Exploitation of intracellular pH gradients in the cellular delivery of macromolecules

Manganese-impregnated mesoporous silica nanoparticles for signal enhancement in MRI cell labelling studies

Mesoporous silica nanoparticles (MSNs) are used in drug delivery and cell tracking applications. As Mn(2+) is already implemented as a "positive" cell contrast agent in preclinical imaging procedures (in the form of MnCl2 for neurological studies), the introduction of Mn in the porous network of MSNs would allow labelling cells and tracking them using MRI. These particles are in general internalized in endosomes, an acidic environment with high saline concentration. In addition, the available MSN porosity could also serve as a carrier to deliver medical/therapeutic substances through the labelled cells. In the present study, manganese oxide was introduced in the porous network of MCM-48 silica nanoparticles (Mn-M48SNs). The particles exhibit a narrow size distribution (~140 nm diam.) and high porosity (~60% vol.), which was validated after insertion of Mn. The resulting Mn-M48SNs were characterized by TEM, N2 physisorption, and XRD. Evidence was found with H2-TPR, and XPS characterization, that Mn(II) is the main oxidation state of the paramagnetic species after suspension in water, most probably in the form of Mn-OOH. The colloidal stability as a function of time was confirmed by DLS in water, acetate buffer and cell culture medium. In NMR data, no significant evidence of Mn(2+) leaching was found in Mn-M48SNs in acidic water (pH 6), up to 96 hours after suspension. High longitudinal relaxivity values of r1 = 8.4 mM(-1) s(-1) were measured at 60 MHz and 37 °C, with the lowest relaxometric ratios (r2/r1 = 2) reported to date for a Mn-MSN system. Leukaemia cells (P388) were labelled with Mn-M48SNs and nanoparticle cell internalization was confirmed by TEM. Finally, MRI contrast enhancement provided by cell labelling with escalated incubation concentrations of Mn-M48SNs was quantified at 1 T. This study confirmed the possibility of efficiently confining Mn into M48SNs using incipient wetness, while maintaining an open porosity and relatively high pore volume. Because these Mn-labelled M48SNs express strong "positive" contrast media properties at low concentrations, they are potentially applicable for cell tracking and drug delivery methodologies.

Poly(2 deoxy 2 methacrylamido glucopyranose) b Poly(methacrylate amine)s: Optimization of Diblock Glycopol ycations for Nucleic Acid Delivery

A series of nine poly(2-deoxy-2-methacrylamido glucopyranose)-b-poly(methacrylate amine) diblock copolycations The cationic block was varied in length and in the degree of methyl group substitution (secondary, tertiary, quaternary) on the pendant amine in an effort to optimize the structure and activity for plasmid DNA delivery. Upon a thorough kinetic study of polymerization for each polymer, the glycopolymers were prepared with well-controlled Mn and Ð. The binding and colloidal stability of the polymer-pDNA nanocomplexes at different N/P ratios and in biological media has been investigated using gel electrophoresis and light scattering techniques. The toxicity and transfection efficiency of the polyplexes has been evaluated with Hep G2 (human liver hepatocellular carcinoma) cells; several polymers displayed excellent delivery and toxicity profiles justifying their further development for in vivo gene therapy.

Triple stimulus-responsive polypeptide nanoparticles that enhance intratumoral spatial distribution

To address the limited tumor penetration of nanoparticle drug delivery vehicles, we report the first pH-responsive polypeptide micelle that dissociates at the low extracellular pH of solid tumors. This histidine-rich elastin-like polypeptide block copolymer self-assembles at 37 °C into spherical micelles that are stabilized by Zn(2+) and are disrupted as the pH drops from 7.4 to 6.4. These pH-sensitive micelles demonstrate better in vivo penetration and distribution in tumors than a pH-insensitive control.

Endosomal escape and transfection efficiency of PEGylated cationic liposome-DNA complexes prepared with an acid-labile PEG-lipid

Cationic liposome-DNA (CL-DNA) complexes are being pursued as nonviral gene delivery systems for use in applications that include clinic trials. However, to compete with viral vectors for systemic delivery in vivo, their efficiencies and pharmacokinetics need to be improved. The addition of poly (ethylene glycol)-lipids (PEGylation) prolongs circulation lifetimes of liposomes, but inhibits cellular uptake and endosomal escape of CL-DNA complexes. We show that this limits their transfection efficiency (TE) in a manner dependent on the amount of PEG-lipid, the lipid/DNA charge ratio, and the lipid membrane charge density. To improve endosomal escape of PEGylated CL-DNA complexes, we prepared an acid-labile PEG-lipid (HPEG2K-lipid, PEG MW 2000) which is designed to lose its PEG chains at the pH of late endosomes. The HPEG2K-lipid and a similar but acid-stable PEG-lipid were used to prepare PEGylated CL-DNA complexes. TLC and dynamic light scattering showed that HPEG2K-CL-DNA complexes are stable at pH 7.4 for more than 24 h, but the PEG chains are cleaved at pH 5 within 1 h, leading to complex aggregation. The acid-labile HPEG2K-CL-DNA complexes showed enhanced TE over complexes stabilized with the acid-stable PEG-lipid. Live-cell imaging showed that both types of complexes were internalized to quantitatively similar particle distributions within the first 2 h of incubation with cells. Thus, we attribute the increased TE of the HPEG2K-CL-DNA complexes to efficient endosomal escape, enabled by the acid-labile HPEG2K-lipid which sheds its PEG chains in the low pH environment of late endosomes, effectively switching on the electrostatic interactions that promote fusion of the membranes of complex and endosome.

Trafficking of alpha4* nicotinic receptors revealed by superecliptic phluorin: effects of a beta4 amyotrophic lateral sclerosis-associated mutation and chronic exposure to nicotine

We employed a pH-sensitive GFP analog, superecliptic phluorin, to observe aspects of nicotinic acetylcholine receptor (nAChR) trafficking to the plasma membrane (PM) in cultured mouse cortical neurons. The experiments exploit differences in the pH among endoplasmic reticulum (ER), trafficking vesicles, and the extracellular solution. The data confirm that few α4β4 nAChRs, but many α4β2 nAChRs, remain in neutral intracellular compartments, mostly the ER. We observed fusion events between nAChR-containing vesicles and PM; these could be quantified in the dendritic processes. We also studied the β4R348C polymorphism, linked to amyotrophic lateral sclerosis (ALS). This mutation depressed fusion rates of α4β4 receptor-containing vesicles with the PM by ∼2-fold, with only a small decrease in the number of nAChRs per vesicle. The mutation also decreased the number of ER exit sites, showing that the reduced receptor insertion results from a change at an early stage in trafficking. We confirm the previous report that the mutation leads to reduced agonist-induced currents; in the cortical neurons studied, the reduction amounts to 2-3-fold. Therefore, the reduced agonist-induced currents are caused by the reduced number of α4β4-containing vesicles reaching the membrane. Chronic nicotine exposure (0.2 μM) did not alter the PM insertion frequency or trafficking behavior of α4β4-laden vesicles. In contrast, chronic nicotine substantially increased the number of α4β2-containing vesicle fusions at the PM; this stage in α4β2 nAChR up-regulation is presumably downstream from increased ER exit. Superecliptic phluorin provides a tool to monitor trafficking dynamics of nAChRs in disease and addiction.

Synthesis and characterization of lipid-polymer hybrid nanoparticles with pH-triggered poly(ethylene glycol) shedding

Novel lipid-polymer hybrid nanoparticles are designed with a poly(ethylene glycol) (PEG) coating that is shed in response to a low pH trigger. This allows the nanoparticles to be stable during systemic circulation and at neutral pH, but destabilize and fuse with lipid membranes in acidic environments. The hybrid nanoparticles consist of a poly(lactic-co-glycolic acid) core with a lipid and lipid-PEG monolayer shell. To make the hybrid nanoparticles pH sensitive, a lipid-(succinate)-mPEG conjugate is synthesized to provide a hydrolyzable PEG stealth layer that is shed off the particle surface at low pH. The pH-sensitivity of the nanoparticles is tunable using the molar concentration of the lipid-(succinate)-mPEG incorporated in the lipid shell of the particles. Possible uses of these pH-sensitive nanoparticles include aggregating in acidic tumor microenvironments, escaping acidified endosomes, or aggregating in deep lung tissue for improved inhalation administration.

Cell type-specific responses of peripheral blood mononuclear cells to silver nanoparticles

Silver nanoparticles (Ag-NP) are increasingly used in biomedical applications because of their remarkable antimicrobial activity. In biomedicine, Ag-NP are coated onto or embedded in wound dressings, surgical instruments and bone substitute biomaterials, such as silver-containing calcium phosphate cements. Free Ag-NP and silver ions are released from these coatings or after the degradation of a biomaterial, and may come into close contact with blood cells. Despite the widespread use of Ag-NP as an antimicrobial agent, there is a serious lack of information on the biological effects of Ag-NP on human blood cells. In this study, the uptake of Ag-NP by peripheral monocytes and lymphocytes (T-cells) was analyzed, and the influence of nanosilver on cell biological functions (proliferation, the expression of adhesion molecules, cytokine release and the generation of reactive oxygen species) was studied. After cell culture in the presence of monodispersed Ag-NP (5-30μgml(-1) silver concentration), agglomerates of nanoparticles were detected within monocytes (CD14+) but not in T-cells (CD3+) by light microscopy, flow cytometry and combined focused ion beam/scanning electron microscopy. The uptake rate of nanoparticles was concentration dependent, and the silver agglomerates were typically found in the cytoplasm. Furthermore, a concentration-dependent activation (e.g. an increased expression of adhesion molecule CD54) of monocytes at Ag-NP concentrations of 10-15μgml(-1) was observed, and cytotoxicity of Ag-NP-treated monocytes was observed at Ag-NP levels of 25μgml(-1) and higher. However, no modulation of T-cell proliferation was observed in the presence of Ag-NP. Taken together, our results provide the first evidence for a cell-type-specific uptake of Ag-NP by peripheral blood mononuclear cells (PBMC) and the resultant cellular responses after exposure.

Functional lipids and lipoplexes for improved gene delivery

Cationic lipids are the most common non-viral vectors used in gene delivery with a few currently being investigated in clinical trials. However, like most other synthetic vectors, these vectors suffer from low transfection efficiencies. Among the various approaches to address this challenge, functional lipids (i.e., lipids responding to a stimuli) offer a myriad of opportunities for basic studies of nucleic acid-lipid interactions and for in vitro and in vivo delivery of nucleic acid for a specific biological/medical application. This manuscript reviews recent advances in pH, redox, and charge-reversal sensitive lipids.

Nanostructure formation in aqueous solution of amphiphilic copolymers of 2-(N,N-dimethylaminoethyl)methacrylate and alkylacrylate: Characterization, antimicrobial activity, DNA binding, and cytotoxicity studies

Three amphiphilic random copolymers poly(2-(dimethylaminoethyl)methacrylate-co-alkylacrylate) (where, alkyl = hexyl, octyl, dodecyl) with 16 mol% hydrophobic substitution were synthesized. Surface tension, viscosity, fluorescence probe, dynamic light scattering (DLS), as well as transmission electron microscopic (TEM) techniques were utilized to investigate self-assembly formation by the hydrophobically modified polymers (HMPs) in pH 5. Formation of hydrophobic domains through inter-polymer chain interaction of the copolymer in dilute solution was confirmed by fluorescence probe studies. Average hydrodynamic diameter of the copolymer aggregates at different polymer concentration was measured by DLS studies. The copolymer with shorter hydrophobic chain exhibits larger hydrodynamic diameter in dilute solution, which decreased with either increase of concentration or increase of hydrophobic chain length. TEM images of the dilute solutions of the copolymers with shorter as well as with longer hydrophobic chain exhibit spherical aggregates of different sizes. The antimicrobial activity of the copolymers was evaluated by measuring the minimum inhibitory concentration value against one Gram-positive bacterium Bacillus subtilis and one Gram-negative bacterium Escherichia coli. The copolymer with the octyl group as pendent hydrophobic chain was found to be more effective in killing these microorganisms. The interaction of the cationic copolymers with calf-thymus DNA was studied by fluorescence quenching method. The polymer-DNA binding was found to be purely electrostatic in nature. The hydrophobes on the polymer backbone were found to have a significant influence on the binding process. Biocompatibility studies of the copolymers in terms of cytotoxicity measurements were finally performed at different concentrations of the HMPs to evaluate their potential application in biomedical fields.

Strategies for Neoglycan conjugation to human acid α-glucosidase

Engineering proteins for selective tissue targeting can improve therapeutic efficacy and reduce undesired side effects. The relatively high dose of recombinant human acid α-glucosidase (rhGAA) required for enzyme replacement therapy of Pompe disease may be attributed to less than optimal muscle uptake via the cation-independent mannose 6-phosphate receptor (CI-MPR). To improve muscle targeting, Zhu et al. (1) conjugated periodate oxidized rhGAA with bis mannose 6-phosphate bearing synthetic glycans and achieved 5-fold greater potency in a murine Pompe efficacy model. In the current study, we systematically evaluated multiple strategies for conjugation based on a structural homology model of GAA. Glycan derivatives containing succinimide, hydrazide, and aminooxy linkers targeting free cysteine, lysines, and N-linked glycosylation sites on rhGAA were prepared and evaluated in vitro and in vivo. A novel conjugation method using enzymatic oxidation was developed to eliminate side oxidation of methionine. Conjugates derived from periodate oxidized rhGAA still displayed the greatest potency in the murine Pompe model. The efficiency of conjugation and its effect on catalytic activity were consistent with predictions based on the structural model and supported its use in guiding selection of appropriate chemistries.

Comparison of hydrazone heterobifunctional cross-linking agents for reversible conjugation of thiol-containing chemistry

Reversible covalent conjugation chemistries that allow site- and condition-specific coupling and uncoupling reactions are attractive components in nanotechnologies, bioconjugation methods, imaging, and drug delivery systems. Here, we compare three heterobifunctional cross-linkers, containing both thiol- and amine-reactive chemistries, to form pH-labile hydrazones with hydrazide derivatives of the known and often published water-soluble polymer, poly[N-(2-hydroxypropyl methacrylamide)] (pHPMA), while subsequently coupling thiol-containing molecules to the cross-linker via maleimide addition. Two novel cross-linkers were prepared from the popular heterobifunctional cross-linking agent, succinimidyl-4-(N-maleimidomethyl) cyclohexane-1-carboxylate (SMCC), modified to contain either terminal aldehyde groups (i.e., 1-(N-3-propanal)-4-(N-maleimidomethyl) cyclohexane carboxamide, PMCA) or methylketone groups (i.e., 1-(N-3-butanone)-4-(N-maleimidomethyl) cyclohexane carboxamide, BMCA). A third cross-linking agent was the commercially available N-4-acetylphenyl maleimide (APM). PMCA and BMCA exhibited excellent reactivity toward hydrazide-derivatized pHPMA with essentially complete hydrazone conjugation to polymer reactive sites, while APM coupled only ∼60% of available reactive sites on the polymer despite a 3-fold molar excess relative to polymer hydrazide groups. All polymer hydrazone conjugates bearing these bifunctional agents were then further reacted with thiol-modified tetramethylrhodamine dye, confirming cross-linker maleimide reactivity after initial hydrazone polymer conjugation. Incubation of dye-labeled polymer conjugates in phosphate buffered saline at 37 °C showed that hydrazone coupling resulting from APM exhibited the greatest difference in stability between pH 7.4 and 5.0, with hydrolysis and dye release increased at pH 5.0 over a 24 h incubation period. Polymer conjugates bearing hydrazones formed from cross-linker BMCA exhibited intermediate stability with hydrolysis much greater at pH 5.0 at early time points, but hydrolysis at pH 7.4 was significant after 5 h. Hydrazones formed with the PMCA cross-linker showed no difference in release rates at pH 7.4 and 5.0.

pH-Responsive nanoparticles for drug delivery

First-generation nanoparticles (NPs) have been clinically translated as pharmaceutical drug delivery carriers for their ability to improve on drug tolerability, circulation half-life, and efficacy. Toward the development of the next-generation NPs, researchers have designed novel multifunctional platforms for sustained release, molecular targeting, and environmental responsiveness. This review focuses on environmentally responsive mechanisms used in NP designs, and highlights the use of pH-responsive NPs in drug delivery. Different organs, tissues, and subcellular compartments, as well as their pathophysiological states, can be characterized by their pH levels and gradients. When exposed to these pH stimuli, pH-responsive NPs respond with physicochemical changes to their material structure and surface characteristics. These include swelling, dissociating or surface charge switching, in a manner that favors drug release at the target site over surrounding tissues. The novel developments described here may revise the classical outlook that NPs are passive delivery vehicles, in favor of responsive, sensing vehicles that use environmental cues to achieve maximal drug potency.

Thermodynamics of polyethylenimine-DNA binding and DNA condensation

In this study, polyethylenimine (PEI) binding to DNA was examined by isothermal titration calorimetry. Two types of binding modes were found to describe the interactions between these polyelectrolytes in buffers and in water. One type of binding involves PEI binding to the DNA groove because the enthalpy change of this binding mode is positive, and PEI is deprotonated to bind to DNA. Another likely binding mode involves external binding of PEI to the DNA phosphate backbone, accompanied with DNA condensation. The enthalpy change is negative and PEI is protonated when it binds to DNA in this mode. The intrinsic enthalpy change of first binding mode is 1.1 kJ/mol and -0.88 kJ/mol for the second binding mode. This result implies that the PEI is rearranged from the groove to the phosphate backbone of DNA when DNA is condensed. The mechanism of DNA condensation caused by PEI is discussed in this study.

Effects of experimental setup on the apparent concentration dependency of active efflux transport in in vitro cell permeation experiments

P-Glycoprotein mediated efflux is one of the barriers limiting drug absorption from the intestine. Predictions of the intestinal P-glycoprotein function need to take into account the concentration dependency because high intestinal drug concentrations may saturate P-glycoprotein. However, the substrate binding site of P-glycoprotein lies inside the cells and the drug concentration at the binding site cannot be measured directly. Therefore, rigorous determination of concentration dependent P-glycoprotein kinetics is challenging. In this study, the effects of the aqueous boundary layers, extracellular pH and cellular retention on the apparent saturation kinetics of P-glycoprotein mediated transport of quinidine in an in vitro cell permeation setting were explored. The changes in the experimental conditions caused 1 order of magnitude variation in the apparent affinity to P-glycoprotein (K(m,app)) and a 5-fold difference in the maximum effective P-glycoprotein mediated transport rate of quinidine (V(max,app)). However, fitting the concentration data into a compartmental model which accounted for the aqueous boundary layers, cell membranes and cellular retention suggested that the P-glycoprotein function per se was not altered, it was the differences in the passive transfer of quinidine which changed the apparent transport kinetics. These results provide further insight into the dynamics of the P-glycoprotein mediated transport and on the roles of several confounding factors involved in in vitro experimental setting. Further, the results confirm the applicability of compartmental model based data analysis approach in the determination of active transporter kinetics.

Dynamics and mechanisms of quantum dot nanoparticle cellular uptake

BACKGROUND

The rapid growth of the nanotechnology industry and the wide application of various nanomaterials have raised concerns over their impact on the environment and human health. Yet little is known about the mechanism of cellular uptake and cytotoxicity of nanoparticles. An array of nanomaterials has recently been introduced into cancer research promising for remarkable improvements in diagnosis and treatment of the disease. Among them, quantum dots (QDs) distinguish themselves in offering many intrinsic photophysical properties that are desirable for targeted imaging and drug delivery.

RESULTS

We explored the kinetics and mechanism of cellular uptake of QDs with different surface coatings in two human mammary cells. Using fluorescence microscopy and laser scanning cytometry (LSC), we found that both MCF-7 and MCF-10A cells internalized large amount of QD655-COOH, but the percentage of endocytosing cells is slightly higher in MCF-7 cell line than in MCF-10A cell line. Live cell fluorescent imaging showed that QD cellular uptake increases with time over 40 h of incubation. Staining cells with dyes specific to various intracellular organelles indicated that QDs were localized in lysosomes. Transmission electron microscopy (TEM) images suggested a potential pathway for QD cellular uptake mechanism involving three major stages: endocytosis, sequestration in early endosomes, and translocation to later endosomes or lysosomes. No cytotoxicity was observed in cells incubated with 0.8 nM of QDs for a period of 72 h.

CONCLUSIONS

The findings presented here provide information on the mechanism of QD endocytosis that could be exploited to reduce non-specific targeting, thereby improving specific targeting of QDs in cancer diagnosis and treatment applications. These findings are also important in understanding the cytotoxicity of nanomaterials and in emphasizing the importance of strict environmental control of nanoparticles.

Nanoparticle-based biocompatible and long-life marker for lysosome labeling and tracking

In this paper, a novel biocompatible and long-life lysosome labeling and tracking method based on dye entrapped silica nanoparticles (DSiNPs) has been put forward. Through colocalization studies using LysoTracker Green as the standard lysosome marker, it has been demonstrated that DSiNPs selectively accumulated in lysosomes of Hela cells and the photostability of DSiNPs associated with lysosomes was detectable, at least, 30 times as long as that of LysoTracker Green involved in lysosomes. By comparison with LysoTracker Green and Alexa 488-dextran, the fluorescence of DSiNPs could be detected over a 5-day postrecultivation period and the staining pattern in lysosomes could be well retained after cell fixation and permeabilization. In addition, results from MTT assays showed that DSiNPs did not affect the viability of Hela cells at the concentration for lysosome labeling. Primary applications of DSiNPs were then further performed in lysosome tracking in chloroquine-treated Hela cells, and lysosome labeling of differnet cell lines, including MCF-7 cells, MEAR cells, and MSC cells. These results indicated that DSiNPs, therefore, can be used as a biocompatible, long-life, and highly photostable lysosome marker for lysosome-related studies.

Effect of cerium oxide nanoparticles on inflammation in vascular endothelial cells

Because vascular endothelial cell inflammation is critical in the development of cardiovascular pathology, we hypothesized that direct exposure of human aortic endothelial cells (HAECs) to ultrafine particles induces an inflammatory response. To test the hypothesis, we incubated HAECs for 4 h with different concentrations (0.001-50 microg/ml) of CeO(2) nanoparticles and subsequently measured mRNA levels of the three inflammatory markers intercellular adhesion molecule 1 (ICAM-1), interleukin (IL)-8, and monocyte chemotactic protein (MCP-1) using real-time polymerase chain reaction (PCR). Ceria nanoparticles caused very little inflammatory response in HAECs, even at the highest dose. This material is apparently rather benign in comparison with Y(2)O(3) and ZnO nanoparticles that we have studied previously. These results suggest that inflammation in HAECs following acute exposure to metal oxide nanoparticles depends strongly on particle composition.

Long-circulating, pH-sensitive liposomes

A major limiting factor for the wide application of pH-sensitive liposomes is their recognition and sequestration by the phagocytes of the reticulo-endothelial system, which conditions a very short circulation half-life. Typically prolonged circulation of liposomes is achieved by grafting their membranes with pegylated phospholipids (PEG-lipids), which have been shown, however, to deteriorate membrane integrity on one hand and to hamper the pH-responsiveness on the other. Hence, the need for novel alternative surface modifying agents to ensure effective half-life prolongation of pH-sensitive liposomes is a subject of intensive research. A series of copolymers having short blocks of lipid-mimetic units has been shown to sterically stabilize conventional liposomes based on different phospholipids. This has prompted us to broaden their utilization to pH-sensitive liposomes, too. The present contribution gives thorough account on the chemical synthesis of these copolymers their incorporation in DOPE:CHEMs pH-sensitive liposomes and detailed explanation on the battery of techniques for the biopharmaceutical characterization of the prepared formulations in terms of pH-responsiveness, cellular internalization, in vivo pharmacokinetics and biodistribution.

Dynamic visualization of membrane-inserted fraction of pHluorin-tagged channels using repetitive acidification technique

BACKGROUND

Changes in neuronal excitability, synaptic efficacy and generally in cell signaling often result from insertion of key molecules into plasma membrane (PM). Many of the techniques used for monitoring PM insertion lack either spatial or temporal resolution.

RESULTS

We improved the imaging method based on time-lapse total internal reflection fluorescence (TIRF) microscopy and pHluorin tagging by supplementing it with a repetitive extracellular acidification protocol. We illustrate the applicability of this method by showing that brief activation of NMDA receptors ("chemical LTP") in cultured hippocampal neurons induced a persistent PM insertion of glutamate receptors containing the pHluorin-tagged GluR-A(flip) subunits.

CONCLUSION

The repetitive acidification technique provides a more accurate way of monitoring the PM-inserted fraction of fluorescently tagged molecules and offers a good temporal and spatial resolution.

Lipid-based systemic delivery of siRNA

RNAi technology has brought a new category of treatments for various diseases including genetic diseases, viral diseases, and cancer. Despite the great versatility of RNAi that can down regulate almost any protein in the cells, the delicate and precise machinery used for silencing is the same. The major challenge indeed for RNAi-based therapy is the delivery system. In this review, we start with the uniqueness and mechanism of RNAi machinery and the utility of RNAi in therapeutics. Then we discuss the challenges in systemic siRNA delivery by dividing them into two categories-kinetic and physical barriers. At the end, we discuss different strategies to overcome these barriers, especially focusing on the step of endosome escape. Toxicity issues and current successful examples for lipid-based delivery are also included in the review.

Synergistic effect of co-exposure to carbon black and Fe2O3 nanoparticles on oxidative stress in cultured lung epithelial cells

Background

There is a need to better understand synergism in the biological effects of particles composed of multiple substances. The objective of this study was to determine if the oxidative stress in cultured cells caused by co-exposure to carbon black and Fe₂O₃ nanoparticles was significantly greater than the additive effects of exposure to either type of particles alone; and to determine a possible cause for such synergistic effect if one was found. Cultured A549 human lung epithelial cells were exposed to (1) carbon black nanoparticles alone, (2) Fe₂O₃ nanoparticles alone, and (3) both types of particles simultaneously. Protein oxidation, lipid peroxidation, and cellular uptake of Fe in these cells were measured after 25 hours of exposure. The reduction of solubilized Fe³⁺ by the carbon black nanoparticles was measured separately in a cell-free assay, by incubating the carbon black and the Fe₂O₃ nanoparticles in 0.75 M sulfuric acid at 40°C and measuring the amount of reduced Fe³⁺ at different time points up to 24 hours.

Results

Cells exposed to carbon black particles alone did not show protein oxidation, nor did the cells exposed to Fe₂O₃ particles alone, relative to the control. However, cells co-exposed to both carbon black and Fe₂O₃ particles showed up to a two-fold increase in protein oxidation relative to the control. In addition, co-exposure induced significant lipid peroxidation, although exposure to either particle type alone did not. No significant difference in cellular iron uptake was found between single exposure and co-exposure, when the Fe₂O₃ dosing concentration was the same in each case. In the cell-free assay, significant reduction of Fe³⁺ ions by carbon black nanoparticle was found within 2 hour, and it progressed up to 24 hours. At 24 hours, the carbon black nanoparticles showed a reductive capacity of 0.009 g/g, defined as the mass ratio of reduced Fe³⁺ to carbon black.

Conclusion

Co-exposure to carbon black and Fe₂O₃ particles causes a synergistic oxidative effect that is significantly greater than the additive effects of exposures to either particle type alone. The intracellular redox reaction between carbon black and Fe³⁺ is likely responsible for the synergistic oxidative effect. Therefore elemental carbon particles and fibres should be considered as potential reducing agents rather than inert materials in toxicology studies. Acidified cell organelles such as the lysosomes probably play a critical role in the solubilization of Fe₂O₃. Further research is necessary to better understand the mechanisms.

Assessing the sensitivity of commercially available fluorophores to the intracellular environment

The use of fluorescence has become commonplace in the biological sciences, with many studies utilizing probes based on commercially available fluorophores to provide insight into cell function and behavior. As these imaging applications become more advanced, it becomes increasingly important to acquire accurate quantitative measurements of the fluorescence signal. Absolute quantification of fluorescence, however, requires the fluorophores themselves to be insensitive to environmental factors such as nonspecific protein interactions and pH. Here, we present a method for characterizing the sensitivity of fluorophores to the cytosolic environment by comparing their fluorescent intensity to an environment-insensitive reference signal before and after intracellular delivery. Results indicated that although the fluorescent intensity of a few fluorophores, e.g., fluorescein, were highly susceptible to the intracellular environment, other fluorophores, e.g., Dylight 649, Alexa647, and Alexa750, were insensitive to the intracellular environment. It was also observed that the sensitivity of the fluorophore could be dependent on the biomolecule to which it was attached. In addition to assessing the environmental sensitivity of fluorophores, a method for quantifying the amount of fluorophores within living cells is also introduced. Overall, the present study provides a means to select fluorophores for studies that require an absolute quantification of fluorescence in the intracellular environment.

Evaluation of the interaction of propranolol with 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) liposomes: the Langmuir model

The interaction of the amine containing beta-receptor blocking agent propranolol (Ppn) with dimyristoylphosphatidylcholine (DMPC) vesicles was studied. Using a centrifugation assay, the protonated as well as unprotonated amount of the drug sorbed was verified, whereas the binding of the protonated Ppn was deduced from the surface charge density of the vesicles as calculated from electrophoretic mobility measurements. Assuming a 1:1 binding, a Langmuir model with only two parameters was found to be sufficient to fit all experimental data. Sensitivity analysis revealed that the estimated values of these parameters were reliable and independent from each other. These parameters were truly intrinsic, as electrostatic interactions were accounted for in the model. It was found that the pKa of Ppn shifted from 9.24, when dissolved in water, downward by 1.34 units upon sorption, indicating that the intrinsic partition coefficient of the unprotonated Ppn was about 22 times higher than that of the protonated analog. In addition, a significant increase in the affinity of both Ppn analogs with increasing salt concentration was found. Theoretical analysis revealed that the Langmuir sorption model may be considered as a partitioning model with decreasing partition coefficient as the sorbed amount increases. Thus, the Langmuir model provides a better fit than a simple partition model at conditions that induce a substantial amount of propranolol sorbed, such as high pH and high propranolol concentrations.

Cationic polystyrene nanosphere toxicity depends on cell-specific endocytic and mitochondrial injury pathways

The exponential increase in the number of new nanomaterials that are being produced increases the likelihood of adverse biological effects in humans and the environment. In this study we compared the effects of cationic nanoparticles in five different cell lines that represent portal-of-entry or systemic cellular targets for engineered nanoparticles. Although 60 nm NH(2)-labeled polystyrene (PS) nanospheres were highly toxic in macrophage (RAW 264.7) and epithelial (BEAS-2B) cells, human microvascular endothelial (HMEC), hepatoma (HEPA-1), and pheochromocytoma (PC-12) cells were relatively resistant to particle injury. While the death pathway in RAW 264.7 cells involves caspase activation, the cytotoxic response in BEAS-2B cells is more necrotic in nature. Using fluorescent-labeled NH(2)-PS, we followed the routes of particle uptake. Confocal microscopy showed that the cationic particles entered a LAMP-1 positive lysosomal compartment in RAW 264.7 cells from where the particles could escape by lysosomal rupture. A proton pump inhibitor interfered in this pathway. Subsequent deposition of the particles in the cytosol induced an increase in mitochondrial Ca(2+) uptake and cell death that could be suppressed by cyclosporin A (CsA). In contrast, NH(2)-PS toxicity in BEAS-2B cells did not involve the LAMP-1 endosomal compartment, stimulation of proton pump activity, or an increase in mitochondrial Ca(2+). Particles were taken up by caveolae, and their toxicity could be disrupted by cholesterol extraction from the surface membrane. Although the particles induced mitochondrial damage and ATP depletion, CsA did not affect cytotoxicity. Cationic particles were taken up into HEPA-1, HMEC, and PC-12 cells, but this did not lead to lysosomal permeabilization, increased Ca(2+) flux, or mitochondrial damage. Taken together, the results of this study demonstrate the importance of cell-specific uptake mechanisms and pathways that could lead to sensitivity or resistance to cationic particle toxicity.

Breaking the bonds: non-viral vectors become chemically dynamic

The delivery of a variety of nucleic acids such as plasmid DNA (pDNA) and small interfering RNA (siRNA) to mammalian cells is both an important research tool and potential therapeutic approach. Synthetic vehicles (SVs) that include lipoplexes and polyplexes, are widely used for non-viral delivery. A promising method of improving the efficacy of this approach is to create SVs that are chemically dynamic, so that delivery is enabled by the cleavage of chemical bonds upon exposure to various physiological environments or external stimuli. An example of this approach is the use of masked endosomolytic agents (MEAs) that improve the release of nucleic acids from endosomes, a key step during transport. When the MEA enters the acidic environment of the endosome, a pH-labile bond is broken, releasing the agent';s endosomolytic capability. Another challenge has been to develop SVs that enable in vivo delivery. Recently, an MEA that was used within dynamic polyconjugates (DPCs) enabled the efficient delivery of siRNA into hepatocytes in vivo. The use of labile bonds to mask endosomolytic agents, provides a critical design feature, because it enables efficient in vivo delivery without sacrificing endosomolytic function for release into the cytoplasm.

Cytosolic delivery of membrane-impermeable molecules in dendritic cells using pH-responsive core-shell nanoparticles

Polycations that absorb protons in response to the acidification of endosomes can theoretically disrupt these vesicles via the "proton sponge" effect. To exploit this mechanism, we created nanoparticles with a segregated core-shell structure for efficient, noncytotoxic intracellular drug delivery. Cross-linked polymer nanoparticles were synthesized with a pH-responsive core and hydrophilic charged shell designed to disrupt endosomes and mediate drug/cell binding, respectively. By sequestering the relatively hydrophobic pH-responsive core component within a more hydrophilic pH-insensitive shell, nontoxic delivery of small molecules and proteins to the cytosol was achieved in dendritic cells, a key cell type of interest in the context of vaccines and immunotherapy.

Sheddable coatings for long-circulating nanoparticles

Nanoparticles, such as liposomes, polymeric micelles, lipoplexes and polyplexes are frequently studied as targeted drug carrier systems. The ability of these particles to circulate in the bloodstream for a prolonged period of time is often a prerequisite for successful targeted delivery. To achieve this, hydrophilic ‘stealth’ polymers, such as poly(ethylene glycol) (PEG), are used as coating materials. Such polymers shield the particle surface and thereby reduce opsonization by blood proteins and uptake by macrophages of the mononuclear phagocyte system. Yet, after localizing in the pathological site, nanoparticles should deliver their contents in an efficient manner to achieve a sufficient therapeutic response. The polymer coating, however, may hinder drug release and target cell interaction and can therefore be an obstacle in the realization of the therapeutic response. Attempts have been made to enhance the therapeutic efficacy of sterically stabilized nanoparticles by means of shedding, i.e. a loss of the coating after arrival at the target site. Such an ‘unmasking’ process may facilitate drug release and/or target cell interaction processes. This review presents an overview of the literature regarding different shedding strategies that have been investigated for the preparation of sterically stabilized nanoparticulates. Detach mechanisms and stimuli that have been used are described.

Flow Cytometry of HEK 293T Cells Interacting with Polyelectrolyte Multilayer Capsules Containing Fluorescein-Labeled Poly(acrylic acid) as a pH Sensor

Polyelectrolyte multilayer sensor capsules, 5 microm in diameter, which contained fluorescein-labeled poly(acrylic acid) (PAAAF) as pH-sensitive reporter molecules, were fabricated and employed to explore their endocytotic uptake into HEK 293T cells by flow cytometry. The percentage of capsules residing in the endolysosomal compartment was estimated from the fluorescence intensity decrease caused by acidification. Capsules attached to the extracellular surface of the plasma membrane were identified by trypan blue quenching. The number of capsules in the cytoplasm was rather small, being below the detection limit of the method. The advantages of polyelectrolyte multilayer capsules are that the fluorophore is protected from interaction with cellular compartments and that the multilayer can be equipped with additional functions.

Characterization of dual-wavelength seminaphthofluorescein and seminapthorhodafluor dyes for pH sensing under high hydrostatic pressures

Hydrostatic pressure is an important physical parameter in biology, with pressures in the few-hundred-atm range having significant effects on cellular morphology, metabolism, and viability. To ensure valid results when studying pressure effects using fluorescence spectroscopy and imaging methods, metabolic probes need to be characterized for high-pressure use. Of interest is the sensing of pH at high pressures due to the key role that pH plays in cellular function. Despite the availability of pH-sensitive dyes, only a few have been characterized for high-pressure use. Here we present the effects of pressure on the acid-base equilibria of four dual-wavelength seminaphthorhodafluor and seminaphthofluorescein dyes (pK(a)=6.6-7.8). Using phosphate buffers as high-pressure pH references, we investigate the pressure dependence of pK(a) for these dyes and determine the volume change associated with the acid-dissociation reaction. We find that if pressure-induced pK(a) changes are not accounted for during interpretation of emission spectra, systematic errors of up to 0.02 pH units per 100atm would result, comparable to previously measured pressure-induced pH changes in vivo. Results are validated by correctly sensing pH changes in Tris and acetate solutions. Methods presented here are applicable to other metabolic probes utilizing dual-wavelength ratiometric sensing modes.

Recent approaches to intracellular delivery of drugs and DNA and organelle targeting

Intracellular delivery of various drugs, including DNA, and drug carriers can sharply increase the efficiency of various treatment protocols. However, the receptor-mediated endocytosis of drugs, drug carriers, and DNA results in their lysosomal delivery and significant degradation. The problem can be solved and therapy efficacy still further increased if the approaches for direct intracytoplasmic delivery that bypass the endocytic pathway are developed. This is especially important for many anticancer drugs (proapoptotic drugs whose primary action site is the mitochondrial membrane) and gene therapy (nuclear or mitochondrial genomes should be targeted). This review considers several current approaches for intracellular drug delivery: the use of pH-sensitive liposomes, the use of cell-penetrating proteins and peptides, and the use of immunoliposomes targeting intracellular antigens. Among intracellular targets, nuclei (gene therapy), mitochondria (proapoptotic cancer therapy and targeting of the mitochondrial genome), and lysosomes (lysosomal targeting of enzymes for the therapy of the lysosomal storage diseases) are considered. Examples of successful intracellular and organelle-specific delivery of biologically active molecules, including DNA, are presented; unanswered questions, challenges, and future trends are also discussed.

pH at the micellar interface: Synthesis of pH probes derived from salicylic acid, acid–base dissociation in sodium dodecyl sulfate micelles, and Poisson–Boltzmann simulation

The study of the H+ concentration at the micellar interface is a convenient system for modeling the distribution of H+ at interfaces. We have synthesized salicylic acid derivatives to analyze the proton dissociation of both the carboxylic and phenol groups of the probes, determining spectrophotometrically the apparent pK(a)'s (pK(ap)) in sodium dodecyl sulfate, SDS, micelles with and without added salt. The synthesized probes were 2-hydroxy-5-(2-trimethylammoniumacetyl)benzoate; 2-hydroxy-5-(2-dimethylhexadecylammoniumacetyl)benzoate; 2-hydroxy-5-(2-dimethylhexadecylammoniumhexanoyl)benzoate; 2-hydroxy-5-(2-dimethylhexadecylammoniumundecanoyl)benzoate; 2-hydroxy-5-acetylbenzoic acid; and 2-hydroxy-5-dodecanoylbenzoic acid. Upon incorporation into SDS micelles the pK(ap)'s of both carboxylic and phenol groups increased by ca. 3 pH units and NaCl addition caused a decrease in the probe-incorporated pK(ap). The experimental results were fitted with a cell model Poisson-Boltzmann (P-B) equation taking in consideration the effect of salt on the aggregation number of SDS and using the distance of the dissociating group as a parameter. The conformations of the probes were analyzed theoretically using two dielectric constants, e.g., 2 and 78. Both the P-B analysis and conformation calculations can be interpreted by assuming that the acid groups dissociate very close to, or at, the interface. Our results are consistent with the assumption that the intrinsic pK(a)'s of both carboxylic and phenol groups of the salicylic acid probes used here can be taken as those in water. Using this assumption the micellar and salt effects on the pK(ap)'s of the (trialkylammonium)benzoate probes were described accurately using a cell model P-B analysis.

Recent advances in non-viral gene delivery

Gene therapy has been deemed the medicine of the future due to its potential to treat many types of diseases. However, many obstacles remain before gene delivery is optimized to specific target cells. Over the last several decades, many approaches to gene delivery have been closely examined. By understanding the factors that determine the efficiency of gene uptake and expression as well as those that influence the toxicity of the vector, we are better able to develop new vector systems. This chapter will provide a brief overview of recent advances in gene delivery, specifically on the development of novel non-viral vectors. The following chapters will provide additional details regarding the evolution of non-viral gene delivery systems.

Cytosolic delivery of macromolecules

Three tertiary amine-based detergents with zero, one, or two hydroxyl groups at various positions in their head group were characterized for their ability to promote the cytosolic delivery of macromolecules. Critical micellar concentrations (CMC) and membrane-bound pKa values of the lipid constructs increased with increasing head group polarity, ranging from 1-5 microM and 5.9 to 6.3, respectively. Fluorescence resonance energy transfer (FRET) and calcein leakage experiments revealed that when the amine group is protonated introduction of -OH moieties to detergent head groups enhanced their ability to interact with and permeabilize anionic, endosome-mimicking vesicles. Different formulations of a diethanolamine-based lipid (DEL) were further evaluated for pH-dependent hemolytic activity and ability to promote cytosolic delivery of macromolecules in vitro. Intact liposomes containing DEL at its maximum limit of incorporation were less efficient than DEL-containing micelles in promoting hemoglobin leakage from human erythrocytes at acidic pH. In HeLa cells, DEL-containing detergent micelles facilitated efficient cytosolic release of endocytosed macromolecules such as fluorescein-labeled dextran of MW 10 kDa. This observation was further corroborated by a functional assay based on antisense-mediated up-regulation of enhanced green fluorescent protein (EGFP). Taken together, our findings emphasize the key role of polar head groups and micellar architecture of pH-sensitive detergents in mediating endosomal permeabilization and the efficient cytosolic delivery of macromolecules.

Strategies for targeting protein therapeutics to selected tissues and cells

The advent of recombinant biotechnology and the recent sequencing of the human genome now allow for identification of scores of potential protein therapeutics along with the capacity to produce them in quantities and purities required for clinical application. Thus, clinical development of potential protein therapeutics has become as commonplace as development efforts of classical small molecule therapeutics. Whereas small molecule therapeutic lead candidates are identified through screens of large sets of possibilities, therapeutic protein candidates are defined by genetic information as a single composition (or a limited set of isoforms). Small molecule leads are identified through the combined assessment of desired selectivity, biodistribution and pharmacokinetic properties. In essence, these selection parameters emulate the actions of protein therapeutics that function as systemic hormones through their ability to target selective cells and tissues of the body via selective receptor interaction with minimal actions elsewhere. However, many, if not most, potential protein therapeutics do not normally circulate through the body to reach their target cell or tissue; rather, they are frequently synthesised at local sites, act at that site and are degraded without reaching appreciable systemic levels. Dose-limiting adverse events are associated with systemic administration of many of these proteins, restricting their clinical potential. This review examines current strategies to reduce these dose-limiting events by possibly focusing the delivery of potential protein therapeutics to discrete tissues and cells.

Caco-2 permeability of weakly basic drugs predicted with the Double-Sink PAMPA method

The aim of this study was to analyze pH-dependent permeability of cationic drugs in Caco-2 cell monolayers using the pK(a)(flux) method and to correlate the results with those obtained in PAMPA (parallel artificial membrane permeability assay). The pH-dependent permeability of verapamil and propranolol was studied in Caco-2 cell monolayers. The data were subsequently processed using software developed for the PAMPA pK(a)(flux) method. Literature values for an additional nine cationic drugs were also analyzed. Double-Sink PAMPA data were also obtained for the same cationic drugs, to compare with the Caco-2 data. The Algorithm Builder program was then used to develop a predictive model of Caco-2 permeability based on PAMPA permeability and calculated Abraham molecular descriptors. From the relationship between permeability and pH it was shown that in PAMPA only the uncharged form of the drugs permeated across the membrane barrier, while charged and ionized forms of the drugs were significantly permeable in Caco-2. The charged-form permeability, P(i), was therefore determined and subsequently subtracted from all permeability coefficients in Caco-2 prior to the comparison with PAMPA. The resulting intrinsic permeability coefficients (P(o)) obtained in Caco-2 were successfully related to those derived from the PAMPA model. In this study we have shown that permeability coefficients obtained in PAMPA can predict the passive transcellular permeability in Caco-2.