of potentially controlling such diseases. Oligonucleotides can be designed to repair genetic mutations, to modify genomic sequences in order to compensate for gene deletions, or to modify RNA processing in order to improve the effects of the underlying gene mutation. Steric block ON approach have proven to be effective in experimental model for various diseases. Here, we describe our experience in investigating two strategies for ON delivery: ON conjugation with basic peptides and lipid-based particulate system (lipoplex). Basic peptides or Cell Penetrating Peptides (CPP) such as the TAT-derived peptide appear to circumvent many problems associated with ON and drug delivery. This strategy may represent the next paradigm in our ability to modulate cell function and offers a unique avenue for the treatment of disease. Lipoplexes result from the intimate interaction of ON with cationic lipids leading to ON carrying particles able to be taken up by cells and to release ON in the cytoplasm. We have used as an experimental model the correction of a splicing alteration of the mutated beta-globin intron causing thalassemia. Data on cell penetration and efficacy of correction of specific steric block ON delivered either by basic peptides or lipoplex are described. A comparison of the properties of both delivery systems is made respective to the use of this new class of therapeutic molecules.

Local RNA target structure influences siRNA efficacy: a systematic global analysis

The efficiency with which small interfering RNAs (siRNAs) down-regulate specific gene expression in living cells is variable and a number of sequence-governed, biochemical parameters of the siRNA duplex have been proposed for the design of an efficient siRNA. Some of these parameters have been clearly identified to influence the assembly of the RNA-induced silencing complex (RISC), or to favour the sequence preferences of the RISC endonuclease. For other parameters, it is difficult to ascertain whether the influence is a determinant of the siRNA per se, or a determinant of the target RNA, especially its local structural characteristics. In order to gain an insight into the effects of local target structure on the biological activity of siRNA, we have used large sets of siRNAs directed against local targets of the mRNAs of ICAM-1 and survivin. Target structures were classified as accessible or inaccessible using an original, iterative computational approach and by experimental RNase H mapping. The effectiveness of siRNA was characterized by measuring the IC50 values in cell culture and the maximal extent of target suppression. Mean IC50 values were tenfold lower for accessible local target sites, with respect to inaccessible ones. Mean maximal target suppression was improved. These data illustrate that local target structure does, indeed, influence the activity of siRNA. We suggest that local target screening can significantly improve the hit rate in the design of biologically active siRNAs.

Cellular Uptake of Unconjugated TAT Peptide Involves Clathrin-dependent Endocytosis and Heparan Sulfate Receptors

Delivery of macromolecules mediated by protein transduction domains (PTDs) attracts a lot of interest due to its therapeutic and biotechnological potential. A major reevaluation of the mechanism of PTD-mediated internalization and the role of endocytosis in this mechanism has been recently initiated. Here, we demonstrate that the entry of TAT peptide (one of the most widely used PTDs) into different primary cells is ATPand temperature-dependent, indicating the involvement of endocytosis. Specific inhibitors of clathrin-dependent endocytosis partially inhibit TAT peptide uptake, implicating this pathway in TAT peptide entry. In contrast, the caveolin-dependent pathway is not essential for the uptake of unconjugated TAT peptide as evidenced by the efficient internalization of TAT in the presence of the known inhibitors of raft/caveolin-dependent pathway and for cells lacking or deficient in caveolin-1 expression. Whereas a significant part of TAT peptide uptake involves heparan sulfate receptors, efficient internalization of peptide is observed even in their absence, indicating the involvement of other receptors. Our results suggest that unconjugated peptide might follow endocytic pathways different from those utilized by TAT peptide conjugated to different proteins.

Tat peptide-mediated cellular delivery: back to basics

Peptides are emerging as attractive drug delivery tools. The HIV Tat-derived peptide is a small basic peptide that has been successfully shown to deliver a large variety of cargoes, from small particles to proteins, peptides and nucleic acids. The 'transduction domain' or region conveying the cell penetrating properties appears to be confined to a small (9 amino acids) stretch of basic amino acids, with the sequence RKKRRQRRR [S. Ruben, A. Perkins, R. Purcell, K. Joung, R. Sia, R. Burghoff, W.A. Haseltine, C.A. Rosen, Structural and functional characterization of human immunodeficiency virus tat protein, J. Virol. 63 (1989) 1-8; S. Fawell, J. Seery, Y. Daikh, C. Moore, L.L. Chen, B. Pepinsky, J. Barsoum, Tat-mediated delivery of heterologous proteins into cells, Proc. Natl. Acad. Sci. U. S. A. 91 (1994) 664-668; E. Vives, P. Brodin, B. Lebleu, A truncated HIV-1 Tat protein basic domain rapidly translocates through the plasma membrane and accumulates in the cell nucleus, J. Biol. Chem. 272 (1997) 16010-16017; S. Futaki, T. Suzuki, W. Ohashi, T. Yagami, S. Tanaka, K. Ueda, Y. Sugiura, Arginine-rich peptides. An abundant source of membrane-permeable peptides having potential as carriers for intracellular protein delivery, J. Biol. Chem. 276 (2001) 5836-5840.]. The mechanism by which the Tat peptide adheres to, and crosses, the plasma membrane of cells is currently a topic of heated discussion in the literature, with varied findings being reported. This review aims to bring together some of those findings. Peptide interactions at the cell surface, and possible mechanisms of entry, will be discussed together with the effects of modifying the basic sequence and attaching a cargo.

Oligonucleotide-based strategies to inhibit human hepatitis C virus

Hepatitis C virus (HCV) infection represents a worldwide problem, and current antiviral regimens are not satisfactory. The need to develop novel, specific, anti-HCV antiviral drugs is clear. Antisense oligonucleotides (AS-ON), ribozymes, and more recently, small interfering RNAs (siRNAs) have been widely used to control gene expression, and several clinical trials are in progress. The potential to use AS-ON as tools to control HCV infection, either by promoting an RNase H mediated cleavage of viral genomic RNA or by interfering with the assembly of a translation initiation complex on the internal ribosome entry site (IRES) is reviewed. Extensive knowledge of IRES structure and conservation among HCV genotypes have rendered the HCV IRES (and, in particular, its IIId loop) particularly attractive for antisense approaches. Encouraging data have been obtained with IRES-targeted RNase H-competent and incompetent ON analogs. We demonstrate here that very short steric blocking ONs can inhibit the formation of translation preinitiation complexes on the IRES and block IRES-mediated translation in a cell-free translation assay and in a transfected hepatoma cell line.