Membrane destabilization by ricin

Current Development of siRNA Bioconjugates: From Research to the Clinic

Small interfering RNAs (siRNAs) acting via RNA interference mechanisms are able to recognize a homologous mRNA sequence in the cell and induce its degradation. The main problems in the development of siRNA-based drugs for therapeutic use are the low efficiency of siRNA delivery to target cells and the degradation of siRNAs by nucleases in biological fluids. Various approaches have been proposed to solve the problem of siRNA delivery in vivo (e.g., viruses, cationic lipids, polymers, nanoparticles), but all have limitations for therapeutic use. One of the most promising approaches to solve the problem of siRNA delivery to target cells is bioconjugation; i.e., the covalent connection of siRNAs with biogenic molecules (lipophilic molecules, antibodies, aptamers, ligands, peptides, or polymers). Bioconjugates are "ideal nanoparticles" since they do not need a positive charge to form complexes, are less toxic, and are less effectively recognized by components of the immune system because of their small size. This review is focused on strategies and principles for constructing siRNA bioconjugates for in vivo use.

Quantitative analysis of receptor-mediated uptake and pro-apoptotic activity of mistletoe lectin-1 by high content imaging

Ribosome inactivating proteins (RIPs) are highly potent cytotoxins that have potential as anticancer therapeutics. Mistletoe lectin 1 (ML1) is a heterodimeric cytotoxic protein isolated from European Mistletoe and belongs to RIP class II. The aim of this project was to systematically study ML1 cell binding, endocytosis pathway(s), subcellular processing and apoptosis activation. For this purpose, state of the art cell imaging equipment and automated image analysis algorithms were used. ML1 displayed very fast binding to sugar residues on the membrane and energy-dependent uptake in CT26 cells. The co-staining with specific antibodies and uptake blocking experiments revealed involvement of both clathrin-dependent and -independent pathways in ML1 endocytosis. Co-localization studies demonstrated the toxin transport from early endocytic vesicles to Golgi network; a retrograde road to the endoplasmic reticulum. The pro-apoptotic and antiproliferative activity of ML1 were shown in time lapse movies and subsequently quantified. ML1 cytotoxicity was less affected in multidrug resistant tumor cell line 4T1 in contrast to commonly used chemotherapeutic drug (ML1 resistance index 6.9 vs 13.4 for doxorubicin; IC₅₀: ML1 1.4 ng/ml vs doxorubicin 24000 ng/ml). This opens new opportunities for the use of ML1 as an alternative treatment in multidrug resistant cancers.

Trileucine residues in a ligand-CPP-based siRNA delivery platform improve endosomal escape of siRNA

siRNA entrapment within endosomes is a significant problem encountered with siRNA delivery platforms that co-opt receptor-mediated entry pathways. Attachment of a cell-penetrating peptide (CPP), such as nona-arginine (9R) to a cell receptor-binding ligand like the Rabies virus glycoprotein, RVG, allows effective siRNA delivery to the cytoplasm by non-endocytic pathways, but a significant amount of siRNA complexes also enters the cell by ligand-induced receptor endocytosis and remain localized in endosomes. Here, we report that the incorporation of trileucine (3 Leu) residues as an endo-osmolytic moiety in the peptide improves endosomal escape and intracellular delivery of siRNA. The trileucine motif did not affect early non-endosomal mechanism of cytoplasmic siRNA delivery but enhanced target gene silencing by >20% only beyond 24 h of transfection when siRNA delivery is mostly through the endocytic route and siRNA trapped in the endosomes at later stages were subject to release into cytoplasm. The mechanism may involve endosomal membrane disruption as trileucine residues lysed RBCs selectively under endosomal pH conditions. Interestingly <3 Leu or >3 Leu residues were not as effective, suggesting that 3 Leu residues are useful for enhancing cytoplasmic delivery of siRNA routed through endosomes.

pH-Dependent Formation and Disintegration of the Influenza A Virus Protein Scaffold To Provide Tension for Membrane Fusion

Influenza virus is taken up from a pH-neutral extracellular milieu into an endosome, whose contents then acidify, causing changes in the viral matrix protein (M1) that coats the inner monolayer of the viral lipid envelope. At a pH of ~6, M1 interacts with the viral ribonucleoprotein (RNP) in a putative priming stage; at this stage, the interactions of the M1 scaffold coating the lipid envelope are intact. The M1 coat disintegrates as acidification continues to a pH of ~5 to clear a physical path for the viral genome to transit from the viral interior to the cytoplasm. Here we investigated the physicochemical mechanism of M1's pH-dependent disintegration. In neutral media, the adsorption of M1 protein on the lipid bilayer was electrostatic in nature and reversible. The energy of the interaction of M1 molecules with each other in M1 dimers was about 10 times as weak as that of the interaction of M1 molecules with the lipid bilayer. Acidification drives conformational changes in M1 molecules due to changes in the M1 charge, leading to alterations in their electrostatic interactions. Dropping the pH from 7.1 to 6.0 did not disturb the M1 layer; dropping it lower partially desorbed M1 because of increased repulsion between M1 monomers still stuck to the membrane. Lipid vesicles coated with M1 demonstrated pH-dependent rupture of the vesicle membrane, presumably because of the tension generated by this repulsive force. Thus, the disruption of the vesicles coincident with M1 protein scaffold disintegration at pH 5 likely stretches the lipid membrane to the point of rupture, promoting fusion pore widening for RNP release.

IMPORTANCE

Influenza remains a top killer of human beings throughout the world, in part because of the influenza virus's rapid binding to cells and its uptake into compartments hidden from the immune system. To attack the influenza virus during this time of hiding, we need to understand the physical forces that allow the internalized virus to infect the cell. In particular, we need to know how the protective coat of protein inside the viral surface reacts to the changes in acid that come soon after internalization. We found that acid makes the molecules of the protein coat push each other while they are still stuck to the virus, so that they would like to rip the membrane apart. This ripping force is known to promote membrane fusion, the process by which infection actually occurs.

Increase in transgene expression by pluronic L64-mediated endosomal/lysosomal escape through its membrane-disturbing action

For efficient transgene delivery and expression, internalized nucleic acids should quickly escape from cellular endosomes and lysosomes to avoid enzymatic destruction and degradation. Here, we report a novel strategy for safe and efficient endosomal/lysosomal escape of transgenes mediated by Pluronic L64, a neutral amphiphilic triblock copolymer. L64 enhanced the permeability of biomembranes by structural disturbance and pore formation in a concentration- and time-dependent manner. When applied at optimal concentration, it rapidly reached the endosome/lysosome compartments, where it facilitated escape of the transfection complex from the compartments and dissociation of the complex. Therefore, when applied properly, L64 not only significantly increased polyethylenimine- and liposome-mediated transgene expression, but also decreased the cytotoxicity occasioned by transfection process. Our studies revealed the function and mechanism of neutral amphiphilic triblock copolymer as potent mediator for safe and efficient gene delivery.

Basics and recent advances in peptide and protein drug delivery

While the peptide and protein therapeutic market has developed significantly in the past decades, delivery has limited their use. Although oral delivery is preferred, most are currently delivered intravenously or subcutaneously due to degradation and limited absorption in the gastrointestinal tract. Therefore, absorption enhancers, enzyme inhibitors, carrier systems and stability enhancers are being studied to facilitate oral peptide delivery. Additionally, transdermal peptide delivery avoids the issues of the gastrointestinal tract, but also faces absorption limitations. Due to proteases, opsonization and agglutination, free peptides are not systemically stable without modifications. This review discusses oral and transdermal peptide drug delivery, focusing on barriers and solutions to absorption and stability issues. Methods to increase systemic stability and site-specific delivery are also discussed.

A computational perspective of molecular interactions through virtual screening, pharmacokinetic and dynamic prediction on ribosome toxin A chain and inhibitors of Ricinus communis

Background:

Ricin is considered to be one of the most deadly toxins and gained its favor as a bioweapon that has a serious social and biological impact, due to its widespread nature and abundant availability. The hazardous effects of this toxin in human being are seen in almost all parts of the organ system. The severe consequences of the toxin necessitate the need for developing potential inhibitors that can effectively block its interaction with the host system.

Materials and Methods:

In order to identify potential inhibitors that can effectively block ricin, we employed various computational approaches. In this work, we computationally screened and analyzed 66 analogs and further tested their ADME/T profiles. From the kinetic and toxicity studies we selected six analogs that possessed appropriate pharmacokinetic and dynamic property. We have also performed a computational docking of these analogs with the target.

Results:

On the basis of the dock scores and hydrogen bond interactions we have identified analog 64 to be the best interacting molecule. Molecule 64 seems to have stable interaction with the residues Tyr80, Arg180, and Val81. The pharmacophore feature that describes the key functional features of a molecule was also studied and presented.

Conclusion:

The pharmacophore features of the drugs provided suggests the key functional groups that can aid in the design and synthesis of more potential inhibitors.

Interaction of nuclease colicins with membranes: insertion depth correlates with bilayer perturbation

BACKGROUND

Protein transport across cellular membranes is an important aspect of toxin biology. Escherichia coli cell killing by nuclease colicins occurs through DNA (DNases) or RNA (RNases) hydrolysis and to this end their cytotoxic domains require transportation across two sets of membranes. In order to begin to unravel the molecular mechanisms underlying the membrane translocation of colicin nuclease domains, we have analysed the membrane association of four DNase domains (E9, a charge reduction E9 mutant, E8, and E7) and one ribosomal RNase domain (E3) using a biomembrane model system.

PRINCIPAL RESULTS

We demonstrate, through the use of large unilamellar vesicles composed of synthetic and E. coli lipids and a membrane surface potential sensor, that the colicin nuclease domains bind anionic membranes only, with micromolar affinity and via a cooperative binding mechanism. The evaluation of the nuclease bilayer insertion depth, through a fluorescence quenching analysis using brominated lipids, indicates that the nucleases locate to differential regions in the bilayer. Colicin DNases target the interfacial region of the lipid bilayer, with the DNase E7 showing the deepest insertion, whereas the ribosomal RNase E3 penetrates into the hydrophobic core region of the bilayer. Furthermore, the membrane association of the DNase E7 and the ribosomal RNase E3 induces vesicle aggregation, lipid mixing and content leakage to a much larger extent than that of the other DNases analysed.

CONCLUSIONS/SIGNIFICANCE

Our results show, for the first time, that after the initial electrostatically driven membrane association, the pleiotropic membrane effects induced by colicin nuclease domains relate to their bilayer insertion depth and may be linked to their in vivo membrane translocation.

Nucleic-acid based gene therapeutics: delivery challenges and modular design of nonviral gene carriers and expression cassettes to overcome intracellular barriers for sustained targeted expression

The delivery of nucleic acid molecules into cells to alter physiological functions at the genetic level is a powerful approach to treat a wide range of inherited and acquired disorders. Biocompatible materials such as cationic polymers, lipids, and peptides are being explored as safer alternatives to viral gene carriers. However, the comparatively low efficiency of nonviral carriers currently hampers their translation into clinical settings. Controlling the size and stability of carrier/nucleic acid complexes is one of the primary hurdles as the physicochemical properties of the complexes can define the uptake pathways, which dictate intracellular routing, endosomal processing, and nucleocytoplasmic transport. In addition to nuclear import, subnuclear trafficking, posttranscriptional events, and immune responses can further limit transfection efficiency. Chemical moieties, reactive linkers or signal peptide have been conjugated to carriers to prevent aggregation, induce membrane destabilization and localize to subcellular compartments. Genetic elements can be inserted into the expression cassette to facilitate nuclear targeting, delimit expression to targeted tissue, and modulate transgene expression. The modular option afforded by both gene carriers and expression cassettes provides a two-tier multicomponent delivery system that can be optimized for targeted gene delivery in a variety of settings.

Lipopolyamine treatment increases the efficacy of intoxication with saporin and an anticancer saporin conjugate

Saporin is a type I ribosome-inactivating protein that is often appended with a cell-binding domain to specifically target and kill cancer cells. Urokinase plasminogen activator (uPA)-saporin, for example, is an anticancer toxin that consists of a chemical conjugate between the human uPA and native saporin. Both saporin and uPA-saporin enter the target cell by endocytosis and must then escape the endomembrane system to reach the cytosolic ribosomes. The latter process may represent a rate-limiting step for intoxication and would therefore directly affect toxin potency. In the present study, we document two treatments (shock with dimethylsulfoxide and lipopolyamine coadministration) that generate substantial cellular sensitization to saporin/uPA-saporin. With the use of lysosome-endosome X (LEX)1 and LEX2 mutant cell lines, an endosomal trafficking step preceding cargo delivery to the late endosomes was identified as a major site for the dimethylsulfoxide-facilitated entry of saporin into the cytosol. Dimethylsulfoxide and lipopolyamines are known to disrupt the integrity of endosome membranes, so these reagents could facilitate the rapid movement of toxin from permeabilized endosomes to the cytosol. However, the same pattern of toxin sensitization was not observed for dimethylsulfoxide- or lipopolyamine-treated cells exposed to diphtheria toxin, ricin, or the catalytic A chain of ricin. The sensitization effects were thus specific for saporin, suggesting a novel mechanism of saporin translocation by endosome disruption. Lipopolyamines have been developed as in vivo gene therapy vectors; thus, lipopolyamine coadministration with uPA-saporin or other saporin conjugates could represent a new approach for anticancer toxin treatments.

Ricin A Chain from Ricinus sanguineus: DNA Sequence, Structure and Toxicity

The gene coding for the ricin A-chain from Ricinus sanguineus (RsTA) was isolated and sequenced (GB: DQ661048). Comparison of RsTA with the ricin A-chain from Ricinus communis (RTA) revealed the presence of five differences in the gene sequence. At the protein level only two differences were noticed, the replacement of Asn136 by Ser (N136S) and Ile173 by Val (I174 V). From the RsTA structure model (PMDB: PM0074652), the N136S mutation was predicted to have no effect on RsTA structure. The I174V mutation is believed to have no effect on the RsTA structure or on toxicity since this replacement was found in the Ricinus agglutinin's A-chain and the latter has a comparable toxicity to RTA. The Ser221 of the putative lipase active site was found. Toxicity experiments showed that RsTA and RTA have similar toxicities. This finding proves that the N136S and I174 V mutations have no effect on RsTA toxicity.